Structural Design and Electronic Modulation of Transition-Metal-Carbide Electrocatalysts toward Efficient Hydrogen Evolution

In-situ revealed inhibition of W2C to excessive oxidation of CoOOH for high-efficiency alkaline overall water splitting

The design of low-cost, efficient, and stable multifunctional basic catalysts to replace the high-cost noble metal catalysts remains a challenge. In this work, we report a dual-component Co-W2C catalytic system which achieves excellent properties of hydrogen evolution reaction (HER, η10 = 63 mV), oxygen evolution reaction (OER, η10 = 259 mV) and overall water splitting (η10 = 1.53 V) by adjusting the interfacial electronic structure of the material. Further density functional theory (DFT) calculations indicate that the efficient electronic modulation at the W2C/Co interface leads to the generation of favorable hydroxyl and hydrogen species energetics on the hybrid surface. The results of the in-situ Raman spectra show that W2C can suppress the excessive oxidation of the active site during the OER process, and the existence of core-shell structure also protects the W2C substrate. The stable and efficient catalytic performance of Co-W2C is attributed to the common advantages of structural and interface manipulation.

A cobalt porphyrin-bridged covalent triazine polymer-derived electrode for efficient hydrogen production

Pronounced compositional regulation and microstructure evolution have a significant influence on hydrogen electrocatalysis. Herein, for the first time, we demonstrate that N,Co-codoped carbon supported Co5.47N nanoparticles (Co5.47N/N,Co-C-800) derived from a nitrogen-rich porphyrin-bridged covalent triazine polymer (CoTAPPCC) are an effective electrocatalyst for the HER in 1.0 M KOH when compared to CoCo2O4/N,Co-C-900 (pyrolysis at 900 °C) and CoO/N,Co-C-1000 (pyrolysis at 1000 °C). The structural and morphological variations of CoTAPPCC at different heat treatment temperatures were investigated through various spectroscopic techniques. We reveal that electrocatalytic HER activity is temperature- and component-dependent. The overpotentials for Co5.47N/N,Co-C-800 to reach current densities of 10 and 100 mA cm-2 were determined to be 76 and 229 mV, respectively, outperforming many other state-of-the-art HER electrocatalysts. This work also sheds light on the influence of calcination temperature on the electrocatalytic HER of final samples.

Recent Advancements in Ascribing Several Platinum Free Electrocatalysts Pertinent to Hydrogen Evolution from Water Reduction

The advancement of a sustainable and scalable catalyst for hydrogen production is crucial for the future of the hydrogen economy. Electrochemical water splitting stands out as a promising pathway for sustainable hydrogen production. However, the development of Pt-free electrocatalysts that match the energy efficiency of Pt while remaining economical poses a significant challenge. This review addresses this challenge by highlighting latest breakthroughs in Pt-free catalysts for the hydrogen evolution reaction (HER). Specifically, we delve into the catalytic performance of various transition metal phosphides, metal carbides, metal sulphides, and metal nitrides toward HER. Our discussion emphasizes strategies for enhancing catalytic performance and explores the relationship between structural composition and the performance of different electrocatalysts. Through this comprehensive review, we aim to provide insights into the ongoing efforts to overcome barriers to scalable hydrogen production and pave the way for a sustainable hydrogen economy.

Carbon Nanotube Support, Carbon Loricae and Oxygen Defect Co-Promoted Superior Activities and Excellent Durability of RuO2 Nanoparticles Towards the pH-Universal H2 Evolution

This work reports a strategy that integrates the carbon nanotube (CNT) supporting, ultrathin carbon coating and oxygen defect generation to fabricate the RuO2 based catalysts toward the pH-universal hydrogen evolution reaction (HER) with high efficiencies. Specifically, the CNT supported RuO2 nanoparticles with ultrathin carbon loricae and rich oxygen vacancies at the surface (C@OV-RuO2/CNTs-325) have been synthesized. The C@OV-RuO2/CNTs-325 shows superior activities and excellent durability for the HER. It only requires overpotentials of 36.1, 18.0, and 19.3 mV to deliver -10 mA cm-2 in the acidic, neutral, and alkaline media, respectively. Its HER activities are comparable to that of the Pt/C in the acidic media but higher than those of the Pt/C in the neutral and alkaline media. The C@OV-RuO2/CNTs-325 shows excellent HER durability with no activity losses for > 500 h in the acidic, neutral or alkaline media at -250 mA cm-2. The density-functional-theory calculations indicate that the CNT supporting, the carbon coating, and the OVs can modulate the d-band centers of Ru, increasing the HER activities of C@OV-RuO2/CNTs-325, and stabilize the Ru atoms in the catalyst, increasing the durability of the C@OV-RuO2/CNTs-325. More interestingly, the C@OV-RuO2/CNTs-325 shows great potential for practical applications toward overall seawater splitting.

Two-Dimensional MXene-Based Electrocatalysts: Challenges and Opportunities

MXene, regarded as cutting-edge two-dimensional (2D) materials, have been widely explored in various applications due to their remarkable flexibility, high specific surface area, good mechanical strength, and interesting electrical conductivity. Recently, 2D MXene has served as a ideal platform for the design and development of electrocatalysts with high activity, selectivity, and stability. This review article provides a detailed description of the structural engineering of MXene-based electrocatalysts and summarizes the uses of 2D MXene in hydrogen evolution reactions, nitrogen reduction reactions, oxygen evolution reactions, oxygen reduction reactions, and methanol/ethanol oxidation. Then, key issues and prospects for 2D MXene as a next-generation platform in fundamental research and real-world electrocatalysis applications are discussed. Emphasis will be given to material design and enhancement techniques. Finally, future research directions are suggested to improve the efficiency of MXene-based electrocatalysts.

Manipulating electron redistribution between iridium and Co6Mo6C bridging with a carbon layer leads to a significantly enhanced overall water splitting performance at industrial-level current density

Nowadays, alkaline water electrocatalysis is regarded as an economical and highly effective approach for large-scale hydrogen production. Highly active electrocatalysts functioning under large current density are urgently required for practical industrial applications. In this work, we present a meticulously designed methodology to anchor Ir nanoparticles on Co6Mo6C nanofibers (Co6Mo6C-Ir NFs) bridging with nitrogen-doped carbon as efficient bifunctional electrocatalysts with both excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity and stability in alkaline media. With a low Ir content of 5.9 wt%, Co6Mo6C-Ir NFs require the overpotentials of only 348 and 316 mV at 1 A cm-2 for the HER and OER, respectively, and both maintain stability for at least 500 h at ampere-level current density. Consequently, an alkaline electrolyzer based on Co6Mo6C-Ir NFs only needs a voltage of 1.5 V to drive 10 mA cm-2 and possesses excellent durability for 500 h at 1 A cm-2. Density functional theory calculations reveal that the introduction of Ir nanoparticles is pivotal for the enhanced electrocatalytic activity of Co6Mo6C-Ir NFs. The induced interfacial electron redistribution between Ir and Co6Mo6C bridging with nitrogen-doped carbon dramatically modulates the electron structure and activates inert atoms to generate more highly active sites for electrocatalysis. Moreover, the optimized electronic structure is more conducive to the balance of the adsorption and desorption energies of reaction intermediates, thus significantly promoting the HER, OER and overall water splitting performance.

