Hydroxide Is Not a Promoter of C2+ Product Formation in the Electrochemical Reduction of CO on Copper

Recent advances in microenvironment regulation for electrocatalysis

High-efficiency electrocatalysis could serve as the bridge that connects renewable energy technologies, hydrogen economy and carbon capture/utilization, promising a sustainable future for humankind. It is therefore of paramount significance to explore feasible strategies to modulate the relevant electrocatalytic reactions and optimize device performances so as to promote their large-scale practical applications. Microenvironment regulation at the catalytic interface has been demonstrated to be capable of effectively enhancing the reaction rates and improving the selectivities for specific products. In this review we summarize the latest advances in microenvironment regulation in typical electrocatalytic processes (including water electrolysis, hydrogen-oxygen fuel cells, and carbon dioxide reduction) and the related in situ/operando characterization techniques and theoretical simulation methods. At the end of this article, we present an outlook on development trends and possible future directions.

Electrochemical CO2 Reduction in Acidic Electrolytes: Spectroscopic Evidence for Local pH Gradients

Inspired by recent advances in electrochemical CO2 reduction (CO2R) under acidic conditions, herein we leverage in situ spectroscopy to inform the optimization of CO2R at low pH. Using attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and fluorescent confocal laser scanning microscopy, we investigate the role that alkali cations (M+) play on electrochemical CO2R. This study hence provides important information related to the local electrode surface pH under bulk acidic conditions for CO2R, both in the presence and absence of an organic film layer, at variable [M+]. We show that in an acidic electrolyte, an appropriate current density can enable CO2R in the absence of metal cations. In situ local pH measurements suggest the local [H+] must be sufficiently depleted to promote H2O reduction as the competing reaction with CO2R. Incrementally incorporating [K+] leads to increases in the local pH that promotes CO2R but only at proton consumption rates sufficient to drive the pH up dramatically. Stark tuning measurements and analysis of surface water structure reveal no change in the electric field with [M+] and a desorption of interfacial water, indicating that improved CO2R performance is driven by suppression of H+ mass transport and modification of the interfacial solvation structure. In situ pH measurements confirm increasing local pH, and therefore decreased local [CO2], with [M+], motivating alternate means of modulating proton transport. We show that an organic film formed via in situ electrodeposition of an organic additive provides a means to achieve selective CO2R (FECO2R ∼ 65%) over hydrogen evolution reaction in the presence of strong acid (pH 1) and low cation concentrations (≤0.1 M) at both low and high current densities.

Hydroxyl on Stepped Copper and its Interaction with Water

We describe the hydroxyl and mixed hydroxyl-water structures formed on a stepped copper surface following the reaction of adsorbed O with water at a low temperature and compare them to the structures found previously on plane copper surfaces. Thermal desorption profiles, STM, and low-energy electron diffraction show that water reacts with O at temperatures below 130 K on Cu(511). Two well-defined phases appear as the OH/H2O layer is heated to desorb excess water, a 1OH:1H2O phase and a pure OH phase. The 1OH:1H2O structure consists of 1D chains binding across two adjacent copper steps, with a double period along the step. Electronic structure calculations show that the structure has a zigzag chain of water along the terrace, stabilized by hydrogen bonds to OH groups adsorbed in the step bridge sites. This structure binds OH in its favored site and is similar to the structure observed on other open faces of Cu and Ni, suggesting that this structural arrangement may be common on other surfaces that have steps or rows of close packed metal atoms. The hydroxyl/water chains decompose at 210 K to leave OH adsorbed in the Cu step bridge site, with some forming H-bonded trimers that bridge between two Cu steps. Heating the surface causes hydroxyl to disproportionate near 300 K, desorbing water to leave chemisorbed O.

Structuring Cu Membrane Electrode for Maximizing Ethylene Yield from CO2 Electroreduction

Electrocatalytic ethylene (C2H4) evolution from CO2 reduction is an intriguing route to mitigate both the energy and environmental crises; however, to acquire industrially relevant high productivity and selectivity at low energy cost remains to be challenging. Membrane assembly electrode has shown great prospect and tailoring its architecture for maximizing C2H4 yield at minimum voltage with long-term stability becomes critical. Here a freestanding Cu membrane cathode is designed and constructed by electrochemically depositing mesoporous Cu film on Cu foam to simultaneously manage CO2, electron, water, and product transport, which shows an extraordinary C2H4 Faradaic efficiency of 85.6% with a full cell power conversion efficiency of 33% at a current density of 368 mA cm-2, heading the techno-economic viability for electrocatalytic C2H4 production.

