Spectroscopic Evidence of Size-Dependent Buffering of Interfacial pH by Cation Hydrolysis during CO2 Electroreduction

Alkali Metal Cations Induce Structural Evolution on Au(111) During Cathodic Polarization

The activity of the electrocatalytic CO2 reduction reaction (CO2RR) is substantially affected by alkali metal cations (AM+) in electrolytes, yet the underlying mechanism is still controversial. Here, we employed electrochemical scanning tunneling microscopy and in situ observed Au(111) surface roughening in AM+ electrolytes during cathodic polarization. The roughened surface is highly active for catalyzing the CO2RR due to the formation of surface low-coordinated Au atoms. The critical potential for surface roughening follows the order Cs+ > Rb+ > K+ > Na+ > Li+, and the surface proportion of roughened area decreases in the order of Cs+ > Rb+ > K+ > Na+ > Li+. Electrochemical CO2RR measurements demonstrate that the catalytic activity strongly correlates with the surface roughness. Furthermore, we found that AM+ is critical for surface roughening to occur. The results unveil the unrecognized effect of AM+ on the surface structural evolution and elucidate that the AM+-induced formation of surface high-activity sites contributes to the enhanced CO2RR in large AM+ electrolytes.

Integrating Active Learning and DFT for Fast-Tracking Single-Atom Alloy Catalysts in CO2-to-Fuel Conversion

Electrocatalytic carbon dioxide reduction (CO2RR) technology enables the conversion of excessive CO2 into high-value fuels and chemicals, thereby mitigating atmospheric CO2 concentrations and addressing energy scarcity. Single-atom alloys (SAAs) possess the potential to enhance the CO2RR performance by full utilization of atoms and breaking linear scaling relationships. However, quickly screening high-performance metal portfolios of SAAs remains a formidable challenge. In this study, we proposed an active learning (AL) framework to screen high-performance catalysts for CO2RR to yield fuels such as CH4 and CH3OH. After four rounds of AL iterations, the ML model attained optimal prediction performance with the test set R2 of approximately 0.94 and successful prediction was achieved for the binding free energy of *CHO, *COH, *CO, and *H on 380 catalyst surfaces with an accuracy within 0.20 eV. Subsequent analysis of the SAA catalysts' activity, selectivity, and stability led to the discovery of eight previously unexplored SAA catalysts for CO2RR. Notably, the surface activity of Ti@Cu(100), Au@Pt(100), and Ag@Pt(100) shines prominently. Utilizing DFT calculations, we elucidated the complete reaction pathway of the CO2RR on the surfaces of these catalysts, confirming their high catalytic activity with limiting potentials of -0.11, -0.34, and -0.46 eV, respectively, which are significantly lower than those of pure Cu catalysts. The results showcase the exceptional predictive prowess of AL, providing a valuable reference for the design of CO2RR catalysts.

Anchoring Cs+ Ions on Carbon Vacancies for Selective CO2 Electroreduction to CO at High Current Densities in Membrane Electrode Assembly Electrolyzers

Electrolyte cations have been demonstrated to effectively enhance the rate and selectivity of the electrochemical CO2 reduction reaction (CO2RR), yet their implementation in electrolyte-free membrane electrode assembly (MEA) electrolyzer presents significant challenges. Herein, an anchored cation strategy that immobilizes Cs+ on carbon vacancies was designed and innovatively implemented in MEA electrolyzer, enabling highly efficient CO2 electroreduction over commercial silver catalyst. Our approach achieves a CO partial current density of approximately 500 mA cm-2 in the MEA electrolyzer, three-fold enhancement compared to pure Ag. In situ Raman and theoretical analyses, combined with machine learning potentials, reveal anchored Cs induces an electric field that significantly promotes the adsorption of *CO2 - intermediates through performing muti-point energy calculations on each structure. Furthermore, reduced adsorption of *OH intermediates effectively hampers competing hydrogen evolution reaction, as clarified by disk electrode experiments and density functional theory studies. Additionally, coupling our system with commercial polysilicon solar cells yields a notable solar-to-CO energy conversion efficiency of 8.3 %. This study opens a new avenue for developing effective cation-promoting strategy in MEA reactors for efficient CO2RR.

Electroanalytical Strategies for Local pH Sensing at Solid-Liquid Interfaces and Biointerfaces

Obtaining analytical information about chemical species at interfaces is fundamentally important to improving our understanding of chemical reactions and biological processes. pH at solid-liquid interfaces is found to deviate from the bulk solution value, for example, in electrocatalytic reactions at surfaces or during the corrosion of metals. Also, in the vicinity of living cells, metabolic reactions or cellular responses cause changes in pH at the extracellular interface. In this review, we collect recent progress in the development of sensors with the capability to detect pH at or close to solid-liquid and bio interfaces, with spatial and time resolution. After the two main principles of pH detection are presented, the different classes of molecules and materials that are used as active components in these sensors are described. The review then focuses on the reported electroanalytical techniques for local pH sensing. As application examples, we discuss model studies that exploit local pH sensing in the area of electrocatalysis, corrosion, and cellular interfaces. We conclude with a discussion of key challenges for wider use of this analytical approach, which shows promise to improve the mechanistic understanding of reactions and processes at realistic interfaces.

Time-Resolved Infrared Spectroscopic Evidence for Interfacial pH-Dependent Kinetics of Formate Evolution on Cu Electrodes

By deployment of rapid-scan (second time scale) electrochemical FT-IR reflection-absorption spectroscopy, we studied the reduction of CO2 in 0.1 M Na2SO4 in deuterated water at a pD of 3.7. We report on the impact of dynamic changes in the bicarbonate equilibrium concentration in the vicinity of a polycrystalline Cu electrode, induced by step changes in applied electrode potential. We correlate these changes in interfacial composition and concentrations of dissolved species to the formation rate of formate, and provide evidence for the following conclusions: (i) the kinetics for the conversion of dissolved CO2 to formate (formic acid) are fast, (ii) bicarbonate is also converted to formate, but with less favorable kinetics, and (iii) carbonate does not yield any formate. These results reveal that formate formation requires (mildly) acidic conditions at the interface for CO2 to undergo a proton-coupled conversion step, and we postulate that bicarbonate reduction to formate is driven by catalytic hydrogenation via in situ formed H2. Interestingly CO was not observed, suggesting that the kinetics of the CO2 to CO reaction are significantly less favorable than formate formation under the experimental conditions (pH and applied potential). We also analyzed the feasibility of pulsed electrolysis to enhance the (average) rate of formation of formate. While a short positive potential pulse enhances the CO2 concentration, this also leads to the formation of basic copper carbonates, resulting in electrode deactivation. These observations demonstrate the potential of rapid-scan EC-IRRAS to elucidate the mechanisms and kinetics of electrochemical reactions, offering valuable insights for optimizing catalyst and electrolyte performance and advancing CO2 reduction technologies.

