5 wt.% of platinum (Pt) nanoparticles are highly dispersed on the surface of IrO2 by chemical reduction, and the catalyst is mixed with Pt black to be used as a novel bifunctional oxygen electrocatalyst for the unitized regenerative fuel cell (URFC). The novel cell has been evaluated in the hydrogen and oxygen fuel cell and water electrolysis modes, and compared to a similar cell with an oxygen electrode using conventional mixed Pt black and IrO2 catalyst. With the novel oxygen electrode catalyst, the highest fuel cell power density is 1160 mW cm(-2) at 2600 mA cm(-2); the overall performance is close to that with the commercial Pt supported on carbon catalyst and about 1.8 times higher than that with the conventional mixed Pt black and IrO2 catalyst. Additionally, the cell performance for water electrolysis is also slightly improved, which is probably the result of lower interparticle catalyst resistance with 5 wt.% Pt on IrO2 compared to no Pt on IrO2. (C) 2009 Elsevier B.V. All rights reserved.