Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy

Polypyrrole-decorated Vulcan XC-72R support enables a low platinum content PtRu catalyst toward alkaline hydrogen oxidation reaction

PtRu alloys are promising catalysts for the hydrogen oxidation reaction (HOR) in alkaline hydroxide exchange membrane fuel cells (HEMFCs) for commercial application. However, there is a need to lower the loading of platinum while simultaneously increasing its activity. Herein, we successfully prepared a PtRu alloy catalyst featuring a precisely controlled Pt-to-Ru atomic ratio of 1:5, which is typically 1:1 or 1:3, supported on polypyrrole (Ppy) decorated Vulcan XC-72R (XC). The catalyst, named Pt1Ru5/Ppy-XC, exhibits a remarkable mass catalytic activity of 10.69 ± 1.40 mA μgPGM-1, which is 7.2-fold and 2.6-fold higher than those of commercial PtRu/C and Pt1Ru5 nanoparticles supported on raw XC, respectively. Moreover, the HEMFC with Pt1Ru5/Ppy-XC anode achieves a peak power density of 1.53 W cm-2 (0.15 mgPGM cmanode-2), outperforming that of Pt1Ru5/XC (1.26 W cm-2, 0.18 mgPGM cmanode-2). The combined experimental characterization and theoretical calculations reveal that Ppy significantly enhances the active site density due to the decrease in the proportion of micropores while optimizing the binding strength of *H and *OH species on Pt1Ru5/Ppy-XC, resulting in excellent catalytic performance even with a low Pt usage. This work provides a novel strategy for developing high-performance electrocatalysts by employing functionalized XC support to fine-tune catalyst/support interactions and control over the pore structure of carbon supports.

Electrocatalyst of PdNi Particles on Carbon Black for Hydrogen Oxidation Reaction in Alkaline Membrane Fuel Cell

This work reports the synthesis of PdNi bimetallic particles and Pd on Carbon black (Vulcan XC-72) by reverse microemulsion and the chemical reduction of metallic complexes. The physicochemical characterization techniques used for the bimetallic and metallic materials were TGA, STEM, ICP-OES, and XRD. Also, the electrocatalysts were studied by electrochemical techniques such as anodic CO stripping and β-NiOOH reduction to elucidate the Pd and Ni surface sites participation in the reactions. The electrocatalysts were evaluated in the anodic reaction in anion-exchange membrane fuel cells (AEMFC) and the hydrogen oxidation reaction (HOR) in alkaline media. The results indicate that PdNi/C electrocatalysts exhibited higher electrocatalytic activity than Pd/C electrocatalysts in both the half-cell test and in the AEMFC, even with the same Pd loading, which is attributed to the bifunctional mechanism that provides OH- groups in oxophilic sites associated to Ni, that can facilitate the desorption of Hads in the Pd sites for the bimetallic material.

Water in Electrocatalysis

Renewable electricity-powered electrocatalysis technologies occupy a central position in clean energy conversion and the pursuit of a net-zero carbon emission future. Water can serve multiple roles in electrocatalytic reactions, for instance, as a reaction medium, reactant, modifier, promoter, etc. This significantly influences the mass transport, active site, intermediate adsorption and reaction kinetics, ultimately determining the electrocatalytic performance (e.g., activity, selectivity, and stability) as well as device efficiency. As the heart location where electrocatalytic reactions occur, the typical electrical-double layer is established at a water-electrode interface. Therefore, the comprehension and regulation of water are crucial topics in electrocatalysis, which encourages us to organize this review. We begin with the fundamental understanding on structure of water and its behavior under electrochemical conditions. Subsequently, we delve into the "water effect" by elucidating specific functions of water in electrocatalysis. Recent advances in manipulating water to enhance electrocatalytic efficiency of representative reactions such as hydrogen evolution/oxidation, oxygen evolution/reduction, CO2 reduction, N2 reduction and organic electrosynthesis, are also highlighted. We finally discuss the remaining challenges and future opportunities in this field.

Highly Distorted High-Entropy Alloy Aerogels for High-Efficiency Hydrogen Oxidation Reaction

The development of efficient electrocatalysts for alkaline hydrogen oxidation reaction (HOR) is essential for anion exchange membrane fuel cells and advancing the hydrogen economy. Herein, we demonstrated PtRuRhPdIr high-entropy alloy aerogels (HEAAs) with highly distorted structure as efficient HOR electrocatalysts and realized effective control of PtRu-based metallic aerogels (MAs) with elemental components ranging from two to seven. Specially, PtRuRhPdIr HEAAs on carbon (PtRuRhPdIr HEAAs/C) exhibit excellent HOR activity, with Pt group metal (PGM)-normalized mass activity (5.75 A mgPGM-1) at 50 mV and exchange current density normalized by electrochemical surface area (0.69 mA cm-2), approximately 16.9 and 4.1 times that of the commercial Pt/C (0.34 A mgPGM-1, 0.17 mA cm-2), respectively. The mechanism study shows that the highly distorted PtRuRhPdIr HEAAs provide abundant unsaturated sites for HOR, and the synergistic effect of multiple-active sites balances the adsorption of H* and *OH, boosting the HOR performance.

The Roles of Surface Hydrogen and Hydroxyl in Alkaline Hydrogen Oxidation on Ni-Based Electrocatalysts

One important target for anion exchange membrane fuel cells (AEMFCs) is to enable the application of anode non-precious metal hydrogen oxidation reaction (HOR) catalyst. Nickel presents a promising candidate for alkaline HOR; yet, its practical application is hampered by the intrinsically sluggish activity and poor stability. Herein, a series of Ni-based metals (Ni5Mo, Ni25Co, Ni14W and Ni) are electrodeposited as model catalysts to systematically explore the alkaline HOR by considering the role of adsorbed hydroxyl (OHad). Spectroscopic studies together with density functional theory calculations shed light on the beneficial effect of transition metal M (M=Mo, Co, W) alloying/doping on HOR by introducing the charge transfer from M to Ni and down shifting Ni 3d band center. The HOR specific activities on Ni-based catalysts reveal a volcano-type relationship with the hydrogen binding energy (HBE). The strongly adsorbed OHad is proven to induce deactivation for Ni active sites, and the deactivation potential is OHad binding energy (OHBE) dependent. This study adds new insight into the HOR mechanism and stability of Ni-based electrocatalysts, providing a new avenue for the rational design of highly efficient and robust alkaline HOR catalysts.

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.

