Cation- and pH-Dependent Hydrogen Evolution and Oxidation Reaction Kinetics

Super-Concentrated Alkali Hydroxide Electrolytes for Rechargeable Zn Batteries

Increasing the salt concentration of an electrolyte to over 10 M brings new solute-solvent interactions that define an emerging class of super-concentrated electrolytes for rechargeable batteries. To this class we introduce a super-concentrated alkaline electrolyte. Nearly saturated with KOH (at 15 M), the aqueous solution displays a broad electrochemical stability window (>2.5 V on Au) while retaining an exceptionally high ionic conductivity (>0.27 S/cm at 25 °C). Without a solid-electrolyte interphase, we can confirm the role of electron transfer kinetics in determining the stability as opposed to the thermodyanmic effect computed based on activity coefficients. The compositionally simple solution also allows spectroscopies and ab-initio molecular dynamics simulation to identify a novel mechanism of OH- structural diffusion, where unlike the conventional Grotthuss mechanism through hydrogen-bond networks, electrostatic forces sustain proton hopping and break the common stability-conductivity tradeoff. A high ZnO solubility in the electrolyte further mitigate the issue of passivation when a Zn anode is deeply discharged. These unique properties enable a NiOOH||Zn battery to deliver a cumulative capacity >10 Ah/cm2 at 40 mA/cm2 to meet practical needs of safe, inexpensive energy storage.

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.

Enhancing Built-In Electric Field via Balancing Interfacial Atom Orbit Hybridization at Boride@Sulfide Heterostructure for Hydrogen Evolution Reaction

Exploring nonprecious metal-based catalysts for cathodic hydrogen evolution reaction (HER) has facilitated the realization of hydrogen economy toward water electrolysis in alkaline media. However, the difficult water dissociation process for the Volmer step (H2O → H* + OH*) and the subsequent unsuitable OH* adsorption energy on nonprecious metal-based catalysts severely reduce the kinetics of HER. Herein, the universal synthesis for a series of transition metal (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W)-based boride@sulfide heterostructured catalysts is realized by using the molten-salt method to conduct the in situ boronization of commercial sulfides. Significantly, WB2@WS2 heterostructured catalyst exhibits excellent catalytic activity and stability for HER. Balancing interfacial atom orbit hybridization between W(d)-B(s,p) and W(d)-S(s,p) at WB2@WS2 heterostructured interface enhances the built-in electric field. In situ Raman spectroscopy and density functional theory calculation results reveal that the strong built-in electric field in WB2@WS2 optimizes the adsorption and desorption of OH* intermediate, reducing the energy barrier of the rate-determining step (OH* desorption step), and thus favoring the enhancement of catalytic performance toward HER.

Reorganizing the Pt Surface Water Structure for Highly Efficient Alkaline Hydrogen Oxidation Reaction

The hydrogen oxidation reaction (HOR) in alkaline electrolytes exhibits markedly slower kinetics than that in acidic electrolytes. This poses a critical challenge for alkaline exchange membrane fuel cells (AEMFCs). The slower kinetics in alkaline electrolytes is often attributed to the more sluggish Volmer step (hydrogen desorption). It has been shown that the alkaline HOR activity on the Pt surface can be considerably enhanced by the presence of oxophilic transition metals (TMs) and surface-adsorbed hydroxyl groups on TMs (TM-OHad), although the exact role of TM-OHad remains a topic of active debates. Herein, using single-atom Rh-tailored Pt nanowires as a model system, we demonstrate that hydroxyl groups adsorbed on the Rh sites (Rh-OHad) can profoundly reorganize the Pt surface water structure to deliver a record-setting alkaline HOR performance. In situ surface characterizations, together with theoretical studies, reveal that surface Rh-OHad could promote the oxygen-down water (H2O↓) that favors more hydrogen bond with Pt surface adsorbed hydrogen (H2O↓···Had-Pt) than the hydrogen-down water (OH2↓). The H2O↓ further serves as the bridge to facilitate the formation of an energetically favorable six-membered-ring transition structure with neighboring Pt-Had and Rh-OHad, thus reducing the Volmer step activation energy and boosting HOR kinetics.

Highly Tension-Strained Copper Concentrates Diluted Cations for Selective Proton-Exchange Membrane CO2 Electrolysis

Electrolysis of carbon dioxide (CO2) in acid offers a promising route to overcome CO2 loss in alkaline and neutral electrolytes, but requires concentrated alkali cations (typical ≥3 M) to mitigate the trade-off between low pH and high hydrogen evolution reaction (HER) rate, causing salt precipitation. Here we report a strategy to resolve this problem by introducing tensile strain in a copper (Cu) catalyst, which can selectively reduce CO2 to valuable multicarbon products, particularly ethylene, in a pH 1 electrolyte with 1 M potassium ions. We find that the tension-strained Cu creates an electron-rich surface that concentrates diluted potassium ions, contributing to CO2 activation and HER suppression. With this catalyst, we show constant ethylene Faradaic efficiency (FE) of 44.3 % over 100 hours at 400 mA cm-2 and a cell voltage of 3.1 volts in a proton-exchange membrane electrolyser. Moreover, selective electrosynthesis of ethylene oxide using the as-produced ethylene was demonstrated in an integrated system.

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.

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.

