A unifying mechanism for cation effect modulating C1 and C2 productions from CO2 electroreduction

A tutorial on the modeling of the heterogenous captured CO2 electroreduction reaction and first principles electrochemical modeling

As the energy demands of the world continue to grow, the electroreduction of captured CO2 (c-CO2RR) is an appealing alternative to the traditional CO2 reduction reaction (CO2RR) as it does not include the energetically unfavorable release of CO2 from the capture agent. In this tutorial we cover the motivation behind the c-CO2RR and CO2RR, their respective mechanisms, and computational tools that have been used to model these reactions and to compare their reactivities. Emphasis is given to methods that have already been used to model the c-CO2RR but a comparison to the methods used to explore the more understood CO2RR is covered as well.

Stabilized Triple-Phase Interface at CF4 Plasma Bombarded Cu Gas Diffusion Electrode for CO2-to-C2H4 Valorization

Cu-based gas diffusion electrodes (GDEs) hold the potential to produce carbon-neutral fuels and value-added chemicals from CO2 greenhouse gas at substantial current densities, yet they are challenged by the sluggish reaction kinetics toward C2+ product production and the fierce competition of H2 evolution from electrowetted/flooded catalyst layers. Herein, we develop a roughened hydrophobic Cu/PTFE GDE via CF4 plasma bombardment and demonstrate its effectiveness in facilitating CO2-to-C2H4 valorization. Online electrochemical mass spectrometry reveals that the enhanced C2H4 electrosynthesis is correlated with the increased rates of CO2 consumption and CO utilization, as well as a reduction in H2 generation upon CFx modification. Molecular dynamics simulations highlight the significant promotional effect of electrolyte management sustaining a high local [CO2]/[H2O] ratio near the CFx-Cu surface, where the improved C-C coupling kinetics is attributable to the abundant Cuδ+ sites adjacent to surface-bonded fluorocarbons with electron-withdrawing character.

Non-isothermal CO2 electrolysis enables simultaneous enhanced electrochemical and anti-precipitation performance

Electrochemical conversion of CO2 into fuels represents an important pathway for addressing the challenges of climate change and energy storage. However, large-scale applications remain hindered by the instability and inefficiency of CO2 reduction systems, particularly under highly alkaline electrolytes and high current densities. One primary obstacle is the cathodic salt precipitation, which hinders mass transfer and blocks active sites limiting the lifespan of these systems. Here, we present a non-isothermal strategy that leverages a thermal gradient across the membrane electrode assembly to enhance electrochemical performance and suppress salt precipitation. By maintaining a cooler cathode and warmer anode, we exploit the Soret effect to drive cations away from the cathode, mitigating salting-out while boosting anodic activity and cathodic CO2 solubility. The non-isothermal case has demonstrated over 200 h of stable operation at 100 mA cm-2 under highly alkaline conditions, outperforming conventional isothermal systems. Techno-economic analysis reveals reductions in CO2-to-CO production costs, supporting the scalability of this strategy. These findings enable the practical deployment of stable, high-efficiency CO2 electrolysis systems.

DFT-CES2: Quantum Mechanics Based Embedding for Mean-Field QM/MM of Solid-Liquid Interfaces

The solid-liquid interface plays a crucial role in governing complex chemical phenomena, such as heterogeneous catalysis and (photo)electrochemical processes. Despite its importance, acquiring atom-scale information about these buried interfaces remains highly challenging, which has led to an increasing demand for reliable atomic simulations of solid-liquid interfaces. Here, we introduce an innovative first-principles-based multiscale simulation approach called DFT-CES2, a mean-field QM/MM method. To accurately model interactions at the interface, we developed a quantum-mechanics-based embedding scheme that partitions complex noncovalent interactions into Pauli repulsion, Coulomb (including polarization), and London dispersion energies, which are described using atom-dependent transferable parameters. As validated by comparison with high-level quantum mechanical energies, DFT-CES2 demonstrates chemical accuracy in describing interfacial interactions. DFT-CES2 enables the investigation of complex solid-liquid interfaces while avoiding extensive parametrization. Therefore, we expect DFT-CES2 to be broadly applicable for elucidating atom-scale details of large scale solid-liquid interfaces for multicomponent systems.

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.

Mechanistic Insights into Cation Effects in Electrolytes for Electrocatalysis

Traditional understanding of electrocatalytic reactions has primarily focused on the covalent interactions between adsorbates and catalyst surfaces, often overlooking the significant influence of the electrolyte on the catalytic process. Recently, researchers have highlighted the pivotal role of cations in the electrolyte, demonstrating their capability to modulate the reaction pathways and thus the activity and selectivity. These findings underscore the need for a deeper, atomic-level understanding of the electrode-electrolyte interface. This perspective presents the mechanisms through which cations affect electrocatalysis, with a focus on the hydrogen-bond network, the adsorption behavior of reaction intermediates, the electric field within the electrochemical double layer, and the local electronic properties of catalysts. It provides a summary of the influences of cations on various electrocatalytic reactions, including hydrogen oxidation reaction (HOR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), nitrogen reduction reaction (NRR), nitrate reduction reaction (NO3 -RR), and carbon dioxide reduction reaction (CO2RR). At the end, future research directions are provided to maximize the potential of leveraging the cation effects in the design of more efficient electrolyte systems.

