Accelerating acidic CO2 electroreduction: strategies beyond catalysts

Carbon dioxide electrochemical reduction (CO2RR) into high-value-added chemicals offers an alternative pathway toward achieving carbon neutrality. However, in conventional neutral or alkaline electrolyte systems, a significant portion of CO2 is converted into (bi)carbonate due to the thermodynamically favorable acid–base neutralization reaction between CO2 and hydroxide ions. This results in the single-pass carbon efficiency (SPCE) being theoretically capped at 50%, presenting challenges for practical applications. Acidic CO2RR can completely circumvent the carbonate issue and theoretically achieve 100% SPCE, garnering substantial attention from researchers in recent years. Nevertheless, acidic CO2RR currently lags behind traditional neutral/alkaline systems in terms of product selectivity, stability, and energy efficiency, primarily because the abundance of H+ ions exacerbates the hydrogen evolution reaction (HER). Encouragingly, significant breakthroughs have been made to address these challenges, with numerous studies indicating that the regulation of the local catalytic environment may be more crucial than the catalyst itself. In this review, we will discuss the main challenges and latest strategies for acidic CO2RR, focusing on three key aspects beyond the catalyst: electrolyte regulation, local catalytic environment modification, and novel designs of gas diffusion electrodes (GDEs)/electrolyzers. We will also conclude the current advancement for acidic CO2RR and provide an outlook, with the hope that this technology will contribute to achieving carbon neutrality and advance towards practical application.


Introduction
The correlation between CO 2 emissions and global warming has been extensively validated and recognized by the scientic community.The 28th Conference of the Parties to the United Nations Framework Convention on Climate Change (COP28), held in Dubai, UAE in 2023, highlighted that the current global efforts in various domains of climate action, including greenhouse gas reduction, are insufficient.2][13] Consequently, acidic CO 2 RR has garnered considerable attention from researchers.
3][24][25] However, although high carbon efficiency can be achieved in acidic CO 2 RR, it still faces signicant technical and economic challenges for industrial applications.Firstly, since the reduction of H + is more kinetically favorable than the CO 2 RR, especially in strong acidic electrolytes, the erce competition from the HER poses a signicant challenge to achieving high selectivity for C 2+ products.Therefore, effectively suppressing the HER becomes a critical task for acidic CO 2 RR.Secondly, the addition of alkaline ions to inhibit the HER can also lead to (bi)carbonate precipitation locally, potentially decreasing the stability of the GDE, especially for membrane electrode assembly (MEA) electrolyzers.This is primarily due to the presence of a pH gradient near the electrode.As soon as the CO 2 RR commences, a locally elevated pH inevitably arises, leading to some of the CO 2 to react with OH − ions to form carbonate or bicarbonate.These species can further interact with alkali metal cations migrating from the anode, producing (bi)carbonates.If these (bi)carbonates are not fully dissolved in the electrolyte, the remaining portion will accumulate in the GDE or ow channel, thereby impacting the hydrophobicity and stability of the interface. 26To avoid (bi)carbonate formation in the presence of alkali cations, there seems to be a balance between the local and bulk pH.Notably, recent studies show that quaternary ammonium cations on the catalyst surface could replace the function of alkali cations, thereby efficient CO 2 RR can occur without metal cations. 27This method shows a promising ability to avoid the formation of (bi)carbonates and maintain long-term stability.Thirdly, it should be noted that most catalysts reported in the literature are still derived directly from neutral or alkaline systems, and since most metal oxides are not thermodynamically stable in acid and the active site might be destroyed, the stability of the catalyst in an acidic environment still needs to be enhanced. 18,19For example, since Koper et al. 28 proved that CO 2 electroreduction does not occur without metal cations, high concentrations of alkali metal cations in electrolytes are essential for acidic CO 2 RR in most reported studies.However, the simultaneous effect of anions (e.g., Cl − in KCl solution), which we discussed previously, 29 can also signicantly inuence the structural evolution of catalysts and the performance of the CO 2 RR in acidic electrolytes, yet this aspect remains underexplored.Furthermore, Chen et al. 30 reported a novel hollow-ber GDE using only Cu metal as the catalyst, achieving a nearly 80% FE for C 2+ products with a partial current density exceeding 2 A cm −2 .The above research indicates that achieving practical industrial applications of acidic CO 2 RR requires more than just the development of a stable catalyst.Increasing evidence suggests that factors such as the electrolyte, GDE, and electrolyzer design are even more critical for achieving efficient acidic CO 2 RR. 31 These aspects urgently require further in-depth investigation in the future.
In this review, we will investigate the mechanisms and primary strategies to enhance acidic CO 2 RR beyond the catalyst itself.We rst summarize the state-of-the-art performance of the CO 2 RR under near-neutral, alkaline, and acidic electrolyte conditions, and then identify the main challenges of acidic CO 2 RR in three areas: selectivity, stability, and energy efficiency.Subsequently, we explore methods to address these challenges, including electrolyte regulation, local catalytic environment modication, and innovative GDE/electrolyzer designs (Fig. 1).Although existing review articles on acidic CO 2 RR provide valuable insights, [31][32][33][34][35][36][37][38][39] our focus differs.For instance, Gu et al. 31 discussed methods to improve the selectivity of acidic CO 2 RR via mass transport and electrode reactions, such as catalyst surface decoration, nanostructuring, and electronic structure modulation.Wang et al. 38 addressed the carbonate issue primarily from the perspective of electrocatalysts for acidic CO 2 RR.Yan et al. 32 focused on accelerating acidic CO 2 RR through the rational design of electrodes/catalysts and the local catalytic environment.Xia et al. 39 explored the regulation of the reaction environment based on catalysts, electrodes, and electrolytes.Considering the primary challenges faced by acidic CO 2 RR from an industrial application viewpoint, we concentrate on factors beyond the catalyst itself, including the electrolyte, local environment, and GDE/electrolyzer.Our goal is to deepen the understanding of acidic CO 2 RR in terms of system design and process optimization, thereby promoting its industrial application in real-world environments.

Challenges of acidic CO 2 RR
Compared to traditional neutral/alkaline CO 2 RR systems, the main advantages of acidic CO 2 electroreduction are as follows: high carbon efficiency and high energy efficiency.By avoiding the formation of carbonates, which reduces CO 2 loss, higher carbon efficiency can be achieved.Meanwhile, the ohmic losses in acidic electrolytes are lower than those in near-neutral electrolytes, which is crucial for achieving high energy efficiency at high current densities.However, the industrial application of acidic CO 2 electroreduction still requires overcoming challenges such as poor product selectivity, inadequate system stability, and low energy efficiency (Fig. 2 and Tables 1 and 2).

