Electrochemical Synthesis of Nickel-Molybdenum Alloys and Nickel-Molybdenum-Oxygen Composites from Citrate-Ammonium Baths and Their Electrocatalytic Properties for Hydrogen Evolution Reaction

Abstract

<p>Renewable energy is essential to minimize climate changes and improve sustainability. Especially, hydrogen is considered as an alternative fuel to fossil fuels due to its high energy density and zero pollutant emission. Besides the complicated fossil-fuel-derived synthesis methods, hydrogen gas can be generated simultaneously with oxygen gas through electrochemical water splitting. To maximize the efficiency, electrocatalysts need to be optimized to further reduce the overpotential for hydrogen and accelerate the rate of water splitting. Many strategies have been approached to enhance the catalyst efficiency, such as modification of the electronic structure as well as increase in electron transports and the number of active sites. These parameters can be controlled by synthesis methods and composition. In addition to the morphology and composition control as other synthesis methods, electrodeposition is a scalable and low-cost fabrication technique which not only provides excellent tunability of electronic structures by deposition at non-equilibrium but also can enhance the charge transfer of catalysts by direct deposition of material onto conductive substrate. Owing to the affinity of unpaired d-band electrons for chemisorption of hydrogen atoms and multiple oxidation states, transition metals have been combined with various elements (e.g., metals and oxygen) to tune the electronic structures and create the synergistic effects of the electrocatalysts. Additionally, heterostructures of the catalysts can facilitate the adsorption of different intermediates which are generated from constituent reactions of the water splitting.</p><p>Nickel with minimum energy for hydrogen adsorption among various nonprecious metals exhibits the most efficient catalytic activities as a binary alloy with molybdenum in alkaline HER. Hydrogen spillover and averaging effect between the hydrogen adsorption energies of nickel and molybdenum facilitate the catalytic activity of nickel molybdenum alloy. Moreover, the incorporation of oxygen to form Ni-Mo-O composites promotes the dissociation of water and the production of hydrogen intermediates. The overarching goal of the research is to develop a cost-effective and scalable electrodeposition techniques to synthesize nickel-based electrocatalysts for hydrogen evolution reaction. The objective can be achieved by firstly correlating the synthesis parameters (i.e., electrolyte composition and electrodeposition parameters) and the material properties of deposits (e.g., composition, morphology, and crystal structure). The electrocatalytic activities (e.g., overpotential, onset potential, Tafel slope) are correlated with the material properties.</p>