Strong effect-correlated electrochemical CO2 reduction

Electrochemical CO2 reduction (ECR) holds great potential to alleviate the greenhouse effect and our dependence on fossil fuels by integrating renewable energy for the electrosynthesis of high-value fuels from CO2. However, the high thermodynamic energy barrier, sluggish reaction kinetics, inadequate CO2 conversion rate, poor selectivity for the target product, and rapid electrocatalyst degradation severely limit its further industrial-scale application. Although numerous strategies have been proposed to enhance ECR performances from various perspectives, scattered studies fail to comprehensively elucidate the underlying effect-performance relationships toward ECR. Thus, this review presents a comparative summary and a deep discussion with respect to the effects strongly-correlated with ECR, including intrinsic effects of materials caused by various sizes, shapes, compositions, defects, interfaces, and ligands; structure-induced effects derived from diverse confinements, strains, and fields; electrolyte effects introduced by different solutes, solvents, cations, and anions; and environment effects induced by distinct ionomers, pressures, temperatures, gas impurities, and flow rates, with an emphasis on elaborating how these effects shape ECR electrocatalytic activities and selectivity and the underlying mechanisms. In addition, the challenges and prospects behind different effects resulting from various factors are suggested to inspire more attention towards high-throughput theoretical calculations and in situ/operando techniques to unlock the essence of enhanced ECR performance and realize its ultimate application.

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.

Tethered Alkylammonium Dications as Electrochemical Interface Modifiers: Chain Length Effect on CO2 Reduction Selectivity at Industry-Relevant Current Density

The electrochemical reduction of CO2 (CO2RR) has the potential to be an economically viable method to produce platform chemicals synergistically with renewable energy sources. Copper is one of the most commonly used electrocatalysts for this purpose, as it allows C-C bond formation, yielding a broad product distribution. Controlling selectivity is a stepping stone on the way to its industrial application. The kinetics of the reaction can be modified to favor the rates of certain products quickly and inexpensively by applying additives such as ionic liquids and coelectrolytes that directly affect the electrocatalytic interface. In this work, we propose tethered tetraalkylammonium salts as double-charged cationic modifiers of the electrochemical double layer to control CO2RR product selectivity. A novel setup comprising a gas diffusion electrode (GDE) flow cell coupled with real-time mass spectroscopy was used to study the effect of a library of the selected salts. We emphasize how the length of an alkyl linker effectively controls the selectivity of the reaction toward C1, C2, or C3 products at high relevant current densities (Jtotal = -400 mA cm-2) along with the inhibition of the parasitic hydrogen evolution reaction. Standard long-term experiments were performed for quantitative validation and stability evaluation. These results have broad implications for further tailoring an effective catalytic system for selective CO2 reduction reaction.

Cation Effects on the Adsorbed Intermediates of CO2 Electroreduction Are Systematic and Predictable

The electrode-electrolyte interface, and in particular the nature of the cation, has considerable effects on the activity and product selectivity of the electrochemical reduction of CO2. Therefore, to improve the electrocatalysis of this challenging reaction, it is paramount to ascertain whether cation effects on adsorbed intermediates are systematic. Here, DFT calculations are used to show that the effects of K+, Na+, and Mg2+, on single carbon CO2 reduction intermediates can either be stabilizing or destabilizing depending on the metal and the adsorbate. Because systematic trends are observed, cation effects can be accurately predicted in simple terms for a wide variety of metals, cations and adsorbed species. These results are then applied to the reduction of CO2 to CO on four different catalytic surfaces (Au, Ag, Cu, Pd) and activation of weak-binding metals is consistently observed by virtue of the stabilization of the key intermediate *COOH.

Rational Design of Local Reaction Environment for Electrocatalytic Conversion of CO2 into Multicarbon Products

The electrocatalytic conversion of CO2 into multi-carbon (C2+) products provides an attractive route for storing intermittent renewable electricity as fuels and feedstocks with high energy densities. Although substantial progress has been made in selective electrosynthesis of C2+ products via engineering the catalyst, rational design of the local reaction environment in the vicinity of catalyst surface also acts as an effective approach for further enhancing the performance. Here, we discuss recent advances and pertinent challenges in the modulation of local reaction environment, encompassing local pH, the choice of the species and concentrations of cations and anions as well as local reactant/intermediate concentrations, for achieving high C2+ selectivity. In addition, mechanistic understanding in the effects of the local reaction environment is also discussed. Particularly, the important progress extracted from in situ and operando spectroscopy techniques provides insights into how local reaction environment affects C-C coupling and key intermediates formation that lead to reaction pathways toward a desired C2+ product. The possible future direction in understanding and engineering the local reaction environment is also provided.

Tuning hydrogen bond network connectivity in the electric double layer with cations

Hydrogen bond (H-bond) network connectivity in electric double layers (EDLs) is of paramount importance for interfacial HER/HOR electrocatalytic processes. However, it remains unclear whether the cation-specific effect on H-bond network connectivity in EDLs exists. Herein, we report simulation evidence from ab initio molecular dynamics that cations at Pt(111)/water interfaces can tune the structure and the connectivity of H-bond networks in EDLs. As the surface charge density σ becomes more negative, we show that the connectivity of the H-bond networks in EDLs of the Na+ and Ca2+ systems decreases markedly; in stark contrast, the connectivity of the H-bond networks in EDLs of the Mg2+ system increases slightly. Further analysis revealed that the interplay between the hydration of cations and the interfacial water structure plays a key role in the connectivity of H-bond networks in EDLs. These findings highlight the key roles of cations in EDLs and electrocatalysis.

Cation effects in hydrogen evolution and CO2-to-CO conversion: A critical perspective

The rates of many electrocatalytic reactions can be strongly affected by the structure and dynamics of the electrochemical double layer, which in turn can be tuned by the concentration and identity of the supporting electrolyte's cation. The effect of cations on an electrocatalytic process depends on a complex interplay between electrolyte components, electrode material and surface structure, applied electrode potential, and reaction intermediates. Although cation effects remain insufficiently understood, the principal mechanisms underlying cation-dependent reactivity and selectivity are beginning to emerge. In this Perspective, we summarize and critically examine recent advances in this area in the context of the hydrogen evolution reaction (HER) and CO2-to-CO conversion, which are among the most intensively studied and promising electrocatalytic reactions for the sustainable production of commodity chemicals and fuels. Improving the kinetics of the HER in base and enabling energetically efficient and selective CO2 reduction at low pH are key challenges in electrocatalysis. The physical insights from the recent literature illustrate how cation effects can be utilized to help achieve these goals and to steer other electrocatalytic processes of technological relevance.