Recent Advances in Revealing the Electrocatalytic Mechanism for Hydrogen Energy Conversion System

In light of the intensifying global energy crisis and the mounting demand for environmental protection, it is of vital importance to develop advanced hydrogen energy conversion systems. Electrolysis cells for hydrogen production and fuel cell devices for hydrogen utilization are indispensable in hydrogen energy conversion. As one of the electrolysis cells, water splitting involves two electrochemical reactions, hydrogen evolution reaction and oxygen evolution reaction. And oxygen reduction reaction coupled with hydrogen oxidation reaction, represent the core electrocatalytic reactions in fuel cell devices. However, the inherent complexity and the lack of a clear understanding of the structure-performance relationship of these electrocatalytic reactions, have posed significant challenges to the advancement of research in this field. In this work, the recent development in revealing the mechanism of electrocatalytic reactions in hydrogen energy conversion systems is reviewed, including in situ characterization and theoretical calculation. First, the working principles and applications of operando measurements in unveiling the reaction mechanism are systematically introduced. Then the application of theoretical calculations in the design of catalysts and the investigation of the reaction mechanism are discussed. Furthermore, the challenges and opportunities are also summarized and discussed for paving the development of hydrogen energy conversion systems.

Rapid synthesis of high-purity molybdenum carbide with controlled crystal phases

The synthesis of phase-pure carbide nanomaterials is crucial for understanding their structure-performance relationships, and for advancing their application in catalysis. Molybdenum carbides, in particular, have garnered increasing interest due to their Pt-like surface electronic properties and high catalytic activity. Traditional methods for synthesizing molybdenum carbide are often lengthy and energy-intensive, leading to an uncontrolled phase, low purity, and excessive carbon coverage, which hinder their catalytic performance improvement. This work introduces a novel pulsed Joule heating (PJH) technique that overcomes these limitations, enabling the controlled synthesis of high-purity molybdenum carbides (β-Mo2C, η-MoC1-x, and α-MoC1-x) within seconds by using MoOx/4-Cl-o-phenylenediamine as the hybrid precursor. The PJH method allows precise control over the diffusion of carbon species in the Mo-C system, resulting in a significantly improved phase purity of up to 96.89 wt%. Moreover, the electronic structure of platinum catalysts on molybdenum carbide was modulated through electron metal-support interaction (EMSI) between Pt and MoxC, and contributed to enhanced catalytic performance compared to carbon-supported Pt catalysts during the hydrogen evolution reaction. Overall, this work paves the way for efficient production of high-quality molybdenum carbide nanomaterials, and thus is expected to accelerate their industrial deployments in practical catalytic reactions.

Alkali cation stabilization of defects in 2D MXenes at ambient and elevated temperatures

Transition metal carbides have been adopted in energy storage, conversion, and extreme environment applications. Advancements in their 2D counterparts, known as MXenes, enable the design of unique structures at the ~1 nm thickness scale. Alkali cations have been essential in MXenes manufacturing processing, storage, and applications, however, exact interactions of these cations with MXenes are not fully understood. In this study, using Ti3C2Tx, Mo2TiC2Tx, and Mo2Ti2C3Tx MXenes, we present how transition metal vacancy sites are occupied by alkali cations, and their effect on MXene structure stabilization to control MXene's phase transition. We examine this behavior using in situ high-temperature x-ray diffraction and scanning transmission electron microscopy, ex situ techniques such as atomic-layer resolution secondary ion mass spectrometry, and density functional theory simulations. In MXenes, this represents an advance in fundamentals of cation interactions on their 2D basal planes for MXenes stabilization and applications. Broadly, this study demonstrates a potential new tool for ideal phase-property relationships of ceramics at the atomic scale.

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.

Cr-doping promoted surface reconstruction of Ni3N electrocatalysts toward efficient overall water splitting

Understanding and utilizing the dynamic changes of electrocatalysts under working conditions are important for advancing the sustainable hydrogen production. Here, we for the first time report that Cr-doping can promote the in situ reconstruction of a self-supported Ni3N electrocatalyst (Cr-Ni3N/NF) during oxygen and hydrogen evolution reactions (OER and HER), and therefore improve the electrocatalytic water splitting performance. As identified by in situ measurements and theoretical calculations, Cr-doping enhances OH- adsorption during OER at anode and thereby boosts the transformation of Ni3N pre-catalysts to defect-rich nickel oxyhydroxide (NiOOH) active species. Meanwhile, it facilitates the generation of Ni3N/Ni(OH)2 at cathodes due to effective H2O activation, leading to the fast HER kinetics on the Ni3N/Ni(OH)2 interfaces. Notably, the optimal Cr-Ni3N/NF displays good OER and HER performance in 1.0 M KOH electrolytes, with low overpotentials of 316 and 188 mV to achieve the current density of ± 100 mA cm-2, respectively. Benefiting from its bi-functionality and self-supporting property, an alkaline electrolyzer equipped with Cr-Ni3N/NF as both anode and cathode affords a small voltage of 1.72 V at 100 mA cm-2, along with 100 h operation stability. Elucidating that Cr-doping can boost in situ reconfiguration and consequently the electrocatalytic activity, this work would shed new light on the rational design and synthesis of electrocatalysts via directional reconstructions.

Predictive Synthesis of Transition Metal Carbide via Thermochemical Oxocarbon Equilibrium

Fabricating nanoscale metal carbides is a great challenge due to them having higher Gibbs free energy of formation (ΔG°) values than other metal compounds; additionally, these carbides have harsh calcination conditions, in which metal oxidation is preferred in the atmosphere. Herein, we report oxocarbon-mediated calcination for the predictive synthesis of nanoscale metal carbides. The thermochemical oxocarbon equilibrium of CO-CO2 reactions was utilized to control the selective redox reactions in multiatomic systems of Mo-C-O, contributing to the phase-forming and structuring of Mo compounds. By harnessing the thermodynamically predicted processing window, we controlled a wide range of Mo phases (MoO2, α-MoC1-x, and β-Mo2C) and nanostructures (nanoparticle, spike, stain, and core/shell) in the Mo compounds/C nanofibers. By inducing simultaneous reactions of C-O (selective C combustion) and Mo-C (Mo carbide formation) in the nanofibers, Mo diffusion was controlled in C nanofibers, acting as a template for the nucleation and growth of Mo carbides and resulting in precise control of the phases and structures of Mo compounds. The formation mechanism of nanostructured Mo carbides was elucidated according to the CO fractions of CO-CO2 calcination. Moreover, tungsten (W) and niobium (Nb) carbides/C nanofibers have been successfully synthesized by CO-CO2 calcination. We constructed the thermodynamic map for the predictive synthesis of transition metal carbides to provide universal guideline via thermochemical oxocarbon equilibrium. We revealed that our thermochemical oxocarbon-mediated gas-solid reaction enabled the structure and phase control of nanoscale transition metal compounds to optimize the material-property relationship accordingly.

Crystalline Phase Engineering to Modulate the Interfacial Interaction of the Ruthenium/Molybdenum Carbide for Acidic Hydrogen Evolution

Ruthenium (Ru) is an ideal substitute to commercial Pt/C for the acidic hydrogen evolution reaction (HER), but it still suffers from undesirable activity due to the strong adsorption free energy of H* (ΔGH*). Herein, we propose crystalline phase engineering by loading Ru clusters on precisely prepared cubic and hexagonal molybdenum carbide (α-MoC/β-Mo2C) supports to modulate the interfacial interactions and achieve high HER activity. Advanced spectroscopies demonstrate that Ru on β-Mo2C shows a lower valence state and withdraws more electrons from the support than that of Ru on α-MoC, indicative of a strong interfacial interaction. Density functional theory reveals that the ΔGH* of Ru/β-Mo2C approaches 0 eV, illuminating an enhancement mechanism at the Ru/β-Mo2C interface. The resultant Ru/β-Mo2C exhibits an encouraging performance in a proton exchange membrane water electrolyzer with a low cell voltage (1.58 V@ 1.0 A cm-2) and long stability (500 h@ 1.0 A cm-2).