Selectivity
In acidic environments, the HER from H + reduction typically dominates, making it challenging to selectively produce specic products, especially for high valued C 2+ products such as ethylene, ethanol, etc.It can be observed from Fig. 2a and b that although the faradaic efficiency (FE) of C 1 products under acidic conditions is comparable to that of neutral and alkaline conditions, its current density remains signicantly lower than that of alkaline systems.Additionally, for C 2+ products, both the current density and FE are lower compared to neutral and alkaline systems.These ndings indicate that product selectivity under acidic conditions remains a focal research direction to overcome.Although current studies suggest that cation effects and local pH effects are critical in inuencing the selectivity of the CO 2 RR in acidic environments, more fundamental or novel theoretical frameworks need further renement.

Stability
The durability of electrodes and electrolyzers is critical to the industrial application of the CO 2 RR.It has been reported that the operating time of CO 2 electrolysis cells should be comparable to that of hydrogen production electrolysis cells (over 50 000 h) and has a considerable decay rate. 78Specically, the FE for single products (e.g., CO, HCOOH, and C 2 H 4 ) should be maximized (greater than 80%) and kept stable over extended periods (DFE/Dt: <0.1% per 1000 h), while ensuring a low voltage decay rate (<10 mV h −1 ). 79At present, the longest stability for single C1 products in neutral, alkaline, and acidic electrolytes is 4000, 2400, and 5200 h respectively (Fig. 2c).It is exciting to note that Xia et al. 54 recently reported a durable CO 2 conversion in the proton-exchange membrane system.They utilized recyclable lead as the cathode catalyst, coupled with the hydrogen oxidation reaction (HOR), capable of running at over 5000 h at 600 mA cm −2 in acidic electrolytes while maintaining an FE (HCOOH) of over 90% and a voltage of around 2.2 V.This study demonstrates the feasibility of achieving ultra-long stability in the CO 2 RR under acidic conditions.Apart from the intrinsic high stability of lead catalysts, the high stability of the GDE triple-phase interface, maintained by its surface hydrophobicity (crucially inuenced by PTFE or carbon nanoparticles), plays a signicant role.Moreover, by utilizing the HOR rather than the water oxidation reaction (WOR) at the anode, the overall voltage was decreased, and, more importantly, the generation of harmful hydrogen peroxide was avoided, which could degrade and even destroy the polymer electrolyte membrane (PEM).
However, in the case of C 2+ products, the stability of all products is signicantly lower than that of C1 products, with the highest stability currently below 1000 h (Fig. 2d).Additionally, in acidic systems, the FE and stability of C 2+ products are also lower than in neutral or alkaline environments, indicating that the generation of C 2+ products still poses a signicant challenge in terms of stability.This phenomenon may primarily be attributed to the complex formation of C 2+ products, which results in a more unstable three-phase interface.Factors such as local high pH-induced carbonation, 80 electrowetting, 81 liquid products crossing the membrane, 10 and intermediate adsorption-induced catalyst reconstruction 8 contribute to this instability.It is important to note that while acidic electrolytes can prevent carbonate precipitation and enhance CO 2 conversion, current ndings indicate that even in strongly acidic electrolytes (e.g., pH = 1), local pH variations can still lead to carbonate precipitation.This not only increases CO 2 reduction activity but also causes carbonate accumulation and affects product selectivity when the current density surpasses a certain threshold. 80

Energy efficiency (EE)
Another key issue for the industrialization of the CO 2 RR is energy efficiency.Fig. 2e and f present an estimate of the energy required to produce 1 mole of CO and C 2 H 4 based on a ow cell system using different media (acidic, near-neutral, and alkaline) and a proton exchange membrane MEA in CO 2 electroreduction.The gure compares the energy consumption under each condition at a partial current density of 200 mA cm −2 .In acidic medium, the electroreduction of carbon dioxide is more efficient due to the inhibition of the hydrogen evolution reaction by alkali metal ions, resulting in lower energy demand.However, the energy consumption in near-neutral and alkaline media is relatively high, possibly due to energy efficiency loss caused by carbonate formation.For instance, alkaline electrolytes such as KOH consume CO 2 , resulting in a carbon efficiency of less than 10% and the energy consumed for regeneration exceeds the electric energy consumed by the electrolyzer itself, which makes the CO 2 RR using alkaline electrolytes uneconomical. 12ote that due to the constant equilibrium potentials of CO 2 / CO and CO 2 /ethylene on the RHE scale, the overpotential increases with decreasing pH.Consequently, the energy consumption due to overpotential loss at the cathode follows the order: acidic > near-neutral > alkaline.Despite this, the overall energy consumption is lowest in acidic medium because Fig. 2 The state-of-the-art performance of the CO 2 RR in neutral, alkaline, and acidic electrolytes.The FE of single C 1 and C 2+ products versus (a and b) the corresponding partial current density and (c and d) the stability.Energy consumption (kJ mol −1 ) required to produce 1 mole of (e) carbon monoxide and (f) ethylene, assuming a partial current density of 200 mA cm −2 .Systems based on a flow cell with acidic, near-neutral or alkaline medium, and an MEA with an anion exchange membrane, are compared. 12f lower losses in other categories (Fig. 2e and f). 12 In alkaline medium, the KOH electrolyte regeneration is the most energyintensive, resulting in an overall energy consumption three times higher than in acidic medium.In near-neutral medium, ohmic and anode energy losses are higher than in acidic medium, leading to an overall energy consumption approximately 14% higher than in acidic medium. 12

Electrolyte regulation
In previous discussions, we have explored the interfacial electrolyte effects on the CO 2 RR in neutral and alkaline environments, identifying local pH and alkali metal cation effects as the two most signicant factors.When transitioning to an acidic electrolyte system, some mechanisms remain relevant, such as the persistence of local concentration gradients and the critical role of alkali metal cations.However, the proton concentration gradient in acidic systems introduces additional effects on reaction kinetics.In this section, we will examine how local pH and alkali metal cation effects inuence the CO 2 RR under acidic conditions and discuss the future development prospects of cation-free acidic electrolyte systems.

