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

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 (similar to 1000 K). (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.