Reaction microenvironment control in membrane electrode assemblies for CO 2 electrolysis

CO 2 electrolysis is an emerging and promising carbon neutrality technology, but currently suﬀers from challenging selectivity issues at industrially relevant reaction rates. Selectivity control in CO 2 electrolysis relies on the molecular understanding and manipulation of multiple parallel reaction pathways that are equally governed by catalytically active sites and the reaction microenvironments in their vicinity. In this perspective, we summarize and discuss the latest achievements in reaction microenvironment control for active, selective, energy-and carbon-eﬃcient CO 2 electrolysis, with particular attention being paid to that in membrane electrode assembly electrolyzers operating at industrial current densities ( Z 200 mA cm (cid:2) 2 ). The eﬀects and underlying catalytic mechanisms of reaction microenvironments tailored by functional organic molecules/polymers and reactant feed compositions on the activity and selectivity of CO 2 electrolysis are discussed using selected examples. The eﬀorts made to tailor acidic reaction microenvironments by controlling the transport of reactive species for carbon-eﬃcient CO 2 electrolysis are also exemplified. Finally, we illustrate current challenges and future opportunities in the mechanistic understanding and rational design of reaction microenvironments for improving CO 2 electrolysis performance.

the rational design and precise preparation of highly efficient catalytic materials. 6,7][10][11] The reaction microenvironments can modulate specific transition states and control the transport of reactive species to and from catalytically active sites, via multiple physicochemical effects. 9ationally manipulating reaction microenvironments offers an alternative strategy to break the linear scaling relationships that are widely present in heterogeneous electrocatalysis over bare metal surfaces. 124][15][16][17][18] Such properties are also relevant to the configurations of electrodes and electrolyzers used for CO 2 electrolysis. 19,202][23] In an MEA electrolyzer, liquid electrolytes or pure water and the CO 2 gas are fed to the anode and the cathode, respectively.CO 2 electrolysis proceeds at the cathode in the presence of reactive species (e.g., water and cations) transported from the anode through an ion exchange membrane.Compared to the liquid-solid interfaces in traditional electrochemistry, the gas-liquid-solid three-phase electrochemical interfaces in MEA electrolyzers become much more complicated, resulting in notably distinct reaction microenvironments.In this perspective, we discuss recent advances in the reaction microenvironment control for CO 2 electrolysis, with a special emphasis on that in MEA electrolyzers operating at industrial current densities that should be Z200 mA cm À2 in order to minimize the capital-expenditure of an electrolyzer to economically compelling levels. 24Tailored reaction microenvironments created by organic molecules and polymers as well as reactant feed compositions for active and selective CO 2 electrolysis, as well as acidic microenvironments for carbonefficient CO 2 electrolysis will be exemplified using selected examples (Fig. 1).

Tailoring reaction microenvironments by molecular modifications
In the colloidal synthesis of size-and shape-selected nanoparticle catalysts, capping agents such as small molecule ligands and polymers are commonly used to maintain specific nanostructures by binding to metal atoms and minimizing surface energies. 257][28][29][30] These molecular modifications are expected to control reaction pathways via manipulating electrode hydrophilicity/hydrophobicity, modulating interfacial cation solvation, tuning the adsorption of reaction intermediates, inducing additional chemical activation of CO 2 as well as increasing ion conductivity in MEA electrolyzers.

Organic molecules and polymers
As water is a reactant for both CO 2 electrolysis and the hydrogen evolution reaction (HER), the relative local concentration of water versus CO 2 is a key factor affecting catalytic selectivity.