Local pH effect
In acidic systems, similar to traditional neutral or alkaline electrolyte systems, a local alkaline environment forms on the electrode surface during the CO 2 RR (Fig. 3a), 11 leading to a gradient distribution of proton concentration (Fig. 3b). 22evious studies have demonstrated that local pH primarily inuences CO 2 solubility and reaction kinetics. 29Generally, when the CO 2 RR causes an increase in the local OH − concentration, H + from the surrounding electrolyte diffuses to the surface to neutralize OH − .However, at higher current densities (e.g., above 150 mA cm −2 ), the diffusion rate of H + cannot keep up with the generation rate of OH − , leading to an increase in local pH.Consequently, a signicant amount of CO 2 forms (bi) carbonates with OH − instead of undergoing the CO 2 RR.A recent operando synchrotron wide-angle X-ray scattering (WAXS) study demonstrated that substantial (bi)carbonate precipitation occurs in GDEs even in strongly acidic electrolyte (pH = 1). 80onetheless, if the proton concentration in the bulk phase is sufficiently high or proton diffusion is enhanced, the generated (bi)carbonates can react with H + to re-form CO 2 as a reactant.This is the key reason that acidic CO 2 RR can signicantly increase the conversion rates.As can be seen, acidic CO 2 RR requires a trade-off between local and bulk pH, where the local pH remains relatively alkaline to promote the CO 2 RR while inhibiting the HER, 11 without causing excessive accumulation of (bi)carbonates, and thereby guaranteeing sufficient CO 2 at the reaction interface.Indeed, Wang et al. 82 modelled local pH changes in the vicinity of the electrode surface at varied current density with different bulk pH (Fig. 3c).They proposed that bulk pH 2 might be available to balance high local CO 2 and high current density without signicant carbonate formation.
In fact, Koper et al. initiated the study of acidic CO 2 RR in 2015, exploring the relationship between proton dependence The state-of-the-art performance of the CO and the selectivity of various products. 84From a thermodynamic perspective, the HER can proceed via proton reduction or water reduction, as any Brønsted acid can act as a proton donor. 29,85owever, Koper et al. posited that under typical CO 2 reduction conditions, water reduction remains the predominant form of the HER.This conclusion is primarily based on their observation that the onset potential for water reduction is signicantly inuenced by the CO 2 reaction, whereas the onset potential for proton reduction remains relatively unaffected under the same conditions, even in an acidic electrolyte (pH = 2.5). 86On the other hand, when proton transport kinetics are considered, such as at lower pH levels, H + reduction also becomes a dominant pathway for the HER. 31 Koper et al. 83 recently investigated acidic CO 2 RR on planar Au electrodes, detecting the changes of FE varied with interfacial pH.They demonstrated that the primary HER shis from proton reduction to water reduction as the local environment transitions from acidic to basic.Meanwhile, the CO 2 RR initiates in the proton reduction region and dominates under the near-neutral conditions.Since protons are consumed by OH − generated from CO 2 reduction; proton reduction diminishes while the CO 2 RR persists (Fig. 3d).Overall, the above theory suggests that the HER in acidic CO 2 RR can be suppressed by either increasing the local pH or limiting the concentration of interfacial water molecules.
To increase the local pH and promote the acidic CO 2 RR, Gu et al. posited that the key is to limit the migration of protons rather than to retard the kinetics of H + reduction, because the onset potential of H + reduction is more positive than that of the CO 2 RR and it always reaches the mass-transport limit under CO 2 reduction conditions. 31,82They proposed three different ways to regulation the mass transport process, which include creating high local pH by the CO 2 RR, suppressing diffusion of H + and migration of H + by alkali cations (the cation effect will be discussed in Section 3.2). 31More recently, they demonstrated that the key role of the Ni-N-C catalyst in acidic tandem CO 2 RRto-C 2+ is as a local pH modulator, instead of solely producing the CO intermediate for Cu.The catalyst-induced high local pH is the major reason for the improved C 2+ formation under acidic conditions. 87However, to further improve the selectivity of single C 2+ products, e.g., ethanol, the adsorption and coverage of *CO cannot be ignored, based on a recent study which showed that the presence of hydronium ions (H 3 O + ) in acidic electrolyte might weaken the *CO binding energy and induce a low coverage on Cu. 88 Given the above discussion, the direct measurement of local pH becomes vital for the deep understanding of pH effects.Several methods have been reported, for example, the Raman or IR peak area ratio of HCO 3 − (1014 cm −1 ) to CO 3 2− (1067 cm −1 ) is linearly correlated to the surface pH in the vicinity of the

Alkali cation effect
Since local pH is proved vital for acidic CO 2 RR, as a matter of fact, once it reaches a local alkaline environment, the concentration and type of alkali metal cations become the key factors to determine the selectivity of C 2+ formation.5][96] Three main theories have been proposed including the modication of the local electric eld (Fig. 4a), the local pH buffer (Fig. 4b), and the stabilization of key intermediates (Fig. 4c).We will not delve into the specic analysis of the three aforementioned mechanisms in this study in detail.Instead, our focus will be on examining the applicability of these theories in acidic electrolyte systems and their crucial role in inhibiting the migration of H + ions (Fig. 4d).
The larger cation effect was rst investigated by Frumkin 97 in 1959; the larger cations could increase the overall current density, which they attributed to higher specic adsorption of larger cations and the increased potential in OHP.However, this theory cannot explain the selectivity of the CO 2 RR over the HER.Moreover, since the equilibrium adsorption potential of alkaline cations (e.g., K + ) is more negative than that of the CO 2 RR, their specic adsorption might not occur under these conditions. 98arkovic et al. 99 ruled out the specic adsorption theory and proposed that the accumulation of cations near the surface was due to the noncovalent interactions, which nally result in the high local electric elds.Ringe et al. 100 and Resasco et al. 101 then proved that larger cations are less likely hydrated and prefer to adsorb in the OHP, which can increase the surface charge density and the corresponding electronic eld (Fig. 4a).Meanwhile, in acidic electrolyte, the cation-induced local electric eld increases

