Progress of Metal Chalcogenides as Catalysts for Efficient Electrosynthesis of Hydrogen Peroxide

Hydrogen peroxide (H2O2) is a high-demand chemical, valued as a powerful and eco-friendly oxidant for various industrial applications. The traditional industrial method for producing H2O2, known as the anthraquinone process, is both costly and environmentally problematic. Electrochemical synthesis, which produces H2O2 using electricity, offers a sustainable alternative, particularly suited for small-scale, continuous on-site H2O2 generation due to the portability of electrocatalytic devices. For efficient H2O2 electrosynthesis, electrocatalysts must exhibit high selectivity, activity, and stability for the two-electron pathway-oxygen reduction reaction (2e− ORR). Transition-metal chalcogenide (TMC)-based materials have emerged as promising candidates for effective 2e− ORR due to their high activity in acidic environments and the abundance of their constituent elements. This review examines the potential of TMC-based catalysts in H2O2 electrosynthesis, categorizing them into noble-metal and non-noble-metal chalcogenides. It underscores the importance of achieving high selectivity, activity, and stability in 2e− ORR. By reviewing recent advancements and identifying key challenges, this review provides valuable insights into the development of TMC-based electrocatalysts for sustainable H2O2 production.

To date, around 95% of industrial H 2 O 2 is produced using the anthraquinone process.However, this method necessitates expensive noble-metal catalysts, primarily palladium, and poses environmental issues due to its toxic byproducts [1,2,4].Additionally, its continuous operation under high temperature and pressure renders it complex, large-scale, and energy intensive [11].Another method to directly synthesize H 2 O 2 is chemically catalyzing a liquid-phase mixture of H 2 and O 2 [12].However, due to the safety hazards associated with H 2 and O 2 mixtures, the H 2 gas must be diluted in CO 2 or N 2 , significantly limiting production efficiency [3].Addressing these challenges, the electrosynthesis of H 2 O 2 has attracted attention due to its green chemistry approach, as it can be conducted under ambient conditions and avoids the use of toxic solvents and intermediates associated with traditional methods [3,13,14].This process primarily involves the reduction of oxygen in aqueous electrolytes, resulting in minimal byproducts and waste [15].Furthermore, the electrochemical route can be powered by renewable energy sources, significantly reducing the carbon footprint of H 2 O 2 production [16].In addition, the electrochemical process can be integrated with practical fuel cells for renewable energy and simultaneous power generation.Due to the flexibility of the electrochemical system and the portability of electrocatalytic devices, it is also suitable for small-scale, continuous on-site H 2 O 2 production [13][14][15].
The efficient electrosynthesis of H 2 O 2 through the oxygen reduction reaction (ORR) requires electrocatalysts with high two-electron pathway (2e − ) selectivity, activity, and stability, while also being cost-effective for practical applications [3,13,15].Recently, carbon materials have been extensively studied as potential electrocatalysts, achieving significant advancements in the electrosynthesis of H 2 O 2 through 2e − ORR due to their high surface area, tunable electronic properties, low conjugated π-system and excellent electrical conductivity [17][18][19][20].However, most carbon nanomaterials have exhibited high performance in alkaline media and showed poor stability in acidic conditions [21][22][23].
To overcome these limitations, transition-metal chalcogenide (TMC)-based electrocatalysts have emerged for the 2e − ORR owing to their strong stability against acid conditions [23][24][25].TMCs can be classified into two categories: those composed of noble metals such as palladium (Pd) and platinum (Pt), and those made up of non-noble metals like iron (Fe), cobalt (Co), nickel (Ni), manganese (Mn), and copper (Cu).These metals are further combined with chalcogenides, such as sulfur (S), selenium (Se), and tellurium (Te).Noble-metal chalcogenides (e.g., PdS, PtSe) are known for their high catalytic activity and stability, while their practical applications are often limited by the high cost and scarcity of noble metals [26,27].In contrast, non-noble-metal chalcogenides (e.g., Cu 2 Se, CoSe 2 , NiTe 2 ) offer a cost-effective and abundant alternative, whereas their 2e − ORR activity is still in need to be improved [28][29][30].
This review examines the potential of TMC-based catalysts for efficient H 2 O 2 electrosynthesis via the ORR.First, we will discuss the mechanism of H 2 O 2 synthesis through 2e − ORR.This includes the competitive reaction, reaction kinetics, and pH-dependent variation in 2e − ORR.Next, recent advancements in TMC electrocatalysts will be covered, which highlight the benefits of phase engineering and the achievements of high performance in terms of selectivity, activity, and stability.This review also covers a comprehensive examination of recent research, including innovative strategies such as chemical transformation and surface coordination tuning, which have shown promising results in various studies.By offering a detailed overview of the current state of research and identifying critical challenges and future directions, this review aims to offer valuable insights into the advancements in TMC-based electrocatalysts for sustainable and efficient H 2 O 2 production.

