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

Water splitting will be one of the most strategictechniques in the upcoming hydrogen-based economy. In thiscontext, the development of efficient and low-cost Pt-freeelectrocatalysts is crucial to make it economically viable. Thepresent work proposes a low-cost and scalable methodology toproduce electrocatalytic layers based on nickel phosphide forhydrogen evolution reaction. In particular, a nickel−phosphorussolid solution is electrolytically codeposited together with redphosphorus particles. This approach overcomes the compositionallimit typical of electrodeposited Ni−P by providing a supple-mentary phosphorous source directly embedded in the layer andmakes it possible to synthesize high-P phosphides such as Ni12P5and Ni2P. The obtained composites are subjected to differentannealing cycles to precipitate phosphides, evidencing a major influence of process conditions on thefinal phase composition. X-rayphotoelectron 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 areinvestigated. Samples with an optimized content of Ni2P evidence the lowest overpotential values, with 224 mV at 10 mA/cm2, andgood stability over time.