Electrochemical Lithiation Regulates the Active Hydrogen Supply on Ru-Sn Nanowires for Hydrogen Evolution Toward the High-Performing Anion Exchange Membrane Water Electrolyzer

Designing a rational electrocatalyst/electrolyte interface with superb active hydrogen supply is of significant importance for the alkaline hydrogen evolution reaction (HER) and anion exchange membrane water electrolyzers (AEMWEs). Here, we propose a strategy to tune the interfacial active hydrogen supply via inducing dissoluble cation into electrocatalysts to boost HER in alkali, with electrochemical lithiated sub-2 nm RuSn0.8 nanowires (NWs) as a proof of concept. It is found that a part of Li+ could dissolve in situ from lithiated RuSn0.8 NWs during HER, which tends to affect the interfacial structure and facilitate the proton transport. Among all the Li-Ru-Sn and Ru-Sn NWs, the best-performing Li3.0RuSn0.8 NWs exhibit the lowest initial overpotential of 66 mV at 100 mA cm-2 in 1.0 M KOH, which could be further reduced to 38 mV after the 30 000 cycles accelerated stability test (AST). In situ Raman spectroscopy and operando X-ray adsorption spectroscopy indicate that the pristine Li3.0RuSn0.8 NWs are highly active toward water dissociation and the dissolved Li+ during AST could further enhance the flexibility of the hydrogen bond network for proton transportation. Ab initio molecular dynamics simulations and density functional theory calculations disclose that the incorporation of Li into the Ru-Sn lattice is beneficial to lower the water dissociation barrier, while dissolved Li+ at the interface significantly increases the population of interfacial water molecules, thereby providing sufficient active hydrogens for H2 production. The AEMWE equipped with the Li3.0RuSn0.8 NWs cathode delivers an extremely low cell voltage (1.689 V) at an industrial-scale current density (1 A cm-2) and outstanding stability (56 μV h-1 loss at 1 A cm-2 after 1000 h galvanostatic test), representing one of the best alkaline HER electrocatalysts ever reported.

Kinetic cation effect in alkaline hydrogen electrocatalysis and double layer proton transfer

Unveiling the so far ambiguous mechanism of the significant dependence on the identity of alkali metal cation would prompt opportunities to solve the more than two orders of magnitude slowdown of hydrogen electrocatalytic kinetics in base relative to acid, which has hampered the effort to reduce the precious metal usage in fuel cells by using the hydroxide exchange membrane. Herein, we present atomic-scale evidences from ab-initio molecular dynamics simulation and in-situ surface-enhanced infrared absorption spectroscopy which show that it is the apparent discrepancies in the electric double-layer structures induced by differently sized cations that lead to largely different interfacial proton transfer barriers and therefore hydrogen electrocatalytic kinetics in base. Concretely, severe accumulation of larger cation in electric double-layer causes more discontinuous interfacial water distribution and H-bond network, thus rendering the proton transfer from bulk to interface more obstructed. Such notion is strikingly different from the previously envisioned impact of cation-intermediate interactions on the energetics of surface steps, providing a unique interfacial perspective for understanding the ubiquitous cation specificity in electrocatalysis.

Bias Dependence of the Transition State of the Hydrogen Evolution Reaction

The hydrogen evolution reaction (HER) is one of the most prominent electrocatalytic reactions of green energy transition. However, the kinetics across materials and electrolyte pH and the impact of hydrogen coverage at high current densities remain poorly understood. Here, we study the HER kinetics over a large set of nanoparticle catalysts in industrially relevant acidic and alkaline membrane electrode assemblies that are only operated with pure water humidified gases. We discover distinct kinetic fingerprints between the iron triad (Fe, Ni, Co), coinage (Au, Cu, Ag), and platinum group metals (Ir, Pt, Pd, Rh). Importantly, the applied bias changes not only the activation energy (EA) but also the pre-exponential factor (A). We interpret these changes as entropic changes in the interfacial solvent that differ between acid and base and entropic changes on the surface due to a changing hydrogen coverage. Finally, we observe that anions can induce Butler-Volmer behavior for the coinage metals in acid. Our results provide a new foundation to understand HER kinetics and, more broadly, highlight the pressing need to update common understanding of basic concepts in the field of electrocatalysis.

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.

Transforming Adsorbate Surface Dynamics in Aqueous Electrocatalysis: Pathways to Unconstrained Performance

Developing highly efficient catalysts to accelerate sluggish electrode reactions is critical for the deployment of sustainable aqueous electrochemical technologies, yet remains a great challenge. Rationally integrating functional components to tailor surface adsorption behaviors and adsorbate dynamics would divert reaction pathways and alleviate energy barriers, eliminating conventional thermodynamic constraints and ultimately optimizing energy flow within electrochemical systems. This approach has, therefore, garnered significant interest, presenting substantial potential for developing highly efficient catalysts that simultaneously enhance activity, selectivity, and stability. The immense promise and rapid evolution of this design strategy, however, do not overshadow the substantial challenges and ambiguities that persist, impeding the realization of significant breakthroughs in electrocatalyst development. This review explores the latest insights into the principles guiding the design of catalytic surfaces that enable favorable adsorbate dynamics within the contexts of hydrogen and oxygen electrochemistry. Innovative approaches for tailoring adsorbate-surface interactions are discussed, delving into underlying principles that govern these dynamics. Additionally, perspectives on the prevailing challenges are presented and future research directions are proposed. By evaluating the core principles and identifying critical research gaps, this review seeks to inspire rational electrocatalyst design, the discovery of novel reaction mechanisms and concepts, and ultimately, advance the large-scale implementation of electroconversion technologies.

Hydroxyl-Binding Induced Hydrogen Bond Network Connectivity on Ru-based Catalysts for Efficient Alkaline Hydrogen Oxidation Electrocatalysis

Understanding the role of adsorbed intermediates at the polarized catalyst-electrolyte interface on the structure of electrical double layer (EDL) is essential for developing highly efficient electrocatalysts. Here, we prepared a series of unconventional face-centered-cubic (fcc) phase Ru-based catalysts (i.e. fcc-Ru, fcc-RuCr, and fcc-RuCrW) by rational tuning the binding energetics of hydroxyl intermediate to engineer the electrochemical interface and boost the performance of alkaline hydrogen oxidation reaction (HOR). The introduction of oxyphilic metals Cr and W can regulate the orbital occupation of Ru, promote the adsorption of hydroxyl species, resulting in an anomalous behavior that HOR performance under alkaline media exceeds acidic media. Experimental results and theoretical calculations unravel that the modulated adsorption of hydroxyl species on the electrode surface are responsible for the reconstruction of interfacial water structure and dynamic evolution of free water molecules from nearest to the electrode surface to above the gap region in the EDL, thereby leading to significantly increased water connectivity and hydrogen bond network. Our work reveals a new understanding of the surface intermediates in controlling the dynamic process of interfacial water and hydrogen bonding network in HOR electrocatalysis, and will guide rational design of advanced electrocatalysts through electrochemical interfacial engineering.

