A New Class of Electrocatalysts for Hydrogen Production from Water Electrolysis: Metal Monolayers Supported on Low-Cost Transition Metal Carbides

Low-Loading of Pt Nanoparticles on 3D Carbon Foam Support for Highly Active and Stable Hydrogen Production

Minimizing Pt loading is essential for designing cost-effective water electrolyzers and fuel cell systems. Recently, three-dimensional macroporous open-pore electroactive supports have been widely regarded as promising architectures to lower loading amounts of Pt because of its large surface area, easy electrolyte access to Pt sites, and superior gas diffusion properties to accelerate diffusion of H₂ bubbles from the Pt surface. However, studies to date have mainly focused on Pt loading on Ni-based 3D open pore supports which are prone to corrosion in highly acidic and alkaline conditions. Here, we investigate electrodeposition of Pt nanoparticles in low-loading amounts on commercially available, inexpensive, 3D carbon foam (CF) support and benchmark their activity and stability for electrolytic hydrogen production. We first elucidate the effect of deposition potential on the Pt nanoparticle size, density and subsequently its coverage on 3D CF. Analysis of the Pt deposit using scanning electron microscopy images reveal that for a given deposition charge density, the particle density increases (with cubic power) and particle size decreases (linearly) with deposition overpotential. A deposition potential of -0.4 V vs. standard calomel electrode (SCE) provided the highest Pt nanoparticle coverage on 3D CF surface. Different loading amounts of Pt (0.0075-0.1 mg_Pt/cm²) was then deposited on CF at -0.4 V vs. SCE and subsequently studied for its hydrogen evolution reaction (HER) activity in acidic 1M H₂SO₄ electrolyte. The Pt/CF catalyst with loading amounts as low as 0.06 mg_Pt/cm² (10-fold lower than state-of-the-art commercial electrodes) demonstrated a mass activity of 2.6 ampere per milligram Pt at 200 mV overpotential, nearly 6-fold greater than the commercial Pt/C catalyst tested under similar conditions. The 3D architectured electrode also demonstrated excellent stability, showing <7% loss in activity after 60 h of constant current water electrolysis at 100 mA/cm².

Recent Progresses in Electrocatalysts for Water Electrolysis

Abstract

The study of hydrogen evolution reaction and oxygen evolution reaction electrocatalysts for water electrolysis is a developing field in which noble metal-based materials are commonly used. However, the associated high cost and low abundance of noble metals limit their practical application. Non-noble metal catalysts, aside from being inexpensive, highly abundant and environmental friendly, can possess high electrical conductivity, good structural tunability and comparable electrocatalytic performances to state-of-the-art noble metals, particularly in alkaline media, making them desirable candidates to reduce or replace noble metals as promising electrocatalysts for water electrolysis. This article will review and provide an overview of the fundamental knowledge related to water electrolysis with a focus on the development and progress of non-noble metal-based electrocatalysts in alkaline, polymer exchange membrane and solid oxide electrolysis. A critical analysis of the various catalysts currently available is also provided with discussions on current challenges and future perspectives. In addition, to facilitate future research and development, several possible research directions to overcome these challenges are provided in this article.

Graphical Abstract

Oxygen Evolution Reaction Kinetic Barriers on Nitrogen-Doped Carbon Nanotubes

We investigate kinetic barriers for the oxygen evolution reaction (OER) on singly and doubly nitrogen-doped single-walled carbon nanotubes (NCNTs) using the climbing image nudged elastic band method with solvent effects represented by a 45-water-molecule droplet. The studied sites were chosen based on a previous study of the same systems utilizing a thermodynamic model which ignored both solvent effects and kinetic barriers. According to that model, the two studied sites, one on a singly nitrogen-doped CNT and the other on a doubly doped CNT, were approximately equally suitable for OER. For the four-step OER process, however, our reaction barrier calculations showed a clear difference in the rate-determining *OOH formation step between the two systems, with barrier heights differing by more than 0.4 eV. Thus, the simple thermodynamic model may alone be insufficient for identifying optimal OER sites. Of the remaining three reaction steps, the two H₂O forming ones were found to be barrierless in all cases. We also performed solvent-free barrier calculations on NCNTs and undoped CNTs. Substantial differences were observed in the energies of the intermediates when the solvent was present. In general, the observed low activation energy barriers for these reactions corroborate both experimental and theoretical findings of the utility of NCNTs for OER catalysis.

