Electric Field Effects in Electrochemical CO2 Reduction

Quantifying the Driving Force for the Surface Reconstruction of Copper under Electrochemical Reduction of CO2 by First-Principles Simulations

Cu is one of the most widely used catalysts in the electrochemical CO2 reduction reaction (CO2RR) due to its unique ability to convert CO2 to C2+ products. However, surface reconstruction of Cu significantly affects the activity and stability of Cu catalyst. In this work, density functional theory (DFT) coupled with implicit solvation ab initio molecular dynamics (AIMD) was employed to unveil the possible migration pathways of surface Cu atoms during the structural evolution processes under CO2RR. Surface energy, as the intrinsic thermodynamic driving force of surface reconstruction, is distributed to individual surface Cu atoms and shows a quasi-linear relationship with their generalized coordination number (GCN), demonstrating a maximum driving force of ∼1.10 eV under the applied electric field. The *CO adsorbate weakens the binding of surface Cu atoms, resulting in a maximum vertical displacement of Cu atoms of up to 0.8 Å. In contrast, *H on Cu(100) at high coverage induces a horizontal extruding effect on the surface Cu atoms, causing them to move up to 2 Å. The observable migration of surface Cu atoms in the AIMD run occurs only on Cu adatoms, with the adsorption of pure *H or coadsorption of *CO and *H.

Reactor operating parameters and their effects on the local reaction environment of CO(2) electroreduction

Low temperature aqueous electrochemical CO(2) reduction (ECR) emerged as a pathway to close the carbon cycle with the integration of renewable energy. However, activity, selectivity, and stability barriers prevent ECR from entering industrial scale operation. While catalyst design has made meaningful progress towards selective and active production of many products including CO, formate, and ethylene, operating conditions during catalyst testing have not been standardized. Operational parameters drastically impact the local reaction environment of the ECR and thus the performance of ECR. Herein, we summarize the prevailing operational variability of ECR and their interconnectedness. We first analyze reactant availability via tuning of cell geometry and CO(2) pressures. Then, optimization towards electrolyzer components including electrolyte, electrodes, and bipolar plates is discussed. We further assess the electrochemical protocols to enhance the performance or accelerate the degradation of ECR and the considerations required to scale up ECR to pilot scale. Finally, we provide perspectives on the current challenges of ECR and their promising solutions.

Exploring the Linear Energy Relationships between Activation Energy and Reaction Energy under an Electric Field

Electric-field (EF)-mediated chemistry has recently garnered increasing attention partly owing to its capability to catalyze a broad range of chemical reactions. How the EF affects the kinetics and thermodynamics of target reactions is a critical question. Herein, both density functional theory (DFT) and MP2 calculations suggest that the change of activation energy ΔΔE‡ and the change of reaction energy ΔΔErxn under an EF display a linear energy relationship (LER) ΔΔE‡ = mΔΔErxn. This has been tested against several reactions such as SN2 and proton transfer reactions, including neutral and charged systems and endothermic and exothermic processes. The linear coefficient m approximates to the ratio of the dipole moment change, i.e., Δμ‡/Δμrxn, of the studied reactions. The LER holds well at EF strengths up to ≈1 V/nm but deviates from the DFT-calculated results at larger EFs. Such deviations are mainly caused by the molecular geometry changes under an EF. Systems with larger polarizability experience greater geometry changes under an EF, thus leading to larger deviations. In addition, we propose that the reaction barrier can be predicted by -Δμ‡F - 0.5Δα‡F2, while it is well approximated by -Δμ‡F for small EF strengths. The proposed LER and the field-dependent barrier estimation promise broad applicability in EF-mediated chemical reactions.

Mechanistic Insights Into H2O Dissociation in Overall Photo-/Electro-Catalytic CO2 Reduction

Photo-/electro-catalytic CO2 reduction with H2O to produce fuels and chemicals offers a dual solution to address both environmental and energy challenges. For a long time, catalyst design in this reaction system has primarily focused on optimizing reduction sites to improve the efficiency or guide the reaction pathway of the CO2 reduction half-reaction. However, less attention has been paid to designing activation sites for H2O to modulate the H2O dissociation half-reaction. Impressively, the rate-determining step in overall CO2 reduction is the latter, and it influences the evolution direction and formation energy of carbon-containing intermediates through the proton-coupled electron transfer process. Herein, we summarize the mechanism of the H2O dissociation half-reaction in modulating CO2 reduction performance based on cutting-edge research. These analyses aim to uncover the potential regulatory mechanisms by which H2O activation influences CO2 reduction pathways and conversion efficiency, and to establish a mechanism-structure-performance relationship that can guide the design and development of high-efficiency catalytic materials. A summary of advanced characterization techniques for investigating the dissociation mechanism of H2O is presented. We also discuss the challenges and offer perspectives on the future design of activation sites to improve the performance of photo-/electro-catalytic CO2 reduction.

Electrolyte Effect on Electrocatalytic CO2 Reduction

Electrocatalytic CO2 reduction reaction shows great potential for converting CO2 into high-value chemicals and fuels at normal temperature and pressure, combating climate change and achieving carbon neutrality goals. However, the complex reaction pathways involve the transfer of multiple electrons and protons, resulting in poor product selectivity, and the existence of competitive hydrogen evolution reactions further increases the associated difficulties. This review illustrates the research progress on the micro mechanism of electrocatalytic CO2 reduction reaction in the electrolyte environment in recent years. The reaction pathways of the products, pH effects, cation effects and anion effects were systematically summarized. Additionally, further challenges and difficulties were also pointed out. Thus, this review provides a theoretical basis and future research direction for improving the efficiency and selectivity of electrocatalytic CO2 reduction reaction.

Space Charge, Modulating the Catalytic Activity of Single-Atom Metal Catalysts

Potential-induced electrode charging is a prerequisite to initiate electrochemical reactions at the electrode-electrolyte interface. The 'interface space charge' could dramatically alter the reaction environment and the charge density of the active site, both of which potentially affect the electrochemical activity. However, our understanding of the electrocatalytic role of space charge has been limited. Here, we separately modulate the amount of space charge (characterized by the areal density, σ) with maintaining the electrochemical potential for the oxygen reduction reaction (ORR) at the same level, by exploiting the unique structural feature of MeNC. We reveal that changes in σ control the ORR activity, which is computationally explained by the inductive polarization of the charge density at the active sites, affecting their turnover rates. To guide catalyst design including the space charge effect, we develop a new descriptor, explaining the activity trend in various metal centers and pH conditions using a single volcano. These findings offer fresh insights into the role of space charge in electrocatalysis, providing a new framework for optimizing catalyst design and performance.

