Hydrous Cobalt-Iridium Oxide Two-Dimensional Nanoframes as High Activity and Stability Acidic Oxygen Evolution Catalysts
Date
2020-08
Authors
Ying, Yuanfang
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Abstract
Acidic oxygen evolution reaction (OER) electrocatalysts that have high activity, extended durability, and lower costs are needed to further the development and wide-scale adoption of proton-exchange membrane water electrolyzers. In this work, hydrous cobalt-iridium oxide two-dimensional nanoframes were demonstrated to exhibit higher oxygen evolution activity and similar stability compared with commercial IrO2. Self-supported iridium−cobalt oxide was synthesized by heat treatment of iridium decorated Co(OH)2 nanosheets followed by an acid leaching step. The synthesis process resulted in interconnected cobalt-iridium 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 the OER potential range, the predominately bimetallic alloy surface was transformed to oxide/hydroxide surface. Oxygen evolution activities determined using a rotating disk electrode configuration showed that the hydrous cobalt-iridium oxide nanoframes provided ~17 times higher OER mass and specific activities compared with those of commercial IrO2. The higher OER activities are attributed the interaction of Ir with Co within the hydrous iridium-cobalt oxide surface and subsurface CoIr alloy that tunes the surface atomic and electronic structure. In addition to higher activity, the CoIr nanoframes exhibited similar stability as commercial IrO2 using an accelerated durability testing protocol.
The effect of heat treatment on the structure and the electrocatalytic activity and stability toward the oxygen evolution reaction were studied. The Co(OH)2:Ir was treated under H2/Ar atmosphere at 200ºC, 300ºC and 400ºC, followed by an acid leaching step. The heat treatment temperature was show to significant influence the morphological, structural properties as well as the OER catalytic activity and stability of cobalt-iridium catalysts. The Scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDS) images show that the iridium to oxygen atomic ratio in the catalyst treated at 400ºC was 1:2 which is lower than 1:1 obtained for the catalysts treated at 200 and 300 ºC. X-ray diffraction analysis of the CoIr catalysts confirmed a lattice compression compared to iridium. The lattice constant decreased, and the particle size increased as the treatment temperature increased. The catalyst treated at 200ºC was found to have the highest electrochemical surface area. Catalytic activity and accelerated durability tests of the investigated catalysts indicate that the catalyst treated at 300ºC had the best performance on activity and stability. Thus, 300ºC was determined as the best heat treatment temperature regarding electrocatalytic activity and stability. This work supports that appropriately designed Co-Ir bimetallic oxyhydroxide structures can provide high activity and stability as acidic OER electrocatalysts, and the heat treatment temperature has a significant influence in the structure and electrocatalytic performance of the catalysts.
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Keywords
two-dimensional materials, heat treatment, iridium oxide, electrocatalyst, oxygen evolution reaction, proton exchange membrane electrolyzer
Citation
Ying, Y. (2020). Hydrous cobalt-iridium oxide two-dimensional nanoframes as high activity and stability acidic oxygen evolution catalysts (Unpublished dissertation). Texas State University, San Marcos, Texas.