Mechanism of rate-limiting step switchover for reversible solid oxide cells in H2/H2O atmosphere

Abstract

Hydrogen-water are the primary reactant-product pair in the fuel electrode of reversible solid oxide cells. A different dependence of polarization resistance on pH2O in fuel cell and electrolysis modes has been proven experimentally, revealing different rate-limiting steps in fuel electrodes in these two modes. Despite extensive studies on solid oxide fuel cells or solid oxide electrolysis cells, existing literature is still hard to interpret this phenomenon. To understand the reaction mechanism of reversible solid oxide cells in H2/H2O atmosphere during current direction switch, we develop an elementary reaction mechanistic model of a nickel-patterned electrode button cell coupling charge transfer reactions, surface heterogeneous chemical reactions, and surface diffusion. We use a two-step hydrogen spillover mechanism, i.e., H(Ni)+O2−(YSZ) ↔ (Ni)+OH−(YSZ) +e− and H(Ni)+OH−(YSZ) ↔ (Ni)+H2O(YSZ)+e−, to describe charge transfer reactions. This model can well interpret this phenomenon, and further, reveal the inherent relationship between the operating condition and cell performance. Model calculation reveals that the limitation of OH−(YSZ) surface coverage (<1%) results that the charge transfer reaction generating OH−(YSZ) dominates electrochemical reaction rate no matter which mode a cell operates. To enhance the electrochemical performance of fuel electrode, it is the key to understand how to enhance the surface concentration of OH−(YSZ).