Electrochemical impedance spectroscopy study of a hydrogen electrode reaction at a Zn electrode in a molten LiCl-KCl-LiH system

The hydrogen electrode reaction involving hydride ion, H^-, at a Zn electrode is investigated in a molten LiCl-KCl-LiH system at 673 K. The charge-transfer resistances were measured by electrochemical impedance spectroscopy in the overpotential region of 0.10 ≤ η ≤ 0.35 V and over the H^- concentrations of 1.5 × 10^-4 ≤ CH ^- ≤ 1.2 × 10^-3 mol cm^-3. The logarithm plot of the charge-transfer resistance against the overpotential at C _H^- = 3.0 × 10^-4 mol cm^-3 gives the symmetry factor, β, of 0.50 and the exchange current density, j ₀, of 5.8 × 10^-3A cm^-2, respectively. Analysis of the dependence of j₀ on H^- concentration independently gives a β of 0.55. The reasonable β values indicate that the H^- ⇋ H_ad(M) + e^- step is rate-determining.