Stepwise Understanding on Hydrolysis Formation of the IrO_x Nanoparticles as Highly Active Electrocatalyst for Oxygen Evolution Reaction

In this study, we have investigated the synthesis of supported iridium oxide (IrO_x) nanoparticles (NPs) through hydrolysis in a surfactant-free aqueous bath as a possible route for the large-scale production of highly active electrocatalyst for oxygen evolution reaction (OER) in acidic water electrolyzers. The process involves (i) formation of Ir-hydroxides complex from an Ir precursor in basic media followed by (ii) protonation in acidic media to form colloidal hydrated IrO_x NPs and (iii) conversion and deposition of IrO_x NPs on the surface of carbon or TiN support by probe sonication. The IrO_x NPs produced through hydrolysis route form highly stable colloidal solution. Since it is essential to precipitate the catalyst NPs from the colloidal solution for their use in water electrolyzer electrode development, here, we investigate the optimal reaction conditions, e.g., pH, temperature, time, and presence of support, for efficient synthesis of the catalyst NPs. The reaction intermediates formed at different reaction steps are explored to get insights into the chemistry of the process. Under the optimal synthesis conditions, 100% precipitation of IrO_x NPs was achieved. Further, the precipitated TiN supported IrO_x NPs exhibited high OER activity, superior to that of the commercial benchmark IrO₂ electrocatalyst. The study provides a scalable synthesis route for highly active, low Ir-content OER electrocatalysts for acidic water electrolyzers. Graphical Abstract: (Figure presented.)