Control by atomic layer deposition over the chemical composition of nickel cobalt oxide for the oxygen evolution reaction

Anion exchange membrane water electrolysis (AEMWE) is a promising technology for renewable electricity- driven water splitting towards hydrogen production. However, application of AEMWE at industrial scale requires the development of oxygen evolution reaction (OER) electrocatalysts showing long term stability under mild alkaline conditions. Among these, nickel cobalt oxide thin films are considered promising candidates. The ideal chemical composition of these oxides remains debatable, with recent literature indicating that rock-salt NiCoO2 may exhibit similar OER activity as the traditional spinel NiCo2O4. In this work, we present the development of a plasma- enhanced atomic layer deposition (ALD) process of nickel cobalt oxide thin films (~20 nm) with focus on the role of their chemical composition and crystal structure on the OER activity. The film composition is tuned using a supercycle approach built upon CoOx cycles with CoCp2 as precursor and O2 plasma as co-reactant and NiOx cycles with Ni(MeCp)2 as precursor and O2 plasma as co-reactant. The films exhibit a change in crystallographic phase from rock-salt to spinel structure for increasing cobalt at. %. This change is accompanied by an increase in the Ni3+-to-Ni2+ ratio. Interestingly, an increase in electrical conductivity is observed for the mixed oxides, with an optimum of (2.4±0.2) ·102 S/cm at 64 at. % Co, outperforming both NiO and Co3O4 by several orders of magnitude. An optimal electrocatalytic performance is observed for the 80 at.% Co films. Cyclic voltammetry measurements simultaneously show a strong dependence of the OER-catalytic performance on the electrical conductivity. The present study highlights the merit of ALD in controlling the nickel cobalt oxide chemical composition and crystal structure to gain insight into its electrocatalytic performance. Moreover, these results suggest that it is important to disentangle conductivity effects from the electrocatalytic activity in future work