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

Unveiling the Electrolyte Cations Dependent Kinetics on CoOOH-Catalyzed Oxygen Evolution Reaction

The electrolyte cations-dependent kinetics have been widely observed in many fields of electrocatalysis, however, the exact mechanism of the influence on catalytic performance is still a controversial topic of considerable discussion. Herein, combined with operando X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), we verify that the electrolyte cations could intercalate into the layer of pristine CoOOH catalyst during the oxygen evolution reaction (OER) process, while the bigger cations lead to enlarged interlayer spacing and increased OER activity, following the order Cs+ >K+ >Na+ >Li+ . X-ray absorption spectroscopy (XAS), in situ Raman, in situ Ultraviolet-visible (UV/Vis) spectroscopy, in situ XAS spectroscopy, cyclic voltammetry (CV), and theoretical calculations reveal that the intercalation of electrolyte cations efficiently modify the oxidation states of Co by enlarging the Co-O bonds, which in turn enhance the d-band center of Co, optimize the adsorption strength of oxygen intermediates, facilitate the formation of OER active Co(IV) species, and reduce the energy barrier of the rate-determing step (RDS), thereby enhancing the OER activity. This work not only provides an informative picture to understand the complicated dependence of OER kinetics on electrolyte cations, but also sheds light on understanding the mechanism of other electrolyte cation-targeted electrocatalysis.

Optimizing Pt-Based Alloy Electrocatalysts for Improved Hydrogen Evolution Performance in Alkaline Electrolytes: A Comprehensive Review

The splitting of water through electrocatalysis offers a sustainable method for the production of hydrogen. In alkaline electrolytes, the lack of protons forces water dissociation to occur before the hydrogen evolution reaction (HER). While pure Pt is the gold standard electrocatalyst in acidic electrolytes, since the 5d orbital in Pt is nearly fully occupied, when it overlaps with the molecular orbital of water, it generates a Pauli repulsion. As a result, the formation of a Pt-H* bond in an alkaline environment is difficult, which slows the HER and negates the benefits of using a pure Pt catalyst. To overcome this limitation, Pt can be alloyed with transition metals, such as Fe, Co, and Ni. This approach has the potential not only to enhance the performance but also to increase the Pt dispersion and decrease its usage, thus overall improving the catalyst's cost-effectiveness. The excellent water adsorption and dissociation ability of transition metals contributes to the generation of a proton-rich local environment near the Pt-based alloy that promotes HER. Significant progress has been achieved in comprehending the alkaline HER mechanism through the manipulation of the structure and composition of electrocatalysts based on the Pt alloy. The objective of this review is to analyze and condense the latest developments in the production of Pt-based alloy electrocatalysts for alkaline HER. It focuses on the modified performance of Pt-based alloys and clarifies the design principles and catalytic mechanism of the catalysts from both an experimental and theoretical perspective. This review also highlights some of the difficulties encountered during the HER and the opportunities for increasing the HER performance. Finally, guidance for the development of more efficient Pt-based alloy electrocatalysts is provided.

Unveiling the Solvent Effect in Plasmon Enhanced Electrochemistry via the Nanoparticle-Impact Technique

Plasmon-enhanced electrochemistry (PEEC) has been observed to facilitate energy conversion systems by converting light energy to chemical energy. However, comprehensively understanding the PEEC mechanism remains challenging due to the predominant use of ensemble-based methodologies on macroscopic electrodes, which fails to measure electron-transfer kinetics due to constraints from mass transport and the averaging effect. In this study, we have employed nanoparticle impact electrochemistry (NIE), a newly developed electroanalytical technique capable of measuring electrochemical dynamics at a single-nanoparticle level under optimal mass transport conditions, along with microscopic electron-transfer theory for data interpretation. By investigating the plasmon enhanced hydrogen evolution reaction (HER) at individual silver nanoparticles (AgNPs), we have clearly revealed the previously unknown influence of solvent effects within the PEEC mechanism. This finding suggests an additional approach to optimize plasmon-assisted electrocatalysis and electrosynthesis systems.