Aqueous-S vs Organic-S Battery: Volmer-Step Involved Sulfur Reaction

Aqueous-S batteries (ASBs) are emerging as promising energy storage technologies due to their high safety, low cost, and high theoretical energy density. However, the present understanding of sulfur evolution in water relies on experience derived from conventional organic electrolyte-based sulfur batteries (OSBs). The gap between ASB and OSB has impeded progress in advancing the rational design of sulfur catalysts in the aqueous phase. Herein, we reveal the unique interaction between H2O and S species, which is fundamentally distinguishable from the organic counterparts. A series of spectroscopy analyses discloses that elemental sulfur is initially reduced to polysulfides (mainly S42-), which subsequently react with H2O to generate HS-, involving both polysulfide conversion and the Volmer step of water dissociation. Combined electrochemical and computational analysis further proposes an aqueous-S catalyst selection metric based on simultaneous polysulfide adsorption and Volmer-step catalysis. As a proof of concept, we have successfully prioritized the Mo2C-catalyzed ASBs with a superior rate capability of 1040 mAh g-1 than the Fe3C (693 mAh g-1) and pure C (510 mAh g-1) at a high current density of 5 A g-1. This work provides insights into the aqueous-S charge storage mechanism and establishes a foundational catalyst research paradigm for advancing the following ASBs.

Cluster-Scale Multisite Interface Reinforces Ruthenium-Based Anode Catalysts for Alkaline Anion Exchange Membrane Fuel Cells

Ruthenium (Ru) is a more cost-effective alternative to platinum anode catalysts for alkaline anion-exchange membrane fuel cells (AEMFCs), but suffers from severe competitive adsorption of hydrogen (Had) and hydroxyl (OHad). To address this concern, a strongly coupled multisite electrocatalyst with highly active cluster-scale ruthenium-tungsten oxide (Ru-WOx) interface, which could eliminate the competitive adsorption phenomenon and achieve high coverage of OHad and Had at Ru and WOx domains, respectively, is designed. The experimental and theoretical results demonstrate that WOx domain functions as a proton sponge to perpetually accommodate the activated hydrogen species that spillover from the adjacent Ru domain, and the resulting WO-Had species are readily coupled with Ru-OHad at the heterointerface to finish the hydrogen oxidation reaction with faster kinetics via the thermodynamically favorable Tafel-Volmer mechanism. The AEMFC delivers a high peak power density of 1.36 W cm-2 with a low anode catalyst loading of 0.05 mgRu cm-2 and outstanding durability (negligible voltage decay over 80-h operation at 500 mA cm-2). This work offers completely new insights into understanding the alkaline HOR mechanism and designing advanced anode catalysts for AEMFCs.

Isolated Metal Centers Activate Small Molecule Electrooxidation: Mechanisms and Applications

Electrochemical oxidation of small molecules shows great promise to substitute oxygen evolution reaction (OER) or hydrogen oxidation reaction (HOR) to enhance reaction kinetics and reduce energy consumption, as well as produce high-valued chemicals or serve as fuels. For these oxidation reactions, high-valence metal sites generated at oxidative potentials are typically considered as active sites to trigger the oxidation process of small molecules. Isolated atom site catalysts (IASCs) have been developed as an ideal system to precisely regulate the oxidation state and coordination environment of single-metal centers, and thus optimize their catalytic property. The isolated metal sites in IASCs inherently possess a positive oxidation state, and can be more readily produce homogeneous high-valence active sites under oxidative potentials than their nanoparticle counterparts. Meanwhile, IASCs merely possess the isolated metal centers but lack ensemble metal sites, which can alter the adsorption configurations of small molecules as compared with nanoparticle counterparts, and thus induce various reaction pathways and mechanisms to change product selectivity. More importantly, the construction of isolated metal centers is discovered to limit metal d-electron back donation to CO 2p* orbital and reduce the overly strong adsorption of CO on ensemble metal sites, which resolve the CO poisoning problems in most small molecules electro-oxidation reactions and thus improve catalytic stability. Based on these advantages of IASCs in the fields of electrochemical oxidation of small molecules, this review summarizes recent developments and advancements in IASCs in small molecules electro-oxidation reactions, focusing on anodic HOR in fuel cells and OER in electrolytic cells as well as their alternative reactions, such as formic acid/methanol/ethanol/glycerol/urea/5-hydroxymethylfurfural (HMF) oxidation reactions as key reactions. The catalytic merits of different oxidation reactions and the decoding of structure-activity relationships are specifically discussed to guide the precise design and structural regulation of IASCs from the perspective of a comprehensive reaction mechanism. Finally, future prospects and challenges are put forward, aiming to motivate more application possibilities for diverse functional IASCs.

Disentangling Multiple pH-Dependent Factors on the Hydrogen Evolution Reaction at Au(111)

Understanding how the electrolyte pH affects electrocatalytic activity is a topic of crucial importance in a large variety of systems. However, unraveling the origin of the pH effects is complicated often by the fact that both the reaction driving forces and reactant concentrations in the electric double layer (EDL) change simultaneously with the pH value. Herein, we employ the hydrogen evolution reaction (HER) at Au(111)-aqueous solution interfaces as a model system to disentangle different pH-dependent factors. In 0.1 M NaOH, the HER current density at Au(111) in the potential range of -0.4 V < E RHE < 0 V is up to 60 times smaller than that in 0.1 M HClO4. A reaction model with proper consideration of the local reaction conditions within the EDL is developed. After correcting for the EDL effects, the rate constant for HER is only weakly pH-dependent. Our analysis unambiguously reveals that the observed pH effects are mainly due to the pH-dependent reorganization free energy, which depends on the electrostatic potential and the local reaction conditions within the EDL. Possible origins of the pH and temperature dependence of the activation energy and the electron transfer coefficients are discussed. This work suggests that factors influencing the intrinsic pH-dependent kinetics are easier to understand after proper corrections of EDL effects.

Dynamic surface reconstruction engineers interfacial water structure for efficient alkaline hydrogen oxidation

Investigating the dynamic evolution of the catalyst and regulating the structure of interfacial water molecules participating in the hydrogen oxidation reaction (HOR) are essential for developing highly efficient electrocatalysts toward the practical application of anion exchange membrane fuel cells. Herein, we report an efficient strategy to activate hexagonal close-packed PtSe catalyst through in situ reconstruction that undergoes dynamic Se leaching and phase transition during linear sweep voltammetry cycles. The obtained Pt-Se catalyst presents as a surface Se atom-modified face-centered-cubic Pt-based nanocatalyst, and it exhibited remarkable catalytic performance in the alkaline HOR, showing an intrinsic activity of 0.552 mA cm-2 (j 0,s) and a mass activity of 1.084 mA μg-1 (j k,m @ 50 mV). The experimental results, including in situ surface-enhanced infrared absorption spectroscopy and density functional theory calculations suggest that the accumulated electrons on the surface-decorated Se of Pt-Se can induce the regulation of the interfacial water structure between the electrode and electrolyte surface in the electric double-layer region. Consequently, the migration of OH- species from the electrolyte to the catalyst surface can be apparently accelerated within this disordered water network, which together with the optimized intermediate thermodynamic binding energies, contribute to the enhanced alkaline HOR activity.