Mechanism of Electrocatalytic 2e − ORR
The electrochemical synthesis of H 2 O 2 primarily involves the ORR.The ORR can proceed via two pathways: the two-electron (2e − ) pathway producing H 2 O 2 , and the four-electron (4e − ) pathway producing water (H 2 O) [16].The 2e − pathway leads to the formation of H 2 O 2 , which is useful in various chemical synthesis and environmental applications.The 4e − pathway is preferred for applications like fuel cells where the complete reduction of oxygen to water is desired, as it provides the highest energy efficiency [31,32].The desired pathway for H 2 O 2 production is the 2e − ORR, which can be represented by the following reactions [33]: In acidic media: In alkaline media: where SHE is a Standard Hydrogen Electrode.However, the 2e − pathway is thermodynamically less favorable compared to the 4e − pathway.The stability of the produced H 2 O 2 is another challenge, as it can be further reduced to H 2 O or decompose chemically.The 4e − pathway for H 2 O production can be represented by the following reactions [33]: In acidic media: In alkaline media: The efficient electrochemical synthesis of H 2 O 2 hinges on the development of specialized catalysts that can selectively facilitate the 2e − ORR pathway, thereby preventing the further reduction of H 2 O 2 to H 2 O.This necessitates a focus on materials that not only demonstrate high catalytic performance but also maintain stability under operational conditions.
In catalytic reactions, factors influencing reaction pathways include electrostatic interactions governing catalyst-reactant affinity and steric hindrance dictating reactant adsorption on catalysts [3,34].The mechanism of electrochemical ORR has been extensively investigated and is depicted in Figure 1.In the ORR, the way oxygen binds to the catalyst, whether in side-on or end-on configurations, dictates pathway preference.The end-on binding of O 2 triggers the formation of *OOH as an intermediate species (Equation ( 5)), regardless of the eventual product outcome.If the interaction between *OOH and the catalyst surface remains weak or intermediate, H 2 O 2 is produced as the ultimate product via the 2e − reaction pathway (Equation ( 6)).Conversely, if there is a strong binding between *OOH and the catalyst surface, the O-O bond dissociates, resulting in the formation of * O as an intermediate species (Equation (7)).This leads to consecutive 2e − reactions, finally resulting in the 4e − reaction pathway to produce H 2 O as the final product (Equation ( 8)).
O 2 + * + (H + + e − ) → *OOH (5) In the case of a side-on reaction, O 2 strongly absorbs onto the surface, leading to the dissociation of the O-O bond and subsequent formation of the *O intermediate species (Equation ( 9)).The *O intermediate then evolves to form *OH (Equation ( 10)), ultimately resulting in the formation of H 2 O via the 4e − reaction pathway (Equation ( 11)).
According to this ORR mechanism, adjusting the binding orientation of O 2 or the affinity between the catalyst and reactants is crucial to produce H 2 O 2. Effective catalyst design strategies must focus on maintaining the integrity of the O-O bond while also managing factors such as mass transport and electrolyte pH to ensure H 2 O 2 stability [16].The adsorption energy refers to the energy change when an O 2 molecule binds to the surface of the catalyst.It should be sufficiently strong to ensure that O 2 molecules are effectively adsorbed and activated for the reaction.However, it must not be so strong that it impedes the subsequent desorption of the reaction intermediates of the final products, which would otherwise hinder the overall catalytic efficiency [3,15,32,33].To achieve optimal selectivity toward H 2 O 2 , catalysts must minimize the kinetic barriers for O 2 adsorption and *OOH desorption, while maximizing the kinetic barriers for *OOH dissociation to *O and *OH.This balance ensures high reduction activity while maintaining selectivity toward H 2 O 2 .Geometric effects significantly influence the activity and selectivity of the ORR [35][36][37].These effects pertain to the spatial arrangement and physical structure of the catalyst surface, which play a crucial role in the adsorption and reaction dynamics of O 2 molecules during the ORR process [38,39].The atomic arrangement on the catalyst surface, determined by its crystallographic facets, exhibits different structural characteristics [40][41][42].Catalysts with well-structured surfaces that expose a high number of active sites tend to provide more opportunities for O 2 adsorption and reaction, thereby demonstrating superior performance [43].Furthermore, the presence of large groups on the catalyst surface can obstruct O 2 molecules from accessing the active sites.Such steric hindrance can reduce the number of accessible active sites while also influencing the preferred orientation of the adsorbed O 2 molecules [44].To optimize the surface structure, several strategies can be employed, including the creation of porous structures [45,46], introduction of defects [47,48], and construction of heterostructures [49,50].These methods can adjust the geometric effects and enhance catalytic performance.Moreover, rational reactor and process designs, including conditions of forced convection and low catalyst loading, are crucial for maintaining these conditions.
Materials 2024, 17, x FOR PEER REVIEW 4 the subsequent desorption of the reaction intermediates of the final products, w would otherwise hinder the overall catalytic efficiency [3,15,32,33].To achieve opt selectivity toward H2O2, catalysts must minimize the kinetic barriers for O2 adsorp and *OOH desorption, while maximizing the kinetic barriers for *OOH dissociation t and *OH.This balance ensures high reduction activity while maintaining selectivity ward H2O2.Geometric effects significantly influence the activity and selectivity of the O [35][36][37].These effects pertain to the spatial arrangement and physical structure of the alyst surface, which play a crucial role in the adsorption and reaction dynamics of O2 ecules during the ORR process [38,39].The atomic arrangement on the catalyst sur determined by its crystallographic facets, exhibits different structural characteristics 42].Catalysts with well-structured surfaces that expose a high number of active sites to provide more opportunities for O2 adsorption and reaction, thereby demonstrating perior performance [43].Furthermore, the presence of large groups on the catalyst sur can obstruct O2 molecules from accessing the active sites.Such steric hindrance can red the number of accessible active sites while also influencing the preferred orientation o adsorbed O2 molecules [44].To optimize the surface structure, several strategies ca employed, including the creation of porous structures [45,46], introduction of de [47,48], and construction of heterostructures [49,50].These methods can adjust the metric effects and enhance catalytic performance.Moreover, rational reactor and pro designs, including conditions of forced convection and low catalyst loading, are cru for maintaining these conditions.

Reaction Kinetics and Volcano Plot Analysis for 2e − ORR
Based on scaling relationships among various descriptors, specifically ΔG(*OO ΔG(*OH) + 3.2 and ΔG(*O) = 2ΔG(*OH), the adsorption energies of *OH and *OOH s as crucial indicators for 2e − ORR and 4e − ORR activities [51].This scaling issue can be igated by adjusting the covalency of the catalyst.As electrochemical catalysts have vanced, the complexity of electrode materials has increased, evolving to include do carbons [52], noble metal alloys [53], and metal oxides [54].The development of com tational methods, such as density functional theory (DFT) calculations, has significa enhanced the prediction of material activity for H2O2 electrosynthesis, thereby accel ing catalyst material advancements.
Figure 2a illustrates volcano plots for 2e − ORR and 4e − ORR on metal surfaces metal-nitrogen/carbon (M-N/C) structures, with dashed lines representing the stand   [51].This scaling issue can be mitigated by adjusting the covalency of the catalyst.As electrochemical catalysts have advanced, the complexity of electrode materials has increased, evolving to include doped carbons [52], noble metal alloys [53], and metal oxides [54].The development of computational methods, such as density functional theory (DFT) calculations, has significantly enhanced the prediction of material activity for H 2 O 2 electrosynthesis, thereby accelerating catalyst material advancements.
Figure 2a illustrates volcano plots for 2e − ORR and 4e − ORR on metal surfaces and metal-nitrogen/carbon (M-N/C) structures, with dashed lines representing the standard equilibrium potentials of these reactions.The step of O 2 reduction to *OOH limits the reaction on the weak-binding, right-hand leg of the volcano, while *OOH reduction to H 2 O 2 limits the reaction on the strong-binding, left-hand leg.Among the pure-metal surfaces, only Au is positioned on the weak-binding leg, whereas among M-N/C structures, Cu-N/C, Ni-N/C, Pd-N/C, and Pt-N/C also lie on the weak-binding leg.Metals with low oxygen affinity maintain low hybridization between metal and oxygen bonds, thus retaining high selectivity for the 2e − ORR [55,56].Conversely, metals with strong *OH and *OOH adsorption energies promote O-O bond dissociation, limiting the reduction of *OOH to H 2 O 2 and favoring the 4e − pathway [57].These differences arise from intrinsic properties such as the number of valence electrons and spin states [58,59].The optimal catalyst should exhibit moderate *OOH adsorption, balancing the interaction between the catalyst surface and reaction intermediates to achieve excellent performance.The ideal Gibbs adsorption free energy for an H 2 O 2 synthesis catalyst is approximately 4.22 eV, with a theoretical thermodynamic limit potential of 0.7 V vs. RHE [60].According to Figure 2a, no material possesses the optimal electronic structure for H 2 O 2 production.However, alloying Pt and Pd with Hg can adjust the *OOH binding to achieve high activity for H 2 O 2 production (Figure 2b).Modifying the electronic and geometric effects of catalysts is a valuable approach to controlling catalyst activity and selectivity.Nevertheless, Pt and Pd are rare and expensive metals, imposing significant economic burdens for large-scale industrial applications [61,62].Hg, while cheaper, is a hazardous metal with considerable environmental and health risks, limiting its industrial use [63].In contrast, TMCs are significantly cheaper than noble-metal catalysts and are composed of elements abundantly available on Earth, making them economically feasible for large-scale production.Additionally, TMCs exhibit distinctive properties in charge density control, which can be adjusted through various doping and alloying techniques [64].This capability allows for the development of catalysts with high ORR activity and selectivity.Inconsistencies exist in the kinetic descriptions of different electrode interfaces, such as rotating ring-disk electrodes (RRDEs) and gas-diffusion electrodes (GDEs) [3,14].In GDEs, the restriction of oxygen diffusion to the reaction site decreases the Faraday efficiency for H 2 O 2 as catalyst loading increases.High catalyst loading slows the diffusion kinetics of H 2 O 2 , leading to its in situ decomposition within the high-pH electrocatalyst layer [65][66][67].The mass transport of reaction species or products within the electrode interface microenvironment significantly affects the 2e − ORR kinetics [68].Electrode structure and interfacial hydrophobicity can also profoundly impact 2e − ORR kinetics [69].Constructing three-phase sites, tailoring local solution environments, and designing flow-through reactors can facilitate O 2 uptake and proton transfer, thereby enhancing the 2e − ORR kinetics [3,34,70].

