CATHODIC BEHAVIOR OF IRO2 ELECTRODES IN ALKALINE-SOLUTION .2. KINETICS AND ELECTROCATALYSIS OF H-2 EVOLUTION

H-2 evolution on Ti/IrO2 electrodes, prepared by the thermal decomposition of IrCl3 in the temperature range 300-degrees-C-500-degrees-C, has been investigated in 1 mol dm-3 NaOH by means of polarization curves and reaction order determination. Electrodes prepared at T < 400-degrees-C are poorly active, being subjected mainly to bulk oxide reduction. Those prepared at T greater-than-or-equal-to 400-degrees-C show two different Tafel slopes (about 40 mV and 120 mV) at low and high overpotentials respectively. The observed reaction order of -0.5 with the Tafel slope of 40 mV suggests a mechanism in which the second electron transfer is rate determining while the oxide surface experiences acid-base equilibria. Tafel slopes determined between 5-degrees-C and 45-degrees-C have shown temperature dependence close to theoretical expectations. which suggests that these surfaces conform to classical kinetic models better than metal surfaces since double-layer and impurity effects are minimized. Open-circuit potentials before and after H-2 evolution, the origin of uncompensated ohmic drops and electrocatalytic activity as a function of calcination temperature are also discussed.