Revealing the role of double-layer microenvironments in pH-dependent oxygen reduction activity over metal-nitrogen-carbon catalysts

A standing puzzle in electrochemistry is that why the metal-nitrogen-carbon catalysts generally exhibit dramatic activity drop for oxygen reduction when traversing from alkaline to acid. Here, taking FeCo-N6-C double-atom catalyst as a model system and combining the ab initio molecular dynamics simulation and in situ surface-enhanced infrared absorption spectroscopy, we show that it is the significantly distinct interfacial double-layer structures, rather than the energetics of multiple reaction steps, that cause the pH-dependent oxygen reduction activity on metal-nitrogen-carbon catalysts. Specifically, the greatly disparate charge densities on electrode surfaces render different orientations of interfacial water under alkaline and acid oxygen reduction conditions, thereby affecting the formation of hydrogen bonds between the surface oxygenated intermediates and the interfacial water molecules, eventually controlling the kinetics of the proton-coupled electron transfer steps. The present findings may open new and feasible avenues for the design of advanced metal-nitrogen-carbon catalysts for proton exchange membrane fuel cells.

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.

Electric Double Layer Effects in Electrocatalysis: Insights from Ab Initio Simulation and Hierarchical Continuum Modeling

Structures of the electric double layer (EDL) at electrocatalytic interfaces, which are modulated by the material properties, the electrolyte characteristics (e.g., the pH, the types and concentrations of ions), and the electrode potential, play crucial roles in the reaction kinetics. Understanding the EDL effects in electrocatalysis has attracted substantial research interest in recent years. However, the intrinsic relationships between the specific EDL structures and electrocatalytic kinetics remain poorly understood, especially on the atomic scale. In this Perspective, we briefly review the recent advances in deciphering the EDL effects mainly in hydrogen and oxygen electrocatalysis through a multiscale approach, spanning from the atomistic scale simulated by ab initio methods to the macroscale by a hierarchical approach. We highlight the importance of resolving the local reaction environment, especially the local hydrogen bond network, in understanding EDL effects. Finally, some of the remaining challenges are outlined, and an outlook for future developments in these exciting frontiers is provided.

Action at a distance: organic cation induced long range organization of interfacial water enhances hydrogen evolution and oxidation kinetics

Engineering efficient electrode-electrolyte interfaces for the hydrogen evolution and oxidation reactions (HOR/HER) is central to the growing hydrogen economy. Existing descriptors for HOR/HER catalysts focused on species that could directly impact the immediate micro-environment of surface-mediated reactions, such as the binding energies of adsorbates. In this work, we demonstrate that bulky organic cations, such as tetrapropyl ammonium, are able to induce a long-range structure of interfacial water molecules and enhance the HOR/HER kinetics even though they are located outside the outer Helmholtz plane. Through a combination of electrokinetic analysis, molecular dynamics and in situ spectroscopic investigations, we propose that the structure-making ability of bulky hydrophobic cations promotes the formation of hydrogen-bonded water chains connecting the electrode surface to the bulk electrolyte. In alkaline electrolytes, the HOR/HER involve the activation of interfacial water by donating or abstracting protons. The structural diffusion mechanism of protons in aqueous electrolytes enables water molecules and cations located at a distance from the electrode to influence surface-mediated reactions. The findings reported in this work highlight the prospect of leveraging the nonlocal mechanism to enhance electrocatalytic performance.

Bifunctional Near-Neutral Electrolyte Enhances Oxygen Evolution Reaction

Performance of electrocatalytic reactions depends on not only the composition and structure of the active sites, but also their local environment, including the surrounding electrolyte. In this work, we demonstrate that BF2 (OH)2 - anion is the key fluoroborate species formed in the mixed KBi/KF (KBi=potassium borate) electrolyte to enhance the rate of the oxygen evolution reaction (OER) at near-neutral pH. Through a combination of electrokinetic and in situ spectroscopic studies, we show that the mixed KBi/KF electrolyte promotes the OER via two pathways: 1) stabilizing the interfacial pH during the proton-producing reaction with its high buffering capacity; and 2) activating the interfacial water via strong hydrogen bonds with F-containing species. With the KBi/KF electrolyte, electrodeposited Co(OH)2 is able to achieve 100 mA/cm2 at 1.74 V, which is among the highest reported activities with earth-abundant electrocatalysts at near neutral conditions. These findings highlight the potential of leveraging electrolyte-engineering for improving the electrochemical performance of the OER.

