Direct Ethanol Fuel Cell for Clean Electric Energy: Unravelling the Role of Electrode Materials for a Sustainable Future

Direct ethanol fuel cells (DEFCs) are better than others in commercially used FCs due to easy availability, less toxicity, and C‐2‐type alcohol. Ethanol has a high theoretical efficiency of 97% and is a safe, plentiful, and renewable resource that can be stored and controlled using the infrastructure that is in place now. Nevertheless, low functional efficiencies and the release of carbon dioxide (CO2), acetaldehyde, and byproducts of acetic acid must be addressed if DEFCs are to grow and become more commercially viable. To overcome these problems, new anode and cathode catalysts are needed, so this review article discusses the introduction of FCs with their structure, working and mechanism. Further, the report covers various types of FC catalysts, and their application in FC technology is explained. The role of the catalyst (such as anode and cathode), similarities and differences between Pt/Pd‐based catalysts, and the importance of supporting materials (such as carbon, transition metal dichalcogenides, MXene, and black phosphorus‐based materials) in DEFCs are described. In addition, the applications, advantages, and disadvantages of the DEFCs are discussed. Finally, the proposed theme is concluded with the existing challenges in this field and the future prospect of DEFCs.


Introduction
The necessity for energy rises gradually; as a result, fossil fuel is used rapidly.The pollution problem is the primary concern related to fossil fuels by the release of poisonous gases and some particulate matter.Research is going on in this area and gives a valuable alternative for this problem: the invention of the alcohol fuel cell (FC).This FC has high calorific value and no or fewer environmental issues.Ethanol (EtOH) is the most suitable option among the available alcohols as it is a C-2-type alcohol with multipurpose use.Research is going on for EtOH in various areas, including transportation fuels. [1]The use of gasoline decreased by using EtOH, as done in the United States-this increase of alcohol as fuel increases daily.Alcohols also give less transportation cost of energy, emitting less greenhouse gas than gasoline.Due to the smaller carbon chain of EtOH as compared to other alcohols (except methanol (MeOH)) and the less toxic effect of EtOH as compared to MeOH, direct ethanol FCs (DEFCs), it produced more energy than direct methanol FCs (DMFCs). [2]pproximately 8.0 kW h À1 kg À1 energy is produced from the complete oxidation of EtOH, while it is 6.1 kW h À1 kg À1 in the case of MeOH. [3]econd, EtOH is less toxic than MeOH because MeOH sometimes destroys the central nervous system and leads to blindness.EtOH is readily available and less costly as it is produced mainly from agricultural activities.The alteration of EtOH into carbon dioxide (CO 2 ) by a whole oxidation reaction is the main objective of DEFCs.By this process, 12 electrons are produced during one reaction. [4]The performance of DEFCs depends on the membrane electrode assembly (MEA).MEA consists of electrolyte, support material, and electrocatalyst. [5]To enhance the activity of DEFCs, we need some electrode catalysts.These catalysts provide a larger surface area and decrease the activation energy for the EtOH electrooxidation reaction.Hence, the catalyst also increases the rate of reaction.A catalyst, i.e., anode and cathode, is used at the electrode.5d,6] The reactions EOR and ORR affect the activity of DEFCs.The half-reaction of the anode and cathode are different; hence, preparing the catalyst in various shapes gives optimal activity. [7]For the applications of FCs, a particular category of hybrid electrode catalysts based on conducting polymer (CP)-supported nanomaterials combines the advantageous qualities of both nanoparticles (NPs) and CPs.Much work has been put into creating platinum (Pt)-free catalysts deposited on CPs. [8]Still, because low-Pt NPs have such excellent catalytic performance, they are also loaded onto polymers.Graphene has garnered significant interest due to its large surface area and high theoretical capacitance. [9]Graphene, a 2D macromolecule, is extensively researched for its outstanding properties: high electrical conductivity (2000 S cm À1 ), large theoretical surface area (2630 m 2 g À1 ), excellent stability, mechanical flexibility, low weight, and cost-effectiveness. [10]Reduced graphene oxide, a derivative of graphene oxide, is gaining significant attention as a viable alternative to graphene. [11]Also, electroactive CPs, like polyaniline, stand out for applications in secondary batteries, FCs, electrochromic devices, supercapacitors, electrocatalysis, sensors, and biosensors due to their high conductivity, unique redox properties, stability, electrocatalytic activity, and strong adherence to electrode surfaces. [12]For CP-supported hybrid catalysts, numerous research groups have successfully created a variety of synthetic techniques, including metal NPs, multimetallic NPs, and metal oxides that can be employed as anodes, cathodes, and electrolyte membranes. [13]In this article, we will discuss the brief history of the FC, their working mechanism, various types of electrode catalysts, and their electrochemical performance toward the DEFCs.

History and the General Outlook of the FC
Innovative work that ultimately prompted a functional FC returns to the middle 1800s through the excellent work of Sir William Grove, i.e., electrolyzer/FC experimental demonstration.He was a physicist and patent barrister widely regarded as the father of FC technology.He also developed a reverse procedure that might generate power using electrolysis knowledge.Considering this theory, Grove triumphed and successfully designed a system that utilizes oxygen and hydrogen to generate electricity (rather than isolating them by using power).Once known as a gas battery, the appliance became an FC. [14]Fossil fuels fulfil approximately 81% of the energy economy of the world.Burning of fossil fuel also emits lethal and toxic materials/gases, which leads to the search for alternate energy sources, i.e., hydrogen-based energy.Hydrogen acts as a clean source of energy with zero emission.Hydrogen production by electrolysis of water is more efficient and highly economical.The show, transportation, and storage of hydrogen do not create any environmental pollution and are environmentally friendly.Hydrogen can be produced by electrolysis of water using a variety of electrolyzer technologies, such as alkaline electrolyzers, solid oxide electrolyzers, and proton-exchange membrane (PEM) electrolyzers.Fundamentally, an FC works like a battery because it has an electrolyte positioned in between the two electrodes.Dissimilar to the battery, the energy in an FC has a longer life or needs to be recharged up to capacity levels. [15]Oxygen is used from one terminal, while hydrogen is used from the other terminal, which produces power, water, and heat.The FC "anode" is used as a source of hydrogen fuel, and oxygen (or air) enters the FC through the terminal, which is cathodic in polarization. [16]FCs reduce the amount of regulated toxic emissions.The emissions of currently controlled pollutants, such as carbon monoxide, nitrous oxides, sulfur oxides, and particulates, are well below the standards for current air quality and are typically almost nonexistent. [17] Working and the Mechanism of the FC FCs are energy converters based on electrochemistry.Without a thermoelectric transitional step, an FC's chemical energy is instantly transformed into electrical energy.In the FC, an exothermic chemical reaction occurs, wherein the responding molecules or atoms exchange electrons or electrical charges instantly.For instance, hydrogen can spontaneously oxidize with oxygen, releasing heat due to the high energy content of the reaction.In an FC, the fuel does not respond directly to the oxidant.However, it transports its electrons to the anode.The electrons then, at that point, flow toward the cathode, where they are taken up by the other reaction accomplice, commonly barometrical oxygen.This allows for regulating the reaction and high levels of current proficiency.Low-and high-temperature FCs are the two main types of FCs.Suppose the operating temperature, that is, approximately 120 °C, is the category of low temperature; when the temperature lies between 600 and 1000 °C, it is called hightemperature FCs.[18] The FC has two terminals, including the cathode and anode.The cathodes allow the passage of hydrogen and oxygen into a bed of concentrated NaOH solution.However, it could take a very long time to produce any energy because of the slow reaction rate.In this way, the synergistic energy of platinum (Pt) or palladium (Pd) is sufficient for accelerating the reaction.The logical methodology determines how assuming the surface area is expanded, the reaction rate is also accelerated.Stimuli are finely divided to enhance the surface area of the catalyst and the rate of reactions divided.[19] The hydrogen gas is ionized at the anode, giving a free electron and H + particles.Each hydrogen particle brought to the anode surface is separated into two H atoms by the catalytic property of the electrode.These moves into electrolyte solution as hydrogen particles release 2e À , which go through the outer circuit to the cathode (positive electrode).

Anodic reaction∶ H
At the cathode, oxygen combines with the electrolyte's water and the electrons that have been transmitted to form hydroxyl (OH À ) ions.
The overall chemical reaction of an FC illustrates that the supplied water, electrical energy, and heat are produced. [20]he working mechanism of DEFC with catalyst is different in different mediums, i.e., acidic and basic mediums.It was found that in the case of redox reactions, an alkaline medium shows a better and more interesting mechanism than an acidic medium. [21]In the acidic media, the OH ad is present due to the activation of water that blocks the O 2 adsorption directly over the active site.On the other hand, in the alkaline medium, the OH ad is present due to specific adsorption of OH À species that promote the transfer of electrons by the outer-sphere mechanism and result in the formation of intermediate product peroxide.Further, the interaction between OH À moieties present at the surface and cluster of O 2 .(H 2 O) n creates some nonspecific nature that cannot identify the particular electrode materials.The result of the nonspecificity of alkaline media makes it more suitable for works with multiple-range non-noble metals (NNMs), their oxides as electrode materials, and also accelerates the kinetics of reaction compared to acidic media.The outer-sphere mechanism requires a highly catalytic surface based on Pt and does not apply to a wide range of NNMs because it is absent in acidic media. [22]Further, for comparing both media, some Pt-and Pd-based catalysts based on carbon support, i.e., Pt 1Àx Pd x /C, have been fabricated using the formic acid reduction approach.For this purpose, different ratios of Pt and Pd are used, like x at% equals 0, 27, 53, 77, 100.When compared to acidic media, the electrochemical performance of Pt 1Àx Pd x /C catalyst for the electrooxidation of EtOH performs better in alkaline media.In the alkaline medium, Pt 23 Pd 77 /C catalyst shows higher electrocatalytic activity with 2453.7 mA mg PtPd À1 value of mass-specific peak current, while it is 339.7 mA mg PtPd À1 in the acidic medium with catalyst Pt 77 Pd 23 /C. [23]his result indicated that DEFCs required both the medium for working according to the material used, but it works more suitably in an alkaline medium than in an acidic medium.Ptbased catalysts are typically used in acidic medium DEFCs, whereas nonprecious metal catalysts are frequently used in alkaline medium DEFCs.Nonprecious metal catalysts provide an affordable alternative for alkaline conditions, although they have certain durability issues.Pt-based catalysts exhibit high activity and durability in acidic environments. [24]The current study aims to improve catalyst stability and performance in acidic and alkaline media for robust and efficient DEFC systems.

Types of the FC
FCs classified into several sorts based on the reactant type utilized (hydrogen, MeOH, methane, carbon monoxide, and other natural substances), the electrolyte type, and the working temperatures. [25]DEFCs are polymer electrolyte membrane FCs (PEMFCs), which utilize a solid polymer membrane as the electrolyte. [26]This membrane permits protons to pass through while blocking the path of electrons, producing an electric current which can be harnessed for different applications.
One of the key benefits of DEFCs over other FC types is using EtOH as fuel.EtOH is a renewable and readily unrestricted alcohol produced from different biomass sources, such as corn, sugarcane, and cellulosic materials. [27]Unlike hydrogen, typically employed in other FCs, ethanol is nontoxic, easily controlled, and has a higher energy density (ED), causing it more suitable for storage and transportation.EtOH electrochemical oxidation at the anode and oxygen reduction at the cathode are impacted by a DEFC.At the anode, EtOH is converted to protons, CO 2 , and electrons.While the electrons must travel via an external circuit to induce an electric current that can be used to power appliances or charge batteries, the protons pass through the electrolyte membrane and reach the cathode. [28]At the cathode, oxygen from the air reacts with the protons and electrons to form water.
Out of the above-discussed FCs, various other categories of FCs are also known, which have been reviewed in multiple pieces of literature frequently. [29]The pictorial form of different FCs is shown in Figure 1a-c for solid oxide FC (SOFC), polymer electrolyte FC (PEFC), and biofuel cell (BFC), respectively. [30]he FC works in both mediums, i.e., in acidic and basic mediums.The working temperature, kind of electrolyte used, their working efficiency, and applications of different FCs are different.Some of them are arranged in brief in Table 1.
Out of the above-discussed FCs, we focused more on the DEFC in our present article due to its higher efficiency than DMFCs.Their catalysts took into consideration as they enhance the electrochemical performance of DEFCs.

DEFC
Relating to alcohol, EtOH stands out as an appealing and promising fuel because of its availability in nature, lack of toxicity, and high-power density.Considering that technology-wise, DEFCs seem more attractive than DMFCs.The DEFCs are divided into passive and active fuel distribution and handling categories.A dynamic system of DEFCs needs moving components to  and c) BFC.Reproduced with permission. [30]Copyright 2017, Multidisciplinary Digital Publishing Institute.
utilize oxidizing agents and fuel, which provide power to the cell for its working.An active approach is more expensive, has lower ED, and is more suitable for large FCs.To achieve all cycles with no additional power consumption, the passive framework is utilized for diffusion, evaporation, air breathing, and natural capillary forces.Therefore, the passive system of FCs is much more significant for portable power sources. [31]n different applications, FCs are generally perceived as highly appealing devices to get direct electric energy by the combustion of chemicals, i.e., fuels.Low-temperature FCs, commonly considered a proton electrolyte layer, appear ready to be utilized and have enormous scope in power applications.On the other hand, MeOH has been considered a harmful product for some time.Nevertheless, a possible ecological issue concerning MeOH is its enormous mixing in water.EtOH provides a desirable alternate or option as a fuel in absolute temperature energy components because it is the primary sustainable biofuel derived from biomass fermenting and may be produced in vast amounts from agricultural products.The EtOH delivered to the anodic chamber can pass by the electrolyte to the cathode, like the instance of MeOH in DMFCs. [32]The reactions that take place in DEFCs are shown by Equation ( 5)-( 7): [33] At anode∶ The DEFC, as the DMFC, works by EtOH oxidation over the catalyst's surface to form CO 2 and H 2 O. Protons (H + ) are carried toward the anode via a PEM and react with oxygen to form H 2 O. Electrons (e À ) are taken toward the cathode from the anode via the outer circuit, giving the capacity to associated devices. [34]his DEFCs system can lower EtOH crossover, minimize catalyst poisoning because of highly active ORR, and introduce efficient hydrogel electrolytes with ion conductivity and good compatibility in an alkaline medium.The exemplary conduct indicates the versatile DEFC system's enormous potential for use in various applications, including electronics.Pt nanocrystal has shown excellent performance with their distinctive polyhedral concave architecture. [35]A few factors decide the exhibitions and durability of the DEFC.Among them, EtOH crossover is a significant challenge.Overall, EtOH crossover is comparatively less as compared to MeOH due to EtOH's greater atomic weight. [36]1.Types of DEFC DEFCs come in two variations: alkaline and acidic.Alkaline DEFCs incorporate an alkaline membrane, whereas acidic DEFCs utilize an acidic membrane.In the acidic DEFC, a membrane partitions the positive and negative electrode chambers.There is an acid solution within the positive electrode chamber, while the negative electrode chamber holds an alkali solution.[37] 5.1.1.Acidic DEFC The acidic-type DEFC shows great potential as a technology for portable power sources.By employing EtOH as its fuel, DEFC takes advantage of a nontoxic, renewable resource with a high ED.Advancements in acidic membranes, catalysts, and a comprehensive understanding of reaction mechanisms play crucial roles in enhancing the performance of acidic DEFCs and paving the way for their commercialization. [38]In the acidic-type DEFC, the oxidant in the fuel stream is H 2 SO 4 . [39]5c] It reduces the energy needs for hydrogen production and enhances FC performance by lowering the anodic charge transfer resistance.Yet, a heightened H 2 SO 4 concentration can dissolve the anodic catalyst, resulting in significant FC degradation.The study examined H 2 SO 4 addition to the anolyte for electrochemical EtOH reforming in a PEM electrolysis cell.Tests with Pt-Sn/C and Pt/C MEAs, Nf membrane, revealed an optimal H 2 SO 4 concentration (4 mol L À1 EtOH, 0.01 mol L À1 H 2 SO 4 ), improving electrocatalytic activity and stability by reducing energy needs for hydrogen production. [40]hile the acidic DEFC holds promise for clean and efficient power generation, additional research is essential to fine-tune the H 2 SO 4 addition and address technical and economic hurdles. [41]ble 1.Different types of FCs and their working conditions.

