Hexavalent iridium (Ir^VI) oxide is predicted to be more active and stable than any other Ir oxide for the oxygen evolution reaction in acid; however, its experimental realization remains challenging. Here, we report the synthesis, characterization, and application of atomically dispersed Ir^VI oxide (Ir^VI-ado) for proton-exchange membrane (PEM) water electrolysis. The Ir^VI-ado was synthesized by oxidatively substituting the ligands of K₂IrCl₆ with manganese oxide. The mass-specific activity (1.7 × 10⁵ A g_Ir^-1) and turnover number (1.5 × 10⁸) exceeded those of benchmark Ir oxides, and in-siu X-ray analysis during PEM operations manifested the durability of Ir^VI at current densities up to 2.3 A cm^-2. The high activity and stability of Ir^VI-ado showcase its promise as an anode material for PEM electrolysis.

Nafion 115 polymer membranes were used to fabricate the membrane electrode assembly (MEA). Prior to use, the Nafion membrane was boiled sequentially for 1 h each in 3 wt.% H₂O₂, Milli-Q water, 1.0 M H₂SO₄, and Milli-Q water to remove possible contaminants and ensure that the membrane was completely protonated. Ir^VI-ado samples with loading amounts of 0.02, 0.04, and 0.08 mg_Ir cm^-2 were used as an anode and coated with 10 wt. % Nafion ionomer before being assembled into the MEAs. Pt/C catalyst (20 wt% Pt on carbon black, Fuel Cell Earth) coated on hydrophobic carbon paper (TGP-H-060, Toray) with a loading amount of 0.2 mg_Pt cm^-2 was used as a cathode. The Pt/C catalyst slurry used for coating was prepared by mixing Pt/C, water, ethanol, and Nafion solution (10 wt% in H₂O, Sigma-Aldrich) with a Pt/C to Nafion ionomer ratio of 3:1. The MEAs with an active area of 1 cm² were prepared by hot-pressing the anode and cathode on a Nafion membranes at 135 °C at a mold clamping force of 2 MPa for 3 min. Electrolysis tests were conducted using a single-cell PEM electrolyzer (WE-4S-RICW, FC Development, Japan). The current-voltage and long-term durability were conducted using a potentiostat (HZ-7000, Hokuto Denko) equipped with a 30A-booster (HZAP1230, Hokuto Denko). All tests were conducted at 80 °C with water fed into the anode side of the PEM electrolyzer.