Theories for Electrolyte Effects in CO2 Electroreduction

Unveiled the Structure-Selectivity Relationship for Carbon Dioxide Reduction Triggered by Bi-Doped Cu-Based Nanocatalysts

To investigate synergistic effect between geometric and electronic structures on directing CO2RR selectivity, water phase synthetic protocol and surface architecture engineering strategy are developed to construct monodispersed Bi-doped Cu-based nanocatalysts. The strongly correlated catalytic directionality and Bi3+ dopant can be rationalized by the regulation of [*COOH]/[*CO] adsorption capacities through the appropriate doping of Bi3+ electronic modulator, resulting in volcano relationship between FECO/TOFCO and surface EVBM values. Spectroscopic study reveals that the dual-site binding mode ([Cu─μ─C(═O)O─Bi3+]) enabled by Cu1Bi3+ 2 motif in single-phase Cu150Bi1 nanocatalyst drives CO2-to-CO conversion. In contrast, the study of dynamic Bi speciation and phase transformation in dual-phase Cu50Bi1 nanocatalyst unveils that the Bi0-Bi0 contribution emerges at the expense of BOC phase, suggesting metallic Bi0 phase acting as [H]˙ formation center switches CO2RR selectivity toward CO2-to-HCOO- conversion via [*OCHO] and [*OCHOK] intermediates. This work provides significant insight into how geometric architecture cooperates with electronic effect and catalytic motif/phase to guide the selectivity of electrocatalytic CO2 reduction through the distinct surface-bound intermediates and presents molecular-level understanding of catalytic mechanism for CO/HCOO- formation.

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.

Computational electrocatalysis beyond conventional hydrogen electrode model: CO2 reduction to C2 species on copper facilitated by dynamically formed solvent halide ions at the solid-liquid interface

The reduction of CO2 into value-added chemicals and fuels has been actively studied as a promising strategy for mitigating carbon dioxide emissions. However, the dilemma for the experimentalist in choosing an appropriate reaction medium and neglecting the effect of solvent ions when using a simple thermochemical model, normally leads to the disagreement between experimental observations and theoretical calculations. In this work, by considering the effects of both the anion and cation, a more realistic CO2 reduction environment at the solid-liquid interface between copper and solvent ions has been systematically studied by using ab initio molecular dynamics and density functional theory. We revealed that the co-occurrence of alkali ions (K+) and halide ions (F-, Cl-, Br-, and I-) in the electric double layer (EDL) can enhance the adsorption of CO2 by more than 0.45 eV compared to that in pure water, and the calculated energy barrier for CO-CO coupling also decreases 0.32 eV in the presence of I ion on a negatively charged copper electrode. The hydrated ions can modulate the distribution of the charge near the solid-liquid interface, which significantly promotes CO2 reduction and meanwhile impedes the hydrogen evolution reaction. Therefore, our work unveils the significant role of halide ions at the electrode-electrolyte interface for promoting CO2 reduction on copper electrode.

Exploring how cation entropy influences electric double layer formation and electrochemical reactivity

Electric double layers are crucial to energy storage and electrocatalytic device performance. While double layer formation originates in electrostatic interactions, electric double layer properties are governed by a balance of both electrostatic and entropic driving forces. Favorable ion-surface electrostatic interactions attract counterions to charged surfaces to compensate, or "screen," potentials, but the confinement of these same ions from a bulk reservoir to the interface incurs an entropic penalty. Here, we use a dicationic imidazolium ionic liquid and its monovalent analogue to explore how cation valence and entropy influence double layer formation and electrochemical reactivity using CO2 electroreduction as a model reaction. We find that divalent and monovalent cations display similar CO2 reduction kinetics but differ vastly in steady-state reactivity due to rapid electrochemically induced precipitation of insulating dicationic (bi)carbonate films. Using in situ surface-enhanced Raman scattering spectroscopy, we find that potential-dependent cation reorientation occurs at similar potentials between the two ionic liquids, but the introduction of a covalent link in the divalent cation imparts a more ordered double layer structure that favors (bi)carbonate precipitation. In mixed monovalent-divalent electrolytes, we find that the divalent cations dominate interfacial properties by preferentially accumulating at surfaces even at very low relative concentrations. Our findings confirm that ion entropy plays a key role in modulating local electrochemical environments. Furthermore, we highlight how double layer properties are sensitive to the properties of counterions that pay the lowest entropic penalty to accumulate at interfaces. Overall, we illustrate that ion entropy provides a new knob to tune reaction microenvironments and unveil how entropy plays a major role in modulating electrochemical reactivity in mixed ion electrolytes.

