Insights into the pH effect on hydrogen electrocatalysis

Hydrogen electrocatalytic reactions, including the hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR), play a crucial role in a wide range of energy conversion and storage technologies. However, the HER and HOR display anomalous non-Nernstian pH dependent kinetics, showing two to three orders of magnitude sluggish kinetics in alkaline media compared to that in acidic media. Fundamental understanding of the origins of the intrinsic pH effect has attracted substantial interest from the electrocatalysis community. More critically, a fundamental molecular level understanding of this effect is still debatable, but is essential for developing active, stable, and affordable fuel cells and water electrolysis technologies. Against this backdrop, in this review, we provide a comprehensive overview of the intrinsic pH effect on hydrogen electrocatalysis, covering the experimental observations, underlying principles, and strategies for catalyst design. We discuss the strengths and shortcomings of various activity descriptors, including hydrogen binding energy (HBE) theory, bifunctional theory, potential of zero free charge (pzfc) theory, 2B theory and other theories, across different electrolytes and catalyst surfaces, and outline their interrelations where possible. Additionally, we highlight the design principles and research progress in improving the alkaline HER/HOR kinetics by catalyst design and electrolyte optimization employing the aforementioned theories. Finally, the remaining controversies about the pH effects on HER/HOR kinetics as well as the challenges and possible research directions in this field are also put forward. This review aims to provide researchers with a comprehensive understanding of the intrinsic pH effect and inspire the development of more cost-effective and durable alkaline water electrolyzers (AWEs) and anion exchange membrane fuel cells (AMFCs) for a sustainable energy future.

Exploring dynamic solvation kinetics at electrocatalyst surfaces

The interface between electrocatalyst and electrolyte is highly dynamic. Even in absence of major structural changes, the intermediate coverage and interfacial solvent are bias and time dependent. This is not accounted for in current kinetic models. Here, we study the kinetics of the hydrogen evolution, ammonia oxidation and oxygen reduction reactions on polycrystalline Pt with distinct intrinsic rates and intermediates (e.g. *H, *OH, *NH2, *N). Despite these differences, we discover shared relationships between the pre-exponential factor and the activation energy that we link to solvation kinetics in the presence of electronic excess charge and charged intermediates. Further, we study dynamic changes of these kinetic parameters with a millisecond time resolution during electrosorption and double layer charging and dynamic *N and *NO poisoning. Finally, we discover a pH-dependent activation entropy that explains non-Nernstian overpotential shifts with pH. In sum, our results demonstrate the importance of accounting for a bias and time-dependent interfacial solvent and catalyst surface.

The electrocatalytic activity for the hydrogen evolution reaction on alloys is determined by element-specific adsorption sites rather than d-band properties

Trends of the electrocatalytic activities for the hydrogen evolution reaction (HER) across transition metals are typically explained by d-band properties such as center or upper edge positions in relation to Fermi levels. Here, the universality of this relation is questioned for alloys, exemplified for the AuPt system which is examined with electrocatalytic measurements and density functional theory (DFT) calculations. At small overpotentials, linear combinations of the pure-metals' Tafel kinetics normalized to the alloy compositions are found to precisely resemble the measured HER activities. DFT calculations show almost neighbor-independent adsorption energies on Au and Pt surface-sites, respectively, as the adsorbed hydrogen influences the electron density mostly locally at the adsorption site itself. In contrast, the density of states of the d-band describe the delocalized conduction electrons in the alloys, which are unable to portray the local electronic environments at adsorption sites and related bonding strengths. The adsorption energies at element-specific surface sites are related to overpotential-dependent reaction mechanisms in a multidimensional reinterpretation of the volcano plot for alloys, which bridges the found inconsistencies between activity and bonding strength descriptors of the common electrocatalytic theory for alloys.