Influence of Cations on HCOOH and CO Formation during CO2 Reduction on a PdMLPt(111) Electrode

Understanding the role of cations in the electrochemical CO2 reduction (CO2RR) process is of fundamental importance for practical application. In this work, we investigate how cations influence HCOOH and CO formation on PdMLPt(111) in pH 3 electrolytes. While only (a small amount of adsorbed) CO forms on PdMLPt(111) in the absence of metal cations, the onset potential of HCOOH and CO decreases with increasing cation concentrations. The cation effect is stronger on HCOOH formation than that on CO formation on PdMLPt(111). Density functional theory simulations indicate that cations facilitate both hydride formation and CO2 activation by polarizing the electronic density at the surface and stabilizing *CO2-. Although the upshift of the metal work function caused by high coverage of adsorbates limits hydride formation, the cation-induced electric field counterbalances this effect in the case of *H species, sustaining HCOOH production at mild negative potentials. Instead, at the high *CO coverages observed at very negative potentials, surface hydrides do not form, preventing the HCOOH route both in the absence and presence of cations. Our results open the way for a consistent evaluation of cationic electrolyte effects on both activity and selectivity in CO2RR on Pd-Pt catalysts.

Green carbon science: fundamental aspects

Carbon energy has contributed to the creation of human civilization, and it can be considered that the configuration of the carbon energy system is one of the important laws that govern the operation of everything in the universe. The core of the carbon energy system is the opposition and unity of two aspects: oxidation and reduction. The operation of oxidation and reduction is based on the ternary elemental system composed of the three elements of carbon, hydrogen and oxygen. Its operation produces numerous reactions and reaction products. Ancient Chinese philosophy helps us to understand in depth the essence of green carbon science, to explore its scientific basis, and to identify the related platforms for technology development.

Revealing the role of interfacial water and key intermediates at ruthenium surfaces in the alkaline hydrogen evolution reaction

Ruthenium exhibits comparable or even better alkaline hydrogen evolution reaction activity than platinum, however, the mechanistic aspects are yet to be settled, which are elucidated by combining in situ Raman spectroscopy and theoretical calculations herein. We simultaneously capture dynamic spectral evidence of Ru surfaces, interfacial water, *H and *OH intermediates. Ru surfaces exist in different valence states in the reaction potential range, dissociating interfacial water differently and generating two distinct *H, resulting in different activities. The local cation tuning effect of hydrated Na+ ion water and the large work function of high-valence Ru(n+) surfaces promote interfacial water dissociation. Moreover, compared to low-valence Ru(0) surfaces, high-valence Ru(n+) surfaces have more moderate adsorption energies for interfacial water, *H, and *OH. They, therefore, facilitate the activity. Our findings demonstrate the regulation of valence state on interfacial water, intermediates, and finally the catalytic activity, which provide guidelines for the rational design of high-efficiency catalysts.

Enhanced CO Functionality on Carbon Papers Ensures Lowering Nucleation Delay of ALD for Ru towards Unprecedented Alkaline HER Activity

The success in lowering the nucleation delay for Atomic Layer Deposition (ALD) of Ru on carbon surfaces is mitigated by constructive pretreatments resulting enhancement of CO functionality. Treatment of the carbon papers (CP) allowed Ru species deposition for minimum number of ALD cycles (25 cycles) with good conformality. The development of electrocatalysts from single atoms to nanoparticles (NPs) on conductive supports with low metal loadings, thus improving performance, is essential in electrocatalysis. For alkaline hydrogen evolution reaction, ALD decorated CPs with Ru exhibit low onset potentials of ≈4.7 mV versus reversable hydrogen electrode (RHE) (at 10 mA cm-2 ) and a high turnover frequency of 1.92 H2 s-1 at 30 mV versus RHE. The Ru decorated CPs show comparable to higher catalytic activity than of Platinum (Pt) decorated CP also developed by ALD. The current representation of unfamiliar catalytic activities of Ru active centers developed by ALD, pave a bright and sustainable path for energy conversion reactions.

Machine learning-aided atomic structure identification of interfacial ionic hydrates from AFM images

Relevant to broad applied fields and natural processes, interfacial ionic hydrates have been widely studied by using ultrahigh-resolution atomic force microscopy (AFM). However, the complex relationship between the AFM signal and the investigated system makes it difficult to determine the atomic structure of such a complex system from AFM images alone. Using machine learning, we achieved precise identification of the atomic structures of interfacial water/ionic hydrates based on AFM images, including the position of each atom and the orientations of water molecules. Furthermore, it was found that structure prediction of ionic hydrates can be achieved cost-effectively by transfer learning using neural network trained with easily available interfacial water data. Thus, this work provides an efficient and economical methodology that not only opens up avenues to determine atomic structures of more complex systems from AFM images, but may also help to interpret other scientific studies involving sophisticated experimental results.