Impact of Anodic Oxidation Reactions in the Performance Evaluation of High-Rate CO2 /CO Electrolysis

The membrane-electrode assembly (MEA) approach appears to be the most promising technique to realize the high-rate CO2 /CO electrolysis, however there are major challenges related to the crossover of ions and liquid products from cathode to anode via the membrane and the concomitant anodic oxidation reactions (AORs). In this perspective, by combining experimental and theoretical analyses, several impacts of anodic oxidation of liquid products in terms of performance evaluation are investigated. First, the crossover behavior of several typical liquid products through an anion-exchange membrane is analyzed. Subsequently, two instructive examples (introducing formate or ethanol oxidation during electrolysis) reveals that the dynamic change of the anolyte (i.e., pH and composition) not only brings a slight shift of anodic potentials (i.e., change of competing reactions), but also affects the chemical stability of the anode catalyst. Anodic oxidation of liquid products can also cause either over- or under-estimation of the Faradaic efficiency, leading to an inaccurate assessment of overall performance. To comprehensively understand fundamentals of AORs, a theoretical guideline with hierarchical indicators is further developed to predict and regulate the possible AORs in an electrolyzer. The perspective concludes by giving some suggestions on rigorous performance evaluations for high-rate CO2 /CO electrolysis in an MEA-based setup.

Cation-induced changes in the inner- and outer-sphere mechanisms of electrocatalytic CO2 reduction

The underlying mechanism of cation effects on CO2RR remains debated. Herein, we study cation effects by simulating both outer-sphere electron transfer (OS-ET) and inner-sphere electron transfer (IS-ET) pathways during CO2RR via constrained density functional theory molecular dynamics (cDFT-MD) and slow-growth DFT-MD (SG-DFT-MD), respectively. Our results show without any cations, only OS-ET is feasible with a barrier of 1.21 eV. In the presence of K+ (Li+), OS-ET shows a very high barrier of 2.93 eV (4.15 eV) thus being prohibited. However, cations promote CO2 activation through IS-ET with the barrier of only 0.61 eV (K+) and 0.91 eV (Li+), generating the key intermediate (adsorbed CO[Formula: see text]). Without cations, CO2-to-CO[Formula: see text](ads) conversion cannot proceed. Our findings reveal cation effects arise from short-range Coulomb interactions with reaction intermediates. These results disclose that cations modulate the inner- and outer-sphere pathways of CO2RR, offering substantial insights on the cation specificity in the initial CO2RR steps.

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.

InBi Bimetallic Sites for Efficient Electrochemical Reduction of CO2 to HCOOH

Formic acid is receiving intensive attention as being one of the most progressive chemical fuels for the electrochemical reduction of carbon dioxide. However, the majority of catalysts suffer from low current density and Faraday efficiency. To this end, an efficient catalyst of In/Bi-750 with InOx nanodots load is prepared on a two-dimensional nanoflake Bi2 O2 CO3 substrate, which increases the adsorption of * CO2 due to the synergistic interaction between the bimetals and the exposure of sufficient active sites. In the H-type electrolytic cell, the formate Faraday efficiency (FE) reaches 97.17% at -1.0 V (vs reversible hydrogen electrode (RHE)) with no significant decay over 48 h. A formate Faraday efficiency of 90.83% is also obtained in the flow cell at a higher current density of 200 mA cm-2 . Both in-situ Fourier transform infrared spectroscopy (FT-IR) and theoretical calculations show that the BiIn bimetallic site can deliver superior binding energy to the * OCHO intermediate, thereby fundamentally accelerating the conversion of CO2 to HCOOH. Furthermore, assembled Zn-CO2 cell exhibits a maximum power of 6.97 mW cm-1 and a stability of 60 h.

