The Polarisation Behaviour of Silver in Potassium Hydroxide Solution at Elevated Temperatures

The Polarisation behaviour of silver in 1 mol kg-1 KOH solution over the temperature range 295-478 K was studied using cyclic voltammetry, potentiostatic, galvanostatic and a.c. impedance techniques. Thermodynamic data for the silver/water system at temperatures up to 573 K is presented in the form of potential-pH diagrams which assist in the interpretation of results at elevated temperatures. The cyclic voltammograms and galvanostatic charging curves indicate the principal changes to be the appearance of additional peaks and arrests, respectively, as the temperature is increased, followed by their disappearance at higher temperatures. The data obtained from cyclic voltammetry and also potentiostatic polarisation are analysed in terms of previously-derived relations for limiting rate control, in particular that due to diffusion. The impedance data are examined using equivalent circuit models from which it is shown that surface roughness of the electrodes has a significant effect on the reaction kinetics over the temperature range studied. It is proposed that substantial changes in the electrochemistry of silver at 478 K are associated with extensive surface roughness that a change in the kinetics of growth of the Ag2O multilayer takes place at elevated temperatures with the rate of nucleation of Ag2O growth centres becoming more important in relation to the rate of solid-state diffusion.