Reorganization energy in a polybromide ionic liquid measured by scanning electrochemical cell microscopy

Room temperature ionic liquids (RT-ILs) are promising electrolytes for electrocatalysis. Understanding the effects of the electrode–electrolyte interface structure on electrocatalysis in RT-ILs is important. Ultrafast mass transport of redox species in N-methyl- N-ethyl-pyrrolidinium polybromide (MEPBr2n+1) enabled evaluation of the reorganization energy ( λ), which reflects the solvation structure in the inner Helmholtz plane (IHP). λ was achieved by fitting the electron transfer rate-limited voltammogram at a Pt ultramicroelectrode (UME) to the Marcus–Hush–Chidsey model for heterogeneous electron transfer kinetics. However, it is time-consuming or even impossible to prepare electrode materials, including alloys of numerous compositions in the form of UME, for each experiment. Herein, we report a method to evaluate the λ of MEPBr2n+1 by scanning electrochemical cell microscopy (SECCM), which allows high throughput electrochemical measurements using a single electrode with high spatial resolution. Fast mass transport in the nanosized SECCM tip is critical for achieving heterogeneous electron transfer-limited voltammograms. Furthermore, investigating λ on a high-entropy alloy materials library composed of Pt, Pd, Ru, Ir, and Ag suggests a negative correlation between λ and the work function. Given that the potential of zero charge correlates with the work function of electrodes, this can be attributed to the surface-charge sensitive ionic structure in the IHP of MEPBr2n+1, modulating the solvation energy of the redox-active species in the IHP.

Mechanism of Cations Suppressing Proton Diffusion Kinetics for Electrocatalysis

Proton transfer is crucial for electrocatalysis. Accumulating cations at electrochemical interfaces can alter the proton transfer rate and then tune electrocatalytic performance. However, the mechanism for regulating proton transfer remains ambiguous. Here, we quantify the cation effect on proton diffusion in solution by hydrogen evolution on microelectrodes, revealing the rate can be suppressed by more than 10 times. Different from the prevalent opinions that proton transport is slowed down by modified electric field, we found water structure imposes a more evident effect on kinetics. FTIR test and path integral molecular dynamics simulation indicate that proton prefers to wander within the hydration shell of cations rather than to hop rapidly along water wires. Low connectivity of water networks disrupted by cations corrupts the fast-moving path in bulk water. This study highlights the promising way for regulating proton kinetics via a modified water structure.

Diversity of platinum-sites at platinum/fullerene interface accelerates alkaline hydrogen evolution

Membrane-based alkaline water electrolyser is promising for cost-effective green hydrogen production. One of its key technological obstacles is the development of active catalyst-materials for alkaline hydrogen-evolution-reaction (HER). Here, we show that the activity of platinum towards alkaline HER can be significantly enhanced by anchoring platinum-clusters onto two-dimensional fullerene nanosheets. The unusually large lattice distance (~0.8 nm) of the fullerene nanosheets and the ultra-small size of the platinum-clusters (~2 nm) leads to strong confinement of platinum clusters accompanied by pronounced charge redistributions at the intimate platinum/fullerene interface. As a result, the platinum-fullerene composite exhibits 12 times higher intrinsic activity for alkaline HER than the state-of-the-art platinum/carbon black catalyst. Detailed kinetic and computational investigations revealed the origin of the enhanced activity to be the diverse binding properties of the platinum-sites at the interface of platinum/fullerene, which generates highly active sites for all elementary steps in alkaline HER, particularly the sluggish Volmer step. Furthermore, encouraging energy efficiency of 74% and stability were achieved for alkaline water electrolyser assembled using platinum-fullerene composite under industrially relevant testing conditions.

Nanoscale Electron Transfer Variations at Electrocatalyst-Electrolyte Interfaces Resolved by in Situ Conductive Atomic Force Microscopy

Rational innovation of electrocatalysts requires detailed knowledge of spatial property variations across the solid-electrolyte interface. We introduce correlative atomic force microscopy (AFM) to simultaneously probe, in situ and at the nanoscale, electrical conductivity, chemical-frictional, and morphological properties of a bimetallic copper-gold system for CO2 electroreduction. In air, water, and bicarbonate electrolyte, current-voltage curves reveal resistive CuOx islands in line with local current contrasts, while frictional imaging indicates qualitative variations in the hydration layer molecular ordering upon change from water to electrolyte. Nanoscale current contrast on polycrystalline Au shows resistive grain boundaries and electrocatalytically passive adlayer regions. In situ conductive AFM imaging in water shows mesoscale regions of low current and reveals that reduced interfacial electric currents are accompanied by increased friction forces, thus indicating variations in the interfacial molecular ordering affected by the electrolyte composition and ionic species. These findings provide insights into how local electrochemical environments and adsorbed species affect interfacial charge transfer processes and support building in situ structure-property relationships in catalysis and energy conversion research.

