Revisiting the Activity Gap of Iridium Electrocatalysts for Acidic Water Oxidation.

Iridium electrocatalysts have been extensively studied for the acidic water oxidation reaction (2HO → O + 4H + 4, also known as the oxygen evolution reaction, OER) in recent years. However, the activity of different iridium catalysts, such as amorphous, crystalline, and metallic ones, varies significantly, and there is no common explanation for the origin of this difference. Here four types of iridium catalysts were selected as models and characterized by various techniques. The redox behavior of iridium catalysts and oxidation of hydrogen peroxide (in the form of OOH) were applied to probe the adsorption energy of oxygen reaction intermediates (*OH, *O, and *OOH) on iridium catalysts under the OER conditions. Structure-activity analysis suggested that the more optimal and broader distribution of adsorption energies on metallic iridium (iridium black) and its good conductivity are the origin of its highest activity among the four different iridium catalysts.