Cathodic Electrodeposition of Ni-Mo on Semiconducting NiFe 2 O 4 for Photoelectrochemical Hydrogen Evolution in Alkaline Media.

Photocathodes for hydrogen evolution from water were made by electrodeposition of Ni-Mo layers on NiFe ₂ O ₄ substrates, deposited by spin coating on F:SnO ₂ -glass. Analysis confirmed the formation of two separate layers, without significant reduction of NiFe ₂ O ₄ . Bare NiFe ₂ O ₄ 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/NiFe ₂ O ₄ 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 NiFe ₂ O ₄ 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 NiFe ₂ O ₄ 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/NiFe ₂ O ₄ photocathodes.
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