The mechanism of electrolytic hydrogen evolution—evidence for the participation of proton tunneling

Abstract

The protium-tritium separation factor SH,T for hydrogen evolution was measured as a function of potential, temperature and solution composition on Hg in acid solution. Tafel measurements and the determination of the activation energy for hydrogen evolution on Hg were also carried out. The protium-tritium separation factor was found to be linearly dependent on potential for temperatures in the range +50° to −71°C. At 27°C in 7·5 N HCl the linear region extended from −0.8 to −1.3 V overpotential with a flattening out at high cathodic overpotentials to a value of SH,T 3·4. The rate of decrease of SH,T with increase in cathodic overpotential is given by dSH,inT/dη = 10·0 V−1 for 7·5 N HCl and dSH,T/dη = 8·5 V− for dilute H2SO4. New Tafel data was obtained for 7·5 N HCl in the region −10 to −71°C and new data for the dependence of io on temperature was obtained in this region. The strong dependence of SH,T on potential is ascribed to the effects of proton tunneling. An anomalous dependence of Tafel slope and io on temperature at temperatures below 5°C in 7·5 N HCl is ascribed to specific adsorption of Cl− ions.