Entropic contributions to the stability of electrochemically adsorbed anion layers on Au(111): a microcalorimetric study

We measure the entropy of formation of the interface upon anion adsorption (Cl^-, Br^- I^- and SO₄^2-) on Au(111) as an important indicator for the structure, order and composition of the interface. The entropy of formation of the interface exhibits a rather universal behaviour for all anions with a steep decrease upon initial adsorption followed by a shallow minimum at intermediate anion coverages and a strong increase close to the completion of the adsorbate adlayer. The strong variation of the entropy signals significant entropic contributions to the free enthalpy of the adsorption process and thus the stability of the adsorbed phase. At low anion coverages, close to the potential of zero charge, we attribute the entropy variations to the rearrangement of the interfacial water structure. At intermediate and high anion coverages, a comparison with the results of a lattice-gas model, considering pairwise repulsive interactions within the quasi-chemical approximation, shows that the entropy changes upon anion adsorption can be explained by the configurational entropy of the adsorbed phase. Thus, entropic contributions from both the solvent and the adsorbate are important for the stability of surface phases, particularly for disordered systems.

Advances in Spin Catalysts for Oxygen Evolution and Reduction Reactions

Searching for high effective catalysts has been an endless effort to improve the efficiency of green energy harvesting and degradation of pollutants. In the past decades, tremendous strategies are explored to achieve high effective catalysts, and various theoretical understandings are proposed for the improved activity. As the catalytic reaction occurs at the surface or edge, the unsaturated ions may lead to the fluctuation of spin. Meanwhile, transition metals in catalysts have diverse spin states and may yield the spin effects. Therefore, the role of spin or magnetic moment should be carefully examined. In this review, the recent development of spin catalysts is discussed to give an insightful view on the origins for the improved catalytic activity. First, a brief introduction on the applications and advances in spin-related catalytic phenomena, is given, and then the fundamental principles of spin catalysts and magnetic fields-radical reactions are introduced in the second part. The spin-related catalytic performance reported in oxygen evolution/reduction reaction (OER/ORR) is systematically discussed in the third part, and general rules are summarized accordingly. Finally, the challenges and perspectives are given. This review may provide an insightful understanding of the microscopic mechanisms of catalytic phenomena and guide the design of spin-related catalysts.

Surface Water as an Initial Proton Source for the Electrochemical CO Reduction Reaction on Copper Surfaces

We have employed in situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and density functional theory (DFT) calculations to study the CO reduction reaction (CORR) on Cu single-crystal surfaces under various conditions. Coadsorbed and structure-/potential-dependent surface species, including *CO, Cu-O_ad , and Cu-OH_ad , were identified using electrochemical spectroscopy and isotope labeling. The relative abundance of *OH follows a "volcano" trend with applied potentials in aqueous solutions, which is yet absent in absolute alcoholic solutions. Combined with DFT calculations, we propose that the surface H₂ O can serve as a strong proton donor for the first protonation step in both the C₁ and C₂ pathways of CORR at various applied potentials in alkaline electrolytes, leaving adsorbed *OH on the surface. This work provides fresh insights into the initial protonation steps and identity of key interfacial intermediates formed during CORR on Cu surfaces.

Interfacial Structure of Pt(110) Electrode during Hydrogen Evolution Reaction in Alkaline Solutions

In alkaline solutions, interfacial cations affect the hydrogen evolution reaction (HER) activity of platinum electrodes. However, the effects of cations on the HER activity have not been previously investigated based on interfacial structures. In situ surface X-ray diffraction was performed on Pt(110), of which the HER activity is the highest in the low-index planes of Pt, at hydrogen evolution potentials in alkaline solutions, and revealed the interfacial structure of alkali metal cations (Li⁺ and Cs⁺). The interfacial structure of the Pt(110) electrode changed reversibly depending on the electrode potential. In the LiOH solution, where the HER activity was higher, the densely packed water layer in the electrical double layer acted as a hydrogen supplier. In the CsOH solution, where the HER activity was lower, the Cs⁺ cations were aligned in the missing rows of the 1 × 2 reconstructed Pt(110) surface, suggesting that the Cs⁺ hindered water from accessing the surface, resulting in a lower HER activity.

