The influence of equilibrium potential on the hydrogen oxidation kinetics of SOFC anodes

Fundamental electrochemical relations predict that the kinetic properties of an electrochemical charge-transfer reaction depend on reactant and product concentrations due to electrical equilibrium-potential (Nernst potential) effects. This paper discusses the consequences for the interpretation of observed reaction rates and orders of the electrochemical hydrogen oxidation at solid oxide fuel cell (SOFC) Ni/YSZ anodes. A thermodynamic model of the three-phase boundary is developed that describes the coupling of electroactive intermediates with global gas-phase reactants and products. The model is used to study the behavior of various reaction pathways proposed before, including hydrogen spillover, oxygen spillover, and interstitial hydrogen transfer. The results are compared with literature experimental data. The well-established activating effect of water on the SOFC anode kinetics can be explained by equilibrium-potential effects alone, without the necessity of assuming any additional kinetic or catalytic effect. (c) 2006 Elsevier B.V. All rights reserved.