Alkaline DEFC
In the electrooxidation of EtOH, alkaline DEFCs have demonstrated success using Pt catalysts, and research indicates promising results with Pd as a catalyst in these FCs.The growing research focus on alkaline DAFCs is attributed to their advantages over acidic mediums.These include enhanced catalyst stability, diminished corrosion concerns in DAFCs, reduced alcohol crossover, and the utilization of cost-effective metal current collectors. [33]Utilizing NaOH as the electrolyte enables effective energy conversion and yields high power densities. [42]Enhancing alkaline DEFC performance involves optimizing the anodic flow field design, a critical factor influencing FC performance, particularly in lower temperatures. [43]Polyvinyl alcohol (PVA)-based sodium ion conducting membranes have been explored for alkaline DEFCs, exhibiting promising outcomes in power density and performance compared to alternative membrane types. [44]verall, alkaline DEFCs using NaOH as the electrolyte offer a sustainable and efficient technology for directly converting EtOH into electricity. [45]Zakaria et al. [46] compared the catalysts Pt/C, Pd/C, PtRu/C, and PdNi/C for their activity in alcohol oxidation reactions in both acidic and alkaline media.The Pt/C catalyst exhibited enhanced reactivity, further improved with the binary catalyst PtRu/C.Peak current densities using PtRu/C for MeOH, EtOH, and glycerol oxidation reactions were measured at 6.8, 17, and 1.1 mA cm À2 , respectively.Wang et al. [47] employed catalysts Pt/CNT, Pd/CNT, and Au/CN for electrooxidation of MeOH, EtOH, ethylene glycol, n-propanol, 2-propanol, and glycerol.In an alkaline medium, Pt/CNT, Pd/CNT, and Au/CNT exhibited high activity in the oxidation reactions of alcohols.Conversely, Pd/CNT and Au/CNT demonstrated low activity and almost no reactivity in an acidic medium.

What Is Ethanol Crossover?
Fuel crossover is a common criticism of any primary alcohol FC.Fuel crossover occurs when fuel travels through the electrolyte from the anode to the cathode without being desired.EtOH crossover also affects the performance and durability of DEFCs.EtOH has notable advantages because of the reduced rate of fuel crossover and influencing the cathode activity less acutely in regard to MeOH due to its more modest porousness by the Nafion (Nf ) layer and its sluggish electrochemical oxidation energy at the cathode of Pt/C.Similar to MeOH crossover, EtOH crossover is defined as the electrolyte membrane allowing EtOH to move from the anode toward the cathode.The drawbacks of EtOH hybrid FCs include cathode depolarization and decreased cathode potential.The general productivity of direct-EtOH energy units will diminish because of EtOH crossover.
Consequently, it wastes fuel while in use-the EtOH crossover rate changes with temperature, current density (CD), and provided concentration.5c] The prepared catalysts were precoated onto two pieces of carbon cloth and placed between the sodium polyacrylate (PANa) hydrogel to assemble the DEFC (Figure 2a,b). [48]The anodic carbon cloth was fastened with a small sponge for sufficient EtOH fuel.The exposed electrolyte portions were covered with a preservative film to maintain geometrical integrity and sealed with sealing film.Our solid-state DEFC offers long-term discharge endurance for approximately 120 h with a small EtOH ED of up to 13.63 mWh cm À2 , even outperforming various stiff ones (3 mL).Additionally, the completed FC is adaptable and has an adding-and-run feature (when EtOH is poured into the anodic compartment, the FC reacts).
Figure 2c depicts these dealloyed catalysts normalized CV curves in an H 2 SO 4 (0.5 M) solution.In an explanation of KOH (1.0 M) þ EtOH (0.5 M), the electrocatalytic performances of these treated samples toward EOR were investigated (Figure 2d).The anodic CD (If ) of the forward scan for np-AlPdNiCuMo is obtained at 2.67 A mg À1 Pd , which is 15.71-fold as compared to np-AlPdNi (ternary/0.17A mg À1 Pd ) and 1.82-fold that of np-AlPdNiMo (quaternary/1.47A mg À1 Pd ) (Figure 2e), demonstrating the considerable activity improvement by producing a several constituent high-entropy alloy (HEA) having a proper element mix.Chronoamperometric (CA) tests were carried out for 250 min over 0.8 V (vs reversible hydrogen electrode [RHE]).Throughout the testing, it was discovered that the np-AlPdNiCuMo displayed a constant and enhanced CD (Figure 2f ), indicating increased catalytic activity and high stability.
We must utilize distinct membranes in MEA in order to stop fuel crossover and waste.Alkali anion exchange membranes, also known as PEMs, are arranged in a layer called a MEA, which uses a catalyst and a level plate electrode as its energy components. [49]node dispersion film, anode catalyst coating, electrolyte membrane, cathode catalyst coating, and cathode diffusion membrane are the five hypothetical sections that make up MEAs. [50]The diffusion layers give the fundamental mechanical design to the cathode and anode.Furthermore, they simultaneously work or act as a transport channel for the products and reactants as well as a conductor and collector of current.Most normal layers utilized are produced using Nf.When NaOH or KOH is used to modify Nf, its ionic conductivity and stability toward heat are increased.According to a hypothesis, the ionic conductivity of KOH is more excellent than NaOH.The catalyst layer (CL) can be placed directly over the membrane or the electrode can be separated using the two commonly used methods for creating MEAs.The first method involves rolling, spraying, or printing the catalyst directly onto the electrolyte membrane to form a catalyst-coated membrane (CCM).Next, the CCM-coated diffusion membrane is heated to press the MEAs together. [51]

Mechanism to Prevent Ethanol Crossover
Alcohol crossover is the most significant problem in DEFCs.It leads to the wastage of fuel and affects the efficiency of FC.To overcome related challenges, various polymer electrolyte membranes are used.For example, to prevent EtOH crossover and improve cell performance and the working efficiency of EtOH fuel, it is suggested to use a composite anode with an outer and an inner CL.The inner CL is the reactive EtOH filter with a thin layer of Pt 50 -Sn 50 NPs directly deposited via the impregnation-reduction method on the Nf membrane surface. [52]he Nf membrane is the standard polymer, discussed in the next section, out of the known polymer electrolyte membrane.Several types and manufacturing processes are used to manufacture electrolyte membranes for FC applications, such as composite-based membranes, blending, nanocomposite-based membranes, and so on.

Nafion Membrane
By mimicking proton conductors, Nf ionomers are dispersed at the CL mediating between the electrodes in the MEA design to augment the electrochemically dynamic locale for the CL.The Nf layer acts as a hydrophilic agent inside the FC and significantly enhances the mechanical stability of the cell. [53]The perfluorinated polymer known as Nf is composed of a polytetrafluoroethylene (PTFE) spine and uniformly separated extended perfluoro vinyl ether pendant side chains terminated by sulfonic or carboxylic ionic functional groups.The hydrophobic PTFE is genuinely the spine of Nf because it provides stability toward heat and chemicals, and its perfluorinated side chain shows a water-loving nature and works for the conduction of protons.Nonetheless, it has significant weaknesses, including 1) its high cost of production, 2) lower conductivity at higher temperatures or lower humidity, 3) mechanical stability loss at higher temperatures, 4) increased alcohol porousness, and 5) limited working temperatures.
The most well-known method to solve all the drawbacks is the modification of Nf 's physical qualities using polymer nanocomposite technology. [54]hus, the pretreatment method, protonation level, and manufacturing process can all directly impact the Nf characteristics for FC applications.Various techniques are used to prepare the Nf membrane.One of them is electrospinning.Electrospinning is an appropriate method for creating polymer fibers by introducing an outer electric field in a polymer solution and ejecting it using a needle.This technique reduces Nf size to the nanoscale and performs better.Dong et al. [55] are trying to find highly pure Nf fibers for FC applications.By using the electrospinning process, they introduced a superconductive Nf were determined.f ) CA graphs for the EOR at 0.8 V in a solution of EtOH (0.5 M) and KOH (1 M) that are saturated by N 2 (vs RHE).Reproduced with permission. [48]Copyright 2020, John Wiley and Sons.
membrane.High-purity Nf was created using a carrier polymer having less content (higher subatomic weighted polyethylene oxide, i.e., 8000 kg mol À1 ).Their findings showed that high atomic weight carrier polymer could frame very pure Nf fiber, while polymer with the lower carrier, i.e., 400 g mol À1 , gives beaded fibers.Polyethylene oxide systems with polar solvents like water and alcohols exhibit gel-like behavior and are not electrospinnable due to hydrogen bonding. [56]

Composite-Based Nafion Membrane
For a composite-based membrane fabrication, a protonconductive ionomer, such as Nf, fills a porous neutral polymer.Combining the desired properties of electrolyte membranes through such a process, Yu et al. [57] used the porous PTFE film that will be impregnated with a modified Nf solution.In spite of this, they selected a different solvent that depends on the dissolvability of Nf membrane.It is a solubility parameter (δ = 9.5(cal cm À3 ) 1/2 ; they chose a different solvent for the Nf membrane.Among other solvents used in their work, MeOH, EtOH, and propane-2-ol, the final remaining one (propane-2ol), because of its solubility parameter close to the backbone of Nf, was selected as a suitable solvent for Nf.A commercially available Nf solution is used in their work.The prepared composite membranes performed well based on their results compared to Nafion 122, 117, and 112 commercial Nf membranes. [56] weak interfacial connection between the catalyst and membrane is made for the proton transfer.Proton transport is always impacted by maintaining solid contact among the catalyst and the membrane.The membrane's surface characteristics and flexibility must be carefully considered, mainly when built in a tubular pattern.The impact of membrane materials is another critical consideration.Because of this, numerous scientists have created composite-based Nf membranes, Nf-membrane based on surface pattern, and even non-Nf membranes, as illustrated in Figure 3. [58]

Modified Nafion Membrane
Hybridizing or creating natural or inorganic materials in various shapes, sizes, and structures with Nf grids can modify Nf membranes like a polymer film for DAFCs.To develop novel nanoarchitecture Nf hybrid membranes or nanocomposites, natural or inorganic NPs from a variety of materials, including SiO 2 , oxides of metal, phosphate of zirconium, zeolites, PVAs, clay, poly-pyrrole, and some other materials, can be added with an Nf polymer in a variety of architectures.These categories of Nf composite membranes, which include two and three components, can be divided into Nf dual hybrid membranes and Nf triple hybrid membranes.Nf films can also be changed on the surface of Nf and in multifaceted frameworks to reduce MeOH crossover.54a]

Catalyst-Coated Membrane
In order to improve the contact area between the polymer electrolyte and catalyst, a Nf layer was added to the interface between the membrane and the CL, resulting in a modified CCM structure.The FC performance was greatly enhanced using the fivelayer CCM.The five current layers are: layer one, cathode layer one, layer two, anode layer two, and SPE membrane.Layer two, the CL, arises from layer one, which is connected to the membrane and is composed of Nf solution.For layer one preparation, a section of the Na þ form membrane was put on a vacuum table.Nf solution (5 wt%, EW 1100, Dupont) was then sprayed over the membrane at 60 °C.Consequently, the dry Nf loading of layer one was approximately 0.6 mg cm À2 .Layer one was covered with catalyst ink.To create two layers, Pt-Ru black is loaded at approximately 3.7 AE 0.1 mg cm À2 for the anode and Pt is loaded at approximately 2.3 AE 0.1 mg cm À2 for the cathode. [59]The impact of catalyst ink solvent types upon the activity of MEA synthesized through the CCM process was examined.The key physical qualities were the dielectric constant, swelling proportion, Figure 3. Advancement in the Nf membrane.Reproduced with permission. [58]Copyright 2021, Elsevier Ltd. viscosity, and boiling temperature of the solvent utilized in catalyst ink synthesis.The solvents' viscosity and boiling point affected the catalyst ink's viscosity.Pt utilization can be increased by using solvents that have the best density and evaporation rate to distribute the CL uniformly across the membrane.Due to the high solvent absorption capacity of the membrane, there can be an improvement in the proton transportation and ionic resistance as a result of better membrane-catalyst interaction. [60]

Gas Diffusion Electrode
A CL is placed on top of a gas diffusion layer (GDL) to create a gas diffusion electrode (GDE).Enzymes and various types of catalysts, such as metal, metal-free, and molecular catalysts, can be accommodated in GDEs. [61]The gas flow channel or field and the CL are separated by a porous assembly called the GDL.GDL gives the catalyst physical support while facilitating gas flow to the CL.Gas passage to the CL is facilitated by the hydrophobic nature of the GDL, which prevents the electrolyte from clogging its pores.There are two categories for GDLs: single-and dual-layer GDLs.A dual-layer GDL combines an microporous substrate (MPS) and a microporous layer (MPL) in place of a single-layer GDL, which consists solely of a MPL or MPS (Figure 4). [62]n order to look into the EtOH oxidation activity, two-layer GDEs were made using the electrocatalysts. [63]Over a carbon cloth (PWB-3, Stackpole), a diffusion layer of carbon powder (Vulcan XC-72R) and 15% (w/w) PTFE was applied.Above this layer was the electrocatalyst, which was a uniform dispersion of isopropanol (Merck) and Nf solution (5 wt%, Aldrich).Every electrode has been created using 1 mg of Pt cm À2 .Electrodes were hot pressed on equal edges of a Nf 115 membrane for 2 min at 125 °C and 50 kg cm À2 for the direct-EtOH single-cell experiments. [64]3.6.PEMs PEMs, or PEMFC membranes, are a crucial segment of FC systems.They act as an electrolyte, relieving the transfer of protons (H þ ) while blocking the passage of electrons.PEMs play a critical function in the electrochemical reactions within FCs, allowing the alteration of chemical energy within electrical energy.[65] PEMs typically comprise a thin, solid polymer material with excellent proton conductivity and high chemical stability.[66] The most commonly employed polymer for PEMs is a perfluoro sulfonic acid polymer known as Nf, generated by DuPont.Nf includes a Teflon-like backbone with sulfonic acid (-SO 3 H) groups linked to the polymer chain.The system of PEMs is typically a porous, hydrated polymer film.Water molecules within the membrane are essential for preserving its ionic conductivity.[67] The water content affects the membrane's performance, as extreme drying can lead to reduced proton conductivity, while effective water content can inhibit the transport of reactant gases.
PEM's function is based on the principles of proton transport.When hydrogen gas is provided to the anode of a PEMFC, it dissociates into H + and electrons (e À ).The PEM selectively permits the protons to relocate via the membrane while controlling the path of electrons.This generates an electric current that can be used for a variety of purposes by creating a positive charge on the anode electrode and causing electrons to flow through an external circuit.The oxygen from the air combines with the protons carried by the PEM and enters the FC's cathode. [68]Water is produced as a byproduct at the cathode when protons, electrons, and oxygen combine.The overall reaction in a PEMFC can be categorized as the electrochemical oxidation of hydrogen fuel to produce water and electricity.
PEMs need to show specific effects to provide an efficient FC process.These include the following: 1) The initial condition of a PEM is high proton conductivity, permitting quick and efficient transfer of protons across the membrane. [69]This provides a low internal resistance in the FC, resulting in high power output.
2) PEMs should be chemically stable under the working necessities of the FC.They must resist degradation from the reactant gases, high temperatures, and the acidic conditions produced by the presence of protons. [70]3) PEMs must be mechanically robust to endure the stresses and strains encountered during the FC process. [71]They should preserve their structural integrity over vast periods without compromising performance.4) PEMs should have the tiniest gas permeability to control reactant gas crossover among the anode and cathode.Gas crossover can result in smaller fuel efficiency and reduced performance. [72]) Effective water control is essential for optimal PEMFC performance.The membrane must balance water content to ensure sufficient hydration for proton conduction without flooding or drying out. [73]hile Nf remains the most broadly utilized PEM material, investigators repeatedly study alternative membranes to overcome restrictions and enhance FC performance.These include nonfluorinated polymers, composite membranes, and inorganic materials, for example, phosphoric acid-doped polybenzimidazole membranes.Efforts are also underway to progress PEMs that can work at higher temperatures, known as high-temperature PEMs (HTPEMs). [74]These HTPEMs offer advantages such as enhanced electrode kinetics, improved tolerance to fuel impurities, and simplified water management. .Basic diagrams of a GDE.The cathodic section of a CO 2 electrolyzer is usually where the stacked assembly of a GDE fitted between the liquid electrolyte and the gas flow field is located.Reproduced with permission. [62]Copyright 2020, Multidisciplinary Digital Publishing Institute.