Principles and Methods for the Rational Design of Core-Shell Nanoparticle Catalysts with Ultralow Noble Metal Loadings

Conspecuts Commercial and emerging renewable energy technologies are underpinned by precious metal catalysts, which enable the transformation of reactants into useful products. However, the noble metals (NMs) comprise the least abundant elements in the lithosphere, making them prohibitively scarce and expensive for future global-scale technologies. As such, intense research efforts have been devoted to eliminating or substantially reducing the loadings of NMs in various catalytic applications. These efforts have resulted in a plethora of heterogeneous NM catalyst morphologies beyond the traditional supported spherical nanoparticle. In many of these new architectures, such as shaped, high index, and bimetallic particles, less than 20% of the loaded NMs are available to perform catalytic turnovers. The majority of NM atoms are subsurface, providing only a secondary catalytic role through geometric and ligand effects with the active surface NM atoms. A handful of architectures can approach 100% NM utilization, but severe drawbacks limit general applicability. For example, in addition to problems with stability and leaching, single atom and ultrasmall cluster catalysts have extreme metal-support interactions, discretized d-bands, and a lack of adjacent NM surface sites. While monolayer thin films do not possess these features, they exhibit such low surface areas that they are not commercially relevant, serving predominantly as model catalysts. This Account champions core-shell nanoparticles (CS NPs) as a vehicle to design highly active, stable, and low-cost materials with high NM utilization for both thermo- and electrocatalysis. The unique benefits of the many emerging NM architectures could be preserved while their fundamental limitations could be overcome through reformulation via a core-shell morphology. However, the commercial realization of CS NPs remains challenging, requiring concerted advances in theory and manufacturing. We begin by formulating seven constraints governing proper core material design, which naturally point to early transition metal ceramics as suitable core candidates. Two constraints prove extremely challenging. The first relates to the core modifying the shell work function and d-band. To properly investigate materials that could satisfy this constraint, we discuss our development of a new heat, quench, and exfoliation (HQE) density functional theory (DFT) technique to model heterometallic interfaces. This technique is used to predict how transition metal carbides can favorably tune the catalytic properties of various NM monolayer shell configurations. The second challenging constraint relates to the scalable manufacturing of CS NP architectures with independent synthetic control of the thickness and composition of the shell and the size and composition of the core. We discuss our development of a synthetic method that enables high temperature self-assembly of tunable CS NP configurations. Finally, we discuss how these principles and methods were used to design catalysts for a variety of applications. These include the design of a thermally stable sub-monolayer CS catalyst, a highly active methanol electrooxidation catalyst, CO-tolerant Pt catalysts, and a hydrogen evolution catalyst that is less expensive than state-of-the-art NM-free catalysts. Such core-shell architectures offer the promise of ultralow precious metal loadings while ceramic cores hold the promise of thermodynamic stability and access to unique catalytic activity/tunability.

Enhanced Stability of PtML/MML/WC(0001) Multilayer Alloys under Electrochemical Conditions: A First Principle Study

A platinum (Pt) monolayer catalyst could greatly reduce the use of Pt. However, the core or subsurface transition metal could easily segregate to the surface and eventually selectively dissolve in the working conditions. In this work, a type of Pt_ML/M_ML/tungsten carbide (WC) multilayer structure catalyst has been designed using the density functional theory method. The results show that the stability of W-terminated core catalysts is lower than that of the corresponding C-terminated one. More importantly, the C-terminated Pt_ML/Co_ML/WC alloy could also maintain a stable Pt monolayer structure in various electrolyte solutions (e.g., perchloric acid, phosphoric acid, and alkaline solutions) and via different oxygen reduction reaction (ORR) pathways, in addition to the previously well-known precious alloy elements, such as Ru, Ir, and so forth. The segregation of the M metal will be suppressed by introducing a high electronegativity nonmetal element in the core such as C through enhanced interaction between the C and M interlayers. The improved ORR activity and the stability of C-terminated Pt_ML/Co(Ru)_ML/WC(0001) multilayer structures highlight the importance of surface chemistry of the substrate in rational design Pt monolayer catalysts.