Boosting Alkaline Hydrogen Oxidation Kinetics through Interfacial Environments Induced Surface Migration of Adsorbed Hydroxyl

Constructing bifunctional sites through heterojunction engineering to accelerate water formation has become a pivotal strategy to improve the alkaline hydrogen oxidation reaction (HOR) kinetics, which is mainly focused on the synergistic effect of neighboring sites and the energetics of the surface reaction steps. However, the roles of the surface migration of key intermediates that go beyond the bifunctional mechanism limited to neighboring atoms have usually been ignored. Using the heterostructured Ni3C-Ni catalyst as a model, we found that the rapid surface migration of OHad species from the positively charged Ni3C to the negatively charged Ni component played a decisive role in facilitating water formation. Such unprecedented surface migration of OHad is induced by the large discrepancy between the local surface charge densities and interfacial environments of the Ni3C and Ni components under operating conditions. Benefiting from this, the resultant Ni3C-Ni exhibited outstanding mass activity for the alkaline HOR, which was approximately 19-fold and 21-fold higher than those of Ni and Ni3C, respectively. These findings not only provide novel insights into the alkaline HOR mechanism of heterostructured catalysts but also open new avenues for developing advanced electrocatalysts for alkaline fuel cells.

Hydrogen Electrode Reactions in Energy-Related Electrocatalysis Systems

Hydrogen electrode reactions, including hydrogen evolution reactions and hydrogen oxidation reactions, are fundamental and crucial within aqueous electrochemistry. Particularly in energy-related electrocatalysis processes, there is a consistent involvement of hydrogen-related electrochemical processes, underscoring the need for in-depth study. This review encompasses significant reports, delving into elementary steps and reaction mechanisms of hydrogen electrode reactions, as well as catalyst design strategies. In addition, we focus on the application of hydrogen electrode reaction mechanism in different energy-related electrocatalytic reactions, and the significance of the promotion and suppression of reaction kinetics in different reaction systems. It thoroughly elucidated the significance of these reactions and the need for a deeper understanding, offering a novel perspective for the future development of this field.

How to Assess and Predict Electrical Double Layer Properties. Implications for Electrocatalysis

The electrical double layer (EDL) plays a central role in electrochemical energy systems, impacting charge transfer mechanisms and reaction rates. The fundamental importance of the EDL in interfacial electrochemistry has motivated researchers to develop theoretical and experimental approaches to assess EDL properties. In this contribution, we review recent progress in evaluating EDL characteristics such as the double-layer capacitance, highlighting some discrepancies between theory and experiment and discussing strategies for their reconciliation. We further discuss the merits and challenges of various experimental techniques and theoretical approaches having important implications for aqueous electrocatalysis. A strong emphasis is placed on the substantial impact of the electrode composition and structure and the electrolyte chemistry on the double-layer properties. In addition, we review the effects of temperature and pressure and compare solid-liquid interfaces to solid-solid interfaces.

In situ ammonium formation mediates efficient hydrogen production from natural seawater splitting

Seawater electrolysis using renewable electricity offers an attractive route to sustainable hydrogen production, but the sluggish electrode kinetics and poor durability are two major challenges. We report a molybdenum nitride (Mo2N) catalyst for the hydrogen evolution reaction with activity comparable to commercial platinum on carbon (Pt/C) catalyst in natural seawater. The catalyst operates more than 1000 hours of continuous testing at 100 mA cm-2 without degradation, whereas massive precipitate (mainly magnesium hydroxide) forms on the Pt/C counterpart after 36 hours of operation at 10 mA cm-2. Our investigation reveals that ammonium groups generate in situ at the catalyst surface, which not only improve the connectivity of hydrogen-bond networks but also suppress the local pH increase, enabling the enhanced performances. Moreover, a zero-gap membrane flow electrolyser assembled by this catalyst exhibits a current density of 1 A cm-2 at 1.87 V and 60 oC in simulated seawater and runs steadily over 900 hours.

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.

The Role of Phosphorus on Alkaline Hydrogen Oxidation Electrocatalysis for Ruthenium Phosphides

Transition metal/p-block compounds are regarded as the most essential materials for electrochemical energy converting systems involving various electrocatalysis. Understanding the role of p-block element on the interaction of key intermediates and interfacial water molecule orientation at the polarized catalyst-electrolyte interface during the electrocatalysis is important for rational designing advanced p-block modified metal electrocatalysts. Herein, taking a sequence of ruthenium phosphides (including Ru2P, RuP and RuP2) as model catalysts, we establish a volcanic-relation between P-proportion and alkaline hydrogen oxidation reaction (HOR) activity. The dominant role of P for regulating hydroxyl binding energy is validated by active sites poisoning experiments, pH-dependent infection-point behavior, in situ surface enhanced infrared absorption spectroscopy, and density functional theory calculations, in which P could tailor the d-band structure of Ru, optimize the hydroxyl adsorption sites across the Ru-P moieties, thereby leading to improved proportion of strongly hydrogen-bonded water and facilitated proton-coupled electron transfer process, which are responsible for the enhanced alkaline HOR performance.