Mechanism of DEFC
While we think of more growth in FC technology, it is evident to know the catalytic mechanism of electrocatalysts to develop more suitable catalysts with good catalytic activity.The mechanism of catalysts for the hydrogen evolution reaction (HER) and ORR is being studied in great detail using a variety of experimental methods and theoretical calculations.This section will discuss some proposed catalytic mechanisms of FC catalysts with their chemical stability, activeness, and structural-based pathway.While studying the mechanism of EtOH oxidation, Schmidt et al. [75] detected that the presence of Ru into the material partially suppresses the development of chemisorbed moieties that are obtained from dissolved EtOH.Due to this, oxidation occurs by the ineffectually adsorbed moieties, and hence the selectivity for EtOH formation was found to be higher than bare Pt.In favor of the above-discussed outcome, Camara et al. [76] observed that the presence of Ru also blocks the dissociative adsorption of EtOH.The above-explained effect may be a reduction of Pt active sites in the vicinity that are mandatory for the breakdown of the C─C bond.
Moreover, Ru functions as an enhancer that supports the electrooxidation of strongly firmed adsorbed intermediates to produce a higher yield of CO 2 than Pt, according to Fujiwara et al. [77] Using cyclic voltammetry (CV), Lee et al. [78] examined the consequence of temperature on the electrooxidation of EtOH over carbon-based Pt and Pt-Ru.They found that when temperature increases from 25 ºC to 80 ºC, the current increases 8 times more in the case of Pt-Ru/C and only 4 times more in the case of Pt/C.Furthermore, research in the field of electrocatalysts gives better results with Pd as compared to Pt.Further, the EOR's electrocatalytic performance was studied using conventional CV for Pd 2 Ru/C and Pd/C electrocatalysts in NaOH (1.0 M) and EtOH (1.0 M).Two oxidation pathways appeared during the positive and negative potential scans of electrode catalyst Pd 2 Ru/C.The current begins to increase at a lower potential value of À0.8 V and exhibits a shoulder that reaches À0.40 V.This is followed by the main EtOH oxidation peak, which was found to be almost at À0.10 V.
Presently, it is found that electrooxidation of EtOH occurs by two parallel approaches known as C 1 and C 2 paths.In the C 2 path, the breakage of the C─C bond does not happen, and EtOH oxidized to acetaldehyde (CH 3 CHO), followed by further oxidation into acetic acid in the alkaline medium.As shown in Equation (8), In the C 1 path, the breakage of the C─C bond occurs; due to this breakage, two species CH x and CO are obtained that can be oxidized in an alkaline medium to give CO 2 or carbonate.As shown in Equation ( 9), [79] C 2 H 5 OH !C 2 H 5 OH ðadÞ !COðadÞ, CH X ðadÞ !CO 2 (9) Lai et al. [80] advised that in basic solution, the ion of acetaldehyde ion after the removal of the proton, i.e., (CH 3 CHOHO À ), is the main reactive ion for electrooxidation of CH 3 CHO and electrooxidation of EtOH takes place by the following mechanism, as shown in Figure 5.
During the electrooxidation of EtOH in an aqueous electrolyte, three products are obtained, i.e., CH 3 CHO (2e À ), acetic acid (4e À ), and CO 2 (12e À ) at the lower temperature, as shown in Figure 6.The incomplete oxidation of EtOH gives acetic acid as a significant product for multi metal-based Pt-catalysts because this needs lower energy in comparison to CO 2 formation.Further, the electrooxidation of EtOH that produces acetic acid and confirms the decarbonization of the system follows two different steps: 1) oxidative dehydrogenation of EtOH into CH 3 CHO; 2) radical autooxidation of CH 3 CHO into acetic acid, which shows the oxophillic effect due to hetero-metal other than Pt in the electrocatalyst. [81]

Catalysts for DEFC Applications
The MEA is the primary aspect in determining performance in the DEFC framework.The catalyst component of MEA plays an important role in improving the efficiency of FC.The DEFC framework's catalyst is a site with a large, active surface where EtOH electrooxidation occurs at a higher rate and with lower starting energy.Catalysts are needed at the both electrodes of the MEA. [7]EORs occur over the anode, whereas ORR occurs at the cathode, and the rate of these reactions is affected by a catalyst.Copyright 2022, Elsevier Ltd.

Anode Catalyst
While comparing the activity of FCs based on lower alcohols, it was found that FCs operating with EtOH perform better than MeOH.Further, for improving DEFCs performance, more emphasis is given to fabricating a catalyst for anode, which acts as an electrooxidation of alcohols. [32]The main problem with DEFCs is the catalyst polarization and fragmentation of the C─C network within the EtOH.The catalyst further improved the performance of the cell.Most catalysts are based on Pt and Pd with some base materials.

Platinum-Based Catalyst
Pt is broadly utilized as a catalyst in FC advancement and has shown excellent outcomes in the DMFC framework.Pt also exhibits an encouraging oxidizing property for EtOH, especially in the corrosive medium.Researchers used, which Pt oxidizes EtOH to CO 2 but only in minor quantities, using sulfuric acid (1.0 M) solution as the buffer.However, Pt alloyed with other metals demonstrated superior performance to Pt alone.The implication was that Pt surfaces are quickly contaminated, but Ru alloys on surfaces increase the anode's resistance to contaminating species.At 80 °C, Pt alone converts to CO 2 more readily than other catalysts at higher temperatures.Accordingly, the ideal method for keeping up with the EOR at consistent execution is utilizing a bimetallic Pt-based catalyst at moderately low temperatures. [7]Neto and co-workers [82] fabricated a Pt nanowire (NW)-based catalyst that improved the performance of DEFCs.
Figure 7 demonstrates the transmission electron microscope (TEM) for commercially available Pt/C (Figure 7a) and the micrographs for synthesized catalyst Pt/C NWs (Figure 7b).As illustrated in the pictures, HCOOH is used as a reductant methodology for efficiently producing the well-obtained manyequipped Pt NWs.In Figure 7c, Pt NWs have numerous short arms and are around 20 nm in length and have a diameter of 4 nm.The evenly distributed Pt NWs with the carbon base may show an optimum interaction in the metal and the uncovered crystallite carbon planes.A sequence of highlighted, intense rings ascribed to the face-centered cubic (FCC) arrangements of the Pt crystal, which is like the bulk structure of Pt, can be seen in the pattern of the selected area electron diffraction (SAED).The SAED pattern (Figure 7d insertion) depicts a series of brightening concentric circles associated with the FCC construction of Pt crystals, similar to the Pt's bulk.These magnificent rings, which point to the planes (outer and inner rings), attest to the highly crystalline NWs created using this technique.The CO stripping voltammetry for catalyst with NPs morphology Pt/C NPs of the first scan is shown in Figure 7e, and it was found that peak II is not present.But peak III is present for both catalysts, i.e., Pt/C NPs (0.82 V) and Pt/C NWs (0.83 V).These peaks for the oxidation of CO adsorbed in individual NPs of Pt.
The CV for the adsorption of EtOH at the active site of the catalyst surface with an initial voltage of 0.05 V is exhibited in Figure 7f.When the morphology of the catalyst changes from NPs to NWs, a significant change is observed in the CV, i.e., an overall potential smaller value of EOR shown by catalyst Pt/C NPs with an onset potential of approximately 0.73 V and Reproduced with permission. [82]Copyright 2020, Elsevier Ltd. having a CD of 5 mA cm À2 .On the other hand, fabricated Pt/C NWs displayed 5 times more EOR CD over a low onset potential of 0.57 V.
As per Ghumnan et al. [83] Pt is an oxidizing agent for EtOH in an acidic medium.For this purpose, they took H 2 SO 4 (1M) as a buffer solution, and the result shows that EtOH is oxidized into CO 2 completely by using Pt in a small amount.It was observed that the performance of Pt was enhanced by slowing Pt with various metals.The inference was that the Pt surface is easily polluted, but when Pt has alloyed with Ru, the tolerance power of the anode surface increases from various contaminants such as CO ads at lower temperatures and CD.In contrast, Pt has a higher power of conversion to CO 2 at high temperatures, i.e., 80 ºC, as compared to other catalysts due to a more active site at the surface of Pt and increases with an increase in temperature.
In other research for EOR, Beyhan et al. [84] fabricated various trimetallic and bimetallic anode catalysts for EOR based on Pt and Sn with carbon support, i.e., PtSnM/C (M = Pd, Ni, Rh, Co,) and PtX/C (X = Ni, Sn, Co, Pd, Rh) by using Bönnemann's colloidal precursor approach when the as-prepared catalyst is compared with each other by comparing CV curves in HClO 4 (0.1 M), as exhibited in Figure 8a-d Zhou et al. [85] fabricated various Pt-based catalysts in which activated XC-72R was used as support material using a new approach.The incorporated material was characterized by X-ray diffraction (XRD) (Figure 9a  .Reproduced with permission. [84]Copyright 2013, Elsevier Ltd.
better performance toward EOR in DEFC as anode catalysts.This diverse activity is due to the bifunctional process and Pt and additive's electronic interaction.The CV data show that PtSn/C had the most pronounced peak and was associated with PtPd/C, PtRu/C, PtW/C, and the control Pt/C catalyst.
Huynh et al. [86] showed that Pt 3 Ir/Ti 0.7 W 0.3 O 2 is an efficient electrode catalyst for ORR and EOR in the DEFCs.The catalyst Pt 3 Ir/Ti 0.7 W 0.3 O 2 was created by combining the Pt 3 Ir nanoalloy with the tungsten-incorporated TiO 2 nanosupport.The Pt 3 Ir/ Ti 0.7 W 0.3 O 2 -based catalyst exhibited superior activity than the Pt/C catalyst in terms of ORR performance, with a positive E onset of 0.99 V RHE and an increase mass activity (MA) of 802.45 mA mgPt À1 at 0.9 V RHE .Numerous research indicates that Pt-M catalysts (M stands for Cu, [87] Co, [88] and other elements) are substantially more effective for EOR than Pt alone.

Palladium-Based Catalyst
Researchers are now observing that a significant obstacle to the commercialization of energy components is the high cost of catalysts, especially Pt, even though this metal has been successful in Me FC research.Pd, a metal that belongs to the same group as Pt but is somewhat slightly electropositive, has gained popularity.According to a few studies, Pd is the utmost appropriate applicant substitution for Pt for DEFC.In an acidic electrolyte, Pd is the least responsive catalyst; in an alkaline solution, Pd outperformed Pt.Price-wise, Pd is less costly than Pt, [7] which could lessen the expense of the research and experimentation.Additionally, various cocatalysts are available for Pd because it needs a pH environment higher than 8 to perform well. [7]nfrared reflection absorption spectroscopy studies revealed that EtOH oxidation does not occur entirely over electrodes of Pd, with acetate as the actual production.Moreover, electrooxidation of EtOH selectively occurs into CO 2 at a rate of less than 2.5% in a potential window of 0.60-0 V, although it is still somewhat higher than Pt in the alkaline medium, according to studies using Fourier transform infrared spectroscopy (FTIR). [94]y using an electrochemical process, the fabrication of reduced graphene oxide (RGO), as well as nitrogen-doped RGO (N-RGO), takes place, which is subsequently decorated with palladium NPs (Pd NPs) using a solvothermal process to create Pd/RGO and Pd/NRGO.Solvothermal techniques for wet chemical nitrogen doping, nitrogen plasma treatment, and hydrothermal approach for NGQDs in ammonia have all been used to add nitrogen to the carbon structure.Based on the N 1s electron binding energies in X-ray photoelectron spectroscopy (XPS) studies, N might be mixed with some designs of carbon like pyridinic-N, graphitic-N (quaternary) centers, and pyrrolic-N.The TEM pictures in Figure 11a-d displayed the morphological structure of  [90] Copyright 2012, Elsevier Ltd.
RGO, N-RGO, Pd/RGO, and Pd/N-RGO.TEM images show the uniform dispersion of Pd NPs over N-RGO; the particle size is estimated at 4.92 nm with AE0.51 nm for the Pd/RGO, and for Pd/N-RGO it is 3.6 nm with AE0.52 nm.Furthermore, to know the constancy or durability of the as-prepared electrocatalysts, CV is done former and later of longstanding cycling.It was found that there are no clear changes in the CV diagram of RGO and N-RGO (Figure 11e,f ); this shows that electrocatalysts are highly durable and establish tolerance for EtOH in basic solution.In a solution of EtOH (0.5 M) and KOH (aq.) (1 M), the CVs of Pd/RGO and Pd/NRGO with varying scan rates ranging from 10 to 250 mV s À1 are depicted in Figure 11g,h.
An increase in scan rate resulted in an increase in the forward peak CD and peak potential.The forward peaks were linked to the EtOH oxidation, and the inverse peaks were attributed to the midproduct oxidation.The MA of Pd/NRGO is 1422.5 mA, and the onset voltage is À0.605 V versus Ag/AgCl during the forward scan.In comparison to Ag/AgCl, mg À1 Pd outperformed Pd/RGO, which has 956.5 mA mg À1 Pd over an onset potential of À0.654 V.This result also demonstrates that Pd/N-RGO has better electrocatalytic performance, tolerance to poisoning agents, and a far more effective removal of carbonaceous intermediate species from the electrode surface. [95]ing NaBH 4 as a reducing agent, Pd and Pd-Ni NPs supported on carbon with a varied atomic ratio of Pd/Ni, or Pd 2 Ni 3 /C, were created by the simultaneous reduction technique.TEM pictures show that metal NPs are appropriately dispersed throughout the powdered carbon.The homogeneous dispersion of Ni around Pd is revealed by the energy-dispersive X-ray analysis.XPS analysis was used to examine different chemical states of nickel, including metallic nickel, nickel oxide, nickel, and nickel (OH) 2 .
The stability and higher catalytic performance of catalyst Pd 2 Ni 3 /C toward electrooxidation of EtOH were shown by CV and chronopotentiometry compared to the Pd/C catalyst in a basic medium.FC performance displays that the fabricated anode catalyst in alkaline DEFCs provides a maximum power density of 90 mW cm À2 at 60 °C. [96]Due to its comparatively straightforward apparatus and time-saving working procedure, in materials science, the ultrasonic-assisted approach is acknowledged as a powerful and efficient way to produce highly active noble metal-based electrocatalysts.Due to acoustic cavitation, ultrasonic irradiation has peculiar physical and chemical effects; metal ions are reduced using a brief pulse of current in a technique for the sonoelectrochemical production of all nanosize metals.An eco-friendly one-pot synthesis Reproduced with permission. [95]Copyright 2021, Elsevier Ltd.
technique is used to decorate Pd-Ag NPs with a tiny size over the graphene (Pd-Ag/G).This involves heating the graphene suspension to 25 °C and then adding silver formate.The suspension is then supplemented with PdCl 2 while stirred to create Pd-Ag/G.TEM images (Figure 12a-d) provide strong evidence that NPs with an average size of about 2-3 nm will disperse evenly.The Pd/C and Pd-Ag/C catalysts' electrocatalytic activity was evaluated using CV analysis (Figure 12e,f ) in KOH (1.0 M) at 25 °C.Both catalysts hold hydrogen adsorption and desorption peaks at À1-À0.6 V.The electrochemical active surface area (ECSA) for Pd/C is 53.4 m 2 g À1 , and for Pd-Ag/G, it is 92.1 m 2 g À1 .The higher ECSA is further supported by the hydrogen sorption peaks linked to Pd-Ag/G, which are significantly larger than those linked to Pd/C. [97]everal Pd-based nanocatalysts are made using various synthesis techniques, including the impregnation approach, chemical reduction approach, microwave aid approach, coreduction, breakdown in the presence of heat, and so on, which had been established to investigate the impacts of multiple structures with well-ordered morphologies.Precursors employed in the EOR procedure that uses seed-mediated growth synthesis at 25 °C over Pd@Cd x -Ag y core-shell (CS)-based nanocatalysts include PdCl 2 , CdCl 2 •2.5H 2 O, and AgNO 3 described in Figure 13a.According to the research, Pd (II) can be converted to NPs of Pd as the core, and Cd-Ag NPs can grow as the external layer to create Pd@Cd x -Ag y .According to XPS and XRD investigations, Pd@Cd 1 -Ag 1 is ascribed to the novel performance due to its distinctive CS nanoarchitecture, more active sites over the surface, and the double-impact of Cd as well as Ag.Because of the tiny size of the particle, structure, and synergetic consequences, all the synthesized materials showed significant CO removal toward EOR.Using TEM, the catalysts' element dispersion morphologies were examined.The TEM images of Pd@Cd 1 Ag 1 electrocatalyst (b,b 1 ), Pd@Ag (c,c 1 ), and Pd@Cd (d,d 1 ) nanocatalysts, with corresponding sizes of 50 and 20 nm, show the CS nanostructure (Figure (b-d)).
The catalysts have a particular CS structure, with a small size distributed evenly.The distinct boundaries of the Pd NPs in Pd@CdAg show that the Cd-Ag is made up of a thin layer of connected NPs that follow the Pd that is produced.As illustrated in Figure 13e,f, the electrocatalytic performance of synthesized electrocatalysts Pd@Cd, Pd@Cd x -Ag y , Pd/C, and Pd@Ag can be improved in the presence of EtOH (1 M) with NaOH (1.0 M).In the CV curve, two peaks are observed, first at À0.05 V for the forward scan and second at À0.41 V for the backward scan.These peaks show the electrochemical oxidation of EtOH and the relative intermediate product, respectively.In order to examine the nanocrystals of Pd@Cd x -Ag y consecutive sweeps for the entire EOR process depicted in Figure 13g, 600 consistent scans of the Pd@Cd 1 -Ag 1 nanocatalyst's CVs were conducted at a sweep rate of 50 mV s À1 in an electrolyte of NaOH (1.0 M) þ C 2 H 5 OH (1.0 M).When the sweep cycle is increased, the electrocatalytic activity peak gradually rises till 550 cycles.The higher ECSA at the start of the EOR is the cause of the increase, which decreases at higher sweep cycles.Figure 13h demonstrates how Pd@Cd1-Ag1 nanocatalysts have improved over supplementary fabricated catalysts and Pd/C (JM). [98]everal Pd-based anode catalysts are known for the electrooxidation of EtOH like PtM/C (M = Sn, Ru, Pd, W), [99] PdNi based on exfoliated GO (PdNi/EGO) composite, [100] carbon-reinforced Pd-based nanocatalysts, [101] PdAuAgCu tubular catalysts, [102] etc. .Reproduced with permission. [97]Copyright 2019, Elsevier Ltd.

