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 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.