Phase-Engineered Dichalcogenides/Fluorine-Free V4C3Tx (T = OH, O) Heterostructures for pH-Universal Hydrogen Evolution Reaction

This research addresses the pH-dependency limitation in electrocatalytic hydrogen evolution reactions (HER) by creating heterostructures through the chemical bonding between 2D-dichalcogenides and V4C3Tx (T = OH, O) planes. The one-step solvothermal synthesis employed in this study constructs a synergistically interacted 1T phase of, e.g., MoS2 and V4C3Tx MXene, demonstrating an omnidirectional improvement on catalytic stability, active site exposure, surface area enlargement, electrical conductivity, and hence enhancement of water dissociation activities. Despite the notable progress in creating hydrogen production catalysts with ground breaking performances, a significant gap remains in the availability of catalysts capable of functioning effectively under high current densities. The catalyst 1T MoS2@V4C3Tx shows remarkable activities under the current density of 1000 mA cm-2, which require overpotentials of 16, 24, and 37 mV in 0.5 m H2SO4, 1 m KOH, and 0.1 m PBS electrolytes, respectively at 10 mA cm-2, and exhibits excellent HER performance with small overpotentials of 103.16 and 138 mV to achieve current densities of 500 and 1000 mA cm-2, respectively, with outstanding stability for 1000 cylic voltammetric cycle HER test without degradation in acidic media. Enhanced HER performance has also been observed in other 2D-dichalcogenides/V4C3Tx heterostructures, providing prospects for phase-engineered dichalcogenides/fluorine-free V4C3Tx composites for pH-universal HER.

Molten-Salt Electrochemical Preparation of Co2B/MoB2 Heterostructured Nanoclusters for Boosted pH-Universal Hydrogen Evolution

Boosting the hydrogen evolution reaction (HER) activity of α-MoB2 at large current densities and in pH-universal medium is significant for efficient hydrogen production. In this work, Co2B/MoB2 heterostructured nanoclusters are prepared by molten-salt electrolysis (MSE) and then used as a HER catalyst. The composition, structure, and morphology of Co2B/MoB2 can be modulated by altering the stoichiometries of raw materials and synthesis temperatures. Impressively, the obtained Co2B/MoB2 at optimized conditions exhibits a low overpotential of 297 and 304 mV at 500 mA cm-2 in 0.5 m H2SO4 and 1 m KOH, respectively. Moreover, the Co2B/MoB2 catalyst possesses a long-term catalytic stability of over 190 h in both acidic and alkaline medium. The excellent HER performance is due to the modified electronic structure at the Co2B/MoB2 heterointerface where electrons are accumulated at the Mo sites to strengthen the H adsorption. Density functional theory (DFT) calculations reveal that the formation of the Co2B/MoB2 heterointerface decreases the H adsorption and H2O dissociation free energies, contributing to the boosted HER intrinsic catalytic activity of Co2B/MoB2. Overall, this work provides an experimental and theoretical paradigm for the design of efficient pH-universal boride heterostructure electrocatalysts.

Mo2C-Based Ceramic Electrode with High Stability and Catalytic Activity for Hydrogen Evolution Reaction at High Current Density

Developing robust electrodes with high catalytic performance is a key step for expanding practical HER (hydrogen evolution reaction) applications. This paper reports on novel porous Mo2C-based ceramics with oriented finger-like holes directly used as self-supported HER electrodes. Due to the suitable MoO3 sintering additive, high-strength (55 ± 6 MPa) ceramic substrates and a highly active catalytic layer are produced in one step. The in situ reaction between MoO3 and Mo2C enabled the introduction of O in the Mo2C crystal lattice and the formation of Mo2C(O)/MoO2 heterostructures. The optimal Mo2C-based electrode displayed an overpotential of 333 and 212 mV at 70 °C under a high current intensity of 1500 mA cm-2 in 0.5 m H2SO4 and 1.0 m KOH, respectively, which are markedly better than the performance of Pt wire electrode; furthermore, its price is three orders of magnitude lower than Pt. The chronopotentiometric curves recorded in the 50 - 1500 mA cm-2 range, confirmed its excellent long-term stability in acidic and alkaline media for more than 260 h. Density functional theory (DFT) calculations showed that the Mo2C(O)/MoO2 heterostructures has an optimum electronic structure with appropriate *H adsorption-free energy in an acidic medium and minimum water dissociation energy barrier in an alkaline medium.

Polyoxometalate-Structured Materials: Molecular Fundamentals and Electrocatalytic Roles in Energy Conversion

Polyoxometalates (POMs), a kind of molecular metal oxide cluster with unique physical-chemical properties, have made essential contributions to creating efficient and robust electrocatalysts in renewable energy systems. Due to the fundamental advantages of POMs, such as the diversity of molecular structures and large numbers of redox active sites, numerous efforts have been devoted to extending their application areas. Up to now, various strategies of assembling POM molecules into superstructures, supporting POMs on heterogeneous substrates, and POMs-derived metal compounds have been developed for synthesizing electrocatalysts. From a multidisciplinary perspective, the latest advances in creating POM-structured materials with a unique focus on their molecular fundamentals, electrocatalytic roles, and the recent breakthroughs of POMs and POM-derived electrocatalysts, are systematically summarized. Notably, this paper focuses on exposing the current states, essences, and mechanisms of how POM-structured materials influence their electrocatalytic activities and discloses the critical requirements for future developments. The future challenges, objectives, comparisons, and perspectives for creating POM-structured materials are also systematically discussed. It is anticipated that this review will offer a substantial impact on stimulating interdisciplinary efforts for the prosperities and widespread utilizations of POM-structured materials in electrocatalysis.

Metal-organic framework derived transition metal sulfides grown on carbon nanofibers as self-supported catalysts for hydrogen evolution reaction

Metal-organic framework (MOF) derived transition metal-based electrocatalysts have received great attention as substitutes for noble metal-based hydrogen evolution catalysts. However, the low conductivity and easy detachments from electrodes of raw MOF have seriously hindered their applications in hydrogen evolution reaction. Herein, we report the facile preparation of Co-NSC@CBC84, a porous carbon-based and self-supported catalyst containing Co9S8 active species, by pyrolysis and sulfidation of in-situ grown ZIF-67 on polydopamine-modified biomass bacterial cellulose (PDA/BC). As a binder-free and self-supported electrocatalyst, Co-NSC@CBC84 exhibits superior electrocatalytic properties to other reported cobalt-based sulfide catalytic materials and has good stability in 0.5 M H2SO4 electrolyte. At the current density of 10 mA cm-2, only an overpotential of 138 mV was required, corresponding to a Tafel slope of 123 mV dec-1, owing to the strong synergy effect between Co-NSC nanoparticles and CBC substrate. This work therefore provides a feasible approach to prepare self-supported transition metal sulfides as HER catalysts, which is helpful for the development of noble metal-free catalysts and biomass carbon materials.