Nitrile Groups as Build-In Molecular Sensors for Interfacial Effects at Electrocatalytically Active Carbon-Nitrogen Materials

Electrocatalytic reactions are influenced by various interfacial phenomena including nonspecific interaction forces. For many examples, their contributions to the catalytic cycle have yet to be identified. Noncovalent interactions between the electrode and the electrolyte can be described by the local electric field environment at the interface and are experimentally accessible based on the Vibrational Stark Effect. We herein present a carbon-based C2N-type electrocatalyst that is active for the hydrogen evolution reaction and that contains nitrile functions as Stark reporter groups. With this system, we expand the range of electrocatalytically active systems suitable for electrochemical Stark spectroscopy while taking a step away from pure model systems. The stretching mode ν(C≡N) was analyzed via experimental and calculated Raman spectroscopy, revealing a defect character of the inherent CN groups. The ν(C≡N) peak position was furthermore studied via in situ electrochemical Raman spectroscopy. At noncatalytic conditions, a linear dependence between an applied electric potential and ν(C≡N) peak shift is observed, resulting in a red-shift at a more negative potential. At catalytic conditions, deviations from the linearity occur, and a semipermanent blue-shift of the CN peak is observed after electrocatalysis, implying a restructuring of the electrochemical double layer and therefore a change in the local electric field environment due to the catalytic turnover and the associated interfacial processes.

Enhancing the oxygen evolution reaction by tuning the electrode-electrolyte interface in nickel-based electrocatalysts

A comprehensive understanding of the electrode-electrolyte interface in energy conversion systems remains challenging due to the complex and multifaceted nature of interfacial processes. This complexity hinders the development of more efficient electrocatalysts. In this work, we propose a hybrid approach to the theoretical description of the OER process on nickel-iron-based oxyhydroxides (γ-Ni1-xFexOOH) electrodes in alkaline media as a model system. Multiple reaction pathways represented by the single- and dual-site mechanisms were investigated by taking into account the realistic structure of the catalyst, the doping, and the solvation effects using a simple and computationally feasible strategy. Accounting for the variable solvation effects considerably affects the predicted overpotential in a roughly linear relationship between overpotential and dielectric constant. By incorporating quantum chemical simulations with kinetic modeling, we demonstrate that tuning the local solvation environment can significantly enhance the OER activity, opening new routine ways for elucidation of the emerging issues of OER processes on transition metal oxide surfaces and design of cost-effective, efficient electrocatalytic systems.

Dimensionality Reduction of Metal-Organic Frameworks to Monolayers for Enhanced Electrocatalysis

Metal-organic frameworks (MOFs) are potential candidates for electrocatalysis due to their well-defined, tunable structures, and ability to incorporate diverse active sites. However, their inherent insulating nature restricts electron transfer from electrode to remote active sites, leading to diminished catalytic performance. In this work, we present a novel strategy to overcome this limitation by reducing 3D MOFs (3D_MOFs) into monolayered MOFs (monoMOFs) with a thickness of ∼1.8 nm, maximizing the exposure of catalytic sites to the electrode and enhancing electrocatalytic performance. We designed and synthesized a monoMOF incorporating cobalt(II)-porphyrin sites in the linker (monoMOF-Co) for CO2 electroreduction. After being grafted onto graphene oxide, the monoMOF-Co exhibited a peak faradaic efficiency for CO production (FECO = 93%), surpassing the performance of a 3D_MOF incorporating the same porphyrin-Co-based linker (3D_MOF-Co, FECO = 51%). Additionally, monoMOF-Co achieved a turnover frequency of 10 600 h-1 at -0.8 V versus the reversible hydrogen electrode (RHE) and maintained stability over 47 h in a near-neutral aqueous solution. In situ spectroscopic studies further confirmed the distinct electric field environment in the Stern layer between monoMOF-Co and 3D_MOF-Co. Furthermore, similar enhancement effects of monoMOFs over 3D_MOFs were observed in the nitrate and oxygen electroreduction reactions, highlighting the broader applicability of monoMOFs in electrocatalysis.

Light-induced Enhancement of Energetic Charge Carrier Extraction and Modulation of Local Charge Density to Impact Selectivity in Plasmonic Nanometals

Localized surface plasmon resonance (LSPR) metals exhibit remarkable light-absorbing property and unique catalytic activity, attracting significant attention in photocatalysts recently. However, the practical application of plasmonic nanometal is hindered by challenge of energetic electrons extraction and low selectivity. The energetic carriers generated in nanometal under illumination have extremely short lifetimes, leading to rapid energy loss. In this work, silver nanometals modified with five distinct sulfhydryl ligands (re-Ag-S-R) were synthesized via photoreduction of superlattice precursors. Modified surface efficiently extracts and preserves excited state electrons of plasmonic nanometals. By modulation the local charge density at catalytic active sites through substituents with varying electron-donating and electron-withdrawing properties, the selectivity of the photocatalytic carbon dioxide reduction reaction and hydrogen evolution reaction was influenced. The results demonstrated opposite selectivity between methoxy-modified re-Ag-S-OCH3 (CO selectivity of 96.73 %) and amino-modified re-Ag-S-NH2 (H2 selectivity of 96.66 %) despite their similar structures. The changes in excited states and surface contact potentials induced by LSPR were monitored using femtosecond transient absorption (fs-TA) spectroscopy and Kelvin probe force microscopy (KPFM). Meanwhile, the detailed discussion of the LSPR mechanism in plasmonic nanometals will serve as valuable references and foundational elements for future research in this area.

Non-Nernstian Effects in Theoretical Electrocatalysis

Electrocatalysis is one of the principal pathways for the transition to sustainable chemistry, promising greater energy efficiency and reduced emissions. As the field has grown, our theoretical understanding has matured. The influence of the applied potential on reactivity has developed from the first-order predictions based on the Nernst equation to the implicit inclusion of second-order effects including the interaction of reacting species with the interfacial electric field. In this review, we explore these non-Nernstian field effects in electrocatalysis, aiming to both understand and exploit them through theory and computation. We summarize the critical distinction between Nernstian and non-Nernstian effects and outline strategies to address the latter in theoretical studies. Subsequently, we examine the specific energetic contributions of the latter on capacitive and faradaic processes separately. We also underscore the importance of considering non-Nernstian effects in catalyst screening and mechanistic analysis. Finally, we provide suggestions on how to experimentally unravel these effects, offering insights into practical approaches for advancing the field.

Single-Atom-Embedded Nitrogen-Doped Graphene as Efficient Electrocatalysts for the CO2 Reduction Reaction

Single-atom catalysts (SACs) have displayed unprecedented activity and selectivity for electrochemical CO2 reduction reaction (CO2RR). Herein, a series of metal single atoms embedded on nitrogen-doped graphene (M-N4G, where M = In, Tl, Ge, Sn, Pb, Sb, and Bi) is systematically evaluated as CO2RR electrocatalysts by density functional theory (DFT) calculations. The computational results show that most M-N4G exhibit better CO2RR selectivity over the hydrogen evolution reaction (HER). Ge/Pb-N4G exhibits excellent electrocatalytic performance in the generation of HCOOH from the CO2RR with low limiting potentials of -0.292 and -0.306 V, which surpass the performance of the vast majority of electrocatalysts. Adsorption energy of the key intermediate *HCOO can be used as an effective reactivity reaction descriptor to screen promising CO2RR catalysts. The results of this work highlight M-N4G as an ideal electrochemical for the electrocatalytic CO2RR.