Other Anode Catalysts
Besides Pt and Pd, other noble metals and NNMs are also used as an anode catalyst in DEFCs for the electrooxidation of EtOH.Some are Au, Cd, Ni, Ir, W, etc.In this section, we go through these types of anode catalysts.Tungsten carbide (WC) is used as an anode catalyst or an anode for EtOH oxidation.Oh et al. [103] prepared carbon nanofiber (CNF) linked to WC using a conventional reduction process.During CV examination, WC shows a similar consequence as Pd with slightly lower peaks than the  and d,d 1 ) 50 and 20 nm Pd@Cd nanocatalysts, correspondingly.e) Measured CVs of various Pd@Cd x -Ag y catalysts within NaOH + C 2 H 5 OH; f ) CVs of Pd@Cd 1 -Ag 1 , Pd@Cd, Pd@Ag, and Pd/C (JM) nanocatalysts; g) 600 successive scans of CVs at the Pd@Cd 1 -Ag 1 nanomaterial attained in NaOH (1 M) + C 2 H 5 OH (1 M); h) comparison of the catalytic performance of distinct Pd@Cd x -Ag y nanomaterials with Pd@Cd, Pd@Ag, and marketable Pd/C (JM) nanocatalysts, at a sweep rate of 50 mV s À1 .Reproduced with permission. [98]Copyright 2021, Elsevier Ltd.
Pd highest peak.They also concluded that when WC is used as a cocatalyst with Pd, it significantly increases the EOR due to the increased ECSA.Because of its significant electrochemical active surface area, WC offers a good surface area for oxygen adsorption.On the other hand, Au is less expensive and readily available as compared to Pt and Pd.Research toward Au as an electrocatalyst for DAFC is going on.
In this way, Chen et al. [104] fabricated an Au-based catalyst for the electrooxidation of EtOH, MeOH, and ethylene glycol (EG).An impregnation method is used to prepare the catalysts Au-Ru/ C, Au/C, Pd-Ru/C, and Pd-Ru/C with a metal incorporating of 20%.The XRD of catalysts in Figure 14 shows the (220) peak for face-centered cubic crystal construction.The size of units is considered by using the Scherrer equation.For the Au-based catalysts, the estimated particle diameter was around 24 nm, while for Pd/C, it was around 10 nm.The Pd-Ru/C substances looked tiny, and the XRD measurements would not be used to govern the particle size.Both Pd and Au exhibit good activity during the oxidation of MeOH, EtOH, and EG into alkaline solution.The combination of Ru considerably increased the performance of Pd and Au.Significantly, the EtOH oxidation activity on Pd-Ru is nearly 4 times higher compared to Pt-Ru.
Transition metals (TMs) are more cost-effective and abundant than noble/precious metals; among all the known TMs, Co-doped catalytic performance in various chemical reactions.
Considering this, Barakat et al. [105] fabricated a Cd-doped-Co anode catalyst, i.e., CdAc/CoAc/PVA sol-gel, for the electrooxidation of EtOH by using the sol-gel approach.The graphite shell's thickness of roughly 5 nm is confirmed by TEM measurements.The catalyst-to-ore ratio (CV) diagram in KOH (1 M) for 50 runs at a 100 mV s À1 sweep rate of Cd-doped Co/C (Figure 15b) and Co NPs (Figure 15a) demonstrates that the addition of Cd significantly boosts the catalytic activity toward EOR.The voltammograms in Figure 15c for the EOR in linear scan demonstrate that the incorporated substance's equivalent onset voltage is approximately 365 mV (vs Ag/AgCl, or approximately 585 vs normal hydrogen electrode [NHE]).Furthermore, the associated CD is also higher, i.e., 70 mA cm À2 .The OH and CO adsorbed coating on the electrodes' exterior is the primary factor in the robustness of the onset potential; this gas layer causes overpotential.To appropriately pattern the presented NPs' sturdiness, a continual voltage assessment was done at an operating voltage of 0.6 V (Figure 15d).
To fabricate a noble Fe-N-C NNM-based catalyst toward ORR, a challenging templating technique in which Fe(II)-phthalocyanine is used.The tolerance toward EtOH and the stability in an alkaline medium were also evaluated to confirm the high potentiality of the fabricated catalyst to be utilized within an aqueous DEFC.The consequences indicated that the ORR happens primarily after the straight 4 e À drop to OH À and that the Fe-N-C compound shows higher tolerance toward EtOH and good stability.For low-intermediate currents, the catalyst ink performs better and has a maximum PD of 62 mW cm À2 . [106]s a competent anode electrode catalyst for EOR, Ni-CeO 2 NRs with different weight ratios were cocatalyzed with CeO 2 and produced via a modified polyol method.XRD, SEM, TEM, and XPS studies were used to characterize the prepared electrode catalysts, and the results show that the catalysts were successfully fabricated.It used chronoamperometry, LSV, and CV techniques to study the catalysts' electrochemical characterizations.Chronoamperometry experiments reveal that the Ni-CeO 2 -NRs-2 electrocatalyst has the highest CD of 14.02 mA cm À2 among all prepared catalysts.The incorporation of CeO 2 NRs improves catalytic stability through a cumulative synergetic effect. [107]Several groups have recently investigated the part of Au in boosting the EOR of Pd nanomaterials by using an electron beam to irradiate bimetallic PdAu electrocatalysts. [108]ccording to Geraldes et al. [109] compared to pure Pd (25 mW cm À2 at 85 °C), a Pd:Au ratio of 90:10 yields the highest PD (44 mW cm À2 at 85 °C).
The peak PD of the ternary catalyst Pd:Ni:Au was discovered by Dutta et al. [110] to be roughly threefold that of the monometallic Pd material.Owing to its better electrical performance and exceptional durability compared to other conjugated polymers, poly(3,4-ethylene dioxythiophene) (PEDOT) is one of the most effective conductive polymers for integration into graphene sheets.Numerous studies have shown that adding conductive polymers to graphene sheets can considerably improve the ionic transport to the active positions and the electrochemical cyclic constancy of constituents, which is helpful for the electrocatalytic reactions occurring in FCs.Thus, using PEDOT/graphene amalgams as catalyst sustenance is anticipated to allow for the benefits of both PEDOT and graphene films to progress the electrocatalytic efficacy of EtOH oxidation. [111]Various types of anode catalysts for DEFCs are summarized in Table 2.

Cathode Catalyst
The second compartment of DEFCs is the cathode that plays an important part in the cell performance as the anode oxidizes the are the first six combinations.Reproduced with permission. [104]opyright 2007, Elsevier Ltd.
EtOH linked with the cathode, slowly reducing oxygen and producing poor cell activity.So, for the better performance of DEFCs, improvement in the cathode is also essential.Nowadays, Pt is commonly used cathode in acidic as well as in basic mediums. [112]The cathode of Pt is relatively stable, having better oxygen reduction capability.There are only some studies for developing cathode in DEFCs but maximum studies related to Pt due to its good performance.The material must have an intense action for the ORR and a high EtOH oxidation to be used as a DEFC cathode.In low-temperature FCs that run on hydrogen, a few composites of the first-row TMs exhibit a better performance toward ORR than Pt.Both mathematical (reducing the Pt bond distance) and electrical variables (increment of Pt d-electron valency) improved the ORR observed while constructing Pt-M alloy electrocatalysts.The adsorption of oxygen and EtOH has completed the surface area.
The presence of a few contiguous Pt gatherings is necessary for the dissociative chemical adsorption of EtOH, just as it is for MeOH.Atoms of the second metal near Pt active locations may be resistant to EtOH adsorption on Pt locations because of the thinning effect.As a result, MeOH oxidation on the electrocatalyst with a binary component is more challenging.However, oxygen adsorption, which is typically thought of as dissociative chemical adsorption, only requires two adjacent sites and is unaffected by the following metal.Despite having a high ORR movement, Pt-Ni and Pt-Co blends among other catalysts also have a good MeOH tolerance.The ORR movement of the Pt-Pd/C catalyst is similar to that of Pt, although it is more resistant to EtOH. [32]

Platinum-Based Catalyst
In DEFCs, Pt/C acts as a cathode and exhibits moderate performance to ORR.To advance the cathode activity, like anode, Lopes et al. [28a] concluded that it is done by adding some cocatalysts at the cathode.They noted in their report that the oxygen reduction by the catalyst is further strengthened when Pd is alloyed with Pt.This enhancement in the ORR activity is owing to the distraction of Pt-Pt arrangement and the distance between them.It then results in the accessible attraction of oxygen, mainly the electrons present in the sp 2 in the O 2 assembly, onto the catalyst's exterior, which is active because of the openings in the orbital.Furthermore, it was concluded that adding Pd as a cocatalyst decreased the affinity for OH ads and CO ads species with Pt.Other than Pd, it was found that adding Co also enhances the ability to reduce oxygen of the Pt catalyst. [113]he EtOH tolerance of Pt does not increase by adding Co because the EOR performance of Pt/C and PtCo/C is comparable; it mainly improves the ORR performance on the surface active.On the other hand, by adding Pd, the tolerance of EtOH of Pt with ORR performance was enhanced.It is well known that a catalyst's overall ORR activity increases with increased tolerance to EOR activity.This report is also accepted by Beyhan et al. [114] who concluded that incorporating Sn and Ni  [105] Copyright 2013, Elsevier Ltd.
as cocatalysts increases ORR activity.Like the addition of Pd, it also decreases the adsorption of moieties like OH ads and CO ads , and then increases the adsorption of O 2 molecules.In the same manner, the reaction also comprises the flagging of the linkage within oxygen molecule, i.e., O─O bond, which decreases the problems for the adsorption to take place.The research found that Pt-Sn/C exhibits the highest reactivity toward oxygen, even higher than the trimetal-based catalysts of Pt-Sn-Ni/C.The overlap of the ORR process's surface-active locations on the trimetalbased catalyst is the cause.
The catalytic performance of preabsorbed CO ads on as-synthesized Pt 22 Pd 27 Cu 51 , Pt 36 Pd 41 Cu 23 , and Pt/C materials was first evaluated in order to understand the EOR mechanism upon the tri-PtPdCu NWs.As demonstrated in Figure 16a, the CO disrobing peaks of the as-synthesized Pt 22 Pd 27 Cu 51 and Pt 36 Pd 41 Cu 23 NWs discovered at an additional unfavorable place around 60 and 135 mV compared to Pt/C, simultaneously.It is familiar that a primary EOR intermediary of CH 3 CHO may be additionally oxidized to develop either CH 3 COOH or CO 2 . [94]he CH 3 CHO electrooxidation performances of these materials were additionally studied.As displayed in Figure 16b  are related to one another may form a network on top of the catalyst coating that can offer excellent electron conductive pathways without the need for carbon reinforcement.It is commonly known that the stability of the material can be primarily impacted by the oxidation and breakdown of the carbon substrate during the DEFC process. [115]While the incorporated single cell was estimated at 80 °C, the PD is 21.7 mW cm À2 (Figure 16f ), which is ≈3.9 times bigger compared to a single cell manufactured by utilizing Pt/C as catalyst (2 mg Pt cm À2 for each electrode, 5.6 mW cm À2 ) (Figure 16g). [116]2.2.Palladium-Based Catalyst Xu et al. [117] reported that Pd-Au-based alloys are used as the cathode, indicating that this catalyst has activity equivalent to Pt cathodes.In the alkaline media of KOH, the rate of oxygen reduction increased to its maximum level of 50 mA cm À2 at a scan rate of 50 mV s À1 . He alo reported that the causes of Pd-Au better activity, like cathode, are due to a lower rate of EtOH oxidation, which is explained in some well-known studies.[104,118] This is because there are Au cocatalysts present, and as Au has a lower oxidizing activity, it has a reverse reaction for the reduction activity.

