Quantum isotope effect in “hydrogen solution in palladium-hydroxide proton conductor” heterojunctions

Abstract

A hydrogen isotope effect in electrode potentials has been observed for the first time at 368 ± 0.5 K in solid hydroxide protonic PdH_x|KOH · nH₂O (x < 0.02, n = 0.5 and 1) heterojunctions obtained upon electrochemical activation of Pd|KOH · nH₂O|M heterostructures, where M = Pd or C (graphite). The chemical composition of the second (M) electrode did not influence the isotope shift of the electrode potential upon the complete replacement of protium by deuterium, which amounted to 105 ± 15 and 120 ± 20 mV for n = 0.5 and 1, respectively. The isotope shift in the electrode potential of the “hydrogen solution in palladium-hydroxide proton conductor” heterojunction implies the presence of quantum effects. The observed isotope shift is correlated with the isotope effects in the zero-point energies of (i) protons involved in the O-H chemical bonds in the structural units of electrolyte and (ii) hydrogen atoms dissolved in palladium.