CO2 Electrolyzers

CO2 electrolyzers have progressed rapidly in energy efficiency and catalyst selectivity toward valuable chemical feedstocks and fuels, such as syngas, ethylene, ethanol, and methane. However, each component within these complex systems influences the overall performance, and the further advances needed to realize commercialization will require an approach that considers the whole process, with the electrochemical cell at the center. Beyond the cell boundaries, the electrolyzer must integrate with upstream CO2 feeds and downstream separation processes in a way that minimizes overall product energy intensity and presents viable use cases. Here we begin by describing upstream CO2 sources, their energy intensities, and impurities. We then focus on the cell, the most common CO2 electrolyzer system architectures, and each component within these systems. We evaluate the energy savings and the feasibility of alternative approaches including integration with CO2 capture, direct conversion of flue gas and two-step conversion via carbon monoxide. We evaluate pathways that minimize downstream separations and produce concentrated streams compatible with existing sectors. Applying this comprehensive upstream-to-downstream approach, we highlight the most promising routes, and outlook, for electrochemical CO2 reduction.

Cation-Induced Interfacial Hydrophobic Microenvironment Promotes the C-C Coupling in Electrochemical CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO2RR) toward C2 products is a promising way for the clean energy economy. Modulating the structure of the electric double layer (EDL), especially the interfacial water and cation type, is a useful strategy to promote C-C coupling, but atomic understanding lags far behind the experimental observations. Herein, we investigate the combined effect of interfacial water and alkali metal cations on the C-C coupling at the Cu(100) electrode/electrolyte interface using ab initio molecular dynamics (AIMD) simulations with a constrained MD and slow-growth approach. We observe a linear correlation between the water-adsorbate stabilization effect, which manifests as hydrogen bonds, and the corresponding alleviation in the C-C coupling free energy. The role of a larger cation, compared to a smaller cation (e.g., K+ vs Li+), lies in its ability to approach the interface through desolvation and coordinates with the *CO+*CO moiety, partially substituting the hydrogen-bonding stabilizing effect of interfacial water. Although this only results in a marginal reduction of the energy barrier for C-C coupling, it creates a local hydrophobic environment with a scarcity of hydrogen bonds owing to its great ionic radius, impeding the hydrogen of surrounding interfacial water to approach the oxygen of the adsorbed *CO. This skillfully circumvents the further hydrogenation of *CO toward the C1 pathway, serving as the predominant factor through which a larger cation facilitates C-C coupling. This study unveils a comprehensive atomic mechanism of the cation-water-adsorbate interactions that can facilitate the further optimization of the electrolyte and EDL for efficient C-C coupling in CO2RR.

Modulating Cation and Water Transports for Enhanced CO Electrolysis via Ionomer Coating

Electrification of the chemical industry has been considered an enabler for energy transition on a massive scale. In this context, carbon monoxide electroreduction (COR) to produce multi-carbon (C2+ ) products is considered one of the forefront emerging technologies. The key challenge in COR comes from the excessive cation crossover to the cathode via electromigration and water diffusion, which limits CO availability and impedes product selectivity. Commercial anion exchange membrane (AEM) suppresses the electromigration of cations, however, suffers from water diffusion which facilitates cation crossover. Here, we tackled these problems emerging from cation crossover and water diffusion by directly depositing an ultrathin Nafion ionomer on the cathode (sputtered Cu) surface. Our approach enables full-cell energy efficiency of 21 % for converting CO into ethylene (C2 H4 ) at 100 mA/cm2 with over 200 hours of stable operation. We also exhibited record high energy efficiency for ethanol (C2 H5 OH) production with CO-to-C2 H5 OH electrolysis efficiency of 17 %. This approach to directly depositing ultrathin ionomer on the cathode to enhance system performance may benefit other electrochemical systems to overcome challenges associated with scalability, stability, and efficiency to produce high-value chemicals.

