Cathodic Electrodeposition of Ni−Mo on Semiconducting NiFe₂O₄ for Photoelectrochemical Hydrogen Evolution in Alkaline Media

Photocathodes for hydrogen evolution from water were made by electrodeposition of Ni−Mo layers on NiFe₂O₄ substrates, deposited by spin coating on F:SnO₂-glass. Analysis confirmed the formation of two separate layers, without significant reduction of NiFe₂O₄. Bare NiFe₂O₄ was found to be unstable under alkaline conditions during (photo)electrochemistry. To improve the stability significantly, the deposition of a bifunctional Ni−Mo layer through a facile electrodeposition process was performed and the composite electrodes showed stable operation for at least 1 h. Moreover, photocurrents up to −2.1 mA cm⁻² at −0.3 V vs. RHE were obtained for Ni−Mo/NiFe₂O₄ under ambient conditions, showing that the new combination functions as both a stabilizing and catalytic layer for the photoelectrochemical evolution of hydrogen. The photoelectrochemical response of these composite electrodes decreased with increasing NiFe₂O₄ layer thickness. Transient absorption spectroscopy showed that the lifetime of excited states is short and on the ns timescale. An increase in lifetime was observed for NiFe₂O₄ of large layer thickness, likely explained by decreasing the defect density in the primary layer(s), as a result of repetitive annealing at elevated temperature. The photoelectrochemical and transient absorption spectroscopy results indicated that a short charge carrier lifetime limits the performance of Ni−Mo/NiFe₂O₄ photocathodes.