Multilayer Ni/Fe thin films as oxygen evolution catalysts for solar fuel production

The slow kinetics and high overpotential of the oxygen evolution reaction is one of the main limiting factors to achieve the minimum required performances of the so-called photoelectrochemical water splitting systems. An oxygen evolution catalyst (OEC) becomes essential in order to perform this process with higher efficiency. Herein, we report the physical, optical and electrochemical characterization of multilayer Ni/Fe thin films as earth-abundant OEC, to avoid the use of platinum group metals (PGM). Uniform films of thicknesses ranging from 1 to 10 nm were fabricated by sequential and alternate thermal evaporation of Ni and Fe. It was found that the successive deposition allows the fabrication of a Ni terminated surface that does not need activation due to the Fe underlayer. The lowest overpotential achieved for NiFe was 370 mV at 10 mA cm−2 and a Tafel slope of 37 mV dec−1 with 1 nm thickness and 95% transmittance. Finally, NiFe OEC was implemented on top of Mo:BiVO4 photoanodes which resulted in a reduction of the open circuit potential of 0.2 V and up to five fold increase of the oxidation efficiency at 0.7 VRHE. The results presented facilitate the practical implementation of BiVO4 photoanodes in tandem configuration for bias free photoassisted water splitting.