Water is usually in excess, while CO 2 is insufficient under high current densities owing to limited gas diffusion from bulk liquid electrolytes or gas-liquid interfaces. 24Organic molecules and polymers with remarkable hydrophobic character can repel water molecules surrounding catalyst surfaces and allow CO 2 gas to easily access catalyst surfaces.Mougel and co-workers developed a superhydrophobic Cu dendrite catalyst modified with 1-octadecanethiol, inspired by the gas-trapping cuticles of subaquatic spiders. 31The hydrophobic dendritic Cu surface pushes the liquid electrolyte away to form a gas-liquid-solid triple-phase boundary at the electrode, resulting in enhanced CO 2 mass transport and thus an increased local CO 2 concentration (Fig. 2a).The bio-inspired hydrophobic electrode achieves an ethylene Faradaic efficiency (FE) of 56% and an ethanol FE of 17%, compared to 9% and 4% on an unmodified, wettable counterpart.Hydrophobic molecules are also argued to prevent interfacial water from reorientation under the action of a cathodic electric field, resulting in unfavorable water dissociation to form protons for the HER. 32Polymers (including ionomers) are often used as binders in the preparation of catalyst layers in GDEs, and Nafion (with both hydrophilic and hydrophobic functional groups) is the most widely used binder as revealed by the knowledge from well-established fuel cell technology.Luo and co-workers further investigated two other polymer binders for Cu catalysts: polyacrylic acid (PAA, with a hydrophilic -COOH group) and fluorinated ethylene propylene (FEP, with a hydrophobic -CF x group). 33In a flow cell, the Cu-FEP electrode exhibited a peak FE of 52% for C 2+ products with a partial current density of over 600 mA cm On the other hand, water is not always in excess and is likely insufficient in the case of MEA electrolyzers operating at industrial current densities when a liquid electrolyte or pure water is fed to the anode and dry CO 2 gas is fed to the cathode.Reproduced with the permission. 36Copyright 2023, Wiley.(c) Free energy differences between physisorbed and chemisorbed CO 2 for Ag-NOLI and bare Ag surface.(d) Potassium K-edge XANES of Ag-NOLI, Ag foil, and carbon paper.Reproduced with the permission. 43Copyright 2020, Springer Nature.(e) Molecular structures of N-arylpyridinium additives.(f) Correlation between ethylene FE and the ratio of *CO atop /*CO bridge over Cu electrodes modified by dimers of N-arylpyridiniums in (e).Reproduced with the permission. 45Copyright 2020, Springer Nature.
In MEA electrolyzers, water needed for CO 2 electrolysis comes from the anode through diffusion and electro-osmotic transport. 34,35While humidified CO 2 is used in some cases, the major source of water is still the anolyte. 33,34The excessive consumption of water at high reaction rates would significantly decrease the water concentration at the cathode, so that the low water availability limits the reaction, especially for hydrophobic catalytic materials.Using hydrophobic Ni-N-C as a model catalyst, our group investigated the role of interfacial water in CO 2 electrolysis in a custom-made alkaline MEA electrolyzer. 36he amount of interfacial water (i.e., electrode hydrophilicity/ hydrophobicity) is facilely adjusted using three polymer binders: Nafion, polytetrafluoroethylene (PTFE, containing nonionic surfactants), and quaternary ammonia poly(N-methyl-piperidine-co-pterphenyl) (QAPPT), as evidenced by in situ environmental scanning electron microscopy (ESEM) and contact angle measurements.In contrast to the Cu electrodes measured in H-cells and flow cells, 31,33,37 the hydrophilic Ni-N-C-QAPPT electrode shows the highest CO partial current density, up to 665 mA cm À2 , outperforming all previously reported Ni-N-C catalysts for CO 2 electrolysis (Fig. 2b).In situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy results indicate that increasing the amount of interfacial water modulates the mechanism of CO formation from the *COO À pathway to the *COOH pathway. 36It is unclear whether the structure of interfacial water is also modified by different polymers, but it seems that the strategy that the HER can be suppressed by interfacial water structure modification with organic molecules or electrolyte additives in H-cells 32,38,39 does not necessarily apply to the case of MEA electrolyzers.This exceptional finding further highlights the importance of reaction microenvironment control in MEAs.