The Essential Role of Well-Defined Materials in Assessing Electrocatalyst Structure and Function

Advancements in heterogeneous electrocatalysis are essential for the transition to a sustainable future. However, current catalysts often face limitations in activity, selectivity, stability, and cost-effectiveness, highlighting the need for a detailed understanding of catalyst behavior to construct rational design strategies. This perspective focuses on the role of well-defined materials, particularly ordered intermetallics, in enhancing catalytic performance. Ordered intermetallic materials enable precise control over geometric and electronic properties, which are critical for tailoring reactivity. Their structured atomic configurations not only allow for rational control of catalytic activity but also integrate seamlessly with computational models, facilitating the design of improved materials. Additionally, we examine future research directions for intermetallics in electrocatalysis, identifying opportunities for significant breakthroughs in the field.

Heating dictates the scalability of CO2 electrolyzer types

Electrochemical CO2 reduction offers a promising method of converting renewable electrical energy into valuable hydrocarbon compounds vital to hard-to-abate sectors. Significant progress has been made on the lab scale, but scale-up demonstrations remain limited. Because of the low energy efficiency of CO2 reduction, we suspect that significant thermal gradients may develop in industrially relevant dimensions. We describe here a model prediction for non-isothermal behavior beyond the typical 1D models to illustrate the severity of heating at larger scales. We develop a 2D model for two membrane electrode assembly (MEA) CO2 electrolyzers; a liquid anolyte fed MEA (exchange MEA) and a fully gas fed configuration (full MEA). Our results indicate that full MEA configurations exhibit very poor electrochemical performance at moderately larger scales due to non-isothermal effects. Heating results in severe membrane dehydration, which induces large Ohmic losses in the membrane, resulting in a sharp decline in the current density along the flow direction. In contrast, the anolyte employed in the exchange MEA configuration is effective in preventing large thermal gradients. Membrane dehydration is not a problem for the exchange MEA configuration, leading to a nearly constant current density over the entire length of the modeled domain, and indicating that exchange MEA configurations are well suited for scale-up. Our results additionally indicate that a balance between faster kinetics, higher ionic conductivity, smaller pH gradients and lower CO2 solubility causes an optimum operating temperature between 60 and 70 °C.

Dominant Role of Coexisting Ruthenium Nanoclusters Over Single Atoms to Enhance Alkaline Hydrogen Evolution Reaction

Developing efficient and cost-effective electrocatalysts to replace expensive carbon-supported platinum nanoparticles for the alkaline hydrogen evolution reaction remains an important challenge. Recently, an innovative catalyst, composed of ruthenium single atoms (Ru1) integrated with small Ru nanoclusters (RuNC), has attracted considerable attention from the scientific community. However, because of its complexity, this catalyst remains a topic of some debate. Here, a method is reported of precisely controlling the ratios of Ru1 to RuNC on a nitrogenated carbon (NC)-based porous organic framework to produce Ru/NC catalysts, by using different amounts (0, 5, 10 wt.%) of reducing agent. The Ru/NC-10 catalyst, formed with 10 wt.% reducing agent, delivered the best performance under alkaline conditions, indicating that RuNC played a significant role in actual alkaline hydrogen evolution reaction (HER). An anion exchange membrane water electrolyzer (AEMWE) system using the Ru/NC-10 catalyst required a significantly lower operating voltage (1.72 V) than the commercial Pt/C catalyst (1.95 V) to achieve 500 mA cm-2. Moreover, the system can be operated at 100 mA cm-2 without notable performance decay for over 180 h. Theoretical calculations supported these experimental findings that Ru1 contributed to the water dissociation process, while RuNC is more actively associated with the hydrogen recombination process.

Isotopic labelling of water reveals the hydrogen transfer route in electrochemical CO2 reduction

Understanding the hydrogenation pathway in electrochemical CO2 reduction is important for controlling product selectivity. The Eley-Rideal mechanism involving proton-coupled electron transfer directly from solvent water is often considered to be the primary hydrogen transfer route. However, in principle, hydrogenation can also occur via the Langmuir-Hinshelwood mechanism using surface-adsorbed *H. Here, by performing CO2 reduction with Cu in H2O-D2O mixtures, we present evidence that the Langmuir-Hinshelwood mechanism is probably the dominant hydrogenation route. From this, we estimate the extent to which each mechanism contributes towards the formation of six important CO2 reduction products. Through computational simulations, we find that the formation of C-H bonds and O-H bonds is governed by the Langmuir-Hinshelwood and Eley-Rideal mechanism, respectively. We also show that promoting the Eley-Rideal pathway could be crucial towards selective multicarbon product formation and suppressing hydrogen evolution. These findings introduce important considerations for the theoretical modelling of CO2 reduction pathways and electrocatalyst design.

Electrochemical Random-Access Memory: Progress, Perspectives, and Opportunities

Non-von Neumann computing using neuromorphic systems based on analogue synaptic and neuronal elements has emerged as a potential solution to tackle the growing need for more efficient data processing, but progress toward practical systems has been stymied due to a lack of materials and devices with the appropriate attributes. Recently, solid state electrochemical ion-insertion, also known as electrochemical random access memory (ECRAM) has emerged as a promising approach to realize the needed device characteristics. ECRAM is a three terminal device that operates by tuning electronic conductance in functional materials through solid-state electrochemical redox reactions. This mechanism can be considered as a gate-controlled bulk modulation of dopants and/or phases in the channel. Early work demonstrating that ECRAM can achieve nearly ideal analogue synaptic characteristics has sparked tremendous interest in this approach. More recently, the realization that electrochemical ion insertion can be used to tune the electronic properties of many types of materials including transition metal oxides, layered two-dimensional materials, organic and coordination polymers, and that the changes in conductance can span orders of magnitude has further attracted interest in ECRAM as the basis for analogue synaptic elements for inference accelerators as well as for dynamical devices that can emulate a wide range of neuronal characteristics for implementation in analogue spiking neural networks. At its core, ECRAM shares many fundamental aspects with rechargeable batteries, where ion insertion materials are used extensively for their ability to reversibly store charge and energy. Computing applications, however, present drastically different requirements: systems will require many millions of devices, scaled down to tens of nanometers, all while achieving reliable electronic-state tuning at scaled-up rates and endurances, and with minimal energy dissipation and noise. In this review, we discuss the history, basic concepts, recent progress, as well as the challenges and opportunities for different types of ECRAM, broadly grouped by their primary mobile ionic charge carrier, including Li, protons, and oxygen vacancies.