Effects of Electrolyte pH on ORR Mechanism
The pH of the electrolyte significantly impacts the activity and selectivity of catalysts used in the ORR.This is because the behavior of the reaction pathway, including multiple electron transfer steps and the polarity of solvated water, varies under different pH conditions [71][72][73].Understanding these effects is crucial for designing and optimizing efficient electrocatalysts.
Figure 3 illustrates the double-layer structure scenario at the electrode/electrolyte interface during ORR.The interface between the electrode and electrolyte forms a doublelayer structure, comprising the inner Helmholtz plane (IHP) and the outer Helmholtz plane (OHP).The IHP is populated by specifically adsorbed hydroxyl species, solvent water dipoles oriented with the oxygen towards the electrode surface, and chemisorbed O 2 .Beyond the OHP, charged species are loosely scattered in the aqueous solution [74].The species occupying the IHP and OHP vary under different pH conditions, and their interactions promote either an inner-sphere electron transfer (ISET) process or an outersphere electron transfer (OSET) process [4,75].

Effects of Electrolyte pH on ORR Mechanism
The pH of the electrolyte significantly impacts the activity and selectivity of catalysts used in the ORR.This is because the behavior of the reaction pathway, including multiple electron transfer steps and the polarity of solvated water, varies under different pH conditions [71][72][73].Understanding these effects is crucial for designing and optimizing efficient electrocatalysts.
Figure 3 illustrates the double-layer structure scenario at the electrode/electrolyte interface during ORR.The interface between the electrode and electrolyte forms a doublelayer structure, comprising the inner Helmholtz plane (IHP) and the outer Helmholtz plane (OHP).The IHP is populated by specifically adsorbed hydroxyl species, solvent water dipoles oriented with the oxygen towards the electrode surface, and chemisorbed O2.Beyond the OHP, charged species are loosely scattered in the aqueous solution [74].The species occupying the IHP and OHP vary under different pH conditions, and their interactions promote either an inner-sphere electron transfer (ISET) process or an outer-sphere electron transfer (OSET) process [4,75].In acidic environments, the ORR generally follows an ISET mechanism.In this mechanism, molecular O 2 strongly chemisorbs onto the catalyst surface, proceeding through a series of reduction steps.The 4e − pathway is predominant in acidic media and involves the following reaction steps [22]: The process includes intermediates such as *O 2 , *OOH, *O, and *OH, which remain adsorbed on the active site until the final product, H 2 O, is desorbed into the bulk electrolyte.The specific steps are as follows: All intermediates stay bound to the catalyst surface, ensuring efficient electron transfer and product formation.In alkaline media, the ORR often follows an OSET mechanism.Here, solvated molecular oxygen (O 2 •(H 2 O) n ) aq interacts with surface hydroxyl species (*OH) via hydrogen bonding.This promotes the formation of superoxide and hydroperoxide intermediates, which are crucial for the 2e − pathway.For the 2e − pathway specific to alkaline conditions, the reactions are as follows [4]: In this reaction, the superoxide ion (*OOH − ) acts as a major intermediate, easily converting to H 2 O 2 .The weak hydrogen bonds between solvated molecular oxygen and surface hydroxyl species facilitate the outer-sphere electron transfer, promoting the formation of H 2 O 2 .The bond dissociation energy (BDE) of O 2 is 498 kJ/mol, larger than the BDE of many typical nonmetal-O bonds.Thus, the dissociation of O 2 is more energetically favorable on a catalyst surface with moderate adsorption energy of O 2 through various intermediates such as adsorbed oxygen (O ad ), superoxide ion (O 2 − ), and HO 2,ad , among others [33].
The pH conditions in which the ORR occurs play a crucial role in determining the reaction pathways and the stability of the reaction products.This understanding is vital for developing catalysts optimized for specific pH conditions, particularly in electrochemical H 2 O 2 production.In alkaline media, ORR predominantly follows the 2e − pathway, leading to the production of H 2 O 2 [4,76].However, a significant challenge associated with this pathway is the inherent instability of H 2 O 2 in alkaline environments, where it is prone to further reduction or decomposition into H 2 O.This instability can severely limit the efficiency of H 2 O 2 production.To address this, strategies such as rational reactor design or the introduction of H 2 O 2 stabilizers are being explored to enhance the stability and yield of H 2 O 2 in alkaline conditions [4].Conversely, in acidic conditions, ORR tends to follow the 4e − pathway, which results in the complete reduction of O 2 to H 2 O.While the 2e − pathway that leads to H 2 O 2 formation is less active in acidic environments, the H 2 O 2 that is produced is generally more stable and less prone to decomposition [77].The stability offers a distinct advantage in applications where the production and accumulation of H 2 O 2 are desired.
The decomposition of H 2 O 2 varies significantly between acidic and alkaline conditions.In acidic media, the following reaction occurs [33]: In alkaline media, the decomposition proceeds via the following: These reactions illustrate the different pathways through which H 2 O 2 can be decomposed depending on the pH, underscoring the importance of developing catalysts that possess high stability and performance in acidic environments to effectively inhibit these decomposition reactions.Such advancements could significantly improve the efficiency of H 2 O 2 production, with wide-ranging implications for its use in chemical synthesis and environmental applications.
The utilization of carbon materials that exhibit low current densities in acidic media presents significant limitations.In the electrosynthesis of H 2 O 2 , the choice of electrolyte is a crucial factor.In alkaline electrolytes, H 2 O 2 decomposes rapidly, necessitating immediate use or the addition of stabilizers such as phosphoric acid for long-term use [4].Conversely, the electrosynthesis of H 2 O 2 in acidic electrolytes offers greater stability by preventing the decomposition of H 2 O 2 [94].This eliminates the need for stabilizers, thereby reducing production costs and making it immediately applicable to various processes such as the Electro-Fenton method [25,95].In acidic media, Pt-and Pd-based alloys and single-atom catalysts exhibit significant 2e − ORR activity and selectivity [84,85,93].However, Pt-and Pd-based alloys are hindered by issues of scarcity and high costs.Single-atom catalysts (SACs) are prone to agglomeration, leading to catalyst deactivation [51].Additionally, the preparation of efficient SACs involves complex processes, including high-temperature annealing and acid treatment [96][97][98][99].Therefore, extensive research is being conducted to discover alternative electrocatalysts that are earth-abundant, cost-effective, and exhibit excellent activity and stability in acidic electrolytes.
TMCs have garnered significant research interest over the past few decades due to their earth abundance, versatile redox properties, and facile synthesis, making them suitable for large-scale applications.Numerous studies have extensively investigated TMCs as active catalysts for electrocatalytic reactions such as the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) [100,101], and 4e − ORR [64], demonstrating high activity and significant potential in electrosynthesis [102].However, investigations on 2e − ORR catalysts are still in their early stages.Several studies have highlighted that TMCs, such as CoS 2 , CoSe 2 , MoTe 2 , and NiS 2 [103][104][105][106][107], demonstrate promising selectivity and activity in acidic environments, emphasizing their potential as 2e − ORR catalysts.These materials not only exhibit excellent selectivity and activity but also possess remarkable stability in acidic conditions.The ability of TMCs to exhibit these characteristics can be attributed to their unique structural properties, such as high designability of morphology, electronic structures, and rich active sites [108,109].In the 2e − ORR mechanism, achieving the appropriate adsorption energy for the reaction intermediate *OOH on the active sites is crucial [10,110].TMCs can provide diverse and specific surface structural motifs with controllable tunability, enabling the rational design of TMCs with optimal *OOH adsorption energy.Such a design would prevent O-O bond cleavage and facilitate the desorption of hydrogen peroxide during the reduction process.Therefore, further research on TMCs for the 2e − ORR is urgent.
In this chapter, we classify ORR catalysts into noble-metal chalcogenides and nonnoble-metal chalcogenides and present the reaction mechanisms for the 2e − ORR pathway in both acidic and alkaline environments.We also discuss the key factors that enhance selectivity and provide theoretical analyses.