Role of water structure in alkaline water electrolysis

Herein, with the help of experimental and first-principles density functional theory (DFT)-based studies, we have shown that structural changes in the water coordination in electrolytes having high alkalinity can be a possible reason for the reduced catalytic activity of platinum (Pt) in high pH. Studies with polycrystalline Pt electrodes indicate that electrocatalytic HER activity reduces in terms of high overpotential required, high Tafel slope, and high charge transfer resistances in concentrated aqueous alkaline electrolytes (say 6 M KOH) in comparison to that in low alkaline electrolytes (say 0.1 M KOH), irrespective of the counter cations (Na+, K+, or Rb+) present. The changes in the water structure of bulk electrolytes as well as that in electrode-electrolyte interface are studied. The results are compared with DFT-based analysis, and the study can pave new directions in studying the HER process in terms of the water structure near the electrode-electrolyte interface.

Heterogeneous electron transfer reorganization energy at the inner Helmholtz plane in a polybromide redox-active ionic liquid

In ionic liquids (ILs), the electric double layer (EDL) is where heterogeneous electron transfer (ET) occurs. Nevertheless, the relationship between the EDL structure and its kinetics has been rarely studied, especially for ET taking place in the inner Helmholtz plane (IHP). This is largely because of the lack of an appropriate model system for experiments. In this work, we determined the reorganization energy (λ) of Br2 reduction in a redox-active IL 1-ethyl-1-methylpyrrolidinium polybromide (MEPBr2n+1) based on the Marcus-Hush-Chidsey model. Exceptionally fast mass transport of Br2 in MEPBr2n+1 allows voltammograms to be obtained in which the current plateau is regulated by electron-transfer kinetics. This enables investigation of the microscopic environment in the IHP of the IL affecting electrocatalytic reactions through reorganization energy. As a demonstration, TiO2-modified Pt was employed to show pH-dependent reorganization energy, which suggests the switch of major ions at the IHP as a function of surface charges of electrodes.

Ultraefficient Electrocatalytic Hydrogen Evolution from Strain-Engineered, Multilayer MoS2

This paper reports an approach to repurpose low-cost, bulk multilayer MoS₂ for development of ultraefficient hydrogen evolution reaction (HER) catalysts over large areas (>cm²). We create working electrodes for use in HER by dry transfer of MoS₂ nano- and microflakes to gold thin films deposited on prestrained thermoplastic substrates. By relieving the prestrain at a macroscopic scale, a tunable level of tensile strain is developed in the MoS₂ and consequently results in a local phase transition as a result of spontaneously formed surface wrinkles. Using electrochemical impedance spectroscopy, we verified that electrochemical activation of the strained MoS₂ lowered the charge transfer resistance within the materials system, achieving HER activity comparable to platinum (Pt). Raman and X-ray photoelectron spectroscopy show that desulfurization in the multilayer MoS₂ was promoted by the phase transition; the combined effect of desulfurization and the lower charge resistance induced superior HER performance.

Ensemble representation of catalytic interfaces: soloists, orchestras, and everything in-between

Catalytic systems are complex and dynamic, exploring vast chemical spaces on multiple timescales. In this perspective, we discuss the dynamic behavior of fluxional, heterogeneous thermal and electrocatalysts and the ensembles of many isomers which govern their behavior. We develop a new paradigm in catalysis theory in which highly fluxional systems, namely sub-nano clusters, isomerize on a much shorter timescale than that of the catalyzed reaction, so macroscopic properties arise from the thermal ensemble of isomers, not just the ground state. Accurate chemical predictions can only be reached through a many-structure picture of the catalyst, and we explain the breakdown of conventional methods such as linear scaling relations and size-selected prevention of sintering. We capitalize on the forward-looking discussion of the means of controlling the size of these dynamic ensembles. This control, such that the most effective or selective isomers can dominate the system, is essential for the fluxional catalyst to be practicable, and their targeted synthesis to be possible. It will also provide a fundamental lever of catalyst design. Finally, we discuss computational tools and experimental methods for probing ensembles and the role of specific isomers. We hope that catalyst optimization using chemically informed descriptors of ensemble nature and size will become a new norm in the field of catalysis and have broad impacts in sustainable energy, efficient chemical production, and more.