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the CO 2 activation kinetics such that it is more pronounced than that in the neutral or alkaline electrolytes.Sargent et al. 22 showed that the Tafel slope decreased with the increase of K + concentration in H 3 PO 4 electrolytes.And more importantly, the Tafel slope exhibited an extremely high value in the absence of K + , indicating the key role of cations in facilitating the kinetics of the CO 2 RR. 283][104] However, these conclusions are always reached in ow cells, which may not be directly used in MEA electrolyzers.Due to the absence of electrolyte in the cathode electrode, the higher concentration of alkaline cations could also promote (bi)carbonate precipitation in GDEs, thereby affecting the long-term stability. 10Pan et al. 105 recently demonstrated that an optimal concentration of H + and Cs + in acidic MEA must be maintained to balance carbonate deposition and CO 2 RR performance.A recent work from Bao's group 52 also showed that the coexistence of H + and K + can synergistically stabilize the *CO 2 intermediate and promote the formation of CO.
The pH buffer effect of cations was initially proposed by Singh et al. 106 in 2016.In contrast to the local electric eld effect mentioned above, they demonstrated that under negative potentials, larger cations could induce stronger electric elds.Consequently, the pK a value of hydrated cations decreases, allowing them to function as a pH buffer by providing more protons to neutralize the locally generated OH − .This ability to tailor the local pH prevents it from becoming excessively high, which could reduce the reactive CO 2 concentrations (Fig. 4b).The pH buffer effect suggests that maintaining a stable and balanced local pH may be more advantageous for the CO 2 RR.This effect is particularly pronounced in acidic electrolytes, where a higher local pH can effectively suppress the HER, but excessively high pH levels may result in CO 2 loss, undermining the benets of high CO 2 conversion rates in acidic systems.Yan et al. 32 have also touched upon this concept in a recent review, although further research is needed to elucidate the specic mechanism of the pH buffer effect in acidic environments.On the other hand, it is worth noting that previous studies have indicated that the pH buffer effect may not always be reliable.For instance, by using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), Ayemoba et al. 107 suggested that while the pH buffer effect may exist, its magnitude could be overestimated.
The third theory of stabilizing the key intermediates of the CO 2 RR was rst proposed by Chen et al. and further developed by Resasco et al. (Fig. 4c).Particularly, Koper et al. 28 later precluded the effect of local electronic eld and pH buffer on hydrated cations, because they found that the CO 2 RR does not occur in the absence of metal cations in the solution.Since the local electronic eld and pH buffer effect should only affect the reaction kinetics the CO 2 RR should still take place in cation-free electrolytes.This nding supports the theory that the hydrated cations could stabilize the key intermediates (e.g., CO 2 − , *OCCO, *OCOH, and *CHO) effectively via electrostatic interactions.Indeed, Huang et al. 108 reported that K + could stabilize the *OCOH intermediate and promote HCOOH formation in strong acid media (pH = 1).More recently, Sun et al. 109 demonstrated that the regulation of the CO 2 RR in acid by cation effects also involves alterations in the water structure.Specically, Li + effectively promotes the adsorption of CO 2 but slows down the hydrogenation rate, while larger cations such as Na + accelerate the CO 2 RR through a more exible water network.This study indicates that cations can inuence the adsorption and activation of CO 2 by modulating the interfacial water structure.
The fourth theory, repulsing the migration of H + , was recently proposed in acidic CO 2 RR (Fig. 4d).Since the biggest obstacle is the suppression of the HER, Gu et al. 110 investigated the role of alkali cations in suppressing the reduction rates of H + in acidic electrolytes.They showed that when the concentration of cations (e.g., Li + ) is higher than that of H + in the bulk electrolyte, the migration of H + could be completely suppressed.Since the H + ions are continuously consumed via the HER near the electrode surface, while the alkali cations are chemically inert, the accumulation of alkali cations dominated in the OHP than H + . 31As a result, the suppression of H + migration could also signicantly affect the local pH near the surface.Recent simulation results show that once the H + reduction reaches the limiting mass-transport condition (plateau current density), when approaching the cathode surface, H + ions are rstly consumed near the cathode, leading to a reduction in H + and a consequent increase in local pH.Subsequently, due to electrostatic attraction, hydrogen ions migrate back towards the cathode, causing a decrease in local pH.The maximum local pH is observed approximately 100 nanometers from the cathode, while the pH at the cathode surface remains signicantly lower than the bulk pH (Fig. 5a).However, when alkali metal ions are present, the maximum local pH value appears approximately 10 nanometers away from the cathode, with the pH on the cathode surface being higher than the bulk electrolyte pH (Fig. 5b).Notably, the local pH of the cathode's OHP increases when the cathode potential shis negatively, but this occurs only in acidic solutions with alkali metal ions.In contrast, in acidic solutions without alkali metal ions, regardless of the overpotential, continuous H + migration replenishes H + at the cathode surface, maintaining the local pH at the cathode OHP at −0.9.Review Chemical Science

Cation-free electrolyte
Recent studies, including the above observations, indicate that alkali cations are indispensable in acidic CO 2 RR.However, the primary advantage of acidic electrolysis is to avoid the formation of (bi)carbonate and enhance carbon efficiency.The presence of alkali cations introduces two main challenges to the sustainability of the CO 2 RR.The rst issue is the formation of (bi)carbonate precipitates due to local pH increase and subsequent electrostatic attraction with accumulated alkali cations.

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These hydrophilic precipitates disrupt the hydrophobicity of the GDE, leading to cathode ooding.While reducing the concentration of alkali cations can mitigate this issue, it also compromises the selectivity of the CO 2 RR.The second issue is pH variation in both the catholyte and anolyte.As the CO 2 RR progresses, the pH of the catholyte approaches neutral or alkaline, while the anolyte becomes more acidic.This variation poses signicant challenges for the long-term electrolysis of acidic CO 2 RR. 31 Fortunately, recent studies have found some ways via using metal cation-free electrolytes to solve the above problem.The critical role of alkali metal cations in acidic CO 2 RR lies in their accumulation on the cathode surface, which subsequently alters the local electric eld, proton concentration, and reaction kinetics.The local electric eld effect of these cations is pivotal for the stability of key intermediates.In light of this, Gu et al. 25 initially explored the possibility of xing cations on the catalyst surface as an alternative to introducing cations in the bulk electrolyte.They proposed using the high-density quaternary ammonium cations in poly-dimethyl-diallyl-ammonium chloride (PDDA) to mimic the local electric eld effect of alkali metal cations (Fig. 6a).Due to the water solubility of PDDA, to prevent it from being washed away by the electrolyte during the reaction, it was cross-linked (c-PDDA) and immobilized on the catalyst surface (Fig. 6b).This approach ultimately demonstrated excellent CO generation selectivity and stability in pure H 2 SO 4 electrolyte.The bulk pH of the catholyte and anolyte is also relatively stable during 10 h electrolysis in pure H 2 SO 4 electrolyte.Similarly, Li et al. 113 xed the PDDA on graphene oxide (GO) via electrostatic interactions, they achieved an FE of 85% and carbon efficiency of 93% for CO formation in pure H 2 SO 4 .More importantly, they also conducted the test in pure water electrolyte, and obtained 78% FE for CO formation at 100 mA cm −2 .
Sinton et al. 111 sprayed Aemion + ionomer, containing quaternary ammonium cations, onto the surface of Cu/PTFE, resulting in a surface electric eld strength equivalent to that of alkali metal ions (Fig. 6c and d).This treatment achieved an 80% C 2+ FE at a current density of 100 mA cm −2 in pure H 2 SO 4 (pH = 0.4) electrolyte (Fig. 6e).Additionally, Fontecave et al. 112 deposited 1-ethyl-3-methylimidazolium tetrauoroborate (EMIMBF 4 ) ionic liquid, containing imidazolium cations, onto the Cu surface to simulate the alkali metal K + ion layer.The movement of H + and the HER can also be suppressed effectively (Fig. 6f and g).Interestingly, Zhu et al. 114 recently discovered that in pure acidic electrolyte (pH = 1), even without the modication of the aforementioned organic cation layer, the Co-N site in cobalt phthalocyanine can effectively stabilize the *CO 2 intermediate and generate CO with an FE of 60%, even though the selectivity still needs to be improved.

