Electrolytic hydriding of Pd79.5Rh20.5 alloy

The electrolytic insertion of hydrogen into Pd79.5Rh20.5 (at.%) 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 108Pa. The desorption isotherm of β hydride obtained in 0.5M 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 β hydride phase were found only after cathodic loading in alkaline electrolytes. The phenomenon is explained by co-deposition of hydrogen and alkali metal on the alloy, which partially hinders the exit of loaded hydrogen.