Electrolytic hydriding of Pd79.5Rh20.5 alloy

The electrolytic insertion of hydrogen into Pd79.5Rh20.5 (atomic %) alloy from acid and alkaline electrolytes was investigated in the temperature range 25-95°C. It was found that, at 25°C, the alloy can be loaded up to hydrogen to metal (H/M) ratios (0.90 in acid, 0.95 in alkaline electrolytes) comparable to those achieved by H2 pressures above 108 Pa. The desorption isotherm of b hydride obtained in 0.5 M H2SO4 at 25°C and the thermal decomposition of this phase do not differ greatly from the expectations of the thermodynamics of the Pd80Rh20 system. At temperatures above 25°C, high H/M ratios (from 0.95 to 0.93) and the unpredicted thermal stability of the b hydride phase were found only after cathodic loading in alkaline electrolytes. The phenomenon is explained by codeposition of hydrogen and alkali metal on the alloy, which partially hinders the exit of loaded hydrogen.