Recent advances in the role of interfacial liquids in electrochemical reactions

The interfacial liquid, situated in proximity to an electrode or catalyst, plays a vital role in determining the activity and selectivity of crucial electrochemical reactions, including hydrogen evolution, oxygen evolution/reduction, and carbon dioxide reduction. Thus, there has been a growing interest in better understanding the behavior and the catalytic effect of its constituents. This minireview examines the impact of interfacial liquids on electrocatalysis, specifically the effects of water molecules and ionic species present at the interface. How the structure of interfacial water, distinct from the bulk, can affect charge transfer kinetics and transport of species is presented. Furthermore, how cations and anions (de)stabilize intermediates and transition states, compete for adsorption with reaction species, and act as local environment modifiers including pH and the surrounding solvent structure are described in detail. These effects can promote or inhibit reactions in various ways. This comprehensive exploration provides valuable insights for tailoring interfacial liquids to optimize electrochemical reactions.

Tuning the Interfacial Reaction Environment for CO2 Electroreduction to CO in Mildly Acidic Media

A considerable carbon loss of CO2 electroreduction in neutral and alkaline media severely limits its industrial viability as a result of the homogeneous reaction of CO2 and OH- under interfacial alkalinity. Here, to mitigate homogeneous reactions, we conducted CO2 electroreduction in mildly acidic media. By modulating the interfacial reaction environment via multiple electrolyte effects, the parasitic hydrogen evolution reaction is suppressed, leading to a faradaic efficiency of over 80% for CO on the planar Au electrode. Using the rotating ring-disk electrode technique, the Au ring constitutes an in situ CO collector and pH sensor, enabling the recording of the Faradaic efficiency and monitoring of interfacial reaction environment while CO2 reduction takes place on the Au disk. The dominant branch of hydrogen evolution reaction switches from the proton reduction to the water reduction as the interfacial environment changes from acidic to alkaline. By comparison, CO2 reduction starts within the proton reduction region as the interfacial environment approaches near-neutral conditions. Thereafter, proton reduction decays, while CO2 reduction takes place, as the protons are increasingly consumed by the OH- electrogenerated from CO2 reduction. CO2 reduction reaches its maximum Faradaic efficiency just before water reduction initiates. Slowing the mass transport lowers the proton reduction current, while CO2 reduction is hardly influenced. In contrast, appropriate protic anion, e.g., HSO4- in our case, and weakly hydrated cations, e.g., K+, accelerate CO2 reduction, with the former providing extra proton flux but higher local pH, and the latter stabilizing the *CO2- intermediate.

Determination of local pH in CO2 electroreduction

The electrocatalytic conversion of CO2 and H2O into fuels and valuable chemicals has gained significant interest as a prospective method for the storage of renewable energy and the utilization of captured CO2. In the process of electroreduction of CO2, pH near the surface of the electrocatalysts plays an important role in the catalytic selectivity and activity. However, to elucidate the local pH effect on the fundamental reaction mechanism and modify the catalytic CO2 reduction performance, the localized pH determination method is highly desirable. In this minireview, we present the recent advances in the strategies of the local pH probe for CO2 electrolysis in both H-type cell reactors and GDE-type flow electrolyzers, followed with a better understanding of the local reaction environment in CO2 reduction. Additionally, pertinent advantages and drawbacks of the different localized pH probe techniques are discussed, and perspectives on future research efforts are also provided in this minireview.

Studying the cation dependence of CO2 reduction intermediates at Cu by in situ VSFG spectroscopy

The nature of the electrolyte cation is known to have a significant impact on electrochemical reduction of CO2 at catalyst|electrolyte interfaces. An understanding of the underlying mechanism responsible for catalytic enhancement as the alkali metal cation group is descended is key to guide catalyst development. Here, we use in situ vibrational sum frequency generation (VSFG) spectroscopy to monitor changes in the binding modes of the CO intermediate at the electrochemical interface of a polycrystalline Cu electrode during CO2 reduction as the electrolyte cation is varied. A CObridge mode is observed only when using Cs+, a cation that is known to facilitate CO2 reduction on Cu, supporting the proposed involvement of CObridge sites in CO coupling mechanisms during CO2 reduction. Ex situ measurements show that the cation dependent CObridge modes correlate with morphological changes of the Cu surface.

Smart Electrode Surfaces by Electrolyte Immobilization for Electrocatalytic CO2 Conversion

The activity and selectivity of molecular catalysts for the electrochemical CO2 reduction reaction (CO2RR) are influenced by the induced electric field at the electrode/electrolyte interface. We present here a novel electrolyte immobilization method to control the electric field at this interface by positively charging the electrode surface with an imidazolium cation organic layer, which significantly favors CO2 conversion to formate, suppresses hydrogen evolution reaction, and diminishes the operating cell voltage. Those results are well supported by our previous DFT calculations studying the mechanistic role of imidazolium cations in solution for CO2 reduction to formate catalyzed by a model molecular catalyst. This smart electrode surface concept based on covalent grafting of imidazolium on a carbon electrode is successfully scaled up for operating at industrially relevant conditions (100 mA cm-2) on an imidazolium-modified carbon-based gas diffusion electrode using a flow cell configuration, where the CO2 conversion to formate process takes place in acidic aqueous solution to avoid carbonate formation and is catalyzed by a model molecular Rh complex in solution. The formate production rate reaches a maximum of 4.6 gHCOO- m-2 min-1 after accumulating a total of 9000 C of charge circulated on the same electrode. Constant formate production and no significant microscopic changes on the imidazolium-modified cathode in consecutive long-term CO2 electrolysis confirmed the high stability of the imidazolium organic layer under operating conditions for CO2RR.

Molecular understanding of the critical role of alkali metal cations in initiating CO2 electroreduction on Cu(100) surface

Molecular understanding of the solid-liquid interface is challenging but essential to elucidate the role of the environment on the kinetics of electrochemical reactions. Alkali metal cations (M+), as a vital component at the interface, are found to be necessary for the initiation of carbon dioxide reduction reaction (CO2RR) on coinage metals, and the activity and selectivity of CO2RR could be further enhanced with the cation changing from Li+ to Cs+, while the underlying mechanisms are not well understood. Herein, using ab initio molecular dynamics simulations with explicit solvation and enhanced sampling methods, we systematically investigate the role of M+ in CO2RR on Cu surface. A monotonically decreasing CO2 activation barrier is obtained from Li+ to Cs+, which is attributed to the different coordination abilities of M+ with *CO2. Furthermore, we show that the competing hydrogen evolution reaction must be considered simultaneously to understand the crucial role of alkali metal cations in CO2RR on Cu surfaces, where H+ is repelled from the interface and constrained by M+. Our results provide significant insights into the design of electrochemical environments and highlight the importance of explicitly including the solvation and competing reactions in theoretical simulations of CO2RR.