Material Design for Efficient Electrocatalysts
As illustrated in Table 1, the activity and selectivity of TMC-based 2e − ORR electrocatalysts have been greatly enhanced due to the implementation of diverse material design approaches.The pure TMCs are generally inefficient as electrocatalysts for H 2 O 2 production.To improve their performance, strategies such as alloying, element doping, defect engineering, and facet engineering have been employed to modulate the electronic structure, thereby adjusting the binding energy of the *OOH intermediate, which prevents O-O bond cleavage and enhances selectivity.The electronic structure plays a crucial role in determining the performance of electrocatalysts and can be modified through strategies like doping, defect engineering, and other approaches.Altering the electronic structure leads to the redistribution of spin and charge, which in turn enhances electrocatalytic performance [115].The presence of defects in heterogeneous catalysts can disrupt the original stoichiometric ratio, cause distortions in the electric field, and alter the electronic distribution.These changes have been extensively studied to adjust intrinsic physicochemical properties and improve electrocatalytic performance.By carefully creating defects of various types, quantities, and locations, it is possible to precisely control the resulting electrocatalytic performance [116].
Guo et al. synthesized PtCu catalysts with Cu-vacancy defects through an electrochemical etching process.These PtCu catalysts, which have an ultrafine size and self-supporting rigid structure, exhibited significant structural stability.Density functional theory (DFT) calculations showed that the introduction of Cu vacancies on the surface of Pt-Cu (111) catalysts enhances the adsorption capacity of Pt atoms for the *OH intermediate while simultaneously weakening the adsorption for the *O intermediate.This catalyst demonstrated a mass activity 14.1 times higher than that of the commercial Pt/C catalyst (20 wt%, JM) [117].Wang et al. synthesized selenium-vacancy-enriched Fe-CoSe-HT catalysts through a straightforward hydrothermal process followed by high-temperature annealing.The doping of trace amounts of Fe was found to influence crystal growth and induce the formation of Se vacancies during the annealing process.When applied to the 2e − ORR, the Fe-CoSe-HT catalyst achieved a H 2 O 2 production rate of 18.37 mmol L −1 over 120 min under neutral conditions, with a Faraday efficiency of 83.2%.The selenium vacancies in Fe-CoSe-HT significantly improved the selectivity of the 2e − ORR and accelerated Fe re-duction, leading to enhanced •OH radical generation and the effective treatment of organic pollutants [118].

Noble-Metal Chalcogenides
Noble-metal catalysts are widely used in many electrochemical catalysis fields due to their high stability and excellent performance [119][120][121].However, they are well known for favoring the 4e − pathway in ORR reactions, resulting in low selectivity for the electrochemical synthesis of hydrogen peroxide [122,123].Therefore, research is being conducted to develop catalysts that preferentially facilitate the 2e − pathway.
To induce the 2e − pathway in ORR, noble metal alloys such as Pt-Hg and Pd-Hg are widely used [55,85].However, Hg is highly toxic and easily leaches out during the ORR reaction, making practical applications challenging.Compared to metal alloys, noble metal-based chalcogenides with high electronegativity can easily modulate the electronic structure and exhibit high catalytic activity, showing great potential for future applications in hydrogen peroxide production.Song et al. [27] introduced individual chalcogens into platinum metal and compared the catalytic activity for the 2e − pathway based on the atomic radius and chemical properties of the chalcogen elements, confirming the order PtTe 2 /C < a-PtS 2 /C < PtSe 2 /C (Figure 4a).Among these, PtSe 2 /C exhibited high selectivity (~91%) and high mass activity (439 A/g Pt ) for the 2e − pathway (Figure 4b).They effectively modulated the surface atomic arrangement and electronic structure of the active sites using chalcogens.Platinum chalcogenides with isolated active sites facilitated end-on adsorption of O 2 , and by adjusting the binding energy of *OOH through changes in electronic structure, PtSe 2 /C favored the 2e − pathway.

Noble-Metal Chalcogenides
Noble-metal catalysts are widely used in many electrochemical catalysis fields due to their high stability and excellent performance [119][120][121].However, they are well known for favoring the 4e − pathway in ORR reactions, resulting in low selectivity for the electrochemical synthesis of hydrogen peroxide [122,123].Therefore, research is being conducted to develop catalysts that preferentially facilitate the 2e − pathway.
To induce the 2e − pathway in ORR, noble metal alloys such as Pt-Hg and Pd-Hg are widely used [55,85].However, Hg is highly toxic and easily leaches out during the ORR reaction, making practical applications challenging.Compared to metal alloys, noble metal-based chalcogenides with high electronegativity can easily modulate the electronic structure and exhibit high catalytic activity, showing great potential for future applications in hydrogen peroxide production.Song et al. [27] introduced individual chalcogens into platinum metal and compared the catalytic activity for the 2e − pathway based on the atomic radius and chemical properties of the chalcogen elements, confirming the order PtTe2/C < a-PtS2/C < PtSe2/C (Figure 4a).Among these, PtSe2/C exhibited high selectivity (~91%) and high mass activity (439 A/gPt) for the 2e⁻ pathway (Figure 4b).They effectively modulated the surface atomic arrangement and electronic structure of the active sites using chalcogens.Platinum chalcogenides with isolated active sites facilitated end-on adsorption of O2, and by adjusting the binding energy of *OOH through changes in electronic structure, PtSe2/C favored the 2e − pathway.Utilizing the advantages of metal selenides, Dong et al. [26] successfully synthesized ultrathin single-crystal PtSe2 nanosheets through a selenization process using commercial Pt/C as a precursor.As shown in Figure 4c, these PtSe2 nanosheets exhibit an interlayer atomic arrangement of Se-Pt-Se within a hexagonal crystal structure.Unlike Pt/C, which follows a 4-electron pathway, PtSe2 demonstrates an exclusive 2e − pathway, showing high Utilizing the advantages of metal selenides, Dong et al. [26] successfully synthesized ultrathin single-crystal PtSe 2 nanosheets through a selenization process using commercial Pt/C as a precursor.As shown in Figure 4c, these PtSe 2 nanosheets exhibit an interlayer atomic arrangement of Se-Pt-Se within a hexagonal crystal structure.Unlike Pt/C, which follows a 4-electron pathway, PtSe 2 demonstrates an exclusive 2e − pathway, showing high selectivity for H 2 O 2 and high current density across a wide pH range (up to 94.1%) (Figure 4d).In situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy confirmed the catalytic active center of Pt, with the adsorption of the reaction intermediate *OOH and the formation of HOOH clearly observed (Figure 4e).Despite this, many studies have been conducted on the use of platinum chalcogenide electrocatalysts as cathode catalysts in fuel cells, favoring the 4e − reaction [124,125].Therefore, further research in this area will be necessary.

