Semiconductor membrane fuel cells are a new promising R&D for solid oxide fuel cells and proton ceramic fuel cells. There is a challenge of the electronic short circuit issue by using semiconductor to replace conventional electrolyte membrane. In this work, type II band alignment of the semiconductor heterostructure based on Mg-doped ZnO and ZnO can, on one hand, block electrons passing through the junction, and on the other hand, trigger the ionic properties of membrane to boost the fuel cell performance. The Mg doping into ZnO creates more oxygen vacancies at the surface of ZnO, leading to enhanced ionic transport, and meaningful fuel cell performance of 673 mW/cm2; while the Mg-doped ZnO/ZnO heterostructure fuel cell has delivered 997 mW/cm2 and OCV 1.04 V at 520 oC. It is worth highlighting that the constructed heterostructure interface, especially the band bending and constituted build-in electric field, plays a pivotal role in enhancing the ionic transport and suppressing the electron passing through the internal device. First principal calculations using density functional theory confirmed the doping of Mg and the formation of heterostructure with ZnO to help for enhancing charge carriers and separations. This work suggests that the constructed type II band alignment or the semiconductor heterostructure is useful for developing advanced fuel cells.

Perovskite oxide doping may modulate the energy bandgap strongly affecting the oxygen reduction activity and electrical properties with high promise for use as a low-temperature solid oxide fuel cell (LT-SOFC) electrolyte. Here, we show that a small amount of Al-doping into SrTiO_3−δ may tune the energy band structure of SrTiO_3−δ, triggering the electrochemical mechanism and fuel cell performance. The synthesized SrTiO_3−δ and Al-SrTiO_3−δ electrolytes are sandwiched between two symmetrical electrodes (Ni foam pasted NCAL). Ni-NCAL/SrTiO_3−δ/NCAL-Ni and Ni-NCAL/Al-SrTiO_3−δ/NCAL-Ni structures delivered a maximum power density of 0.52 W and 0.692 W and high ionic conductivity of 0.11 S cm⁻¹ and 0.153 S cm⁻¹, respectively, under H₂/Air atmosphere at low operational temperature of 520 °C. Highways of ion transport along the surface and grain boundary (interface) are identified as the main reason for good oxygen ion conduction. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), UV visible (UV), and X-ray photoelectron spectroscopy (XPS) were performed to investigate the structural, electrochemical, morphological, surface and interfacial properties of Al-SrTiO_3−δ. The obtained results suggest that a certain amount of Al (20%) doping into SrTiO₃ (SrTi_0.8Al_0.2O_3−δ) improves the fuel cell performance and is a promising electrolyte candidate for LT-SOFC.

Reducing the operating temperature of Solid Oxide Fuel Cells (SOFCs) to 300–600 °C is a great challenge for the development of SOFC. Among the extensive research and development (R&D) efforts that have been done on lowering the operating temperature of SOFCs, nanomaterials have played a critical role in improving ion transportation in electrolytes and facilitating electrochemical catalyzation of the electrodes. This work reviews recent progress in lowering the temperature of SOFCs by using semiconductor-ionic conductor nanomaterial, which is typically a composition of semiconductor and ionic conductor, as a membrane. The historical development, as well as the working mechanism of semiconductor-ionic membrane fuel cell (SIMFC), is discussed. Besides, the development in the application of nanostructured pure ionic conductors, semiconductors, and nanocomposites of semiconductors and ionic conductors as the membrane is highlighted. The method of using nano-structured semiconductor-ionic conductors as a membrane has been proved to successfully exhibit a significant enhancement in the ionic conductivity and power density of SOFCs at low temperatures and provides a new way to develop low-temperature SOFCs.