Apart from hydrophilic/phobic modulation, organic molecules and polymers can tune activity and selectivity of CO 2 electrolysis by varying local pH.Gewirth and co-workers developed a Cu-polyamine hybrid catalyst through co-electroplating and achieved an ethylene FE of 87% at À0.47 V vs. the reversible hydrogen electrode (RHE) in 10 M KOH electrolyte. 40The remarkable ethylene selectivity is attributed to the higher *CO coverage and higher local pH in the presence of polyamine retained on the Cu electrode surface as revealed by the in situ Raman spectroscopy results.Xie and co-workers employed an anti-swelling anion exchange ionomer (AEI) to manipulate the reaction microenvironment of an oxide-derived Cu nanosheet catalyst. 41The OH À -accumulated -N(CH 3 ) 3 + groups and antiswelling backbone of AEI synergistically regulated local pH and the water amount, resulting in a C 2+ FE of 85.1% at a current density of 800 mA cm À2 .Bell and co-workers tailored the reaction microenvironment of a sputtered Cu catalyst using bilayer cation-and anion-conducting ionomer coatings. 10In combination with pulsed CO 2 electrolysis, the tailored reaction microenvironment results in a 2.5-fold increase in the C 2+ production rate with a C 2+ FE as high as 90% compared with static electrolysis over bare Cu, owing to the higher local pH (via Donnan exclusion) and increased local CO 2 /H 2 O concentration ratio (via ionomer properties).Although the current densities reported in this work were low (10 to 20 mA cm À2 ), the fundamental understanding may also be favorable for the design of membranes and ionomers for high performance MEA electrolyzers.For instance, Mallouk and co-workers introduced a weak-acid cation exchange layer via layer-by-layer (LBL) assembly into bipolar-membrane-based MEA electrolyzers. 42ocal pH measurements in MEA electrolyzers indicate a sharp pH increase from B2 to B5 within the first few bilayers and a steady pH in the bulk of the cation exchange layer.Thus, the presence of the weak-acid layer increased local pH near the catalyst layer and suppressed the HER current density without affecting CO 2 electroreduction.The ligands could improve CO 2 electrolysis performance by modulating the composition, concentration, and solvation structure of interfacial cations at the electrode/electrolyte interfaces.Yang and co-workers developed a nanoparticle/orderedligand interlayer (NOLI) structure composed of an Ag nanoparticle surface and a detached layer of tetradecylphosphonic acid ligands in its vicinity. 43The reversible adsorption/desorption of the ligand layer creates a catalytic pocket at the metal/ligand interface, where hydrated cations are intercalated and then lose their hydration shell due to the strong electrostatic interaction between K + and anionic phosphonate groups of the ligands (Fig. 2c and d).Thus, dehydrated K + cations are anchored close to the metal surface, stabilizing the *COO À intermediate, consistent with the cation effects correlated with hydration free energy over bare metal surfaces. 44The modularity of NOLI is demonstrated with other anionic ligands with a long hydrocarbon chain (e.g., oleic acid) and other noble metals (e.g., Au, Pd).With GDE and a neutral electrolyte, the NOLI achieved a CO FE of 98.1% at 400 mA cm À2 .
Molecular modification provides an alternative strategy of tuning the stabilities of key intermediates to facilitate desired reaction pathways.Sargent and co-workers functionalized the surface of Cu catalysts with a library of dimers of N-arylpyridiniums (Fig. 2e) through electro-dimerization. 45 These adhered molecules improved the stabilization of the *CO atop intermediate (Fig. 2f), thus favoring the C-C coupling pathway via the dimerization of *CO atop and *CO bridge .The optimal Cu catalyst modified with N-aryl-substituted tetrahydro-bipyridine oligomeric films achieved an impressive CO 2 -to-ethylene conversion with an ethylene FE of 72% in a flow cell and a full-cell energy efficiency of 20% in an MEA electrolyzer.7][48][49][50][51] The *CO intermediate can further be enriched at the metal-organic interfaces using a tandem catalyst through functionalizing Cu surfaces with a group of porphyrin-based metallic complexes capable of reducing CO 2 to CO. 52 The interactions between ligands and complex C 2 intermediates at the molecular level would also tune relative selectivity among C 2+ products after C-C coupling; however, relevant mechanistic understanding is still missing.4][55] Whether such ligands have other direct electronic and steric effects is not yet well clarified.