Reactions of water and C1 molecules on carbide and metal-modified carbide surfaces

The formation of carbides can significantly modify the physical and chemical properties of the parent metals. In the current review, we summarize the general trends in the reactions of water and C1 molecules over transition metal carbide (TMC) and metal-modified TMC surfaces and thin films. Although the primary focus of the current review is on the theoretical and experimental studies of reactions of C1 molecules (CO, CO₂, CH₃OH, etc.), the reactions of water will also be reviewed because water plays an important role in many of the C1 transformation reactions. This review is organized by discussing separately thermal reactions and electrochemical reactions, which provides insights into the application of TMCs in heterogeneous catalysis and electrocatalysis, respectively. In thermal reactions, we discuss the thermal decomposition of water and methanol, as well as the reactions of CO and CO₂ over TMC surfaces. In electrochemical reactions, we summarize recent studies in the hydrogen evolution reaction, electrooxidation of methanol and CO, and electroreduction of CO₂. Finally, future research opportunities and challenges associated with using TMCs as catalysts and electrocatalysts are also discussed.

Ruthenium-Tungsten Composite Catalyst for the Efficient and Contamination-Resistant Electrochemical Evolution of Hydrogen

A new catalyst, prepared by a simple physical mixing of ruthenium (Ru) and tungsten (W) powders, has been discovered to interact synergistically to enhance the electrochemical hydrogen evolution reaction (HER). In an aqueous 0.5 M H₂SO₄ electrolyte, this catalyst, which contained a miniscule loading of 2-5 nm sized Ru nanoparticles (5.6 μg Ru per cm² of geometric surface area of the working electrode), required an overpotential of only 85 mV to drive 10 mA/cm² of H₂ evolution. Interestingly, our catalyst also exhibited good immunity against deactivation during HER from ionic contaminants, such as Cu²⁺ (over 24 h). We unravel the mechanism of synergy between W and Ru for catalyzing H₂ evolution using Cu underpotential deposition, photoelectron spectroscopy, and density functional theory (DFT) calculations. We found a decrease in the d-band and an increase in the electron work function of Ru in the mixed composite, which made it bind to H more weakly (more Pt-like). The H-adsorption energy on Ru deposited on W was found, by DFT, to be very close to that of Pt(111), explaining the improved HER activity.

Synergistic Effects of C/α-MoC and Ag for Efficient Oxygen Reduction Reaction

It remains challenging to prepare highly active and stable catalysts from earth-abundant elements for the oxygen reduction reaction (ORR). Herein we report a facile method to synthesize cost-effective heterogeneous C/α-MoC/Ag electrocatalysts. Rotating disc electrode (RDE) experiments revealed that the obtained C/α-MoC/Ag exhibited much superior catalytic performance for ORR than that of C/Ag, C/α-MoC, or even the conventional Pt/C. First-principles calculations indicated that the enhanced activity could be attributed to the efficient synergistic effects between Ag and α-MoC/C by which the energy barrier for O₂ dissociation has been substantially reduced. Furthermore, Li-air and Al-air cells were assembled to demonstrate the unprecedented electrochemical performance of C/α-MoC/Ag nanocomposites surpassing the Pt/C. Thus experimental results and theoretical calculations together showed that the heterogeneous C/α-MoC/Ag nanocomposites are a promising alternative to platinum for applications in industrial metal-air batteries.