Unraveling the rate-determining step of C2+ products during electrochemical CO reduction

The electrochemical reduction of CO has drawn a large amount of attention due to its potential to produce sustainable fuels and chemicals by using renewable energy. However, the reaction's mechanism is not yet well understood. A major debate is whether the rate-determining step for the generation of multi-carbon products is C-C coupling or CO hydrogenation. This paper conducts an experimental analysis of the rate-determining step, exploring pH dependency, kinetic isotope effects, and the impact of CO partial pressure on multi-carbon product activity. Results reveal constant multi-carbon product activity with pH or electrolyte deuteration changes, and CO partial pressure data aligns with the theoretical formula derived from *CO-*CO coupling as the rate-determining step. These findings establish the dimerization of two *CO as the rate-determining step for multi-carbon product formation. Extending the study to commercial copper nanoparticles and oxide-derived copper catalysts shows their rate-determining step also involves *CO-*CO coupling. This investigation provides vital kinetic data and a theoretical foundation for enhancing multi-carbon product production.

Site-selective protonation enables efficient carbon monoxide electroreduction to acetate

Electrosynthesis of acetate from CO offers the prospect of a low-carbon-intensity route to this valuable chemical--but only once sufficient selectivity, reaction rate and stability are realized. It is a high priority to achieve the protonation of the relevant intermediates in a controlled fashion, and to achieve this while suppressing the competing hydrogen evolution reaction (HER) and while steering multicarbon (C2+) products to a single valuable product--an example of which is acetate. Here we report interface engineering to achieve solid/liquid/gas triple-phase interface regulation, and we find that it leads to site-selective protonation of intermediates and the preferential stabilization of the ketene intermediates: this, we find, leads to improved selectivity and energy efficiency toward acetate. Once we further tune the catalyst composition and also optimize for interfacial water management, we achieve a cadmium-copper catalyst that shows an acetate Faradaic efficiency (FE) of 75% with ultralow HER (<0.2% H2 FE) at 150 mA cm-2. We develop a high-pressure membrane electrode assembly system to increase CO coverage by controlling gas reactant distribution and achieve 86% acetate FE simultaneous with an acetate full-cell energy efficiency (EE) of 32%, the highest energy efficiency reported in direct acetate electrosynthesis.

Covalent Organic Framework Ionomer Steering the CO2 Electroreduction Pathway on Cu at Industrial-Grade Current Density

CO2 electroreduction holds great promise for addressing global energy and sustainability challenges. Copper (Cu) shows great potential for effective conversion of CO2 toward specific value-added and/or high-energy-density products. However, its limitation lies in relatively low product selectivity. Herein, we present that the CO2 reduction reaction (CO2RR) pathway on commercially available Cu can be rationally steered by modulating the microenvironment in the vicinity of the Cu surface with two-dimensional sulfonated covalent organic framework nanosheet (COF-NS)-based ionomers. Specifically, the selectivity toward methane (CH4) can be enhanced to more than 60% with the total current density up to 500 mA cm-2 in flow cells in both acidic (pH = 2) and alkaline (pH = 14) electrolytes. The COF-NS, characterized by abundant apertures, can promote the accumulation of CO2 and K+ near the catalyst surface, alter the adsorption energy and surface coverage of *CO, facilitate the dissociation of H2O, and finally modulate the reaction pathway for the CO2RR. Our approach demonstrates the rational modulation of reaction interfaces for the CO2RR utilizing porous open framework ionomers, showcasing their potential practical applications.

Regulating the Innocuity of Urea Electro-Oxidation via Cation-mediated Adsorption

Urea electrolysis is an emerging technology that bridges efficient wastewater treatment and hydrogen production with lower electricity costs. However, conventional Ni-based catalysts could easily overoxidize urea into the secondary contaminant NOx - , and enhancing the innocuity of urea electrolysis remains a grand challenge to be achieved. Herein, we tailored the electrode-electrolyte interface of an unconventional cation effect on the anodic oxidation of urea to regulate its activity and selectivity. Smaller cations of Li+ were discovered to increase the Faradaic efficiency (FE) of the innocuous N2 product from the standard value of ~15 % to 45 %, while decreasing the FEs of the over-oxidized NOx - product from ~80 % to 46 %, pointing to a more sustainable process. The kinetic and computational analysis revealed the dominant residence of cations on the outer Helmholtz layer, which forms the interactions with the surface adsorbates. The Li+ hydration shells and rigid hydrogen bonding network interact strongly with the adsorbed urea to decrease its adsorption energy and subjection to C-N cleavage, thereby directing it toward the N2 pathway. This work emphasizes the tuning of the interactions within the electrode-electrolyte interface for enhancing the efficiency and sustainability of electrocatalytic processes.