Insight into the Change in Local pH near the Electrode Surface Using Phosphate Species as the Probe

The difference between solution pH and local pH near an electrode surface greatly determines the electrocatalytic performance. However, there is still a lack of a facile and universal method for the local pH detection of various electrode reactions, leaving the origin of local pH changes unclear. Herein, by using phosphate species in phosphate buffer solution (PBS) as the pH probe, we demonstrate a universal local pH detection strategy through in situ Raman spectroscopy for various electrode reactions. Oxygen evolution is chosen as the example to detect the potential-dependent local pH change. Then the strategy extends to nitrate reduction, nitrobenzene reduction, and benzylamine oxidation. By comparing the local pH changes in different reactions, we reveal that the local pH change is strongly dependent on the reaction current, the ability of the system to replenish the local H+/OH-, and the number of H+/OH- per electron transfer of the electrode reaction.

Gaining Deep Understanding of Electrochemical CO2 RR with In Situ/Operando Techniques

Electrocatalysis for CO2 conversion has been extensively studied to mitigate the energy shortage and environmental issues, which are gaining ever-increasing attention. However, the complicated CO2 reduction process and the dynamic evolution occurring on electrocatalyst surface make it hard to understand the catalytic mechanism. The development of advanced in situ/operando techniques intelligently coupled with electrochemical cells sheds light on the related study via capturing surface atomic rearrangement, tracing chemical state change of catalysts, monitoring the behavior of intermediates and products, and depicting microenvironment near the electrode surface. In this review, fundamentals of the state-of-the-art in situ/operando techniques are clarified first. Case studies on the in situ/operando techniques performed to probe the CO2 reduction reaction processes are then discussed in detail. Finally, conclusions and outlook on this field are presented.

An Interfacial View of Cation Effects on Electrocatalysis Systems

The identity of alkali metal cations in the electrolyte of electrocatalysis systems has been recently introduced as a crucial factor to tailor the kinetics and Faradaic efficiency of many electrocatalytic reactions. In this Minireview, we have summarized the recent advances in the molecular-level understanding of cation effects on relevant electrocatalytic processes such as hydrogen evolution (HER), oxygen evolution (OER), and CO2 electroreduction (CO2 RR) reactions. The discussion covers the effects of electrolyte cations on interfacial electric fields, structural organization of interfacial water molecules, blocking the catalytic active sites, stabilization or destabilization of intermediates, and interfacial pHs. These cation-induced interfacial phenomena have been reported to impact the performance (activity, selectivity, and stability) of electrochemical reactions collaboratively or independently. We describe that although there is almost a general agreement on the relationship between the size of alkali cations and the activities of HER, OER, and CO2 RR, however, the mechanism by which the performance of these electrocatalytic reactions is influenced by alkali metal cations is still in debate.

Alkaline Ionic Liquid Microphase Promotes Deep Reduction of CO2 on Copper

Electrochemical reduction of CO2 to multicarbon (C2+) products using renewable energy sources is an important route to storing sustainable energy and achieving carbon neutrality. It remains a challenge to achieve high C2+ product faraday efficiency (FE) at ampere-level current densities. Herein, we propose the immobilization of an alkaline ionic liquid on copper for promoting the deep reduction of CO2. By this strategy, a C2+ FE of 81.4% can be achieved under a current density of 0.9 A·cm-2 with a half-cell energy conversion efficiency of 47.4% at -0.76 V vs reversible hydrogen electrode (RHE). Particularly, when the current density is as high as 1.8 A·cm-2, the C2+ FE reaches 71.6% at an applied potential of -1.31 V vs RHE. Mechanistic studies demonstrate that the alkaline ionic liquid plays multiple roles of improving the accumulation of CO2 molecules on the copper surface, promoting the activation of the adsorbed CO2, reducing the energy barrier of CO dimerization, stabilizing intermediates, and facilitating the C2+ product formation.

Cation Incorporation into Copper Oxide Lattice at Highly Oxidizing Potentials

Electrolyte cations can have significant effects on the kinetics and selectivity of electrocatalytic reactions. We show an atypical mechanism through which electrolyte cations can impact electrocatalyst performance─direct incorporation of the cation into the oxide electrocatalyst lattice. We investigate the transformations of copper electrodes in alkaline electrochemistry through operando X-ray absorption spectroscopy in KOH and Ba(OH)2 electrolytes. In KOH electrolytes, both the near-edge structure and extended fine-structure agree with previous studies; however, the X-ray absorption spectra vary greatly in Ba(OH)2 electrolytes. Through a combination of electronic structure modeling, near-edge simulation, and postreaction characterization, we propose that Ba2+ cations are directly incorporated into the lattice and form an ordered BaCuO2 phase at potentials more oxidizing than 200 mV vs the normal hydrogen electrode (NHE). BaCuO2 formation is followed by further oxidation to a bulk Cu3+-like BaxCuyOz phase at 900 mV vs NHE. Additionally, during reduction in Ba(OH)2 electrolyte, we find both Cu-O bonds and Cu-Ba scattering persist at potentials as low as -400 mV vs NHE. To our knowledge, this is the first evidence for direct oxidative incorporation of an electrolyte cation into the bulk lattice to form a mixed oxide electrode. The oxidative incorporation of electrolyte cations to form mixed oxides could open a new route for the in situ formation of active and selective oxidation electrocatalysts.

Influence of Cations on HCOOH and CO Formation during CO2 Reduction on a PdMLPt(111) Electrode

Understanding the role of cations in the electrochemical CO2 reduction (CO2RR) process is of fundamental importance for practical application. In this work, we investigate how cations influence HCOOH and CO formation on PdMLPt(111) in pH 3 electrolytes. While only (a small amount of adsorbed) CO forms on PdMLPt(111) in the absence of metal cations, the onset potential of HCOOH and CO decreases with increasing cation concentrations. The cation effect is stronger on HCOOH formation than that on CO formation on PdMLPt(111). Density functional theory simulations indicate that cations facilitate both hydride formation and CO2 activation by polarizing the electronic density at the surface and stabilizing *CO2-. Although the upshift of the metal work function caused by high coverage of adsorbates limits hydride formation, the cation-induced electric field counterbalances this effect in the case of *H species, sustaining HCOOH production at mild negative potentials. Instead, at the high *CO coverages observed at very negative potentials, surface hydrides do not form, preventing the HCOOH route both in the absence and presence of cations. Our results open the way for a consistent evaluation of cationic electrolyte effects on both activity and selectivity in CO2RR on Pd-Pt catalysts.

