Evaluating Operating Conditions and Thermal Management in High Power Density Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) with their high energy conversion efficiencies offer a way to use dense hydrogen-rich liquid fuels for hybrid electric power in transportation applications. Doped ceria electrolytes have high ionic conductivity and can achieve area-specific power-densities above 1.0 W cm-2 at intermediate temperatures below 700 °C. This work reports a physics-based model to characterize the performance of a gadolinia-doped ceria (GDC) electrolyte-based SOFC at high power-density operating scenarios. The model couples defect thermochemistry, charge transfer kinetics for multiple reactions, and porous media transport in both electrodes to quantify how the mixed ionic-electronic conduction of GDC impacts cell performance. The model is experimentally validated with button cell data over a range of operating temperatures (500-650 °C) and pressures (1.0 - 5.0 bar). A down-the-channel version of the model for operation with both hydrogen and autothermally reformed methane gas shows that high power densities necessitate cathode flows with excess air ratios ≥ 5 to sustain axial temperature gradients ≤ 15 K cm-1 Simulations indicate no power density benefit of operating the GDC cell at temperatures above 680 °C due to the rapid increase in polaron leakage current through the electrolyte membrane at those temperatures. Polaron leakage can be mitigated by operating at cell voltages below 0.7 V, although continuous operation at high current densities associated with these voltages may induce shorter cell lifetime. Power densities over 1.0 W cm-2 can be achieved at the cell level for pressurized operation with careful control of operating temperature and excess air ratio. Such power densities can enable stack specific power of over 1 kW kg-1 that can accelerate the adaptation of SOFCs in the transportation sector.