Local catalytic environment modification
Similar to the intrinsic structure of the catalyst, the local catalytic reaction interface is a critical factor inuencing the performance of the CO 2 RR.To address the current challenges faced by acidic CO 2 RR, such as selectivity and stability, recent research primarily focuses on the surface modication of the catalyst and the regulation of the carrier.Sargent et al. 22 proposed using cationic peruorosulfonic acid (PFSA) ionomer (e.g., Naon) to modify the Cu surface.Its acidic -SO 3 H group is expected to exchange its protons with K + from the bulk electrolyte in a nonacidic local environment, maintaining a high K + concentration at the catalyst surface (Fig. 7a).The selectivity of C 2+ products was signicantly enhanced over C 1 products, and they nally achieved the CO 2 RR on Cu at pH < 1 with a singlepass CO 2 utilization of 77%, including a conversion efficiency of 50% toward C 2+ products (Fig. 7b).Moreover, their work demonstrated that the Tafel slope decreases with the increase of alkali cation concentration, leading to faster kinetics of acidic CO 2 RR.This result further proves the key role of accumulated alkali cations in the vicinity of the cathode electrode (Fig. 7c).Similarly, Sinton et al. 24 reported a COF:PFSA-adlayer-modied Cu electrode, creating evenly distributed cation-carrying and hydrophilic-hydrophobic nanochannels that control the catalyst microenvironment (Fig. 7d).The modier acted as a proton-ux-constraining ionomer adlayer.The resulting high local alkalinity and cation-enriched environment enables a C 2+ FE of 75% at 200 mA cm −2 in a strongly acidic electrolyte (pH = 1).This PFSA-modier also shows a similar function in acidic tandem CO 2 RR. 76Zhong et al. 23 recently reported an electrically nonconductive nanoporous SiC-Naon™ layer, which can maintain near-neutral conditions on the surface (Fig. 7e and  f).The SnBi catalyst can also be stabilized without corrosion, and obtain an FE (HCOOH) of >90% at 100 mA cm −2 over 125 h.Zhao et al. 13 used quaternary ammonium poly(N-methyl-piperidine-co-p-ter-phenyl) (QAPPT) and PTFE to co-modify a commercial Ag catalyst, utilizing the electrostatic repulsion of quaternary ammonium salts to reduce the diffusion of H + and K + and promote the selectivity of acidic CO 2 RR in an MEA reactor.It should be noted that most of the above modications are conducted in ow cells, and high concentrations of alkali cations (e.g., KCl) are always indispensable.
Besides ionomer modiers, other substances such as PTFE, various polymers, and carbon supports have been utilized to optimize the local catalytic environment.Interestingly, Li et al. 53 demonstrated that incorporating hydrophobic and nonconductive PTFE into a Ni-SAC catalyst reduces system impedance.The presence of PTFE is believed to regulate the balance between CO 2 and H 2 O, thereby decreasing the thickness of the interfacial diffusion layer and enhancing the selectivity and stability of acidic CO 2 RR (Fig. 8a and b).Lin et al. 115 recently showed that the intrinsic superhydrophobicity of the microporous layer (MPL) in a GDE is inuenced by ionomers, such as Naon, present in the catalyst slurry.Hence, by introducing PTFE suspension and adjusting the electrodeposition time, the hydrophobicity of the catalyst surface can be effectively preserved, thereby enhancing the stability (Fig. 8c and d).In addition, Sargent et al. 76 introduced amide-bearing polymers (e.g., poly(Lys, Phe)) during the electrodeposition of Cu catalysts, which increases the coverage of surface hydroxyl (OH) species.The interaction between OH species and CO intermediates raises the binding energy of CO, thereby facilitating acidic C-C coupling reactions.
Carbon-based materials or supports have demonstrated signicant efficacy in enhancing the performance of acidic CO 2 RR.Similar to the neutral or alkaline conditions, catalysts such as Ni SACs, 53,117,118 Ni 3 N, 117,119 Ag, 120 Ni, 121 and Fe 122 nanoparticles have shown promising capabilities in generating CO in acidic electrolytes.Importantly, in these studies, the intrinsic structure and properties of carbon materials (e.g., hydrophobicity and porosity) are crucial for ensuring excellent CO 2 RR performance in acidic electrolytes, beyond merely serving as dispersion carriers for single atoms and nanoparticles.For instance, Ma et al. 122 employed a porous carbon layer to encapsulate Fe NPs, creating a local hydrophobic environment that suppressed the HER.Gong et al. 123 embedded Ag into hollow carbon spheres and utilized the connement effect of carbon to enrich OH − , thereby inhibiting the HER.Sargent et al. reported a hollow carbon support cobalt phthalocyanine (CoPc/ HC) catalyst, where individual CoPc molecules are evenly anchored (Fig. 8e).More importantly, the presence of HC may

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Review also help promote the mass transfer of CO 2 gas and intermediates, although the specic mechanism has not been declared.Notably, Jiang et al. 116 discovered that adding a conductive carbon black layer could maintain a high FE (90%) of CO formation at lower K + concentrations (0.05-0.5 M).They proposed that the carbon black layer protected the silver active sites from directly interacting with carbonates, and thus promoting the accumulation of K + ions and enabling a high FE and stability at lower K + concentrations (Fig. 8f).