Integrating Physical Principles with Machine Learning for Predicting Field-Enhanced Catalysis

Field-dipole interactions can tune the energetics of polarized species over catalyst nanoparticles (NPs) for sustainable technologies. This can boost the energy efficiency of desired reactions by several orders of magnitude compared with conventional heating. However, the local electric field accumulation over the NPs sharp points and field-dependent adsorption over NPs are not well studied, and the associated computational expense is immense. To address this challenge, we introduce an innovative approach that combines density functional theory (DFT) calculations, DFT-based CO vibrational Stark effects, and physics principles enhanced machine learning (ML). This approach enables precise mapping of local electric fields and integrates the physical principles of the first-order Taylor expansion as a training input into the ML model for predicting field-dependent adsorption, facilitating rapid prediction of field-dependent adsorption energetics with acceptable accuracies, particularly when training data sets are limited. Our methodology reveals the dominant roles of external electric field (EEF), the generalized coordination number (GCN), and NP size in determining the local electric field (LEF) strength. Low-coordinated sites and small NPs size enhanced the LEF by about 4-fold compared to the flat surfaces. Using ML models, we can predict the field-driven adsorption energetics at a given adsorption site of the NPs with high accuracy and efficiency. The integration of ab initio modeling and ML algorithms offers exceptional possibilities to facilitate catalyst development and create the opportunity to enter a new paradigm in field-enhanced catalysis design based on fundamentals rather than trial and error.

Electrochemical CO2 Reduction in Acidic Media: A Perspective

The electrochemical CO2 reduction reaction (eCO2RR) is a promising approach for converting CO2 to useful chemicals and, hence, achieving carbon neutrality. Though high selectivity and activity of products have been achieved recently, all are reported in neutral or alkaline electrolytes. Although these electrolyte media give high selectivity and activity, they face the major challenge of low CO2 utilization because of carbonate formation, which lowers the overall efficiency of the process. Conducting the eCO2RR in acidic media can help overcome the issue of carbonate formation and hence can increase the CO2 utilization efficiency. However, there are many challenges associated with acidic eCO2RR. Two major concerns are the highly competitive hydrogen evolution reaction in acidic media and salt precipitation issues. This Perspective focuses on the fundamentals of acidic eCO2RR, recent catalyst development strategies, and relevant problems that need to be addressed in the future. In the end, we provide a future outlook that will give an idea about the problems to focus on in the future in the field of acidic eCO2RR.

Understanding Electrochemical CO2 Reduction through Differential Electrochemical Mass Spectrometry

The electrochemical reduction of CO2 powered by renewable energy is a viable pathway to produce valuable fuels and chemicals, while simultaneously helping to mitigate greenhouse gas emissions. The strong research interest in improving the selectivity and efficiency of CO2 reduction has led to a multitude of electrocatalyst studies that employ a variety of electrochemical, spectroscopic, spectrometric, and materials characterization analytical techniques. Among these, differential electrochemical mass spectrometry (DEMS) has become an increasingly instrumental tool for investigating electrocatalyst performance by enabling in situ volatile product detection. DEMS has the significant advantages of being able to rapidly screen product distributions in real time as the potential is varied and distinguishing isotopically labeled species for mechanistic studies. There are also challenges for employing DEMS to study CO2 reduction, including cell design limitations for optimal mass transport and high product ion current signal, a lack of nonvolatile product detection, and the difficulty of extracting reliable, quantitative faradaic efficiency measurements. Many researchers have applied DEMS to study the reduction of CO2 on numerous catalysts under a variety of conditions, highlighting cell designs and protocols for overcoming some of these challenges. This review focuses on the implementation of DEMS in the study of electrochemical CO2 reduction, explaining the working principle and the various commonly employed cell designs and highlighting the findings of key reports that were enabled by DEMS.

Alkali Cation Inhibition of Imidazolium-Mediated Electrochemical CO2 Reduction on Silver

Imidazolium-based ionic liquids have led to enhanced CO2 electroreduction activity due to cation effects at the cathode surface, stabilizing the reaction intermediates and decreasing the activation energy. In aqueous media, alkali cations are also known to improve CO2 reduction activity on metals such as Ag, with the enhancement attributed to electrical double layer effects and trending with the size of the alkali cation. However, the effect of a mixed catholyte solution of alkali cations in the presence of an imidazolium-based ionic liquid has not been well-explored. Herein, 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF4], in water was investigated with alkali salts to unravel the interaction effects for CO2 electroreduction on Ag. Although both [EMIM]+ and alkali cations have individually improved CO2 to CO conversion on Ag in water, electrochemical results showed that alkali cations hindered imidazolium-mediated CO2 electroreduction in most conditions. Li+, in particular, was sharply inhibitory compared to other alkali cations and strongly redirected the selectivity to hydrogen evolution. The nature of the alkali cation inhibition was investigated with spectroscopic techniques, including in situ surface-enhanced Raman spectroscopy (SERS) and dynamic electrochemical impedance spectroscopy (DEIS). Along with computational insights from density functional theory (DFT), the electrochemical and spectroscopic data suggest that alkali cations inhibit [EMIM]-mediated CO2 reduction by competing for surface adsorption sites, preventing the potential-dependent structural reorientation of imidazolium, and promoting hydrogen evolution by bringing solvated water to the cathode surface.

Interfacial Hydrogen-Bond Network Regulation Tuned Water Dissociation Enables Selective Chlorination of Alkenes

Electrocatalytically selective chlorination of olefins in Cl--containing solutions is a sustainable method for synthesizing chlorohydrin/vicinal dichloride; however, controlling the selectivity is challenging. Here, aqueous/dimethyl carbonate (DMC) hybrid electrolytes with different H2O/DMC ratios are designed to modulate the ·OH formation to increase the corresponding selectivities. The combined results of in/ex situ spectroscopies and molecular dynamics simulations reveal the origin of high selectivity. TFSI- shields the transportation of free H2O to provide moderate ·OH formation for the synthesis of chlorohydrin. DMC reconstructs hydrogen bonds with free H2O to minimize the interaction between them and the anode, matching the requirements of vicinal dichloride production. Thus, these hybrid electrolytes not only achieve high selectivities of 80% and 76% for chlorohydrin and vicinal dichloride, respectively, but also enable the selective chlorination of other olefins with high isolated yields of up to 74%. This work provides a facile strategy to regulate the selectivity of anodic chlorination via a rational electrolyte design.