Molecular Engineering of Cation Solvation Structure for Highly Selective Carbon Dioxide Electroreduction

Balancing the activation of H2 O is crucial for highly selective CO2 electroreduction (CO2 RR), as the protonation steps of CO2 RR require fast H2 O dissociation kinetics, while suppressing hydrogen evolution (HER) demands slow H2 O reduction. We herein proposed one molecular engineering strategy to regulate the H2 O activation using aprotic organic small molecules with high Gutmann donor number as a solvation shell regulator. These organic molecules occupy the first solvation shell of K+ and accumulate in the electrical double layer, decreasing the H2 O density at the interface and the relative content of proton suppliers (free and coordinated H2 O), suppressing the HER. The adsorbed H2 O was stabilized via the second sphere effect and its dissociation was promoted by weakening the O-H bond, which accelerates the subsequent *CO2 protonation kinetics and reduces the energy barrier. In the model electrolyte containing 5 M dimethyl sulfoxide (DMSO) as an additive (KCl-DMSO-5), the highest CO selectivity over Ag foil increased to 99.2 %, with FECO higher than 90.0 % within -0.75 to -1.15 V (vs. RHE). This molecular engineering strategy for cation solvation shell can be extended to other metal electrodes, such as Zn and Sn, and organic molecules like N,N-dimethylformamide.

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.

Unintended cation crossover influences CO2 reduction selectivity in Cu-based zero-gap electrolysers

Membrane electrode assemblies enable CO₂ electrolysis at industrially relevant rates, yet their operational stability is often limited by formation of solid precipitates in the cathode pores, triggered by cation crossover from the anolyte due to imperfect ion exclusion by anion exchange membranes. Here we show that anolyte concentration affects the degree of cation movement through the membranes, and this substantially influences the behaviors of copper catalysts in catholyte-free CO₂ electrolysers. Systematic variation of the anolyte (KOH or KHCO₃) ionic strength produced a distinct switch in selectivity between either predominantly CO or C₂₊ products (mainly C₂H₄) which closely correlated with the quantity of alkali metal cation (K⁺) crossover, suggesting cations play a key role in C-C coupling reaction pathways even in cells without discrete liquid catholytes. Operando X-ray absorption and quasi in situ X-ray photoelectron spectroscopy revealed that the Cu surface speciation showed a strong dependence on the anolyte concentration, wherein dilute anolytes resulted in a mixture of Cu⁺ and Cu⁰ surface species, while concentrated anolytes led to exclusively Cu⁰ under similar testing conditions. These results show that even in catholyte-free cells, cation effects (including unintentional ones) significantly influence reaction pathways, important to consider in future development of catalysts and devices.

Elucidating the Roles of Nafion/Solvent Formulations in Copper-Catalyzed CO2 Electrolysis

Nafion ionomer, composed of hydrophobic perfluorocarbon backbones and hydrophilic sulfonic acid side chains, is the most widely used additive for preparing catalyst layers (CLs) for electrochemical CO₂ reduction, but its impact on the performance of CO₂ electrolysis remains poorly understood. Here, we systematically investigate the role of the catalyst ink formulation on CO₂ electrolysis using commercial CuO nanoparticles as the model pre-catalyst. We find that the presence of Nafion is essential for achieving stable product distributions due to its ability to stabilize the catalyst morphology under reaction conditions. Moreover, the Nafion content and solvent composition (water/alcohol fraction) regulate the internal structure of Nafion coatings, as well as the catalyst morphology, thereby significantly impacting CO₂ electrolysis performance, resulting in variations of C₂₊ product Faradaic efficiency (FE) by >3×, with C₂₊ FE ranging from 17 to 54% on carbon paper substrates. Using a combination of ellipsometry and in situ Raman spectroscopy during CO₂ reduction, we find that such selectivity differences stem from changes to the local reaction microenvironment. In particular, the combination of high water/alcohol ratios and low Nafion fractions in the catalyst ink results in stable and favorable microenvironments, increasing the local CO₂/H₂O concentration ratio and promoting high CO surface coverage to facilitate C₂₊ production in long-term CO₂ electrolysis. Therefore, this work provides insights into the critical role of Nafion binders and underlines the importance of optimizing Nafion/solvent formulations as a means of enhancing the performance of electrochemical CO₂ reduction systems.

