Nickel Phosphides Fabricated through a Codeposition-Annealing Technique as Low-Cost Electrocatalytic Layers for Efficient Hydrogen Evolution Reaction

Abstract

Water splitting will be one of the most strategic techniques in the upcoming hydrogen-based economy. In this context, the development of efficient and low-cost Pt-free electrocatalysts is crucial to make it economically viable. The present work proposes a low-cost and scalable methodology to produce electrocatalytic layers based on nickel phosphide for hydrogen evolution reaction. In particular, a nickel-phosphorus solid solution is electrolytically codeposited together with red phosphorus particles. This approach overcomes the compositional limit typical of electrodeposited Ni-P by providing a supplementary phosphorous source directly embedded in the layer and makes it possible to synthesize high-P phosphides such as Ni12P5 and Ni2P. The obtained composites are subjected to different annealing cycles to precipitate phosphides, evidencing a major influence of process conditions on the final phase composition. X-ray photoelectron spectroscopy reveals the presence of a phosphorus-depleted region in correspondence of the surface of the samples. Finally, layers are tested to assess their electrocatalytic performances, and the effects of annealing time and catalyst loading are investigated. Samples with an optimized content of Ni2P evidence the lowest overpotential values, with 224 mV at 10 mA/cm2, and good stability over time.