Multiscale CO2 Electrocatalysis to C2+ Products: Reaction Mechanisms, Catalyst Design, and Device Fabrication

Electrosynthesis of value-added chemicals, directly from CO2, could foster achievement of carbon neutral through an alternative electrical approach to the energy-intensive thermochemical industry for carbon utilization. Progress in this area, based on electrogeneration of multicarbon products through CO2 electroreduction, however, lags far behind that for C1 products. Reaction routes are complicated and kinetics are slow with scale up to the high levels required for commercialization, posing significant problems. In this review, we identify and summarize state-of-art progress in multicarbon synthesis with a multiscale perspective and discuss current hurdles to be resolved for multicarbon generation from CO2 reduction including atomistic mechanisms, nanoscale electrocatalysts, microscale electrodes, and macroscale electrolyzers with guidelines for future research. The review ends with a cross-scale perspective that links discrepancies between different approaches with extensions to performance and stability issues that arise from extensions to an industrial environment.

Integrated CO2 capture and electrochemical upgradation: the underpinning mechanism and techno-chemical analysis

Coupling post-combustion CO2 capture with electrochemical utilization (CCU) is a quantum leap in renewable energy science since it eliminates the cost and energy involved in the transport and storage of CO2. However, the major challenges involved in industrial scale implementation are selecting an appropriate solvent/electrolyte for CO2 capture, modeling an appropriate infrastructure by coupling an electrolyser with a CO2 point source and a separator to isolate CO2 reduction reaction (CO2RR) products, and finally selection of an appropriate electrocatalyst. In this review, we highlight the major difficulties with detailed mechanistic interpretation in each step, to find out the underpinning mechanism involved in the integration of electrochemical CCU to achieve higher-value products. In the past decades, most of the studies dealt with individual parts of the integration process, i.e., either selecting a solvent for CO2 capture, designing an electrocatalyst, or choosing an ideal electrolyte. In this context, it is important to note that solvents such as monoethanolamine, bicarbonate, and ionic liquids are often used as electrolytes in CO2 capture media. Therefore, it is essential to fabricate a cost-effective electrolyser that should function as a reversible binder with CO2 and an electron pool capable of recovering the solvent to electrolyte reversibly. For example, reversible ionic liquids, which are non-ionic in their normal forms, but produce ionic forms after CO2 capture, can be further reverted back to their original non-ionic forms after CO2 release with almost 100% efficiency through the chemical or thermal modulations. This review also sheds light on a focused techno-economic evolution for converting the electrochemically integrated CCU process from a pilot-scale project to industrial-scale implementation. In brief, this review article will summarize a state-of-the-art argumentation of challenges and outcomes over the different segments involved in electrochemically integrated CCU to stimulate urgent progress in the field.

Weakly Coordinating Organic Cations Are Intrinsically Capable of Supporting CO2 Reduction Catalysis

The rates and selectivity of electrochemical CO₂ reduction are known to be strongly influenced by the identity of alkali metal cations in the medium. However, experimentally, it remains unclear whether cation effects arise predominantly from coordinative stabilization of surface intermediates or from changes in the mean-field electrostatic environment at the interface. Herein, we show that Au- and Ag-catalyzed CO₂ reduction can occur in the presence of weakly coordinating (poly)tetraalkylammonium cations. Through competition experiments in which the catalytic activity of Au was monitored as a function of the ratio of the organic to metal cation, we identify regimes in which the organic cation exclusively controls CO₂ reduction selectivity and activity. We observe substantial CO production in this regime, suggesting that CO₂ reduction catalysis can occur in the absence of Lewis acidic cations, and thus, coordinative interactions between the electrolyte cations and surface-bound intermediates are not required for CO₂ activation. For both Au and Ag, we find that tetraalkylammonium cations support catalytic activity for CO₂ reduction on par with alkali metal cations but with distinct cation activity trends between Au and Ag. These findings support a revision in electrolyte design rules to include water-soluble organic cation salts as potential supporting electrolytes for CO₂ electrolysis.

Electrochemical Carbon Dioxide Reduction to Ethylene: From Mechanistic Understanding to Catalyst Surface Engineering

Electrochemical carbon dioxide reduction reaction (CO2RR) provides a promising way to convert CO2 to chemicals. The multicarbon (C2+) products, especially ethylene, are of great interest due to their versatile industrial applications. However, selectively reducing CO2 to ethylene is still challenging as the additional energy required for the C-C coupling step results in large overpotential and many competing products. Nonetheless, mechanistic understanding of the key steps and preferred reaction pathways/conditions, as well as rational design of novel catalysts for ethylene production have been regarded as promising approaches to achieving the highly efficient and selective CO2RR. In this review, we first illustrate the key steps for CO2RR to ethylene (e.g., CO2 adsorption/activation, formation of *CO intermediate, C-C coupling step), offering mechanistic understanding of CO2RR conversion to ethylene. Then the alternative reaction pathways and conditions for the formation of ethylene and competitive products (C1 and other C2+ products) are investigated, guiding the further design and development of preferred conditions for ethylene generation. Engineering strategies of Cu-based catalysts for CO2RR-ethylene are further summarized, and the correlations of reaction mechanism/pathways, engineering strategies and selectivity are elaborated. Finally, major challenges and perspectives in the research area of CO2RR are proposed for future development and practical applications.

Direct observation of bicarbonate and water reduction on gold: understanding the potential dependent proton source during hydrogen evolution

The electrochemical conversion of CO2 represents a promising way to simultaneously reduce CO2 emissions and store chemical energy. However, the competition between CO2 reduction (CO2R) and the H2 evolution reaction (HER) hinders the efficient conversion of CO2 in aqueous solution. In water, CO2 is in dynamic equilibrium with H2CO3, HCO3 -, and CO3 2-. While CO2 and its associated carbonate species represent carbon sources for CO2R, recent studies by Koper and co-workers indicate that H2CO3 and HCO3 - also act as proton sources during HER (J. Am. Chem. Soc. 2020, 142, 4154-4161, ACS Catal. 2021, 11, 4936-4945, J. Catal. 2022, 405, 346-354), which can favorably compete with water at certain potentials. However, accurately distinguishing between competing reaction mechanisms as a function of potential requires direct observation of the non-equilibrium product distribution present at the electrode/electrolyte interface. In this study, we employ vibrational sum frequency generation (VSFG) spectroscopy to directly probe the interfacial species produced during competing HER/CO2R on Au electrodes. The vibrational spectra at the Ar-purged Na2SO4 solution/Au interface, where only HER occurs, show a strong peak around 3650 cm-1, which appears at the HER onset potential and is assigned to OH-. Notably, this species is absent for the CO2-purged Na2SO4 solution/gold interface; instead, a peak around 3400 cm-1 appears at catalytic potential, which is assigned to CO3 2- in the electrochemical double layer. These spectral reporters allow us to differentiate between HER mechanisms based on water reduction (OH- product) and HCO3 - reduction (CO3 2- product). Monitoring the relative intensities of these features as a function of potential in NaHCO3 electrolyte reveals that the proton donor switches from HCO3 - at low overpotential to H2O at higher overpotential. This work represents the first direct detection of OH- on a metal electrode produced during HER and provides important insights into the surface reactions that mediate selectivity between HER and CO2R in aqueous solution.