Novel GDE/electrolyzer designs
Despite some breakthroughs having been achieved in catalyst design or surface modication for the acidic CO 2 RR, the stability, one of the biggest challenges that we have discussed in Section 2, is still unsatisfactory mainly due to the electrolyte ooding of the GDL.The commercial GDL typically includes a macro-porous carbon ber layer (CFL) and a MPL.Meanwhile, the commonly known GDE is designed by covering a catalyst layer (CL) on the GDL (Fig. 9a).In fact, these commercial GDLs are originally designed for fuel cells, however, the CO 2 RR is Review Chemical Science much different from those reactions (e.g., HER, ORR) from the aspects of more complex reaction pathways, formation of both gas and liquid products, and so on.Using commercial GDLs to prepare GDEs directly can negatively impact CO 2 RR performance, especially its stability.Therefore, a specialized GDL for the CO 2 RR is critically required.Zhang et al. 124 proposed a hydrophobic modication of a commercial 28BC (Sigracet) GDE (Fig. 9b).They initially used so nylon bristles to uniformly coat a PTFE emulsion on the MPL surface.To prevent PTFE from blocking the channels in the MPL, they subsequently turned the GDE over and used an airbrush for N 2 purging.They then cleaned it in hexane and purged it with N 2 again.Aer repeating these steps several times, the PTFE-coated 28BC was calcined in a tube furnace under an N 2 atmosphere (Fig. 9c), resulting in a hydrophobicity-graded GDE.Bao et al. 52 also reported the preparation of a hydrophobic GDL based on commercial carbon paper (Toray TPG-H-60).They mixed carbon black (Vulcan XC-72R) and PTFE in ethanol to form a homogeneous ink, which was then coated directly on the MPL side of the carbon paper.The modied GDL was subsequently calcined in a muffle furnace at 350 °C for 1 h.Besides the conventional laminate GDE, Peng et al. 125 recently developed a Ni-N-C nanober integrated electrode with a graded porous structure using electrospinning technology (Fig. 10a and b).The surface of the electrode was coated with PTFE through heat treatment to enhance its hydrophobicity.This novel GDE exhibits superior mechanical and chemical stability (resistant to water ooding and salting out), more conductive active sites and gas diffusion channels, and simpler fabrication.This novel GDL enables a good stability for CO formation (273 h) in neutral electrolyte with an MEA electrolyzer.However, the FE of CO decreased signicantly aer 21 h of reaction in acidic electrolyte.This decline may be attributed to salting-out caused by the pH increase at the catalytic interface, thereby affecting the hydrophobic and CO 2 concentration at the interface, and ultimately intensifying the side reaction of the HER.In addition, to address the issue of CO 2 diffusion in acidic solutions (Fig. 10c and d), Yamauchi

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et al. 77 recently proposed a new ultra-thin Cu-based GDL with large pore size and super-hydrophobicity (Fig. 10e).They electrochemically oxidized loose copper sheets under alkaline conditions to form nano-structures with vertical, needle-like surfaces, and then coated them with 1-octadecanethiol to impart waterproof properties.Consequently, the Cu-GDL exhibits a C 2+ product selectivity of up to 87% in acidic electrolytes and a current density of 1.6 A cm −2 .Chen et al. 30 proposed the use of copper hollow bers to enhance electrode penetration.This design compels CO 2 to interact with active sites as it penetrates the porous wall, thereby improving reaction kinetics at the three-phase interface.The enforced CO 2 penetration increases its coverage on the electrode surface, effectively suppressing the HER.As a result, they achieved an FE of 70% for C 2+ products and a single-pass carbon efficiency (SPCE) of 51.8% at a current density of 2 A cm −2 .
In addition to GDE designs, the development of electrochemical cells or electrolyzers is crucial for the industrial application of the CO 2 RR.Based on structural differences, four types of cells can be categorized, as shown in Fig. 11a.Generally, the typical H-cell has been gradually replaced by ow cells or zero-gap MEA electrolyzers, mainly due to their efficient CO 2 mass transport via GDEs.Furthermore, depending on the type of polymer membrane used, electrolyzers can be classied into anion exchange membrane (AEM), CEM, and bipolar membrane (BPM) types, which are commonly employed for the CO 2 RR (Fig. 11b).BPMs can further be divided into reverse and forward types based on the ion movement direction during the water decomposition reaction.
Currently, there are limited studies on reactor design for acidic CO 2 RR.Most research still focuses on ow cells or MEA electrolyzers.In ow cells, the use of high-concentration alkali metal cation electrolytes (e.g., 3 M KCl) at the cathode can also Chemical Science Review leads to the formation of carbonate anions in the local alkaline environment under high current density.These anions react with K + to form (bi)carbonate precipitates, resulting in signicant CO 2 loss in acidic CO 2 RR.Moreover, these hydrophilic precipitates reduce local interface hydrophobicity and may obstruct the ow channels, ultimately decreasing CO 2 RR performance, particularly in terms of selectivity and stability.To address the above problems, Sinton et al. 126 designed a microchannel solid electrolyte (MSE) for acidic CO 2 RR, which can effectively capture and recycle CO 2 and prevent its loss during electrolysis.The MSE comprises an anion-conducting layer, an integrated channel layer, and a cation-conducting layer near the cathode, resembling the structure of a BPM.Protons migrating from the anode react with carbonate anions from the cathode within the integrated channel layer, regenerating CO 2 gas molecules that return to the GDE to participate in the reaction (Fig. 12a).The integrated channel layer is fabricated directly on a cation exchange membrane through photolithography, facilitating the transport of gas-phase CO 2 , liquids, and ions via parallel ows (Fig. 12b).Additionally, by incorporating a xed cation in the poly(aryl piperidinium) anion-conducting layer, the quaternary ammonium piperidinium cation can activate CO 2 reduction in the absence of alkaline cations.The system demonstrated stable operation at a current density of 100 mA cm −2 for 200 h in a pure acidic electrolyte (0.01 M H 2 SO 4 ), achieving a C 2+ product FE of 70%.More recently, Lau et al. 61 proposed an AEM + PEM assembly (APMA) MEA system with pure water as the anolyte to avoid the formation of carbonate/ precipitation (Fig. 12d).The AEM on the cathode surface provides a local alkaline environment by constraining abundant OH − ions.The presence of PEM allows the transport of H + ions while preventing the crossover of anions from the catholyte.Consequently, only water is formed at the APMA interface.This novel structure differs from conventional bipolar membranes (BPMs) in both reverse (Fig. 12c) and forward bias modes (Fig. 12e).In the reverse bias mode, an additional water dissociation catalyst is required at the interface, while in the forward bias mode, the BPM's stability may be compromised due to difficulties in controlling the mechanical strength of the junction, despite the structural similarity to APMA.Finally, they conducted a scaled-up electrolyzer stack, achieving over 1000 h of stability without CO 2 or electrolyte losses and a 50% FE for ethylene at a total current of 10 A. In summary, due to the relatively mature and comprehensive industrial system for water electrolysis and fuel cells, MEAbased electrolyzers are the most promising devices for the industrial application of acidic CO 2 RR. 10 Despite their relatively complex structure, the above novel APMA electrolyzer serves as an excellent example for long-term electrolysis of pure water CO 2 RR.However, for acidic CO 2 RR in MEA electrolyzers, it remains essential to optimize the operating parameters, such as cell orientation, gas humidication, GDL compression, and cathode CO 2 pressure. 127,128These factors are crucial for ensuring the long-term stability of MEA and the efficiency of the CO 2 RR, forming the foundation for its industrial-scale application.

