Two-dimensional modeling for physical processes in direct flame fuel cells

Abstract

Commercial direct flame fuel cells (DFFC) need larger cell surface area for higher power output. In such cases, multi-dimensional effects play significant roles on cell performances. In this work, a two-dimensional numerical model is developed to illustrate physical behaviors associated with the multi-dimensional effects in DFFCs. It is revealed that DFFCs suffer from the negative consequences of non-uniform distributions of temperature, species and voltage in radial direction. Non-uniform distributions of temperature and species results in the decrease of current density at edge regions of DFFCs, owing to lower ionic conductivities and fuel species concentration. And the non-uniform voltage distribution in radial direction causes the decreases of current density at center regions of DFFCs due to the lower over-potential there. Therefore, current density distributions in electrolytes are likely to be M-shaped. The multi-dimensional effects become progressively important with increasing the size of solid oxide fuel cells. Comparing with the DFFC with a SOFC with small cell radius (6.5 mm), a DFFC with a SOFC with large cell radius (33.75 mm) has 25–30% lower maximum power density. We also reveal that cross-over electronic currents in samaria-doped-ceria electrolytes and fuel species starvation due to the secondary oxidation are dominant factors on the cell performance loss at high cell temperatures (∼1000 K).