Electrochemical synthesis of propylene from carbon dioxide on copper nanocrystals

The conversion of carbon dioxide to value-added products using renewable electricity would potentially help to address current climate concerns. The electrochemical reduction of carbon dioxide to propylene, a critical feedstock, requires multiple C-C coupling steps with the transfer of 18 electrons per propylene molecule, and hence is kinetically sluggish. Here we present the electrosynthesis of propylene from carbon dioxide on copper nanocrystals with a peak geometric current density of -5.5 mA cm^-2. The metallic copper nanocrystals formed from CuCl precursor present preponderant Cu(100) and Cu(111) facets, likely to favour the adsorption of key *C₁ and *C₂ intermediates. Strikingly, the production rate of propylene drops substantially when carbon monoxide is used as the reactant. From the electrochemical reduction of isotope-labelled carbon dioxide mixed with carbon monoxide, we infer that the key step for propylene formation is probably the coupling between adsorbed/molecular carbon dioxide or carboxyl with the *C₂ intermediates that are involved in the ethylene pathway.

Operando characterization of continuous flow CO2 electrolyzers: current status and future prospects

The performance of continuous-flow CO₂ electrolyzers has substantially increased in recent years, achieving current density and selectivity (particularly for CO production) meeting the industrial targets. Further improvement is, however, necessary in terms of stability and energy efficiency, as well as in high-value multicarbon product formation. Accelerating this process requires deeper understanding of the complex interplay of chemical-physical processes taking place in CO₂ electrolyzer cells. Operando characterization can provide these insights under working conditions, helping to identify the reasons for performance losses. Despite this fact, only relatively few studies have taken advantage of such methods up to now, applying operando techniques to characterize practically relevant CO₂ electrolyzers. These studies include X-ray absorption- and Raman spectroscopy, fluorescent microscopy, scanning probe techniques, mass spectrometry, and radiography. Their objective was to characterize the catalyst structure, its microenviroment, membrane properties, etc., and relate them to the device performance (reaction rates and product distribution). Here we review the current state-of-the-art of operando methods, associated challenges, and also their future potential. We aim to motivate researchers to perform operando characterization in continuous-flow CO₂ electrolyzers, to understand the reaction mechanism and device operation under practically relevant conditions, thereby advancing the field towards industrialization.

Interfacial pH measurements during CO2 reduction on gold using a rotating ring-disk electrode

Insights into how to control the activity and selectivity of the electrochemical CO₂ reduction reaction are still limited because of insufficient knowledge of the reaction mechanism and kinetics, which is partially due to the lack of information on the interfacial pH, an important parameter for proton-coupled reactions like CO₂ reduction. Here, we used a reliable and sensitive pH sensor combined with the rotating ring-disk electrode technique, in which a functionalized Au ring electrode works as a real-time detector of the OH^- generated during the CO₂ reduction reaction at a gold disk electrode. Variations of the interfacial pH due to both electrochemical and homogeneous reactions are mapped and the correlation of the interfacial pH with these reactions is inferred. The interfacial pH near the disk electrode increases from 7 to 12 with increasing current density, with a sharp increase at around -0.5 V vs. RHE, which indicates a change of the dominant buffering species. Through scan rate-dependent voltammetry and chronopotentiometry experiments, the homogenous reactions are shown to reach equilibrium within the time scale of the pH measurements, so that the interfacial concentrations of different carbonaceous species can be calculated using equilibrium constants. Furthermore, pH measurements were also performed under different conditions to disentangle the relationship between the interfacial pH and other electrolyte effects. The buffer effect of alkali metal cations is confirmed, showing that weakly hydrated cations lead to less pronounced pH gradients. Finally, we probe to which extent increasing mass transport and the electrolyte buffer capacity can aid in suppressing the increase of the interfacial pH, showing that the buffer capacity is the dominant factor in suppressing interfacial pH variations.

Cation-Coordinated Inner-Sphere CO2 Electroreduction at Au-Water Interfaces

Electrochemical CO₂ reduction reaction (CO₂RR) is a promising technology for the clean energy economy. Numerous efforts have been devoted to enhancing the mechanistic understanding of CO₂RR from both experimental and theoretical studies. Electrolyte ions are critical for the CO₂RR; however, the role of alkali metal cations is highly controversial, and a complete free energy diagram of CO₂RR at Au-water interfaces is still missing. Here, we provide a systematic mechanism study toward CO₂RR via ab initio molecular dynamics simulations integrated with the slow-growth sampling (SG-AIMD) method. By using the SG-AIMD approach, we demonstrate that CO₂RR is facile at the inner-sphere interface in the presence of K cations, which promote the CO₂ activation with the free energy barrier of only 0.66 eV. Furthermore, the competitive hydrogen evolution reaction (HER) is inhibited by the interfacial cations with the induced kinetic blockage effect, where the rate-limiting Volmer step shows a much higher energy barrier (1.27 eV). Eventually, a comprehensive free energy diagram including both kinetics and thermodynamics of the CO₂RR to CO and the HER at the electrochemical interface is derived, which illustrates the critical role of cations on the overall performance of CO₂ electroreduction by facilitating CO₂ adsorption while suppressing the hydrogen evolution at the same time.

Electric Double Layer: The Good, the Bad, and the Beauty

The electric double layer (EDL) is the most important region for electrochemical and heterogeneous catalysis. Because of it, its modeling and investigation are something that can be found in the literature for a long time. However, nowadays, it is still a hot topic of investigation, mainly because of the improvement in simulation and experimental techniques. The present review aims to present the classical models for the EDL, as well as presenting how this region affects electrochemical data in everyday experimentation, how to obtain and interpret information about EDL, and, finally, how to obtain some molecular point of view insights on it.