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

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

Facilitating alkaline hydrogen evolution kinetics via interfacial modulation of hydrogen-bond networks by porous amine cages

The electrode-electrolyte interface is pivotal in the electrochemical kinetics. However, modulating the electrochemical interface at the atomic or molecular level is challenging due to the lack of efficient interfacial regulators. Here, we employ a porous amine cage as an interfacial modifier to Pt cluster in a confining configuration, largely enhancing alkaline HER kinetics by facilitating charge transfer. In situ electrochemical surface-enhanced Raman spectra, in combination with the ab initio molecular dynamics simulation, elucidates that the interaction between water and the -NH- moiety of cage frame softens the H-bonds net of interfacial water, making it more flexible for charge transfer. Moreover, our investigation pinpointed that the -NH- moiety acted as a pump for charge transfer by Grotthuss mechanism, lowering the kinetic barrier for hydrogen adsorption. Our findings highlight the strategy of establishing a soft-confining interfacial modifier by porous cage, offering opportunities to optimize electrochemical interfaces and promote reaction kinetics in a targeted way.

Scanning electrochemical probe microscopy: towards the characterization of micro- and nanostructured photocatalytic materials

Platinum-black (Pt-B) has been demonstrated to be an excellent electrocatalytic material for the electrochemical oxidation of hydrogen peroxide (H2O2). As Pt-B films can be deposited electrochemically, micro- and nano-sized conductive transducers can be modified with Pt-B. Here, we present the potential of Pt-B micro- and sub-micro-sized sensors for the detection and quantification of hydrogen (H2) in solution. Using these microsensors, no sampling step for H2 determination is required and e.g., in photocatalysis, the onset of H2 evolution can be monitored in situ. We present Pt-B-based H2 micro- and sub-micro-sized sensors based on different electrochemical transducers such as microelectrodes and atomic force microscopy (AFM)-scanning electrochemical microscopy (SECM) probes, which enable local measurements e.g., at heterogenized photocatalytically active samples. The microsensors are characterized in terms of limits of detection (LOD), which ranges from 4.0 μM to 30 μM depending on the size of the sensors and the experimental conditions such as type of electrolyte and pH. The sensors were tested for the in situ H2 evolution by light-driven water-splitting, i.e., using ascorbic acid or triethanolamine solutions, showing a wide linear concentration range, good reproducibility, and high sensitivity. Proof-of-principle experiments using Pt-B-modified cantilever-based sensors were performed using a model sample platinum substrate to map the electrochemical H2 evolution along with the topography using AFM-SECM.

Electrocatalysis: From Planar Surfaces to Nanostructured Interfaces

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

Pd1Ni2 Trimer Sites Drive Efficient and Durable Hydrogen Oxidation in Alkaline Media

Anion-exchange membrane fuel cell (AEMFC) is a cost-effective hydrogen-to-electricity conversion technology under a zero-emission scenario. However, the sluggish kinetics of the anodic hydrogen oxidation reaction (HOR) impedes the commercial implementation of AEMFCs. Here, we develop a Pd single-atom-embedded Ni3N catalyst (Pd1/Ni3N) with unconventional Pd1Ni2 trimer sites to drive efficient and durable HOR in alkaline media. Integrating theoretical and experimental analyses, we demonstrate that dual Pd1Ni2 sites achieve a "*H on Pd1Ni2-HV + *OH on Pd1Ni2-HN" adsorption mode, effectively weakening the overstrong *H and *OH adsorptions on pristine Ni3N. Owing to the unique coordination mode and atomically dispersed catalytic sites, the resulting Pd1/Ni3N catalyst delivers a high intrinsic and mass activity together with excellent antioxidation capability and CO tolerance. Specifically, the HOR mass activity of Pd1/Ni3N reaches 7.54 A mgPd-1 at the overpotential of 50 mV. The AEMFC employing Pd1/Ni3N as the anode catalyst displays a high power density of 31.7 W mgPd-1 with an ultralow anode precious metal loading of only 0.023 mgPd cm-2. This study provides guidance for the design of high-performance alkaline HOR catalytic sites at the atomic level.

An Interstitial Boron Inserted Metastable Hexagonal Rh Nanocrystal for Efficient Hydrogen Oxidation Electrocatalysis

Constructing metastable phase structure plays an important role in changing the physicochemical properties and improving the catalytic performance of nanocrystals. Unfortunately, the synthesis of metastable phase metallic nanocrystals is highly challenging, mainly due to the thermodynamically unstable ground-state. Here, we report a synthesis of unconventional metastable hexagonal rhodium nanocrystal (Bint-Rhhcp/C) via interstitial boron insertion. The insertion of boron atoms into the interstitial sites of cubic Rh lattice not only induces the atomic arrangements from face-centered cubic (fcc) to hexagonal close-packed (hcp), but also stabilizes the metastable hexagonal Rh structure. Benefiting from the phase transition and interstitial boron doping, the Bint-Rhhcp/C catalyst exhibits remarkable catalytic performance toward hydrogen oxidation reaction (HOR) under alkaline media, with a mass activity of 1.413 mA μgPGM -1. Experimental measurements including in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) and density functional theory (DFT) calculations indicate that the strengthened adsorption of hydroxyl species on the electrode surface of Bint-Rhhcp/C is responsible for the reconstruction of interfacial water structure and increased water proportions in the gap region in the electric double layers. As a result, the increased water connectivity and hydrogen bond network facilitate high-efficiency hydrogen transfer across the interface, thereby boost the alkaline HOR performance.

Research Progress and Perspectives on Anti-Poisoning Hydrogen Oxidation Reaction Electrocatalysts for Hydrogen Fuel Cells

As global demand for clean and sustainable energy continues to rise, fuel cell technology has seen rapid advancement. However, the presence of trace impurities like carbon monoxide (CO) and hydrogen sulfide (H₂S) in hydrogen fuel can significantly deactivate the anode by blocking its active sites, leading to reduced performance. Developing electrocatalysts that are resistant to CO and H₂S poisoning has therefore become a critical priority. This paper provides a comprehensive analysis of the poisoning mechanisms of CO and H₂S and reviews the key strategies developed over the past few decades to enhance the impurity tolerance of anode electrocatalysts. It begins by examining the differences in hydrogen oxidation reaction (HOR) mechanisms in acidic and alkaline environments, focusing on the roles of hydrogen binding energy (HBE) and hydroxide binding energy (OHBE). Next, it outlines three main approaches to mitigate CO poisoning: (I) bifunctional mechanisms, (II) direct mechanisms, and (III) constructing protective blocking layers. The review then shifts to strategies for countering H₂S poisoning, emphasizing both electrocatalyst design and structural improvements in fuel cells. Finally, the paper highlights recent advances in anti-poisoning electrocatalysts, discusses their applications and limitations, and identifies the key challenges and future opportunities for further research in this field.

