Highly Active and Durable Iridium Nickel Oxide Platelets for a Proton Exchange Membrane Water Electrolyzer with Low Iridium Loading

Minimizing the use of iridium (Ir) in proton exchange membrane water electrolyzers (PEMWEs) is essential for hydrogen production without carbon emission. Herein, layered monoclinic iridium nickel oxide (IrNiO_x) platelets were synthesized using the molten salt method and used for the oxygen evolution reaction (OER) in a PEMWE. The IrNiO_x hexagonal platelets consist of the edge-sharing octahedral framework, in which Ni atoms replace Ir sites in the crystalline lattice. Thin IrNiO_x platelets exhibited high OER activity with suppressed Ni dissolution from the bulk lattice in acidic media. When the platelets were applied in a membrane electrode assembly (MEA), they presented improved interconnectivity in the catalyst layer, facilitating electron transfer. Even at a low Ir loading of 0.2 mg_Ir cm^–2, the platelets presented good performance with an initial cell voltage of 1.70 V at a current density of 1 A cm^–2. Despite the use of a Ti porous transport layer (PTL) without Pt coating, the PEMWE operated stably for 150 h, exceeding the performance achievable by commercial Ir oxide and rutile IrO₂. When a Pt-coated Ti PTL was used, the PEMWE could be operated stably for 500 h. Incorporating earth-abundant transition metals into the Ir oxide lattice can be an effective way to minimize the use of Ir in PEMWEs.