Unconventional hcp/fcc Nickel Heteronanocrystal with Asymmetric Convex Sites Boosts Hydrogen Oxidation

Developing non-platinum group metal catalysts for the sluggish hydrogen oxidation reaction (HOR) is critical for alkaline fuel cells. To date, Ni-based materials are the most promising candidates but still suffer from insufficient performance. Herein, we report an unconventional hcp/fcc Ni (u-hcp/fcc Ni) heteronanocrystal with multiple epitaxial hcp/fcc heterointerfaces and coherent twin boundaries, generating rugged surfaces with plenty of asymmetric convex sites. Systematic analyses discover that such convex sites enable the adsorption of *H in unusual bridge positions with weakened binding energy, circumventing the over-strong *H adsorption on traditional hollow positions, and simultaneously stabilizing interfacial *H2O. It thus synergistically optimizes the HOR thermodynamic process as well as reduces the kinetic barrier of the rate-determining Volmer step. Consequently, the developed u-hcp/fcc Ni exhibits the top-rank alkaline HOR activity with a mass activity of 40.6 mA mgNi -1 (6.3 times higher than fcc Ni control) together with superior stability and high CO-tolerance. These results provide a paradigm for designing high-performance catalysts by shifting the adsorption state of intermediates through configuring surface sites.

Effect of ion-specific water structures at metal surfaces on hydrogen production

Water structures at electrolyte/electrode interfaces play a crucial role in determining the selectivity and kinetics of electrochemical reactions. Despite extensive experimental and theoretical efforts, atomic-level details of ion-specific water structures on metal surfaces remain unclear. Here we show, using scanning tunneling microscopy and noncontact atomic force microscopy, that we can visualize water layers containing alkali metal cations on a charged Au(111) surface with atomic resolution. Our results reveal that Li+ cations are elevated from the surface, facilitating the formation of an ice-like water layer between the Li+ cations and the surface. In contrast, K+ and Cs+ cations are in direct contact with the surface. We observe that the water network structure transitions from a hexagonal arrangement with Li+ to a distorted hydrogen-bonding configuration with Cs+. These observations are consistent with surface-enhanced infrared absorption spectroscopy data and suggest that alkali metal cations significantly impact hydrogen evolution reaction kinetics and efficiency. Our findings provide insights into ion-specific water structures on metal surfaces and underscore the critical role of spectator ions in electrochemical processes.

Modulating Surface Cation Concentration via Tuning the Molecular Structures of Ethylene Glycol-Functionalized PEDOT for Improved Alkaline Hydrogen Evolution Reaction

The sluggish catalytic kinetics of nonprecious metal-based electrocatalysts often hinder them from achieving efficient hydrogen evolution reactions (HERs). Poly(3,4-ethylenedioxythiophene) (PEDOT) and its derivatives have been promising materials for various electrochemical applications. Nevertheless, previous studies have demonstrated that PEDOT coatings can be detrimental to HER performance. In this study, we investigated the alkaline HER efficiency of nickel foam coated with three types of ethylene glycol (EG)-functionalized EDOT. Specifically, EDOT derivatives bearing hydroxyl (-OH) and methoxy (-OCH3) end groups on the EG side chain and molecules containing two EDOT units are interconnected via EG moieties. EG groups are selected due to their strong interaction with alkali metal cations. Intriguingly, improved HER performance is observed on all electrodes coated with EG-functionalized EDOTs. Electrochemical impedance spectroscopy, electrochemical quartz crystal microbalance with dissipation, and XPS analysis are employed to explore the origin of enhanced HER efficiency. The results suggest the EG moieties can induce locally concentrated ions near the electrode surface and facilitate water dissociation through noncovalent interactions. The influence of EG chain length is systematically investigated by synthesizing molecules with di-EG, tetra-EG, and hexa-EG functionalities. This study highlights the importance of molecular design in modifying electrode surface properties to promote alkaline HER.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

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.

Unraveling Stoichiometry Effect in Nickel-Tungsten Alloys for Efficient Hydrogen Oxidation Catalysis in Alkaline Electrolytes

Anion-exchange membrane fuel cells provide the possibility to use platinum group metal-free catalysts, but the anodic hydrogen oxidation reaction (HOR) suffers from sluggish kinetics and its source is still debated. Here, over nickel-tungsten (Ni-W) alloy catalysts, we show that the Ni : W ratio greatly governs the HOR performance in alkaline electrolyte. Experimental and theoretical studies unravel that alloying with W can tune the unpaired electrons in Ni, tailoring the potential of zero charge and the catalytic surface to favor hydroxyl adsorption (OHad). The OHad species coordinately interact with potassium (K+) ions, which break the K+ solvation sheath to leave free water molecules, yielding an improved connectivity of hydrogen-bond networks. Consequently, the optimal Ni17W3 alloy exhibits alkaline HOR activity superior to the state-of-the-art platinum on carbon (Pt/C) catalyst and operates steadily with negligible decay after 10,000 cycles. Our findings offer new understandings of alloyed HOR catalysts and will guide rational design of next-generation catalysts for fuel cells.

Emerging Atomistic Modeling Methods for Heterogeneous Electrocatalysis

Heterogeneous electrocatalysis lies at the center of various technologies that could help enable a sustainable future. However, its complexity makes it challenging to accurately and efficiently model at an atomic level. Here, we review emerging atomistic methods to simulate the electrocatalytic interface with special attention devoted to the components/effects that have been challenging to model, such as solvation, electrolyte ions, electrode potential, reaction kinetics, and pH. Additionally, we review relevant computational spectroscopy methods. Then, we showcase several examples of applying these methods to understand and design catalysts relevant to green hydrogen. We also offer experimental views on how to bridge the gap between theory and experiments. Finally, we provide some perspectives on opportunities to advance the field.

Effects of Ionic Interferents on Electrocatalytic Nitrate Reduction: Mechanistic Insight

Nitrate, a prevalent water pollutant, poses substantial public health concerns and environmental risks. Electrochemical reduction of nitrate (eNO3RR) has emerged as an effective alternative to conventional biological treatments. While extensive lab work has focused on designing efficient electrocatalysts, implementation of eNO3RR in practical wastewater settings requires careful consideration of the effects of various constituents in real wastewater. In this critical review, we examine the interference of ionic species commonly encountered in electrocatalytic systems and universally present in wastewater, such as halogen ions, alkali metal cations, and other divalent/trivalent ions (Ca2+, Mg2+, HCO3-/CO32-, SO42-, and PO43-). Notably, we categorize and discuss the interfering mechanisms into four groups: (1) loss of active catalytic sites caused by competitive adsorption and precipitation, (2) electrostatic interactions in the electric double layer (EDL), including ion pairs and the shielding effect, (3) effects on the selectivity of N intermediates and final products (N2 or NH3), and (4) complications by the hydrogen evolution reaction (HER) and localized pH on the cathode surface. Finally, we summarize the competition among different mechanisms and propose future directions for a deeper mechanistic understanding of ionic impacts on eNO3RR.