CO2 electrolysis towards large scale operation: rational catalyst and electrolyte design for efficient flow-cell

The electrochemical CO₂ reduction reaction (CO₂RR) to renewable fuels/chemicals is a potential approach towards addressing the carbon neutral economy. To date, a comprehensive analysis of key performance indicators, such as an intrinsic property of catalyst, reaction environment and technological advancement in the flow cell, is limited. In this study, we discuss how the design of catalyst material, electrolyte and engineering gas diffusion electrode (GDE) could affect the CO₂RR in a gas-fed flow cell. Significant emphasis is given to scale-up requirements, such as promising catalysts with a partial current density of ≥100 mA cm^-2 and high faradaic efficiency. Additional experimental hurdles and their potential solutions, as well as the best available protocols for data acquisition for catalyst activity evaluation, are listed. We believe this manuscript provides some insights into the making of catalysts and electrolytes in a rational manner along with the engineering of GDEs towards CO₂RR.

Recent Progress in Electrochemical Nitrogen Reduction on Transition Metal Nitrides

Distributed electrochemical nitrogen reduction reaction (ENRR) powered by renewable energy for the on-site production of ammonia is an attractive alternative to the industrial Haber-Bosch process, which is responsible for roughly 2 % of global energy consumption. In this Review, we summarize recent progress in the ENRR catalyzed by transition metal nitrides (TMNs). The unique electronic structures of TMNs make them promising ENRR catalysts for active and selective ammonia production, which have been predicted theoretically and demonstrated experimentally. Reaction pathways and deactivation mechanisms of the ENRR on different TMNs are surveyed, and current understanding of structure-activity relations is discussed. To develop highly active, selective, and stable TMN catalysts for industrial-scale ENRR, membrane electrode assembly configuration is recommended in catalyst evaluation. Furthermore, we highlight the importance of developing mechanistic understanding on ENRR with different operando spectroscopic techniques.

Weak CO binding sites induced by Cu-Ag interfaces promote CO electroreduction to multi-carbon liquid products

Electrochemical reduction of carbon monoxide to high-value multi-carbon (C₂₊) products offers an appealing route to store sustainable energy and make use of the chief greenhouse gas leading to climate change, i.e., CO₂. Among potential products, C₂₊ liquid products such as ethanol are of particular interest owing to their high energy density and industrial relevance. In this work, we demonstrate that Ag-modified oxide-derive Cu catalysts prepared via high-energy ball milling exhibit near 80% Faradaic efficiencies for C₂₊ liquid products at commercially relevant current densities (>100 mA cm^-2) in the CO electroreduction in a microfluidic flow cell. Such performance is retained in an over 100-hour electrolysis in a 100 cm² membrane electrode assembly (MEA) electrolyzer. A method based on surface-enhanced infrared absorption spectroscopy is developed to characterize the CO binding strength on the catalyst surface. The lower C and O affinities of the Cu-Ag interfacial sites in the prepared catalysts are proposed to be responsible for the enhanced selectivity for C₂₊ oxygenates, which is the experimental verification of recent computational predictions.