Other Cathode Catalyst
Hypermec K14, a NNM-based material, was described by Zhiani et al. [112a] as the cathode.It was mixed with a small amount of PTFE and then painted onto the carbon exterior.They also reported that K14 has a higher ORR activity associated with Pt/C, particularly in KOH.They demonstrate that, compared to the previously used Pt, the transition materials utilized in the K14 are significantly more responsive in ORR.K14 also shows high inertness toward EtOH.While doing a single-cell test, a surprising crossover occurs by the membrane that results in more Pt activity loss and causes polarization.K14 has a unique Reproduced with permission. [116]Copyright 2021, Elsevier Ltd.
property for ORR with increased tolerance to any EOR because the EtOH-crossover CD over a surface is almost zero when used as the cathode, as shown in Figure 17.
Because the Pt cathode's active surface exhibit reaction selectivity, this will intensely reduce the competition.Due to this consequence, this is one of the better catalysts to utilize as a DEFC cathode because crossover may be reduced.Various cathode catalysts for DEFCs are summarized in Table 3.

Similarities and Differences between Pt-and Pd-Based Materials
Both Pt and Pd noble metal-based materials are for EtOH electrooxidation in DEFCs.These electrocatalysts show corrosion resistance and also for CO poisoning.Both enhance the EOR and hence increase the CD. [119]t and Pd are TMs usually used as catalysts in various chemical reactions.While they have some similarities, there are also significant differences between Pt-and Pd-based catalysts.Let us explore these similarities and differences.
The following are the similarities between Pt-and Pd-based catalysts: 1) Pt and Pd show outstanding catalytic activity, making them widely used in different industrial operations.They are beneficial in catalyzing oxidation and reduction reactions. [120]) Pt and Pd are highly stable metals, resistant to corrosion and oxidation.This stability permits them to keep their catalytic performance over extended periods, even beneath intense reaction conditions. [121]3) Pt and Pd catalysts generally work via parallel mechanisms.They both undergo surface adsorption of reactant molecules, followed by activation and reaction phases. [122]he following are the differences between Pt-and Pd-based catalysts: 1) One of the most essential distinctions between Pt and Pd is their comparative cost.Platinum is more costly than palladium due to its rarity and higher need.This cost aspect usually affects the selection of catalysts in industrial applications.2) Pd is typically more reactive than Pt.This higher reactivity can be attributed to Pd's electronic network and atomic size, making it more inclined to bond with other atoms or molecules.Therefore, Pd catalysts may show higher activity for specific reactions than Pt catalysts. [123]3) While Pt and Pd can catalyze various reactions, their selectivity can vary.Due to their electronic and structural distinctions, Pt and Pd catalysts may show inconsistent choices for specific reaction paths, guiding to different product allocations.This selectivity can be necessary in fine-tuning the expected outcome of a chemical approach. [124]4) Pt and Pd catalysts can be modified by comprising ligands (organic or inorganic molecules) that attach to the metal center.These ligands can affect the catalyst's performance, selectivity, and stability. [125]evertheless, the selection and effects of ligands can vary among Pt and Pd catalysts due to their inconsistent electronic structures.5) Pt and Pd catalysts find applications in various industries and ; cathode fed by 250 KPa pressure of N 2 ; 0% relative humidity; 200 mL min À1 .112a] Copyright 2011, Elsevier Ltd. reactions.Platinum catalysts are often employed in FCs, electrolyzers, and automotive emission control systems. [126]Palladium catalysts, on the other hand, are often used in hydrogenation, cross-coupling reactions, and organic synthesis.Pt and Pd have some differences based on various factors like path of action, stability, etc., which are discussed in Table 4.
However, differences in cost, reactivity, selectivity, ligand effects, and applications set them apart.The selection between Pt and Pd catalysts depends on the specific reaction conditions, cost concerns, and desired results.

Role of Supporting Materials in Electrocatalytic Performance of DEFC
The support materials' role in FC electrocatalytic performance is also significant.For reinforced metal substances, the support acts as a macromolecular ligand, influencing the composition of the active site and, on occasion, directly affecting the reactivity.Metal classes, such as nanocomposites, groups, or single particles, may be placed on carbon-based constituents for various catalytic processes.A large family of compounds with a wide range of textural and structural variations comprises all the carbon-based materials used as catalyst supports.Recent advances in well-controlled amalgamation procedures, progressive classification methods, and modeling tools enable the correlation of the relations among metal, support, and reactant at the molecular level. [127]The initial conditions for proper FC catalyst sustainment should include: 1) a large surface area, 2) suitable porosity, 3) high electrical conduction, and 4) high durability following FC working conditions.
It is necessary to disperse the catalyst NPs on the conductive support, which is commonly made of carbon.The consensus is that carbon support enhances mass transfer kinetics at the electrode surface, electron transport, and catalyst material scattering. [128]he electrocatalytic performance of a DEFC be contingent on the properties of the catalyst material used and the support material used to hold the catalyst. [129]The role of support materials in the electrocatalytic performance of DEFCs is to provide a stable and conductive substrate for the catalyst material to be deposited on.The support material should have good electrical performance, better surface area, and good chemical stability under the functioning circumstances of the DEFC. [130]Additionally, the support material should have good mechanical steadiness to endure the harsh functioning conditions of the FC.

Numerous types of support materials have been investigated
for use in DEFCs, including carbon-based substances, for example, carbon black (CB), [131] graphene, [100] and CNTs, [132] as well as metal oxides [133] such as titanium dioxide and zirconia.The choice of support material can significantly impact the electrocatalytic performance of the DEFC.For instance, the high electrical conductivity and large surface area of carbon-based support materials can increase the catalytic activity of the catalyst material.But under the FC's operating conditions, these materials can also be vulnerable to oxidation, which over time could cause the support material to deteriorate and perform less well.
The effect of CO on the catalyst surface in EtOH FCs is a critical consideration in optimizing catalytic performance and FC efficiency.Researchers focus on developing CO-tolerant catalysts and employing various strategies to mitigate the detrimental effects of CO adsorption on the catalyst surface.As an example, consider alloying, which is the process of adding specific metals to the catalyst composition to reduce CO adsorption and increase tolerance, or nanostructuring, which is the process of forming catalysts with nanoscale structures to expose more active sites and reduce the impact of CO; and many other materials science advancements, catalyst design, promoter addition and nanostructuring techniques.These solutions aim to optimize catalyst performance, mitigate CO interference, and enhance the overall efficiency of EtOH FCs.Some are discussed below.

Monoatomic Catalyst
Monoatomic catalysts refer to catalysts where the active catalytic sites are composed of individual atoms rather than larger clusters of atoms or NPs.These catalysts are often designed at the atomic scale, providing unique properties and advantages in various catalytic processes.Monoatomic catalysts, including single-site Cu-doped PdSn wavy NWs, are crucial for boosting EtOH FC performance through enhanced catalytic activity, exemplifying a promising avenue for efficiency improvement. [134]Monoatomic catalysts play a crucial role in the oxidation of EtOH.Pd and Pt are two commonly used monoatomic catalysts for this reaction.Pd NPs facilitate the cleavage of C─C bonds inEtOH, and the Ni single-atom catalyst (SAC) efficiently eliminates adsorbed CO intermediates.The Pd NPs@Ni SAC structure demonstrates exceptional EOR performance, showcasing high MA, favorable selectivity for C1 products, and robust electrocatalytic stability. [135]Single-atomic Ir on hcp-PtPb/fcc-Pt core-shell Lattice parameter [nm] 0.3923 0.3890 [201]   Peak potential/V RHE 0.687(Pt/C) 0.417(Pd/C) [202]   Conditions EtOH/KOH/scan rate -0.5 M/0.5 M/50 mV s À1 (Pd/CNTs) [202]   PD [mW cm À2 ] 1113(Pt/C) 508 (Pd/C-NaBH 4 -NH 3 ) [203]   Energy parameter [meV] 9.7894 3.2864 [204]   Cost More expensive Less expensive [205]  hexagonal nanoplates (PtPb@PtIr 1 HNPs), [136] the two investigated nanocatalysts demonstrated outstanding performance in direct alcohol alkaline FC (DAAFCs).Specifically, the core-shell FeCo@Fe@Pd catalysts outperformed single Pd metal on identical substrates.A substantial fourfold rise in power density was noted in the DEFC, with a concurrent threefold increase observed in the DMFC.These improvements were evident when operating passive DAAFCs at moderate temperatures, underscoring the effectiveness of the core-shell nanocatalysts, [137] which exhibited improved catalytic activity and durability.Monoatomic catalysts enhance the EOR by impeding CO adsorption, altering intermediate adsorption, and diminishing the reaction barrier of EOR. [138]Additionally, the monatomic catalysts show enhanced stability and antipoisoning properties, contributing to their improved EtOH FC performance.The construction of low-coordination active sites on shape-controlled nanocrystals and the synergistic effect between different metal atoms in the catalysts further enhance their selectivity and activity toward EtOH oxidation.These findings demonstrate the potential of monatomic catalysts for optimizing the performance of EtOH FCs.So, we summarize those monoatomic catalysts, represented by single-atom catalysts, contribute to EtOH FC efficiency by providing precise control over catalytic sites.The single-atom nature ensures maximum exposure of active sites, enhancing catalytic activity.In summary, monoatomic catalysts offer an accurate and effective solution to the challenges associated with CO in ethanol FCs.Their unique structure provides controlled and selective catalytic activity, minimizing the impact of CO on the catalyst surface.This leads to improved EtOH oxidation, enhanced FC performance, and increased efficiency in energy conversion processes.This design minimizes undesired side reactions and improves selectivity, leading to more efficient EtOH oxidation.The atomic dispersion also contributes to increased stability and prolonged catalyst life.

HEA Catalyst
A HEA catalyst is a type of catalyst based on HEAs, which are materials characterized by the simultaneous presence of multiple elements in approximately equiatomic or near-equiatomic proportions.The inherent stability and mechanical strength of HEAs can contribute to the durability of the catalyst, making it resistant to deactivation or structural changes during catalytic cycles.HEA catalysts have shown great potential for the EOR.Using HEAs in EOR can promote the 12-electron EOR, leading to highly efficient DEFCs. [139]HEAs have been synthesized with specific morphologies and tunable compositions, allowing for controllable synthesis of nanoscale HEAs for advanced catalysts.The researchers successfully synthesized 12 nanoscale HEAs, including 0D NPs, 1D NWs, 2D ultrathin nanorings (UNRs), and 3D nanodendrites, combining diverse elements like Pd, Pt, Ag, Cu, Fe, Co, Ni, Pb, Bi, Sn, Sb, and Ge.The resulting HEA-PdPtCuPbBiUNRs/C demonstrated exceptional electrocatalytic performance for EtOH oxidation, surpassing commercial Pd/C and Pt/C catalysts with substantial improvements in MA and durability. [140]ntroducing a fifth metal in HEAs, such as Rh, can enhance the electron-transfer efficiency and improve the oxidation capability in alkaline environments.The (CoCrFeNiAl) 3 O 4 nanostructure, synthesized through a polyol hydrothermal precipitation-calcination method, exhibits promising features for hydrogen production.The spinel-phase high-entropy oxide (HEO) displays a unique self-reorganization during the reaction, where metals emerge from the HEO bulk phase as active species for hydrogen production.This distinctive behavior, coupled with the formation and enrichment of oxygen vacancies in a hydrogen atmosphere, results in a catalyst with dispersed active sites and high thermal stability.Notably, the isolated metal cations re-enter the parent metal oxide cell after the reaction, preventing agglomeration over the catalyst surface.The catalyst achieves impressive results, with 81% hydrogen yield and 85% H 2 selectivity attained at 600 °C during the electrolysis process, highlighting its potential as an efficient and stable catalyst for hydrogen production. [141]eunghwa Lee et al. [142] demonstrated that electrochemical activation of high-entropy molybdates (HEMo) improved their performance as oxygen evolution catalysts in alkaline media.They show electrochemical activation enhances the performance of HEMo as oxygen evolution catalysts in alkaline conditions.The activated HEMo demonstrates a lower overpotential, achieving 10 mA cm À2 with a 75 mV improvement over pristine HEMo.It maintains high activity during 24 h of electrolysis, attributed to a shift in active sites from Ni to Fe and an increased electrochemical surface area postactivation.This work showcases the application of HEMo in electrocatalysis, introduces a synthesis approach for multication NPs, and provides valuable insights into the efficacy of FeOOH as an active oxygen evolution catalyst.
Men et al. [143] introduced an HEA catalyst, specifically HEA-PdNiRuIrRh, exhibiting remarkable MA (3.25 mA μg À1 ) for alkaline HOR.This represents an eightfold improvement over commercial Pt/C.The atomic structure, challenging to identify, is elucidated using machine learning potential-based Monte Carlo simulation.The dominant PdÀPdÀNi/PdÀPdÀPd bonding environments and Ni/Ru oxophilic sites on the HEA surface contribute to optimized adsorption/desorption of *H and enhanced *OH adsorption, resulting in excellent HOR activity and stability.The bimetallic catalysts also showed high catalytic activity, 4.1 and 3.9 times higher than the commercial Pd/C catalyst.The charge transfer between Pd and Cu(or Ni) in these bimetallic catalysts alters the electron density of Pd and enhances the anti-CO poisoning ability.
So, alloy-based catalysts, like HEAs or bimetallic catalysts, optimize EtOH FC efficiency by tailoring the catalyst composition.Alloying different metals can create synergistic effects, enhance catalytic activity and selectivity, control the impact of CO, and enhance overall efficiency.The diverse composition, synergistic effects, and tunable properties of HEAs provide a promising avenue for addressing challenges associated with CO in FC applications.This leads to improved reaction kinetics, lower overpotentials, and increased durability, ultimately boosting the overall efficiency of EtOH oxidation in FCs.