The Electrocatalytic Activity of Au Electrodes Changes Significantly in Various Na+/K+ Supporting Electrolyte Mixtures

The potential of maximum entropy (PME) is an indicator of extreme disorder at the electrode/electrolyte interface and can predict changes in catalytic activity within electrolytes of varying compositions. The laser-induced current transient technique is employed to evaluate the PME for Au polycrystalline (Aupc) electrodes immersed in Ar-saturated cation electrolyte mixtures containing potassium and sodium ions at pH = 8. Five cation ratios (0.5 M K2SO4:0.5 M Na2SO4 = 0:1, 0.25:0.75, 0.5:0.5, 0.75:0.25, and 1:0) are explored, considering earlier studies that unveil cation-dependent shifts at near-neutral pH. Moreover, for all electrolyte compositions, electrochemical impedance spectroscopy is utilized to determine the double-layer capacitance (C DL), the minimum of which should be close to the potential of zero charge (PZC). By correlating cation molar ratios with the PMEs and PZCs, the impact on the model oxygen reduction reaction (ORR) activity, assessed via the rotating disk electrode method, is analyzed. The results demonstrate a linear relationship between electrolyte cation mixtures and PME, while ORR activity exhibits an exponential trend. This observation validates the PME-activity link hypothesis, underscoring electrolyte components' pivotal role in tailoring interfacial properties for electrocatalytic systems. These findings introduce a new degree of freedom for designing optimal electrocatalytic systems by adjusting various electrolyte components.

Long-Range SECCM Enables High-Throughput Electrochemical Screening of High Entropy Alloy Electrocatalysts at Up-To-Industrial Current Densities

High-entropy alloys (HEAs), especially in the form of compositional complex solid solutions (CCSS), have gained attention in the field of electrocatalysis. However, exploring their vast composition space concerning their electrocatalytic properties imposes significant challenges. Scanning electrochemical cell microscopy (SECCM) offers high-speed electrochemical analysis on surface areas with a lateral resolution down to tens of nm. However, high-precision piezo positioners often used for the motion of the tip limit the area of SECCM scans to the motion range of the piezo positioners which is typically a few tens of microns. To bridge this experimental gap, the study proposes a long-range SECCM system with a rapid gas-exchange environmental cell for high-throughput electrochemical characterization of 100 mm diameter HEA thin-film material libraries (ML) obtained by combinatorial co-sputtering. Due to the gas-liquid interface at the positioned SECCM droplet on the sample, high-throughput evaluation under industrial current density conditions becomes feasible. This allows the direct correlation between electrocatalytic activity and material composition with high statistical reliability. The multidimensional data obtained accelerates materials discovery, development, and optimization.

Atomic Layer Deposition of TiO2 on Si Window Enables In Situ ATR-SEIRAS Measurements in Strong Alkaline Electrolytes

The Si window is the most widely used internal reflection element (IRE) for electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), yet local chemical etching on Si by concentrated OH- anions bottlenecks the reliable application of this method in strong alkaline electrolytes. In this report, atomic layer deposition of a 25 nm nonconductive TiO2 barrier layer on the reflecting plane of a Si prism is demonstrated to address this challenge. In situ ATR-SEIRAS measurement on a Au film electrode with the Si/TiO2 composite IRE in 1 M NaOH reveals reversible global spectral features without spectral distortion at 1000-1300 cm-1, in stark contrast to those obtained with a bare Si window. By applying this structured ATR-SEIRAS, ethanol electrooxidation on a Pt/C catalyst in 1 and 5 M NaOH is explored, manifesting that such high pH values prevent the adsorption of as-formed acetate in the C2 pathway but not that of CO intermediate in the C1 pathway.

Unveiling Nanoscale Heterogeneities at the Bias-Dependent Gold-Electrolyte Interface

Electrified solid-liquid interfaces (SLIs) are extremely complex and dynamic, affecting both the dynamics and selectivity of reaction pathways at electrochemical interfaces. Enabling access to the structure and arrangement of interfacial water in situ with nanoscale resolution is essential to develop efficient electrocatalysts. Here, we probe the SLI energy of a polycrystalline Au(111) electrode in a neutral aqueous electrolyte through in situ electrochemical atomic force microscopy. We acquire potential-dependent maps of the local interfacial adhesion forces, which we associate with the formation energy of the electric double layer. We observe nanoscale inhomogeneities of interfacial adhesion force across the entire map area, indicating local differences in the ordering of the solvent/ions at the interface. Anion adsorption has a clear influence on the observed interfacial adhesion forces. Strikingly, the adhesion forces exhibit potential-dependent hysteresis, which depends on the local gold grain curvature. Our findings on a model electrode extend the use of scanning probe microscopy to gain insights into the local molecular arrangement of the SLI in situ, which can be extended to other electrocatalysts.