The pH-Induced Increase of the Rate Constant for HER at Au(111) in Acid Revealed by Combining Experiments and Kinetic Simulation

Origins of pH effects on the kinetics of electrocatalytic reactions involving the transfer of both protons and electrons, including the hydrogen evolution reaction (HER) considered in this study, are heatedly debated. By taking the HER at Au(111) in acid solutions of different pHs and ionic concentrations as the model systems, herein, we report how to derive the intrinsic kinetic parameters of such reactions and their pH dependence through the measurement of j-E curves and the corresponding kinetic simulation based on the Frumkin-Butler-Volmer theory and the modified Poisson-Nernst-Planck equation. Our study reveals the following: (i) the same set of kinetic parameters, such as the standard activation Gibbs free energy, charge transfer coefficient, and Gibbs adsorption energy for Had at Au(111), can simulate well all the j-E curves measured in solutions with different pH and temperatures; (ii) on the reversible hydrogen electrode scale, the intrinsic rate constant increases with the increase of pH, which is in contrast with the decrease of the HER current with the increase of pH; and (iii) the ratio of the rate constants for HER at Au(111) in x M HClO4 + (0.1 - x) M NaClO4 (pH ≤ 3) deduced before properly correcting the electric double layer (EDL) effects to the ones estimated with EDL correction is in the range of ca. 10 to 40, and even in a solution of x M HClO4 + (1 - x) M NaClO4 (pH ≤ 2) there is a difference of ca. 5× in the rate constants without and with EDL correction. The importance of proper correction of the EDL effects as well as several other important factors on unveiling the intrinsic pH-dependent reaction kinetics are discussed to help converge our analysis of pH effects in electrocatalysis.

Non-precious metal-based heterostructure catalysts for hydrogen evolution reaction: mechanisms, design principles, and future prospects

As a highly promising clean energy source to replace fossil fuels in the 21st century, hydrogen energy has garnered considerable attention, with water electrolysis emerging as a key hydrogen production technology. The development of highly active and stable non-precious metal-based catalysts for the hydrogen evolution reaction (HER) is crucial for achieving efficient and low-cost hydrogen production through electrolysis. Recently, heterostructure composite catalysts comprising two or more non-precious metals have demonstrated outstanding catalytic performance. First, we introduced the basic mechanism of the HER and, based on the reported HER theory, discussed the essence of constructing heterostructures to improve the catalytic activity of non-noble metal-based catalysts, that is, the coupling effect between components effectively regulates the electronic structure and the position of d-band centers. Then three catalytic effects of non-precious metal-based heterogeneous catalysts are described: synergistic effect, electron transfer effect and support effect. Lastly, we emphasized the potential of non-precious metal-based heterogeneous catalysts to replace precious metal-based catalysts, and summarized the future prospects and challenges.

pH Effects in a Model Electrocatalytic Reaction Disentangled

Varying the solution pH not only changes the reactant concentrations in bulk solution but also the local reaction environment (LRE) that is shaped furthermore by macroscopic mass transport and microscopic electric double layer (EDL) effects. Understanding ubiquitous pH effects in electrocatalysis requires disentangling these interwoven factors, which is a difficult, if not impossible, task without physical modeling. Herein, we demonstrate how a hierarchical model that integrates microkinetics, double-layer charging, and macroscopic mass transport can help understand pH effects of the formic acid oxidation reaction (FAOR). In terms of the relation between the peak activity and the solution pH, intrinsic pH effects without consideration of changes in the LRE would lead to a bell-shaped curve with a peak at pH = 6. Adding only macroscopic mass transport, we can already reproduce qualitatively the experimentally observed trapezoidal shape with a plateau between pH 5 and 10 in perchlorate and sulfate solutions. A quantitative agreement with experimental data requires consideration of EDL effects beyond Frumkin correlations. Specifically, the peculiar nonmonotonic surface charging relation affects the free energies of adsorbed intermediates. We further discuss pH effects of FAOR in phosphate and chloride-containing solutions, for which anion adsorption becomes important. This study underpins the importance of a full consideration of multiple interrelated factors for the interpretation of pH effects in electrocatalysis.

