Iridium Incorporation into MnO2 for an Enhanced Electrocatalytic Oxygen Evolution Reaction

Abstract

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 δ-MnO2) and the 3D MnO2 (pyrolusite, β-MnO2). The Ir-incorporated β-MnO2 (Ir/β-MnO2) electrocatalysts were prepared for the first time via a thermally induced phase transition of δ-MnO2 containing 16–22 wt% Ir. This phase transition of δ-MnO2 to β-MnO2 was facilitated by the presence of Ir in the structure, as both Ir in IrO2 and Mn in β-MnO2 could adopt a thermodynamically favored rutile structure. Extended X-ray absorption fine structure (EXAFS) of Ir/β-MnO2 showed that the catalyst consisted of Ir substituted into the crystalline β-MnO2 lattice. 22 wt% Ir/β-MnO2 (60 (Formula presented.)) exhibited an OER overpotential ((Formula presented.)) of 337 mV, lower than the (Formula presented.) for commercial IrO2. This (Formula presented.) was constant for 6 h, at 10 mA (Formula presented.) 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/β-MnO2 had a strained structure containing ∼41 % Mn3+, an OER active species, along with a modified Ir bond covalency consisting of both Ir−O−Ir and Ir−O−Mn.