GAS ELECTRODE-REACTIONS IN MOLTEN CARBONATE MEDIA .4. ELECTRODE-KINETICS AND MECHANISM OF HYDROGEN OXIDATION IN (LI+K)CO3 EUTECTIC

The hydrogen oxidation reaction in (62 + 38) mole percent (Li + K)CO3 at 650°C has been studied at Au, Ag, Cu, Pt, Ir, Pd, Ni, Co, and Fe electrodes by using cyclic voltammetry, ac impedance, potential step, and chronocoulometric methods. Impedance data were analyzed by computer fitting with an equivalent circuit, taking hydrogen adsorption into account. The exchange current density decreased, under the gas composition of [formula omitted], in the following series: Pd(136.1 mA/cm2) > Ni > Pt > Ir > Au > Ag(13.3 mA/cm2). The chronocoulometric method was employed to determine the stoichiometric number v using a new technique described in this paper, and the value obtained was v = 2. The apparent transfer coefficient, a, and exchange current density were also determined from an Allen-Hickling plot, and found to be α = 0.1 ~ 0.25 for Au, Ir, and Ag, and α = 0.5 for Ni and Pt. The concentrations and diffusion coefficients of the reactant and the product species, including the diffusion process of hydrogen dissolved in the Ni lattice, were estimated, based on an estimated value for [formula omitted] from the Wilke-Chang equation. Based on these results, a detailed reaction mechanism is proposed in this paper. © 1990, The Electrochemical Society, Inc. All rights reserved.