The effect of specific adsorption of halide ions on electrochemical CO2 reduction

In the electrochemical CO2 reduction reaction (CO2RR), halide ions could impose a significant effect on multi-carbon (C2+) product production for Cu-based catalysts by a combined contribution from various mechanisms. However, the nature of specific adsorption of halide ions remains elusive due to the difficulty in decoupling different effects. This paper describes a facile method to actively immobilize the morphology of Cu-based catalysts during the CO2RR, which makes it possible to reveal the fundamental mechanism of specific adsorption of halide ions. A stable morphology is obtained by pre-reduction in aqueous KX (X = Cl, Br, I) electrolytes followed by conducting the CO2RR using non-buffered and non-specifically adsorbed K2SO4 as the supporting electrolyte, by which the change of local pH and cation concentration is also maintained during the CO2RR. In situ spectroscopy revealed that the specific adsorption of halide ions enhances the adsorption of *CO intermediates, which enables a high selectivity of 84.5% for C2+ products in 1.0 M KI.

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.

Electrochemical Reduction of Gaseous Nitrogen Oxides on Transition Metals at Ambient Conditions

Mitigating nitrogen oxide (NO_x) emissions is critical to tackle global warming and improve air quality. Conventional NO_x abatement technologies for emission control suffer from a low efficiency at near ambient temperatures. Herein, we show an electrochemical pathway to reduce gaseous NO_x that can be conducted at high reaction rates (400 mA cm^-2) under ambient conditions. Various transition metals are evaluated for electrochemical reduction of NO and N₂O to reveal the role of electrocatalyst in determining the product selectivity. Specifically, Cu is highly selective toward NH₃ formation with >80% Faradaic efficiency in NO electroreduction. Furthermore, the partial pressure study of NO electroreduction revealed that a high NO coverage facilitates the N-N coupling reaction. In acidic electrolytes, the formation of NH₃ is greatly favored, whereas the N₂ production is suppressed. Additional mechanistic studies were conducted by using flow electrochemical mass spectrometry to gain further insights into reaction pathways. This work provides a promising avenue toward abating gaseous NO_x emissions at ambient conditions by using renewable electricity.

On the importance of the electric double layer structure in aqueous electrocatalysis

To design electrochemical interfaces for efficient electric-chemical energy interconversion, it is critical to reveal the electric double layer (EDL) structure and relate it with electrochemical activity; nonetheless, this has been a long-standing challenge. Of particular, no molecular-level theories have fully explained the characteristic two peaks arising in the potential-dependence of the EDL capacitance, which is sensitively dependent on the EDL structure. We herein demonstrate that our first-principles-based molecular simulation reproduces the experimental capacitance peaks. The origin of two peaks emerging at anodic and cathodic potentials is unveiled to be an electrosorption of ions and a structural phase transition, respectively. We further find a cation complexation gradually modifies the EDL structure and the field strength, which linearly scales the carbon dioxide reduction activity. This study deciphers the complex structural response of the EDL and highlights its catalytic importance, which bridges the mechanistic gap between the EDL structure and electrocatalysis.

Steering the Glycerol Electro-Reforming Selectivity via Cation-Intermediate Interactions

Electro-reforming of renewable biomass resources is an alternative technology for sustainable pure H2 production. Herein, we discovered an unconventional cation effect on the concurrent formate and H2 production via glycerol electro-reforming. In stark contrast to the cation effect via forming the double layers in cathodic reactions, the presence of residual cations at the anode were discovered to interact with the glycerol oxidation intermediates to steer its product selectivity. Through a combination of product analysis, transient kinetics, crown ether trapping experiments, in situ IRRAS spectroscopy and DFT calculation, the aldehyde intermediates were discovered to be stabilized by the Li+ cations to favor the non-oxidative C-C cleavage for formate production. The maximal formate efficiency could reach 81.3% under ~ 60 mA/cm2 in LiOH. This work emphasizes the significance of engineering the microenvironment at the electrode-electrolyte interface for efficient electrolytic processes.

Influence of solvent structure and hydrogen bonding on catalysis at solid-liquid interfaces

Solvent molecules interact with reactive species and alter the rates and selectivities of catalytic reactions by orders of magnitude. Specifically, solvent molecules can modify the free energies of liquid phase and surface species via solvation, participating directly as a reactant or co-catalyst, or competitively binding to active sites. These effects carry consequences for reactions relevant for the conversion of renewable or recyclable feedstocks, the development of distributed chemical manufacturing, and the utilization of renewable energy to drive chemical reactions. First, we describe the quantitative impact of these effects on steady-state catalytic turnover rates through a rate expression derived for a generic catalytic reaction (A → B), which illustrates the functional dependence of rates on each category of solvent interaction. Second, we connect these concepts to recent investigations of the effects of solvents on catalysis to show how interactions between solvent and reactant molecules at solid-liquid interfaces influence catalytic reactions. This discussion demonstrates that the design of effective liquid phase catalytic processes benefits from a clear understanding of these intermolecular interactions and their implications for rates and selectivities.