Multiscale Understanding of Anion Exchange Membrane Fuel Cells: Mechanisms, Electrocatalysts, Polymers, and Cell Management

Anion exchange membrane fuel cells (AEMFCs) are among the most promising sustainable electrochemical technologies to help solve energy challenges. Compared to proton exchange membrane fuel cells (PEMFCs), AEMFCs offer a broader choice of catalyst materials and a less corrosive operating environment for the bipolar plates and the membrane. This can lead to potentially lower costs and longer operational life than PEMFCs. These significant advantages have made AEMFCs highly competitive in the future fuel cell market, particularly after advancements in developing non-platinum-group-metal anode electrocatalysts, anion exchange membranes and ionomers, and in understanding the relationships between cell operating conditions and mass transport in AEMFCs. This review aims to compile recent literature to provide a comprehensive understanding of AEMFCs in three key areas: i) the mechanisms of the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) in alkaline media; ii) recent advancements in the synthesis routes and structure-property relationships of cutting-edge HOR and ORR electrocatalysts, as well as anion exchange membranes and ionomers; and iii) fuel cell operating conditions, including water management and impact of CO2. Finally, based on these aspects, the future development and perspectives of AEMFCs are proposed.

Understanding the Roles and Regulation Methods of Key Adsorption Species on Ni-Based Catalysts for Efficient Hydrogen Oxidation Reactions in Alkaline Media

This review focuses on recent advancements in the development and understanding of nickel-based catalysts for the hydrogen oxidation reaction in alkaline media. Given the economic and environmental limitations associated with platinum group metals, nickel-based catalysts have emerged as promising alternatives due to their abundance, lower cost, and comparable catalytic properties. The review begins with an exploration of the fundamental HOR mechanisms, emphasizing the key roles of the reactive species in optimizing the catalytic activity of Ni-based catalysts. Thermodynamic and stability optimizations of nickel-based catalysts are thoroughly examined, focusing on alloying strategies, heteroatom incorporation, and the use of various support materials to enhance their catalytic performance and durability. The review also addresses the challenge of catalyst poisoning, particularly by carbon monoxide, and evaluates the effectiveness of different approaches to improve poison resistance. Finally, the review concludes by summarizing the key findings and proposing future research directions to further enhance the efficiency and stability of nickel-based catalysts for practical applications in anion exchange membrane fuel cells. The insights gained from this comprehensive analysis aim to contribute to the development of cost-effective and sustainable catalysts and facilitate the broader adoption of AEMFCs in the quest for clean energy solutions.

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.

An In-Plane Heterostructure Ni3N/MoSe2 Loaded on Nitrogen-Doped Reduced Graphene Oxide Enhances the Catalyst Performance for Hydrogen Oxidation Reaction

Non-noble metal electrocatalysts for the hydrogen oxidation reaction (HOR) that are both highly active and low-cost are essential for the widespread use of fuel cells. Herein, a simple two-step method for creating an in-plane heterostructure of Ni3N/MoSe2 loaded on N-doped reduced graphene oxide (Ni3N/MoSe2@N-rGO) as an effective electrocatalyst for the HOR is described. The process involves hydrothermal treatment of the Ni and Mo precursors with N-doped reduced graphene oxide, followed by the annealing with urea. The Ni3N/MoSe2@N-rGO catalyst exhibits high activities for the HOR, with current densities of 2.15 and 3.06 mA cm-2 at 0.5 V vs. the reversible hydrogen electrode (RHE) in H2-saturated 0.1 M KOH and 0.1 M HClO4 electrolytes, respectively, which is comparable to a commercial 20% Pt/C catalyst under similar experimental conditions. Furthermore, the catalyst demonstrates excellent durability, maintaining its performance during accelerated degradation tests for 5000 cycles. This work offers a practical framework for the designing and preparing of non-precious metal electrocatalysts for the HOR in fuel cells.

All-round enhancement induced by oxophilic single Ru and W atoms for alkaline hydrogen oxidation of tiny Pt nanoparticles

Anion exchange membrane fuel cells (AEMFCs) are one of the ideal energy conversion devices. However, platinum (Pt), as the benchmark catalyst for the hydrogen oxidation reaction (HOR) of AEMFCs anodes, still faces issues of insufficient performance and susceptibility to CO poisoning. Here, we report the Joule heating-assisted synthesis of a small sized Ru1Pt single-atom alloy catalyst loaded on nitrogen-doped carbon modified with single W atoms (s-Ru1Pt@W1/NC), in which the near-range single Ru atoms on the Ru1Pt nanoparticles and the long-range single W atoms on the support simultaneously modulate the electronic structure of the active Pt-site, enhancing alkaline HOR performance of s-Ru1Pt@W1/NC. The mass activity of s-Ru1Pt@W1/NC is 7.54 A mgPt+Ru-1 and exhibits notable stability in 1000 ppm CO/H2-saturated electrolyte. Surprisingly, it can operate stably in H2-saturated electrolyte for 1000 h with only 24.60 % decay. Theoretical calculations demonstrate that the proximal single Ru atoms and the remote single W atoms synergistically optimize the electronic structure of the active Pt-site, improving the HOR activity and CO tolerance of the catalyst.

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.

Advances in Two-Electron Water Oxidation Reaction for Hydrogen Peroxide Production: Catalyst Design and Interface Engineering

Hydrogen peroxide (H2O2) is a versatile and zero-emission material that is widely used in the industrial, domestic, and healthcare sectors. It is clear that it plays a critical role in advancing environmental sustainability, acting as a green energy source, and protecting human health. Conventional production techniques focused on anthraquinone oxidation, however, electrocatalytic synthesis has arisen as a means of utilizing renewable energy sources in conjunction with available resources like oxygen and water. These strides represent a substantial change toward more environmentally and energy-friendly H2O2 manufacturing techniques that are in line with current environmental and energy goals. This work reviews recent advances in two-electron water oxidation reaction (2e-WOR) electrocatalysts, including design principles and reaction mechanisms, examines catalyst design alternatives and experimental characterization techniques, proposes standardized assessment criteria, investigates the impact of the interfacial milieu on the reaction, and discusses the value of in situ characterization and molecular dynamics simulations as a supplement to traditional experimental techniques and theoretical simulations. The review also emphasizes the importance of device design, interface, and surface engineering in improving the production of H2O2. Through adjustments to the chemical microenvironment, catalysts can demonstrate improved performance, opening the door for commercial applications that are scalable through tandem cell development.