Optical method for quantifying the potential of zero charge at the platinum-water electrochemical interface

When an electrode contacts an electrolyte, an interfacial electric field forms. This interfacial field can polarize the electrode's surface and nearby molecules, but its effect can be countered by an applied potential. Quantifying the value of this countering potential ('potential of zero charge' (pzc)) is, however, not straightforward. Here we present an optical method for determining the pzc at an electrochemical interface. Our approach uses phase-sensitive second-harmonic generation to determine the electrochemical potential where the interfacial electric field vanishes at an electrode-electrolyte interface with Pt-water as a model experiment. Our method reveals that the pzc of the Pt-water interface is 0.23 ± 0.08 V versus standard hydrogen electrode (SHE) and is pH independent from pH 1 to pH 13. First-principles calculations with a hybrid explicit-implicit solvent model predict the pzc of the Pt(111)-water interface to be 0.23 V versus SHE and reveal how the interfacial water structure rearranges as the electrode potential is moved above and below the pzc. We further show that pzc is sensitive to surface modification; deposition of Ni on Pt shifts the interfacial pzc in the cathodic direction by ~360 mV. Our work demonstrates a materials-agnostic approach for quantifying the interfacial electrical field and water orientation at an electrochemical interface without requiring probe molecules and confirms the long-held view that the interfacial electric field is more intense during hydrogen electrocatalysis in alkaline than in acid.

Proton Transfer Dynamics-Mediated CO2 Electroreduction

Electrochemical CO₂ reduction reaction (CO₂ RR) is crucial to addressing environmental crises and producing chemicals. Proton activation and transfer are essential in CO₂ RR. To date, few research reviews have focused on this process and its effect on catalytic performance. Recent studies have demonstrated ways to improve CO₂ RR by regulating proton transfer dynamics. This Concept highlights the use of regulating proton transfer dynamics to enhance CO₂ RR for the target product and discusses modulation strategies for proton transfer dynamics and operative mechanisms in typical systems, including single-atom catalysts, molecular catalysts, metal heterointerfaces, and organic-ligand modified metal catalysts. Characterization methods for proton transfer dynamics during CO₂ RR are also discussed, providing powerful tools for the hydrogen-involving electrochemical study. This Concept offers new insights into the CO₂ RR mechanism and guides the design of efficient CO₂ RR systems.

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.

Enriching Surface-Accessible CO2 in the Zero-Gap Anion-Exchange-Membrane-Based CO2 Electrolyzer

Zero-gap anion exchange membrane (AEM)-based CO₂ electrolysis is a promising technology for CO production, however, their performance at elevated current densities still suffers from the low local CO₂ concentration due to heavy CO₂ neutralization. Herein, via modulating the CO₂ feed mode and quantitative analyzing CO₂ utilization with the aid of mass transport modeling, we develop a descriptor denoted as the surface-accessible CO₂ concentration ([CO₂ ]^SA ), which enables us to indicate the transient state of the local [CO₂ ]/[OH^- ] ratio and helps define the limits of CO₂ -to-CO conversion. To enrich the [CO₂ ]^SA , we developed three general strategies: (1) increasing catalyst layer thickness, (2) elevating CO₂ pressure, and (3) applying a pulsed electrochemical (PE) method. Notably, an optimized PE method allows to keep the [CO₂ ]^SA at a high level by utilizing the dynamic balance period of CO₂ neutralization. A maximum j_CO of 368±28 mA cm_geo ^-2 was achieved using a commercial silver catalyst.

Modeling stepped Pt/water interfaces at potential of zero charge with ab initio molecular dynamics

It is worthwhile to understand the potentials of zero charge (PZCs) and structures of stepped metal/water interfaces, because for many electrocatalytic reactions stepped surfaces are more active than atomically flat surfaces. Herein, a series of stepped Pt/water interfaces are modeled at different step densities with ab initio molecular dynamics (AIMD). It is found that the structures of Pt/water interfaces are significantly influenced by the step density, particularly for the distribution of chemisorbed water. The step sites of metal surfaces are more preferred for water chemisorption than the terrace sites, and until the step density is very low, water will chemisorb on the terrace. In addition, it is revealed that the PZCs of stepped Pt/water interfaces are generally smaller than that of Pt(111), and the difference is mainly attributed to the difference in the work function, providing a simple way to estimate the PZCs of stepped metal surfaces. Finally, it is interesting to see that the Volta potential difference is almost same for Pt/water interfaces with different step densities, although their interface structures and magnitude of charge transfer clearly differ.

Recent advancements in visible-light-driven carboxylation with carbon dioxide

Carbon dioxide as a classic C1 source has long been investigated in organic synthetic chemistry. Diverse catalytic methods for CO2 activation were reported in the past several decades. In this...