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

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

Electrochemical Generation of Te Vacancy Pairs in PtTe for Efficient Hydrogen Evolution

Two-dimensional (2D) van der Waals materials are increasingly seen as potential catalysts due to their unique structures and unmatched properties. However, achieving precise synthesis of these remarkable materials and regulating their atomic and electronic structures at the most fundamental level to enhance their catalytic performance remain a significant challenge. In this study, we synthesized single-crystal bulk PtTe crystals via chemical vapor transport and subsequently produced atomically thin, large PtTe nanosheets (NSs) through electrochemical cathode intercalation. These NSs are characterized by a significant presence of Te vacancy pairs, leading to undercoordinated Pt atoms on their basal planes. Experimental and theoretical studies together reveal that Te vacancy pairs effectively optimize and enhance the electronic properties (such as charge distribution, density of states near the Fermi level, and d-band center) of the resultant undercoordinated Pt atoms. This optimization results in a significantly higher percentage of dangling O-H water, a decreased energy barrier for water dissociation, and an increased binding affinity of these Pt atoms to active hydrogen intermediates. Consequently, PtTe NSs featuring exposed and undercoordinated Pt atoms demonstrate outstanding electrocatalytic activity in hydrogen evolution reactions, significantly surpassing the performance of standard commercial Pt/C catalysts.

Metal-Organic Framework (MOF)-Based Clean Energy Conversion: Recent Advances in Unlocking its Underlying Mechanisms

Carbon neutrality is an important goal for humanity . As an eco-friendly technology, electrocatalytic clean energy conversion technology has emerged in the 21st century. Currently, metal-organic framework (MOF)-based electrocatalysis, including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), carbon dioxide reduction reaction (CO2RR), nitrogen reduction reaction (NRR), are the mainstream energy catalytic reactions, which are driven by electrocatalysis. In this paper, the current advanced characterizations for the analyses of MOF-based electrocatalytic energy reactions have been described in details, such as density function theory (DFT), machine learning, operando/in situ characterization, which provide in-depth analyses of the reaction mechanisms related to the above reactions reported in the past years. The practical applications that have been developed for some of the responses that are of application values, such as fuel cells, metal-air batteries, and water splitting have also been demonstrated. This paper aims to maximize the potential of MOF-based electrocatalysts in the field of energy catalysis, and to shed light on the development of current intense energy situations.

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.

Efficient ammonia synthesis from the air using tandem non-thermal plasma and electrocatalysis at ambient conditions

While electrochemical N2 reduction presents a sustainable approach to NH3 synthesis, addressing the emission- and energy-intensive limitations of the Haber-Bosch process, it grapples with challenges in N2 activation and competing with pronounced hydrogen evolution reaction. Here we present a tandem air-NOx-NOx--NH3 system that combines non-thermal plasma-enabled N2 oxidation with Ni(OH)x/Cu-catalyzed electrochemical NOx- reduction. It delivers a high NH3 yield rate of 3 mmol h-1 cm-2 and a corresponding Faradaic efficiency of 92% at -0.25 V versus reversible hydrogen electrode in batch experiments, outperforming previously reported ones. Furthermore, in a flow mode concurrently operating the non-thermal plasma and the NOx- electrolyzer, a stable NH3 yield rate of approximately 1.25 mmol h-1 cm-2 is sustained over 100 h using pure air as the intake. Mechanistic studies indicate that amorphous Ni(OH)x on Cu interacts with hydrated K+ in the double layer through noncovalent interactions and accelerates the activation of water, enriching adsorbed hydrogen species that can readily react with N-containing intermediates. In situ spectroscopies and density functional theory (DFT) results reveal that NOx- adsorption and their hydrogenation process are optimized over the Ni(OH)x/Cu surface. This work provides new insights into electricity-driven distributed NH3 production using natural air at ambient conditions.

Role of Electrolyte pH on Water Oxidation for Iridium Oxides

Understanding the effect of noncovalent interactions of intermediates at the polarized catalyst-electrolyte interface on water oxidation kinetics is key for designing more active and stable electrocatalysts. Here, we combine operando optical spectroscopy, X-ray absorption spectroscopy (XAS), and surface-enhanced infrared absorption spectroscopy (SEIRAS) to probe the effect of noncovalent interactions on the oxygen evolution reaction (OER) activity of IrOx in acidic and alkaline electrolytes. Our results suggest that the active species for the OER (Ir4.x+-*O) binds much stronger in alkaline compared with acid at low coverage, while the repulsive interactions between these species are higher in alkaline electrolytes. These differences are attributed to the larger fraction of water within the cation hydration shell at the interface in alkaline electrolytes compared to acidic electrolytes, which can stabilize oxygenated intermediates and facilitate long-range interactions between them. Quantitative analysis of the state energetics shows that although the *O intermediates bind more strongly than optimal in alkaline electrolytes, the larger repulsive interaction between them results in a significant weakening of *O binding with increasing coverage, leading to similar energetics of active states in acid and alkaline at OER-relevant potentials. By directly probing the electrochemical interface with complementary spectroscopic techniques, our work goes beyond conventional computational descriptors of the OER activity to explain the experimentally observed OER kinetics of IrOx in acidic and alkaline electrolytes.

Influence of Alkali Metal Cations on the Oxygen Reduction Activity of Pt5Y and Pt5Gd Alloys

Electrolyte species can significantly influence the electrocatalytic performance. In this work, we investigate the impact of alkali metal cations on the oxygen reduction reaction (ORR) on active Pt5Gd and Pt5Y polycrystalline electrodes. Due to the strain effects, Pt alloys exhibit a higher kinetic current density of ORR than pure Pt electrodes in acidic media. In alkaline solutions, the kinetic current density of ORR for Pt alloys decreases linearly with the decreasing hydration energy in the order of Li+ > Na+ > K+ > Rb+ > Cs+, whereas Pt shows the opposite trend. To gain further insights into these experimental results, we conduct complementary density functional theory calculations considering the effects of both electrode surface strain and electrolyte chemistry. The computational results reveal that the different trends in the ORR activity in alkaline media can be explained by the change in the adsorption energy of reaction intermediates with applied surface strain in the presence of alkali metal cations. Our findings provide important insights into the effects of the electrolyte and the strain conditions on the electrocatalytic performance and thus offer valuable guidelines for optimizing Pt-based electrocatalysts.

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.

