Interplay of Activity and Stability within Hydrous Cobalt-Iridium Oxide Oxygen Evolution Electrocatalysts

Abstract

To further the development of proton-exchange membrane electrolyzers, oxygen evolution reaction (OER) electrocatalysts with high activity, extended durability, and lower costs are needed. We have explored iridium-nickel and iridium-cobalt two-dimensional nanoframes where the interaction of iridium with other metal alters the surface electronic structure and influences the activity and stabiity.1,2 In contrast to comparable structures with nickel, hydrous cobalt-iridium oxide two-dimensional nanoframes exhibit higher oxygen evolution activity and similar stability compared to commercial IrO2; however, the bimetallic catalyst undergoes different degradation processes compared to the monometallic catalyst. Bimetallic cobalt-iridium (Co-Ir) two-dimensional nanoframes are composed of interconnected Co-Ir alloy domains within an unsupported, carbon-free, porous nanostructure that allows three-dimensional molecular access to the catalytically active surface sites. After electrochemical conditioning within oxygen evolution potentials, the predominately bimetallic alloy surface is transformed to oxide/hydroxide surface. From rotating disk electrode measurements, hydrous Co-Ir oxide nanoframes provide 17 times higher OER mass activity and 16 times higher specific activity compared to those of commercial IrO2. The higher OER activities of hydrous CoIr nanoframes are attributed the interaction of Ir with Co within the surface and subsurface region that modifies the surface atomic and electronic structure. After accelerated durability testing, IrO2 has a lower specific activity and resulted in partial dissolution of Ir. In contrast, durability testing of hydrous Co-Ir oxide nanoframes resulted in an increase in specific activity, an increase in the relative contribution of surface iridium hydroxide groups, and a higher Ir dissolution rate. Understanding the differences between degradation processes invovled in bimetallic and monometallic catalysts furthers our ability to design high activity and stability acidic OER electrocatalysts. References Godínez-Salomón, F.; Albiter, L.; Mendoza-Cruz, R.; Rhodes, C.P. Bimetallic Two-dimensional Nanoframes: High Activity Acidic Bifunctional Oxygen Reduction and Evolution Electrocatalysts”, ACS Applied Energy Materials, 2020, 3, 2404-2421. http://dx.doi.org/10.1021/acsaem.9b02051 Godínez-Salomón, F.; Albiter, L.; Alia, S.M.; Pivovar, B.S.; Camacho-Forero, L.E.; Balbuena, P.B.; Mendoza-Cruz, R.; Arellano-Jimenez, M.J.; Rhodes, C.P. Self-Supported Hydrous Iridium-Nickel Oxide Two-dimensional Nanoframes for High Activity Oxygen Evolution Electrocatalysts, ACS Catalysis 2018, 8, 10498-10520. DOI: 10.1021/acscatal.8b02171