C-C Coupling Is Unlikely to Be the Rate-Determining Step in the Formation of C2+ Products in the Copper-Catalyzed Electrochemical Reduction of CO

The identity of the rate-determining step (RDS) in the electrochemical CO reduction reaction (CORR) on Cu catalysts remains unresolved because: 1) the presence of mass transport limitation of CO; and 2) the absence of quantitative correlation between CO partial pressure (p_CO ) and surface CO coverage. In this work, we determined CO adsorption isotherms on Cu in a broad pH range of 7.2-12.9. Together with electrokinetic data, we demonstrate that the reaction orders of adsorbed CO at p_CO <0.4 and >0.6 atm are 1^st and 0^th , respectively, for multi-carbon (C₂₊ ) products on three Cu catalysts. These results indicate that the C-C coupling is unlikely to be the RDS in the formation of C₂₊ products in the CORR. We propose that the hydrogenation of CO with adsorbed water is the RDS, and the site competition between CO and water leads to the observed transition of the CO reaction order.

Designing Copper-Based Catalysts for Efficient Carbon Dioxide Electroreduction

The electroreduction of carbon dioxide (CO₂ ) has been emerging as a high- potential approach for CO₂ utilization using renewables. When copper (Cu) based catalysts are used, this platform can produce multi-carbon (C₂₊ ) fuels and chemicals with almost net-zero emission, contributing to the closure of the anthropogenic carbon cycle. Nonetheless, the rational design and development of Cu-based catalysts are critical toward the realization of highly selective and efficient CO₂ electroreduction. In this review, first the latest advances in Cu-catalyzed CO₂ electroreduction in the product selectivity and electrocatalytic activity are briefly summarized. Then, recent theoretical and mechanistic studies of CO₂ electroreduction on Cu-based catalysts are investigated, which serve as programs to design catalysts. Strategies for devising Cu catalysts that aim at promoting different key elementary steps for hydrocarbon and C₂₊ oxygenates production are further summarized. Moreover, challenges in understanding the mechanism, operando investigation of Cu catalysts and reactions, and systems' influences are also presented. Finally, the future prospects of CO₂ electroreduction are discussed.

Electrocatalytic reduction of CO2 and CO to multi-carbon compounds over Cu-based catalysts

The electrocatalytic reduction of CO₂ with H₂O to multi-carbon (C₂₊) compounds, in particular, C₂₊ olefins and oxygenates, which have versatile applications in the chemical and energy industries, holds great potential to mitigate the depletion of fossil resources and abate carbon emissions. There are two major routes for the electrocatalytic CO₂ reduction to C₂₊ compounds, i.e., the direct route and the indirect route via CO. The electrocatalytic CO₂ reduction to CO has been commercialised with solid oxide electrolysers, making the indirect route via CO to C₂₊ compounds also a promising alternative. This tutorial review focuses on the similarities and differences in the electrocatalytic CO₂ and CO reduction reactions (CO₂RR and CORR) into C₂₊ compounds, including C₂H₄, C₂H₅OH, CH₃COO^- and n-C₃H₇OH, over Cu-based catalysts. First, we introduce the fundamental aspects of the two electrocatalytic reactions, including the cathode and anode reactions, electrocatalytic reactors and crucial performance parameters. Next, the reaction mechanisms, in particular, the C-C coupling mechanism, are discussed. Then, efficient catalysts and systems for these two reactions are critically reviewed. We analyse the key factors that determine the selectivity, activity and stability for the electrocatalytic CO₂RR and CORR. Finally, the opportunities, challenges and future trends in the electrocatalytic CO₂RR and CORR are proposed. These insights will offer guidance for the design of industrial-relevant catalysts and systems for the synthesis of C₂₊ olefins and oxygenates.

