A nanosized IrO(2) anode electrocatalyst was prepared by a sulfite-complex route for application in a proton exchange membrane (PEM) water electrolyzer. The physicochemical properties of the IrO(2) catalyst were studied by termogravimetry-differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The electrochemical activity of this catalyst for oxygen evolution was investigated in a single cell PEM electrolyzer consisting of a Pt/C cathode and a Nafion membrane. A current density of 1.26 A cm(-2) was obtained at 1.8 V and a stable behavior during steady-state operation at 80 degrees C was recorded. The Tafel plots for the overall electrochemical process indicated a slope of about 80 mV dec(-1) in a temperature range from 25 degrees C to 80 degrees C. The kinetic and ohmic activation energies for the electrochemical process were 70.46 kJ mol(-1) and 13.45 kJ mol(-1), respectively. A short stack (3 cells of 100 cm(2) geometrical area) PEM electrolyzer was investigated by linear voltammetry, impedance spectroscopy and chrono-amperometric measurements. The amount of H(2) produced was 80 lh(-2) at 60 A under 330 W of applied electrical power. The stack electrical efficiency at 60 A and 75 degrees C was 70% and 81% with respect to the low and high heating value of hydrogen, respectively. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.