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

Solvent-Free, One-Pot Synthesis of Tungsten Semi-Carbide for Stable and Self-Hydrating Short-Side-Chain-Based Polymer Electrolyte Membrane for Low-Humidity Hydrogen Fuel Cells

Polymer electrolyte membranes (PEMs) that promote fast and selective ionic transport at low relative humidity (RH) are of high demand for energy conversion devices, particularly in hydrogen fuel cells. Herein, we report a facile and solvent free synthesis of tungsten semi-carbide (W2C@NC) and its incorporation onto short side chain (SSC)-based membrane matrix to facilitate water holding and water-assisted humidification generated by the reaction of crossover gas molecules. In the present study, the influence of W2C@NC on the membrane matrix is widely investigated through its microstructure, physicochemical properties, proton conductivity, and fuel cell performance. It is demonstrated that addition of W2C@NC facilitates membrane hydration via in situ water generation, thus preventing fuel crossover across the membrane. In addition, W2C@NC contributes toward low-humidity polymer electrolyte fuel cell (PEFC) operation. The study revealed minimal differences in cell performance between fully humidified and low RH conditions for composite membranes, with a noteworthy improvement in performance observed even under completely dry conditions compared to pristine membranes. Apart from good thermal and mechanical stabilities, 81% of initial OCV and 72.86% of current density was retained even after 100 h of accelerated stress test (AST), which opens further perspectives for development of perfluoro sulfonic acid (PFSA) based low RH proton exchange membrane fuel cells (PEMFCs).

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.

MoSx on Nitrogen-Doped Graphene for High-Efficiency Hydrogen Evolution Reaction: Unraveling the Mechanisms of Unique Interfacial Bonding for Efficient Charge Transport and Stability

Functional nanostructures with abundant exposed active sites and facile charge transport through conductive scaffolds to active sites are pivotal for developing an advanced and efficient electrocatalyst for water splitting. In the present study, by coating ∼3 nm MoS_x on nitrogen-doped graphene (NG) pre-engrafted on a flexible carbon cloth (MNG) as a model system, an extremely low Tafel slope of 39.6 mV dec^-1 with cyclic stability up to 5000 cycles is obtained. The specific fraction of N on the NG framework is also analyzed by X-ray photoelectron spectroscopy and X-ray absorption near edge spectroscopy with synchrotron radiation light sources, and it is found that the MoS_x particles are selectively positioned on the specific graphitic N sites, forming the unique Mo-N-C bonding state. This Mo-N-C bonding is founded to facilitate highly effective charge transfer directly to the active sulfur sites on the edges of MoS_x, leading to a highly improved hydrogen evolution reaction (HER) with excellent stability (95% retention @ 5000 cycles). The functional anchoring of MoS_x by such bonding prevents particle aggregation, which plays a significant role in maintaining the stability and activity of the catalyst. Furthermore, it has been revealed that MNG samples with adequately high amounts of both pyridinic and graphitic N result in the best HER performance. This work helps in understanding the mechanisms and bonding interactions within various catalysts and the scaffold electrode.

Synthesis of Mo2C and W2C Nanoparticle Electrocatalysts for the Efficient Hydrogen Evolution Reaction in Alkali and Acid Electrolytes

The synthesis of low cost, high efficacy, and durable hydrogen evolution electrocatalysts from the non-noble metal group is a major challenge. Herein, we establish a simple and inexpensive chemical reduction method for producing molybdenum carbide (Mo2C) and tungsten carbide (W2C) nanoparticles that are efficient electrocatalysts in alkali and acid electrolytes for hydrogen evolution reactions (HER). Mo2C exhibits outstanding electrocatalytic behavior with an overpotential of -134 mV in acid medium and of -116 mV in alkaline medium, while W2C nanoparticles require an overpotential of -173 mV in acidic medium and -130 mV in alkaline medium to attain a current density of 10 mA cm-2. The observed results prove the capability of high- and low-pH active electrocatalysts of Mo2C and W2C nanoparticles to be efficient systems for hydrogen production through HER water electrolysis.

Three-dimensional macroporous W2C inverse opal arrays for the efficient hydrogen evolution reaction

The development of a low-cost and durable non-precious metal-based electrocatalyst for the hydrogen evolution reaction (HER) is important to realize highly efficient overall water splitting. Here, we report the design and fabrication of a binder-free electrocatalyst of three-dimensional macroporous ditungsten carbide (W2C) inverse opal (W2C IO) arrays by a facile thermal carburization process with WO3 IO as a template. The as-fabricated W2C IO exhibits superior electrocatalytic performance in 0.5 M H2SO4 solution in terms of a low overpotential of 146 mV to reach a current density of 10 mA cm-2, a low Tafel slope of 78 mV dec-1 and excellent long-term stability. The superior performance can be attributed to the favorable electronic structure and hydrogen adsorption Gibbs free energy of W2C evidenced by theory calculations and the enhancement of the charge/mass transfer process by the 3D macroporous arrayed electrode design.