Insight into Structural Evolution, Active Site and Stability of Heterogeneous Electrocatalysts

The structure-activity correlation study of electrocatalysts is essential for improving conversion from electrical to chemical energy. Recently, increasing evidences obtained by operando characterization techniques reveal that the structural evolution of catalysts caused by the interplay with electric fields, electrolytes or reactants/intermediates brings about the formation of real active sites. Hence, it is time to summarize the structural evolution-related research advances and envisage their future developments. In this minireview, we first introduce the fundamental concepts associated with structural evolution ( e.g., catalyst, active site/center and stability/lifetime) and their relevance. Then, the multiple inducements of structural evolution and advanced operando characterizations are discussed. Lastly, a brief overview of structural evolution and its reversibility in heterogeneous electrocatalysis, especially for representative electrocatalytic oxygen evolution reaction (OER) and CO 2 reduction reaction (CO 2 RR), along with key challenges and opportunities, is highlighted.

Electrokinetic and in situ spectroscopic investigations of CO electrochemical reduction on copper

Rigorous electrokinetic results are key to understanding the reaction mechanisms in the electrochemical CO reduction reaction (CORR), however, most reported results are compromised by the CO mass transport limitation. In this work, we determined mass transport-free CORR kinetics by employing a gas-diffusion type electrode and identified dependence of catalyst surface speciation on the electrolyte pH using in-situ surface enhanced vibrational spectroscopies. Based on the measured Tafel slopes and reaction orders, we demonstrate that the formation rates of C₂₊ products are most likely limited by the dimerization of CO adsorbate. CH₄ production is limited by the CO hydrogenation step via a proton coupled electron transfer and a chemical hydrogenation step of CO by adsorbed hydrogen atom in weakly (7 < pH < 11) and strongly (pH > 11) alkaline electrolytes, respectively. Further, CH₄ and C₂₊ products are likely formed on distinct types of active sites.

Electrokinetic results are key to understanding the mechanisms in electrochemical CO reduction reaction. Here, the authors determine mass transport free kinetics using a gas-diffusion electrode and identified dependence of copper surface speciation on the electrolyte pH using in situ surface enhanced spectroscopies.

Asymmetric Multipole Plasmon-Mediated Catalysis Shifts the Product Selectivity of CO2 Photoreduction toward C2+ Products

Cu/TiO₂ is a well-known photocatalyst for the photocatalytic transformation of CO₂ into methane. The formation of C₂₊ products such as ethane and ethanol rather than methane is more interesting due to their higher energy density and economic value, but the formation of C-C bonds is currently a major challenge in CO₂ photoreduction. In this context, we report the dominant formation of a C₂ product, namely, ethane, from the gas-phase photoreduction of CO₂ using TiO₂ nanotube arrays (TNTAs) decorated with large-sized (80-200 nm) Ag and Cu nanoparticles without the use of a sacrificial agent or hole scavenger. Isotope-labeled mass spectrometry was used to verify the origin and identity of the reaction products. Under 2 h AM1.5G 1-sun illumination, the total rate of hydrocarbon production (methane + ethane) was highest for AgCu-TNTA with a total C_xH_2x+2 rate of 23.88 μmol g^-1 h^-1. Under identical conditions, the C_xH_2x+2 production rates for Ag-TNTA and Cu-TNTA were 6.54 and 1.39 μmol g^-1 h^-1, respectively. The ethane selectivity was the highest for AgCu-TNTA with 60.7%, while the ethane selectivity was found to be 15.9 and 10% for the Ag-TNTA and Cu-TNTA, respectively. Adjacent adsorption sites in our photocatalyst develop an asymmetric charge distribution due to quadrupole resonances in large metal nanoparticles and multipole resonances in Ag-Cu heterodimers. Such an asymmetric charge distribution decreases adsorbate-adsorbate repulsion and facilitates C-C coupling of reaction intermediates, which otherwise occurs poorly in TNTAs decorated with small metal nanoparticles.