Comparing interfacial cation hydration at catalytic active sites and spectator sites on gold electrodes: understanding structure sensitive CO2 reduction kinetics

Hydrated cations present in the electrochemical double layer (EDL) are known to play a crucial role in electrocatalytic CO2 reduction (CO2R), and numerous studies have attempted to explain how the cation effect contributes to the complex CO2R mechanism. CO2R is a structure sensitive reaction, indicating that a small fraction of total surface sites may account for the majority of catalytic turnover. Despite intense interest in specific cation effects, probing site-specific, cation-dependent solvation structures remains a significant challenge. In this work, CO adsorbed on Au is used as a vibrational Stark reporter to indirectly probe solvation structure using vibrational sum frequency generation (VSFG) spectroscopy. Two modes corresponding to atop adsorption of CO are observed with unique frequency shifts and potential-dependent intensity profiles, corresponding to direct adsorption of CO to inactive surface sites, and in situ generated CO produced at catalytic active sites. Analysis of the cation-dependent Stark tuning slopes for each of these species provides estimates of the hydrated cation radius upon adsorption to active and inactive sites on the Au electrode. While cations are found to retain their bulk hydration shell upon adsorption at inactive sites, catalytic active sites are characterized by a single layer of water between the Au surface and the electrolyte cation. We propose that the drastic increase in catalytic performance at active sites stems from this unique solvation structure at the Au/electrolyte interface. Building on this evidence of a site-specific EDL structure will be critical to understand the connection between cation-dependent interfacial solvation and CO2R performance.

Activation of H2O Tailored by Interfacial Electronic States at a Nanoscale Interface for Enhanced Electrocatalytic Hydrogen Evolution

Despite the fundamental and practical significance of the hydrogen evolution reaction (HER), the reaction kinetics at the molecular level are not well-understood, especially in basic media. Here, with ZIF-67-derived Co-based carbon frameworks (Co/NCs) as model catalysts, we systematically investigated the effects of different reaction parameters on the HER kinetics and discovered that the HER activity was directly dependent not on the type of nitrogen in the carbon framework but on the relative content of surface hydroxyl and water (OH-/H2O) adsorbed on Co active sites embedded in carbon frameworks. When the ratio of the OH-/H2O was close to 1:1, the Co/NC nanocatalyst showed the best reaction performance under the condition of high-pH electrolytes, e.g., an overpotential of only 232 mV at a current density of 10 mA cm-2 in the 1 M KOH electrolyte. We unambiguously identified that the structural water molecules (SWs) in the form of hydrous hydroxyl complexes absorbed on metal centers {OHad·H2O@M+} were catalytic active sites for the enhanced HER, where M+ could be transition or alkaline metal cations. Different from the traditional hydrogen bonding of water, the hydroxyl (hydroxide) groups and water molecules in the SWs were mainly bonded together via the spatial interaction between the p orbitals of O atoms, exhibiting features of a delocalized π-bond with a metastable state. These newly formed surface bonds or transitory states could be new weak interactions that synergistically promote both interfacial electron transfer and the activation of water (dissociation of O-H bonds) at the electrode surface, i.e., the formation of activated H adducts (H*). The capture of new surface states not only explains pH-, cation-, and transition-metal-dependent hydrogen evolution kinetics but also provides completely new insights into the understanding of other electrocatalytic reductions involving other small molecules, including CO2, CO, and N2.

Continuum Modeling of Porous Electrodes for Electrochemical Synthesis

Electrochemical synthesis possesses substantial promise to utilize renewable energy sources to power the conversion of abundant feedstocks to value-added commodity chemicals and fuels. Of the potential system architectures for these processes, only systems employing 3-D structured porous electrodes have the capacity to achieve the high rates of conversion necessary for industrial scale. However, the phenomena and environments in these systems are not well understood and are challenging to probe experimentally. Fortunately, continuum modeling is well-suited to rationalize the observed behavior in electrochemical synthesis, as well as to ultimately provide recommendations for guiding the design of next-generation devices and components. In this review, we begin by presenting an historical review of modeling of porous electrode systems, with the aim of showing how past knowledge of macroscale modeling can contribute to the rising challenge of electrochemical synthesis. We then present a detailed overview of the governing physics and assumptions required to simulate porous electrode systems for electrochemical synthesis. Leveraging the developed understanding of porous-electrode theory, we survey and discuss the present literature reports on simulating multiscale phenomena in porous electrodes in order to demonstrate their relevance to understanding and improving the performance of devices for electrochemical synthesis. Lastly, we provide our perspectives regarding future directions in the development of models that can most accurately describe and predict the performance of such devices and discuss the best potential applications of future models.