Highly Efficient and Stable Hydrogen Production in All pH Range by Two-Dimensional Structured Metal-Doped Tungsten Semicarbides

Transition-metal-doped tungsten semicarbide nanosheets (M-doped W₂C NSs, M=Fe, Co, and Ni) have been synthesized through carburization of the mixture of tungsten trioxide, polyvinylpyrrolidone, and metal dopant. The nanosheets grow directly on the W mesh and have the lateral dimension of several hundreds of nm to a few μm with a thickness of few tens nm. It is demonstrated that the M-doped W₂C NSs exhibit superior electrocatalytic activity for hydrogen evolution reaction (HER). Impressively, the Ni-doped W₂C NSs (2 at% Ni) with the optimized HER activity show extremely low onset overpotentials of 4, 9, and 19 mV and modest Tafel slopes of 39, 51, and 87 mV dec^-1 in acidic (pH=0), neutral (pH=7.2), and basic (pH=14) solutions, respectively, which is close to the commercial Pt/C catalyst. Density functional theory (DFT) calculations also demonstrate that the Gibbs free energy for H adsorption of Ni-W₂C is much closer to the optimal value ∆G_H⁎ = -0.073 eV as compared to -0.16 eV of W₂C. Furthermore, nearly 100% Faradaic efficiency and long-term stability are obtained in those environments. This realization of highly tolerant metal semicarbide catalyst performing on par with commercial Pt/C in all range of pH offers a key step towards industrially electrochemical water splitting.

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

As the key of hydrogen economy, electrocatalytic hydrogen evolution reactions (HERs) depend on the availability of cost-efficient electrocatalysts. Over the past years, there is a rapid rise in noble-metal-free electrocatalysts. Among them, transition metal carbides (TMCs) are highlighted due to their structural and electronic merits, e.g., high conductivity, metallic band states, tunable surface/bulk architectures, etc. Herein, representative efforts and progress made on TMCs are comprehensively reviewed, focusing on the noble-metal-like electronic configuration and the relevant structural/electronic modulation. Briefly, specific nanostructures and carbon-based hybrids are introduced to increase active-site abundance and to promote mass transportation, and heteroatom doping and heterointerface engineering are encouraged to optimize the chemical configurations of active sites toward intrinsically boosted HER kinetics. Finally, a perspective on the future development of TMC electrocatalysts is offered. The overall aim is to shed some light on the exploration of emerging materials in energy chemistry.

Carbon-encapsulated multi-phase nanocomposite of W2C@WC1-x as a highly active and stable electrocatalyst for hydrogen generation

The major challenges related to the activity, stability, and cost of electrocatalysts are being increasingly raised to achieve highly efficient and cost-effective hydrogen generation. Herein, multiphase nanocomposites of W₂C@WC_1-x encapsulated within graphitic carbon layers were prepared via a facile and effective process of electrical explosion of wires and subsequent heat treatment to serve as a highly active and stable electrocatalyst without any noble metal for hydrogen generation. The single-phase comprising less than 15 nm WC_1-x nanoparticles embedded in a lump of amorphous carbon were successfully synthesized via the EEW process in oleic acid used as a carbon source at room temperature. Subsequent heat treatment facilitates the desired phase transition of WC_1-x to W₂C without the formation of any secondary phases, maintaining the initial particle size and simultaneously eliminating excess amorphous carbon adhered to the nanoparticles. The few graphitic carbon layer-encapsulated nanoparticles with the main W₂C phase prepared by this simple method exhibit high efficiency for hydrogen generation with a low overpotential of 240 mV at a current density of 10 mA cm^-2 and a low Tafel slope of 86 mV dec^-1. Moreover, the overpotential is well maintained at a constantly injected current density of 10 mA cm^-2 for 100 h with a low η₁₀₀/η_i value of 1.03 (η_i: initial overpotential, η₁₀₀: overpotential after 100 h), demonstrating superior catalytic stability in acidic media. This work proposes and evaluates a facile strategy for the synthesis of highly efficient electrocatalysts based on metal carbides without noble metals.