Bridging the Gap in the Mechanistic Understanding of Electrocatalysis via In Situ Characterizations

Electrocatalysis offers a promising strategy to take advantage of the increasingly available and affordable renewable energy for the sustainable production of fuels and chemicals. Attaining this promise requires a molecular level insight of the electrical interface that can be used to tailor the selectivity of electrocatalysts. Addressing this selectivity challenge remains one of the most important areas in modern electrocatalytic research. In this Perspective, we focus on the use of in situ techniques to bridge the gap in the fundamental understanding of electrocatalytic processes. We begin with a brief discussion of traditional electrochemical techniques, ex situ measurements and in silico analysis. Subsequently, we discuss the utility and limitations of in situ methodologies, with a focus on vibrational spectroscopies. We then end by looking ahead toward promising new areas for the application of in situ techniques and improvements to current methods.

A few basic concepts in electrochemical carbon dioxide reduction

In this perspective, I discuss a few basic concepts in fundamental mechanistic studies of electrochemical carbon dioxide reduction.

Electrochemical carbon dioxide (CO2) reduction has the potential to sustainably produce carbon-based fuels and chemicals while mitigating the increasing levels of CO2 in the atmosphere. In this comment, the author discusses a few basic concepts in the fundamental mechanistic studies of electrochemical CO2 reduction.

Understanding the electric and nonelectric field components of the cation effect on the electrochemical CO reduction reaction

Electrolyte cations affect the activity of surface-mediated electrocatalytic reactions; however, understanding the modes of interaction between cations and reaction intermediates remains lacking. We show that larger alkali metal cations (excluding the thickness of the hydration shell) promote the electrochemical CO reduction reaction on polycrystalline Cu surfaces in alkaline electrolytes. Combined reactivity and in situ surface-enhanced spectroscopic investigations show that changes to the interfacial electric field strength cannot solely explain the reactivity trend with cation size, suggesting the presence of a nonelectric field strength component in the cation effect. Spectroscopic investigations with cation chelating agents and organic molecules show that the electric and nonelectric field components of the cation effect could be affected by both cation identity and composition of the electrochemical interface. The interdependent nature of interfacial species indicates that the cation effect should be considered an integral part of the broader effect of composition and structure of the electrochemical interface on electrode-mediated reactions.

Oxygen induced promotion of electrochemical reduction of CO2 via co-electrolysis

Harnessing renewable electricity to drive the electrochemical reduction of CO₂ is being intensely studied for sustainable fuel production and as a means for energy storage. Copper is the only monometallic electrocatalyst capable of converting CO₂ to value-added products, e.g., hydrocarbons and oxygenates, but suffers from poor selectivity and mediocre activity. Multiple oxidative treatments have shown improvements in the performance of copper catalysts. However, the fundamental underpinning for such enhancement remains controversial. Here, we combine reactivity, in-situ surface-enhanced Raman spectroscopy, and computational investigations to demonstrate that the presence of surface hydroxyl species by co-electrolysis of CO₂ with low concentrations of O₂ can dramatically enhance the activity of copper catalyzed CO₂ electroreduction. Our results indicate that co-electrolysis of CO₂ with an oxidant is a promising strategy to introduce catalytically active species in electrocatalysis.

Speciation of Cu Surfaces During the Electrochemical CO Reduction Reaction

Cu-catalyzed selective electrocatalytic upgrading of carbon dioxide/monoxide to valuable multicarbon oxygenates and hydrocarbons is an attractive strategy for combating climate change. Despite recent research on Cu-based catalysts for the CO₂ and CO reduction reactions, surface speciation of the various types of Cu surfaces under reaction conditions remains a topic of discussion. Herein, in situ surface-enhanced Raman spectroscopy (SERS) is employed to investigate the speciation of four commonly used Cu surfaces, i.e., Cu foil, Cu micro/nanoparticles, electrochemically deposited Cu film, and oxide-derived Cu, at potentials relevant to the CO reduction reaction in an alkaline electrolyte. Multiple oxide and hydroxide species exist on all Cu surfaces at negative potentials, however, the speciation on the Cu foil is distinct from that on micro/nanostructured Cu. The surface speciation is demonstrated to correlate with the initial degree of oxidation of the Cu surface prior to the exposure to negative potentials. Combining reactivity and spectroscopic results on these four types of Cu surfaces, we conclude that the oxygen containing surface species identified by Raman spectroscopy are unlikely to be active in facilitating the formation of C₂₊ oxygenates in the CO reduction reaction.