Theoretical study on the mechanism of the hydrogen evolution reaction catalyzed by platinum subnanoclusters

The smallest subnanocluster models of platinum colloid (Pt_n) are supposed to diffuse in aqueous media in order to examine their behaviors when they are subjected to the electrocatalytic hydrogen evolution reaction under zero overpotential conditions at pH 0. The DFT approach allows us to clarify the nature of individual proton transfer (PT) and electron transfer (ET) processes together with the importance of relying on concerted proton-electron transfer (CPET) pathways to promote the majority of H* adsorption processes by Pt_n subnanoclusters. Although the CPET processes are closely correlated with the Volmer steps (Pt + H⁺ + e^- → Pt-H*) described so far in electrochemistry, our study for the first time points out the essential capability of the Pt_n clusters to promote the multiple PT steps without the need to transfer any electrons, revealing the fundamentally high basicity of the naked Pt_n clusters (pK_a = 27-28 for Pt₄, Pt₅, and Pt₆). The discrete cluster models adopted herein avoid the structural constraints forced by the standard slab models and enable us to discuss the drastic alterations in the geometric and electronic structures of the intermediates given by the consecutive promotion of multiple CPET steps. The weakening of the Pt-H* bond strength with the increasing number of CPET steps is well rationalized by carefully examining the changes in the ν(Pt-H*) vibrational frequencies, the hydricity, and the H₂ desorption energy. The behaviors are also correlated with the underpotential and overpotential deposited hydrogen atoms (H_UPD and H_OPD) discussed in electrochemical studies for many years.

Improving the HER Activity and Stability of Pt Nanoparticles by Titanium Oxynitride Support

Water electrolysis powered by renewables is regarded as the feasible route for the production of hydrogen, obtained at the cathode side through electrochemical hydrogen evolution reaction (HER). Herein, we present a rational strategy to improve the overall HER catalytic performance of Pt, which is known as the best monometallic catalyst for this reaction, by supporting it on a conductive titanium oxynitride (TiON_x) dispersed over reduced graphene oxide nanoribbons. Characterization of the Pt/TiON_x composite revealed the presence of small Pt particles with diameters between 2 and 3 nm, which are well dispersed over the TiON_x support. The Pt/TiON_x nanocomposite exhibited improved HER activity and stability with respect to the Pt/C benchmark in an acid electrolyte, which was ascribed to the strong metal–support interaction (SMSI) triggered between the TiON_x support and grafted Pt nanoparticles. SMSI between TiON_x and Pt was evidenced by X-ray photoelectron spectroscopy (XPS) through a shift of the binding energies of the characteristic Pt 4f photoelectron lines with respect to Pt/C. Density functional theory (DFT) calculations confirmed the strong interaction between Pt nanoparticles and the TiON_x support. This strong interaction improves the stability of Pt nanoparticles and weakens the binding of chemisorbed H atoms thereon. Both of these effects may result in enhanced HER activity.

Electrocatalytic activity of metal encapsulated, doped, and engineered fullerene-based nanostructured materials towards hydrogen evolution reaction

The utilization of nanostructured materials as efficient catalyst for several processes has increased tremendously, and carbon-based nanostructured materials encompassing fullerene and its derivatives have been observed to possess enhanced catalytic activity when engineered with doping or decorated with metals, thus making them one of the most promising nanocage catalyst for hydrogen evolution reaction (HER) during electro-catalysis. Prompted by these, and the reported electrochemical, electronic and stability advantage, an attempt is put forward herein to inspect the metal encapsulated, doped, and decorated dependent HER activity of C24 engineered nanostructured materials as effective electro-catalyst for HER. Density functional theory (DFT) calculations have been utilized to evaluate the catalytic hydrogen evolution reaction activity of four proposed bare systems: fullerene (C24), calcium encapsulated fullerene (CaencC24), nickel-doped calcium encapsulated fullerene (NidopCaencC24), and silver decorated nickel-doped calcium encapsulated (AgdecNidopCaencC24) engineered nanostructured materials at the TPSSh/GenECP/6-311+G(d,p)/LanL2DZ level of theory. The obtained results divulged that, a potential decrease in energy gap (Egap) occurred in the bare systems, while a sparing increase was observed upon adsorption of hydrogen onto the surfaces, these surfaces where also observed to maintain the least EH-L gap while the AgdecNidopCaencC24 surface exhibited an increased electrocatalytic activity when compared to others. The results also showed that the electronic properties of the systems evinced a correspondent result with their electrochemical properties, the Ag-decorated surface also exhibited a proficient adsorption energy [Formula: see text] and Gibb's free energy (ΔGH) value. The engineered Ag-decorated and Ni-doped systems were found to possess both good surface stability and excellent electro-catalytic property for HER activities.

