Electrochemical Properties of a Dual-Ion Semiconductor-Ionic Co_0.2Zn_0.8O-Sm_0.20Ce_0.80O_2-δComposite for a High-Performance Low-Temperature Solid Oxide Fuel Cell

Semiconductor heterostructures offer a high ionic conduction path enhanced by built-in electric field at the interface, which helps to avoid electronic conduction in low-temperature solid oxide fuel cells (LT-SOFCs). In this study, we synthesized a semiconductor heterostructure based on Co-doped ZnO and Sm0.2Ce0.8O2-δ (SDC) for LT-SOFC application. First, we optimized the composition of the Co-doped ZnO by varying the doping concentration. The cell with Co0.2Zn0.8O composition (σi = 0.158 S cm-1) yielded the best performance of 664 mW cm-2 at 550 °C. This optimized composition of Co-doped ZnO was mixed with a well-known ionic conductor Sm0.2Ce0.8O2-δ (SDC) to further improve the ionic conductivity and performance of the cell. The heterostructure formed between these two semiconductor materials improved the ionic conductivity of this composite material to 0.24 S cm-1 at 550 °C, which is 2 orders higher in magnitude than that of bulk SDC. The fuel cells fabricated with this promising semiconductor-ionic heterostructure material produced an outstanding power density of 928 mW cm-2 at 550 °C. Our further investigation shows protonic conduction (H+) in the Co0.2Zn0.8O-SDC composite, which exhibited protonic conduction 0.088 S cm-1 with a power density of 388 mW cm-2 at 550 °C. A detailed characterization of the material and the fuel cells is performed with the help of different electrochemical (electrochemical impedance spectroscopy (EIS)), spectroscopic (X-ray diffraction (XRD), UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS)), and microscopic techniques (scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectrometry (EDX)). The stability of the cell was tested for 35 h to ensure stable operation of these devices. This semiconductor-ionic heterostructure composite provides insight into the development of electrolyte membranes for advanced SOFCs.