The mechanism of OER activity and stability enhancement in acid by atomically doped iridium in γ-MnO2

Abstract

Construction of iridium (Ir) based active sites on certain acid stable supports now is a general strategy for the development of low-Ir OER catalysts. Atomically doped Ir in the lattice of acid stable γ-MnO₂ has been recently achieved, which shows high activity and stability though Ir usage was reduced more than 95% than that in current commercial proton exchange membrane water electrolyzer (PEMWE). However, the activity and stability enhancement by Ir doping in γ-MnO₂ still remains elusive. Herein, high dispersion of iridium (up to 1.37 atom%) doping in the lattice of γ-MnO₂ has been achieved by optimizing the thermal decomposition of the iridium precursors. Benefiting from atomic dispersive doping of Ir, the optimized Ir-MnO₂ catalyst shows high OER activity, as it has turnover frequency of 0.655 s^–1 at an overpotential of 300 mV in 0.5 mol L⁻¹ H₂SO₄. The catalyst also shows high stability, as it can sustainably work at 100 mA cm⁻² for 24 h. Experimental and theoretical studies reveal that Ir is preferentially doped into β phase rather than R phase, and the Ir site is the active site for OER. The OER active site is postulated to be Ir⁵⁺-O(H)-Mn³⁺ unit structure on the surface. Furthermore, Ir doping changes the potential determining step from the formation of O* to the formation of *OOH, emphasizing the promoting effect toward OER derived from Ir sites. This work not only demonstrates the possibility of achieving atomic-level doping of Ir on the surface of a support to dramatically reduce Ir usage, but also, more importantly, reveals the mechanism behind accounting for the stability and activity enhancement by Ir doping. These important findings may serve as valuable guidance for further development of more efficient, stable and cost-effective low Ir-based OER catalysts for PEMWE.