Switching Electric Double Layer Potential by Phase Structure Control for Advanced Oxygen Reduction Reaction of Cobalt@Nitrogen Doped Carbon Core-Shell

The key to design an advanced oxygen reduction reaction (ORR) electrocatalyst is a well-balance between the adsorption and desorption of oxygen intermediates. This study systematically evaluated the ORR activity of HCP and FCC cobalt core-shell cobalt/N-doped carbon (Cobalt@NC) catalyst via theoretical and experimental studies. The electronic structure calculations using density functional theory (DFT) calculations revealed that the ORR activity of carbon layer can be improved by 1) switching the electrostatic potential in the electrical double layer due to the polarization induced at the carbon-cobalt interface and 2) modulating the electron population in the bonding orbital in the C-O bonds in an ORR. The results revealed that an O atom is bounded stronger to the outer NC shell with FCC Cobalt than HCP Cobalt, which hindered the desorption steps of OH*. Experimentally, plasma-engineered HCP Cobalt@NC also showed remarkably advanced performance toward ORR compared to that FCC Cobalt@NC. The kinetic current density of HCP Cobalt@NC at 0.85 V versus RHE is calculated as 6.24 mA cm-2, which is six folds higher than FCC Cobalt@NC and even outperform 20 wt.% Pt/C. In a practical Aluminium-air battery, HCP Cobalt@NC also exhibited slightly higher peak power density (110.57 mW cm-2) compared to 20 wt.% Pt/C.

Synergistic enhancement of electrocatalytic nitroarene hydrogenation over Mo2C@MoS2 heteronanorods with dual active-sites

Electrocatalytic hydrogenation (ECH) enables the sustainable production of chemicals under ambient conditions, in which catalysts catering for the different chemisorption of reactants/intermediates are desired but still challenging. Here, Mo2C@MoS2 heteronanorods with dual active-sites are developed to accomplish efficient nitroarene ECH according to our theoretical prediction that the binding of atomic H and nitro substrates would be synergistically strengthened on Mo2C-MoS2 interfaces. They afford high faradaic efficiency (>85%), yield (>78%) and selectivity (>99%) for the reduction of 4-nitrostyrene (4-NS) to 4-vinylaniline (4-VA) in neutral electrolytes, outperforming not only the single-component counterparts of Mo2C nanorods and MoS2 nanosheets, but also recently reported noble-metals. Accordingly, in situ Raman spectroscopy combined with electrochemical tests clarifies the rapid ECH of 4-NS on Mo2C-MoS2 interfaces due to the facilitated elementary steps, quickly refreshing active sites for continuous electrocatalysis. Mo2C@MoS2 further confirms efficient and selective ECH toward functional anilines with other well-retained reducible groups in wide substrate scope, underscoring the promise of dual-site engineering for exploring catalysts.

Adiabatic ionization energies of RuC, RhC, OsC, IrC, and PtC

The ionization energies (IEs) of RuC, RhC, OsC, IrC, and PtC are assigned by the measurement of their two-photon ionization thresholds. Although late transition metal-carbon bonds are of major importance in organometallic chemistry and catalysis, accurate and precise fundamental thermochemical data on these chemical bonds are mainly lacking in the literature. Based on their two-photon ionization thresholds, in this work, we assign IE(RuC) = 7.439(40) eV, IE(RhC) = 7.458(32) eV, IE(OsC) = 8.647(25) eV, IE(IrC) = 8.933(74) eV, and IE(PtC) = 9.397(32) eV. These experimentally derived IEs are further confirmed through quantum chemical calculations using coupled-cluster single double perturbative triple methods that are extrapolated to the complete basis set limit using a three-parameter mixed Gaussian/exponential extrapolation scheme and corrected for spin-orbit effects using a semiempirical method. The electronic structure and chemical bonding of these MC species are discussed in the context of these ionization energy measurements. The IEs of RuC, RhC, OsC, and IrC closely mirror the IEs of the corresponding transition metal atoms, suggesting that for these species, the (n + 1)s electrons of the transition metals are not significantly involved in chemical bonding.

Solid-State Synthesis of Niobium Carbide Electrocatalyst in the Presence of Vanadium as Suppressor of Energy Input and Greenhouse Gas Emission

Solid-state reaction (SSR) is a widely adopted method for functional inorganic material syntheses. Unlike intricate systems emerging from chemically unstable precursor usage, the SSR can proceed from stable precursor couples using simple apparatuses. However, this reaction is associated with high temperatures that overcome solid-state diffusion. Moreover, solid-state syntheses of technologically crucial carbides lead to greenhouse gas emissions. Therefore, exploring an extrinsic component to suppress these challenges is vital to confronting global energy and environmental issues. This study reports that the presence of an ordinary element, vanadium (V), changes the routes of the SSR of niobium carbide (NbC), producing NbC efficiently and cleanly. 1000 °C is far below the temperature required to obtain NbC from a precursor couple of Nb2O5 and C, i.e., approximately 1500 °C is required. However, a carbon substitute, vanadium carbide, completely consumed Nb2O5 before reaching 1000 °C and consummated NbC crystallization for 10 h. Furthermore, NbC crystallites were observed using X-ray diffraction from 770 °C, and their formation was primarily accompanied by VNbO4, rather than being routed through NbO2 produced for the Nb2O5-C combination. The obtained NbC contained V as a dopant in the 15-50% range (NbC:V), and the relative abundance was correlated with the preparation temperature. Mass analyses revealed that the formation of NbC/V is barely associated with greenhouse gas emissions because of the sizable thermodynamic driving force for primarily forming vanadium oxide byproducts. Device performance using NbC/V was also assessed for a standard electrochemical hydrogen evolution reaction.

Strategically Designed Pd-Induced Changes in Alkaline Hydrogen Evolution Reaction and Oxygen Evolution Reaction Performances of Electrochemical Water Oxidation by the Galvanically Synthesized MoO2/MoO3 Composite Thin Film

Electrochemical water oxidation is believed to be an effective pathway to produce clean, carbon-free, and environmentally sustainable green energy. In this work, we report a simple, easy-to-construct, facile, low-cost, and single-step galvanic technique to synthesize a Pd-supported temperature-assisted MoOx thin film nanocomposite for effective water oxidation. The most suitable nanocomposite exhibits very low overpotential at 10 mA/cm2 with smaller Tafel slope values for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) processes in an alkaline medium. The formation of a metal oxide-metal junction accelerates the growth of more active sites, promoting induced electronic synergism at the MoOx-Pd interface. This endows higher electrical conductivity and faster electron transfer kinetics, thus accelerating the faster water dissociation reaction following the Tafel-Volmer mechanism to boost the HER process in an alkaline medium. The excellent electrochemical HER and OER performances of our electrocatalyst even supersede the accomplishments of the benchmark catalysts Pt/C and RuO2. Moreover, neither of these two catalysts demonstrates both catalytic reactions, i.e., HER and OER at the same time, which have been observed for our synthesized catalyst. Our findings illustrate the potential of a thin-film MoOx-Pd nanocomposite to be an exceedingly effective electrocatalyst developed by interface engineering strategies. This also provides insight into designing several other semiconductor composite catalysts using simple synthesis techniques for highly efficient HER/OER processes that could be alternatives to benchmark electrocatalysts for water electrolysis.