Excess Cations Alter *CO Intermediate Configuration and Product Selectivity of Cu in Acidic Electrochemical CO2 Reduction Reaction

Concentrated cations are often employed to promote electrochemical CO2 reduction reaction (CO2RR) selectivity in acidic electrolytes. Here, we investigate the influence of excess cations on the *CO adsorption configuration and the product distribution of the CO2RR. Operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) reveals that increasing the Cs+ concentration shifts the preference of the *CO intermediate on the Cu surface from the atop (*COatop) to the bridge (*CObridge) configuration. This transition leads to a sharp decline in C-C coupling and an increase in the hydrogen evolution reaction at high Cs+ concentrations (0.7 and 1.0 M) under acidic conditions. Time-resolved SEIRAS scans show that *COatop is kinetically dominant and the proportion of *CObridge increases gradually only at high cation concentrations. Density functional theory simulations confirm that Cs+ on the Cu surface can interact electrostatically with *CO and stabilize *CObridge over *COatop on the Cu surface. The evolution of *CObridge is also observed on Ag catalysts, indicating that the effect at high concentrations is not limited to Cu. Furthermore, polymeric binders on the Cu surface mitigate these detrimental effects on the CO2RR and restore C2H4 production by preventing the cation from altering the *CO adsorption sites on the catalyst surface. This study provides new insights into the effects of cations on catalyst performance, with implications for catalyst design and operation.

Unraveling the effect of alkali cations on Fe single atom catalysts with high coordination numbers

Fe single atom catalysts (SACs) with high coordination numbers have emerged as high-performance catalysts for the conversion of CO2 to CO. However, the influence of alkali cations at the catalyst-electrolyte interface has not yet been understood clearly. Here, we investigate the role of various alkali metal cations (Na+, K+, Rb+) in catalytic CO2 reduction reaction (CO2RR) behavior on high coordination number Fe SACs (FeN5 and FeN6) obtained from a facile hard template method. We find that larger cations can greatly promote the CO2RR and such effects are enhanced with increasing cation concentration. Nevertheless, the hydrogen evolution side reaction (HER) on co-existing N heteroatom sites will be worsened. This trade-off highlights the importance of manipulating the reactive sites for SACs. From theoretical simulation and in situ spectroscopy results, we confirm that the functioning mechanism of cations on Fe SACs lies in the enhancement of the adsorption of key intermediates through direct coordination and indirect hydrogen bonding effects. With the rationally designed Fe SACs (FeN5) and the electrolyte conditions (1 M KOH), our flow cell test demonstrates a maximum Faraday efficiency of CO (FECO) of approximately 100% at 100 mA cm-2. This research provides significant insights for future SACs and electrolyte design.

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.

Strain-Enabled Band Structure Engineering in Layered PtSe2 for Water Electrolysis under Ultralow Overpotential

This paper describes a simple design methodology to develop layered PtSe2 catalysts for hydrogen evolution reaction (HER) in water electrolysis operating under ultralow overpotentials. This approach relies on the transfer of mechanically exfoliated PtSe2 flakes to gold thin films on prestrained thermoplastic substrates. By relieving the prestrain, a tunable level of uniaxial internal compressive and tensile strain is developed in the flakes as a result of spontaneously formed surface wrinkles, giving rise to band structure modulations with overlapped values of the valence band maximum and conduction band minimum. This strain-engineered PtSe2 with an optimized level of internal tensile strain amplifies the HER performance of the PtSe2, with performance far greater than that of pure platinum due to significantly reduced charge transfer resistance. Density functional theory calculations provide fundamental insight into how strain-induced band structure engineering correlates with the promoted HER activity, especially at the atomic edge sites of the materials.

DESC: An Automated Strategy to Efficiently Account for Dynamic Environment Effects in Solution

The properties and dynamic behavior of molecules in liquid solutions depend critically on the solvent and other species, or cosolutes, including electrolytes (if present), especially when molecular association or pairing occurs. In Quantum Mechanical (QM) calculations, the electronic structure of molecules in liquid solution is typically obtained with implicit solvent models (ISMs). However, ISMs cannot differentiate between, for example, cation types (e.g., Cs+ versus nBu4N+), leading to limited accuracy in capturing possible solute-specific interactions. Addressing this issue in QM calculations often requires an explicit treatment of the cosolute, typically a counterion, a challenging approach due to the definition of representative cosolute positions, numerical convergence, and high computational cost for bulky species. A new computational strategy called Dynamic Environment in Solution by Clustering (DESC) is herein presented, which leverages classical Molecular Dynamics (MD) data to feed QM calculations, enabling the inclusion of counterion-specific effects with greater detail and efficiency than ISMs. DESC is particularly advantageous in cases where ion pairing/aggregation is significant, offering chemically representative QM results at a small fraction of the computational cost associated with the explicit inclusion of counterions in the model. This work presents MD data on polyoxometalate-counterion-solvent systems, introduces the philosophy behind DESC and its operational details, and applies it to polyoxometalate solutions and other relevant systems, comparing outcomes with benchmark QM/ISM calculations.

