(81a) Theoretical Investigation of the H2 Oxidation On the Sr2Fe1.5Mo0.5O6-d (001) Perovskite Structure Under Anodic Solid Oxide Fuel Cell Conditions

Anodic Solid Oxide Fuel Cell Conditions

Suwit Suthirakun, Salai Cheettu Ammal, and Andreas Heyden*

University of South Carolina

Department of Chemical Engineering

301 S. Main St., Columbia, South Carolina 29208, USA

*heyden@cec.sc.edu

Ni-based electrodes have been the most commonly used anode materials in solid oxide fuel cells (SOFCs) because they exhibit excellent electrochemical performance with pure H₂ as fuel. However, when hydrocarbon fuels are used, these anode materials suffer from several limitations such as instability upon redox cycling, nickel sintering, and sulfur and carbon poisoning. Hence, finding alternative anode materials to replace conventional Ni-based cermet electrodes has become an important objective in the development of SOFC technology. Among the novel anode catalysts, perovskite based materials (ABO₃) are of great interest because they can accommodate various dopants and incorporate cations with multiple oxidation states which often improves the electro-catalytic activity and provides a mechanism for mixed electronic and ionic conductivity.

                Nevertheless, perovskite based anode materials usually display low electrochemical performance. For example, Xiao et al. report for Sr₂Fe_1.5Mo_0.5O_6-δ (SFM) anode materials a very low cell performance when using H₂ or CH₄ as fuels; while a significant improvement in performance occurs when Ni particles are dispersed on the catalyst surface (1).

                To better understand the reasons for this experimentally observed anode behavior we performed density functional theory (DFT) calculations to investigate the H₂ oxidation mechanism on the SFM (001) surface.  First, we performed constrained ab initio thermodynamic calculations to obtain a surface model at anodic SOFC conditions. Then, we studied the reaction mechanism from first principles and developed a microkinetic model to identify the rate determining step of the reaction. All calculations performed for this study are based on the plane wave DFT + U theory as implementation in the Vienna Ab initio Simulation Package (VASP 5.2).

We find the H₂ dissociation step to be difficult (it has a high activation barrier of 1.64 eV) and a microkinetic analysis of the reaction mechanism reveals that the H₂ dissociation is indeed rate determining with a Campbell’s degree of rate control of 0.50.  The other rate effecting steps are the H migration and the water desorption with a Campbell’s degree of rate control of 0.25 each.  Overall, we compute an apparent activation barrier of 0.76 eV.

                The calculated results are in excellent agreement with experimental observations that the addition of Ni improves the electrochemical performance of the SOFC cell (1). Adding a transition metal to the SFM surface (e.g. Ni) facilitates H₂ dissociation and therefore improves the overall cell performance.

1. G. Xiao and F. Chen, Electrochem. Commun. 13, 57 (2011).