Enhanced Oxygen-Reaction Electrocatalysis and Corrosion Resistance of CoCrFeNi Thin Films by Tuned Microstructure and Surface Oxidation.

Oxygen electrocatalysts play a key role in renewable and fossil-free energy production. Bifunctional catalysts active for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) allow use of the same material system for both energy production (ORR) and fuel generation (OER). However, optimizing the performance of bifunctional catalysts requires in depth understanding of the catalyst structure, its surface chemistry in terms of active sites and the underlying catalytic mechanism. Here, the catalytic performance of CoCrFeNi thin films is investigated, synthesized using high-power impulse magnetron sputtering, as bifunctional oxygen electrocatalysts. The film crystal structure and morphology, and thereby the catalytic performance, can be tuned by the ion acceleration (bias) to the substrate. To further enhance the catalytic activity, anodization is used to electrochemically modify the films, forming a thicker oxide layer enriched in Co and Ni cations which significantly improves the ORR performance. Anodization improves the catalyst stability during OER, with an OER potential of 1.45 V versus the reversible hydrogen electrode (RHE) at 10 mA cm for more than 24 h. While the corrosion resistance is high both before and after anodization, in terms of catalytic activity the anodized films outperformed the as-deposited ones. This makes anodized films excellent electrocatalyst candidates in corrosive alkaline environments such as fuel cells and electrolyzers.