Digital Composition Control of Cobalt Nickel Oxides by ALD for the Oxygen Evolution Reaction

Abstract

Anion exchange membrane water electrolysis is a potentially low-cost and sustainable technology for renewable-energy driven hydrogen production. Realisation of its full potential relies on the development of cost-effective, earth-abundant OER electrocatalysts alternatives to the state-of-the art noble metal oxides1. Ternary transition metal oxides, such as cobalt nickel oxides, are promising candidates due to their excellent alkali resistance in combination with remarkable catalytic activities. The latter relate to their tuneable oxidation states and variation in coordination of their metal centres. These correlations can be properly investigated when precise control over the layer chemical composition is attained. Atomic layer deposition (ALD) is widely acknowledged to deliver control over film uniformity, thickness and conformality, as well as film chemical composition due to the self-limiting nature of its surface reactions.In this study, an ALD supercycle process based on cobalt cyclopentadienyl (CoCp2) and nickel methylcyclopentadienyl (Ni(MeCp)2) precursors and an oxygen plasma co-reactant has been developed2 to investigate the influence of chemical composition and crystallographic properties of cobalt nickel oxide thin films on their electrocatalytic OER performance. Deposition of ~20 nm binary nickel oxide and cobalt oxide results in the growth of polycrystalline films of the rock-salt and spinel phase, respectively. Ternary cobalt nickel oxide films with various ratios can be deposited uniformly, as indicated by energy dispersive x-ray spectroscopy mapping. The mixed oxides display a transition from the +2 oxidation state-based pure rock-salt phase (<25 at.% Co) to the spinel phase consisting of mixed +2/+3 oxidation states (>75% at.% Co) upon increasing Co at.% as verified by electron diffraction (figure a) and x-ray photoelectron spectroscopy (XPS). XPS furthermore reveals a linear increase in the Ni3+-to-Ni2+ ratio with increasing Co at.%, which indicates that an inverse spinel structure is formed. The formation of a Ni3+-rich inverse spinel cobalt nickel oxide between 55 and 75 at.% Co facilitates the formation of a semi-metallic state (102 S/cm) as opposed to the poorly conductive binary oxides (10-3 S/cm and 10-2 S/cm for NiO and Co3O4, respectively).Cyclic voltammetry (CV) measurements in 1M KOH, reveal chemical composition dependent activation behaviour (figure b). Continuous activation behaviour is observed for rock-salt phase films, whilst increment of the Co at.% results in reduced film activation until virtually no activation of the spinel phase films is observed. The activation of the nickel-rich film is accompanied by irreversible redox changes from the +2 to +3 oxidation state as indicated by the integrated non-catalytic wave. Post CV XPS analysis confirms the transition from Co2+ and Ni2+ oxidation states to Co3+ and Ni3+. The formation of an (oxy)hydroxide state is observed for the rock-salt dominated films, whilst oxidation states characteristic of a spinel- like oxide structure are observed for the cobalt-rich films. No changes in the transition metal ratios are observed after activation. Scanning electron microscopy images furthermore reveal that the transformation of the Ni-rich films is accompanied by surface restructuring. These results indicate that initial composition can significantly influence the electrocatalytic performance of ternary oxides and therefore highlight the importance of techniques such as atomic layer deposition to study their complex behaviour. 1Xu et al., Energy Chem., 4 (2022) 100087 2van Limpt et al., J. Vac. Sci. Technol. A, 41 (2023) 032407 This work has been carried out within the SCALE project (NWA.1237.18.001) funded jointly by the Netherlands Organization for Scientific Research. Authors would like to thank the co-founders of the project ISPT, Syngaschem, VecoPrecision and Vsparticle and international partners Toyota Motor Europe and FORTH institute. Figure:(a) ALD deposited cobalt nickel oxide film show a transition from the +2 oxidation state-based rock-salt phase (NiO) to the spinel structure consisting of mixed +2/+3 oxidation states (Co3O4) upon tuning the chemical composition as identified by electron diffraction. (b) This significantly affects the OER-electrocatalytic performance and activation of the films as represented by the current density at 1.8V vs RHE over 500 CV cycles in 1M KOH. Data labels indicate cobalt concentration as at.% Co/(at.% Co+ at.% Ni). Figure 1