Conversion of low-grade heat from molten carbonate fuel cells to electricity by thermoradiative devices with optimized transport properties

High-temperature fuel cells are known for their excellent output power stability, high efficiency, and modest material requirements, but they also produce significant amounts of waste heat. To address this, they can be paired with thermoradiative devices (TDs) to convert the waste heat into electricity. In this work, we demonstrate that the performance of a combined system — consisting of a molten carbonate fuel cell (MCFC) and a TD — can be substantially improved by optimizing the TD's transport properties. Specifically, we investigate TDs constructed from materials with two regions of differing bandgaps, modeled using three quasi-Fermi levels (QFLs). We account for heat leakage between the MCFC and the environment, deriving expressions for the output power and efficiency of the integrated MCFC-TD system. Our results show that the maximum output power density (MOPD) and corresponding efficiency of MCFCs operating at 923K can be increased by up to 170% and 47%, respectively, by incorporating an optimized TD. These improvements significantly exceed previously reported performance gains from MCFC-TD systems, underscoring the importance of including transport properties in TD modeling. Additionally, we find that the optimal bandgap for the TD semiconductor material (≈0.2077eV) is approximately twice the value previously estimated (≈0.1061eV), offering practical advantages for material fabrication. Furthermore, the observed MOPD improvements for our MCFC-TD model exceed those of other MCFC-based systems. The presented methodology and findings can be readily extended to other high-temperature fuel cells.