ELECTRODEPOSITION OF PALLADIUM AND ADSORPTION OF PALLADIUM-CHLORIDE ONTO SOLID ELECTRODES FROM ROOM-TEMPERATURE MOLTEN-SALTS

The electrodeposition of palladium onto various electrode surfaces was examined in room temperature AlCl3-MEIC molten salts with AlCl3 mole fractions, N, from 0.33 < N < 0.5 (basic melts) to 0.5 < N < 0.67 (acidic melts) and at N = 0.5 (neutral melt). The behavior of palladium electrodeposition was markedly dependent on the mole fraction of AlCl3 in the molten salts. The palladium reduction potential shifts approximately +2.0 V when the melt is changed from basic to acidic. Nucleation overpotentials were evident in basic melts, and to a lesser extent in acidic and neutral melts. In acidic melts, the reduction of the sparingly soluble palladium complex displays characteristics distinctive of an adsorption phenomenon, while the oxidation process shows considerable broadening. Oxidation of a palladium electrode in an N = 0.55 acidic melt produces an insoluble palladium chloride layer (approximately a monolayer) on the electrode surface which protects the underlying metal from further oxidation. Reduction of this surface anchored palladium chloride layer is rapid and provides a high cathodic current density. This behavior in acidic melts is pointedly different from the reduction process in a basic melt where the reduction of the soluble palladium chloro complex exhibits a diffusion wave with nucleation effects.