Enhanced Electrochemical CO2 Reduction to Formate on Poly(4-vinylpyridine)-Modified Copper and Gold Electrodes

Developing active and selective catalysts that convert CO₂ into valuable products remains a critical challenge for further application of the electrochemical CO₂ reduction reaction (CO₂RR). Catalytic tuning with organic additives/films has emerged as a promising strategy to tune CO₂RR activity and selectivity. Herein, we report a facile method to significantly change CO₂RR selectivity and activity of copper and gold electrodes. We found improved selectivity toward HCOOH at low overpotentials on both polycrystalline Cu and Au electrodes after chemical modification with a poly(4-vinylpyridine) (P4VP) layer. In situ attenuated total reflection surface-enhanced infrared reflection-adsorption spectroscopy and contact angle measurements indicate that the hydrophobic nature of the P4VP layer limits mass transport of HCO₃^- and H₂O, whereas it has little influence on CO₂ mass transport. Moreover, the early onset of HCOOH formation and the enhanced formation of HCOOH over CO suggest that P4VP modification promotes a surface hydride mechanism for HCOOH formation on both electrodes.

Investigating the role of potassium cations during electrochemical CO2 reduction

The specific identity of electrolyte cations has many implications in various electrochemical reactions. However, the exact mechanism by which cations affect electrochemical reactions is not agreed upon in the literature. In this report, we investigate the role of cations during the electrochemical reduction of CO₂ by chelating the cations with cryptands, to change the interaction of the cations with the components of the electric double layer. As previously reported we do see the apparent suppression of CO₂ reduction in the absence of cations. However, using in situ-SEIRAS we see that CO₂ reduction does indeed take place albeit at very reduced scales. We also observe that cations play a role in tuning the absorption strengths of not only CO₂ as has been speculated, but also that of reaction products such as CO.

Electrolyte Effects on CO2 Electrochemical Reduction to CO

The electrochemical reduction of CO₂ (CO2RR) constitutes an alternative to fossil fuel-based technologies for the production of fuels and commodity chemicals. Yet the application of CO2RR electrolyzers is hampered by low energy and Faradaic efficiencies. Concomitant electrochemical reactions, like hydrogen evolution (HER), lower the selectivity, while the conversion of CO₂ into (bi)carbonate through solution acid–base reactions induces an additional concentration overpotential. During CO2RR in aqueous media, the local pH becomes more alkaline than the bulk causing an additional consumption of CO₂ by the homogeneous reactions. The latter effect, in combination with the low solubility of CO₂ in aqueous electrolytes (33 mM), leads to a significant depletion in CO₂ concentration at the electrode surface.

The nature of the electrolyte, in terms of pH and cation identity, has recently emerged as an important factor to tune both the energy and Faradaic efficiency. In this Account, we summarize the recent advances in understanding electrolyte effects on CO2RR to CO in aqueous solutions, which is the first, and crucial, step to further reduced products. To compare literature findings in a meaningful way, we focus on results reported under well-defined mass transport conditions and using online analytical techniques. The discussion covers the molecular-level understanding of the effects of the proton donor, in terms of the suppression of the CO₂ gradient vs enhancement of HER at a given mass transport rate and of the cation, which is crucial in enabling both CO2RR and HER. These mechanistic insights are then translated into possible implications for industrially relevant cell geometries and current densities.

Interactions of CO2 Anion Radicals with Electrolyte Environments from First-Principles Simulations

Successful transformation of carbon dioxide (CO2) into value-added products is of great interest, as it contributes in part to the circular carbon economy. Understanding chemical interactions that stabilize crucial reaction intermediates of CO2 is important, and in this contribution, we employ atom centered density matrix propagation (ADMP) molecular dynamics simulations to investigate interactions between CO2 - anion radicals with surrounding solvent molecules and electrolyte cations in both aqueous and nonaqueous environments. We show how different cations and solvents affect the stability of the CO2 - anion radical by examining its angle and distance to a coordinating cation in molecular dynamics simulations. We identify that the strength of CO2 - interactions can be tailored through choosing an appropriate cation and solvent combination. We anticipate that this fundamental understanding of cation/solvent interactions can facilitate the optimization of a chemical pathway that results from selective stabilization of a crucial reaction intermediate.

Quantifying local pH changes in carbonate electrolyte during copper-catalysed [Formula: see text] electroreduction using in operando [Formula: see text] NMR

The electrochemical carbon dioxide reduction on copper attracted considerable attention within the last decade, since Cu is the only elemental transition metal that catalyses the formation of short-chain hydrocarbons and alcohols. Research in this field is mainly focused on understanding the reaction mechanism in terms of adsorbates and intermediates. Furthermore, dynamic changes in the micro-environment of the catalyst, i.e. local pH and [Formula: see text] concentration values, play an equivalently important role in the selectivity of product formation. In this study, we present an in operando [Formula: see text] nuclear magnetic resonance technique that enables the simultaneous measurement of pH and [Formula: see text] concentration in electrode vicinity during electroreduction. The influence of applied potential and buffer capacity of the electrolyte on the formation of formate is demonstrated. Theoretical considerations are confirmed experimentally and the importance of the interplay between catalyst and electrolyte is emphasised.

Modeling Operando Electrochemical CO2 Reduction

Since the seminal works on the application of density functional theory and the computational hydrogen electrode to electrochemical CO₂ reduction (eCO₂R) and hydrogen evolution (HER), the modeling of both reactions has quickly evolved for the last two decades. Formulation of thermodynamic and kinetic linear scaling relationships for key intermediates on crystalline materials have led to the definition of activity volcano plots, overpotential diagrams, and full exploitation of these theoretical outcomes at laboratory scale. However, recent studies hint at the role of morphological changes and short-lived intermediates in ruling the catalytic performance under operating conditions, further raising the bar for the modeling of electrocatalytic systems. Here, we highlight some novel methodological approaches employed to address eCO₂R and HER reactions. Moving from the atomic scale to the bulk electrolyte, we first show how ab initio and machine learning methodologies can partially reproduce surface reconstruction under operation, thus identifying active sites and reaction mechanisms if coupled with microkinetic modeling. Later, we introduce the potential of density functional theory and machine learning to interpret data from Operando spectroelectrochemical techniques, such as Raman spectroscopy and extended X-ray absorption fine structure characterization. Next, we review the role of electrolyte and mass transport effects. Finally, we suggest further challenges for computational modeling in the near future as well as our perspective on the directions to follow.