Hydrophobic Ionic Liquid Engineering for Reversing CO Intermediate Configuration toward Ampere-Level CO2 Electroreduction to C2+ Products

Hydrophobic ionic liquid (HIL) engineering on the catalyst surface represents a simple yet potent direction for optimizing the CO2 electroreduction performance. However, the pivotal role of HIL engineering at an industrial current density is still ambiguous due to limited and conflicting research findings. Herein, HIL-engineered oxide-derived Cu porous nanoparticles with electron-delocalized groups and a specific ultramicropore structure are first constructed to facilitate CO2-to-C2+ electroreduction at ampere-level current densities. The uniformly decorated HIL is innovatively demonstrated by positron annihilation lifetime spectroscopy, which offers unparalleled advantages in ultramicropore characterization. Bader charge-dependent performance analyses and theoretical calculations disclose that the N atoms in the HIL lower the adsorption energy of CO on the atop site from -0.38 to -1.42 eV through electron donation, which inverts the most stable adsorption site and favors the energy-efficient dimerization of atop-bound CO. Operando Raman spectra and in situ attenuated total reflection-surface enhanced infrared absorption spectroscopy indicate that the adhered HIL increases *CO coverage and alters the *CO adsorption configuration to an atop-bound state with an abundant high-frequency band. Furthermore, staircase potential electrochemical impedance spectroscopy unravels the specific arrangement structure of HIL enlarges the electrochemical surface charge by about 1.5 times, thereby accelerating CO2 electroreduction. As a result, the HIL-engineered oxide-derived Cu porous nanoparticles achieve a prominent C2+ productivity with a Faradaic efficiency of 85.1% and a formation rate up to 2512 μmol h-1 cm-2, outperforming most reported Cu-based electrocatalysts.

Carbon Flow in Acidic CO2 Electroreduction

Electrochemical CO2 reduction in acidic media attracts extensive research attention due to its potential in increasing carbon efficiency. In most reports, alkali cations are introduced to suppress hydrogen evolution and to promote CO2 reduction. However, the mass transport of alkali cations through cation exchange membrane induces the change of electrolyte compositions. Herein, the variation of electrolyte compositions and the flow of carbon during CO2 reduction are analyzed quantitatively by simulation and experiments. If the initial amount of alkali cations in the anolyte is higher than the initial amount of H+ in the catholyte, the pH of the catholyte increases remarkably in long-term CO2 reduction electrolysis, resulting in the decrease of carbon efficiency. Bicarbonate salt precipitation on the cathode with alkali cation-containing catholyte is another origin of the decrease of CO2 reduction Faradaic efficiency and carbon efficiency. To maintain high carbon efficiency, the electrolyte should contain low concentration of alkali cations or even be free of alkali cations. Decorating the catalyst of cathode with ionomer with high density of cation sites enables CO2 reduction in pure acid solution, achieving 30-h stable carbon efficiency.

Cation effects on CO2 reduction catalyzed by single-crystal and polycrystalline gold under well-defined mass transport conditions

The presence of alkali metal cations in the electrolyte substantially affects the reactivity and selectivity of electrochemical carbon dioxide (CO2) reduction (CO2R). This study examines the role of cations in CO2R on single-crystal and polycrystalline Au under controlled mass-transport conditions. It establishes that CO2 adsorption is the rate-determining step regardless of cation type or surface structure. Density functional theory calculations show that electron transfer occurs to a solvated CO2-cation complex. A more positive potential of zero charge enhances CO2R activity only on Au with similar surface coordination. The symmetry factor (β) of the rate-determining step varies with surface structure and cation identity, with density functional theory calculations indicating β's sensitivity to surface and double-layer structures. These findings emphasize the importance of both surface and double-layer structures in understanding cation effects on CO2R.

Cation Effect on the Electrochemical Platinum Dissolution

Ensuring the stability of electrocatalysts is paramount to the success of electrochemical energy conversion devices. Degradation is a fundamental process involving the release of positively charged metal ions into the electric double layer (EDL) and their subsequent diffusion into the bulk electrolyte. However, despite its vital importance in achieving prolonged electrocatalysis, the underlying causality of catalyst dissolution with the EDL structure remains largely unknown. Here, we show that electrochemical Pt dissolution is strongly influenced by the identity of the alkali metal cation (AM+) in the electrolyte. By monitoring Pt dissolution in real-time, we found a trend of reduced Pt leaching in the sequence Li+ > Na+ > K+ > Cs+. Our computational predictions suggest that interfacial OH- concentration plays a pivotal role in Pt dissolution, where OH- facilitates the outward diffusion of dissolved Pt ions into the bulk electrolyte by neutralizing the Ptz+ species, thereby screening the migration force for their redeposition. Combined with this theoretical result, we verify a strong correlation between the amount of dissolved Pt and the hydrolysis pKa or acidity of AM+, indicating that the AM+ identity determines the local OH- concentration and thereby modifies the amount of Pt dissolution. Our results underscore the need to tune the EDL structure to achieve durable electrocatalysis, a promising area for future research.

Specific Adsorption of Alkaline Cations Enhances CO-CO Coupling in CO2 Electroreduction

Electrolyte alkaline cations can significantly modulate the reaction selectivity of electrochemical CO2 reduction (eCO2R), enhancing the yield of the valuable multicarbon (C2+) chemical feedstocks. However, the mechanism underlying this cation effect on the C-C coupling remains unclear. Herein, by performing constant-potential AIMD simulations, we studied the dynamic behavior of interfacial K+ ions over Cu surfaces during C-C coupling and the origin of the cation effect. We showed that the specific adsorption of K+ readily occurs at the surface sites adjacent to the *CO intermediates on the Cu surfaces. Furthermore, this specific adsorption of K+ during *CO-*CO coupling is more important than quasi-specific adsorption for enhancing coupling kinetics, reducing the coupling barriers by approximately 0.20 eV. Electronic structure analysis revealed that charge redistribution occurs between the specifically adsorbed K+, *CO, and Cu sites, and this can account for the reduced barriers. In addition, we identified excellent *CO-*CO coupling selectivity on Cu(100) with K+ ions. Experimental results show that suppressing surface K+-specific adsorption using the surfactant cetyltrimethylammonium bromide (CTAB) significantly decreases the Faradaic efficiency for C2 products from 41.1% to 4.3%, consistent with our computational findings. This study provides crucial insights for improving the selectivity toward C2+ products by rationally tuning interfacial cation adsorption during eCO2R. Specifically, C-C coupling can be enhanced by promoting K+-specific adsorption, for example, by confining K+ within a coated layer or using pulsed negative potentials.

