Thermodynamic investigations of a hydrogen electrode reaction in a molten LiCl-KCl-LiH system

Abstract

The equilibrium potential of the H2/H- couple has been measured in a molten LiCl-KCl-LiH system at 643-883 K. The Nernst relation holds for the potential at hydrogen pressure, pH2, of 0.1-1.0 atm and H- ion concentration, XH-, of 0.002-0.080 (anion fraction). The standard formal potential of H2/H-, EH2/H-0′, is evaluated to be 0.258 ± 0.002 V (vs. Li+/Li) (pH2 = 1 atm, XH- = 1) at 673 K. EH2/H-0′ changes linearly with temperature at 643-883 K, having the gradient of -0.575 ± 0.011 mV K-1. Thermodynamic quantities for formation of LiH in the melt are also estimated. When the activity coefficient of H- ion, γH-, is defined as unity in pure liquid LiH (XH- = 1), γH- in the dilute H- region (0.002 ≤ XH- ≤ 0.080) is evaluated to be 7.1.