Investigation of the Stability and Hydrogen Evolution Activity of Dual-Atom Catalysts on Nitrogen-Doped Graphene

Single atom catalysts (SACs) have received a lot of attention in recent years for their high catalytic activity, selectivity, and atomic utilization rates. Two-dimensional N-doped graphene has been widely used to stabilize transition metal (TM) SACs in many reactions. However, the anchored SAC could lose its activity because of the too strong metal-N interaction. Alternatively, we studied the stability and activity of dual-atom catalysts (DACs) for 24 TMs on N-doped graphene, which kept the dispersion state but had different electronic structures from SACs. Our results show that seven DACs can be formed directly compared to the SACs. The others can form stably when the number of TMs is slightly larger than the number of vacancies. We further show that some of the DACs present better catalytic activities in hydrogen evolution reaction (HER) than the corresponding SACs, which can be attributed to the optimal charge transfer that is tuned by the additional atom. After the screening, the DAC of Re is identified as the most promising catalyst for HER. This study provides useful information for designing atomically-dispersed catalysts on N-doped graphene beyond SACs.

Theoretical Investigation of Catalytic Water Splitting by the Arene-Anchored Actinide Complexes

Actinide complexes, which could enable the electrocatalytic H₂O reduction, are not well documented because of the fact that actinide-containing catalysts are precluded by extremely stable actinyl species. Herein, by using relativistic density functional theory calculations, the arene-anchored trivalent actinide complexes (^Me,MeArO)₃ArAn (marked as [AnL]) with desirable electron transport between metal and ligand arene are investigated for H₂ production. The metal center is changed from Ac to Pu. Electron-spin density calculations reveal a two-electron oxidative process (involving high-valent intermediates) for complexes [AnL] (An = P-Pu) along the catalytic pathway. The electrons are provided by both the actinide metal and the arene ring of ligand. This is comparable to the previously reported uranium catalyst (^Ad,MeArO)₃mesU (Ad = adamantine and mes = mesitylene). From the thermodynamic and kinetic perspectives, [PaL] offers appreciably lower reaction energies for the overall catalytic cycle than other actinide complexes. Thus, the protactinium complex tends to be the most reactive for H₂O reduction to produce H₂ and has the advantage of its experimental accessibility.

Models of Electron Transfer at Different Electrode Materials

Electron transfer is the most important electrochemical process. In this review, we present elements of various aspects of electron transfer theory from the early work of Marcus and Hush to recent developments. The emphasis is on the role of the electronic, and to a lesser extent the geometrical, properties of the electrode. A variety of experimental works are discussed in light of these theoretical concepts. Because the field of electron transfer is so vast, this review is far from comprehensive; rather, we focus on systems that offer a special interest and illuminate aspects of the theory.

