Harvesting Clean Energy Through H₂ Production Using Cobalt-Boride-Based Nanocatalyst

Increase in the energy requirement and emission of greenhouse gases have been a growing concern. Hydrogen is recognized as a clean fuel and a promising solution for energy storage. At present, hydrogen required for fuel cell (FC) is mostly produced at industrial scales using the steam reforming of natural gas. These industries possibly leave CO and CO₂ into the atmosphere, which are the major known reasons for the devastating climate changes witnessed today. Moreover, improper separation of these carbon contaminants from H₂, especially CO (even at ppm level), affects the performance of FC by catalyst poisoning. “Hydrolysis of chemical hydrides” and “electrochemical water splitting,” through renewable energy sources, are considered as the cleanest and simplest techniques to produce FC grade H₂ for onboard and off-board applications, respectively. Herein, the role of low-cost cobalt-boride (Co-B)-based nanocatalysts for both these applications is summarized.

Chemical hydrides have high hydrogen storage capacity in terms of volumetric and gravimetric efficiencies and are promising candidates to obtain pure hydrogen at a very high rate at room temperatures for on-broad applications. In the presence of certain catalysts, a large amount of pure hydrogen gas is produced by the hydrolysis of chemical hydrides. Noble metal catalysts (e.g., Ru and Pt) enhance the hydrogen production rate but are not viable for industrial application owing to their high cost and low availability. Low-cost amorphous Co-B nanocatalysts, prepared by reduction of metal salts, have attracted great attention in the catalysis community, owing to their unique properties such as isotropic structure, high concentration of coordinative unsaturated sites, relevant chemical stability, and low cost. However, Co-B nanoparticles agglomeration is a major problem, but it can be solved by introducing transition metals like Mo, W, and Cr as a possible atomic diffusion barrier. These promoter metals, mainly in the form of oxides, are efficient and even a small atomic concentration is able to significantly increase the surface area of the metal-boride catalyst nanoparticles by avoiding agglomeration. Nevertheless recovering and reusing powder catalysts is still an issue, which can be addressed by forming thin films on a substrate. Pulsed laser deposition (PLD) has emerged as a viable method for the production of nanoparticles on the surface of the thin films. By changing the PLD parameters, namely, energy and pulse duration, the morphology and the structure of the film can be optimized for a given application. Co-B catalysts developed by PLD in the form of nanoparticle-assembled films showed a performance similar to that of Pt metal and better than Pd metal for hydrogen production in the hydrolysis reaction.

For off-board purposes, a practical and sustainable way to produce hydrogen is electrolysis of water, driven by clean electric power that can be generated by renewable energy sources, such as photovoltaic and wind. To build highly efficient and cost-effective electrolyzer for this purpose, one of the key challenges is to develop active, stable, inexpensive, and scalable electrocatalysts for the two half reactions of water splitting, namely, oxygen and hydrogen evolution reactions. Although noble metal such as Pt is known as the best hydrogen-evolving catalyst in acidic solutions, the low abundance and high cost of such precious metal limit their large-scale application. Metal borides such as Co-B were also found to be excellent electrocatalysts for hydrogen evolution reaction (HER), active in wide pH ranging from 4 to 9. A significant improvement in activity and stability of Co-B electrocatalyst was obtained after introducing other transition metals, specifically Ni and Mo in Co-B showing electrocatalytic activity comparable to Pt. Co-Mo-B was also found to be equally active for oxygen evolutions in alkaline media. Examples given in this chapter clearly indicate that Co-B-based nanocatalysts can bridge the gap between the noble and nonmetal catalysts, especially for energy carrier generation.