Electrochemical CO2 reduction to liquid fuels: Mechanistic pathways and surface/interface engineering of catalysts and electrolytes

The high energy density of green synthetic liquid chemicals and fuels makes them ideal for sustainable energy storage and transportation applications. Electroreduction of carbon dioxide (CO2) directly into such high value-added chemicals can help us achieve a renewable C cycle. Such electrochemical reduction typically suffers from low faradaic efficiencies (FEs) and generates a mixture of products due to the complexity of controlling the reaction selectivity. This perspective summarizes recent advances in the mechanistic understanding of CO2 reduction reaction pathways toward liquid products and the state-of-the-art catalytic materials for conversion of CO2 to liquid C1 (e.g., formic acid, methanol) and C2+ products (e.g., acetic acid, ethanol, n-propanol). Many liquid fuels are being produced with FEs between 80% and 100%. We discuss the use of structure-binding energy relationships, computational screening, and machine learning to identify promising candidates for experimental validation. Finally, we classify strategies for controlling catalyst selectivity and summarize breakthroughs, prospects, and challenges in electrocatalytic CO2 reduction to guide future developments.

Electrolyte Anions Suppress Hydrogen Generation in Electrochemical CO Reduction on Cu

In this study, we employed electrochemical-mass spectrometry (EC-MS) to elucidate the role of halide anions in electrochemical CO2 and CO reduction. We found that the undesired hydrogen evolution reaction (HER) was significantly suppressed by the anion used. Specifically, the rates of H2 production decreased in the order KF > KCl > KI, meaning that I- most strongly suppressed HER. Interestingly, CO reduction products showed an inverse relationship to HER, with KI leading to the highest rate of CO reduction. By pairing our experimental findings with classical molecular dynamics simulations, we propose a mechanism wherein halide anions influence the dynamic interplay between CO reduction and HER by modulating the competition of H* and CO* for active sites on the Cu surface. We propose that this interaction is enabled by the interfacial concentration of K+ being greater in the presence of F- than in I-. Our results highlight the need to more broadly consider the properties of ions at electrocatalytic interfaces and they point to thus far underappreciated avenues to optimize hydrocarbon production while suppressing hydrogen evolution.

In Situ Probing the Anion-Widened Anodic Electric Double Layer for Enhanced Faradaic Efficiency of Chlorine-Involved Reactions

The electric double layer (EDL), which is directly related to ions, influences the electrocatalytic performance. However, the effects of anions on the anodic EDL and reaction kinetics are unclear, especially in water-mediated electrosynthesis. Here, ClO4- anions are discovered to widen the anodic EDL to inhibit the competitive oxygen evolution reaction (OER) for the gram-scale electrosynthesis of 2-chlorocyclohexanol with a 90% Faradaic efficiency (FE) at 100 mA cm-2. The combined results of molecular dynamics simulations and in situ spectroscopies provide solid evidence for the widened EDL that originates from the repulsion of water molecules from the interface by ClO4-. The addition of ClO4- has a negligible effect on chlorination kinetics because of the electrostatic interaction between the anode and Cl- but obviously suppresses the interaction between water and the anode, leading to high FEs of anodic electrosynthesis by increasing the energy barrier of the undesirable OER. In addition, this method is suitable for other chlorination reactions with enhanced FEs at 100 mA cm-2.

Electrocatalytic reduction of CO2 to produce the C2+ products: from selectivity to rational catalyst design

Electrocatalytic reduction of CO2 (eCO2RR) into valuable multi-carbon (C2+) products is an effective strategy for combating climate change and mitigating energy crises. The high-energy density and diverse applications of C2+ products have attracted considerable interest. However, the complexity of the reaction pathways and the high energy barriers to C-C coupling lead to lower selectivity and faradaic efficiency for C2+ products than for C1 products. Therefore, a thorough understanding of the underlying mechanisms and identification of reaction conditions that influence selectivity, followed by the rational design of catalysts, are considered promising methods for the efficient and selective synthesis of multi-carbon products. This review first introduces the critical steps involved in forming multi-carbon products. Then, we discuss the reaction conditions that influence the selectivity of C2+ products and explore different catalyst design strategies to enhance the selective production of C2+ products. Finally, we summarize the significant challenges currently facing the eCO2RR field and suggest future research directions to address these challenges.

Parameter Dependency of Electrochemical Reduction of CO2 in Acetonitrile - A Data Driven Approach

The electrochemical CO2 reduction reaction (CO2RR) is a promising technology for the utilization of captured CO2. Though systems using aqueous electrolytes is the state-of-the-art, CO2RR in aprotic solvents are a promising alternative that can avoid the parallel hydrogen evolution reaction (HER). While system parameters, such as electrolyte composition, electrode material, and applied potential are known to influence the reaction mechanism, there is a lack of intuitive understanding as to how. We show that by using multivariate data analysis on a large dataset collected from the literature, namely random forest modelling, the most important system parameters can be isolated for each possible product. We find that water content, current density, and applied potential are powerful determinants in the reaction pathway, and therefore in the Faradaic efficiency of CO2RR products.

Electrocatalysis: From Planar Surfaces to Nanostructured Interfaces

The reactions critical for the energy transition center on the chemistry of hydrogen, oxygen, carbon, and the heterogeneous catalyst surfaces that make up electrochemical energy conversion systems. Together, the surface-adsorbate interactions constitute the electrochemical interphase and define reaction kinetics of many clean energy technologies. Practical devices introduce high levels of complexity where surface roughness, structure, composition, and morphology combine with electrolyte, pH, diffusion, and system level limitations to challenge our ability to deconvolute underlying phenomena. To make significant strides in materials design, a structured approach based on well-defined surfaces is necessary to selectively control distinct parameters, while complexity is added sequentially through careful application of nanostructured surfaces. In this review, we cover advances made through this approach for key elements in the field, beginning with the simplest hydrogen oxidation and evolution reactions and concluding with more complex organic molecules. In each case, we offer a unique perspective on the contribution of well-defined systems to our understanding of electrochemical energy conversion technologies and how wider deployment can aid intelligent materials design.

The Impact of Electric Fields on Processes at Electrode Interfaces

The application of external electric fields to influence chemical reactions at electrode interfaces has attracted considerable interest in recent years. However, the design of electric fields to achieve highly efficient and selective catalytic systems, akin to the optimized fields found at enzyme active sites, remains a significant challenge. Consequently, there has been substantial effort in probing and understanding the interfacial electric fields at electrode/electrolyte interfaces and their effect on adsorbates. In this review, we examine recent advances in experimental, computational, and theoretical studies of the interfacial electric field, the origin of the vibrational Stark effect of adsorbates on electrode surfaces, and the effects of electric fields on reactions at electrode/electrolyte interfaces. We also discuss recent advances in control of charge transfer and chemical reactions using magnetic fields. Finally, we outline perspectives on key areas for future studies.