Theoretical Modeling of Electrochemical Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) plays an essential role in a wide range of electrocatalytic processes. A vast array of theoretical and computational methods have been developed to study electrochemical PCET. These methods can be used to calculate redox potentials and pK_a values for molecular electrocatalysts, proton-coupled redox potentials and bond dissociation free energies for PCET at metal and semiconductor interfaces, and reorganization energies associated with electrochemical PCET. Periodic density functional theory can also be used to compute PCET activation energies and perform molecular dynamics simulations of electrochemical interfaces. Various approaches for maintaining a constant electrode potential in electronic structure calculations and modeling complex interactions in the electric double layer (EDL) have been developed. Theoretical formulations for both homogeneous and heterogeneous electrochemical PCET spanning the adiabatic, nonadiabatic, and solvent-controlled regimes have been developed and provide analytical expressions for the rate constants and current densities as functions of applied potential. The quantum mechanical treatment of the proton and inclusion of excited vibronic states have been shown to be critical for describing experimental data, such as Tafel slopes and potential-dependent kinetic isotope effects. The calculated rate constants can be used as input to microkinetic models and voltammogram simulations to elucidate complex electrocatalytic processes.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Adiabatic versus non-adiabatic electron transfer at 2D electrode materials

2D electrode materials are often deployed on conductive supports for electrochemistry and there is a great need to understand fundamental electrochemical processes in this electrode configuration. Here, an integrated experimental-theoretical approach is used to resolve the key electronic interactions in outer-sphere electron transfer (OS-ET), a cornerstone elementary electrochemical reaction, at graphene as-grown on a copper electrode. Using scanning electrochemical cell microscopy, and co-located structural microscopy, the classical hexaamineruthenium (III/II) couple shows the ET kinetics trend: monolayer > bilayer > multilayer graphene. This trend is rationalized quantitatively through the development of rate theory, using the Schmickler-Newns-Anderson model Hamiltonian for ET, with the explicit incorporation of electrostatic interactions in the double layer, and parameterized using constant potential density functional theory calculations. The ET mechanism is predominantly adiabatic; the addition of subsequent graphene layers increases the contact potential, producing an increase in the effective barrier to ET at the electrode/electrolyte interface.

Nanostructured Carbon Electrocatalysts for Energy Conversions

The growing energy demand worldwide has led to increased use of fossil fuels. This, in turn, is making fossil fuels dwindle faster and cause more negative environmental impacts. Thus, alternative, environmentally friendly energy sources such as fuel cells and electrolyzers are being developed. While significant progress has already been made in this area, such energy systems are still hard to scale up because of their noble metal catalysts. In this concept paper, first, various scalable nanocarbon-based electrocatalysts that are being synthesized for energy conversions in these energy systems are introduced. Next, notable heteroatom-doping and nanostructuring strategies that are applied to produce different nanostructured carbon materials with high electrocatalytic activities for energy conversions are discussed. The concepts used to develop such materials with different structures and large density of dopant-based catalytic functional groups in a sustainable way, and the challenges therein, are emphasized in the discussions. The discussions also include the importance of various analytical, theoretical, and computational methods to probe the relationships between the compositions, structures, dopants, and active catalytic sites in such materials. These studies, coupled with experimental studies, can further guide innovative synthetic routes to efficient nanostructured carbon electrocatalysts for practical, large-scale energy conversion applications.

Multifunctional Electrocatalysis on Single-Site Metal Catalysts: A Computational Perspective

Multifunctional electrocatalysts are vastly sought for their applications in water splitting electrolyzers, metal-air batteries, and regenerative fuel cells because of their ability to catalyze multiple reactions such as hydrogen evolution, oxygen evolution, and oxygen reduction reactions. More specifically, the application of single-atom electrocatalyst in multifunctional catalysis is a promising approach to ensure good atomic efficiency, tunability and additionally benefits simple theoretical treatment. In this review, we provide insights into the variety of single-site metal catalysts and their identification. We also summarize the recent advancements in computational modeling of multifunctional electrocatalysis on single-site catalysts. Furthermore, we explain each modeling step with open-source-based working examples of a standard computational approach.

Infusing theory into deep learning for interpretable reactivity prediction

Despite recent advances of data acquisition and algorithms development, machine learning (ML) faces tremendous challenges to being adopted in practical catalyst design, largely due to its limited generalizability and poor explainability. Herein, we develop a theory-infused neural network (TinNet) approach that integrates deep learning algorithms with the well-established d-band theory of chemisorption for reactivity prediction of transition-metal surfaces. With simple adsorbates (e.g., *OH, *O, and *N) at active site ensembles as representative descriptor species, we demonstrate that the TinNet is on par with purely data-driven ML methods in prediction performance while being inherently interpretable. Incorporation of scientific knowledge of physical interactions into learning from data sheds further light on the nature of chemical bonding and opens up new avenues for ML discovery of novel motifs with desired catalytic properties.