Quantitative Analysis and Manipulation of Alkali Metal Cations at the Cathode Surface in Membrane Electrode Assembly Electrolyzers for CO2 Reduction Reactions

The stable operation of the CO2 reduction reaction (CO2RR) in membrane electrode assembly (MEA) electrolyzers is known to be hindered by the accumulation of bicarbonate salt, which are derived from alkali metal cations in anolytes, on the cathode side. In this study, we conducted a quantitative evaluation of the correlation between the CO2RR activity and the transported alkali metal cations in MEA electrolyzers. As a result, although the presence of transported alkali metal cations on the cathode surface significantly contributes to the generation of C2+ compounds, the rate of K+ ion transport did not match the selectivity of C2+, suggesting that a continuous supply of high amount of K+ to the cathode surface is not required for C2+ formation. Based on these findings, we achieved a faradaic efficiency (FE) and a partial current density for C2+ of 77 % and 230 mA cm-2, respectively, even after switching the anode solution from 0.1 M KHCO3 to a dilute K+ solution (<7 mM). These values were almost identical to those when 0.1 M KHCO3 was continuously supplied. Based on this insight, we successfully improved the durability of the system against salt precipitation by intermittently supplying concentrated KHCO3, compared with the continuous supply.

The Loss of Interfacial Water-Adsorbate Hydrogen Bond Connectivity Position Surface-Active Hydrogen as a Crucial Intermediate to Enhance Nitrate Reduction Reaction

The electrochemical nitrate reduction reaction (NO3RR) offers a promising solution for remediating nitrate-polluted wastewater while enabling the sustainable production of ammonia. The control strategy of surface-active hydrogen (*H) is extensively employed to enhance the kinetics of the NO3RR, but atomic understanding lags far behind the experimental observations. Here, we decipher the cation-water-adsorbate interactions in regulating the NO3RR kinetics at the Cu (111) electrode/electrolyte interface using AIMD simulations with a slow-growth approach. We demonstrate that the key oxygen-containing intermediates of the NO3RR (e.g., *NO, *NO2, and *NO3) will stably coordinate with the cations, impeding their integration with the hydrogen bond network and further their hydrogenation by interfacial water molecules due to steric hindrance. The *H can migrate across the interface with a low energy barrier, and its hydrogenation barrier with oxygen-containing species remains unaffected by cations, offering a potent supplement to the hydrogenation process, playing the predominant factor by which the *H facilitates NO3RR reaction kinetic. This study provides valuable insights for understanding the reaction mechanism of NO3RR by fully considering the cation-water-adsorbate interactions, which can aid in the further development of the electrolyte and electrocatalysts for efficient NO3RR.

Strong effect-correlated electrochemical CO2 reduction

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

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.

Quantifying effects of second-sphere cationic groups on redox properties of dimolybdenum quadruple bonds

A series of four dimolybdenum paddlewheel complexes supported by anionic N,N-dimethylglycinate (DMG) or zwitterionic N,N,N-trimethylglycine (TMG) ligands was synthesised to examine the effects of charged groups in the second coordination sphere on redox properties of MoMo bonds. An average shift in reduction potential of +35 mV per cationically charged group was measured, which is approximately half of what would be expected for an analogous mononuclear complex.

Recent advances in computational prediction of molecular properties in food chemistry

The combination of food chemistry and computational simulation has brought many impacts to food research, moving from experimental chemistry to computer chemistry. This paper will systematically review in detail the important role played by computational simulations in the development of the molecular structure of food, mainly from the atomic, molecular, and multicomponent dimension. It will also discuss how different computational chemistry models can be constructed and analyzed to obtain reliable conclusions. From the calculation principle to case analysis, this paper focuses on the selection and application of quantum mechanics, molecular mechanics and coarse-grained molecular dynamics in food chemistry research. Finally, experiments and computations of food chemistry are compared and summarized to obtain the best balance between them. The above review and outlook will provide an important reference for the intersection of food chemistry and computational chemistry, and is expected to provide innovative thinking for structural research in food chemistry.

Identification of K+-determined reaction pathway for facilitated kinetics of CO2 electroreduction

Cations such as K+ play a key part in the CO2 electroreduction reaction, but their role in the reaction mechanism is still in debate. Here, we use a highly symmetric Ni-N4 structure to selectively probe the mechanistic influence of K+ and identify its interaction with chemisorbed CO2-. Our electrochemical kinetics study finds a shift in the rate-determining step in the presence of K+. Spectral evidence of chemisorbed CO2- from in-situ X-ray absorption spectroscopy and in-situ Raman spectroscopy pinpoints the origin of this rate-determining step shift. Grand canonical potential kinetics simulations - consistent with experimental results - further complement these findings. We thereby identify a long proposed non-covalent interaction between K+ and chemisorbed CO2-. This interaction stabilizes chemisorbed CO2- and thus switches the rate-determining step from concerted proton electron transfer to independent proton transfer. Consequently, this rate-determining step shift lowers the reaction barrier by eliminating the contribution of the electron transfer step. This K+-determined reaction pathway enables a lower energy barrier for CO2 electroreduction reaction than the competing hydrogen evolution reaction, leading to an exclusive selectivity for CO2 electroreduction reaction.