Non-Noble-Metal Chalcogenides
Noble metal compounds are known for their high selectivity and stability for H 2 O 2 production even at small overpotentials.However, their high cost poses challenges for practical industrial applications.In contrast, non-noble transition-metal chalcogenides have surface structures with controllable tunability and are abundant and easily obtainable, making them promising catalysts for efficient H 2 O 2 production [126].Recent studies have confirmed that non-noble transition-metal chalcogenide catalysts exhibit high 2e − ORR activity.By comparing their selectivity of 2e − ORR, stability, and yields, future research directions for practical applications can be better envisioned.

Cobalt Chalcogenides
Among the transition metals (Mn, Fe, Co, Ni, Cu), cobalt, with its optimal d band center, is favorable for the 2e − reaction because its binding strength with the intermediate (*OOH) is neither too strong nor too weak [93].Consequently, cobalt has been extensively studied as a 2e − ORR catalyst.
Sheng et al. [103] demonstrated through computational and experimental studies that cobalt pyrite (CoS 2 ) nanomaterials, which are abundant on Earth, are highly active and selective for the 2e − ORR.CoS 2 nanomaterials drop-casted on a RRDE exhibited a high onset potential and selectivity (0.69 V vs. RHE onset potential, ~70% selectivity) in 0.05 M H 2 SO 4 electrolytes (Figure 5a).DFT modeling revealed that the surface energy of the CoS 2 (100) facet was lower than that of the (110) or (111) facets, indicating higher stability.Additionally, on the (100) facet, the binding of *OOH at the Co active sites occurred more readily, resulting in an energy barrier of 0.59 eV, which was lower than the binuclear dissociation barrier between neighboring Co sites (0.78 eV) (Figure 5b).This structural characteristic of CoS 2 ensures optimal binding of *OOH without dissociation of the O-O bond, making CoS 2 favorable for hydrogen peroxide production.Furthermore, while most metal surfaces interact with the *OOH intermediate in a side-on manner, the isolated active sites on the CoS 2 surface allow for the end-on binding of * OOH (Figure 5c).X-ray Photoelectron Spectroscopy (XPS) analysis conducted before and after operational stability tests in both acidic and neutral electrolytes confirmed that the binding energies of the predominant Co 2p (778.9 and 794.0 eV) and S 2p (163.0 and 164.0 eV) signals matched the literature values (Figure 5d,e).This consistency indicates that the surface chemical states remained unchanged, demonstrating excellent stability.Additionally, CoS 2 nanowires directly grown on high-surface-area carbon fiber paper demonstrated high hydrogen peroxide accumulation rates and excellent durability in a three-electrode H-cell setup (Figure 5f).
As noted in many papers on fuel cell catalysts, metal chalcogenide materials do not always induce a 2e − ORR.However, by adding new materials or modifying the surface to redistribute electron density, it is possible to increase the selectivity for the 2e − reaction in catalysts that are otherwise favorable for the 4e − reaction.Zheng et al. [105] demonstrated this through the synthesis of BP/CoSe 2 , where the low selectivity of CoSe 2 nanobelts for hydrogen peroxide was improved by functionalizing them with few-layer black phosphorus (BP) nanosheets, enabling surface charge redistribution in CoSe 2 (Figure 6a).As a result, the presence of BP increased the selectivity for hydrogen peroxide production from 30% to 90% (Figure 6b,c).In the DFT calculations, Bader charge depth analysis showed that Co atoms on the BP/CoSe 2 surface have increased electron density, while Se atoms exhibit decreased charge density (Figure 6d).These computational results suggest that BP induces charge redistribution on the CoSe 2 surface by facilitating electron transfer from Se to Co atoms.In Figure 6e As noted in many papers on fuel cell catalysts, metal chalcogenide materials do not always induce a 2e − ORR.However, by adding new materials or modifying the surface to redistribute electron density, it is possible to increase the selectivity for the 2e − reaction in catalysts that are otherwise favorable for the 4e − reaction.Zheng et al. [105] demonstrated this through the synthesis of BP/CoSe2, where the low selectivity of CoSe2 nanobelts for hydrogen peroxide was improved by functionalizing them with few-layer black phosphorus (BP) nanosheets, enabling surface charge redistribution in CoSe2 (Figure 6a).As a result, the presence of BP increased the selectivity for hydrogen peroxide production from 30% to 90% (Figure 6b,c).In the DFT calculations, Bader charge depth analysis showed that Co atoms on the BP/CoSe2 surface have increased electron density, while Se atoms maintain approximately 90% H2O2 selectivity over a 5 h period.Therefore, optimizing the atomic structure and electronic configuration confirms that the efficient electrochemical synthesis of hydrogen peroxide can be achieved.

Nickel Chalcogenides
Nickel is an earth-abundant element known for its high activity and strong resistance to corrosion, making it an attractive energy electrocatalyst.Among transition metals, nickel has a preferential adsorption capacity for oxygen molecules [23,127].Additionally, nickel chalcogenide catalysts have been reported to exhibit high stability and good efficiency in strongly acidic electrolytes without undergoing corrosion.
Liang et al. [107] successfully synthesized a NiS2 catalyst with high selectivity for the 2e − ORR in acidic electrolytes (up to 99% H2O2 selectivity) from a hydroxide precursor through a low-temperature sulfurization reaction strategy (Figure 7a). Figure 7b shows that *OOH binds to a single oxygen atom on the surface of NiS2, with an energy barrier of 0.97 eV for dissociation, indicating high selectivity towards H2O2 production by preventing O-O bond cleavage.To further enhance the catalytic performance of the 2e − ORR, defect engineering has been employed to induce charge-transfer and energy-band structure changes, thereby improving catalyst performance in several studies (e.g., Fe2O3, Ni2−xP)

