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 MnO 2 , (birnessite, nominally δ-MnO 2 ) and the 3D MnO 2 (pyrolusite, β-MnO 2 ). The Ir-incorporated β-MnO 2 (Ir/β-MnO 2 ) electrocatalysts were prepared for the first time via a thermally induced phase transition of δ-MnO 2 containing 16-22 wt% Ir. This phase transition of δ-MnO 2 to β-MnO 2 was facilitated by the presence of Ir in the structure, as both Ir in IrO 2 and Mn in β-MnO 2 could adopt a thermodynamically favored rutile structure. Extended X-ray absorption fine structure (EXAFS) of Ir/β-MnO 2 showed that the catalyst consisted of Ir substituted into the crystalline β-MnO 2 lattice. 22 wt% Ir/β-MnO 2 (60 mg Ir cm À 2 geo ) exhibited an OER overpotential (h) of 337 mV, lower than the h for commercial IrO 2 . This h was constant for 6 h, at 10 mA cm À 2 geo in 0.5 M H 2 SO 4 . 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-MnO 2 had a strained structure containing ~41 % Mn 3 + , an OER active species, along with a modified Ir bond covalency consisting of both IrÀ OÀ Ir and IrÀ OÀ Mn.

Thermal‐Driven Orderly Assembly of Ir‐atomic Chains on α‐MnO2 with Enhanced Performance for Acidic Oxygen Evolution

“…As shown in Figure 7b, the O 1s spectrum of the α‐MnO 2 material can be split into two peaks, with the main peak at 529.7 eV corresponding to the surface lattice oxygen (labeled as O latt ), in the form of O 2− , and 531.5 eV attributed to the adsorbed oxygen (labeled as O sur ), in the form of O 2 2− , O − or hydroxyl analogous group form [4a] . The calculated O sur /O latt molar ratios are shown in Table S2, which vary in the order IrCeMnO@Ir (3.96)>IrO 2 (1.79)>α‐CeMnO 2 (0.43)>α‐MnO 2 (0.35).…”

“…At present, many supports of Ir‐based catalysts were studied, including metal oxides, such as antimony doped tin oxide (Sb‐SnO 2 ), [5] TiO 2 , [6] Co 3 O 4 , [7] MnO 2 [4a,8] and other materials [1b,9] with good corrosion resistance and electrical conductivity. Among various electrocatalyst development strategies, the design and preparation of one‐dimensional nanostructured electrocatalyst systems, such as nanowires (NWs), [10] nanorods (NRs), [11] and nanotubes (NT), [12] have unique advantages.…”