Bayesian learning of chemisorption for bridging the complexity of electronic descriptors

Building upon the d-band reactivity theory in surface chemistry and catalysis, we develop a Bayesian learning approach to probing chemisorption processes at atomically tailored metal sites. With representative species, e.g., *O and *OH, Bayesian models trained with ab initio adsorption properties of transition metals predict site reactivity at a diverse range of intermetallics and near-surface alloys while naturally providing uncertainty quantification from posterior sampling. More importantly, this conceptual framework sheds light on the orbitalwise nature of chemical bonding at adsorption sites with d-states characteristics ranging from bulk-like semi-elliptic bands to free-atom-like discrete energy levels, bridging the complexity of electronic descriptors for the prediction of novel catalytic materials.

Developing a generalizable model to describe adsorption processes at metal surfaces can be extremely challenging due to complex phenomena involved. Here the authors introduce a Bayesian learning approach based on ab initio data and the d-band model to capture the essential physics of adsorbate–substrate interactions.

Perspective on experimental evaluation of adsorption energies at solid/liquid interfaces

Almost 15 years ago, first papers appeared, in which the density functional theory (DFT) was used to predict activity trends of electrocatalytic reactions. That was a major contribution of computational chemistry in building the theory of electrocatalysis. The possibility of computational electrocatalyst design had a massive impact on the way of thinking in modern electrocatalysis. At the same time, substantial criticism towards popular DFT models was developed during the years, due to the oversimplified view on electrified interfaces. Having this in mind, this work proposes an experimental methodology for quantitative description of adsorption energies at solid/liquid interfaces based on the Kelvin probe technique. The introduced approach already gives valuable trends in adsorption energies while in the future should evolve into an additional source of robust values that could complement existing DFT results. The pillars of the new methodology are established and verified experimentally with very promising initial results.

What is the trigger for the hydrogen evolution reaction? - towards electrocatalysis beyond the Sabatier principle

The mechanism of the hydrogen evolution reaction, although intensively studied for more than a century, remains a fundamental scientific challenge. Many important questions are still open, making it elusive to establish rational principles for electrocatalyst design. In this work, a comprehensive investigation was conducted to identify which dynamic phenomena at the electrified interface are prerequisite for the formation of molecular hydrogen. In fact, what we observe as an onset of the macroscopic faradaic current originates from dynamic structural changes in the double layer, which are entropic in nature. Based on careful analysis of the activation process, an electrocatalytic descriptor is introduced, evaluated and experimentally confirmed. The catalytic activity descriptor is named as the potential of minimum entropy. The experimentally verified catalytic descriptor reveals significant potential to yield innovative insights for the design of catalytically active materials and interfaces.

Hydrogen Evolution Reaction-From Single Crystal to Single Atom Catalysts

Hydrogen evolution reaction (HER) is one of the most important reactions in electrochemistry. This is not only because it is the simplest way to produce high purity hydrogen and the fact that it is the side reaction in many other technologies. HER actually shaped current electrochemistry because it was in focus of active research for so many years (and it still is). The number of catalysts investigated for HER is immense, and it is not possible to overview them all. In fact, it seems that the complexity of the field overcomes the complexity of HER. The aim of this review is to point out some of the latest developments in HER catalysis, current directions and some of the missing links between a single crystal, nanosized supported catalysts and recently emerging, single-atom catalysts for HER.

Advances and challenges for experiment and theory for multi-electron multi-proton transfer at electrified solid–liquid interfaces

Understanding microscopic mechanism of multi-electron multi-proton transfer reactions at complexed systems is important for advancing electrochemistry-oriented science in the 21st century.