Rechargeable Hydrogen Gas Batteries: Fundamentals, Principles, Materials, and Applications

The growing demand for renewable energy sources has accelerated a boom in research on new battery chemistries. Despite decades of development for various battery types, including lithium-ion batteries, their suitability for grid-scale energy storage applications remains imperfect. In recent years, rechargeable hydrogen gas batteries (HGBs), utilizing hydrogen catalytic electrode as anode, have attracted extensive academic and industrial attention. HGBs, facilitated by appropriate catalysts, demonstrate notable attributes such as high power density, high capacity, excellent low-temperature performance, and ultralong cycle life. This review presents a comprehensive overview of four key aspects pertaining to HGBs: fundamentals, principles, materials, and applications. First, detailed insights are provided into hydrogen electrodes, encompassing electrochemical principles, hydrogen catalytic mechanisms, advancements in hydrogen catalytic materials, and structural considerations in hydrogen electrode design. Second, an examination and future prospects of cathode material compatibility, encompassing both current and potential materials, are summarized. Third, other components and engineering considerations of HGBs are elaborated, including cell stack design and pressure vessel design. Finally, a techno-economic analysis and outlook offers an overview of the current status and future prospects of HGBs, indicating their orientation for further research and application advancements.

Insight Into Intermediate Behaviors and Design Strategies of Platinum Group Metal-Based Alkaline Hydrogen Oxidation Catalysts

Hydrogen oxidation reaction (HOR) can effectively convert the hydrogen energy through the hydrogen fuel cells, which plays an increasingly important role in the renewable hydrogen cycle. Nevertheless, when the electrolyte pH changes from acid to base, even with platinum group metal (PGM) catalysts, the HOR kinetics declines with several orders of magnitude. More critically, the pivotal role of reaction intermediates and interfacial environment during intermediate behaviors on alkaline HOR remains controversial. Therefore, exploring the exceptional PGM-based alkaline HOR electrocatalysts and identifying the reaction mechanism are indispensable for promoting the commercial development of hydrogen fuel cells. Consequently, the fundamental understanding of the HOR mechanism is first introduced, with emphases on the adsorption/desorption process of distinct reactive intermediates and the interfacial structure during catalytic process. Subsequently, with the guidance of reaction mechanism, the latest advances in the rational design of advanced PGM-based (Pt, Pd, Ir, Ru, Rh-based) alkaline HOR catalysts are discussed, focusing on the correlation between the intermediate behaviors and the electrocatalytic performance. Finally, given that the challenges standing in the development of the alkaline HOR, the prospect for the rational catalysts design and thorough mechanism investigation towards alkaline HOR are emphatically proposed.

Construction of Mo2TiC2Tx MXene as a Superior Carrier to Support Ru-CuO Heterojunctions for Improving Alkaline Hydrogen Oxidation

The sluggish anodic hydrogen oxidation reaction (HOR) of the hydroxide exchange membrane fuel cell (HEMFC) is a significant barrier for practical implementation. Herein, we designed a catalyst of Mo2TiC2Tx MXene-supported Ru-CuO heterojunctions (named as Ru-CuO/MXene). The XPS spectra and the d-band center data of the different amounts of Cu of the Ru-CuO/MXene suggested that there existed a strongly electronic metal-support interaction between the active species and the substrate with MXene as the excellent carrier. Furthermore, the in situ electrochemical experimental results (operando electrochemical impedance spectroscopy and in situ electrochemical Raman spectroscopy) and density functional theory (DFT) calculations showed that Ru and CuO were the optimal adsorption sites for surface species *H and *OH, respectively, endowing Ru-CuO/MXene with the ability to weaken the hydrogen binding energy (HBE) and strengthen the hydroxide binding energy (OHBE). Remarkably, the optimized catalyst modified an impressive HOR activity in the 0.1 M KOH electrolyte with a kinetic and an exchange current density of 7.63 and 1.37 mA cm-2 at 50 mV overpotential (vs. RHE), respectively, which were 1.98- and 1.2-fold higher than those of commercial Pt/C (20 wt %). Finally, the as-prepared catalyst also exhibited superior durability and exceptional CO antipoisoning ability in 1000 ppm of the CO/H2-saturated 0.1 M KOH electrolyte. This work provides an inspiring strategy to design high-activity HOR electrocatalyst-based metallic Ru in alkaline environments.

Alkaline Hydrogen Evolution Reaction Electrocatalysts for Anion Exchange Membrane Water Electrolyzers: Progress and Perspective

For the aim of achieving the carbon-free energy scenario, green hydrogen (H2) with non-CO2 emission and high energy density is regarded as a potential alternative to traditional fossil fuels. Over the last decades, significant breakthroughs have been realized on the alkaline hydrogen evolution reaction (HER), which is a fundamental advancement and efficient process to generate high-purity H2 in the laboratory. Based on this, the development of the practical industry-oriented anion exchange membrane water electrolyzer (AEMWE) is on the rise, showing competitiveness with the incumbent megawatt-scale H2 production technologies. Still, great challenges lie in exploring the electrocatalysts with remarkable activity and stability for alkaline HER, as well as bridging the gap of performance difference between the three-electrode cell and AEMWE devices. In this perspective, we systematically discuss the in-depth mechanisms for activating alkaline HER electrocatalysts, including electronic modification, defect construction, morphology control, synergistic function, field effect, etc. In addition, the current status of AEMWE is reviewed, and the underlying bottlenecks that impede the application of HER electrocatalysts in AEMWE are summarized. Finally, we share our thoughts regarding the future development directions of electrocatalysts toward both alkaline HER and AEMWE, in the hope of advancing the commercialization of water electrolysis technology for green H2 production.

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.

Recent advances in microenvironment regulation for electrocatalysis

High-efficiency electrocatalysis could serve as the bridge that connects renewable energy technologies, hydrogen economy and carbon capture/utilization, promising a sustainable future for humankind. It is therefore of paramount significance to explore feasible strategies to modulate the relevant electrocatalytic reactions and optimize device performances so as to promote their large-scale practical applications. Microenvironment regulation at the catalytic interface has been demonstrated to be capable of effectively enhancing the reaction rates and improving the selectivities for specific products. In this review we summarize the latest advances in microenvironment regulation in typical electrocatalytic processes (including water electrolysis, hydrogen-oxygen fuel cells, and carbon dioxide reduction) and the related in situ/operando characterization techniques and theoretical simulation methods. At the end of this article, we present an outlook on development trends and possible future directions.