Cation Effects on Hydrogen Oxidation Reaction on Pt Single-Crystal Electrodes in Alkaline Media

The exact mechanism behind the cation-assisted hydrogen oxidation reaction (HOR) on platinum electrodes in alkaline media remains disputed. We show that the cation effects at platinum display a remarkable structure sensitivity: not only the H adsorption but also the HOR activity on (111) terrace sites are independent of the nature of cation and cation concentration. On (110) step sites, at low cation concentration and mildly alkaline media, cations promote the HOR, whereas at more alkaline pH and consequently higher near-surface cation concentrations, the HOR is inhibition by the cations. Moreover, the role of the cation on terrace-OHad is different from that on step-OHad, as can also be observed from the inhibition of the HOR current by terrace-OHad at higher potentials. These results suggest that near the onset potential, HOR mainly takes place on steps, but under diffusion-limited conditions at higher overpotential, HOR mainly takes place on terraces.

Influence of electric double layer rigidity on CO adsorption and electroreduction rate

Understanding the structure of the electric double layer (EDL) is critical for designing efficient electrocatalytic processes. However, the interplay between reactant adsorbates and the concentrated ionic species within the EDL remains an aspect that has yet to be fully explored. In the present study, we employ electrochemical CO reduction on Cu as a model reaction to reveal the significant impact of EDL structure on CO adsorption. By altering the sequence of applying negative potential and elevating CO pressure, we discern two distinct EDL structures with varying cation density and CO coverage. Our findings demonstrate that the EDL comprising densely packed cations substantially hinders CO adsorption on the Cu as opposed to the EDL containing less compact cations. These two different EDL structures remained stable over the course of our experiments, despite their identical initial and final conditions, suggesting an insurmountable kinetic barrier present in between. Moreover, we show that the size and identity of cations play decisive roles in determining the properties of the EDL in CO electroreduction on Cu. This study presents a refined adaptation of the classical Gouy-Chapman-Stern model and highlights its catalytic importance, which bridges the mechanistic gap between the EDL structure and cathodic reactions.

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

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

Key Role of Cations in Stabilizing Hydrogen Radicals for CO2-to-CO Conversion via a Reverse Water-Gas Shift Reaction

Electrochemically converting CO2 into valuable chemicals and fuels in acidic media is argued as a promising energy- and carbon-efficient route. Although several key roles of alkali cations have been unveiled, the alkali cation trends for CO2 reduction remain largely elusive. With decreasing cation size from Cs+ to Li+, here we show that the apparent proton diffusion coefficient in 3.0 M Li+ is tens-fold lower than in 3.0 M K+ and 3.0 M Cs+ acidic electrolytes. Although Li+ has the strongest inhibition ability for proton transport, it acts the worst for both the CO2-to-CO conversion and partial current density on Au catalysts. Unexpectedly, K+ with a higher proton transport performs the best for CO2-to-CO conversion. We thus revisit the roles of alkali cations and find that hydrated K+ can stabilize hydrogen radicals benefiting CO2 conversion at the electrode interface while for Li+ this is not the case. This study proposes that cation-stabilized atomic hydrogen assists in activating CO2 via a reverse water-gas shift route under electrochemical conditions.

Impact of Pt(hkl) Electrode Surface Structure on the Electrical Double Layer Capacitance

The classical theory of the electrical double layer (EDL) does not consider the effects of the electrode surface structure on the EDL properties. Moreover, the best agreement between the traditional EDL theory and experiments has been achieved so far only for a very limited number of ideal systems, such as liquid metal mercury electrodes, for which it is challenging to operate with specific surface structures. In the case of solid electrodes, the predictive power of classical theory is often not acceptable for electrochemical energy applications, e.g., in supercapacitors, due to the effects of surface structure, electrode composition, and complex electrolyte contributions. In this work, we combine ab initio molecular dynamics (AIMD) simulations and electrochemical experiments to elucidate the relationship between the structure of Pt(hkl) surfaces and the double-layer capacitance as a key property of the EDL. Flat, stepped, and kinked Pt single crystal facets in contact with acidic HClO4 media are selected as our model systems. We demonstrate that introducing specific defects, such as steps, can substantially reduce the EDL capacitances close to the potential of zero charge (PZC). Our AIMD simulations reveal that different Pt facets are characterized by different net orientations of the water dipole moment at the interface. That allows us to rationalize the experimentally measured (inverse) volcano-shaped capacitance as a function of the surface step density.

Constant Potential Thermodynamic Integration for Obtaining the Free Energy Profile of Electrochemical Reaction

In this work, we advanced an efficient free energy sampling method based on constrained ab initio molecular dynamics (cAIMD) with a fully explicit solvent layer to depict the electrochemical reaction process at constant surface charge density, named the "Constant-Potential Thermodynamic Integration (CPTI)" method. For automatically adjusting surface charge density at different states, we built an "on-the-fly" procedure which is capable of managing all the necessary steps during cAIMD simulations, including the system pre-equilibrium, surface charge density updating, and force sampling. We applied it to predict the potential-dependent free energy profiles of CO2 adsorption on a single-atom catalyst. The results show that our method can not only account for changes in electrostatic potential energy associated with potential but also consider the potential-induced solvation effects. Our approach enables the accurate simulation of electrochemical environment by presenting the complete solid-liquid interface and efficient computation of electrocatalytic reaction energetics based on a robust potential descriptor.

Tuning the apparent hydrogen binding energy to achieve high-performance Ni-based hydrogen oxidation reaction catalyst

High-performance platinum-group-metal-free alkaline hydrogen oxidation reaction catalysts are essential for the hydroxide exchange membrane fuel cells, which generally require high Pt loadings on the anode. Herein, we report a highly active hydrogen oxidation reaction catalyst, NiCuCr, indicated by the hydroxide exchange membrane fuel cell with a high peak power density of 577 mW cm-2 (18 times as high as the Ni/C anode) and a stability of more than 150 h (a degradation rate slower by 7 times than the Ni/C anode). The spectroscopies demonstrate that the alloy effect from Cu weakens the hydrogen binding, and the surface Cr2O3 species enhance the interfacial water binding. Both effects bring an optimized apparent hydrogen binding energy and thus lead to the high hydrogen oxidation reaction performance of NiCuCr. These results suggest that the apparent hydrogen binding energy determines the hydrogen oxidation reaction performance and that its tuning is beneficial toward high electrocatalytic performance.