The Solvation-Induced Onsager Reaction Field Rather than the Double-Layer Field Controls CO2 Reduction on Gold

The selectivity and activity of the carbon dioxide reduction (CO₂R) reaction are sensitive functions of the electrolyte cation. By measuring the vibrational Stark shift of in situ-generated CO on Au in the presence of alkali cations, we quantify the total electric field present at catalytic active sites and deconvolute this field into contributions from (1) the electrochemical Stern layer and (2) the Onsager (or solvation-induced) reaction field. Contrary to recent theoretical reports, the CO₂R kinetics does not depend on the Stern field but instead is closely correlated with the strength of the Onsager reaction field. These results show that in the presence of adsorbed (bent) CO₂, the Onsager field greatly exceeds the Stern field and is primarily responsible for CO₂ activation. Additional measurements of the cation-dependent water spectra using vibrational sum frequency generation spectroscopy show that interfacial solvation strongly influences the CO₂R activity. These combined results confirm that the cation-dependent interfacial water structure and its associated electric field must be explicitly considered for accurate understanding of CO₂R reaction kinetics.

Theories for Electrolyte Effects in CO2 Electroreduction

Electrochemical CO₂ reduction (eCO₂R) enables the conversion of waste CO₂ to high-value fuels and commodity chemicals powered by renewable electricity, thereby offering a viable strategy for reaching the goal of net-zero carbon emissions. Research in the past few decades has focused both on the optimization of the catalyst (electrode) and the electrolyte environment. Surface-area normalized current densities show that the latter can affect the CO₂ reduction activity by up to a few orders of magnitude.In this Account, we review theories of the mechanisms behind the effects of the electrolyte (cations, anions, and the electrolyte pH) on eCO₂R. As summarized in the conspectus graphic, the electrolyte influences eCO₂R activity via a field (ε) effect on dipolar (μ) reaction intermediates, changing the proton donor for the multi-step proton-electron transfer reaction, specifically adsorbed anions on the catalyst surface to block active sites, and tuning the local environment by homogeneous reactions. To be specific, alkali metal cations (M⁺) can stabilize reaction intermediates via electrostatic interactions with dipolar intermediates or buffer the interfacial pH via hydrolysis reactions, thereby promoting the eCO₂R activity with the following trend in hydrated size (corresponding to the local field strength ε)/hydrolysis ability: Cs⁺ > K⁺ > Na⁺ > Li⁺. The effect of the electrolyte pH can give a change in eCO₂R activity of up to several orders of magnitude, arising from linearly shifting the absolute interfacial field via the relationship U_SHE = U_RHE - (2.3k_BT)pH, homogeneous reactions between OH^- and desorbed intermediates, or changing the proton donor from hydronium to water along with increasing pH. Anions have been suggested to affect the eCO₂R reaction process by solution-phase reactions (e.g., buffer reactions to tune local pH), acting as proton donors or as a surface poison.So far, the existing models of electrolyte effects have been used to rationalize various experimentally observed trends, having yet to demonstrate general predictive capabilities. The major challenges in our understanding of the electrolyte effect in eCO₂R are (i) the long time scale associated with a dynamic ab initio picture of the catalyst|electrolyte interface and (ii) the overall activity determined by the length-scale interplay of intrinsic microkinetics, homogeneous reactions, and mass transport limitations. New developments in ab initio dynamic models and coupling the effects of mass transport can provide a more accurate view of the structure and intrinsic functions of the electrode-electrolyte interface and the corresponding reaction energetics toward comprehensive and predictive models for electrolyte design.

The Role of Cation Acidity on the Competition between Hydrogen Evolution and CO2 Reduction on Gold Electrodes

CO₂ electroreduction (CO₂RR) is a sustainable alternative for producing fuels and chemicals. Metal cations in the electrolyte have a strong impact on the reaction, but mainly alkali species have been studied in detail. In this work, we elucidate how multivalent cations (Li⁺, Cs⁺, Be²⁺, Mg²⁺, Ca²⁺, Ba²⁺, Al³⁺, Nd³⁺, and Ce³⁺) affect CO₂RR and the competing hydrogen evolution by studying these reactions on polycrystalline gold at pH = 3. We observe that cations have no effect on proton reduction at low overpotentials, but at alkaline surface pH acidic cations undergo hydrolysis, generating a second proton reduction regime. The activity and onset for the water reduction reaction correlate with cation acidity, with weakly hydrated trivalent species leading to the highest activity. Acidic cations only favor CO₂RR at low overpotentials and in acidic media. At high overpotentials, the activity for CO increases in the order Ca²⁺ < Li⁺ < Ba²⁺ < Cs⁺. To favor this reaction there must be an interplay between cation stabilization of the *CO₂^– intermediate, cation accumulation at the outer Helmholtz plane (OHP), and activity for water reduction. Ab initio molecular dynamics simulations with explicit electric field show that nonacidic cations show lower repulsion at the interface, accumulating more at the OHP, thus triggering local promoting effects. Water dissociation kinetics is increasingly promoted by strongly acidic cations (Nd³⁺, Al³⁺), in agreement with experimental evidence. Cs⁺, Ba²⁺, and Nd³⁺ coordinate to adsorbed CO₂ steadily; thus they enable *CO₂^– stabilization and barrierless protonation to COOH and further reduction products.

Time-Resolved Local pH Measurements during CO2 Reduction Using Scanning Electrochemical Microscopy: Buffering and Tip Effects

The electrochemical reduction of CO₂ is widely studied as a sustainable alternative for the production of fuels and chemicals. The electrolyte's bulk pH and composition play an important role in the reaction activity and selectivity and can affect the extent of the buildup of pH gradients between the electrode surface and the bulk of the electrolyte. Quantifying the local pH and how it is affected by the solution species is desirable to gain a better understanding of the CO₂ reduction reaction. Local pH measurements can be realized using Scanning Electrochemical Microscopy (SECM); however, finding a pH probe that is stable and selective under CO₂ reduction reaction conditions is challenging. Here, we have used our recently developed voltammetric pH sensor to perform pH measurements in the diffusion layer during CO₂ reduction using SECM, with high time resolution. Using a 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) functionalized gold ultramicroelectrode, we compare the local pH developed above a gold substrate in an argon atmosphere, when only hydrogen evolution is taking place, to the pH developed in a CO₂ atmosphere. The pH is monitored at a fixed distance from the surface, and the sample potential is varied in time. In argon, we observe a gradual increase of pH, while a plateau region is present in CO₂ atmosphere due to the formation of HCO₃ ^- buffering the reaction interface. By analyzing the diffusion layer dynamics once the sample reaction is turned "off", we gain insightful information on the time scale of the homogeneous reactions happening in solution and on the time required for the diffusion layer to fully recover to the initial bulk concentration of species. In order to account for the effect of the presence of the SECM tip on the measured pH, we performed finite element method simulations of the fluid and reaction dynamics. The results show the significant localized diffusion hindrance caused by the tip, so that in its absence, the pH values are more acidic than when the tip is present. Nonetheless, through the simulation, we can account for this effect and estimate the real local pH values across the diffusion layer.