Benchmark and Earth-Abundant Electrocatalysts for Acidic Water Oxidation

Abstract

Solar-mediated hydrogen technology is a promising candidate for the low-carbon production of chemical fuels. Solar-mediated water electrolysis is a mature form of H2 technology that functions by rupturing water molecules to release H2 and O2 gas. Water electrolysis can be performed at any pH, but presently only acidic conditions offer sufficient gas separation and solution conductivity. Large-scale integration of solarmediated acidic water electrolysis is limited by the oxygen evolution reaction (OER), which requires oxidatively stable catalysts to proceed at appreciable rates and energies. The only materials known to possess acceptable levels of activity, stability and selectivity for acidic OER are iridium oxide (IrOx)-based systems. IrOx serves as a “benchmark” material against which other OER catalysts are compared. The performance of IrOx is largely determined by its method of preparation and the conditions under which it is tested, neither of which are controlled for in the literature. To catalogue the impact of fabrication and electrolyte conditions on IrOx OER performance, IrOx films were prepared using common literature techniques and evaluated under equivalent electrolyte conditions. Further, the OER performance of IrOx films was evaluated in different electrolytes and molarities. The results of this study revealed preparation-dependent OER performances and film-dependent sensitivities to electrolyte composition. Identification of fabrication- and electrolyte-dependent properties for IrOx films allows for better comparison between literature electrocatalyst systems. The scarcity and cost of Ir limits the sustainability and scalability of water electrolysis. Earth abundant (EA) alternatives are required for this technology to be an economical large-scale energy solution. The current repertoire of EA materials for acidic OER all show poor stability and/or activity compared to analogous IrOx-based systems. Titanium diboride (TiB2) is a corrosion-resistant material with theorized and demonstrated catalytic behaviour. Its suitability as an acid-stable electrocatalyst for OER was investigated. It was found to have the highest chemical stability of known active EA materials for acidic OER and superior activity relative to other electrochemically-stable EA systems. However, TiB2 films showed limited mechanical stability and delaminated after ~ 10 h operation.