Nickel Chalcogenides
Nickel is an earth-abundant element known for its high activity and strong resistance to corrosion, making it an attractive energy electrocatalyst.Among transition metals, nickel has a preferential adsorption capacity for oxygen molecules [23,127].Additionally, nickel chalcogenide catalysts have been reported to exhibit high stability and good efficiency in strongly acidic electrolytes without undergoing corrosion.
Liang et al. [107] successfully synthesized a NiS 2 catalyst with high selectivity for the 2e − ORR in acidic electrolytes (up to 99% H 2 O 2 selectivity) from a hydroxide precursor through a low-temperature sulfurization reaction strategy (Figure 7a). Figure 7b shows that *OOH binds to a single oxygen atom on the surface of NiS 2 , with an energy barrier of 0.97 eV for dissociation, indicating high selectivity towards H 2 O 2 production by preventing O-O bond cleavage.To further enhance the catalytic performance of the 2e − ORR, defect engineering has been employed to induce charge-transfer and energy-band structure changes, thereby improving catalyst performance in several studies (e.g., Fe 2 O 3 , Ni 2−x P) [128,129].In metal chalcogenide catalysts, to increase catalytic selectivity in alkaline electrolytes, Wang et al. [111] successfully synthesized nickel diselenide (NiSe 2 -V Se ) with intentionally charge-polarized anion vacancies through a sequential solvothermal calcination-selenization-annealing process (Figure 7c). Figure 7d,e show that NiSe 2 -V Se has ∆G* OOH values closer to the ideal ones (4.28 and 3.58 eV at 0 V and 0.70 V, respectively) compared to NiSe 2 (4.548 and 3.848 eV), indicating enhanced catalytic performance for the 2e − ORR.In Figure 7f,g, the differential charge density distributions between adsorbed *OOH and the surfaces of NiSe 2 and NiSe 2 -V Se indicate stronger binding in NiSe 2 -V Se , with shorter bond lengths of 1.8677 Å compared to 1.9148 Å in NiSe 2 .The combined analysis of DFT calculations confirmed that the presence of charge-polarized Se vacancies influenced the optimization of free adsorption energy of reaction intermediates while reducing electron transfer rates.Also, it demonstrated impressive stability, exhibiting only a minor reduction in selectivity after undergoing 5000 cycles of accelerated degradation testing (ADT).Similarly, Sun et al. [108] compared the catalytic performance of nickel chalcogenides with different anion species (Se 2− , S 2− , O 2− ).Like cobalt chalcogenides, this study confirms that the d-band center of metal selenides at −2.24 eV is higher, leading to better electrocatalytic performance.

Copper Chalcogenides
Cu-based nanostructures have been proposed as ORR catalysts in alkaline electrolytes, showing promise for electrochemical catalytic activity.Liu et al. [30] demonstrated that the rich phase structure of Cu 2 Se allows for the tuning of 2e − or 4e − ORR activity (Figure 8a).Pure tetragonal and cubic Cu 2 Se nanowires (NWs) were obtained at 220 and 280 • C, respectively, by using trioctylphosphine (TOP) as a surfactant to accelerate the transformation reaction.The tetragonal Cu 2 Se NWs follow the 4e − ORR mechanism, while the cubic phase exhibits a dual-path mechanism that includes both 2e − and 4e − pathways.This indicates the potential of copper chalcogenide catalysts for the 2e − pathway.The high selectivity for hydrogen peroxide production in copper selenide was also confirmed in the paper by Yuan et al. [112].They synthesized a series of nanosized M-Se catalysts (M = Cu, Ni, Co) using a laser ablation method and achieved flexible control over the 2e − or 4e − ORR reactions by engineering the alloying components (Figure 8b).In Figure 8c, these metal chalcogenides (CoSe, NiSe, Cu 7.2 Se 4 ) are visualized, showing that surface metal atoms are surrounded by Se atoms, thereby eliminating hollow sites for O adsorption.This arrangement leads to the weaker binding of intermediates, enhancing hydrogen peroxide production.As seen in Figure 8d, the scaling lines for ∆G* OOH and ∆G* O for all three metal selenides shift toward weaker *O binding, significantly improving H 2 O 2 selectivity compared to pure metals.Among them, Cu 2−x Se exhibits the weakest *O binding, showing the highest selectivity for the 2e − reaction (Figure 8e).A crystal orbital Hamiltonian population analysis (COHP) comparing the effects of electronic structures revealed that the bonding state energy of O-Cu 2−x Se ( 111) is higher than that of O-Cu (111), and the filling of antibonding states is more significant, resulting in weaker O-Cu 2−x Se (111) bonding.This observation aligns with the experimental data for the 2 e − reaction in Cu 2−x Se (Figure 8f).(111).Upper and lower peaks indicate bonding and antibonding states, respectively.Reproduced with permission [112].Copyright 2021, American Chemical Society.
Zhao et al. demonstrated that 2H-phase MoTe 2 nanoflakes are highly efficient 2e − ORR electrocatalysts, achieving a selectivity of around 93% [113].These MoTe 2 nanoflakes were synthesized using a top-down liquid-phase exfoliation (LPE) method, where the bulk material was exfoliated into a few stabilized layers during ultrasonic treatment, disrupting the Van der Waals interactions, as shown in Figure 9a.The exfoliated MoTe 2 nanoflakes showed excellent performance, with an onset potential of approximately 0.56 V in a 0.5 M sulfuric acid electrolyte (Figure 9b).DFT calculations further support that the 2e − ORR is promoted by the favorable binding energy of the *OOH intermediate at the zigzag edge of 2H-MoTe 2 .Zhang et al. synthesized a titanium-doped zinc-cobalt sulfide hollow superstructure (Ti-ZnCoS HSS) that demonstrated high selectivity (98%) and strong activity (1 mA cm −2 at 0.774 V) for H 2 O 2 production in alkaline electrolytes [114].Figure 9c shows Ti-ZnCoS HSS was created through a two-step synthesis process using NH 2 -MIL-125@ZnCo-ZIF (Zeolitic Imidazolate Framework) as the precursor.Titaniumdoping facilitated the formation of a hollow ZnCoS nanocage superstructure.This design optimizes the d-band center and the adsorption energy of intermediates (3.68 eV), resulting in numerous active sites and enhanced 2e − ORR performance (Figure 9d).In addition to the proposed TMCs, there have been many studies on the ORR performance of various non-precious-metal chalcogenide compounds such as MnSe [130], Co 1-x Zn x Se@NCF [131], and GaSe [132].However, most of these studies have focused on the electrochemical properties of the 4e − pathway rather than the 2e − pathway.Therefore, further research on the electrochemical properties of the 2e − ORR pathway in these promising TMCs is highly anticipated.
Zhao et al. demonstrated that 2H-phase MoTe2 nanoflakes are highly efficien ORR electrocatalysts, achieving a selectivity of around 93% [113].These MoTe2 nanofl were synthesized using a top-down liquid-phase exfoliation (LPE) method, where bulk material was exfoliated into a few stabilized layers during ultrasonic treatment rupting the Van der Waals interactions, as shown in Figure 9a.The exfoliated M nanoflakes showed excellent performance, with an onset potential of approximately V in a 0.5 M sulfuric acid electrolyte (Figure 9b).DFT calculations further support tha 2e − ORR is promoted by the favorable binding energy of the *OOH intermediate a zigzag edge of 2H-MoTe2.Zhang et al. synthesized a titanium-doped zinc-cobalt su hollow superstructure (Ti-ZnCoS HSS) that demonstrated high selectivity (98%) strong activity (1 mA cm −2 at 0.774 V) for H2O2 production in alkaline electrolytes [ Figure 9c shows Ti-ZnCoS HSS was created through a two-step synthesis process u NH2-MIL-125@ZnCo-ZIF (Zeolitic Imidazolate Framework) as the precursor.Titan doping facilitated the formation of a hollow ZnCoS nanocage superstructure.This de optimizes the d-band center and the adsorption energy of intermediates (3.68 eV), re ing in numerous active sites and enhanced 2e − ORR performance (Figure 9d).In add to the proposed TMCs, there have been many studies on the ORR performance of var non-precious-metal chalcogenide compounds such as MnSe [130], Co1-xZnxSe@NCF [ and GaSe [132].However, most of these studies have focused on the electrochemical p erties of the 4e − pathway rather than the 2e − pathway.Therefore, further research on electrochemical properties of the 2e − ORR pathway in these promising TMCs is hi anticipated.

