Core-Shell-Structured Tungsten Carbide Encapsulated within Nitrogen-Doped Carbon Spheres for Enhanced Hydrogen Evolution

Vertical graphene nanowalls supported hybrid W2C/WOx composite material as an efficient non-noble metal electrocatalyst for hydrogen evolution

Research for the development of noble metal-free electrodes for hydrogen evolution has blossomed in recent years. Transition metal carbides compounds, such as W2C, have been considered as a promising alternative to replace Pt-family metals as electrocatalysts towards hydrogen evolution reaction (HER). Moreover, hybridization of TMCs with graphene nanostructures has emerged as a reliable strategy for the preparation of compounds with high surface to volume ratio and abundant active sites. The present study focuses in the preparation of tungsten carbide/oxide compounds deposited in a three-dimensional vertical graphene nanowalls (VGNW) substrate via chemical vapor deposition, magnetron sputtering and thermal annealing processes. Structural and chemical characterization reveals the partial carburization and oxidation of the W film sputtered on the VGNWs, due to C and O migration from VGNWs towards W during the high temperature annealing process. Electrochemical characterization shows the enhanced performance of the nanostructured hybrid W2C/WOx on VGNW compound towards HER, when compared with planar W2C/WOx films. The W2C/WOx nanoparticles on VGNWs require an overpotential of -252 mV for the generation of 10 mA cm-2. Chronoamperometry tests in high overpotentials reveal the compounds stability while sustaining high currents, in the order of hundreds of mA. Post-chronoamperometry test XPS characterization unveils the formation of a W hydroxide layer which favours hydrogen evolution in acidic electrolytes. We aspire that the presented insights can be valuable for those working on the preparation of hybrid electrodes for electrochemical processes.

Urchin-Like Structured MoO2 /Mo3 P/Mo2 C Triple-Interface Heterojunction Encapsulated within Nitrogen-Doped Carbon for Enhanced Hydrogen Evolution Reaction

The development of highly efficient and cost-effective hydrogen evolution reaction (HER) catalysts is highly desirable to efficiently promote the HER process, especially under alkaline condition. Herein, a polyoxometalates-organic-complex-induced carbonization method is developed to construct MoO₂ /Mo₃ P/Mo₂ C triple-interface heterojunction encapsulated into nitrogen-doped carbon with urchin-like structure using ammonium phosphomolybdate and dopamine. Furthermore, the mass ratio of dopamine and ammonium phosphomolybdate is found critical for the successful formation of such triple-interface heterojunction. Theoretical calculation results demonstrate that such triple-interface heterojunctions possess thermodynamically favorable water dissociation Gibbs free energy (ΔG_H2O ) of -1.28 eV and hydrogen adsorption Gibbs free energy (ΔG_H* ) of -0.41 eV due to the synergistic effect of Mo₂ C and Mo₃ P as water dissociation site and H* adsorption/desorption sites during the HER process in comparison to the corresponding single components. Notably, the optimal heterostructures exhibit the highest HER activity with the low overpotential of 69 mV at the current density of 10 mA cm^-2 and a small Tafel slope of 60.4 mV dec^-1 as well as good long-term stability for 125 h. Such remarkable results have been theoretically and experimentally proven to be due to the synergistic effect between the unique heterostructures and the encapsulated nitrogen-doped carbon.

Recent Advancements in Two-Dimensional Layered Molybdenum and Tungsten Carbide-Based Materials for Efficient Hydrogen Evolution Reactions

Green and renewable energy is the key to overcoming energy-related challenges such as fossil-fuel depletion and the worsening of environmental habituation. Among the different clean energy sources, hydrogen is considered the most impactful energy carrier and is touted as an alternate fuel for clean energy needs. Even though noble metal catalysts such as Pt, Pd, and Au exhibit excellent hydrogen evolution reaction (HER) activity in acid media, their earth abundance and capital costs are highly debatable. Hence, developing cost-effective, earth-abundant, and conductive electrocatalysts is crucial. In particular, various two-dimensional (2D) transition metal carbides and their compounds are gradually emerging as potential alternatives to noble metal-based catalysts. Owing to their improved hydrophilicity, good conductivity, and large surface areas, these 2D materials show superior stability and excellent catalytic performances during the HER process. This review article is a compilation of the different synthetic protocols, their impact, effects of doping on molybdenum and tungsten carbides and their derivatives, and their application in the HER process. The paper is more focused on the detailed strategies for improving the HER activity, highlights the limits of molybdenum and tungsten carbide-based electrocatalysts in electro-catalytic process, and elaborates on the future advancements expected in this field.