The ligands may chemically activate the inert CO 2 molecule via specific interactions between CO 2 and functional groups such as amino, pyridyl, and imidazolium. 30For instance, the cysteamine ligand can promote CO 2 chemisorption through hydrogen bonding with the amino group, giving rise to 53-fold enhancement in turnover frequency (TOF) for CO production over Ag. 56 By generating a highly reactive N-heterocyclic carbene (NHC) structure under reducing conditions, the imidazolium ligand can capture CO 2 to form carboxylate intermediates. 57Wang and co-workers put forward a polyquinone modification strategy that activated CO 2 via quasi-reversible bonding between electrochemically reduced quinone groups and the CO 2 molecule. 58urther incorporating the polyquinone into a Cu GDE resulted in an ethylene partial current density of 325 mA cm À2 at a cell voltage of 3.5 V in an MEA electrolyzer fed with pure water.In addition to CO 2 activation, the ligands may also play a physical role as a CO 2 -enriching layer when dilute CO 2 is fed to GDE. 59,60

Bifunctional ionomers
As discussed above, ionomers such as Nafion and QAPPT have been used as binders in the preparation of catalyst layers used in MEAs.However, when liquid electrolytes (e.g., KHCO 3 , KOH) are fed to MEA electrolyzers, 36,48 the ion conductivity of ionomers is not a governing factor for CO 2 electrolysis performance.In contrast, when pure water is used, the introduction of ionically conductive ionomers into catalyst layers can remarkably enlarge the electrochemical interface, as revealed by other modern polymer electrolyte-based technologies like fuel cells.CO 2 electrolysis with pure water completely avoids electrolyte consumption and corrosion issues. 21It is highly desirable to integrate ion conductivity and chemical activation of CO 2 into one ionomer/polymer.Zhuang and co-workers developed such a bifunctional ionomer, namely, quaternary ammonia poly(ether ether ketone) (QAPEEK) with carbonyl groups in the polymer chain. 61In situ ATR-SEIRAS measurements indicated that the carbonyl groups close to the electrode surface played a similar role to alkali metal cations, which helped in activating CO 2 and stabilizing the *COO À intermediate (Fig. 3a and b).The remarkable advantage of the bifunctional QAPEEK ionomer versus other anion exchange ionomers (e.g., QAPPT, Sustainion, Fumasep, and PiperION) was verified by its drastically improved ethylene production (Fig. 3c and d).With a porous Cu catalyst, the QAPEEK ionomer helped achieving an industrial-scale ethylene partial current density of 420 mA cm À2 at a low cell voltage of 3.54 V without any electrolyte consumption.

Tailoring reaction microenvironments by reactant feed compositions
While pure CO 2 is usually used in fundamental studies of CO 2 electrolysis, the realistic composition of flue gas streams (e.g., from coal-fired power plants) is very complicated.The CO 2 concentration is only 10-25% in flue gas which contains considerable amounts of N 2 , unconverted O 2 , trace amounts of SO x and NO x as well as CO (due to incomplete combustion).Direct electrolysis of CO 2 from such industrial sources is necessary but very challenging.On the other hand, the presence of these impurities in the feed would also change the reaction microenvironments for CO 2 electrolysis.
The reduction of O 2 is thermodynamically favorable over that of CO 2 ; so it is urgently needed to develop O 2 -tolerant electrodes.A polymer of intrinsic microporosity 62 and a hydrated ionomer catalyst coating 63 have been developed to locally slow O 2 transport as a CO 2 -selective layer on the electrodes.The presence of O 2 in the feed is also argued to facilitate CO 2 electrolysis by stabilizing surface Cu oxide or oxyhydroxide species. 64,65The presence of a trace amount of NO x shows a negligible impact on CO 2 electrolysis despite loss in FE due to the facile electroreduction of NO x , 65,66 while SO x could poison Cu catalysts and suppress the C 2+ production. 65,67Another important feature of flue gas is its low CO 2 concentration which limits CO 2 electrolysis rate owing to the first-order reaction kinetics.This issue stimulates a new research direction, namely, integrated CO 2 capture and electroreduction technologies. 59,68,69oating a layer of organic ligands onto catalyst surfaces to enrich CO 2 and increase the local CO 2 concentration in reaction microenvironments is an effective method to improve electrolysis performance of dilute CO 2 feed. 59he flue gas from steel plants contains a large fraction of CO, and simultaneous conversion of CO 2 and CO is desirable.The co-electrolysis of CO 2 /CO is rarely studied to date, and the underlying co-electrolysis mechanism is also under debate.][72][73][74] These studies are conducted in H-cells with neutral electrolytes.
Our group conducted CO 2 /CO co-electrolysis in an alkaline MEA electrolyzer where reaction microenvironments were quite different from those in H-cells. 65An obvious difference was the available CO 2 /CO concentration which was close to that in the CO 2 /CO feeds in MEAs without being affected by CO 2 /CO solubility.With increasing CO pressure in the feed, the major product gradually shifted from ethylene to acetate and meanwhile the current density increased drastically (Fig. 4a).Under optimized reaction conditions, C 2+ FE and partial current density reached 90.0% and 3.1 A cm À2 , respectively.Structural characterization, control experiments with Ar/CO co-feeds, isotopic labeling experiments, and operando Raman spectroscopy measurements (Fig. 4b-e) indicated that the feed-compositiondependent selectivity changes were not ascribed to any structural changes, but were induced by reaction microenvironments, herein, *CO coverage (by the pressure of CO) and local pH (by the ratio of CO 2 ).It should be noted that CO 2 -CO cross-coupling did occur in CO 2 /CO co-electrolysis under MEA conditions, 65 but did not improve the formation of ethylene. 70,74The insights presented in our work highlight the great importance of both reactant feed composition and electrolyzer configuration in the reaction microenvironment control for selective production of single products.