Design Strategies for Large Current Density Hydrogen Evolution Reaction

Hydrogen energy is considered one of the cleanest and most promising alternatives to fossil fuel because the only combustion product is water. The development of water splitting electrocatalysts with Earth abundance, cost-efficiency, and high performance for large current density industrial applications is vital for H2 production. However, most of the reported catalysts are usually tested within relatively small current densities (< 100 mA cm-2), which is far from satisfactory for industrial applications. In this minireview, we summarize the latest progress of effective non-noble electrocatalysts for large current density hydrogen evolution reaction (HER), whose performance is comparable to that of noble metal-based catalysts. Then the design strategy of intrinsic activities and architecture design are discussed, including self-supporting electrodes to avoid the detachment of active materials, the superaerophobicity and superhydrophilicity to release H2 bubble in time, and the mechanical properties to resist destructive stress. Finally, some views on the further development of high current density HER electrocatalysts are proposed, such as scale up of the synthesis process, in situ characterization to reveal the micro mechanism, and the implementation of catalysts into practical electrolyzers for the commercial application of as-developed catalysts. This review aimed to guide HER catalyst design and make large-scale hydrogen production one step further.

Opportunities for Electrocatalytic CO2 Reduction Enabled by Surface Ligands

To achieve high selectivity in enzyme catalysis, nature carefully controls both the catalyst active site and the pocket or environment that mediates access and the geometry of a reactant. Despite the many advantages of heterogeneous catalysis, active sites on a surface are rarely defined with atomic precision, making it difficult to control reaction selectivity with the molecular precision of homogeneous systems. In colloidal nanoparticle synthesis, structural control is accomplished using a surface ligand or capping layer that stabilizes a specific particle morphology and prevents nanoparticle aggregation. Usually, these surface ligands are considered detrimental for catalysis because they occupy otherwise active surface sites. However, a number of examples have shown that surface ligands can play a beneficial role in defining the catalytic environment and enhancing performance by a variety of mechanisms. This perspective summarizes recent advances and opportunities using surface ligands to enhance the performance of nanocatalysts for electrochemical CO₂ reduction. Several mechanisms are discussed, including selective permeability, modulating interfacial solvation structure and electric fields, chemical activation, and templating active site selection. These examples inform strategies and point to emerging opportunities to design nanocatalysts toward molecular level control of electrochemical CO₂ conversion.

Accelerated interfacial proton transfer for promoting the electrocatalytic activity

Interfacial pH is critical to electrocatalytic reactions involving proton-coupled electron transfer (PCET) processes, and maintaining an optimal interfacial pH at the electrochemical interface is required to achieve high activity. However, the interfacial pH varies inevitably during the electrochemical reaction owing to slow proton transfer at the interfacial layer, even in buffer solutions. It is therefore necessary to find an effective and general way to promote proton transfer for regulating the interfacial pH. In this study, we propose that promoting proton transfer at the interfacial layer can be used to regulate the interfacial pH in order to enhance electrocatalytic activity. By adsorbing a bifunctional 4-mercaptopyridine (4MPy) molecule onto the catalyst surface via its thiol group, the pyridyl group can be tethered on the electrochemical interface. The pyridyl group acts as both a good proton acceptor and donor for promoting proton transfer at the interfacial layer. Furthermore, the pK_a of 4MPy can be modulated with the applied potentials to accommodate the large variation of interfacial pH under different current densities. By in situ electrochemical surface-enhanced Raman spectroscopy (in situ EC-SERS), we quantitatively demonstrate that proton transfer at the interfacial layer of the Pt catalyst coated with 4MPy (Pt@4MPy) remains ideally thermoneutral during the H⁺ releasing electrocatalytic oxidation reaction of formic acid (FAOR) at high current densities. Thus, the interfacial pH is controlled effectively. In this way, the FAOR apparent current measured from Pt@4MPy is twice that measured from a pristine Pt catalyst. This work establishes a general strategy for regulating interfacial pH to enhance the electrocatalytic activities.

Adsorbing 4MPy on Pt surface promotes proton transfer at the interfacial layer, maintaining an optimal interfacial pH and promotes electrocatalytic reactions involving proton-coupled electron transfer (PCET) processes.