Boosting the Electroreduction of CO2 to CO by Ligand Engineering of Gold Nanoclusters

The electrochemical CO2 reduction reaction (CO2RR) has been widely studied as a promising means to convert anthropogenic CO2 into valuable chemicals and fuels. In this process, the alkali metal ions present in the electrolyte are known to significantly influence the CO2RR activity and selectivity. In this study, we report a strategy for preparing efficient electrocatalysts by introducing a cation-relaying ligand, namely 6-mercaptohexanoic acid (MHA), into atom-precise Au25 nanoclusters (NCs). The CO2RR activity of the synthesized Au25(MHA)18 NCs was compared with that of Au25(HT)18 NCs (HT=1-hexanethiolate). While both NCs selectively produced CO over H2, the CO2-to-CO conversion activity of the Au25(MHA)18 NCs was significantly higher than that of the Au25(HT)18 NCs when the catholyte pH was higher than the pKa of MHA, demonstrating the cation-relaying effect of the anionic terminal group. Mechanistic investigations into the CO2RR occurring on the Au25 NCs in the presence of different catholyte cations and concentrations revealed that the CO2-to-CO conversion activities of these Au25 NCs increased in the order Li+ Collapse

Key Words

• CO2 reduction reaction (CO2RR)
• gold
• ligand engineering
• nanocluster

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Grants

• National Research Foundation of Korea (NRF) grants (no. NRF-2022R1A2C3003610) Ministry of Science and ICT, South Korea
• Carbon-to-X Project (Project no. 2020M3H7A1096388) Ministry of Science and ICT, South Korea
• Post Doc. Researcher Supporting Program (project no.: 2024-12-0026) Yonsei University

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Affiliation(s)

Sang Myeong Han Department of Chemistry, Yonsei University, Seoul, 03722, Republic of Korea
Minyoung Park Department of Chemistry, Yonsei University, Seoul, 03722, Republic of Korea
Jiyoung Kim Department of Chemistry, Yonsei University, Seoul, 03722, Republic of Korea
Dongil Lee Department of Chemistry, Yonsei University, Seoul, 03722, Republic of Korea

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Collaborators Collapse

Accelerating QM/MM simulations of electrochemical interfaces through machine learning of electronic charge densities

A crucial aspect in the simulation of electrochemical interfaces consists in treating the distribution of electronic charge of electrode materials that are put in contact with an electrolyte solution. Recently, it has been shown how a machine-learning method that specifically targets the electronic charge density, also known as SALTED, can be used to predict the long-range response of metal electrodes in model electrochemical cells. In this work, we provide a full integration of SALTED with MetalWalls, a program for performing classical simulations of electrochemical systems. We do so by deriving a spherical harmonics extension of the Ewald summation method, which allows us to efficiently compute the electric field originated by the predicted electrode charge distribution. We show how to use this method to drive the molecular dynamics of an aqueous electrolyte solution under the quantum electric field of a gold electrode, which is matched to the accuracy of density-functional theory. Notably, we find that the resulting atomic forces present a small error of the order of 1 meV/Å, demonstrating the great effectiveness of adopting an electron-density path in predicting the electrostatics of the system. Upon running the data-driven dynamics over about 3 ns, we observe qualitative differences in the interfacial distribution of the electrolyte with respect to the results of a classical simulation. By greatly accelerating quantum-mechanics/molecular-mechanics approaches applied to electrochemical systems, our method opens the door to nanosecond timescales in the accurate atomistic description of the electrical double layer.

Solid Electrolytes for Low-Temperature Carbon Dioxide Valorization: A Review

One of the most promising approaches to address the global challenge of climate change is electrochemical carbon capture and utilization. Solid electrolytes can play a crucial role in establishing a chemical-free pathway for the electrochemical capture of CO2. Furthermore, they can be applied in electrocatalytic CO2 reduction reactions (CO2RR) to increase carbon utilization, produce high-purity liquid chemicals, and advance hybrid electro-biosystems. This review article begins by covering the fundamentals and processes of electrochemical CO2 capture, emphasizing the advantages of utilizing solid electrolytes. Additionally, it highlights recent advancements in the use of the solid polymer electrolyte or solid electrolyte layer for the CO2RR with multiple functions. The review also explores avenues for future research to fully harness the potential of solid electrolytes, including the integration of CO2 capture and the CO2RR and performance assessment under realistic conditions. Finally, this review discusses future opportunities and challenges, aiming to contribute to the establishment of a green and sustainable society through electrochemical CO2 valorization.

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

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

Origin of copper as a unique catalyst for C-C coupling in electrocatalytic CO2 reduction

High yields of C2 products through electrocatalytic CO2 reduction (eCO2R) can only be obtained using Cu-based catalysts. Here, we adopt the generalized frontier molecular orbital (MO) theory based on first-principles calculations to identify the origin of this unique property of Cu. We use the grand canonical ensemble (or fixed potential) approach to ensure that the calculated Fermi level, which serves as the frontier orbital of the metal catalyst, accurately represents the applied electrode potentials. We determine that the key intermediate OCCO assumes a U-shape configuration with the two C atoms bonded to the Cu substrate. We identify the frontier MOs that are involved in the C-C coupling. The good alignment of the Fermi level of Cu with these frontier MOs is perceived to account for the excellent catalytic performance of Cu for C-C coupling. It is expected that these new insights could provide useful guidance in tuning Cu-based catalysts as well as designing non-Cu catalysts toward high-efficiency eCO2R.