Carbon-Based Catalysts
Because of their unique large areas, superior chemical durability, and significant relationship to catalysts, carbon-based catalysts, such as CB, expanded graphite, graphene, and CNTs, have long been considered as specific support substances toward the EOR catalysts.The emergence of a possible metal-carbon interaction may lead to a strong correlation between metal catalysts and carbon-based materials.The most popular retail CB that is frequently used as reinforcement for Pt-and Pd-based materials is Vulcan XC-72R. [144]tOH was used as the reducing agent in a simple and environmentally friendly method to prepare PdNPs decorated upon 3D N-doped CNTs (referred to as Pd/3DNCNTs), as illustrated in Figure 18a.As PdNPs and N 2 -containing functional groups have better relationships and the 3D network system helps, the Pd/3DNCNTs catalysts provide better catalytic activity and stability for EOR (Figure 18b).Disregarding the remarkable boundaries of CNTs, problems persist with regard to CNT self-dispersity and catalyst homogeneous diffusion into the CNT chain.Additionally, a series of CNT-doped Pd-based materials, including Pd/CNT, trimetallic PdSnNi/CNT, and bimetallic PdSn/CNT, were produced by in situ reduction and microwave-supported polyol.It was discovered that the Pd-based NPs were evenly distributed with CNT assistance and did not appear to accumulate (Figure 18c), guaranteeing a significant ECSA.PdSnNi/CNT catalysts show enhanced electrocatalytic performance and long-term stability for the EOR in an alkaline condition when compared to the Pd/C catalyst, in addition to the beneficial transformation of electronic networks by Sn and Ni alloying.Additionally, Pd NP catalysts have been supported by carbon-doped nanocomposites of MWCNTs and extended graphite (Figure 18d).Furthermore, Pd 5 nanoclusters (≈1 nm) doped with MWCNTs and loading as low as 2% show excellent catalytic activity for electrochemical redox (EOR).Interestingly, rGO nanosheets generally form accumulations within the solution owing to the π-π relations among adjoining nanosheets.It may lead to a reduction in the surface areas and electrical conductivity of rGO, making it difficult to optimize the catalytic activity of metal-based catalysts reinforced with rGO.CNTs coupled with Pd-NiO NPs were simulated and integrated with rGO reinforcement in order to counteract the detrimental effects caused by rGO accumulation (Figure 18e).When it comes to EOR in an alkaline medium, the Pd-NiO/CNTs/rGO amalgam materials perform noticeably better (90.9 mA cm À2 ) than hybrid Pd/CNTs/rGO (43.1 mA cm À2 ) or retail Pd/C catalysts (28.0 mA cm À2 ).Furthermore, there has been a significant improvement in the mixed materials' stability (Figure 18f ). [145]onversely, metal oxide-based composites are more durable beneath the working necessities of the FC but can have lower electrical conductivity and surface area than carbon-based substances.Thus, a proportion of electrical conductivity, surface area, and durability is essential when choosing a backing substance for a DEFC. [146]s shown in Figure 19a, the AuPd NCs reinforced on aminefunctionalized Vulcan XC-72R CBs (AuPd/CB H-A ) were created by coreducing HAuCl   [145] Copyright 2019, Multidisciplinary Digital Publishing Institute.
Au 0.4 Pd 0.6 /CB H-A has the highest ECSA, approximately 1.6, 5.0, and 2.8 times higher than the Au 0.4 Pd 0.6 /CB A (54.71 m 2 g À1 ), Au 0.4 Pd 0.6 /CB H (17.75 m 2 g À1 ), and marketable Pd/C (31.09 m 2 g À1 ), respectively (Figure 19f ).The ultrafine particle dimensions of Au 0.4 Pd 0.6 NCs, which may provide more active sites toward catalytic responses, are the main factors influencing the enhanced ECSA of Au 0.4 Pd 0.6 /CB H-A .In a 1.0 M KOH solution with 1.0 M EtOH, the electrocatalytic performances of the as-synthesized materials toward EOR were examined, and they correlated with Pd/C and Pt/C retail catalysts.The CVs of a range of stimuli in 1.0 M KOH and 1.0 M EtOH are shown in Figure 19g.The forward and backward scans exhibit one-one peaks, which were attributed to the oxidation of recently chemisorbed species from EtOH adsorption and the subtraction of residual carbonaceous species included within the forward scan, correspondingly. [147]The loading quantity of AuPd matched the EOR activity for different materials, and the ECSA normalized the specific and mass activities accordingly.The high specific performances of 5.98 and 7.43 mA cmAuPd À2 that the Au 0.4 Pd 0.6 / CB H-A and Au 0.4 Pd 0.6 /CB A exhibit, as shown in Figure 19h, are significantly higher than those of the Au 0.4 Pd 0. ). [148]

2D Materials
Due to their remarkable properties and potential applications in various fields, such as energy conversion and storage, 2D materials have attracted significant attention in recent years.DEFCs are an encouraging technology for clean energy production, and investigators have studied using various 2D materials as reinforcing materials in DEFCs.2D materials have drawn interest as direct oxidation FC anode catalysts because of their intriguing electrochemical features, which include exceptional mechanical qualities, high surface area, superior electron transport, active sites, and tunable electronic states.2D materials such as graphitic carbon nitride, hexagonal boron nitride, phosphorene, TM dichalcogenides (TMDs), and 2D TM carbides/nitrides or carbonitride (MXenes) have been discovered. [149]4.1.TMDs Nanocomposites TMDs are 2D layered materials with chalcogen (sulfur and selenium) and TM (tungsten and molybdenum) atoms sandwiched between them.Due to their unique characteristics, which include their large surface area, excellent electrical conductivity, and catalytic performance, TMDs offer promise for a variety of energyrelated applications.Efficient electrocatalysts are crucial for EtOH oxidation in DEFCs to yield electricity.TMDs have attained considerable attention as potential electrocatalytic materials for DEFCs due to their remarkable properties and catalytic activity.[150] TMDs, such as molybdenum disulfide (MoS 2 ) and tungsten diselenide (WSe 2 ), are another class of 2D materials investigated for DEFCs.These materials possess unique electronic and catalytic properties.TMDs can support catalyst NPs, enhancing the overall catalytic activity and stability in DEFCs.TMDs can also serve as efficient electrocatalysts, promoting EtOH oxidation reactions.[151] TMDs have proven to have outstanding electrochemical performance in applications involving energy storage.MoS 2 has a low electrical conductivity despite having a high specific capacity of 950 mA h g À1 at a rate of 0.1 A g À1 .Consequently, MoS 2 composites containing highquality electrically conductive materials are practical ways to boost Li storage capacity that is nearly equal to theoretical capacity.A MoS 2 /rGO composite demonstrated a capacity increase to 1100 mA h g À1 , and an even higher capacity of 1200 mA h g À1  [148] Copyright 2022, Elsevier Ltd.
was observed in a MoS 2 /MXene Ti 3 C 2 composite. [152]TMDCs have drawn attention as possible substitutes for Pt because of their natural abundance, exceptional catalytic capabilities owing to favorable interactions with metal catalysts and good stability.Because of its many interesting properties, such as its easy synthesis, long active sites, affordability, and basal planes, MoS 2 is a good choice for creating hybrid electrocatalysts.Au@Pt/MoS 2 -2 nanocomposites with a thick Pt shell outperformed catalysts with a thin Pt shell (Au@Pt/MoS 2 -1) in terms of ECSA and electrocatalytic activity.151a] It is important to note that while TMDs show potential for DEFC applications, further research is needed to optimize their performance, stability, and integration into practical FC devices.Continued efforts in understanding the fundamental aspects of TMD-based electrocatalysis will pave the way for their effective utilization in DEFCs and foster advancements in renewable energy technologies.

MXene-Based Materials
MXenes are a relative of 2D TM carbides, nitrides, and carbonitrides that show high electrical conductivity and tunable surface chemistry.They have displayed potential as supporting materials in DEFCs.MXenes can enhance the catalytic performance of the FC electrodes, improve the mass transport of reactants, and contribute to the stability of the catalysts. [153]The generic chemical formula for MAX phases is M n þ 1 AX n (n = 1, 2, 3), where M is an element that is an early TM (like Ti or Sc), A is an element that is in group 13 or 14, like Si or Al, and X is C, N, or their mixtures.The various phase of MAX is divided into 211, 312, and 413 structures.A cross-sectional picture of the 211, 312, and 413 MAX phases and the corresponding MXene structures is presented in Figure 20. [154]The structure of MXenes consists of three parts: 1) an intralayer skeleton layer where ionic bonds are formed by the alternate stacking of Ti and C atoms; 2) an interlayer skeleton region where hydrogen bonds amid O or F atoms involved to the surface and van der Waals interactions between atoms build the interaction among the layers; and 3) surface dismissing groups that are arbitrarily dispersed upon the surface of the main MXene structure. [155]Xenes are also used in technologies related to sustainable energy.Hydrofluoric acid etching produces MXenes such as Mo 2 CT x and Ti 2 CT x by removing the Ga and Al atoms from the parent ternary carbides Mo 2 Ga 2 C and Ti 2 AlC.The electrochemical activities were demonstrated using three-electrode electrochemical cells with MXenes drop cast on the glassy carbon electrode (GCE).Mo 2 CT x , with a CD of about 10 mA cm À2 , was the more promising HER catalyst at an overpotential of 283 mV, as shown in Figure 21a.With a CD of 10 mA cm À2 , the Ti 2 CT x MXene fared much worse at an overpotential of 609 mV.HER activity of Ti 2 CT x declines further, suggesting instability, whereas Mo 2 CT x has a minor initial decrease but maintains a steady HER, reaching 10 mA cm À2 at an overpotential of 305 mV.As can be observed in Figure 21b, neither Mo 2 CT x nor Ti 2 CT x exhibited a significant change in overpotential; the corresponding XPS in Figure 21c illustrates the changes in the chemical state of Mo 2 CT x that were investigated prior to and following HER performance.The electronic structures, charges, and geometries of various surfaces are thoroughly analyzed computationally to explain their properties.In comparison to their O-ended counterparts, the F-ended surfaces should exhibit greater ORR activity but less stability (Figure 21d).Utilizing a separated pyrolysis synthesis technique, the researchers created a NNM composited catalyst of Fe-N-C@Ti 3 C 2 T x (Figure 21e).Through experimental research, they found that Ti 3 C 2 T x MXene not only functions well as a conductive substrate but also significantly reduces Fe-N-C agglomeration and breakdown after carbonization, enriching the ORR performance and durability of the Fe-N-C catalyst. [156]4.3.Black Phosphorus-Based Materials Black phosphorus (BP), a 2D layered material, belongs to the family of elemental phosphorus allotropes.Its puckered structure is composed of phosphorus atoms placed in a layered lattice.With its high carrier mobility, adjustable bandgap, and large specific surface area, BP exhibits superior electrical, optical, and mechanical properties.These properties make it a fascinating prospect for energy-related applications, including FCs. [157] BP is a layered semiconductor that has accumulated attention for its high charge carrier mobility and tunable bandgap.BP-based supporting materials can improve the electrocatalytic performance of DEFCs, especially in the EOR.The unique properties of BP, such as its high surface area and favorable electron transfer kinetics, make it an optimistic prospect for DEFC applications.[158] However, the efficient electrocatalysts needed for EtOH oxidation in DEFCs still need to be improved.BP and its by-products have garnered significant concentration in Reproduced with permission.[154] Copyright 2021, Elsevier Ltd.
recent years as potential electrocatalytic materials due to their unique properties and advantageous application performance. [159]he use of BP-based materials as electrocatalysts in DEFCs suggests several advantages.First, BP shows superior electrocatalytic performance and selectivity toward EORs.Its unique structure and surface properties enable EtOH molecules' adsorption and activation, efficiently converting chemical energy into electrical energy.Second, BP-based materials can be easily synthesized and functionalized, permitting the tailoring of their electrochemical properties and equilibrium. [160]hen exposed to visible light radiation during the EOR process, the Pd/BP-CB [6] catalyst exhibits better results.As shown in Figure 22a, Liu et al. [161] proposed a mechanism by which the photosupported electrocatalysis approach significantly improves EtOH oxidation.The electrocatalytic oxidation of EtOH exhibits two characteristic peaks in the CV arcs, as shown in Figure 22b, at approximately 0 and À0.1 V. Peak CD of Pd/BP-CB [6] is 1900 mA mg Pd À1 , while Pd/BP has a CD of 708 mA mg Pd

À1
below visible light brightness.These results suggest that electrocatalytic performance is improved by CB [6].For FC applications, their stability and catalytic activity are also major issues.The CA curves for prepared Pd/BP and Pd/BP-CB [6] were examined on À0.25 V for 3600 s in both dark and visible light conditions, as shown in Figure 22c.Furthermore, in Ar-saturated 1 M CH 3 CH 2 OH/1 M KOH media, the catalytic performance of the synthesized electrocatalysts and Pd/C for EtOH oxidation was investigated at a voltage range of À0.8-0.2V and a scan rate of 50 mV s À1 .Generally, the electrocatalytic activity of electrodes for EOR was assessed using the forward peak CD.Pd/BP 1 -CB [6] 1 has a forward peak CD of approximately 1420 mA mg Pd À1 .
Under the same environmental conditions, the related forward peak CDs of Pd/BP 1 -CB [6] 2 (731 mA mg Pd À1 ), Pd/BP 2 -CB [6] 1 (657 mA mg Pd À1 ), and Pd/C (350 mA mg Pd À1 ) are smaller than Pd/BP 1 -CB [6] 1 (Figure 22d).As a result, for the following studies, the as-synthesized Pd/BP-CB [6] containing an equal quantity of BP and CB [6] was used.The peak CDs of Pd/CB [6], Pd/BP, and Pd/C are, in order, 90.2, 510, and 350 mA mg Pd À1 , according to Figure 22e.To keep the comparison homogenous, the corresponding MA and specific activity of the Pd/BP-CB [6], Pd/ CB [6], Pd/BP, and Pd/C electrodes standardized with Pd loading and ECSA are outlined.The results are displayed in Figure 22f.
Despite these promising features, challenges remain in developing and utilizing BP-based electrocatalysts for DEFCs.The long-term stability, scalability of synthesis, and integration into practical FC devices require further investigation and optimization.However, continuous research efforts are focused on addressing these challenges and exploring the full potential of BP materials in advancing DEFC technology.The utilization of BP in DEFCs has the potential to contribute to developing sustainable and environmentally friendly energy conversion technologies, bringing us closer to a cleaner and greener energy future.
To improve the efficiency of EtOH electrooxidation, Ibrahim and Abdalla [162] looked into BP/Pd-functionalized carbon aerogel nanocomposite.By using N 2 physisorption at 77 K, the textural effects of the CA, Pd/CA, and BP/Pd/CA specimens are investigated.The samples' N 2 adsorption/desorption isotherms are shown in Figure 23a.Type IV isotherms, which exhibit capillary The termination effects of Pt/v-Ti n + 1 C n T 2 MXene surfaces for ORR catalysis are depicted graphically.e) A Fe-N-C@Ti 3 C 2 T x catalyst based on NNM was created using a simple separated pyrolysis technique.Reproduced with permission. [156]Copyright 2021, Nature.condensation at high P/P 0 and monolayer-multilayer adsorption at low P/P 0 , are characteristic of mesoporous materials.The specimens' XRD patterns are displayed in Figure 23b.The patterns show a broad peak at 2θ ≼ 26°, which corresponds to graphite's (002) diffraction peak.When compared to a perfect graphite crystal, the average space between the (002) planes (d (002) -spacing) is found to be more significant, indicating that the specimens have an amorphous carbon network with a low f ) histograms of the as-synthesized unique probes were swept at a rate of 50 mV s À1 into 1 M CH 3 CH 2 OH/1 M KOH media.Reproduced with permission. [161]Copyright 2022, Elsevier Ltd. .For assessment, the models' contextual electrochemical presentation (black curves) in 1.0 M KOH is also shown.f ) The specimens' mass CDs.Reproduced with permission. [162]Copyright 2022, Royal Society of Chemistry.degree of graphitization.In a 1.0 M KOH/0.5 M C 2 H 5 OH electrolyte, the catalytic activity of CA, Pd/CA, and BP/Pd/CA as anode catalysts for EtOH electrooxidation is investigated at a sweep rate of 60 mV s À1 .The newly chemisorbed EtOH species' oxidation at the integrated electrode is responsible for the anodic peak.On the other hand, the cathodic height is associated with the oxidation and adsorption of intermediate carbonaceous species into the forward scan that are not completely oxidized.The onset voltage of EtOH oxidation during the forward scan is approximately À0.60 V for both Pd/CA and BP/Pd/CA.According to Figure 23c-e, the CDs at the anodic peaks of CA, Pd/CA, and BP/Pd/CA are, respectively, 0.7, 27.7, and 35.9 mA cm À2 .They standardized the CD by the total amount of Pd (0.125 mg) packed upon the material to obtain the catalytic mass performance in order to adequately resemble the electrochemical performance of the materials with the findings of other investigations (Figure 23f ).
Finally, the role of support materials in the electrocatalytic performance of DEFCs is critical, as they provide a stable and conductive substrate for the catalyst material to be deposited on. [163]electing an appropriate support material is essential to ensure high catalytic activity and durability of the DEFC over its operational lifetime.