Silver-doped molybdenum carbide catalyst with high activity for electrochemical water splitting

A hybrid catalyst composed of silver (Ag) doped wire-like molybdenum carbide (Mo_xC_y) with pure β-phase and carbon nanotubes (CNTs) was coated well on a carbon rod electrode for the hydrogen evolution reaction (HER). The effects of Ag loading amount and carbonization temperature on the crystal form of Mo_xC_y were investigated in detail. It is found that the Mo_xC_y crystal form can be tuned by adjusting the preparation conditions, and nanostructured wire-like Mo₂C with pure β-phase was obtained at a temperature over 750 °C. Ag/Mo_xC_y composite nanomaterials were investigated by X-ray diffraction, UV/vis spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy and Brunauer-Emmett-Teller surface area analysis. The hybrid catalyst was further deposited on the carbon nanotube (CNT) modified carbon rod substrate. Due to the high surface area and 3D porous network-like microstructure, the Ag/Mo₂C/CNTs hybrid electrode showed enhanced catalytic performance when comparing with the corresponding pure one. Particularly, for the Ag-doped Mo₂C/CNTs hybrid electrode with an optimum 1 Ag : 5 Mo molar ratio of the precursors, a current density of 10 mA cm^-2 was obtained by applying an overpotential of 142 mV in 0.5 mol L^-1 H₂SO₄ solution. It is expected that such a hybrid electrode can be widely applied for effective electrolysis of water to produce hydrogen.

Tuning core–shell interactions in tungsten carbide–Pt nanoparticles for the hydrogen evolution reaction

Titanium doping enhances the stability and activity of tungsten carbide core–platinum shell nanoparticles for hydrogen evolution

An extremely low Pt loading cathode for a highly efficient proton exchange membrane water electrolyzer

Direct self-terminated Pt electrodeposition on carbon paper enables precise control of loading Pt mass, from the sub-microgram to the sub-milligram scale. This can provide insight into the low limits of Pt use for reasonable performance of a proton exchange membrane water electrolyzer.

A Microribbon Hybrid Structure of CoOx-MoC Encapsulated in N-Doped Carbon Nanowire Derived from MOF as Efficient Oxygen Evolution Electrocatalysts

Developing highly efficient electrocatalysts for oxygen evolution is vital for renewable and sustainable energy production and storage. Herein, nitrogen-doped carbon encapsulated CoOx-MoC heterostructures are reported for the first time as high performance oxygen evolution electrocatalysts. The composition can be tuned by the addition of a Mo source to form a nanowire-assembled hierarchically porous microstructure, which can enlarge the specific surface area, thus exposing more active sites, facilitating mass transport and charge transfer. Moreover, it is demonstrated that the formation of CoOx-MoC heterostructures and the resulting synergistic effect between MoC and Co facilitate the reaction kinetics, leading to significantly improved oxygen evolution reaction (OER) activity with an onset overpotential of merely 290 mV, and a low overpotential of 330 mV to afford a current density of 10 mA cm^-2 . The well-constructed microarchitecture contributes to superior long term stability electrochemical behaviors. This work provides a facile strategy via composition tuning and structure optimization for the development of next-generation nonprecious metal-based OER electrocatalysts.

New and Efficient Electrocatalyst for Hydrogen Production from Water Splitting: Inexpensive, Robust Metallic Glassy Ribbons Based on Iron and Cobalt

Efficient, stable electrocatalysts are required to promote the hydrogen evolution reaction (HER) to obtain hydrogen as a clean, sustainable fuel via water splitting. In the present work, ribbons of the metallic glass Fe₄₀Co₄₀P₁₃C₇ were produced using a conventional melt-spinning technique and assessed as electrocatalysts for HER. In 0.5 M H₂SO₄, these ribbons generated an overpotential of 118 mV at a current density of 10 mA cm^-2. This overpotential remained essentially constant over 20 h under these conditions. On the basis of the excellent properties, these glassy ribbons represent a new type of highly active, robust HER catalyst suitable for practical applications.

Universal, In Situ Transformation of Bulky Compounds into Nanoscale Catalysts by High-Temperature Pulse

The synthesis of nanoscale metal compound catalysts has attracted much research attention in the past decade. The challenges of preparation of the metal compound include the complexity of the synthesis process and difficulty of precise control of the reaction conditions. Herein, we report an in situ synthesis of nanoparticles via a high-temperature pulse method where the bulk material acts as the precursor. During the process of rapid heating and cooling, swift melting, anchoring, and recrystallization occur, resulting in the generation of high-purity nanoparticles. In our work, the cobalt boride (Co₂B) nanoparticles with a diameter of 10-20 nm uniformly anchored on the reduced graphene oxide (rGO) nanosheets were successfully prepared using the high temperature pulse method. The as-prepared Co₂B/rGO composite displayed remarkable electrocatalytic performance for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). We also prepared molybdenum disulfide (MoS₂) and cobalt oxide (Co₃O₄) nanoparticles, thereby demonstrating that the high-temperature pulse is a universal method to synthesize ultrafine metal compound nanoparticles.