Confined Water for Catalysis: Thermodynamic Properties and Reaction Kinetics

Water is a salient component in catalytic systems and acts as a reactant, product and/or spectator species in the reaction. Confined water in distinct local environments can display significantly different behaviors from that of bulk water. Therefore, the wide-ranging chemistry of confined water can provide tremendous opportunities to tune the reaction kinetics. In this review, we focus on drawing the connection between confined water properties and reaction kinetics for heterogeneous (electro)catalysis. First, the properties of confined water are presented, where the enthalpy, entropy, and dielectric properties of water can be regulated by tuning the geometry and hydrophobicity of the cavities. Second, experimental and computational studies that investigate the interactions between water and inorganic materials, such as carbon nanotubes (1D confinement), charged metal or metal oxide surfaces (2D), zeolites and metal-organic frameworks (3D) and ions/solvent molecules (0D), are reviewed to demonstrate the opportunity to create confined water structures with unique H-bonding network properties. Third, the role of H-bonding structure and dynamics in governing the activation free energy, reorganization energy and pre-exponential factor for (electro)catalysis are discussed. We highlight emerging opportunities to enhance proton-coupled electron transfer by optimizing interfacial H-bond networks to regulate reaction kinetics for the decarbonization of chemicals and fuels.

Effect of Interfacial Electric Field on 2D Metal/Graphene Electrocatalysts for CO2 Reduction Reaction

Understanding the influence of local electric fields on electrochemical reactions is crucial for designing highly selective electrocatalysts for CO2 reduction reactions (CO2RR). In this study, we provide a theoretical investigation of the effect of the local electric field induced by the negative-biased electrode and cations in the electrolyte on the energetics and reaction kinetics of CO2RR on 2D hybrid metal/graphene electrocatalysts. Our findings reveal that the electronic structures of the CO2 molecule undergo substantial modification, resulting in the increased adsorption energy of CO2 on metal/graphene structures, thus reducing the initial barrier of the CO2RR mechanism. This field-assisted CO2RR mechanism promotes CO production while suppressing HCOOH production. Our findings highlight the potential of manipulating electric fields to tailor the pathways of CO2RR, providing new avenues designing selective electrocatalysts.

Microenvironment engineering by targeted delivery of Ag nanoparticles for boosting electrocatalytic CO2 reduction reaction

Creating and maintaining a favorable microenvironment for electrocatalytic CO2 reduction reaction (eCO2RR) is challenging due to the vigorous interactions with both gas and electrolyte solution during the electrocatalysis. Herein, to boost the performance of eCO2RR, a unique synthetic method that deploys the in situ reduction of precoated precursors is developed to produce activated Ag nanoparticles (NPs) within the gas diffusion layer (GDL), where the thus-obtained Ag NPs-Skeleton can block direct contact between the active Ag sites and electrolyte. Specifically, compared to the conventional surface loading mode in the acidic media, our freestanding and binder free electrode can achieve obvious higher CO selectivity of 94%, CO production rate of 23.3 mol g-1 h-1, single-pass CO2 conversion of 58.6%, and enhanced long-term stability of 8 hours. Our study shows that delivering catalysts within the GDL does not only gain the desired physical protection from GDL skeleton to achieve a superior local microenvironment for more efficient pH-universal eCO2RR, but also manifests the pore structures to effectively address gas accumulation and flood issues.

A perspective on field-effect in energy and environmental catalysis

The development of catalytic technologies for sustainable energy conversion is a critical step toward addressing fossil fuel depletion and associated environmental challenges. High-efficiency catalysts are fundamental to advancing these technologies. Recently, field-effect facilitated catalytic processes have emerged as a promising approach in energy and environmental applications, including water splitting, CO2 reduction, nitrogen reduction, organic electrosynthesis, and biomass recycling. Field-effect catalysis offers multiple advantages, such as enhancing localized reactant concentration, facilitating mass transfer, improving reactant adsorption, modifying electronic excitation and work functions, and enabling efficient charge transfer and separation. This review begins by defining and classifying field effects in catalysis, followed by an in-depth discussion on their roles and potential to guide further exploration of field-effect catalysis. To elucidate the theory-structure-activity relationship, we explore corresponding reaction mechanisms, modification strategies, and catalytic properties, highlighting their relevance to sustainable energy and environmental catalysis applications. Lastly, we offer perspectives on potential challenges that field-effect catalysis may face, aiming to provide a comprehensive understanding and future direction for this emerging area.

Dielectric profile at the Pt(111)/water interface

The dielectric constant, although a simplified concept when considering atomic scales, enters many mean-field, electrochemical interface models and constant potential models as an important parameter. Here, we use ab initio and machine-learned molecular dynamics to scrutinize the behavior of the electronic contribution to ɛr(z) as a function of distance z from a Pt(111) surface. We show that the resulting dielectric profile can largely be explained as a sum of the metallic response and the density-scaled water response at the interface. A slight enhancement of the dielectric response close to the surface can be explained by elongated, strongly polarizable orbitals induced by metal/water bonding. In spite of this enhancement, our results suggest the presence of a region with a very low dielectric constant close to the surface (where the orientational dielectric response does not kick in yet), even for water in contact with hydrophilic metallic interfaces. This region will restrict the double layer capacitance to relatively low values even at potentials where dielectric saturation does not play a role yet. This finding has implications on possible interpretations of double layer capacitances, the dependence of surface electric fields on the ion size, and on electrochemical kinetics.

The Melamine-Driven Solvation Effect Promotes Oxygen Reduction on a Platinum Catalyst: Machine Learning-Aided Free Energy Calculations

The modification of Pt surfaces with organic compounds like melamine enhances oxygen reduction reaction activity and catalyst durability. Through first-principles free energy calculations utilizing thermodynamic integration and finite-temperature molecular dynamics, enhanced by machine learning force fields for efficient sampling of nanosecond-scale interfacial water fluctuations and incorporating corrections to accurately reproduce first-principles free energies, we demonstrate that melamine destabilizes OH adsorbates, facilitating their removal and enhancing catalytic activity. Unlike alloys, where OH destabilization is driven by changes in electronic structure and surface strain, melamine disrupts hydrogen bonding between OH and interfacial water. Structural and vibrational analyses reveal that melamine alters the water solvation structure, which is evident in modified radial distribution functions and a blue shift in the O-H stretching vibrations. These findings indicate that manipulating interfacial solvation with organic compounds could be a promising approach to enhance catalytic activity without compromising durability.

