Cathodic Electrodeposition of Ni−Mo on Semiconducting NiFe2 O4 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.