Elucidating the Role of Atomically Dilute Copper Centers Impregnating a Phosphamide Polymer for the Preferential Hydrogen Evolution Reaction over CO2 Reduction

Organic polymers have attracted considerable interest in designing a multifunctional electrocatalyst. However, the inferior electro-conductivity of such metal-free polymers is often regarded as a shortcoming. Herein, a nitrogen- and phosphorus-rich polymer with phosphamide functionality (PAP) in the repeating unit has been synthesized from diaminopyridine (DAP) and phenylphosphonic dichloride (PPDC) precursors. The presence of phosphamide oxygen and pyridine nitrogen in the repeating unit of PAP leads to the coordination of the CuII ion and the incorporation of 3.29 wt % in the polymer matrix (Cu30@PAP) when copper salt is used to impregnate the polymer. Combined with a spectroscopic, microscopic, and DFT study, the coordination and geometry of copper in the PAP matrix has been established to be a distorted square planar CuII in a N2O2 ligand environment where phosphamide oxygen and pyridine nitrogen of the PAP coordinate to the metal center. The copper incorporation in the PAP modulates its electrocatalytic activity. On the glassy carbon electrode, PAP shows inferior activity toward the hydrogen evolution reaction (HER) in 0.5 M H2SO4 while 3 wt % copper incorporation (Cu30@PAP) significantly improves the HER performance with an overpotential of 114 mV at 10 mA cm-2. The notable electrochemical activity with Cu30@PAP occurs due to the impregnation of Cu(II) in PAP, improved electro-kinetics, and better charge transfer resistance (Rct). When changing the electrolyte from H2SO4 to CO2-saturated bicarbonate solution at nearly neutral pH, PAP shows HER as the dominant pathway along with the partial reduction of CO2 to formate. Moreover, the use of Cu30@PAP as an electrolcatalyst could not alter the predominant HER path, and only 20% Faradaic efficiency for the CO2 reduced products has been achieved. Post-chronoamperometric characterization of the recovered catalyst suggests an unaltered valence state of the copper ion and the intact chemical structure of PAP. DFT studies unraveled that the copper sites of Cu30@PAP promote water adsorption while phosphamide-NH of the PAP can weakly hold the CO2 adduct via a hydrogen bonding interaction. A detailed calculation has pointed out that the tetra-coordinated copper centers present in the PAP frame are the reactive sites and that the formation of the [CuI-H] intermediate is the rate-limiting step for both HER and its competitive side reaction, i.e., CO2 reduction to formate or CO formation. The high proton concentration in the electrolyte of pH < 7 leads to HER as the predominant pathway. This combined experimental and theoretical study has highlighted the crucial role of copper sites in electrocatalysis, emphasizing the plausible reason for electrocatalytic selectivity.

Ruthenium and Iron Co-doped Molybdenum Carbide as a Stable Hydrogen Evolution Electrocatalyst in Harsh Electrolyte

Electrocatalytic water splitting is one of the most commercially valuable pathways of hydrogen production especially combined with renewable electricity; however, efficient and durable electrocatalysts are urgently needed to reduce electric energy consumption. Here, we reported a Ru and Fe co-doped Mo2 C on nitrogen doped carbon via a controllable two-step method, which can be used for efficient and enduring hydrogen evolution reaction. At 10, 100 and 200 mA cm-2 in acidic electrolyte, the resultant Ru-Fe/Mo2 C@NC delivered low overpotentials of 31, 78 and 103 mV, respectively, which are comparable to that of the commercial Pt/C (20 wt %). At an applied current density of 100 mA cm-2 , stable hydrogen production was conducted for 120 h without obvious degradation. In alkaline media, Ru-Fe/Mo2 C@NC can also deliver a current density of 100 mA cm-2 for more than 100 h. Furthermore, the Ru-Fe/Mo2 C@NC electrocatalyst was used as cathode in an anion exchange membrane water electrolyzer under industrial environments for robust hydrogen production. The characterization and electrochemical results prove the synergism effects between Ru, Fe dopants and Mo2 C for promoting hydrogen evolution activity. This work would pave a new avenue to fabricate low-cost, high-performance hydrogen evolution electrocatalysts for industrial water electrolyzers.

Construction of urchin-like bimetallic phosphides induced by carbon dots for efficient wide pH hydrogen production

The development of an efficient noble-metal-free and pH-universal electrocatalyst for the hydrogen evolution reaction (HER) would be highly significant for hydrogen (H2) production via electrocatalytic water splitting. However, developing such a catalyst remains a formidable task. Herein, a strategy is proposed for the in situ fabrication of a novel urchin-like NiCoP microsphere catalyst (0.5CDs-NiCoP/NF) on nickel foam (NF) using carbon dots (CDs) as a directing agent. The strong bonding between the CDs and metals provides additional active sites, giving 0.5CDs-NiCoP/NF excellent electrocatalytic hydrogen evolution performance in environments ranging from acidic to basic. Moreover, the unique structure of 0.5CDs-NiCoP/NF endows this catalyst with low Tafel slopes of 73, 146 and 74 mV dec-1 for HER in acidic, neutral and alkaline conditions, respectively. This performance exceeds that of numerous other reported non-precious HER catalysts. In summary, this work offers a novel and efficient strategy for the design and synthesis of low-cost, efficient, and robust transition metal phosphides (TMPs) electrocatalysts.

Spectroscopic Signature of the Carbon-Carbon Coupling Reaction between Carbon Monoxide and Nickel Carbide

Spectroscopic characterization of ketenylidene complexes is of essential importance for understanding the structure-reactivity relationships of the catalytic sites. Here, we report a size-specific photoelectron velocity map imaging spectroscopic study of the reactions of carbon monoxide with nickel carbide. Quantum chemical calculations have been conducted to search for the energetically favorable isomers and to recognize the experimental spectra. The target products with the chemical formula of NiC(CO)n- (n = 3-5) are characterized to have an intriguing ketenylidene CCO unit. The evolution from NiC(CO)3- to NiC(CO)4- involves the breaking and formation of the Ni-C bond and the coordination conversion between the terminal and bridging carbonyls. Experimental and theoretical analyses reveal an efficient C-C bond formation process within the reactions of carbon monoxide and laser-vaporized nickel carbide. This work highlights the pivotal roles played by metal carbides in the C-C bond formation and also proposes new ideas for the design and chemical control of a broad class of complexes with unique physical and chemical properties.

Pt nanoparticles-decorated molybdenum nitrides for efficient hydrogen evolution reaction

Exploring low cost and high efficiency catalysts for hydrogen production from electrochemical water splitting is preferable and remains a significant challenge. As an alternative to Pt-based catalysts, molybdenum nitrides have attracted more attention for their hydrogen evolution reaction (HER). However, their performance is restricted due to the strong bonding of Mo-H. Herein, molybdenum nitrides with Pt-doping are fabricated to enhance the catalytic activity for HER in acidic solution. As expected, Pt (5 wt%)-MoNx delivers a low overpotential of 47 mV at a current density of 10 mA cm-2 with a high exchange current density (j0 = 0.98 mA cm-2). The superior performance is attributed to the modified electronic structure of Mo with Pt incorporation.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Modulating the electronic structure of Mo2C/MoP heterostructure to boost hydrogen evolution reaction in a wide pH range

Interface engineering is an effective strategy for the design of electrochemical catalysts with attractive performance for hydrogen evolution reaction. Herein, the Molybdenum carbide/molybdenum phosphide (Mo2C/MoP) heterostructure deposited on nitrogen (N), phosphorous (P) co-doped carbon substrate (Mo2C/MoP-NPC) is fabricated by one-step carbonization. The electronic structure of Mo2C/MoP-NPC is changed by optimizing the ratio of phytic acid and aniline. The calculation and experimental results also show that there is an electron interaction on the Mo2C/MoP interface, which optimizes the adsorption free energy of hydrogen (H) and improves the performance of hydrogen evolution reaction. Mo2C/MoP-NPC exhibits significant low overpotentials at 10 mA·cm-2 current density, 90 mV in 1 M KOH and 110 mV in 0.5 M H2SO4, respectively. In addition, it shows superior stability over a broad pH range. This research provides an effective method for the construction of novel heterogeneous electrocatalysts and is conducive to the development of green energy.