NNM-Based Materials
Researchers have focused on creating NNM catalysts for ORR in order to address the problem of using Pt as a cathodic catalyst in PEMFCs.Several ORR activity approaches were used to synthesize carbonaceous compounds doped with nitrogen and TMs (M-N-C, where M = Fe, Co, and Mn).The catalyst advantage of running PEMFC in an alkaline configuration was highlighted by these materials, which showed noticeably higher ORR activity in alkaline than in acidic conditions.These M-N-C materials are especially desirable for use at DAFC cathodes because they have a high tolerance to the presence of alcohols (high ORR selectivity) in comparison to Pt-based catalysts.This helps to prevent the negative crossover effect. [106]Many likely materials are already known, including nitrides, carbides, oxynitrides, TMs, and chalcogenides.Because ED is higher around the Fermi level due to d band contraction, the electrical structure of Group 4-6 TM nitrides is favorable for ORR because it simplifies electron donation to adsorbed oxygen.Nitrogen-doped graphene sheets with cobalt nitride (Co 4 N) NP decoration were obtained by nitrogen doping of a graphene-oxide precursor and simultaneous nitride production.With its potent electrocatalytic oxygen reduction activity, the nonprecious metal catalyst created in this one-step synthesis could serve as a viable substitute for the conventional Pt/C alkaline FC cathode catalysts.Composites were made from cobalt (II) acetate and lyophilized graphene-oxide nanosheets in an ammonia environment at 600 °C.The average particle size of Co 4 N changed from 14 to 201 nm as the cobalt content increased.The greatest reduction CD under alkaline conditions was found to be 4.1 mA cm À2 , with an electron transfer number of 3.6. [164]xcellent ORR activity is exhibited by Co-N-CNFs in both alkaline and acidic electrolytes.Concurrently, it was demonstrated that single-atom Co was a more practical active component for TM-N-C catalysts than single-atom Fe, based on the structure-activity-durability relationship of Co-N-CNFs.A novel form of Co@SACo-N-C catalyst was reported by Cheng et al. [165] in which Co NPs are inserted into single Co and N atom codoped CNFs, along with the catalyst's preparation figure.
Additionally, as a useful ORR material for alkaline membrane DEFCs, Rauf et al. [166] synthesized nitrogen-doped CNTs with integrated Fe NPs. Figure 24a illustrates the (Bg-CA-M) -Fe/ N/C material's synthesis process.Two crucial characteristics of the electrocatalysts for functional application in direct-acid FCs (DAFCs) are their longevity and their resilience to the effects of alcohol crossover.The durability of the catalyst was first tested in O 2 -saturated 0.1 M NaOH, with potential cycling ranges of 0.6 and 1.0 V at 50 mV s À1 .According to Figure 24b, (Bg-CA-M)-Fe/N/C@800 °C exhibits a minor negative change in E 1/2 by 12 mV after 10 000 potential cycles.This is noticeably less than the profit-oriented Pt/C material, which exhibits a change in E 1/2 of 56 mV.Similarly, in an O 2 -saturated 0.1 M NaOH medium at 900 rpm, the durability was determined at a constant voltage of 0.80 V.After the 100 h test, the (Bg-CA-M)-Fe/N/C@800 °C produced 29% of the primary performance (Figure 24c).
High alcohol tolerance is shown by (Bg-CA-M)-Fe/N/ C@800 °C. Figure 24d shows that the ORR polarization curve recorded in the 0.1 M NaOH is similar to those assessed in the 0.1 M NaOH electrolytes containing 0.25 and 0.50 M EtOH.At 800 °C, EtOH had no effect on the ORR for (Bg-CA-M)-Fe/N/C.Strong C─H bonds can be broken by electrooxidation of alcohols, a process that is not catalyzed by Fe/N/C materials.In addition, similar to Pt/C, in 0.1 M NaOH þ 0.25 M EtOH media, the ORR current is subordinated to the EtOH oxidation current (Figure 24e).These results imply that the (Bg-CA-M)-Fe/N/C catalyst has a useful use in DAFCs.
It has also been extensively researched to use mixed-metal oxides (bimetal, trimetal, or more) to catalyze ORR because they naturally contain metals with different valence states.Of these mixed-metal oxides, spinel-and perovskite-type oxides have been studied the most. [167]Sunarso et al. [168] for instance, reported that ORR activities of La-based perovskite oxides in alkaline media were investigated.LaCrO 3 < LaFeO 3 < LaNiO 3 < LaMnO 3 < LaCoO 3 was the increasing order of ORR activity for La-based perovskite oxides with different TM ions in B-sites.These researchers also noticed that the ORR onset potentials of the substituted oxides improved, suggesting that substitution by two TM cations was beneficial, and that the order of ORR activity changes to LaNiO 3 < LaNi 0.5 Fe 0.5 O 3 < LaNi 0.5 Co 0.5 O 3 < LaNi 0.5 Cr 0.5 O 3 < LaNi 0.5 Mn 0.5 O 3 when half of the B-site metals are replaced with Ni.

Applications of the FCs
FC research and technology have advanced significantly over the last 10 years, especially in some industries that utilize them, such as transportation and dependable power sources. [128]pplications that require high power reliability (telecom, hightech manufacturing facilities, data centers, and call centers) and can reduce or eliminate emissions (urban areas, industrial facilities, airports, and vehicles) for remote and portable regions with little access to the utility grid. [25]In May 2007, researchers from the University of Offenburg in Germany demonstrated the world's first compact prototype car powered by DEFC at the European Shell Eco-Marathon. [169]For portable devices, replacing current Li-ion technology and other progressive batteries with DAFC technology-more especially, DMFC-aims to do away with some of the disadvantages of batteries, such as their low autonomy, long recharge times (often in the order of independence), and short lifespan-usually between 300 and 1000 charge/discharge cycleswhich reduces them to consumables. [170]

Portable Applications
The primary factor driving the portable power generation market is the growing demand for higher quality power supplies, denser capacity, and longer duration.Because products in this industry must be more compact, less expensive, lighter, and equipped with a growing number of features, there is constant competition between the various technologies.Moreover, an entirely dependable electrical supply is required because the sectors of telecommunications, computers, the Internet, and social networks have become indispensable to people.FC portable applications are primarily targeted at two markets: the first is for moveable power generators made for casual outdoor personal use, such as camping and rising.Consumer electronics (including computers, mobile phones, transistors, camcorders, i-pods, and essentially any other electronic gadget that typically operates on a battery) make up the second market. [25]Before making any significant Consistency experiments of (Bg-CA-M)-Fe/N/C@800 °C and Pt/C catalysts complete 10 000 potential runs amid 0.6 and 1.0 V at b) 50 mV s À1 and persistent voltage at c) 0.8 V for 100 h in O 2 -saturated 0.1 M NaOH media.Spin rate: 900 rpm; sweep rate: 10 mV s À1 .Reproduced with permission. [166]Copyright 2017, Elsevier Ltd. advances in the portable industry, it is necessary to address problems with heat dissipation, emissions dissipation, noise, combined fuel storage and distribution, shock and vibration durability, reaction period to abrupt and repetitive request variations, the process under numerous operating circumstances, tolerance to air filths, reusability and recyclability of fuel vessels, and area uncovered to oxygenated air. [14]

Stationary Applications
FCs have good performance and can produce energy right away from fuel.They alter the combustion-supported electric generating systems for stationary uses of the heated engine, where energy breakdowns occur, including the power dynamo.They could produce heat and electricity for domestic, commercial, and manufacturing units. [128]An FC's size is closely connected to the load's required power for stationary applications.The stationary FC industry comprises a small number of kWs and fewer (microgeneration) to more significant numbers of MWs because this sector includes everything from small backup systems to major facilities.In the upcoming years, the stationary FC market will experience rapid growth on a global scale (2025).The global FC market in 2018 was categorized by a sharp increase in FC shipments, which grew to 57 500 units with a total power of 240 MW.The bazaar had a 5% surge in exports and an 8% increase in connected power compared to 2017.
A clearer picture emerges when the market in 2018 is contrasted with the situation in 2012: FC sales in that year totaled 125 MW, meaning that the market has grown by 92% since 2012. [171]A dependable amount of stationary power plant projects has been positively established over the last 10 years.The major power plant is the Ballard Generation Systems, which has a 250 kW production power.Its PEM FC, powered by natural gas, is an efficient backup power supply for small neighborhoods or vital infrastructure (such as hospitals).Natural gas or other conventional fuels are frequently used in stationary power plants, which are typically very operationally efficient. [172]

Transportation Application
The FC is a viable urban transportation option that improves the environment and quality of life.The FC buses have various benefits over internal combustion engine buses, including the following: direct energy conversion eliminates the need for combustion, reducing pollution emissions.FCs have a worldwide efficiency of roughly 48%, diesel engines between 30% and 35%, and gasoline engines between 13% and 25%. [173]In the United States, the transportation industry is responsible for 28% of all greenhouse gas releases, 34% of CO 2 releases, 36-78% of the primary pollutants in smog, and 68% of oil usage.Three main societal objectives for the light-duty vehicle (LDV) transport subdivision were established as the starting point of our methodology.The ultimate fleet of passenger vehicles would eventually 80% lessen the level of greenhouse gas pollution compared to 1990.Almost all regulated urban air criterion pollutants should be eliminated.Attain "quasi-energy individuality", which is characterized by reducing the amount of oil used by LDVs to the extent that, in an emergency, US national oil production and oil imports from only the American hemisphere would suffice to meet all petroleum needs aside from transportation and a small amount for LDVs.All biofuels, including biodiesel, butanol, and MeOH from biomass, are substituted with EtOH.The estimated maximum annual production capacity of EtOH in the United States ranges significantly from 45 to 140 billion gallons. [174].4.Industrial and Residential Power Generation Applications DEFCs are FC that uses EtOH as a fuel source.EtOH is a renewable fuel from various sources, including corn, sugarcane, and cellulosic biomass. [175]DEFCs have several advantages over other types of FCs, including low-cost, high ED, and the ability to use a wide range of fuels.

Industrial Power Generation Applications
DEFCs can be used for industrial power generation applications.For example, DEFCs can power industrial processes that require electricity, such as manufacturing, chemical production, and water treatment.DEFCs can also be used as backup power systems for industrial facilities in case of power outages. [176]

Residential Power Generation Applications
DEFCs can also be used for residential power generation applications.For example, DEFCs can be used to power homes in remote areas that are not connected to the grid.DEFCs can also be used as backup power systems for homes in case of power outages. [177]Additionally, DEFCs can be used with solar panels or wind turbines to provide a reliable power source.
Overall, DEFCs have the potential to be a cost-effective and sustainable source of power for both industrial and residential applications.

Ethanol Fuel Is Cost-Effective as Compared to Various Other Biofuels
EtOH fuel is a straightforwardly most affordable energy source and the reason being every nation can create it.Most nations produce corn, sugar sticks, or grain fillings, which make the creation conservative in comparison to petroleum products.Petroleum products can play against the economy of most nations, particularly emerging nations that cannot investigate them.It, subsequently, seems OK for these developing economies to harp on the creation of EtOH fuel to tone down on the reliance on petroleum derivatives to save income.To evaluate the effectiveness of combination FC buses (FCB), two separate buses, designated FCB A and FCB B, were built.These buses work on energy (FCB A) and for power (FCB B) hybrid structure.The primary benefit to fuel economy is demonstrated by FCB A's PEMFC and works on highly competent range PEMFC engine.However, cycle testing done for consumption of hydrogen FCB A consumes 7.9 kg/100 km and FCB B consumes 9.8 kg/ 100 km, while these buses running with a speed of 40 kmph for continuous speed testing, the hydrogen ingesting is 3.3 kg/100 km (FCB A) and 4.0 kg/100 km (FCB B).This leads to the conclusion that it is essential to divide the power among PEMFC and an appropriate, robustness battery. [178].1.2.Ecologically Effective As EtOH comprises a lot of oxygen in its chemical construction, it consumes neatly.Including modest quantities (ordinarily one section of EtOH, nine sections of gas) to the gas that powers our vehicles, it decreases greenhouse releases like CO and nitrogen oxides.According to Argonne National Laboratory data, the use of EtOH fuel alone resulted in a 10 ton (9.07 metric ton) reduction in emissions of substances that deplete the ozone layer in 2007. [179].1.3.Produce Useable Byproducts The two main byproducts of EtOH synthesis are CO 2 and dried distillers' grains (DDGs).When CO 2 catch technologies are applied to EtOH production, they may very well be used to produce dry ice, cryogenic freezing, and as a specialist for pneumatic structures.DDGs are substituted for cornmeal or soybean meal in animal food stocks.According to the Corn and Soybean Digest, one metric tons of DDGs could substitute 1.22 metric tons of corn and soybean meal used as food ingredients.
For every unit of energy input in 2007, the maize EtOH that was shipped to the United States produced 1.3 units more energy.Different types of EtOH, like sugarcane EtOH in Brazil, are considerably higher.For each unit of energy input, sugarcane EtOH generates about 8 units of energy.Cellulosic EtOH is a different kind of EtOH that is significantly more powerful.It can produce up to 36 units of energy for every unit of energy input, depending on the creation strategy used. [180].1.4

. Source of Hydrogen
There are three principal receptive paths for delivering hydrogen straight from EtOH: one of them is a synergist, which is contrasted by the coreactant utilized, the cycle science; what is more, the most extreme hydrogen yields reachable, to be specific, steam improving, fractional oxidation and its mixture, and auto-thermal reforming.The maximum hydrogen manufacture may be attained by EtOH steam reforming, where EtOH responds with water steam to provide CO 2 and hydrogen. [181] 3 CH 2 OH þ 3H 2 O ! 2CO 2 þ 6H 2 (10)   12.2.Disadvantages of the FC

Spikes the Food Prices
The massive amount of bushel that this group uses.This composed with the detail that a huge quantity of the people in numerous emerging nations (such as some of the fastest-rising ones alike Brazil and India) are deprived; the current and unexpected increase in nutrition costs has been causing significant communal and political issues everywhere.It is also investigated if the quantity of land utilized for EtOH construction in the United States and Brazil has an impact on food costs.If land use for EtOH production increases at the expenditure of land used for food manufacture, one would a priori anticipate positive connections. [182].2.2.Hard to Vaporize It is difficult to vaporize pure EtOH.Because it is practically impossible to control a car in a cold climate, many car owners make it a point to always have some petrol on hand.Consider E85 vehicles, which run on 85% EtOH and 15% petrol.These days, the most common mixture is E85, which is composed of 85% EtOH and 15% gasoline.This mixture offers poorer mileage than either pure gasoline or the E10 (10% EtOH) blend.However, the greatest pros of switching to the E85 blend are that the oil remains tidy and impurities-free for an extended period; the engine is under less stress, which consequently results in a proportionate decrease in engine maintenance.These little advantages enable the cost of decreased mileage to be offset.Not to mention the overall decrease in carbon impact, which is the one advantage of EtOH fuel consumption that everyone should strive for this technology. [183].Conclusion, Challenges, and Future Prospective This review article focusses on the fabrication of various anode and cathode catalysts for DEFCs.These FC catalysts are based on Pt, Pd, TM, carbon composite, etc.The effect of morphology of catalyst also discussed in brief.In addition to this portable, stationery and transportation application of DEFCs are also discussed with advantage and disadvantage of DEFCs.In the energy sector, EtOH is the much efficient source of fuel having higher commercial value.While we study continuously in this topic, DEFCs technology emerges out as green vehicles technology and helps to reduce environmental pollution.The challenge related to DEFCs is the cost of the catalyst, i.e., catalyst based on noble metals like Pt, Pd, etc., shows better performance toward EOR, but these metals are costly in nature.To overcome this problem, alloying of noble metals is done with TMs like Ru, Cu, Au, etc.The next issue is related to EtOH transport, i.e., maintaining the suitable concentration level in the anode CL to increase the efficiency of DEFCs.The next issue has to do with controlling CO 2 , or how its presence lowers the pH of the cathode CL and thus modifies ORR kinetics.Furthermore, as CO 2 lowers the ionic conductivity in the membrane and the cathode CL, cell resistance also rises in response to CO 2 .Nevertheless, the primary difficulty with DEFCs is to identify a suitable catalyst that promotes the cleavage of the C─C bond.Although DEFCs are less active than DMFCs due to the presence of this bond between two heavy atoms, it is theoretically proven that DEFCs produce more energy than other fuel sources.The lower cleavage ability of the C─C bond results in the formation of aldehyde and acetic acid as an intermediate product that results in incomplete oxidation, decreasing energy production.The formed intermediate product may pollute the catalyst's surface, leading to the catalyst's degradation.The finding of a catalyst that shows more tolerance toward surface poisoning is also a significant challenge in DEFCs technology responsible for the catalyst's durability.Hence, easy diffusion of catalyst takes place between electrodes and membrane.Due to this, the cell also has control over the by-product formation.
By applying passive forces or controlling gas flow rates, the direction of oxygen transport aims to improve the oxygen supply to the cathode CL and the removal of water from the cathode diffusion layer.To optimize cell performance, the cathode CL's hydrogen peroxide concentration should be kept at a suitable level.However, before hydrogen peroxide can be used as an oxidant in FC technology, the problem of hydrogen peroxide decomposition must be resolved.This problem can be solved by maximizing the electrochemical properties of electrode materials, the pH value, the concentration of hydrogen peroxide, and the temperature of the aqueous solution.
The DEFCs technology has a very bright future once it is commercialized because it is the best environmentally friendly alternative in place of fossil fuel.If DEFCs come to light in future, they will emerge as a primary and popular source of energy in future.The higher cost of electrocatalysts is also a problemrelated FC technology, so there is a need to find cheap alternate catalysts for anode and cathode in the future.One great way to increase ionic conductivity in the polymer electrolyte membrane is to introduce new, alternative multiple functional groups.To further improve this chemical and mechanical stability, a functional group is grafted onto the polymer backbone.For a quick redox reaction and improved cell performance, the polymer electrolyte membrane and catalysts must be properly paired.Therefore, choosing the right catalyst is one of the most difficult factors in determining the ideal setup for a great redox reaction while DEFCs are in operation.In the long run, the combination of FCs, a promising and appealing technology, and EtOH, a renewable resource, will improve air quality and reduce emissions from an environmental perspective.It will also increase energy security and generate employment opportunities from a social one.The key to achieving the direct use of EtOH as fuel for FCs is a significant advancement in the development of electrolytes and electrocatalysts.