Room-Temperature Wet Chemical Synthesis of Au NPs/TiH2/Nanocarved Ti Self-Supported Electrocatalysts for Highly Efficient H2 Generation

Self-supported electrocatalysts are a new class of materials exhibiting high catalytic performance for various electrochemical processes and can be directly equipped in energy conversion devices. We present here, for the first time, sparse Au NPs self-supported on etched Ti (nanocarved Ti substrate self-supported with TiH₂) as promising catalysts for the electrochemical generation of hydrogen (H₂) in KOH solutions. Cleaned, as-polished Ti substrates were etched in highly concentrated sulfuric acid solutions without and with 0.1 M NH₄F at room temperature for 15 min. These two etching processes yielded a thin layer of TiH₂ (the corrosion product of the etching process) self-supported on nanocarved Ti substrates with different morphologies. While F^--free etching process led to formation of parallel channels (average width: 200 nm), where each channel consists of an array of rounded cavities (average width: 150 nm), etching in the presence of F^- yielded Ti surface carved with nanogrooves (average width: 100 nm) in parallel orientation. Au NPs were then grown in situ (self-supported) on such etched surfaces via immersion in a standard gold solution at room temperature without using stabilizers or reducing agents, producing Au NPs/TiH₂/nanostructured Ti catalysts. These materials were characterized by scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), grazing incidence X-ray diffraction (GIXRD), and X-ray photoelectron spectroscopy (XPS). GIXRD confirmed the formation of Au₂Ti phase, thus referring to strong chemical interaction between the supported Au NPs and the substrate surface (also evidenced from XPS) as well as a titanium hydride phase of chemical composition TiH₂. Electrochemical measurements in 0.1 M KOH solution revealed outstanding hydrogen evolution reaction (HER) electrocatalytic activity for our synthesized catalysts, with Au NPs/TiH₂/nanogrooved Ti catalyst being the best one among them. It exhibited fast kinetics for the HER with onset potentials as low as -22 mV vs. RHE, high exchange current density of 0.7 mA cm^-2, and a Tafel slope of 113 mV dec^-1. These HER electrochemical kinetic parameters are very close to those measured here for a commercial Pt/C catalyst (onset potential: -20 mV, Tafel slope: 110 mV dec^-1, and exchange current density: 0.75 mA cm^-2). The high catalytic activity of these materials was attributed to the catalytic impacts of both TiH₂ phase and self-supported Au NPs (active sites for the catalytic reduction of water to H₂), in addition to their nanostructured features which provide a large-surface area for the HER.

Dodecahedral W@WC Composite as Efficient Catalyst for Hydrogen Evolution and Nitrobenzene Reduction Reactions

Core-shell composites with strong phase-phase contact could provide an incentive for catalytic activity. A simple, yet efficient, H₂O-mediated method has been developed to synthesize a mesoscopic core-shell W@WC architecture with a dodecahedral microstructure, via a one-pot reaction. The H₂O plays an important role in the resistance of carbon diffusion, resulting in the formation of the W core and W-terminated WC shell. Density functional theory (DFT) calculations reveal that adding W as core reduced the oxygen adsorption energy and provided the W-terminated WC surface. The W@WC exhibits significant electrocatalytic activities toward hydrogen evolution and nitrobenzene electroreduction reactions, which are comparable to those found for commercial Pt/C, and substantially higher than those found for meso- and nano-WC materials. The experimental results were explained by DFT calculations based on the energy profiles in the hydrogen evolution reactions over WC, W@WC, and Pt model surfaces. The W@WC also shows a high thermal stability and thus may serve as a promising more economical alternative to Pt catalysts in these important energy conversion and environmental protection applications. The current approach can also be extended or adapted to various metals and carbides, allowing for the design and fabrication of a wide range of catalytic and other multifunctional composites.