Utilization of Electric Fields to Modulate Molecular Activities on the Nanoscale: From Physical Properties to Chemical Reactions

As a primary energy source, electricity drives broad fields from everyday electronic circuits to industrial chemical catalysis. From a chemistry viewpoint, studying electric field effects on chemical reactivity is highly important for revealing the intrinsic mechanisms of molecular behaviors and mastering chemical reactions. Recently, manipulating the molecular activity using electric fields has emerged as a new research field. In addition, because integration of molecules into electronic devices has the natural complementary metal-oxide-semiconductor compatibility, electric field-driven molecular devices meet the requirements for both electronic device miniaturization and precise regulation of chemical reactions. This Review provides a timely and comprehensive overview of recent state-of-the-art advances, including theoretical models and prototype devices for electric field-based manipulation of molecular activities. First, we summarize the main approaches to providing electric fields for molecules. Then, we introduce several methods to measure their strengths in different systems quantitatively. Subsequently, we provide detailed discussions of electric field-regulated photophysics, electron transport, molecular movements, and chemical reactions. This review intends to provide a technical manual for precise molecular control in devices via electric fields. This could lead to development of new optoelectronic functions, more efficient logic processing units, more precise bond-selective control, new catalytic paradigms, and new chemical reactions.

Fundamentals and Challenges of Ligand Modification in Heterogeneous Electrocatalysis

The development of efficient catalytic materials in the energy field could promote the structural transformation from traditional fossil fuels to sustainable energy. In heterogeneous catalytic reactions, ligand modification is an effective way to regulate both electronic and steric structures of catalytic sites, thus paving a prospective avenue to design the interfacial structures of heterogeneous catalysts for energy conversion. Although great achievements have been obtained for the study and applications of heterogeneous ligand-modified catalysts, the systematical refinements of ligand modification strategies are still lacking. Here, we reviewed the ligand modification strategy from both the mechanistic and applicable scenarios by focusing on heterogeneous electrocatalysis. We elucidated the ligand-modified catalysts in detail from the perspectives of basic concepts, preparation, regulation of physicochemical properties of catalytic sites, and applications in different electrocatalysis. Notably, we bridged the electrocatalytic performance with the electronic/steric effects induced by ligand modification to gain intrinsic structure-performance relations. We also discussed the challenges and future perspectives of ligand modification strategies in heterogeneous catalysis.

Polyelectrolyte Additive-Modulated Interfacial Microenvironment Boosting CO2 Electrolysis in Acid

Acidic CO2 electrolysis offers a promising strategy to achieve high carbon utilization and high energy efficiency. However, challenges still remain in suppressing the competitive hydrogen evolution reaction (HER) and improving product selectivity. Although high concentrations of potassium ions (K+) can suppress HER and accelerate CO2 reduction, they still inevitably suffer from salt precipitation problems. In this study, we demonstrate that the sulfonate-based polyelectrolyte, polystyrene sulfonate (PSS), enables to reconstruct the electrode-electrolyte interface to significantly enhance the acidic CO2 electrolysis. Mechanistic studies reveal that PSS induces high local K+ concentrations through the electrostatic interaction between PSS anions and K+. In situ spectroscopy reveals that PSS reshapes the interfacial hydrogen-bond (H-bond) network, which is attributed to the H-bonds between PSS anions and hydrated proton, as well as the steric hindrance of the additive molecules. This greatly weakens proton transfer kinetics and leads to the suppression of undesirable HER. As a result, a Faradaic efficiency of 93.9 % for CO can be achieved at 250 mA cm-2, simultaneous with a high single-pass carbon efficiency of 72.2 % on commercial Ag catalysts in acid. This study highlights the important role of the electrode-electrolyte interface induced by polyelectrolyte additives in promoting electrocatalytic reactions.

Electrolyte Effects on Electrochemical CO2 Reduction Reaction at Sn Metallic Electrode

Understanding the electrolyte factors governing the electrochemical CO2 reduction reaction (CO2RR) is fundamental for selecting the optimized electrolyte conditions for practical applications. While noble metals are frequently studied, the electrolyte effects on the CO2RR on Sn catalysts are not well explored. Here, we studied the electrolyte effect on Sn metallic electrodes, investigating the impact of electrolyte concentration, cation identity, and anion properties, and how it shapes the CO2RR activity and selectivity. The activity for formic acid and carbon monoxide increases with the cation concentration and size at mild acid conditions. In contrast, hydrogen production is not strongly influenced by the cathodic potential, electrolyte concentration, and cation size. Furthermore, we have compared the CO2RR performance at a constant cation concentration in K2SO4 (pH 4) and KHCO3 (pH 7), where we show that the reaction rate toward HCOOH and CO are minimally impacted by the anion identity on the SHE scale, while being affected by the cations in solution, which we attribute to the reaction being limited by cation-coupled electron transfer steps rather than by a proton-coupled electron transfer step. We propose that the HCOOH forms via adsorbed hydrides leading to *OCHO intermediate, while CO forms through an electron transfer step, producing *CO2 δ-. Cations facilitate both processes by stabilizing the negatively charged intermediates, and the difference in the extent of the promotion of HCOOH over CO formation would stem from the stronger cation effects on *H compared with *CO2 δ- species. Additionally, the presence of HCO3 - at high concentrations (1.0 mol L-1) is shown to significantly enhance the production of H2 at high overpotentials (>-1.0 V vs RHE) due to bicarbonate ions acting as protons donors, outcompeting water reduction. These findings underscore the significance of electrolyte engineering for enhanced formic acid synthesis, offering valuable insights for optimizing the CO2RR processes on Sn electrocatalysts.

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.

Surface coverage and reconstruction analyses bridge the correlation between structure and activity for electrocatalysis

Electrocatalysis is key to realizing a sustainable future for our society. However, the complex interface between electrocatalysts and electrolytes presents an ongoing challenge in electrocatalysis, hindering the accurate identification of effective/authentic structure-activity relationships and determination of favourable reaction mechanisms. Surface coverage and reconstruction analyses of electrocatalysts are important to address each conjecture and/or conflicting viewpoint on surface-active phases and their corresponding electrocatalytic origin, i.e., so-called structure-activity relationships. In this review, we emphasize the importance of surface states in electrocatalysis experimentally and theoretically, providing guidelines for research practices in discovering promising electrocatalysts. Then, we summarize some recent progress of how surface states determine the adsorption strengths and reaction mechanisms of occurring electrocatalytic reactions, exemplified in the electrochemical oxygen evolution reaction, oxygen reduction reaction, nitrogen reduction reaction, CO2 reduction reaction, CO2 and N2 co-reductions, and hydrogen evolution reaction. Finally, the review proposes deep insights into the in situ study of surface states, their efficient building and the application of surface Pourbaix diagrams. This review will accelerate the development of electrocatalysts and electrocatalysis theory by arousing broad consensus on the significance of surface states.