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

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

Overcoming Low C2+ Yield in Acidic CO2 Electroreduction: Modulating Local Hydrophobicity for Enhanced Performance

Operating electrochemical CO2 reduction reaction (CO2RR) in acidic media has garnered considerable attention due to its sustainable electrolyte cycling and stable performance. Nevertheless, the severe parasitic hydrogen evolution reaction (HER) and decayed multi-carbon species (C2+) yield still hampers efficient CO2RR in acid. Here, this work investigates the influence of local hydrophobicity on the acidic CO2RR. By employing direct electrodeposition, the hydrophobicity of the catalyst layer can be finely tuned over a wide range without additive. It is revealed that the hydrophobic microenvironment significantly suppressed HER, improved CO2RR performance and boosted C2+ yield. A Faradaic efficiency (FE) of ≈74% for C2+ is achieved in pH = 2 on electrodeposited copper with a highly hydrophobic environment. Moreover, this phenomenon can be extended to industrial application. An ≈81% total FE for the CO2RR, along with a ≈62% FE for C2+ species, is achieved even with commercial copper. Remarkably, the system exhibited stable operation for a continuous period exceeding 50 h at an industrially applied current density of 300 mA cm-2. This work highlights the crucial role of interface hydrophobicity in acidic CO2RR and proposes a facile and universally applicable method for achieving efficient and stable CO2RR to high-value products in acidic media.

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

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

Recent advances in the role of interfacial liquids in electrochemical reactions

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

In-Induced Electronic Structure Modulations of Bi─O Active Sites for Selective Carbon Dioxide Electroreduction to Liquid Fuel in Strong Acid

The formation of carbonate in neutral/alkaline solutions leads to carbonate crossover, severely reducing carbon dioxide (CO2 ) single pass conversion efficiency (SPCE). Thus, CO2 electrolysis is a prospective route to achieve high CO2 utilization under acidic environment. Bimetallic Bi-based catalysts obtained utilizing metal doping strategies exhibit enhanced CO2 -to-formic acid (HCOOH) selectivity in alkaline/neutral media. However, achieving high HCOOH selectivity remains challenging in acidic media. To this end, Indium (In) doped Bi2O2CO3 via hydrothermal method is prepared for in-situ electroreduction to In-Bi/BiOx nanosheets for acidic CO2 reduction reaction (CO2RR). In doping strategy regulates the electronic structure of Bi, promoting the fast derivatization of Bi2O2CO3 into Bi-O active sites to enhance CO2RR catalytic activity. The optimized Bi2 O2 CO3 -derived catalyst achieves the maximum HCOOH faradaic efficiency (FE) of 96% at 200 mA cm-2 . The SPCE for HCOOH production in acid is up to 36.6%, 2.2-fold higher than the best reported catalysts in alkaline environment. Furthermore, in situ Raman and X-ray photoelectron spectroscopy demonstrate that In-induced electronic structure modulation promotes a rapid structural evolution from nanobulks to Bi/BiOx nanosheets with more active species under acidic CO2 RR, which is a major factor in performance improvement.

Accelerating the Reaction Kinetics of CO2 Reduction to Multi-Carbon Products by Synergistic Effect between Cation and Aprotic Solvent on Copper Electrodes

Improving the selectivity of electrochemical CO2 reduction to multi-carbon products (C2+ ) is an important and highly challenging topic. In this work, we propose and validate an effective strategy to improve C2+ selectivity on Cu electrodes, by introducing a synergistic effect between cation (Na+ ) and aprotic solvent (DMSO) to the electrolyte. Based on constant potential ab initio molecular dynamics simulations, we first revealed that Na+ facilitates C-C coupling while inhibits CH3 OH/CH4 products via reducing the water network connectivity near the electrode. Furthermore, the water network connectivity was further decreased by introducing an aprotic solvent DMSO, leading to suppression of both C1 production and hydrogen evolution reaction with minimal effect on *OCCO* hydrogenation. The synergistic effect enhancing C2 selectivity was also experimentally verified through electrochemical measurements. The results showed that the Faradaic efficiency of C2 increases from 9.3 % to 57 % at 50 mA/cm2 under a mixed electrolyte of NaHCO3 and DMSO compared to a pure NaHCO3 , which can significantly enhance the selectivity of the C2 product. Therefore, our discovery provides an effective electrolyte-based strategy for tuning CO2 RR selectivity through modulating the microenvironment at the electrode-electrolyte interface.

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

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

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.