Probing Transition-Metal Silicides as PGM-Free Catalysts for Hydrogen Oxidation and Evolution in Acidic Medium

In this experimental study, we investigate various transition-metal silicides as platinum-group-metal-(PGM)-free electrocatalysts for the hydrogen oxidation reaction (HOR), and for the hydrogen evolution reaction (HER) in acidic environment for the first time. Using cyclic voltammetry in 0.1 M HClO₄, we first demonstrate that the tested materials exhibit sufficient stability against dissolution in the relevant potential window. Further, we determine the HOR and HER activities for Mo, W, Ta, Ni and Mo-Ni silicides in rotating disk electrode experiments. In conclusion, for the HOR only Ni₂Si shows limited activity, and the HER activity of the investigated silicides is considerably lower compared to other PGM-free HER catalysts reported in the literature.

Nitrogen-doped carbon encapsulating γ-MoC/Ni heterostructures for efficient oxygen evolution electrocatalysts

Developing efficient and earth-abundant oxygen evolution reaction (OER) electrocatalysts is of great importance in renewable energy research. Herein, we report nitrogen-doped carbon encapsulating γ-MoC/Ni heterostructures (γ-MoC/Ni@NC) for the first time as efficient OER electrocatalysts. The composites can be controlled by the addition of a Mo source to form a nanoparticle-assembled hierarchically porous nanostructure, which can enlarge the specific surface area, thus exposing more active sites. Moreover, we demonstrate that the formation of γ-MoC@Ni heterostructures and the resulting synergistic effect between γ-MoC and Ni can facilitate the reaction kinetics, leading to significantly improved OER activity with an onset overpotential of merely 240 mV, and a low overpotential of 310 mV to afford a current density of 10 mA cm^-2. This work provides a new insight into the development of highly active and inexpensive OER electrocatalysts.

Non-Noble Metal-based Carbon Composites in Hydrogen Evolution Reaction: Fundamentals to Applications

Hydrogen has been hailed as a clean and sustainable alternative to finite fossil fuels in many energy systems. Water splitting is an important method for hydrogen production in high purity and large quantities. To accelerate the hydrogen evolution reaction (HER) rate, it is highly necessary to develop high efficiency catalysts and to select a proper electrolyte. Herein, the performances of non-noble metal-based carbon composites under various pH values (acid, alkaline and neutral media) for HER in terms of catalyst synthesis, structure and molecular design are systematically discussed. A detailed analysis of the structure-activity-pH correlations in the HER process gives an insight on the origin of the pH-dependence for HER, and provide guidance for future HER mechanism studies on non-noble metal-based carbon composites. Furthermore, this Review gives a fresh impetus to rational design of high-performance noble-metal-free composites catalysts and guide researchers to employ the established electrocatalysts in proper water electrolysis technologies.

Tuning the hydrogen evolution activity of β-Mo2C nanoparticles via control of their growth conditions

The use of water electrocatalysis for hydrogen production is a promising, sustainable and greenhouse-gas-free process to develop disruptive renewable energy technologies. Transition metal carbides, in particular β-phase Mo₂C, are garnering increased attention as hydrogen evolution reaction (HER) catalysts due to their favourable synthesis conditions, stability and high catalytic efficiency. We use a thermodynamic approach in conjunction with density functional theory and a kinetic model of exchange current density to systematically study the HER activity of β-Mo₂C under different experimental conditions. We show that the (011) surface has the highest HER activity, which is rationalized by its lack of strong Mo-based hydrogen adsorption sites. Thus, the HER efficiency of β-Mo₂C can be tuned using nanoparticles (NPs) that expose larger fractions of this termination. We give definite maps between NP morphologies and experimental synthesis conditions, and show that the control of the carbon chemical potential during synthesis can expose up to 90% of the (011) surface, while ambient H₂ has little effect on the NP morphology. The "volcano" plot shows that under these optimum conditions, the NP exchange current density is ∼10^-5 A cm^-2, that is only slightly smaller than that of Pt (111).