Synthesis of a carbon-wrapped microsphere MoO2/Mo2C heterojunction as an efficient electrocatalyst for the oxygen reduction reaction and the hydrogen evolution reaction

The development of non-noble metal catalysts for the optimization of conversion and storage devices is an important research topic. Hence, the microsphere MoO2/Mo2C/C heterojunction composites, which play an important role in the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER), were synthesized using the solvothermal-sintering method. The results revealed that the as-prepared composite exhibited better ORR and HER catalytic performances than those of MoO2/Mo2C and Vulcan XC-72R (carbon black), and approaching that of commercial Pt/C. At the same time, it has a superior methanol tolerance and electrochemical stability than that of the commercial Pt/C. The excellent performance may be attributed to the synergistic effect of the MoO2/Mo2C heterostructure, highly conductive Vulcan XC-72R, and oxygen vacancies (Ov). This research offers new insights into the design and synthesis of cost-effective, environmentally friendly heterojunction composite catalysts used as a high-performance cathode material in fuel cells and water splitting.

Investigating the Structural Evolution and Catalytic Activity of c-Co/Co3Mo Electrocatalysts for Alkaline Hydrogen Evolution Reaction

Transition metal alloys have emerged as promising electrocatalysts due to their ability to modulate key parameters, such as d-band electron filling, Fermi level energy, and interatomic spacing, thereby influencing their affinity towards reaction intermediates. However, the structural stability of alloy electrocatalysts during the alkaline hydrogen evolution reaction (HER) remains a subject of debate. In this study, we systematically investigated the structural evolution and catalytic activity of the c-Co/Co3Mo electrocatalyst under alkaline HER conditions. Our findings reveal that the Co3Mo alloy and H0.9MoO3 exhibit instability during alkaline HER, leading to the breakdown of the crystal structure. As a result, the cubic phase c-Co undergoes a conversion to the hexagonal phase h-Co, which exhibits strong catalytic activity. Additionally, we identified hexagonal phase Co(OH)2 as an intermediate product of this conversion process. Furthermore, we explored the readsorption and surface coordination of the Mo element, which contribute to the enhanced catalytic activity of the c-Co/Co3Mo catalyst in alkaline HER. This work provides valuable insights into the dynamic behavior of alloy-based electrocatalysts, shedding light on their structural stability and catalytic activity during electrochemical reduction processes.

Recent progress of molybdenum carbide based electrocatalysts for electrocatalytic hydrogen evolution reaction

Electrocatalytic hydrogen evolution reaction (HER) is one of the most promising and clean strategies to prepare hydrogen on a large scale. Nevertheless, the efficiency of HER is greatly restricted by the large overpotential at the anode, and it is necessary to develop low cost electrocatalysts with excellent performance and stability. Molybdenum carbide has shown great potential in the field of HER due to its unique electronic structure and physical and chemical properties. In this paper, the current progress of molybdenum carbide-based catalysts for HER is summarized. The influence of phase structure, nanostructure, heterostructure and heteroatoms doping on its catalytic performance is discussed in detail. Especially, the catalytic mechanisms are analyzed according to structural characterization and theoretical calculation results. Finally, the challenges and prospects for the further development of molybdenum carbide-based catalysts for HER are put forward to guide the progress of this field.

Biomass-Derived Carbon-Based Multicomponent Integration Catalysts for Electrochemical Water Splitting

Electrocatalytic water splitting powered by sustainable electricity is a crucial approach for the development of new generation green hydrogen technology. Biomass materials are abundant and renewable, and the application of catalysis can increase the value of some biomass waste and turn waste into fortune. Converting economical and resource-rich biomass into carbon-based multicomponent integrated catalysts (MICs) has been considered as one of the most promising ways to obtain inexpensive, renewable and sustainable electrocatalysts in recent years. In this review, recent advances in biomass-derived carbon-based MICs towards electrocatalytic water splitting are summarized, and the existing issues and key aspects in the development of these electrocatalysts are also discussed and prospected. The application of biomass-derived carbon-based materials will bring some new opportunities in the fields of energy, environment, and catalysis, as well as promote the commercialization of new nanocatalysts in the near future.

Synergistic Activation of Inert Iron Oxide Basal Planes through Heterostructure Formation and Doping for Efficient Hydrogen Evolution

Iron oxides have emerged as a very promising and cost-effective alternative to precious metal catalysts for hydrogen production. However, the inert basal plane of iron oxides needs to be activated to enhance their catalytic efficiency. In this study, we employed heterostructure engineering and doped nickel to cooperatively activate the basal planes of iron oxide (Ni-Fe2 O3 /CeO2 HSs) to achieve high hydrogen evolution reaction (HER) activity. The Ni-Fe2 O3 /CeO2 HSs electrocatalyst demonstrates excellent basic HER activity and stability, such as an extremely low overpotential of 43 mV at 10 mA cm-2 current density and corresponding Tafel slope of 58.6 mV dec-1 . The increase in electrocatalyst activity and acceleration of hydrogen precipitation kinetics arises from the dual modulation of Ni doping and heterostructure, which not only modulates the electrocatalyst's electronic structure, but also increases the number and exposure of active sites. Remarkably, the generation of heterogeneous structure makes the catalyst se. The Ni-doped catalyst has not only increased HER activity but also low-temperature resistance. These results suggest that the synergistic activation of inert iron oxide basal planes through heterostructure formation and doping is a feasible strategy. Furthermore, for efficient electrocatalytic water splitting, this technique can be extended to other non-noble metal oxides.

Synergistically Tuning Surface States of Hierarchical MoC by Pt-N Dual-Doping Engineering for Optimizing Hydrogen Evolution Activity

Catalytic performance can be greatly enhanced by rational modulation of the surface state. In this study, reasonable adjustment of the surface states around the Fermi level (EF ) of molybdenum carbide (MoC) (α phase) via a Pt-N dual-doping process to fabricate an electrocatalyst named as Pt-N-MoC is performed to promote hydrogen evolution reaction (HER) performance over the MoC surface. Systematically experimental and theoretical analyses demonstrate that the synergistic tuning of Pt and N can cause the delocalization of surface states, with an increase in the density of surface states near the EF . This is beneficial for accumulating and transferring electrons between the catalyst surface and adsorbent, resulting in a positively linear correlation between the density of surface states near the EF and the HER activity. Moreover, the catalytic performance is further enhanced by artificially fabricating a Pt-N-MoC catalyst that has a unique hierarchical structure composed of MoC nanoparticles (0D), nanosheets (2D), and microrods (3D). As expected, the obtained Pt-N-MoC electrocatalyst exhibits superb HER activity with an extremely low overpotential of 39 mV@10 mA cm-2 as well as superb stability (over 24 d) in an alkaline solution. This work highlights a novel strategy to develop efficient electrocatalysts via adjusting their surface states.