Conclusions
In this review, we have comprehensively explored the recent advancements in the field of TMCs as electrocatalysts for the ORR.Our discussion delved into the intricate ORR mechanisms, the significant impact of pH on the reaction pathways, and the comparative performance of TMC-based catalysts.The insights gathered from this review highlight the promising potential of TMCs in achieving efficient and selective 2e − ORR, a critical step for sustainable H 2 O 2 production.The ORR mechanism can proceed via the 2e − or 4e − pathway, significantly influenced by the pH of the electrolyte.In acidic media, the ISET mechanism is predominant, whereas in alkaline conditions, the OSET mechanism becomes more significant.Understanding these pathways is crucial for designing effective catalysts.The stability of H 2 O 2 in acidic conditions compared to its rapid decomposition in alkaline media underscores the importance of selecting appropriate electrolytes to enhance ORR efficiency and selectivity.TMCs, including compounds of cobalt, nickel, and copper, have shown remarkable potential due to their tunable surface structures and electronic properties.These materials provide abundant active sites and exhibit high selectivity and stability in both acidic and alkaline environments.Advanced techniques like phase engineering, defect creation, and heterostructure construction have been employed to optimize the ORR performance of TMCs, demonstrating significant improvements in catalytic activity and selectivity.Despite the progress made, several challenges and opportunities remain in the research and development of TMC-based electrocatalysts for ORR.
(1) Development of high-performing electrocatalysts: TMCs have demonstrated superior catalytic efficiency in acidic media, outperforming carbon-based materials, largely due to their electronic structure.Key strategies such as doping and defect engineering, which modulate electron density, are essential for further performance enhancement.Among TMCs, cobalt chalcogenides have garnered significant interest, owing to the favorable adsorption energies associated with cobalt's electron density.Future research should prioritize the exploration of charge redistribution on cobalt chalcogenides' surfaces, employing advanced strategies to drive further performance improvements.(2) H 2 O 2 stability and catalyst durability: Hydrogen peroxide is slightly acidic and highly stable in such conditions, allowing it to be maintained without the need for stabilizers.TMCs like BP/CoSe 2 and NiS 2 show excellent selectivity and activity in acidic electrolytes, making them strong candidates for 2e − ORR electrocatalysts in practical applications.However, the acidic environment also brings heightened risks of corrosion and degradation, which must be addressed.To ensure these catalysts perform reliably in real-world conditions, future research should focus on improving their long-term stability and durability, such as through the development of carbon shells.(3) Mechanistic insights: In-depth mechanistic studies using advanced characterization techniques like in situ ATR-IR, in situ Raman, XANES, and EXAFS are necessary to gain a better understanding of the ORR processes at the atomic level.Such insights will aid in the rational design of more effective catalysts.(4) Scale-up and operational costs: To transition from laboratory-scale experiments to practical large-scale applications, several key challenges must be addressed.These challenges include optimizing reactor design, increasing current density, and ensuring high Faradaic efficiency during prolonged electrolysis.For practical implementation, maintaining high Faradaic efficiency at current densities above 300 mA cm −2 is essential.Additionally, reducing stabilizer use in acidic electrolytes, lowering production costs by using affordable non-precious-metal catalysts, and innovating electrochemical reactor designs-such as gas diffusion electrodes (GDEs) and membrane electrode assemblies (MEAs)-are critical for scaling up the ORR process.
In conclusion, the continued development and optimization of TMC-based electrocatalysts hold great promise for advancing sustainable and efficient H 2 O 2 production through ORR.By addressing the aforementioned challenges and leveraging the

Figure 1 .
Figure 1.Schematic illustration of associative and dissociative ORR mechanisms.In electrochem ORR, the 2e − pathway (red) proceeds via the preservation of the O-O bond on the *OOH rea intermediate.In contrast, the 4e − pathway (blue and black) involves the cleavage of the O-O bo form *O, ultimately leading to the production of H2O.

Figure 1 .
Figure 1.Schematic illustration of associative and dissociative ORR mechanisms.In electrochemical ORR, the 2e − pathway (red) proceeds via the preservation of the O-O bond on the *OOH reaction intermediate.In contrast, the 4e − pathway (blue and black) involves the cleavage of the O-O bond to form *O, ultimately leading to the production of H 2 O.

Figure 2 .
Figure 2. (a) Volcano plots illustrating the limiting potential as a function of ΔG*OOH for packed pure metal slabs (blue) and M-N/C structures (red), derived from DFT calculations.The green curve

Figure 2 .
Figure 2. (a) Volcano plots illustrating the limiting potential as a function of ∆G* OOH for packed pure metal slabs (blue) and M-N/C structures (red), derived from DFT calculations.The green curve represents the 2e − pathway, while the black curve represents the 4e − pathway.(b) Comparison of experimental data and theoretical predictions for pure metals and metal alloys.Circles denote the potential required to achieve a kinetic current density of 1 mA/cm 2 on polycrystalline electrodes.Reproduced with permission [16].Copyright 2018, American Chemical Society.

Figure 3 .
Figure 3. Schematic depiction of the double-layer structure during ORR in acidic (left) and alkaline (right) environments.Insets (a) and (b) illustrate the inner-sphere and outer-sphere electron transfer processes, respectively.The inner Helmholtz plane (IHP) and the outer Helmholtz plane (OHP) are shown, highlighting the distribution of hydroxyl species, water, oxygen alkali metal cations, and supporting electrolyte anions.Reproduced with permission.Copyright 2011 [74], American Chemical Society.

Figure 3 .
Figure 3. Schematic depiction of the double-layer structure during ORR in acidic (left) and alkaline (right) environments.Insets (a) and (b) illustrate the inner-sphere and outer-sphere electron transfer processes, respectively.The inner Helmholtz plane (IHP) and the outer Helmholtz plane (OHP) are shown, highlighting the distribution of hydroxyl species, water, oxygen alkali metal cations, and supporting electrolyte anions.Reproduced with permission.Copyright 2011 [74], American Chemical Society.

23 Figure 5 .
Figure 5. (a) H2O2 selectivity in O2-saturated 0.05 M H2SO4 of drop-casted CoS2 (cobalt loading = 305 µg/cmdisk 2 ) at different rotation rates.(b) Computational modeling of ORR on the CoS2 (100) surface.Free-energy diagram for both 2e − and 4e − ORR at the calculated standard equilibrium reduction potential of 2e − ORR.(c) Side view of the CoS2 surface with adsorbed *OOH.(d) Co 2p and (e) S 2p XPS spectra (structural and compositional characterizations of drop-casted CoS2 before and after operational stability tests in O2-saturated 0.05 M H2SO4 and 0.05 M Na2SO4).(f) Electrocatalytic production of H2O2 on CoS2/CFP in a three-electrode H-cell setup, and chemical quantification of the H2O2 product at 0.5 V vs. RHE in O2 saturated 0.05 M H2SO4 and the corresponding cumulative H2O2 yield.Reproduced with permission [103].Copyright 2019, American Chemical Society.

