Proton exchange membrane steam electrolyzers suffer from insufficient catalyst activity and durability due to the slow reaction kinetics for oxygen evolution reaction (OER) and poor durability under harsh operating environments. Aiming at enhancement of oxygen electrode kinetics and durability, composite support materials for iridium oxide are synthesized via in situ phosphorization reaction on tin doped indium oxide and possess functionalities of high electronic and intrinsic proton conductivity. At 130 °C under a water vapor atmosphere an overall conductivity of 0.72 S cm−1 is achieved with a contribution of around 10−2 S cm−1 proton conductivity. The support structure of three-dimensionally ordered hexagonal arrays displays a high specific surface area of 180 m2 g−1. Benefiting from the mixed conductivities and porous structure in the composite support materials, the supported IrO2 catalysts exhibit about five times enhancement of the OER activity in acidic electrolytes. The improved catalytic performance for the OER was further confirmed by PEM electrolyzer tests at 130 °C. A test of such a steam electrolyzer cell at 350 mA cm−2 shows good durability within a period of up to 1150 hours.