Tailoring reaction microenvironments in acidic CO 2 electrolysis
A challenging issue of CO 2 electrolysis in alkaline and neutral media is carbonate formation and crossover which leads to substantial CO 2 loss, high cost for downstream CO 2 regeneration and separation, as well as reduced long-term electrolyzer stability. 75Strategies such as rinsing GDEs with water, applying periodic regeneration voltage and using bipolar membranes (BPMs) have been proposed to alleviate the effects of carbonate formation, [76][77][78] but failed to address this issue by eliminating its formation intrinsically.The origin of the carbonate formation is the homogeneous reaction of CO 2 with OH À that is present in electrolytes or is formed as a side product of proton consumption via the reduction of CO 2 and water.The latter would be the major OH À source for the carbonate formation in MEA electrolyzers operating at industrial current densities.CO 2 crossover occurs via electromigration of carbonate or bicarbonate ions from the cathode to anode through an anion exchange membrane.The Bjerrum plot of the carbonate system suggests that the formation of carbonate and bicarbonate can be completely suppressed at a pH lower than 4. Therefore, CO 2 electrolysis in acidic media holds great promise in achieving carbon-efficient CO 2 conversion.Given the remarkable interfacial pH increase upon applying high current densities as revealed by the reaction-diffusion modelling results, 79 reaction media should be strongly acidic, ideally with a bulk pH lower than 2, in order to fully eliminate the carbonate formation.However, such harsh reaction conditions give rise to extra difficulty in selectivity control owing to facile HER and unfavorable C-C coupling.
1][82][83] Sinton and co-workers developed a cation-carrying and protonblocking catalyst adlayer to suppress HER and promote CO 2 electrolysis to C 2+ products. 84The adlayer was composed of insulating polymer nanoparticles (IPN, imine-and carbonylfunctionalized covalent organic frameworks) and perfluorinated sulfonic-acid (PFSA) ionomers, and it can restrict proton transport flux to cation-conducting hydrophilic nanochannels and enrich K + near the Cu catalyst surface (Fig. 5a).The resulting high local alkalinity and cation-enriched reaction microenvironment enabled a C 2+ FE of 75% and a single-pass CO 2 utilization of 45% towards C 2+ at 200 mA cm À2 in a slim flow cell.The asymmetric ion migration-adsorption strategy has also been verified via modifying catalyst surfaces with covalent organic frameworks and organic additives. 85,86In addition to manipulating local cation concentrations, such molecular modifications were also expected to exhibit other direct microenvironment effects by interacting with intermediates, similar to the case of alkaline electrolysis discussed above (Section 2).Our group developed an acidic MEA electrolyzer using K 2 SO 4 + H 2 SO 4 anolytes with pH 0-2 and used HER-inactive Ni-N-C as a model catalyst. 87By optimizing pH, K + concentration and CO 2 pressure, acidic CO 2 electrolysis achieved a CO FE of 95% at a total current density of 500 mA cm À2 at pH 0.5 and the CO 2 loss can be reduced by 86% at 300 mA cm À2 , compared with alkaline CO 2 electrolysis (Fig. 5b-d).By further adjusting the flow rate of input CO 2 , the single-pass CO 2 utilization reached as high as 85%, breaking the upper limit of 50% for CO production in alkaline CO 2 electrolysis.The impressive CO selectivity at high current densities was attributed to the presence of high-concentration K + cations that migrated from the anolyte.Li and co-workers also achieved a CO FE of B80% and a single-pass CO 2 utilization of B90% in an acidic MEA electrolyzer using Cs + cations and Ag catalyst. 88It seemed that high-concentration cations were indispensable for efficient acidic CO 2 electrolysis.A couple of mechanisms on cation effects have been proposed, such as interfacial electric field, buffering interfacial pH, stabilizing intermediates and nonelectric field effect. 18All the proposed reaction mechanisms were positively correlated with cation concentration.Further investigations on producing C 2+ products in acidic MEA electrolyzers are still needed, due to relatively facile HER performance of Cu catalysts and the limited local cation concentration in MEA configurations.It should be noted that for acidic CO 2 electrolysis in anolyte-fed MEA electrolyzers, salt deposition still occurred at an extended electrolysis period, especially when using high-concentration cations due to the transfer of cations from the anolyte to cathode through membranes. 870][91] This strategy has recently shown great promise, but whether the catalytic capability of these organic cations is comparable to that of alkali metal cations is still not yet clear.Given that CO does not react with OH À to form carbonate and C 2+ selectivity of CO electrolysis is higher, tandem acidic/alkaline electrolysis, namely, connecting an acidic MEA electrolyzer (CO 2 electrolysis to CO) and an alkaline MEA electrolyzer (CO electrolysis to C 2+ ), has been proposed and demonstrated as an energy-and carbon-efficient route for CO 2 electrolysis to C 2+ products. 87

Conclusions and outlook
In this perspective, we have summarized and discussed the fundamentals and strategies of tuning reaction microenvironments for active, selective, energy-and carbon-efficient CO 2 electrolysis, with particular attention being paid to those examples using MEA electrolyzers towards practical applications.The effects and underlying mechanisms of reaction microenvironments created by molecular modifications with organic molecules/polymers and tuned by reactant feed compositions on the activity and selectivity of CO 2 electrolysis are illustrated in detail.The efforts to rationally control the transport of H + /OH À and cations to modify acidic reaction microenvironments for carbon-efficient CO 2 electrolysis are also discussed.