Figure 5 .
Figure 5. (a) H 2 O 2 selectivity in O 2 -saturated 0.05 M H 2 SO 4 of drop-casted CoS 2 (cobalt loading = 305 µg/cm disk 2 ) at different rotation rates.(b) Computational modeling of ORR on the CoS 2 (100) surface.Free-energy diagram for both 2e − and 4e − ORR at the calculated standard equilibrium reduction potential of 2e − ORR.(c) Side view of the CoS 2 surface with adsorbed *OOH.(d) Co 2p and (e) S 2p XPS spectra (structural and compositional characterizations of drop-casted CoS 2 before and after operational stability tests in O 2 -saturated 0.05 M H 2 SO 4 and 0.05 M Na 2 SO 4 ).(f) Electrocatalytic production of H 2 O 2 on CoS 2 /CFP in a three-electrode H-cell setup, and chemical quantification of the H 2 O 2 product at 0.5 V vs. RHE in O 2 saturated 0.05 M H 2 SO 4 and the corresponding cumulative H 2 O 2 yield.Reproduced with permission [103].Copyright 2019, American Chemical Society.

Figure 6 .
Figure 6.(a) Schematic of the BP-mediated surface charge redistribution of CoSe2 for enhanced 2e − ORR process.(b) Comparison of ORR performance (up) and the simultaneous H2O2 detection current densities at the ring electrode (down) for pure CoSe2, BP, and BP/CoSe2 catalysts in O2-saturated 0.5 m H2SO4 at a sweep rate of 10 mV s −1 .(c) H2O2 selectivity as a function of the applied potential of different catalysts.(d) Bader charge depth profile of Co and Se atoms for the BP/CoSe2 catalyst.The relative Bader charge represents the difference between the average Bader charge of Co/Se atoms in each layer of BP/CoSe2 and CoSe2.(e) Calculated 2e − (solid line) volcano plot for the electrochemical ORR to H2O2 demonstrated with the limiting potential plotted as a function of ΔG*OOH (the equilibrium potential: 0.70 V).(f) Free-energy diagrams of 2e − ORR on pure CoSe2, pure BP, and BP/CoSe2 catalysts in acidic electrolyte at zero potential.(g) Hydrogen adsorption free energy for the protonation of *OOH to H2O2 at different sites of the BP/CoSe2 catalyst.Reproduced with permission [105].Copyright 2022, Wiley-VCH GmbH.

Figure 6 .
Figure 6.(a) Schematic of the BP-mediated surface charge redistribution of CoSe 2 for enhanced 2e − ORR process.(b) Comparison of ORR performance (up) and the simultaneous H 2 O 2 detection current densities at the ring electrode (down) for pure CoSe 2 , BP, and BP/CoSe 2 catalysts in O 2saturated 0.5 m H 2 SO 4 at a sweep rate of 10 mV s −1 .(c) H 2 O 2 selectivity as a function of the applied potential of different catalysts.(d) Bader charge depth profile of Co and Se atoms for the BP/CoSe 2 catalyst.The relative Bader charge represents the difference between the average Bader charge of Co/Se atoms in each layer of BP/CoSe 2 and CoSe 2. (e) Calculated 2e − (solid line) volcano plot for the electrochemical ORR to H 2 O 2 demonstrated with the limiting potential plotted as a function of ∆G* OOH (the equilibrium potential: 0.70 V).(f) Free-energy diagrams of 2e − ORR on pure CoSe 2 , pure BP, and BP/CoSe 2 catalysts in acidic electrolyte at zero potential.(g) Hydrogen adsorption free energy for the protonation of *OOH to H 2 O 2 at different sites of the BP/CoSe 2 catalyst.Reproduced with permission [105].Copyright 2022, Wiley-VCH GmbH.
Figure7d,e show that NiSe2-VSe has ΔG*OOH values closer to the ideal ones (4.28 and 3.58 eV at 0 V and 0.70 V, respectively) compared to NiSe2 (4.548 and 3.848 eV), indicating enhanced catalytic performance for the 2e − ORR.In Figure7f,g, the differential charge density distributions between adsorbed *OOH and the surfaces of NiSe2 and NiSe2-VSe indicate stronger binding in NiSe2-VSe, with shorter bond lengths of 1.8677 Å compared to 1.9148 Å in NiSe2.The combined analysis of DFT calculations confirmed that the presence of charge-polarized Se vacancies influenced the optimization of free adsorption energy of reaction intermediates while reducing electron transfer rates.Also, it demonstrated impressive stability, exhibiting only a minor reduction in selectivity after undergoing 5000 cycles of accelerated degradation testing (ADT).Similarly, Sun et al.[108] compared the catalytic performance of nickel chalcogenides with different anion species (Se 2− , S 2− , O 2− ).Like cobalt chalcogenides, this study confirms that the d-band center of metal selenides at −2.24 eV is higher, leading to better electrocatalytic performance.

Figure 7 .Figure 7 .
Figure 7. (a) Calculated H2O2 selectivity and the electron transfer numbers (inset in (a)) of NiS2 based on the RRDE measurements in O2-saturated 0.05 M H2SO4 electrolytes and 0.10 M HClO4, respectively.(b) The minimum-energy pathway for the dissociation of *OOH to *O + *OH.The blue, yellow, red, and pink spheres denote the Ni, S, O, and H atoms, respectively.Reproduced with permission [107].Copyright 2021, The Royal Society of Chemistry.(c) The synthetic scheme of NiSe2-VSe nanoparticles.Free-energy diagram for 2e − ORR on NiSe2 and NiSe2-VSe at (d) U = 0 V and (e) U = 0.70 V. Differential charge density distribution between adsorbed *OOH and (f) NiSe2 and (g) NiSe2-VSe substrates, where the grey and green spheres represent the Ni and Se atom, respectively,Figure 7. (a) Calculated H 2 O 2 selectivity and the electron transfer numbers (inset in (a)) of NiS 2 based on the RRDE measurements in O 2 -saturated 0.05 M H 2 SO 4 electrolytes and 0.10 M HClO 4 , respectively.(b) The minimum-energy pathway for the dissociation of *OOH to *O + *OH.The blue, yellow, red, and pink spheres denote the Ni, S, O, and H atoms, respectively.Reproduced with permission [107].Copyright 2021, The Royal Society of Chemistry.(c) The synthetic scheme of NiSe 2 -V Se nanoparticles.Free-energy diagram for 2e − ORR on NiSe 2 and NiSe 2 -V Se at (d) U = 0 V and (e) U = 0.70 V. Differential charge density distribution between adsorbed *OOH and (f) NiSe 2 and (g) NiSe 2 -V Se substrates, where the grey and green spheres represent the Ni and Se atom, respectively, while the red-and blue-colored isosurfaces mean the positive and negative charge, respectively.Reproduced with permission [111].Copyright 2022, Wiley-VCH GmbH.

Table 1 .
Summary of the TMC-based catalysts for enhanced 2e − ORR electrocatalysis.