Electronic Structure Engineering of Pt Species over Pt/WO3 toward Highly Efficient Electrocatalytic Hydrogen Evolution

Pt-based supported materials, a widely used electrocatalyst for hydrogen evolution reaction (HER), often experience unavoidable electron loss, resulting in a mismatching of electronic structure and HER behavior. Here, a Pt/WO3 catalyst consisting of Pt species strongly coupled with defective WO3 polycrystalline nanorods is rationally designed. The electronic structure engineering of Pt sites on WO3 can be systematically regulated, and so that the optimal electron-rich Pt sites on Pt/WO3 -600 present an excellent HER activity with only 8 mV overpotential at 10 mA cm-2 . Particularly, the mass activity reaches 7015 mA mg-1 at the overpotential of 50 mV, up to 26-fold higher than that of the commercial Pt/C. The combination of experimental and theoretical results demonstrates that the O vacancies of WO3 effectively mitigate the tendency of electron transfer from Pt sites to WO3 , so that the d-band center could reach an appropriate level relative to Fermi level, endowing it with a suitableΔ G H ∗ $\Delta {G_{{{\rm{H}}^ * }}}$ . This work identifies the influence of the electronic structure on catalytic activity.

Corrosion of Titanium Electrode Used for Solar Saline Electroflotation

The solar electroflotation (EF) processes using saline electrolytes are today one of the great challenges for the development of electrochemical devices, due to the corrosion problems that are generated during the operation by being in permanent contact with Cl^- ions. This manuscript discloses the corrosion behavior of titanium electrodes using a superposition model based on mixed potential theory and the evaluation of the superficial performance of the Ti electrodes operated to 4 V/SHE solar electroflotation in contact with a solution of 0.5 M NaCl. Additionally provided is an electrochemical analysis of Ti electrodes regarding HER, ORR, OER, and CER that occur during the solar saline EF process. The non-linear superposition model by mixed potential theory gives electrochemical and corrosion parameters that complement the information published in scientific journals, the corrosion current density and corrosion potential in these conditions is 0.069 A/m² and -7.27 mV, respectively. The formation of TiO₂ and TiOCl on the anode electrode was visualized, resulting in a reduction of its weight loss of the anode electrode.

Re-Dispersion of Platinum From CNTs Substrate to α-MoC1 - x to Boost the Hydrogen Evolution Reaction

Developing high-performance electrocatalysts toward hydrogen evolution reaction (HER) is important for clean and sustainable hydrogen energy, yet still challenging. Herein, an α-MoC_1 - x induced redispersing strategy to construct a superior HER electrocatalyst (Pt/CNTs-N + α-MoC_1 - x ) by mechanical mixing of α-MoC_1 - x with Pt/CNTs-N followed by thermal reduction is reported. It is found that thermo-activation treatment enables partial Pt atoms to redisperse on α-MoC_1 - x substrate from carbon nanotubes, which creates dual active interfaces of Pt species dispersed over carbon nanotubes and α-MoC_1 - x . Benefiting from the strong electronic interaction between the Pt atom and α-MoC_1 - x , the utilization efficiency of the Pt atom and the zero-valence state of Pt is evidently enhanced. Consequently, Pt/CNTs-N + α-MoC_1 - x catalyst exhibits excellent HER activity with low overpotentials of 17 and 34 mV to achieve a current density of 10 mA cm^-2 in acidic and alkaline electrolytes, respectively. Density functional theory calculations further reveal that the synergistic effect between Pt and α-MoC_1 - x makes it accessible for the dissociation of water molecules and subsequent desorption of hydrogen atoms. This work reveals the crucial roles of α-MoC_1 - x additives, providing practical solutions to enhance platinum dispersion, and thereby enhance the catalytic activity in HER.

Polymer Coated Functional Catalysts for Industrial Applications

Surface engineering of conventional catalysts using polymeric coating has been extensively explored for producing hybrid catalytic material with enhanced activity, high mechanical and thermal stability, enhanced productivity, and selectivity of the desired product. The present review discusses in detail the state-of-the-art knowledge on surface modification of catalysts, namely photocatalysts, electrocatalysts, catalysts for photoelectrochemical reactions, and catalysts for other types of reactions, such as hydrodesulfurization, carbon dioxide cycloaddition, and noble metal-catalyzed oxidation/reduction reactions. The various techniques employed for the polymer coating of catalysts are discussed and the role of polymers in enhancing the catalytic activity is critically analyzed. The review further discusses the applications of biodegradable and biocompatible natural polysaccharide-based polymers, namely, chitosan and polydopamine as prospective coating material.

Electrochemical hydrogen evolution on Pt-based catalysts from a theoretical perspective

Hydrogen evolution reaction (HER) by splitting water is a key technology toward a clean energy society, where Pt-based catalysts were long known to have the highest activity under acidic electrochemical conditions but suffer from high cost and poor stability. Here, we overview the current status of Pt-catalyzed HER from a theoretical perspective, focusing on the methodology development of electrochemistry simulation, catalytic mechanism, and catalyst stability. Recent developments in theoretical methods for studying electrochemistry are introduced, elaborating on how they describe solid-liquid interface reactions under electrochemical potentials. The HER mechanism, the reaction kinetics, and the reaction sites on Pt are then summarized, which provides an atomic-level picture of Pt catalyst surface dynamics under reaction conditions. Finally, state-of-the-art experimental solutions to improve catalyst stability are also introduced, which illustrates the significance of fundamental understandings in the new catalyst design.

Recent Advances in Manganese-Based Materials for Electrolytic Water Splitting

Developing earth-abundant and highly effective electrocatalysts for electrocatalytic water splitting is a prerequisite for the upcoming hydrogen energy society. Recently, manganese-based materials have been one of the most promising candidates to replace noble metal catalysts due to their natural abundance, low cost, adjustable electronic properties, and excellent chemical stability. Although some achievements have been made in the past decades, their performance is still far lower than that of Pt. Therefore, further research is needed to improve the performance of manganese-based catalytic materials. In this review, we summarize the research progress on the application of manganese-based materials as catalysts for electrolytic water splitting. We first introduce the mechanism of electrocatalytic water decomposition using a manganese-based electrocatalyst. We then thoroughly discuss the optimization strategy used to enhance the catalytic activity of manganese-based electrocatalysts, including doping and defect engineering, interface engineering, and phase engineering. Finally, we present several future design opportunities for highly efficient manganese-based electrocatalysts.

Transition metal chalcogenides carbon-based as bifunctional cathode electrocatalysts for rechargeable zinc-air battery: An updated review

The rechargeable alkaline aqueous zinc-air batteries (ZABs) are prospective candidates to supply the energy demand for their high theoretical energy density, inherent safety, and environmental friendliness. However, their practical application is mainly restricted by the unsatisfactory efficiency of the air electrode, leading to an intense search for high-efficient oxygen electrocatalysts. In recent years, the composites of carbon materials and transition metal chalcogenides (TMC/C) have emerged as promising alternatives because of the unique properties of these single compounds and the synergistic effect between them. In this sense, this review presented the electrochemical properties of these composites and their effects on the ZAB performance. The operational fundamentals of the ZABs were described. After elucidating the role of the carbon matrix in the hybrid material, the latest developments in the ZAB performance of the monometallic structure and spinel of TMC/C were detailed. In addition, we report topics on doping and heterostructure due to the large number of studies involving these specific defects. Finally, a critical conclusion and a brief overview sought to contribute to the advancement of TMC/C in the ZABs.