Summary and outlook
Since Sargent et al. 22 achieved a milestone breakthrough in acidic CO 2 RR in 2021, signicant progress has been made over the past three years.For instance, the FE of C 2+ products has increased from 30% to nearly 90%, and the current density has reached the A cm −2 level.However, acidic CO 2 RR still struggles to maintain good stability at higher current densities, leading to low EE, which falls short of industrial application requirements.This instability may be partly due to the structural degradation of the catalyst itself, as the catalyst is less stable in acidic environments and prone to reconstruction. 18Additionally, changes in the microenvironment of the catalytic interface could be a more critical factor affecting stability. 129For example, the H + /OH − concentration gradient in acidic environments is more pronounced, causing signicant local pH changes that impact the stability of the catalyst/electrode interface.Therefore, in this article, we systematically discuss recent strategies for acidic CO 2 RR, focusing on the modication of the electrolyte and local reaction environment as well as the design of GDEs and electrolyzers, to enhance our understanding of the acidic CO 2 RR and meet industrialization requirements in practical applications.
Firstly, electrolyte regulation was one of the earliest and most thoroughly studied aspects, primarily focusing on local pH and cation effects.In traditional neutral or alkaline electrolyte systems, these factors are also crucial and can signicantly inuence the kinetics of the CO 2 RR.For instance, a high local pH promotes C-C coupling, 68 a theory that extends to acidic CO 2 RR as well.Koper et al. 28 demonstrated that alkali cations are essential during the CO 2 RR because CO 2 reduction scarcely occurs without them.Their ndings highlighted the key role of alkali cations in stabilizing crucial CO 2 RR intermediates.This theory is equally applicable to acidic CO 2 RR, as the absence of alkali cations in acidic electrolytes results predominantly in H 2 production.Notably, alkali cations in acidic electrolytes also uniquely repel H + migration.The accumulation of hydrated cations in the OHP inhibits H + transport near the electrode, thereby maintaining the local pH.However, although the effect of alkali cations is crucial for acidic CO 2 RR, it does not ultimately resolve the issue of (bi)carbonate precipitation, as we have discussed in Section 3.2.Interestingly, recent studies have demonstrated that by leveraging the electric eld and repulsion effects of cations, coating the catalyst with a layer of organic cations can substitute for alkali metal cations in the electrolyte.This approach enables efficient CO 2 RR in the absence of alkali metal cations and even allows stable operation in pure water electrolyte.These ndings suggest that acidic CO 2 RR with low or no metal cations, and even pure water electrolysis, will be a signicant research direction in the future.
Secondly, an essential research direction for regulating the local catalytic environment is the surface modication of organic cations, as we have mentioned above.Currently, molecules with organic cationic functional groups mainly include Sustainion, QAPPT, and PDDA.The positive charge density they carry is crucial for their activity.Among them,

Review
Chemical Science PDDA exhibits a higher positive charge density and demonstrates superior HER inhibition in pure acid electrolytes. 25herefore, future research should focus on designing functional groups in polymer molecules to enhance positive charge density and maintain a high and stable local hydrogen ion concentration. 130On the other hand, current research on the regulation of the local catalytic environment is predominantly based on ow cells, which is mainly concentrated on improving reaction activity and selectivity.Meanwhile, less attention is given to the long-term stability under high current densities, particularly for MEA reactors that offer better commercialization prospects. 10ince there is no cathode electrolyte in the MEA electrolyzer, it is more sensitive to potential bicarbonate precipitation caused by the presence of alkali metal cations. 13Therefore, achieving higher activity and selectivity in extremely low-concentration or alkali metal cation-free electrolytes is critical for ensuring longer stability in MEA electrolyzers.Thirdly, regarding GDE/electrolyzer design, current research predominantly relies on porous PTFE or commercial GDLs, both of which have limitations.For instance, PTFE is nonconductive and cannot be directly used as a current collector in an MEA electrolyzer.Additionally, commercial GDLs are typically not designed specically for the CO 2 RR and are susceptible to electrowetting, leading to interface instability.At present, researchers mainly modify commercial carbon paper with PTFE and carbon black to enhance the hydrophobicity and stability of GDEs.Some have developed new structural GDEs, such as integrated carbon bers and copper hollow bers, to improve the selectivity and stability of acidic CO 2 RR by enhancing gas diffusion.However, these GDEs are still only suitable for ow cells.On the other hand, designing a microchannel solid electrolyte layer capable of in situ CO 2 regeneration at the cathode effectively mitigates the carbonate issue. 126dditionally, advancements in traditional commercial BPM can address the need for additional hydrolysis catalysts and interface instability, achieving stability for 1000 h in a pure water system. 61These ndings suggest that for the industrial application of acidic CO 2 RR, the design and development of new GDEs or electrolyzers may be more critical than catalyst research and development. 131The underlying scientic challenges and technologies warrant further in-depth investigation. 6ourthly, previous studies have demonstrated that the microenvironment surrounding the cathode-specically, the local concentration and electric eld distribution-signicantly inuences the performance of the acidic CO 2 RR.However, density functional theory (DFT) simulations are limited in their capacity to elucidate how the CO 2 RR induces alterations in the microenvironment and how these changes subsequently affect the dynamics of the CO 2 RR.To achieve a comprehensive understanding of CO 2 RR performance under acidic conditions, it is crucial to integrate atomic-level DFT simulations with nite element analysis simulations spanning the nanometer to micrometer scale, the latter of which is typically employed to model mass transport and homogeneous reactions.For instance, as illustrated in Fig. 13, DFT-based molecular simulations can rst establish the relationship between key intermediates and the local electric eld; the energetic parameters obtained can then be utilized to model the micro-kinetics of electrode reactions.Finally, the boundary conditions resulting from electrode reactions can be applied to the nite element analysis of the mass transport process, which includes the EDL, local concentration variations, and the effect of cations or anions. 31inally, SPCE remains a crucial indicator for evaluating the performance of acidic CO 2 RR.A higher SPCE indicates a greater conversion of CO 2 .However, in practical applications, regardless of SPCE, the separation of gas and liquid products from the

Chemical Science Review
CO 2 RR presents a technical challenge. 132Consequently, it is essential to balance the cost of product separation with the efficiency loss associated with high SPCE, as high SPCE typically results in reduced gas ow rate, current density, and stability, thereby impacting reaction energy efficiency.Resasco et al. 132 recently suggested that maximizing SPCE should not be the primary goal; instead, greater emphasis should be placed on the concentration of the product outlet, which is more critical for practical industrial applications.Apfel et al. 78 also highlighted the substantial gap between fundamental research and industrial application of the CO 2 RR.For instance, the catalyst's overpotential may constitute only a minor of the total electrolyzer voltage, and thus it should not be the sole criterion for evaluating GDE performance.They recommended that future research reports provide both half-cell and full-cell data.In summary, acidic CO 2 RR holds signicant potential for industrial applications as it can surpass the theoretical 50% conversion limit inherent in traditional neutral or alkaline systems.Despite current technical challenges such as poor selectivity, low energy efficiency, and inadequate stability, it is anticipated that with the ongoing advancement in researchers' understanding of acidic CO 2 RR systems, this technology can achieve practical industrial implementation in future.