Descriptors for Electrochemical CO2 Reduction in Imidazolium-Based Electrolytes

Electrochemical CO2 reduction (CO2R) allows us to close the carbon cycle and store intermittent renewable energy into chemical products. Among these, syngas, a mixture of hydrogen and carbon monoxide, is particularly valuable due to its high market share and the low energy required for its electrocatalytic production. In addition to catalyst optimization, lately, electrolyte modifications to achieve a suitable CO/H2 ratio have also been considered. Ionic liquid (IL)-based electrolytes have enabled high faradaic efficiency toward CO, depending on the chemical properties of the IL. In this work, we rationalized through density functional theory (DFT) descriptors the competition between hydrogen evolution (HER) and CO2R on silver in imidazolium-based electrolytes, developing a DFT-based analytical model. The electrolyte anion regulates the concentration ratio between cationic and carbene species of ILs cation, respectively, between the 1-ethyl-3-methylimidazolium cation (EMIM+) and carbene (EMIM:) species and between the 1-butyl-3-methylimidazolium cation (BMIM+) and carbene (BMIM:). The latter species, if formed, hinders the CO2R by blocking the active sites or trapping CO2 in solution. In the case of weak Lewis base anions as fluorinated ones, EMIM+ (BMIM+) cations, which serve as cocatalysts in CO2R, are more abundant, allowing high CO partial current densities and high electrochemically active surface area. Applying the here-defined descriptors to ILs not yet tested makes it possible to predict the HER and CO2R selectivity on silver, thus enabling guidelines for designing better ILs for CO2R.

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

Ab Initio Kinetics of Electrochemical Reactions Using the Computational Fc0/Fc+ Electrode

The current state-of-the-art electron-transfer modeling primarily focuses on the kinetics of charge transfer between an electroactive species and an inert electrode. Experimental studies have revealed that the existing Butler-Volmer model fails to satisfactorily replicate experimental voltammetry results for both solution-based and surface-bound redox couples. Consequently, experimentalists lack an accurate tool for predicting electron-transfer kinetics. In response to this challenge, we developed a density functional theory-based approach for accurately predicting current peak potentials by using the Marcus-Hush model. Through extensive cyclic voltammetry simulations, we conducted a thorough exploration that offers valuable insights for conducting well-informed studies in the field of electrochemistry.

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.

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.

Switching Reaction Pathways of CO2 Electroreduction by Modulating Cations in the Electrochemical Double Layer

Tuning the selectivity of CO2 electroreduction reaction (CO2RR) solely by changing electrolyte is a very attractive topic. In this study, we conducted CO2RR in different aqueous electrolytes over bulk metal electrodes. It was discovered that controlled CO2RR could be achieved by modulating cations in the electrochemical double layer. Specifically, ionic liquid cations in the electrolyte significantly inhibits the hydrogen evolution reaction (HER), while yielding high Faraday efficiencies toward CO (FECO) or formate (FEformate) depending on the alkali metal cations. For example, the product could be switched from CO (FECO=97.3 %) to formate (FEformate=93.5 %) by changing the electrolyte from 0.1 M KBr-0.5 M 1-octyl-3-methylimidazolium bromide (OmimBr) to 0.1 M CsBr-0.5 M OmimBr aqueous solutions over pristine Cu foil electrode. In situ spectroscopy and theoretical calculations reveal that the ordered structure generated by the assembly of Omim+ under an applied negative potential alters the hydrogen bonding structure of the interfacial water, thereby inhibiting the HER. The difference in selectivity in the presence of different cations is attributed to the hydrogen bonding effect caused by Omim+, which alters the solvated structure of the alkali metal cations and thus affects the stabilization of intermediates of different pathways.

Effect of External Electric Field on Nitrogen Activation on a Trimetal Cluster

Efficient nitrogen (N2) fixation and activation under mild conditions are crucial for modern society. External electric fields (Felectric) can significantly affect N2 activation. In this work, the effect of Felectric on N2 activation by Nb3 clusters supported in a sumanene bowl was studied by density functional theory calculations. Four typical systems at different stages of N-N activation were studied, including two intermediates and two transition states. The impact of Felectric on various properties related to N2 activation was investigated, including the N-N bond length, overlap population density of states (OPDOS), total energy of the system, adsorption energy of N2, decomposition of energy changes, and electron transfer. The sumanene not only functions as a support and protective substrate, but also serves as a donor or acceptor under different Felectric conditions. Negative Felectric is beneficial to N-N bond activation because it promotes electron transfer to the N-N region and improves the d-π* orbital hybridization between metals and N2 in the activation process. Positive Felectric improves d-π* orbital hybridization only when the N-N is nearly dissociated. The microscopic mechanism of Felectric's effects provides insight into N2 activation and theoretical guidance for the design of catalytic reaction conditions for nitrogen reduction reactions (NRR).

Direct Microenvironment Modulation of CO2 Electroreduction: Negatively Charged Ag Sites Going beyond Catalytic Surface Reactions

Electrochemical reduction of CO2 is an important way to achieve carbon neutrality, and much effort has been devoted to the design of active sites. Apart from elevating the intrinsic activity, expanding the functionality of active sites may also boost catalytic performance. Here we designed "negatively charged Ag (nc-Ag)" active sites featuring both the intrinsic activity and the capability of regulating microenvironment, through modifying Ag nanoparticles with atomically dispersed Sn species. Different from conventional active sites (which only mediate the surface processes by bonding with the intermediates), the nc-Ag sites could also manipulate environmental species. Therefore, the sites could not only activate CO2, but also regulate interfacial H2O and CO2, as confirmed by operando spectroscopies. The catalyst delivers a high current density with a CO faradaic efficiency of 97 %. Our work here opens up new opportunities for the design of multifunctional electrocatalytic active sites.

Disentangling heterogeneous thermocatalytic formic acid dehydrogenation from an electrochemical perspective

Heterogeneous thermocatalysis of formic acid dehydrogenation by metals in solution is of great importance for chemical storage and production of hydrogen. Insightful understanding of the complicated formic acid dehydrogenation kinetics at the metal-solution interface is challenging and yet essential for the design of efficient heterogeneous formic acid dehydrogenation systems. In this work, formic acid dehydrogenation kinetics is initially studied from a perspective of electrochemistry by decoupling this reaction on Pd catalyst into two short-circuit half reactions, formic acid oxidation reaction and hydrogen evolution reaction and manipulating the electrical double layer impact from the solution side. The pH-dependences of formic acid dehydrogenation kinetics and the associated cation effect are attributed to the induced change of electric double layer structure and potential by means of electrochemical measurements involving kinetic isotope effect, in situ infrared spectroscopy as well as grand canonical quantum mechanics calculations. This work showcases how kinetic puzzles on some important heterogeneous catalytic reactions can be tackled by electrochemical theories and methodologies.