Superstructures of Organic-Polyoxometalate Co-crystals as Precursors for Hydrogen Evolution Electrocatalysts

Molybdenum-based carbides and nitrides have been considered as catalysts for the hydrogen evolution reaction (HER). One of the challenges in using Mo-based HER electrocatalysts is establishing well-defined precursors which can be transformed into Mo-based carbides/nitrides with controllable structure and porosity. We report the synthesis of a series of superstructures consisting of organic-polyoxometalate co-crystals (O-POCs) as a new type of metal-organic precursor to synthesize Mo-based carbides/nitrides in a controlled fashion and to use them for efficient catalytic hydrogen production. This protocol enables to create electrocatalysts composed of abundant nanocrystallites and heterojunctions with tunable micro- and nanostructure and mesoporosity. The best performing electrocatalyst shows high HER activity and stability with a low overpotential of 162 mV at 100 mA cm-2 (in comparison to Pt/C with 263 mV), which makes it one of the best non-noble metal HER catalysts in alkaline media and seawater.

Stable and Antisintering Tungsten Carbides with Controllable Active Phase for Selective Cleavage of Aryl Ether C-O Bonds

Transition-metal carbides are important materials in heterogeneous catalysis. It remains challenging yet attractive in nanoscience to construct the active phase of carbide catalysts in a controllable manner and keep a sintering-resistant property in redox reactions, especially hydroprocessing. In this work, an integrated strategy was presented to synthesize stable and well-defined tungsten carbide nanoparticles (NPs) by assembling the metal precursor onto carbon nanotubes (CNTs), wrapping a thin polymeric layer, and following a controlled carburization. The polymer served as a soft carbon source to modulate the metal/carbon ratio in the carbides and introduced amorphous carbons around the carbides to prevent the NPs from sintering. The as-built p-W_xC/CNT displayed high stability in the hydrogenolysis of aryl ether C-O bond in guaiacol for more than 150 h. Its activity was more than two and six times higher than those prepared via typical temperature-programmed reduction with gaseous carbon (W_xC/CNT-TPR) and carbothermal reduction with intrinsic carbon support (W_xC/CNT-CTR), respectively. Our p-W_xC/CNT catalyst also achieved high efficiency for selective cleavage of the aryl ether C-O bonds in lignin-derived aromatic ethers, including anisole, dimethoxylphenol, and diphenyl ether, with a robust lifespan.

Core-shell nanostructured electrocatalysts for water splitting

As the cornerstone of the hydrogen economy, water electrolysis consisting of the hydrogen and oxygen evolution reactions (HER and OER) greatly needs cost-efficient electrocatalysts that can decrease the dynamic overpotential and save on energy consumption. Over past years, observable progress has been made by constructing core-shell structures free from or with few noble-metals. They afford particular merits, e.g., a highly-exposed active surface, modulated electronic configurations, strain effects, interfacial synergy, or reinforced stability, to promote the kinetics and electrocatalytic performance of the HER, OER and overall water splitting. So far, a large variety of inorganics (carbon and transition-metal related components) have been introduced into core-shell electrocatalysts. Herein, representative efforts and progress are summarized with a clear classification of core and shell components, to access comprehensive insights into electrochemical processes that proceed on surfaces or interfaces. Finally, a perspective on the future development of core-shell electrocatalysts is offered. The overall aim is to shed some light on the exploration of emerging materials for energy conversion and storage.

Designed synthesis of WC-based nanocomposites as low-cost, efficient and stable electrocatalysts for the hydrogen evolution reaction

In this study, effectively conductive rGO (reduced graphene oxide) was used as the supporter both to promote charge transfer and to refine particle size of WC, to realize efficient and stable HER performance.

Electric Field-Driven Interfacial Alloying for in Situ Fabrication of Nano-Mo2C on Carbon Fabric as Cathode toward Efficient Hydrogen Generation

A binderless composite cathode for efficient electrocatalytic hydrogen evolution reaction (HER), Mo₂C-decorated carbon cloth (denoted as CC/MC), is simply fabricated via a novel and unique strategy which involves a solid-solid phase interfacial electrochemical reaction between carbon fiber and bulk-MoS₂ in molten NaCl-KCl (700 °C). MoS₂, evenly coated on carbon cloth (CC), is electrochemically reduced in situ and readily reacts with the carbon fibers of CC current collector to form a Mo₂C nanoparticle layer. The experiment and calculation results show that the applied electric field results in a declining migration barrier of Mo vacancies in Mo₂C lattice, which promotes the diffusion of Mo atoms into carbon across the interfacial Mo₂C layer, thereby impelling the combination of Mo with C in depth. The electrochemical tests indicate that the optimized cathode (CC/MC-2) exhibits a small overpotential of 134.4 mV at 10 mA cm^-2 and stays stable for HER in acidic media. The catalytic capacity for N₂ reduction of CC/MC-2 is analyzed. In addition, a Ni-doped Mo₂C-modified carbon fabric electrode with enhanced HER activity (η₁₀ = 96.6 mV) can be prepared through a similar process.