Vitamin C-induced CO2 capture enables high-rate ethylene production in CO2 electroreduction

High-rate production of multicarbon chemicals via the electrochemical CO2 reduction can be achieved by efficient CO2 mass transport. A key challenge for C-C coupling in high-current-density CO2 reduction is how to promote *CO formation and dimerization. Here, we report molecularly enhanced CO2-to-*CO conversion and *CO dimerization for high-rate ethylene production. Nanoconfinement of ascorbic acid by graphene quantum dots enables immobilization and redox reversibility of ascorbic acid in heterogeneous electrocatalysts. Cu nanowire with ascorbic acid nanoconfined by graphene quantum dots (cAA-CuNW) demonstrates high-rate ethylene production with a Faradaic efficiency of 60.7% and a partial current density of 539 mA/cm2, a 2.9-fold improvement over that of pristine CuNW. Furthermore, under low CO2 ratio of 33%, cAA-CuNW still exhibits efficient ethylene production with a Faradaic efficiency of 41.8%. We find that cAA-CuNW increases *CO coverage and optimizes the *CO binding mode ensemble between atop and bridge for efficient C-C coupling. A mechanistic study reveals that ascorbic acid can facilitate *CO formation and dimerization by favorable electron and proton transfer with strong hydrogen bonding.

An implicit electrolyte model for plane wave density functional theory exhibiting nonlinear response and a nonlocal cavity definition

We have developed and implemented an implicit electrolyte model in the Vienna Ab initio Simulation Package (VASP) that includes nonlinear dielectric and ionic responses as well as a nonlocal definition of the cavities defining the spatial regions where these responses can occur. The implementation into the existing VASPsol code is numerically efficient and exhibits robust convergence, requiring computational effort only slightly higher than the original linear polarizable continuum model. The nonlinear + nonlocal model is able to reproduce the characteristic "double hump" shape observed experimentally for the differential capacitance of an electrified metal interface while preventing "leakage" of the electrolyte into regions of space too small to contain a single water molecule or solvated ion. The model also gives a reasonable prediction of molecular solvation free energies as well as the self-ionization free energy of water and the absolute electron chemical potential of the standard hydrogen electrode. All of this, combined with the additional ability to run constant potential density functional theory calculations, should enable the routine computation of activation barriers for electrocatalytic processes.

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

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

Realistic Modeling of the Electrocatalytic Process at Complex Solid-Liquid Interface

The rational design of electrocatalysis has emerged as one of the most thriving means for mitigating energy and environmental crises. The key to this effort is the understanding of the complex electrochemical interface, wherein the electrode potential as well as various internal factors such as H-bond network, adsorbate coverage, and dynamic behavior of the interface collectively contribute to the electrocatalytic activity and selectivity. In this context, the authors have reviewed recent theoretical advances, and especially, the contributions to modeling the realistic electrocatalytic processes at complex electrochemical interfaces,  and illustrated the challenges and fundamental problems in this field. Specifically, the significance of the inclusion of explicit solvation and electrode potential as well as the strategies toward the design of highly efficient electrocatalysts are discussed. The structure-activity relationships and their dynamic responses to the environment and catalytic functionality under working conditions are illustrated to be crucial factors for understanding the complexed interface and the electrocatalytic activities. It is hoped that this review can help spark new research passion and ultimately bring a step closer to a realistic and systematic modeling method for electrocatalysis.

Electrochemical carbon-carbon coupling with enhanced activity and racemate stereoselectivity by microenvironment regulation

Enzymes are characteristic of catalytic efficiency and specificity by maneuvering multiple components in concert at a confined nanoscale space. However, achieving such a configuration in artificial catalysts remains challenging. Herein, we report a microenvironment regulation strategy by modifying carbon paper with hexadecyltrimethylammonium cations, delivering electrochemical carbon-carbon coupling of benzaldehyde with enhanced activity and racemate stereoselectivity. The modified electrode-electrolyte interface creates an optimal microenvironment for electrocatalysis-it engenders dipolar interaction with the reaction intermediate, giving a 2.2-fold higher reaction rate (from 0.13 to 0.28 mmol h-1 cm-2); Moreover, it repels interfacial water and modulates the conformational specificity of reaction intermediate by facilitating intermolecular hydrogen bonding, affording 2.5-fold higher diastereomeric ratio of racemate to mesomer (from 0.73 to 1.82). We expect that the microenvironment regulation strategy will lead to the advanced design of electrode-electrolyte interface for enhanced activity and (stereo)selectivity that mimics enzymes.

Probing Cation Effects on *CO Intermediates from Electroreduction of CO2 through Operando Raman Spectroscopy

Cations in an electrolyte modulate microenvironments near the catalyst surface and affect product distribution from an electrochemical CO2 reduction reaction, and thus, their interaction with intermediate states has been tried to be probed. Herein, we directly observed the cation effect on *CO intermediates on the Cu(OH)2-derived catalyst in real time through operando surface-enhanced Raman spectroscopy at high overpotentials (-1.0 VRHE). Atop *CO peaks are composed of low-frequency binding *CO (*COLFB) and high-frequency binding *CO (*COHFB) because of their adsorption sites. These two *CO intermediates are found to have different sensitivities to the cation-induced field, and each *CO is proposed to be suitably stabilized for efficient C-C coupling. The proportions between *COHFB and *COLFB are dependent on the type of alkali cations, and the increases in the *COHFB ratio have a high correlation with selective C2H4 production under K+ and Cs+, indicating that *COHFB is the dominant and fast active species. In addition, as the hydrated cation size decreases, *COLFB is more sensitively red-shifted than *COHFB, which promotes C-C coupling and suppresses C1 products. Through time-resolved operando measurements, dynamic changes between the two *CO species are observed, showing the rapid initial adsorption of *COHFB and subsequently reaching a steady ratio between *COLFB and *COHFB.