Effects of a Ni cocatalyst on the photocatalytic hydrogen evolution reaction of anatase TiO2 by first-principles calculations

Loading Ni_n clusters on anatase TiO₂ surfaces remarkably reduces the Gibbs free energy of the photocatalytic hydrogen evolution reaction.

Quantum proton tunneling in multi-electron/-proton transfer electrode processes

Quantum proton tunneling in multi-electron/-proton transfer electrode processes were investigated in order to understand their possible microscopic mechanisms.

Metallic nanostructures with low dimensionality for electrochemical water splitting

Metallic nanostructures with low dimensionality (one-dimension and two-dimension) possess unique structural characteristics and distinctive electronic and physicochemical properties including high aspect ratio, high specific surface area, high density of surface unsaturated atoms and high electron mobility. These distinctive features have rendered them remarkable advantages over their bulk counterparts for surface-related applications, for example, electrochemical water splitting. In this review article, we highlight the recent research progress in low-dimensional metallic nanostructures for electrochemical water splitting including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Fundamental understanding of the electrochemistry of water splitting including HER and OER is firstly provided from the aspects of catalytic mechanisms, activity descriptors and property evaluation metrics. Generally, it is challenging to obtain low-dimensional metallic nanostructures with desirable characteristics for HER and OER. We hereby introduce several typical methods for synthesizing one-dimensional and two-dimensional metallic nanostructures including organic ligand-assisted synthesis, hydrothermal/solvothermal synthesis, carbon monoxide confined growth, topotactic reduction, and templated growth. We then put emphasis on the strategies adopted for the design and fabrication of high-performance low-dimensional metallic nanostructures for electrochemical water splitting such as alloying, structure design, surface engineering, interface engineering and strain engineering. The underlying structure-property correlation for each strategy is elucidated aiming to facilitate the design of more advanced electrocatalysts for water splitting. The challenges and perspectives for the development of electrochemical water splitting and low-dimensional metallic nanostructures are also proposed.

Hot electron-driven electrocatalytic hydrogen evolution reaction on metal-semiconductor nanodiode electrodes

Hot electrons generated on metal catalysts influence atomic and molecular processes, leading to hot electron-driven catalytic reactions. Here, we show the acceleration of electrocatalytic hydrogen evolution caused by internal injection of hot electrons on Pt/Si metal–semiconductor electrodes. When a forward bias voltage is applied to the Pt/Si contact, hot electrons are injected. The excess energy of these electrons allows them to reach the Pt/electrolyte interface and reduce the adsorbed hydrogen ions to form H₂ (2H⁺ + 2e⁻→H₂). We show that the onset potential of the hydrogen evolution reaction shifts positively by 160 mV while the cathodic current exhibits an 8-fold increase in the presence of hot electrons. The effect disappears when the thickness of the Pt film exceeds the mean free path of the hot electrons. The concept of a hot electron-driven reaction can lead to the development of a novel mechanism for controlling reactivity at liquid–solid interfaces.

The Role of Surface Texture on the Photocatalytic H2 Production on TiO2

It has been often reported that an efficient and green photocatalytic dissociation of water under irradiated semiconductors likely represents the most important goal for modern chemistry. Despite decades of intensive work on this topic, the efficiency of the water photolytic process under irradiated semiconductors is far from reaching significant photocatalytic efficiency. The use of a sacrificial agent as hole scavenger dramatically increases the hydrogen production rate and might represent the classic “kill two birds with one stone”: on the one hand, the production of hydrogen, then usable as energy carrier, on the other, the treatment of water for the abatement of pollutants used as sacrificial agents. Among metal oxides, TiO2 has a central role due to its versatility and inexpensiveness that allows an extended applicability in several scientific and technological fields. In this review we focus on the hydrogen production on irradiated TiO2 and its fundamental and environmental implications.

Metal/graphene heterobilayers as hydrogen evolution reaction cathodes: a first-principles study

Rh atoms in the interaction region facilitate hydrogen evolution reaction, whereas others in the deformation and transition regions do not, due to the interlayer charge transfer between single-layer Rh sheet and graphene.

