Kinetics of the hydrogen electrode reactionon gold in acid sulphate melts

The hydrogen-electrode reaction on gold prepared in different ways has been investigated in molten acid sulphates at temperatures from ca 180 to 345°C. Potentiostatic and galvanostatic measurements under steady conditions were performed and the evolution of the potential of the working electrode when current was switched on and off was recorded. Current/voltage curves followed a Tafel law at cathodic overvoltages higher than 0-050 V. Slopes ranging from RT/2F to about 2RT/F are reported, depending on the preparation of the electrode, temperature and overvoltage region considered. Anodic current/voltage curves exhibit a limiting current. From the linear overvoltage/current region the reaction resistance was estimated for the anodic reaction. The exchange cds calculated from the reaction resistance agree with the values extrapolated from the Tafel lines having a slope RT/2F, if the stoichiometric number involved in the mechanism of the reaction is one. Electrode potential decayed logarithmically with time. Experimental electrode capacitances were evaluated from the time dependence of electrode potential. The hydrogen-electrode reaction is discussed in terms of a mechanism involving a fast hydrogenion discharge followed by a combination reaction as rate-determining step. The behaviour of the hydrogen-electrode reaction on gold in acid sulphate melts resembles that in aqueous acid solutions at ordinary temperatures.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada