Iridium Incorporation into MnO2 for an Enhanced Electrocatalytic Oxygen Evolution Reaction

We have investigated the structure and activity of electrocatalysts for the oxygen evolution reaction (OER) that had low loadings of Ir incorporated into the 2D layered MnO2, (birnessite, nominally delta-MnO2) and the 3D MnO2 (pyrolusite, beta-MnO2). The Ir-incorporated beta-MnO2 (Ir/beta-MnO2) electrocatalysts were prepared for the first time via a thermally induced phase transition of delta-MnO2 containing 16-22 wt% Ir. This phase transition of delta-MnO2 to beta-MnO2 was facilitated by the presence of Ir in the structure, as both Ir in IrO2 and Mn in beta-MnO2 could adopt a thermodynamically favored rutile structure. Extended X-ray absorption fine structure (EXAFS) of Ir/beta-MnO2 showed that the catalyst consisted of Ir substituted into the crystalline beta-MnO2 lattice. 22 wt% Ir/beta-MnO2 (60 mu g(Ir)cm(geo)(-2) exhibited an OER overpotential ( eta ) of 337 mV, lower than the eta for commercial IrO2. This eta was constant for 6 h, at 10 mA cm(geo)(-2) in 0.5 M H2SO4. EXAFS, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption near edge structure (XANES) showed that 22 wt% Ir/beta-MnO2 had a strained structure containing similar to 41 % Mn3+, an OER active species, along with a modified Ir bond covalency consisting of both Ir-O-Ir and Ir-O-Mn.