Figure 2 .
Figure 2. Visual explanation of a) the elastic solid-form structure DEFC and b) how it works.Measuring the np-HEAs electrochemically and recording the CVs at a scan rate of 50 mV s À1 in N 2 -saturated solutions of c) H 2 SO 4 (0.5 M) and d) 0.5 M EtOH + 1.0 M KOH.e) The MA and relative ECSA in (c) and (d)were determined.f ) CA graphs for the EOR at 0.8 V in a solution of EtOH (0.5 M) and KOH (1 M) that are saturated by N 2 (vs RHE).Reproduced with permission.[48]Copyright 2020, John Wiley and Sons.

Figure 4
Figure 4. Basic diagrams of a GDE.The cathodic section of a CO 2 electrolyzer is usually where the stacked assembly of a GDE fitted between the liquid electrolyte and the gas flow field is located.Reproduced with permission.[62]Copyright 2020, Multidisciplinary Digital Publishing Institute.

Figure 5 .
Figure 5.The schematic representation of electrooxidation of EtOH by C 1 and C 2 mechanism.

Figure 6 .
Figure 6.Schematic representation of C 1 and C 2 reaction pathways by electron transfer.Reproduced with permission.[81]Copyright 2022, Elsevier Ltd.

Figure 7 .
Figure 7. TEM micrographs a) NPs of Pt/C NPs, b,c), catalysts made up of Pt/C NW, and d) the patterns of SAED for the catalysts of Pt/C NWs.e) CO single-layer stripping voltammetry of first scan with NPs of Pt/C and novel catalyst Pt/C NWs in H 2 SO 4 (0.5 M) over 20 mV s À1 .f ) CV of second cycle for the electrooxidation of EtOH (0.5 M) dissolved in sulfuric acid for NPs of Pt/C NPs and Pt/C NWs catalysts.ν = 20 mV s À1 at ambient condition.Reproduced with permission.[82]Copyright 2020, Elsevier Ltd.
.In the case of bimetallic and trimetallic catalysts, CV curves stabilize after ten cycles and no important change was observed in the size and shape of voltammograms.The NNM's ability to dissolve in the catalyst was limited by adjusting the upper voltage limit at 500 mV.The desorption and adsorption peaks of hydrogen for PtSn/C are smaller than that of Pt and Pt-based catalysts.This smaller peak of PtSn/C is due to Sn's blockage of the active site of Pt.Furthermore, hydrogen adsorption on Pt was decreased by Co or Ni compared to Pt/C, as displayed in Figure8a,b.The shape interpretations of CV are complex in the case of trimetallic catalysts because of the overlapping of various voltammograms.A little increase in current was observed in respect of trimetallic PtSn-based catalyst as compared to Pt at a potential window of 400-600 mV.The catalytic activity of Pt/C, bimetallic catalyst based on Pt, and trimetallic catalyst based on PtSn for electrochemical oxidation of alcohols was examined by linear sweep voltammetry (LSV) over a slower sweep rate of 1 mV s À1 in a solution of EtOH (1 M) and HClO 4 (aq.)(0.1 M), as demonstrated in Figure8e,f.LSV shows that catalytic activity increases as PtSnNi/ C > PtSnCo/C > PtSnPd/C > PtSn/C ≈ PtSnRh/ C > Pt/C > PtRh/C > PtNi/C > PtCo/C ≈ PtPd/C at 500 mV versus RHE.The catalytic activity of PtSnCo/C increases compared to PtSnNi/C at a potential lower than 500 mV because of the higher oxophillic nature of cobalt versus nickel at a lower voltage.Pt, Sn, and Ni alloy formation results in enhanced PtSnNi/C performance at higher potential.However, at lower voltage, PtSn and PtSn-based trimetallic catalysts have demonstrated better catalytic performance for EOR than Pt and Pt-based bimetallic catalysts.
,b), and the images of TEM show that the fabricated catalyst of the anode is made up of similar nanosized particles having sharp dispersal, and it was noted that Pt lattice parameter decreases by mixing with Pd and Ru, while increases with the W and Sn.The CV (Figure 9c) and single DEFC test simultaneously show that EtOH's electrooxidation increases by Sn, W and Ru with Pt.Among the synthesized catalyst, the performance order toward electrooxidation of EtOH is Pt 1 Sn 1 /C > Pt 1 Ru 1 /C > Pt 1 W 1 /C > Pt 1 Pd 1 /C > Pt/C.Further, Pt 1 Ru 1 /C was adapted with Mo, Sn and W, and the result shows

Figure 12
Figure 12. a,b) TEM pictures of Pd-Ag/G, c) size distribution of NPs shown by histogram, and d) TEM image of the Pd/C.CV of the electrocatalysts Pd-Ag/G (black) and Pd/C (red): e) in the absence of EtOH only in KOH (1.0 M) and f ) in the presence of EtOH, i.e., KOH (1.0 M) þ EtOH (1.0 M) at sweep rate of 50 mV s À1.Reproduced with permission.[97]Copyright 2019, Elsevier Ltd.

Figure 13 .
Figure 13.a) Diagrammatic representation of the ambient condition seed-mediated growing process toward the fabrication of different core-shell nanomaterials.Representative TEM pictures of: b,b 1 ) Pd@Cd 1 -Ag 1 , c,c 1 ) Pd@Ag,and d,d1 ) 50 and 20 nm Pd@Cd nanocatalysts, correspondingly.e) Measured CVs of various Pd@Cd x -Ag y catalysts within NaOH + C 2 H 5 OH; f ) CVs of Pd@Cd 1 -Ag 1 , Pd@Cd, Pd@Ag, and Pd/C (JM) nanocatalysts; g) 600 successive scans of CVs at the Pd@Cd 1 -Ag 1 nanomaterial attained in NaOH (1 M) + C 2 H 5 OH (1 M); h) comparison of the catalytic performance of distinct Pd@Cd x -Ag y nanomaterials with Pd@Cd, Pd@Ag, and marketable Pd/C (JM) nanocatalysts, at a sweep rate of 50 mV s À1 .Reproduced with permission.[98]Copyright 2021, Elsevier Ltd.

Figure 15 .
Figure 15.CV of a) cobalt NPs and b) Co-doped Co/C in KOH (1 M) for 50 runs at a scan rate of 100 mV s À1 .c) CV for the coated electrocatalyst, Pt/C (40%), and pristine cobalt NPs in KOH (1 M) and with EtOH (1.0 M) at a sweep rate of 50 mV s À1 and 25 °C.The oxidation peak of EtOH in the opposite scanning is indicated by the arrow.d) Investigations of persistent voltage in EtOH (2.0 M) at 25 °C and a cell potential of 0.6 V. Reproduced with permission.[105]Copyright 2013, Elsevier Ltd.

Figure 16 .
Figure 16.The electrocatalytic oxidation capacity of as-synthesized Pt 22 Pd 27 Cu 51 , Pt 36 Pd 41 Cu 23 , and Pt/C materials toward intermediaries developed within EOR. a) At a sweep rate of 20 mV s À1 , preabsorbed CO-stripping arcs were contained in N 2 -saturated 0.5 M H 2 SO 4 media.b) The CV profile of specific move contained 0.5 M H 2 SO 4 and 0.5 M CH 3 CHO.c) LSV profiles gathered within N 2 -saturated 0.5 M H 2 SO 4 and 1 M CH 3 CH 2 OH.d) Schematic image and e) single-cell picture of Pt 36 Pd 41 Cu 23 NW-based MEA within a DEFC.The specific polarization and PD arcs for DEFCs of f ) Pt 36 Pd 41 Cu 23 NWs and g) retail Pt/C as materials are processed at 60 and 80 °C simultaneously.Reproduced with permission.[116]Copyright 2021, Elsevier Ltd.

Figure 17 .
Figure 17.EtOH-crossover dimension at 10 weight percent Pt/C and Hypermec K14 in an obsessed mode cell under the following circumstances: fuel specifications: 12 weight percent EtOH D and 10 weight percent KOH; fuel stream: 7 mL min À1; cathode fed by 250 KPa pressure of N 2 ; 0% relative humidity; 200 mL min À1 .Reproduced with permission.[112a]Copyright 2011, Elsevier Ltd.
4 and Na 2 PdCl 4 mixtures with NaBH 4 and HNO 3 and (3-aminopropyl) triethoxysilane -treated CBs (CB H-A ).High-angle annular dark field scanning transmission microscopy (HAADF-STEM) images of Au 0.4 Pd 0.6 /CB H-A , Au 0.4 Pd 0.6 / CB A , and Au 0.4 Pd 0.6 /CB H are shown in Figure 19b-d.CV was used to evaluate the ECSAs of the materials as synthesized.The CV curves of retail Pd/C, Au 0.4 Pd 0.6 /CB H-A , Au 0.4 Pd 0.6 /CB A , and Au 0.4 Pd 0.6 /CB H into Ar-saturated 1.0 M KOH with a sweep rate of 10 mV s À1 are shown in Figure 19e.The texture Pd and Au oxides cause the oxidation peaks to appear during the positive scans, starting at À0.1 V.With an ECSA of 87.61 m 2 g À1 , the

Figure 18 .
Figure 18.a) Pd/3DNCNTs catalyst's manufactured path schematic.b) A Pd/3DNCNT TEM image.The insets are similar to high-resolution TEM (HRTEM) images of the Pd NPs lattice design and size allocation histograms.c) TEM image of PdSnNi NPs that are evenly distributed and anchored on CNTs.d) Pd NPs sustained on extended graphite and MWCNTs as seen in a TEM image.e) An HRTEM image of hybrid materials made of Pd, NiO, CNTs, and rGO shows Pd, CNT, and rGO with a distorted view of the selected region.f ) EOR durability test on Pd/C, Pd-NiO/MWCNTs/rGO, and Pd/MWCNTs/rGO electrodes.Reproduced with permission.[145]Copyright 2019, Multidisciplinary Digital Publishing Institute.

Figure 19 .
Figure 19.a) The synthesis method presentation of various AuPd materials.The HAADF-STEM pictures of b) Au 0.4 Pd 0.6 /CB H-A , c) Au 0.4 Pd 0.6 /CB A , and d) Au 0.4 Pd 0.6 /CB H . e) CV arcs and f ) the agreeing ECSAs of Au 0.4 Pd 0.6 /CB H-A , Au 0.4 Pd 0.6 /CB A , Au 0.4 Pd 0.6 /CB H , and marketable Pd/C.The CV arcs were noted in Ar-saturated in aqueous solution (1.0 M KOH).g) CV arcs of EOR in 1.0 M KOH/1.0M (C 2 H 5 OH, at sweet rate: 50 mV s À1 ) on Au 0.4 Pd 0.6 /CB H-A , Au 0.4 Pd 0.6 /CB A , Au 0.4 Pd 0.6 /CB H , Pd/CB H-A , and profitable Pd/C and Pt/C.h) The mass and specific activities of different synthetic materials that are related.Reproduced with permission.[148]Copyright 2022, Elsevier Ltd.

Figure 20 .
Figure 20.The typical structure of MAX phases and its related MXenes.Reproduced with permission.[154]Copyright 2021, Elsevier Ltd.

Figure 21 .
Figure 21.a) The HER activity of Pt NPs is also shown, and anodic-going iR-corrected LSVs of Mo 2 CT x and Ti 2 CT x on glassy carbon are compared to bare GCE.b) Temporal potential production is required to maintain a consistent CD value of 10 mA cm À2 .c) XPS spectra of Mo 2 CT x before and after HER testing.d)The termination effects of Pt/v-Ti n + 1 C n T 2 MXene surfaces for ORR catalysis are depicted graphically.e) A Fe-N-C@Ti 3 C 2 T x catalyst based on NNM was created using a simple separated pyrolysis technique.Reproduced with permission.[156]Copyright 2021, Nature.

Figure 22 .
Figure 22. a) The photo-assisted electrocatalytic EOR process at the Pd/BP-CB[6] probe is illustrated graphically using visible light.b) CVs and c) CA arcs at À0.25 V of the as-synthesized Pd/BP and Pd/BP-CB[6] materials.d) CVs of the various Pd/BPx-CB[6]y electrodes and retail Pd/C, e) CVs, and f ) the MA ( j f ) and specific activity ( j 0f ) histograms of the as-synthesized unique probes were swept at a rate of 50 mV s À1 into 1 M CH 3 CH 2 OH/1 M KOH media.Reproduced with permission.[161]Copyright 2022, Elsevier Ltd.

Figure 23 .
Figure 23.a) N 2 adsorption/desorption isotherms, b) XRD patterns of the manufactured materials.Room-temperature CVs (red curves) in 1.0 M KOH + 0.5 M C 2 H 5 OH media for c) CA, d) Pd/CA, and e) BP/Pd/CA at a sweep rate of 60 mV s À1.For assessment, the models' contextual electrochemical presentation (black curves) in 1.0 M KOH is also shown.f ) The specimens' mass CDs.Reproduced with permission.[162]Copyright 2022, Royal Society of Chemistry.
, Pt 36 Pd 41 Cu 23 NWs exhibit a more promising catalytic capability toward CH 3 CHO oxidation.As demonstrated in Figure 16c, the outcomes demonstrate a shorter amount of CH 3 COOH created upon the superficial of Pt 36 Pd 41 Cu 23 NWs approximated with Pt 22 Pd 27 Cu 51 NWs and Pt/C.Pt 36 Pd 41 Cu 23 NWs are used as electrocatalysts in both the anode and cathode to produce MEA, illustrating a practical application of them within DEFC (Figure 16d,e).Pt loading for the anode and cathode is 1.2 mg Pt cm À2 .Pt 36 Pd 41 Cu 23 NWs that

Table 4 .
Difference between Pt and Pd electrocatalyst.