Cobalt-Tungsten-Boron as an Active Electrocatalyst for Water Electrolysis

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.

Construct of an Electrodeposited Cobalt-Molybdenum Film and Evaluation of Its Efficiency in Hydrogen Evolution

Hydrogen is a valuable clean energy source, and electrolysis to produce hydrogen from water is a crucial component. However, a major problem of hydrogen generation by electrolysis is its large overpotential and poor economics. To reduce the overpotential, we mainly use nickel foam and Co-Mo ions as feedstock and create an efficient catalytic material by electrodeposition. The Co-Mo interaction improves the current efficiency. In 1 mol/L NaOH solution, the overpotential of the Co-Mo-NF composites was low when the current density is -10 mA/cm², with the best value reaching 45.3 mV, which is less than those of Co-NF (94.4 mV) and Mo-NF (88.2 mV). All deposits had similar Tafel slopes in the 77.9 mV/decade range. The catalyst does not just have a favorable effect on hydrogen formation but also has a surprisingly high double-layer capacitance (up to 180 mF/cm²) and good stability. This research provides an impactful approach for developing a non-precious metal HER catalyst for industrial hydrogen production.

Transition Metal Non-Oxides as Electrocatalysts: Advantages and Challenges

The identification of hydrogen as green fuel in the near future has stirred global realization toward a sustainable outlook and thus boosted extensive research in the field of water electrolysis focusing on the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). A huge class of compounds consisting of transition metal-based nitrides, carbides, chalcogenides, phosphides, and borides, which can be collectively termed transition metal non-oxides (TMNOs), has emerged recently as an efficient class of electrocatalysts in terms of performance and longevity when compared to transition metal oxides (TMOs). Moreover, the superiority of TMNOs over TMOs to effectively catalyze not only OERs but also HERs and ORRs renders bifunctionality and even trifunctionality in some cases and therefore can replace conventional noble metal electrocatalysts. In this review, the crystal structure and phases of different classes of nanostructured TMNOs are extensively discussed, focusing on recent advances in design strategies by various regulatory synthetic routes, and hence diversified properties of TMNOs are identified to serve as next-generation bi/trifunctional electrocatalysts. The challenges and future perspectives of materials in the field of energy conversion and storage aiding toward a better hydrogen economy are also discussed in this review.

Research Progress of Oxygen Evolution Reaction Catalysts for Electrochemical Water Splitting

The development of a low-cost and high-efficiency oxygen evolution reaction (OER) catalyst is essential to meet the future industrial demand for hydrogen production by electrochemical water splitting. Given the limited reserves of noble metals and many competitive applications in environmental protection, new energy, and chemical industries, many studies have focused on exploring new and efficient non-noble metal catalytic systems, improving the understanding of the OER mechanism of non-noble metal surfaces, and designing electrocatalysts with higher activity than traditional noble metals. This Review summarizes the research progress of anode OER catalysts for hydrogen production by electrochemical water splitting in recent years, for noble metal and non-noble metal catalysts, where non-noble metal catalysts are highlighted. The categories are as follows: (1) Transition metal-based compounds, including transition metal-based oxides, transition metal-based layered hydroxides, and transition metal-based sulfides, phosphides, selenides, borides, carbides, and nitrides. Transition metal-based oxides can also be divided into perovskite, spinel, amorphous, rock-salt-type, and lithium oxides according to their different structures. (2) Carbonaceous materials and their composite materials with transition metals. (3) Transition metal-based metal-organic frameworks and their derivatives. Finally, the challenges and future development of the OER process of water splitting are discussed.

Hierarchical Nanostructured Co-Mo-B/CoMoO4-x Amorphous Composite for the Alkaline Hydrogen Evolution Reaction

Transition metal borides (TMBs) are a class of important but less well-explored electrocatalytic materials for water splitting. The lack of an advanced methodology to synthesize complex nanostructured TMBs with tunable surface properties is a major obstacle to the exploration of the full potential of TMBs for electrocatalytic applications. Here, we report the facile fabrication of a cobalt foam (CF)-supported hierarchical nanostructured Co-Mo-B/CoMoO_4-x composite using a hydrothermal method, followed by annealing and NaBH₄ reduction treatments. Our study found that NaBH₄ reduction of CoMoO₄ resulted in the concurrent formation of amorphous Co-Mo-B and an O-vacancy-rich CoMoO_4-x substrate, which cooperatively catalyzed the hydrogen evolution reaction (HER) in an alkaline electrolyte. The hierarchical nanoporous structure derived from the dehydration and partial reduction reactions of the CoMoO₄·nH₂O precursor could offer ample accessible active sites, as well as interconnected channels for rapid mass transfer. In addition, the in situ growth of electrically conductive Co-Mo-B nanoparticles on the defective structured CoMoO_4-x substrate imparted the electrocatalyst with good electrical conductivity. As a result, the Co-Mo-B/CoMoO_4-x/CF catalyst showed impressively high activity and outstanding stability for the alkaline HER, outperforming most reported TMB electrocatalysts. For instance, it required an overpotential of 55 mV to afford 10 mA·cm^-2 and showed a fluctuation of only ±8 mV in a 100 h constant-current test at 100 mA·cm^-2.

Designing transition-metal-boride-based electrocatalysts for applications in electrochemical water splitting

Investigating renewable and clean energy materials as alternatives to fossil fuels can be foreseen as a potential solution to the global problems of energy shortages and environmental pollution. Recently, transition metal boride (TMB)-based materials have emerged as the rising star as efficient electrocatalysts for hydrogen evolution reaction (HER) and/or oxygen evolution reaction (OER). In this review, an overview of the most recent developments in the use of TMB-based materials as electrocatalysts for HER/OER or overall water splitting has been presented. Initially, we provide a comprehensive introduction of the fundamentals of electrochemical water splitting. Then, the synthesis approaches of TMB materials are summarized and compared. Emphasis is put on the various strategies for further improving the electrocatalytic performance of TMBs. Finally, challenges and future perspectives for TMBs in water-splitting applications are proposed.