The presented examples and associated mechanistic insights highlight future opportunities and design strategies towards reaction microenvironment control for improved activity and selectivity of CO 2 electrolysis.
Despite the recent achievements, the intrinsic underlying effects of reaction microenvironments around catalytically active sites are not understood in depth.For instance, molecular modification of catalyst surfaces is the most widely studied case of reaction microenvironment control, but multiple likely effects (e.g., due to hydrophobicity, local pH, intermediate stabilization, and ion transport) often co-exist in one catalyst system and are difficult to be decoupled.Although control experiments can be performed to rule out contributions of some factors, it is hardly realistic to precisely control ligand structures, catalyst surface structures as well as their complicated interactions, especially under industrially relevant harsh conditions.Currently, the enhancement in C 2+ production has been demonstrated with the aid of tailored reaction microenvironments of Cu catalysts and has been mainly ascribed to the enrichment and activation of CO 2 and the improved adsorption of key intermediates such as *CO. 10,36,43,45,61However, how the organic ligands interact with more complex intermediates beyond *CO 2 and *CO at the molecular level is still not yet clarified.The precise molecular control over specific interactions between ligands and intermediates would be favorable for tuning selectivity among C 2+ products and generating specific C 2+ products. 92][95][96][97] In addition to initial states and conditions, reaction microenvironments are also dynamic and closely associated with reaction rates.[100][101][102] The reaction microenvironments in MEAs (e.g., membrane, ionomer, and catalyst layer structure) are more complex, but currently not well-investigated yet, adding extra degrees of freedom for activity and selectivity control, especially in acidic CO 2 electrolysis.We believe that revealing the nature of electrocatalytic interfaces at the mesoscale would help in designing customizable reaction microenvironments for active, selective, energy-and carbon-efficient CO 2 electrolysis.

Fig. 2
Fig.2(a) Illustration of the role of hydrophobicity in promoting CO 2 electroreduction over HER.Reproduced with the permission.31Copyright 2019, Springer Nature.(b) CO partial current densities over QAPPT-, PTFE-and Nafion-modified Ni-N-C electrodes measured in an MEA electrolyzer.Reproduced with the permission.36Copyright 2023, Wiley.(c) Free energy differences between physisorbed and chemisorbed CO 2 for Ag-NOLI and bare Ag surface.(d) Potassium K-edge XANES of Ag-NOLI, Ag foil, and carbon paper.Reproduced with the permission.43Copyright 2020, Springer Nature.(e) Molecular structures of N-arylpyridinium additives.(f) Correlation between ethylene FE and the ratio of *CO atop /*CO bridge over Cu electrodes modified by dimers of N-arylpyridiniums in (e).Reproduced with the permission.45Copyright 2020, Springer Nature.
À2, followed by Cu-Nafion and Cu-PAA.The water contact angle follows the order of Cu-PAA o Cu-Nafion o Cu-FEP, while the captive bubble contact angle for the CO 2 bubble increases from Cu-FEP (471) to Cu-Nafion (731) and Cu-PAA (1171).These results suggest that electrode hydrophobicity and CO 2 -philicity can be easily tuned by a thin layer of polymer binders with distinct functional groups, with the hydrophobic FEP showing the highest local CO 2 /H 2 O concentration ratio.