Hodrogen Oxidation Reaction Mechanisms in Mixed Ionic and Electronic Conducting Oxide Anode for Solid Oxide Fuel Cells

Abstract

Ceramics anode offers certain advantages in terms of impurity tolerance and other properties, as compared to conventional Ni/YSZ anode for solid oxide fuel cells (SOFC). However, ceramic anodes typically have much lower electrochemical performance, and there is very few studies on the hydrogen oxidation reaction (HOR) mechanisms and kinetics in ceramic anodes. In this research, using Doped YCrO3 as baseline material, we systematically investigated the HOR mechanisms in mixed ionic and electronic conducting (MIEC) ceramic anodes, with the focus on the effect of electronic conductivity, electro-catalytic activity, triple-phase-boundary (3PB), and electrolyte, on the HOR kinetics. It is found that both electrical conductivity and electrode performance of yttrium chromites have been enhanced after Co and Ni doping. Electrochemical impedance spectroscopy (EIS) results indicate that charge transfer process at high frequency and surface adsorption/diffusion processes at low frequency domain can be the dominant anode reaction steps. Ni doping accelerates the surface processes by reducing the related activation energy from 1.2 to 0.5 eV. It also substantially improves the charge transfer process probably by increasing the amount of adsorbed H on electrode surface. The resistance of high frequency is found to be dependent on H2 content. The observed reaction order is 1/4 for Co doped and 1/3 ~ 1/2 for Ni doped yttrium chromites. A model of H2oxidation reaction is proposed, revealing this dependence stems from the reaction between adsorbed H and the lattice oxygen. The influence of electrolyte to electrode performance was investigated by replacing YSZ electrolyte with scandium stabilized zirconium (SSZ). Smaller polarization resistances are observed on each electrode in both wet H2 and air atmospheres. Replacement of electrolyte can alter not only the rate of charge transfer process but also in some cases other surface processes not related to the electrolyte directly. It is proposed that the impact of the electrolyte on each electrode process is passed down as in a chain and the charge transfer step functions as the first ring in the chain.