Iridium As Catalyst and Cocatalyst for Oxygen Evolution/Reduction in Acidic Polymer Electrolyte Membrane Electrolyzers and Fuel Cells

Among noble metal electrocatalysts, only iridium presents high activity for both the oxygen reduction reaction (ORR) in acid medium, in the oxide form, and the oxygen evolution reaction (OER) in acid medium, alloyed with first row transition metals. Indeed, platinum, the best catalyst for the ORR, has poor activity for the OER in any form, and ruthenium, the best catalyst for the OER, in the oxide form, possess poor activity for the ORR in any form. In this work, an overview of the application of Ir and Ir-containing catalysts for the OER in proton-exchange membrane water electrolyzer anodes, for the ORR in proton exchange membrane fuel cell cathodes, and for both OER and ORR in unit regenerative fuel cell oxygen electrodes is presented.