Ultrafine Dual-Phased Carbide Nanocrystals Confined in Porous Nitrogen-Doped Carbon Dodecahedrons for Efficient Hydrogen Evolution Reaction

Designing novel non-noble electrocatalysts with controlled structures and composition remains a great challenge for efficient hydrogen evolution reaction (HER). Herein, a rational synthesis of ultrafine carbide nanocrystals confined in porous nitrogen-doped carbon dodecahedrons (PNCDs) by annealing functional zeolitic imidazolate framework (ZIF-8) with molybdate or tungstate is reported. By controlling the substitution amount of MO₄ units (M = Mo or W) in the ZIF-8 framework, dual-phase carbide nanocrystals confined in PNCDs (denoted as MC-M₂ C/PNCDs) can be obtained, which exhibit superior activity toward the HER to the single-phased MC/PNCDs and M₂ C/PNCDs. The evenly distributed ultrafine nanocrystals favor the exposure of active sites. PNCDs as the support facilitate charge transfer and protect the nanocrystals from aggregation during the HER process. Moreover, the strong coupling interactions between MC and M₂ C provide beneficial sites for both water dissociation and hydrogen desorption. This work highlights a new feasible strategy to explore efficient electrocatalysts via engineering on nanostructure and composition.

Robust NiCoP/CoP Heterostructures for Highly Efficient Hydrogen Evolution Electrocatalysis in Alkaline Solution

Electrocatalytic hydrogen evolution reaction, the cornerstone of the emerging hydrogen economy, can be essentially facilitated by robustly heterostructural electrocatalysts. Herein, we report a highly active and stably heterostructural electrocatalyst consisting of NiCoP nanowires decorated with CoP nanoparticles on a nickel foam (NiCoP-CoP/NF) for effective hydrogen evolution. The CoP nanoparticles are strongly interfaced with NiCoP nanowires producing abundant electrocatalytically active sites. Combined with the integrated catalyst design, NiCoP-CoP/NF affords a remarkable hydrogen evolution performance in terms of high activity, enhanced kinetics, and outstanding durability in an alkaline electrolyte, superior to most of the Co (or Ni)-phosphide-based catalysts reported previously. Density functional theory calculations demonstrate that there is an interfacial effect between NiCoP and CoP, which allows a preferable hydrogen adsorption and thus contributes to the significantly enhanced performance. Furthermore, an electrolyzer employing NiCoP-CoP/NF as the cathode and RuO₂/NF as the anode (NiCoP-CoP/NF||RuO₂/NF) exhibits excellent water-splitting activity and outstanding durability, which is comparable to that of the benchmark Pt-C/NF||RuO₂/NF electrolyzer.

Robust Catalysis on 2D Materials Encapsulating Metals: Concept, Application, and Perspective

Great endeavors are undertaken to search for low-cost, rich-reserve, and highly efficient alternatives to replace precious-metal catalysts, in order to cut costs and improve the efficiency of catalysts in industry. However, one major problem in metal catalysts, especially nonprecious-metal catalysts, is their poor stability in real catalytic processes. Recently, a novel and promising strategy to construct 2D materials encapsulating nonprecious-metal catalysts has exhibited inimitable advantages toward catalysis, especially under harsh conditions (e.g., strong acidity or alkalinity, high temperature, and high overpotential). The concept, which originates from unique electron penetration through the 2D crystal layer from the encapsulated metals to promote a catalytic reaction on the outermost surface of the 2D crystal, has been widely applied in a variety of reactions under harsh conditions. It has been vividly described as "chainmail for catalyst." Herein, recent progress concerning this chainmail catalyst is reviewed, particularly focusing on the structural design and control with the associated electronic properties of such heterostructure catalysts, and also on their extensive applications in fuel cells, water splitting, CO₂ conversion, solar cells, metal-air batteries, and heterogeneous catalysis. In addition, the current challenges that are faced in fundamental research and industrial application, and future opportunities for these fantastic catalytic materials are discussed.

Electrospinning Hetero-Nanofibers of Fe3 C-Mo2 C/Nitrogen-Doped-Carbon as Efficient Electrocatalysts for Hydrogen Evolution

Heterostructured electrocatalysts with multiple active components are expected to synchronously address the two elementary steps in the hydrogen evolution reaction (HER), which require varied hydrogen-binding strength on the catalyst surface. Herein, electrospinning followed by a pyrolysis is introduced to design Fe₃ C-Mo₂ C/nitrogen-doped carbon (Fe₃ C-Mo₂ C/NC) hetero-nanofibers (HNFs) with tunable composition, leading to abundant Fe₃ C-Mo₂ C hetero-interfaces for synergy in electrocatalysis. Owing to the strong hydrogen binding on Mo₂ C and the relatively weak one on Fe₃ C, the hetero-interfaces of Fe₃ C-Mo₂ C remarkably promote HER kinetics and intrinsic activity. Additionally, the loose and porous N-doped carbon matrix, as a result of Fe-catalyzed carbonization, ensures the fast transport of electrolytes and electrons, thus minimizing diffusion limitation. As expected, the optimized Fe₃ C-Mo₂ C/NC HNFs afforded a low overpotential of 116 mV at a current density of -10 mA cm^-2 and striking kinetics metrics (onset overpotential: 42 mV, Tafel slope: 43 mV dec^-1 ) in 0.5 m H₂ SO₄ , outperforming most recently reported noble-metal-free electrocatalysts.