Synergistic Effects of the Electric Field Induced by Imidazolium Rotation and Hydrogen Bonding in Electrocatalysis of CO2

The roles of the ionic liquid (IL), 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), and water in controlling the mechanism, energetics, and electrocatalytic activity of CO2 reduction to CO on silver in nonaqueous electrolytes were investigated. The first electron transfer occurs to CO2 at reduced overpotentials when it is trapped between the planes of the [EMIM]+ ring and the electrode surface due to cation reorientation as determined from voltammetry, in situ surface-enhanced Raman spectroscopy, and density functional theory calculations. Within this interface, water up to 0.5 M does not induce significant Faradaic activity, opposing the notion of it being a free proton source. Instead, water acts as a hydrogen bond donor, and the proton is sourced from [EMIM]+. Furthermore, this study demonstrates that alcohols with varying acidities tune the hydrogen bonding network in the interfacial microenvironment to lower the energetics required for CO2 reduction. The hydrogen bonding suppresses the formation of inactive carboxylate species, thus preserving the catalytic activity of [EMIM]+. The ability to tune the hydrogen bonding network opens new avenues for advancing IL-mediated electrocatalytic reactions in nonaqueous electrolytes.

Reactive capture and electrochemical conversion of CO2 with ionic liquids and deep eutectic solvents

Ionic liquids (ILs) and deep eutectic solvents (DESs) have tremendous potential for reactive capture and conversion (RCC) of CO2 due to their wide electrochemical stability window, low volatility, and high CO2 solubility. There is environmental and economic interest in the direct utilization of the captured CO2 using electrified and modular processes that forgo the thermal- or pressure-swing regeneration steps to concentrate CO2, eliminating the need to compress, transport, or store the gas. The conventional electrochemical conversion of CO2 with aqueous electrolytes presents limited CO2 solubility and high energy requirement to achieve industrially relevant products. Additionally, aqueous systems have competitive hydrogen evolution. In the past decade, there has been significant progress toward the design of ILs and DESs, and their composites to separate CO2 from dilute streams. In parallel, but not necessarily in synergy, there have been studies focused on a few select ILs and DESs for electrochemical reduction of CO2, often diluting them with aqueous or non-aqueous solvents. The resulting electrode-electrolyte interfaces present a complex speciation for RCC. In this review, we describe how the ILs and DESs are tuned for RCC and specifically address the CO2 chemisorption and electroreduction mechanisms. Critical bulk and interfacial properties of ILs and DESs are discussed in the context of RCC, and the potential of these electrolytes are presented through a techno-economic evaluation.

How local electric field regulates C-C coupling at a single nanocavity in electrocatalytic CO2 reduction

C-C coupling is of utmost importance in the electrocatalytic reduction of CO2, as it governs the selectivity of diverse product formation. Nevertheless, the difficulties to directly observe C-C coupling pathways at a specific nanocavity hinder the advances in catalysts and electrolyzer design for efficient high-value hydrocarbon production. Here we develop a nano-confined Raman technology to elucidate the influence of the local electric field on the evolution of C-C coupling intermediates. Through precise adjustments to the Debye length in nanocavities of a copper catalyst, the overlapping of electrical double layers drives a transition in the C-C coupling pathway at a specific nanocavity from *CHO-*CO coupling to the direct dimerization of *CO species. Experimental evidence and simulations validate that a reduced potential drop across the compact layer promotes a higher yield of CO and promotes the direct dimerization of *CO species. Our findings provide insights for the development of highly selective catalyst materials tailored to promote specific products.

Hierarchical Modeling of the Local Reaction Environment in Electrocatalysis

ConspectusElectrocatalytic reactions, such as oxygen reduction/evolution reactions and CO2 reduction reaction that are pivotal for the energy transition, are multistep processes that occur in a nanoscale electric double layer (EDL) at a solid-liquid interface. Conventional analyses based on the Sabatier principle, using binding energies or effective electronic structure properties such as the d-band center as descriptors, are able to grasp overall trends in catalytic activity in specific groups of catalysts. However, thermodynamic approaches often fail to account for electrolyte effects that arise in the EDL, including pH, cation, and anion effects. These effects exert strong impacts on electrocatalytic reactions. There is growing consensus that the local reaction environment (LRE) prevailing in the EDL is the key to deciphering these complex and hitherto perplexing electrolyte effects. Increasing attention is thus paid to designing electrolyte properties, positioning the LRE at center stage. To this end, unraveling the LRE is becoming essential for designing electrocatalysts with specifically tailored properties, which could enable much needed breakthroughs in electrochemical energy science.Theory and modeling are getting more and more important and powerful in addressing this multifaceted problem that involves physical phenomena at different scales and interacting in a multidimensional parametric space. Theoretical models developed for this purpose should treat intrinsic multistep kinetics of electrocatalytic reactions, EDL effects from subnm scale to the scale of 10 nm, and mass transport phenomena bridging scales from <0.1 to 100 μm. Given the diverse physical phenomena and scales involved, it is evident that the challenge at hand surpasses the capabilities of any single theoretical or computational approach.In this Account, we present a hierarchical theoretical framework to address the above challenge. It seamlessly integrates several modules: (i) microkinetic modeling that accounts for various reaction pathways; (ii) an LRE model that describes the interfacial region extending from the nanometric EDL continuously to the solution bulk; (iii) first-principles calculations that provide parameters, e.g., adsorption energies, activation barriers and EDL parameters. The microkinetic model considers all elementary steps without designating an a priori rate-determining step. The kinetics of these elementary steps are expressed in terms of local concentrations, potential and electric field that are codetermined by EDL charging and mass transport in the LRE model. Vital insights on electrode kinetic phenomena, i.e., potential-dependent Tafel slopes, cation effects, and pH effects, obtained from this hierarchical framework are then reviewed. Finally, an outlook on further improvement of the model framework is presented, in view of recent developments in first-principles based simulation of electrocatalysis, observations of dynamic reconstruction of catalysts, and machine-learning assisted computational simulations.

Revealing the Size and Potential Dependent D2O Microkinetics on Pt Nanoparticles Using Grand Canonical Ensemble Modeling

Revealing the potential and nanoparticle size effect is significant for understanding the electrochemical microkinetic behaviors under real reaction conditions. Herein, an efficient strategy of combining the robust fully converged constant potential (FCP) algorithm and the size dependent site distribution rule assumption was proposed. A simple reaction of isotopic D2O/H2O adsorption and dissociation on Pt nanoparticles was set as the model reaction. The results show that the cathodic negative potential and the anodic positive potential would result in the D2O orientation of the D-down/O-down physisorption configuration. Microkinetic simulations by this strategy obtained electrochemical widows for D2O/H2O dissociation, and the optimal Pt nanoparticle diameter was predicted to be 1.8 nm, which agrees well with the experimental observation of ∼2 nm threshold. The kinetic isotope effect (KIE) rate constant ratio at the optimal potential of -0.80 V vs SHE was calculated to be ∼1.83. This work provides a guideline in studying electrochemical electrode-electrolyte interactions on nanoparticles.