3-D nitrogen-doped carbon cage encapsulated ultrasmall MoC nanoparticles for promoting simultaneous ZnIn2S4 photocatalytic hydrogen generation and organic wastewater degradation

Simultaneous redox reactions on photocatalysts make it possible to use wastewater for hydrogen production. The controlled synthesis of ultrasmall metal carbides effectively enhances the photocatalytic efficiency under this system. Here, we report a new type of cocatalyst in which a three-dimensional (3-D) nitrogen-doped carbon cage (NGC) of metal-organic framework derivatives encapsulates ultrasmall MoC nanoparticles (MoC@NGC), promoting simultaneous degradation of organic pollutants and hydrogen production by ZnIn₂S₄ (ZIS). Characterization analyses showed that MoC accelerated the separation of the photogenerated carrier and effectively reduced the overpotential of hydrogen evolution, while NGC promoted the good dispersion of MoC particles and provided sufficient sites. The MoC@NGC/ZIS composite exhibited a high hydrogen (H₂) evolution rate of 1012 µmol g^-1h^-1, which exceed that of ZIS loaded with platinum. In the coupled system, where the electron donor was replaced with rhodamine B (RhB), the mechanism analysis showed that RhB and the as-generated intermediates consumed holes and facilitated hydrogen evolution. In addition, we designed a combined photocatalytic anoxic and oxic sequence process to achieve the recovery of hydrogen energy during the treatment of dye wastewater. This study provides a new way for cooperation between energy development and environmental protection.

Ultrafast Synthesis of Mo2C-Based Catalyst by Joule Heating towards Electrocatalytic Hydrogen Evolution Reaction

Developing earth-abundant electrocatalysts useful for hydrogen evolution reactions (HER) is critical for electrocatalytic water splitting driven by renewable energy. Molybdenum carbide (Mo2C) with the crystal structure of hexagonal symmetry has been identified to be an excellent HER catalyst due to its platinum-like electronic structure while the synthesis of Mo2C is generally time consuming and energy intensive. Herein, we demonstrated the ultrafast synthesis of a Mo2C-based electrocatalyst with Joule heating at 1473 K for only 6 s. Benefitting from several advantages including efficient catalytic kinetics, enhanced charge transport kinetics and high intrinsic activity, the as-prepared catalyst exhibited drastically enhanced HER performance compared with commercial Mo2C. It showed an overpotential of 288 mV for achieving a current density of −50 mA cm−2 and good stability, which highlighted the feasibility of the Joule heating method towards preparing efficient electrocatalysts.

Unconventional Phase Synergies with Doping Engineering Over Ni Electrocatalyst Featuring Regulated Electronic State for Accelerated Urea Oxidation

Exploring high-performing Ni-based electrocatalysts for the urea oxidation reaction (UOR) is crucial for developing urea-related energy technologies yet remains a daunting challenge. In this study, a synergistic anomalous hcp phase and heteroatom doping engineering over metallic Ni are found to enhance the UOR. A metal-organic framework-mediated approach is proposed to construct Ni nanoparticles (NPs) with designated crystal phase embedded in N-doped carbon (fcc-Ni/NC and hcp-Ni/NC). Significant crystal phase-dependent catalytic activity for the UOR is observed; hcp-Ni/NC, featuring unusual hcp phase, outperforms fcc-Ni/NC with conventional fcc phase. Moreover, incorporating foreign Mn species in hcp-Ni/NC can further dramatically promote UOR, making it among the best UOR catalysts reported to date. From experimental results and DFT calculations, the specific nanoarchitecture, involving an anomalous hcp phase together with Mn doping engineering, endows hcp-MnNi/NC with abundant exposed active sites, facile charge transfer, and more significantly, optimized electronic state, giving rise to enriched Ni³⁺ active species and oxygen vacancies on the catalyst surface during electrocatalysis. These features collectively contribute to the enhanced UOR activity. This work highlights a potent design strategy to develop advanced catalysts with regulated electronic state through synergistic crystal phase and doping engineering.

Facet Engineering of Advanced Electrocatalysts Toward Hydrogen/Oxygen Evolution Reactions

The crystal facets featured with facet-dependent physical and chemical properties can exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) attributed to their anisotropy. The highly active exposed crystal facets enable increased mass activity of active sites, lower reaction energy barriers, and enhanced catalytic reaction rates for HER and OER. The formation mechanism and control strategy of the crystal facet, significant contributions as well as challenges and perspectives of facet-engineered catalysts for HER and OER are provided.

The electrocatalytic water splitting technology can generate high-purity hydrogen without emitting carbon dioxide, which is in favor of relieving environmental pollution and energy crisis and achieving carbon neutrality. Electrocatalysts can effectively reduce the reaction energy barrier and increase the reaction efficiency. Facet engineering is considered as a promising strategy in controlling the ratio of desired crystal planes on the surface. Owing to the anisotropy, crystal planes with different orientations usually feature facet-dependent physical and chemical properties, leading to differences in the adsorption energies of oxygen or hydrogen intermediates, and thus exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, a brief introduction of the basic concepts, fundamental understanding of the reaction mechanisms as well as key evaluating parameters for both HER and OER are provided. The formation mechanisms of the crystal facets are comprehensively overviewed aiming to give scientific theory guides to realize dominant crystal planes. Subsequently, three strategies of selective capping agent, selective etching agent, and coordination modulation to tune crystal planes are comprehensively summarized. Then, we present an overview of significant contributions of facet-engineered catalysts toward HER, OER, and overall water splitting. In particular, we highlight that density functional theory calculations play an indispensable role in unveiling the structure–activity correlation between the crystal plane and catalytic activity. Finally, the remaining challenges in facet-engineered catalysts for HER and OER are provided and future prospects for designing advanced facet-engineered electrocatalysts are discussed.

Computational screening toward transition metal doped vanadium carbides in different crystal planes for efficient hydrogen evolution: a first-principles study

In the present work, we evaluated the hydrogen evolution reaction (HER) performance of transition metal (Co, Fe, Ni, Mn, and Mo) doped vanadium carbides (VC). In addition, the doping atoms were screened separately on the (100), (110) and (111) crystal planes to analyze the differences in HER activities. Among all the calculated models, Mn-VC(100) exhibited the best catalytic hydrogen evolution performance with a Gibbs free energy for hydrogen adsorption (ΔG_H*) of 0.0012 eV. Doping Mn greatly improved the HER performance of VC(100) by enhancing the adsorption of hydrogen on the catalyst surface. The analysis of the electronic density of states and charge transfer confirmed that doping transition metal atoms into the surfaces of the VC model successfully optimized the electronic structure and promoted catalytic reaction kinetics. Besides, the relationship between the catalytic activity and pH value of different models was considered, and doping Co atoms on the (100) crystal plane could effectively modify the pH value range applicable for the efficient HER. Interestingly, even if the same metal atoms were doped, various active sites of VC models exhibited different catalytic performances due to disparate exposed crystal planes and pH values. This indicates that the main exposed crystal surfaces and the pH range of application need to be considered when selecting the appropriate doping element for the catalyst.

Electrocatalytic NO Reduction to NH3 on Mo2C Nanosheets

Electrocatalytic reduction of NO to NH₃ (NORR) emerges as a promising route for achieving harmful NO treatment and sustainable NH₃ generation. In this work, we first report that Mo₂C is an active and selective NORR catalyst. The developed Mo₂C nanosheets deliver a high NH₃ yield rate of 122.7 μmol h^-1 cm^-2 with an NH₃ Faradaic efficiency of 86.3% at -0.4 V. Theoretical computations unveil that the surface-terminated Mo atoms on Mo₂C can effectively activate NO, promote protonation energetics, and suppress proton adsorption, resulting in high NORR activity and selectivity of Mo₂C.