A Highly-Efficient Boron Interstitially Inserted Ru Anode Catalyst for Anion Exchange Membrane Fuel Cells

Developing high-performance electrocatalysts for alkaline hydrogen oxidation reaction (HOR) is crucial for the commercialization of anion exchange membrane fuel cells (AEMFCs). Here, boron interstitially inserted ruthenium (B-Ru/C) is synthesized and used as an anode catalyst for AEMFC, achieving a peak power density of 1.37 W cm-2 , close to the state-of-the-art commercial PtRu catalyst. Unexpectedly, instead of the monotonous decline of HOR kinetics with pH as generally believed, an inflection point behavior in the pH-dependent HOR kinetics on B-Ru/C is observed, showing an anomalous behavior that the HOR activity under alkaline electrolyte surpasses acidic electrolyte. Experimental results and density functional theory calculations reveal that the upshifted d-band center of Ru after the intervention of interstitial boron can lead to enhanced adsorption ability of OH and H2 O, which together with the reduced energy barrier of water formation, contributes to the outstanding alkaline HOR performance with a mass activity of 1.716 mA µgPGM -1 , which is 13.4-fold and 5.2-fold higher than that of Ru/C and commercial Pt/C, respectively.

Polyolefin-derived substrate-grown carbon nanotubes as binder-free electrode for hydrogen evolution in alkaline media

Switching from a linear mode of waste management to a circular loop by transforming plastic waste into carbon nanotubes (CNTs) is a promising approach to current plastic waste treatment. One of the many applications of CNTs is its use for electrocatalytic water splitting for hydrogen evolution. Existing methods of CNTs-based hydrogen evolution reaction (HER) electrode fabrication involve additives like polymeric binders and additional steps to improve CNT dispersion, which are detrimental to the CNT structure and properties. The in-situ fabrication approach can potentially be a one-pot solution to HER electrode synthesis. In this study, polyolefins pyrolysis gas and a Co:Ni:Mg catalyst were used to fabricate binder-free CNTs-based electrodes on different substrates for HER. The study assessed CNT quality on conductive carbon paper, semiconductive silicon, and dielectric glass substrates, evaluating their HER performance in 1 M KOH. A mixture of hollow-core, bamboo-like, and cup-stacked arrangement nanotubes were synthesized on the substrates, with CNTs on glass and carbon paper substrates possessing better graphitization than CNTs grown on silicon. This is in agreement with HER performance, whereby the as-prepared electrodes required overpotentials of 267 mV, 241 mV, and 216 mV for silicon, glass, and carbon paper, respectively, to achieve 10 mA/cm2. Despite being poorly conductive, the glass substrate electrode achieved a lower overpotential than the silicon electrode. Additionally, the as-prepared silicon electrode faced a delamination issue likely attributed to the lower surface energy of the silicon substrate surface, demonstrating the weaker adhesion between the CNTs and silicon surface. The proposed approach thus showed that the in-situ fabricated electrodes performed better than separately synthesized CNTs prepared into electrodes by 27.4% and 14.2% for carbon paper and glass substrates, respectively. The improved performance of the as-prepared, binder-free electrodes can be linked to the lower charge-transfer resistance and reduced contact resistance between the CNTs and substrate.

Cation Modifies Interfacial Water Structures on Platinum during Alkaline Hydrogen Electrocatalysis

The molecular details of an electrocatalytic interface play an essential role in the production of sustainable fuels and value-added chemicals. Many electrochemical reactions exhibit strong cation-dependent activities, but how cations affect reaction kinetics is still elusive. We report the effect of cations (K+, Li+, and Ba2+) on the interfacial water structure using second-harmonic generation (SHG) and classical molecular dynamics (MD) simulation. The second- (χH2O(2)) and third-order (χH2O(3)) optical susceptibilities of water on Pt are smaller in the presence of Ba2+ compared to those of K+, suggesting that cations can affect the interfacial water orientation. MD simulation reproduces experimental SHG observations and further shows that the competition between cation hydration and interfacial water alignment governs the net water orientation. The impact of cations on interfacial water supports a cation hydration-mediated mechanism for hydrogen electrocatalysis; i.e., the reaction occurs via water dissociation followed by cation-assisted hydroxide/water exchange on Pt. Our study highlights the role of interfacial water in electrocatalysis and how innocent additives (such as cations) can affect the local electrochemical environment.

Exploring the Cation Regulation Mechanism for Interfacial Water Involved in the Hydrogen Evolution Reaction by In Situ Raman Spectroscopy

Interfacial water molecules are the most important participants in the hydrogen evolution reaction (HER). Hence, understanding the behavior and role that interfacial water plays will ultimately reveal the HER mechanism. Unfortunately, investigating interfacial water is extremely challenging owing to the interference caused by bulk water molecules and complexity of the interfacial environment. Here, the behaviors of interfacial water in different cationic electrolytes on Pd surfaces were investigated by the electrochemistry, in situ core-shell nanostructure enhanced Raman spectroscopy and theoretical simulation techniques. Direct spectral evidence reveals a red shift in the frequency and a decrease in the intensity of interfacial water as the potential is shifted in the positively direction. When comparing the different cation electrolyte systems at a given potential, the frequency of the interfacial water peak increases in the specified order: Li+  < Na+  < K+  < Ca2+  < Sr2+. The structure of interfacial water was optimized by adjusting the radius, valence, and concentration of cation to form the two-H down structure. This unique interfacial water structure will improve the charge transfer efficiency between the water and electrode further enhancing the HER performance. Therefore, local cation tuning strategies can be used to improve the HER performance by optimizing the interfacial water structure.