(Invited) Mitigating the Impact of Mixed Ionic-Electronic Conductivity for High-Power Density Operation of GDC-Based Solid Oxide Fuel Cells

Since the publication of the classic paper by Mogensen et al. [1] on the physical properties of pure and doped ceria, many solid-oxide fuel cell (SOFC) development efforts have sought to take advantage of the high ionic conductivities of doped ceria such as Ce0.9Gd0.1O1.95-δ (GDC) to achieve high SOFC power densities at high cell operating voltages. However, high polaron-driven electronic conductivity of GDC at temperatures above 600 °C causes drops in cell voltages due in part to leakage currents, which also increase internal heat generation [2] and place tighter temperature limits on SOFC stacks with GDC electrolytes. To mitigate these challenges and achieve high power densities in GDC-based SOFCs, strategies must be developed to sustain maximum cell temperatures at approximately 650 °C to avoid high leakage currents while supporting relatively high power densities at sustainable cell voltages ≥ 0.7 V/cell. In the operation of an SOFC stack with co-flowing anode and cathode flows, high cathode excess air ratios (λ) can mitigate temperature rise across the stack, but at the expense of significant cathode-side pressure drops. With the operation of SOFC stacks on hydrocarbon fuels such as CH4, the ability to use internal endothermic reforming at realistic fuel utilizations > 60% provides an additional approach to reducing temperature rises over the length of the stack. A collaboration between the Univ. of Maryland, Colorado School of Mines, Alchemity, and RTX Technology Research Center has focused on the design, development and demonstration of high-power density GDC-based stacks for operation on CH4 with inline upstream pre-reforming to control stack inlet conditions such that temperatures across the stack at high power densities can be maintained to mitigate the effects of mixed ionic-electronic conductivity. This study presents down-the-channel modeling and experimental demonstrations of GDC-based SOFCs operating on CH4 with recycled anode exhaust to explore the range of inlet flow conditions that enable GDC-based electrolyte cells to maintain high-power densities > 0.5 W/cm2.