An Interfacial View of Cation Effects on Electrocatalysis Systems

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

Metal Site-Specific Electrostatic Field Effects on a Tricopper(I) Cluster Probed by Resonant Diffraction Anomalous Fine Structure (DAFS)

Studies of multinuclear metal complexes are greatly enhanced by resonant diffraction measurements, which probe X-ray absorption profiles of crystallographically independent metal sites within a cluster. In particular, X-ray diffraction anomalous fine structure (DAFS) analysis provides data that can be interpreted akin to site-specific XANES, allowing for differences in metal K-edge resonances to be deconvoluted even for different metal sites within a homometallic system. Despite the prevalence of Cu-containing clusters in biology and energy science, DAFS has yet to be used to analyze multicopper complexes of any type until now. Here, we report an evaluation of trends using a series of strategically chosen Cu(I) and Cu(II) complexes to determine how energy dependencies of anomalous scattering factors are impacted by coordination geometry, ligand shell, cluster nuclearity, and oxidation state. This calibration data is used to analyze a formally tricopper(I) complex that was found by DAFS to be site-differentiated due to the unsymmetrical influence on different Cu sites of the electrostatic field from a proximal K+ cation.

Surface-immobilized cross-linked cationic polyelectrolyte enables CO2 reduction with metal cation-free acidic electrolyte

Electrochemical CO2 reduction in acidic electrolytes is a promising strategy to achieve high utilization efficiency of CO2. Although alkali cations in acidic electrolytes play a vital role in suppressing hydrogen evolution and promoting CO2 reduction, they also cause precipitation of bicarbonate on the gas diffusion electrode (GDE), flooding of electrolyte through the GDE, and drift of the electrolyte pH. In this work, we realize the electroreduction of CO2 in a metal cation-free acidic electrolyte by covering the catalyst with cross-linked poly-diallyldimethylammonium chloride. This polyelectrolyte provides a high density of cationic sites immobilized on the surface of the catalyst, which suppresses the mass transport of H+ and modulates the interfacial field strength. By adopting this strategy, the Faradaic efficiency (FE) of CO reaches 95 ± 3% with the Ag catalyst and the FE of formic acid reaches 76 ± 3% with the In catalyst in a 1.0 pH electrolyte in a flow cell. More importantly, with the metal cation-free acidic electrolyte the amount of electrolyte flooding through the GDE is decreased to 2.5 ± 0.6% of that with alkali cation-containing acidic electrolyte, and the FE of CO maintains above 80% over 36 h of operation at -200 mA·cm-2.

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

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

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

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

Microscopic Origin of Electrochemical Capacitance in Metal-Organic Frameworks

Electroconductive metal-organic frameworks (MOFs) have emerged as high-performance electrode materials for supercapacitors, but the fundamental understanding of the underlying chemical processes is limited. Here, the electrochemical interface of Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with an organic electrolyte is investigated using a multiscale quantum-mechanics/molecular-mechanics (QM/MM) procedure and experimental electrochemical measurements. Our simulations reproduce the observed capacitance values and reveals the polarization phenomena of the nanoporous framework. We find that excess charges mainly form on the organic ligand, and cation-dominated charging mechanisms give rise to greater capacitance. The spatially confined electric double-layer structure is further manipulated by changing the ligand from HHTP to HITP (HITP = 2,3,6,7,10,11-hexaiminotriphenylene). This minimal change to the electrode framework not only increases the capacitance but also increases the self-diffusion coefficients of in-pore electrolytes. The performance of MOF-based supercapacitors can be systematically controlled by modifying the ligating group.

The importance of a charge transfer descriptor for screening potential CO2 reduction electrocatalysts

It has been over twenty years since the linear scaling of reaction intermediate adsorption energies started to coin the fields of heterogeneous and electrocatalysis as a blessing and a curse at the same time. It has established the possibility to construct activity volcano plots as a function of a single or two readily accessible adsorption energies as descriptors, but also limited the maximal catalytic conversion rate. In this work, it is found that these established adsorption energy-based descriptor spaces are not applicable to electrochemistry, because they are lacking an important additional dimension, the potential of zero charge. This extra dimension arises from the interaction of the electric double layer with reaction intermediates which does not scale with adsorption energies. At the example of the electrochemical reduction of CO₂ it is shown that the addition of this descriptor breaks the scaling relations, opening up a huge chemical space that is readily accessible via potential of zero charge-based material design. The potential of zero charge also explains product selectivity trends of electrochemical CO₂ reduction in close agreement with reported experimental data highlighting its importance for electrocatalyst design.

Doping and pretreatment optimized the adsorption of *OCHO on bismuth for the electrocatalytic reduction of CO2 to formate

Electrocatalytic reduction of CO₂ to formate is considered as a promising method to achieve carbon neutrality, and the introduction of heteroatoms is an effective strategy to improve the catalytic activity and selectivity of catalysts. However, the structural reconstruction behavior of catalysts driven by voltage is usually ignored. Therefore, we used Cu/Bi₂S₃ as a model to reveal the dynamic reduction process in different atmospheric environments. The catalyst showed an outstanding faradaic efficiency of 94% for formate and a long-term stability of 100 h, and exhibited a high current density of 280 mA cm^-2 in a flow cell. The experimental results and theoretical calculations show that the introduction of copper enhances the adsorption of CO₂, accelerates the charge transfer and reduces the formation barrier of *OCHO, thus promoting the formation of formate. This work draws attention to the effects of saturated gases in the electrolyte during structural evolution and provides a possibility for designing catalysts with high catalytic activity.