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.

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.

Dynamic Active Sites in Electrocatalysis

In-depth understanding of the real-time behaviors of active sites during electrocatalysis is essential for the advancement of sustainable energy conversion. Recently, the concept of dynamic active sites has been recognized as a potent approach for creating self-adaptive electrocatalysts that can address a variety of electrocatalytic reactions, outperforming traditional electrocatalysts with static active sites. Nonetheless, the comprehension of the underlying principles that guide the engineering of dynamic active sites is presently insufficient. In this review, we systematically analyze the fundamentals of dynamic active sites for electrocatalysis and consider important future directions for this emerging field. We reveal that dynamic behaviors and reversibility are two crucial factors that influence electrocatalytic performance. By reviewing recent advances in dynamic active sites, we conclude that implementing dynamic electrocatalysis through variable reaction environments, correlating the model of dynamic evolution with catalytic properties, and developing localized and ultrafast in situ/operando techniques are keys to designing high-performance dynamic electrocatalysts. This review paves the way to the development of the next-generation electrocatalyst and the universal theory for both dynamic and static active sites.

Hydrogen spillover inspired bifunctional Platinum/Rhodium Oxide-Nitrogen-Doped carbon composite for enhanced hydrogen evolution and oxidation reactions in base

The poor activity of Pt-based-catalysts for alkaline hydrogen oxidation/evolution reaction (HOR/HER) encourages scientific society to design an effective electrocatalyst to develop alkaline fuel cells/electrolyzers. Herein, platinum/rhodium oxide-nitrogen-doped carbon (Pt/Rh2O3-CNx) composite is prepared for alkaline HER and HOR inspired by hydrogen spillover. The HER performance of Pt/Rh2O3-CNx is ∼ 6 times higher than Pt/C. In HOR, Pt/Rh2O3-CNx possesses an exchange current density of 657.60 mA/mgmetal, which is ∼ 3.4 times higher than Pt/C. Hydrogen and hydroxyl binding energy (HBE and OHBE) contribute equally to alkaline HOR/HER. The experimental and theoretical evidence suggests that the enhanced HER and HOR activity of Pt/Rh2O3-CNx may be due to hydrogen spillover from Pt to Rh2O3. Small work function difference [0.08 eV] of the system suggested hydrogen-spillover is feasible, which has been justified by reaction-free energy calculations. We proposed that the dissociation of hydrogen (H2) and water (H2O) occurs at Pt to form Pt-adsorbed hydrogen species (Pt-Had). Then, some Had moves to Rh2O3 through hydrogen spillover and reacts with neighboring Had or adsorbed hydroxyl species (OHad) to form H2 or H2O, which enhances the HER and HOR activity, respectively. The role of water-metal-hydroxyl species in the electrical double layer was also demonstrated on alkaline HOR/HER. This work may help to design the hydrogen-spillover-based catalysts for several renewable energy technologies.

Buffering Donor Shuttles in Proton-Coupled Electron Transfer Kinetics for Electrochemical Hydrogenation of Hydroxyacetone to Propylene Glycol

Electrochemical hydrogenation reactions demand rapid proton-coupled electron transfer at the electrode surface, the kinetics of which depend closely on pH. Buffer electrolytes are extensively employed to regulate pH over a wide range. However, the specific role of buffer species should be taken into account when interpreting the intrinsic pH dependence, which is easily overlooked in the current research. Herein, we report the electrochemical hydrogenation of hydroxyacetone, derived from glycerol feedstock, to propylene glycol with a faradaic efficiency of 56 ± 5% on a polycrystalline Cu electrode. The reaction activities are comparable in citrate, phosphate, and borate buffer electrolytes, encompassing different buffer identities and pH. The electrokinetic profile reveals that citrate is a site-blocking adsorbate on the Cu surface, thereby decreasing buffer concentration and increasing pH will enhance the reaction rate; phosphate is an explicit proton donor, which promotes the interfacial rate by increasing buffer concentration and decreasing pH, while borate is an innocent buffer, which can be used to investigate the intrinsic pH effect. Combined with in situ SEIRAS, we demonstrate that water is the primary proton source in citrate and borate electrolytes, reiterating the rationality of the proposed mechanism based on the microkinetic modeling. Our results emphasize the intrinsic complexity of the buffer system on the kinetic activity for electrocatalysis. It calls for special care when we diagnose the mechanistic pathway in buffer electrolytes convoluted by different buffer identities and pH.

Ultralow-Loading Ruthenium-Iridium Fuel Cell Catalysts Dispersed on Zn-N Species-Doped Carbon

Developing low-loading platinum-group-metal (PGM) catalysts is one of the key challenges in commercializing anion-exchange-membrane-fuel-cells (AEMFCs), especially for hydrogen oxidation reaction (HOR). Here, ruthenium-iridium nanoparticles being deposited on a Zn-N species-doped carbon carrier (Ru6Ir/Zn-N-C) are synthesized and used as an anodic catalyst for AEMFCs. Ru6Ir/Zn-N-C shows extremely high mass activity (5.87 A mgPGM -1) and exchange current density (0.92 mA cm-2), which is 15.1 and 3.9 times that of commercial Pt/C, respectively. Based on the Ru6Ir/Zn-N-C AEMFCs achieve a peak power density of 1.50 W cm-2, surpassing the state-of-the-art commercial PtRu catalysts and the power ratio of the normalized loading is 14.01 W mgPGM anode -1 or 5.89 W mgPGM -1 after decreasing the anode loading (87.49 µg cm-2) or the total PGM loading (0.111 mg cm-2), satisfying the US Department of Energy's PGM loading target. Moreover, the solvent and solute isotope separation method is used for the first time to reveal the kinetic process of HOR, which shows the reaction is influenced by the adsorption of H2O and OH-. The improvement of the hydrogen bond network connectivity of the electric double layer by adjusting the interfacial H2O structure together with the optimized HBE and OHBE is proposed to be responsible for the high HOR activity of Ru6Ir/Zn-N-C.