Fig. 3
Fig. 3 Local pH effect in acidic CO 2 RR.(a) Suppression of H + reduction by OH − generated from the CO 2 RR. 11(b) Modeling of pH at different distances to the cathode and current density in 1 M H 3 PO 4 and 3 M KCl.22 (c) Surface pH at various applied current densities and bulk pH.82(d) The dominant hydrogen evolution reaction (proton reduction versus water reduction) and CO 2 RR varied with interfacial pH from acidic to alkaline.83

Fig. 4
Fig. 4 Alkali cations effect in acidic CO 2 RR.(a) Local electric field: larger cations are less hydrated and prefer to absorb in the outer Helmholtz plane (OHP).(b) Local pH buffer: the hydrated cations with lower pK a can give more protons to neutralize the locally generated OH − .(c) Stabilize key intermediates: the hydrated cations could stabilize the key intermediates (e.g., CO 2 − ) effectively via electrostatic interactions.(d) Repulsing migration of H + : the accumulated cations in the OHP can repulse the migration of H + and affect the local pH.

Fig. 5
Fig. 5 The pH distribution values near the cathode under conditions where H + reduction reaches the plateau current density (limiting mass transport condition): 31 (a) in acidic electrolyte free of alkali cations, (b) in acidic electrolyte containing alkali cations.

Fig. 6
Fig. 6 Current strategies for acidic CO 2 RR in metal cation-free electrolytes.(a and b) FE of CO during electrolysis with a constant current density of 200 mA cm −2 .Catalysts of bare Ag, Sustainion-, PDDA-, and c-PDDA-decorated Ag with 0.1 M H 2 SO 4 or 0.1 M H 2 SO 4 + 0.4 M K 2 SO 4 as flowing electrolyte. 25(c and d) Computational studies of electric field for cationic groups (CG)-functionalized catalysts in acidic CO 2 RR, comparison of H + , K + , and immobilized CG at OHP.Note that the interfacial electric field generated by CG was of the same order of magnitude as that generated by K + . 111(e) Full-cell-voltage performance of carbon-protected CG-medium Cu in a slim flow cell, at applied current densities from 25 mA cm −2 to 125 mA cm −2 with 0.2 M H 2 SO 4 flowing electrolyte. 111(f) The electrodeposition of an imidazolium-based layer on Cu NPs enables acidic CO 2 RR in the absence of alkali cations. 112(g) The corresponding LSV curves of electrodeposition in 0.5 M EMIMBF 4 aqueous solution using a Cu NP GDE cathode.112

Fig. 7
Fig. 7 Ionomer-modified local catalytic environment.(a) Schematic illustration of ionic environment and transport near the catalyst surface functionalized by the PFSA ionomer. 22(b) FEs toward gaseous CO 2 RR products on bare Cu and PFSA-modified Cu (Cu/PFSA) at 400 mA cm −2 in 1 M H 3 PO 4 with 3 M KCl. 22(c) Tafel slopes obtained in electrolyte with different K + concentrations. 22(d) Schematics of interfacial reactions and proton transport near the catalyst surface via proton-flux-constraining ionomer adlayer design. 24(e) Schematic of catalysts during the CO 2 RR at pH 1. 23 (f) Surface pH vs. SiC-Nafion™ layer thickness.(g) Cross-section SEM image of SiC-Nafion™/SnBi/PTFE.23

Fig. 8
Fig. 8 The modification of the local catalytic environment with other substances, such as PTFE, polymers, and carbon supports.(a) Schematic illustration of the interface microenvironment inside the catalyst layer with added PTFE.An established balance between gaseous CO 2 and liquid electrolyte in the catalytic layer of GDEs. 53(b) CO partial current density of Ni-N-C and Ni-N-C 60% PTFE electrodes at various current densities. 53(c) Cathode potential and FE for C 2+ and H 2 on the Cu/C and Cu/PTFE/C electrodes in an additional 50 h durability test at a current density of 300 mA cm −2 . 115(d) Cu/C and Cu/C/PTFE electrodes before and after the CO 2 RR at −1.2 V vs. RHE.115(e) Cross-sectional SEM image of a CoPc@HC/Cu tandem electrode (left).TEM image of the upper CoPc@HC catalyst layer and SEM image of the lower Cu catalyst layer with the Cu-ionomer interface (right).75 (f) Schematic illustration of the carbon black layer that promotes the accumulation of K + at pH < 1.116

Fig. 9
Fig. 9 Schematic illustration of a commercial GDL and PTFE-modified GDE.(a) The main components of a typical carbon based GDL and GDE.(b) A hydrophobic modification process of a commercial 28BC (Sigracet) GDE with PTFE emulsion. 124(c) A common calcination post-treatment process of a PTFE modified GDE.

Fig. 10 Fig. 11 36 Fig. 12
Fig. 10 Novel GDE structures for acidic CO 2 RR.(a) Conventional laminate GDE configuration composed of the carbon fiber paper, microporous layer and ionomer-bond catalyst layer. 125(b) Novel integral GDE with catalytic sites embedded within the intertwined carbon nanofibers of hierarchical porosity.125(c and d) Illustrations of CO 2 diffusion in the most traditional GDL and the concentration of CO 2 decreasing with diffusion distance (Dx).MCFP is microporous carbon fiber paper and NCBL is a nano-microporous carbon black layer.77(e) A novel ultra-thin Cu-based GDL with large pore size and super-hydrophobicity.77

Fig. 13
Fig.13Schematic illustration of simulation methods and process for acidic CO 2 RR.31

Table 1
The state-of-the-art performance of the CO 2 RR (single C 1 products) in neutral, alkaline, and acidic electrolytes