Enhancing Acidic Oxygen Evolution Activity by Controlling Oxidation State of Iridium

Iridium oxides with high oxidation states have been reported to be effective in enhancing the acidic oxygen evolution reaction (OER) performance. Herein, we develop ultrasmall IrO_x nanoparticles (NPs) over titanium nitride (TiN), which undergoes surface oxidation under oxidative conditions to form oxygen-modified TiN (oxi-TiN), enabling the formation of highly oxidized Ir^δ+ (δ > 4). This IrO_x/oxi-TiN catalyst delivers higher Ir mass activity than commercial IrO₂, while comparable stability is maintained. The superior OER activity of IrO_x/oxi-TiN is further demonstrated in a proton exchange membrane water electrolyzer (PEMWE), requiring only 1.88 V to reach 3 A cm^-2, achieving the U.S. Department of Energy 2025 target (1.90 V at 3 A cm^-2). In situ X-ray absorption spectroscopy (XAS) confirms that the superior OER activity of IrO_x/oxi-TiN originates from highly oxidized Ir^δ+ under OER conditions. Density functional theory (DFT) calculations reveal a general correlation between the oxidation state of Ir and OER overpotential. Specifically, the introduction of interfacial oxygen at the Ir/TiN interface increases the oxidation state of deposited Ir^δ+ from δ < 4 to δ > 4, decreasing the OER overpotential. This study highlights the critical role of high oxidation states of Ir^δ+ in enhancing OER activity, providing guidance for the development of advanced acidic OER catalysts.