Breaking H2 with CeO2: Effect of Surface Termination

The ability of ceria to break H₂ in the absence of noble metals has prompted a number of studies because of its potential applications in many technological fields. Most of the theoretical works reported in the literature are focused on the most stable (111) termination. However, recently, the possibility of stabilizing ceria particles with selected terminations has opened new avenues to explore. In the present paper, we investigate the role of termination in H₂ dissociation on stoichiometric ceria. We model (111)-, (110)-, and (100)-terminated slabs together with the stepped (221) and (331) surfaces. Our results support a dissociation mechanism proceeding via the formation of a hydride/hydroxyl CeH/OH intermediate. Both the stability of such an intermediate and the activation energy depend critically on the termination, the (100)-terminated surfaces being the most reactive: the activation energy is 0.16 eV, and the CeH/OH intermediate is stable by -0.64 eV for the (100) slab, whereas the (111) slab presents 0.75 and 0.74 eV, respectively. We provide structural, energetic, electronic, and spectroscopic data, as well as chemical descriptors correlating structure, energy, and reactivity, to guide in the theoretical and experimental characterization of the Ce-H surface intermediate.

Overcoming Site Heterogeneity In Search of Metal Nanocatalysts

Site heterogeneity of metal nanocatalysts poses grand challenges for catalyst design from first principles. To accelerate catalyst discovery, it is of pivotal importance to develop an approach that efficiently maps catalytic activity of nanoparticles onto geometry-based descriptors while considering the geometric strain and metal ligand of an active site. We demonstrate that there exist linear correlations between orbitalwise coordination numbers CN^α and free formation energies of oxygen species (e.g., *OH and *OOH) at Pt sites. Kinetic analysis along with herein developed structure-activity relationships accurately predicts the activity trend of pure Pt nanoparticles (∼1-7 nm) toward oxygen reduction. Application of the approach to a search of Pt nanoalloys leads to several Pt monolayer core-shell nanostructures with enhanced oxygen reduction activity and reduced cost. The approach presented here facilitates a transition from traditional single-crystal models to nanoparticles in theory-guided catalyst discovery.

Hydrogen adsorption trends on Al-doped Ni2P surfaces for optimal catalyst design

Nanoparticles of nickel phosphide are promising materials to replace the currently used rare Pt-group metals at cathode-side electrodes in devices for electrochemical hydrogen production.

A post Gurney quantum mechanical perspective on the electrolysis of water: ion neutralization in solution

Electron fluxes crossing the interface between a metallic conductor and an aqueous environment are important in many fields; hydrogen production, environmental scanning tunnelling microscopy, scanning electrochemical microscopy being some of them. Gurney (Gurney 1931 Proc. R. Soc. Lond.134, 137 (doi:10.1098/rspa.1931.0187)) provided in 1931 a scheme for tunnelling during electrolysis and outlined conditions for it to occur. We measure the low-voltage current flows between gold electrodes in pure water and use the time-dependent behaviour at voltage switch-on and switch-off to evaluate the relative contribution to the steady current arising from tunnelling of electrons between the electrodes and ions in solution and from the neutralization of ions adsorbed onto the electrode surface. We ascribe the larger current contribution to quantum tunnelling of electrons to and from ions in solution near the electrodes. We refine Gurney's barrier scheme to include solvated electron states and quantify energy differences using updated information. We show that Gurney's conditions would prevent the current flow at low voltages we observe but outline how the ideas of Marcus (Marcus 1956 J. Chem. Phys.24, 966-978 (doi:10.1063/1.1742723)) concerning solvation fluctuations enable the condition to be relaxed. We derive an average barrier tunnelling model and a multiple pathways tunnelling model and compare predictions with measurements of the steady-state current-voltage relation. The tunnelling barrier was found to be wide and low in agreement with other experimental studies. Applications as a biosensing mechanism are discussed that exploit the fast tunnelling pathways along molecules in solution.