Lattice-Disordered High-Entropy Alloy Engineered by Thermal Dezincification for Improved Catalytic Hydrogen Evolution Reaction

A disordered crystal structure is an asymmetrical atomic lattice resulting from the missing atoms (vacancies) or the lattice misarrangement in a solid-state material. It has been widely proven to improve the electrocatalytic hydrogen evolution reaction (HER) process. In the present work, due to the special physical properties (the low evaporation temperature of below 900 °C), Zn is utilized as a sacrificial component to create senary PtIrNiCoFeZn high-entropy alloy (HEA) with highly disordered lattices. The structure of the lattice-disordered PtIrNiCoFeZn HEA is characterized by the thermal diffusion scattering (TDS) in transmission electron microscope. Density functional theory calculations reveal that lattice disorder not only accelerates both the Volmer step and Tafel step during the HER process but also optimizes the intensity and distribution of projected density of states near the Fermi energy after the H2O and H adsorption. Anomalously high alkaline HER activity and stability are proven by experimental measurements. This work introduces a novel approach to preparing irregular lattices offering highly efficient HEA and a TDS characterization method to reveal the disordered lattice in materials. It provides a new route toward exploring and developing the catalytic activities of materials with asymmetrically disordered lattices.

Immobilizing Ordered Oxophilic Indium Sites on Platinum Enabling Efficient Hydrogen Oxidation in Alkaline Electrolyte

Addressing the sluggish kinetics in the alkaline hydrogen oxidation reaction (HOR) is a pivotal yet challenging step toward the commercialization of anion-exchange membrane fuel cells (AEMFCs). Here, we have successfully immobilized indium (In) atoms in an orderly fashion into platinum (Pt) nanoparticles supported by reduced graphene oxide (denoted as O-Pt3In/rGO), significantly enhancing alkaline HOR kinetics. We have revealed that the ordered atomic matrix enables uniform and optimized hydrogen binding energy (HBE), hydroxyl binding energy (OHBE), and carbon monoxide binding energy (COBE) across the catalyst. With a mass activity of 2.3066 A mg-1 at an overpotential of 50 mV, over 10 times greater than that of Pt/C, the catalyst also demonstrates admirable CO resistance and stability. Importantly, the AEMFC implementing this catalyst as the anode catalyst has achieved an impressive power output compared to Pt/C. This work not only highlights the significance of constructing ordered oxophilic sites for alkaline HOR but also sheds light on the design of well-structured catalysts for energy conversion.

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.

Advancing electrocatalytic reactions through mapping key intermediates to active sites via descriptors

Descriptors play a crucial role in electrocatalysis as they can provide valuable insights into the electrochemical performance of energy conversion and storage processes. They allow for the understanding of different catalytic activities and enable the prediction of better catalysts without relying on the time-consuming trial-and-error approaches. Hence, this comprehensive review focuses on highlighting the significant advancements in commonly used descriptors for critical electrocatalytic reactions. First, the fundamental reaction processes and key intermediates involved in several electrocatalytic reactions are summarized. Subsequently, three types of descriptors are classified and introduced based on different reactions and catalysts. These include d-band center descriptors, readily accessible intrinsic property descriptors, and spin-related descriptors, all of which contribute to a profound understanding of catalytic behavior. Furthermore, multi-type descriptors that collectively determine the catalytic performance are also summarized. Finally, we discuss the future of descriptors, envisioning their potential to integrate multiple factors, broaden application scopes, and synergize with artificial intelligence for more efficient catalyst design and discovery.

Iridium-Based Alkaline Hydrogen Oxidation Reaction Electrocatalysts

The hydroxide exchange membrane fuel cells (HEMFCs) are promising but lack of high-performance anode hydrogen oxidation reaction (HOR) electrocatalysts. The platinum group metals (PGMs) have the HOR activity in alkaline medium two to three orders of magnitude lower than those in acid, leading to the high required PGMs amount on anode to achieve high HEMFC performance. The mechanism study demonstrates the hydrogen binding energy of the catalyst determines the alkaline HOR kinetics, and the adsorbed OH and water on the catalyst surface promotes HOR. Iridium (Ir) has a unique advantage for alkaline HOR due to its similar hydrogen binding energy to Pt and enhanced adsorption of OH. However, the HOR activity of Ir/C is still unsatisfied in practical HEMFC applications. Further fine tuning the adsorption of the intermediate on Ir-based catalysts is of great significance to improve their alkaline HOR activity, which can be reasonably realized by structure design and composition regulation. In this concept, we address the current understanding about the alkaline HOR mechanism and summarize recent advances of Ir-based electrocatalysts with enhanced alkaline HOR activity. We also discuss the perspectives and challenges on Ir-based electrocatalysts in the future.

Multidimensional Electrochemistry Decodes the Operando Mechanism of Hydrogen Oxidation

Being an efficient approach to the utilization of hydrogen energy, the hydrogen oxidation reaction (HOR) is of particular significance in the current carbon-neutrality time. Yet the mechanistic picture of the HOR is still blurred, mostly because the elemental steps of this reaction are rapid and highly entangled, especially when deviating from the thermodynamic equilibrium state. Here we report a strategy for decoding the HOR mechanism under operando conditions. In addition to the wide-potential-range I-V curves obtained using gas diffusion electrodes, we have applied the AC impedance spectroscopy to provide independent and complementary kinetic information. Combining multidimensional data sources has enabled us to fit, in mathematical rigor, the core kinetic parameter set in a 5-D data space. The reaction rate of the three elemental steps (Tafel, Heyrovsky, and Volmer reactions), as a function of the overpotential, can thus be distilled individually. Such an undocumented kinetic picture unravels, in detail, how the HOR is controlled by the elemental steps on polarization. For instance, at low polarization region, the Heyrovsky reaction is relatively slow and can be ignored; but at high polarization region, the Heyrovsky reaction will surpass the Tafel reaction. Additionally, the Volmer reaction has been the fastest within overpotentials of interest. Our findings not only offer a better understanding of the HOR mechanism, but also lay the foundation for the development of improved hydrogen energy utilization systems.

Analyzing the Active Site and Predicting the Overall Activity of Alloy Catalysts

As one of the potential catalysts, disordered solid solution alloys can offer a wealth of catalytic sites. However, accurately evaluating their activity localization structure and overall activity from each individual site remains a formidable challenge. Herein, an approach based on density functional theory and machine learning was used to obtain a large number of sites of the Pt-Ru alloy as the model multisite catalyst for the hydrogen evolution reaction. Subsequently, a series of statistical approaches were employed to unveil the relationship between the geometric structure and overall activity. Based on the radial frequency distribution of metal elements and the distribution of ΔGH, we have identified the surface and subsurface sites occupied by Pt and Ru, respectively, as the most active sites. Particularly, the concept of equivalent site ratio predicts that the overall activity is highest when the Ru content is 20-30%. Furthermore, a series of Pt-Ru alloys were synthesized to validate the proposed theory. This provides crucial insights into understanding the origin of catalytic activity in alloys and thus will better guide the rational development of targeted multisite catalysts.