Dominating Role of Ni0 on the Interface of Ni/NiO for Enhanced Hydrogen Evolution Reaction

The research of a robust catalytic system based on single NiO_x electrocatalyst for hydrogen evolution reaction (HER) remains a huge challenge. Particularly, the factors that dominate the catalytic properties of NiO_x-based hybrids for HER have not been clearly demonstrated. Herein, a convenient protocol for the fabrication of NiO_x@bamboo-like carbon nanotube hybrids (NiO_x@BCNTs) is designed. The hybrids exhibit superb catalytic ability and considerable durability in alkaline solution. A benchmark HER current density of 10 mA cm^-2 has been achieved at an overpotential of ∼79 mV. In combination with the experimental results and density functional theory (DFT) calculations, this for the first time definitely validates that the inherent high Ni⁰ ratio and the Ni⁰ on the interface of Ni/NiO play a vital role in the outstanding catalytic performance. Especially, the Ni⁰ on the interface of Ni/NiO performs superior activity for water splitting compared with that of bulk Ni⁰. These conclusions provide guidance for the rational design of the future non-noble metallic catalysts.

Facile Synthesis of Vanadium-Doped Ni3S2 Nanowire Arrays as Active Electrocatalyst for Hydrogen Evolution Reaction

Ni₃S₂ nanowire arrays doped with vanadium(V) are directly grown on nickel foam by a facile one-step hydrothermal method. It is found that the doping can promote the formation of Ni₃S₂ nanowires at a low temperature. The doped nanowires show excellent electrocatalytic performance toward hydrogen evolution reaction (HER), and outperform pure Ni₃S₂ and other Ni₃S₂-based compounds. The stability test shows that the performance of V-doped Ni₃S₂ nanowires is improved and stabilized after thousands of linear sweep voltammetry test. The onset potential of V-doped Ni₃S₂ nanowire can be as low as 39 mV, which is comparable to platinum. The nanowire has an overpotential of 68 mV at 10 mA cm^-2, a relatively low Tafel slope of 112 mV dec^-1, good stability and high Faradaic efficiency. First-principles calculations show that the V-doping in Ni₃S₂ extremely enhances the free carrier density near the Fermi level, resulting in much improved catalytic activities. We expect that the doping can be an effective way to enhance the catalytic performance of metal disulfides in hydrogen evolution reaction and V-doped Ni₃S₂ nanowire is one of the most promising electrocatalysts for hydrogen production.

Seed-Mediated Growth of Colloidal Metal Nanocrystals

Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, composition, and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also determine their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a number of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics. New capabilities and future directions are also highlighted.

Realistic Surface Descriptions of Heterometallic Interfaces: The Case of TiWC Coated in Noble Metals

Noble metal-coated core-shell nanoparticles have been applied to a suite of catalytic applications, with the aim of decreasing the noble metal loading while ideally improving their performance. The chemistry and therefore activity at the surface of these materials are intimately related to the accurate description of the core-shell interface. Using density functional theory, we developed a procedure to obtain realistic surface topology descriptions at the heterometallic junction. This procedure was applied to a topical series of catalysts: Ti_0.1W_0.9C coated in atomically thin monolayers of noble metals. Our quantum chemical calculations provide access to both relevant surface descriptions of these materials and also rationalize several experimental observations. Our general procedure paves the way for the rationalization and prediction of next-generation heterometallic catalysts.

Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting

Better understanding of true electrochemical reaction behaviors in electrochemical energy devices has long been desired. It has been assumed so far that the reactions occur across the entire catalyst layer (CL), which is designed and fabricated uniformly with catalysts, conductors of protons and electrons, and pathways for reactants and products. By introducing a state-of-the-art characterization system, a thin, highly tunable liquid/gas diffusion layer (LGDL), and an innovative design of electrochemical proton exchange membrane electrolyzer cells (PEMECs), the electrochemical reactions on both microspatial and microtemporal scales are revealed for the first time. Surprisingly, reactions occur only on the CL adjacent to good electrical conductors. On the basis of these findings, new CL fabrications on the novel LGDLs exhibit more than 50 times higher mass activity than conventional catalyst-coated membranes in PEMECs. This discovery presents an opportunity to enhance the multiphase interfacial effects, maximizing the use of the catalysts and significantly reducing the cost of these devices.

