Plasma-Assisted Atomic Layer Deposition of Oxygen Evolution Reaction Electrocatalysts

Anion exchange membrane water electrolysis is a potentially low-cost technology for renewable-energy driven hydrogen generation. Realisation of its full potential relies on the development of cost-effective, earth-abundant oxygen evolution reaction (OER) electrocatalysts, as alternative to the state-of-the art noble metal oxides. In this contribution I will address the synthesis of two electrocatalysts, i.e., cobalt phosphate (CoPi) and cobalt nickel oxide by plasma-assisted atomic layer deposition and discuss the merit of digital control over film stoichiometry to generate insight on the activation mechanisms of these electrocatalysts.<br/>CoPi is prepared by combining ALD cycles of CoO_x from cobaltocene (CoCp₂) and O₂ plasma, with cycles of trimethylphosphate ((CH₃O)₃PO) followed by O₂ plasma. We show that the Co-to-P ratio in the film can be tuned by combining the above-mentioned recipe with extra cycles of CoO_x. We demonstrate that ALD CoPi thin films undergo activation with increasing number of cyclic voltammetry (CV) cycles. During activation, the current density increases in parallel with a progressive leaching of phosphorous out of the electrocatalyst. These chemical changes proceed in parallel with structural changes in the electrocatalyst: measurements of the electrochemical surface area (ECSA) reveal that during activation, the ECSA of this film increases and that the electrochemical activity scales linearly with ECSA for all film compositions. Thus, the initial composition affects the activity of the catalyst indirectly by guiding the restructuring of the catalyst during cycling and the ECSA is a critical parameter in determining the activity of CoPi electrocatalysts. The present study discloses an opportunity for ALD in electrocatalysis: its<b> </b>digital control over chemical composition enables unravelling the ECSA-OER activity correlation.<br/>The second study addresses an ALD supercycle process based on CoCp₂and nickel methylcyclopentadienyl (Ni(^MeCp)₂) and an oxygen plasma to investigate the influence of chemical composition and crystallographic properties of cobalt nickel oxide thin films on their electrocatalytic performance. Deposition of binary nickel oxide and cobalt oxide results in the growth of polycrystalline films of rock-salt and spinel phase, respectively. The mixed oxides display a transition from 2+ oxidation state-based pure rock-salt phase (&lt;25 at.% Co) to the spinel phase consisting of mixed +2/+3 oxidation states (&gt;75% at.% Co) upon increasing Co at.%, as verified by electron diffraction and XPS. XPS also 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. When CV measurements are carried out in 1M KOH, 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. 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.