Single-Step Synthesis of W2C Nanoparticle-Dispersed Carbon Electrocatalysts for Hydrogen Evolution Reactions Utilizing Phosphate Groups on Carbon Edge Sites

A novel, one-step protocol for the selective synthesis of W₂C nanoparticles from phosphotungstic acid (H₃PW₁₂O₄₀), a low-cost and commercially available tungsten compound, was developed. The nanoparticles had diameters of 1-50 nm and were dispersed on a carbon substrate. The W₂C nanoparticles were prepared by a simple operation sequence, involving impregnation of carbon black with H₃PW₁₂O₄₀ followed by calcination at 1000 °C. X-ray diffraction study revealed the selective formation of the W₂C phase in the samples prepared, whereas the tungsten carbide (WC) phase was present in the control prepared from H₂WO₄. Stable W₂C nanoparticles were obtained using this method owing to the presence of phosphate at the interfaces between the W₂C nanoparticles and the carbon substrates, which inhibited the diffusion of carbon atoms from the carbon substrates to the W₂C nanoparticles, leading to the formation of WC. The W₂C nanoparticles prepared showed an excellent catalytic activity for the hydrogen evolution reaction (HER), with low Tafel slopes of ∼50 mV/decade. The HER catalytic activity was notably high, being comparable to that of MoS₂, which is a promising alternative to Pt. The present method can potentially be applied to produce highly effective, low-cost, Pt-free electrocatalysts for the HER.

Ultrasmall and phase-pure W2C nanoparticles for efficient electrocatalytic and photoelectrochemical hydrogen evolution

Earlier research has been primarily focused on WC as one of the most promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER), whereas the other compound in this carbide family-W2C-has received far less attention. Our theoretical calculations suggest that such a focus is misplaced and W2C is potentially more HER-active than WC. Nevertheless, the preparation of phase pure and sintering-free W2C nanostructures represents a formidable challenge. Here we develop an improved carburization method and successfully prepare ultrasmall and phase-pure W2C nanoparticles. When evaluated for HER electrocatalysis, W2C nanoparticles exhibit a small onset overpotential of 50 mV, a Tafel slope of 45 mV dec-1 and outstanding long-term cycling stability, which are dramatically improved over all existing WC-based materials. In addition, the integration of W2C nanoparticles with p-type Si nanowires enables highly active and sustainable solar-driven hydrogen production. Our results highlight the great potential of this traditionally non-popular material in HER electrocatalysis.

Combined Photoemission Spectroscopy and Electrochemical Study of a Mixture of (Oxy)carbides as Potential Innovative Supports and Electrocatalysts

Active and stable non-noble metal materials, able to substitute Pt as catalyst or to reduce the Pt amount, are vitally important for the extended commercialization of energy conversion technologies, such as fuel cells and electrolyzers. Here, we report a fundamental study of nonstoichiometric tungsten carbide (WxC) and its interaction with titanium oxycarbide (TiOxCy) under electrochemical working conditions. In particular, the electrochemical activity and stability of the WxC/TiOxCy system toward the ethanol electrooxidation reaction (EOR) and hydrogen evolution reaction (HER) are investigated. The chemical changes caused by the applied potential are established by combining photoemission spectroscopy and electrochemistry. WxC is not active toward the ethanol electrooxidation reaction at room temperature but it is highly stable under these conditions thanks to the formation of a passive thin film on the surface, consisting mainly of WO2 and W2O5, which prevents the full oxidation of WxC. In addition, WxC is able to adsorb ethanol, forming ethoxy groups on the surface, which constitutes the first step for the ethanol oxidation. The interaction between WxC and TiOxCy plays an important role in the electrochemical stability of WxC since specific orientations of the substrate are able to stabilize WxC and prevent its corrosion. The beneficial interaction with the substrate and the specific surface chemistry makes tungsten carbide a good electrocatalyst support or cocatalyst for direct ethanol fuel cells. However, WxC is active toward the HER and chemically stable under hydrogen reduction conditions, since no changes in the chemical composition or dissolution of the film are observed. This makes tungsten carbide a good candidate as electrocatalyst support or cocatalyst for the electrochemical production of hydrogen.