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Ultra‐wide bandgap semiconductor samarium oxide attracts great interest because of its high stability and electronic properties. However, the ionic transport properties of Sm2O3 have rarely been studied. In this work, Ni doping is proposed to be used for electronic structure engineering of Sm2O3. The formation of Ni‐doping defects lowers the Fermi level to induce a local electric field, which greatly enhances the proton transport at the surface. Furthermore, ascribed to surface modification, the high concentration of vacancies and lattice disorder on the surface layer promote proton transport. A high‐performance of 1438 mW cm–2 and ionic conductivity of 0.34 S cm–1 at 550 °C have been achieved using 3% mol Ni doped Sm2O3 as electrolyte for fuel cells. The well‐dispersed Ni doped surface in Sm2O3 builds up continuous surfaces as proton channels for high‐speed transport. In this work, a new methodology is presented to develop high‐performance, low‐temperature ceramic fuel cells.

Fuel cells are highly efficient and green power sources. The typical membrane electrode assembly is necessary for common electrochemical devices. Recent research and development in solid oxide fuel cells have opened up many new opportunities based on the semiconductor or its heterostructure materials. Semiconductor-based fuel cells (SBFCs) realize the fuel cell functionality in a much more straightforward way. This work aims to discuss new strategies and scientific principles of SBFCs by reviewing various novel junction types/interfaces, i.e., bulk and planar p-n junction, Schottky junction, and n-i type interface contact. New designing methodologies of SBFCs from energy band/alignment and built-in electric field (BIEF), which block the internal electronic transport while assisting interfacial superionic transport and subsequently enhance device performance, are comprehensively reviewed. This work highlights the recent advances of SBFCs and provides new methodology and understanding with significant importance for both fundamental and applied R&D on new-generation fuel cell materials and technologies.

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Multifunctional semiconductor cubic silicon carbide (3C-SiC) is employed for fuel cell electrolyte, which has never been used before. n-type 3C-SiC can be individually employed as the electrolyte in fuel cells, but delivers insufficient open circuit voltage and minuscule current density due to its electronic dominant property. By introducing n-type ZnO to form an n–n 3C-SiC/ZnO heterostructure, significant enhancements in the ionic conductivity of 0.12 S/cm and fuel cell performance of 270 mW cm−2 are achieved at 550 °C. It is found that the energy band bending and build-in electric field of the heterostructure play the pivotal role in the ionic transport and suppressing the electronic conduction of 3C-SiC, leading to a markable material ionic property and fuel cell performance. These findings suggest that 3C-SiC can be tuned to ionic conducting electrolyte for fuel cell applications through the heterostructure approach and energy band alignment methodology.

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• Ceria-based heterostructure composite for novel semiconductor-ionic fuel cells. • Superionic conduction at interfaces is associated with the crossover of band structure. • Band alignment/bending resultant built-in field plays a significant role in superionic conduction. ABSTRACT Ceria-based heterostructure composite (CHC) has become a new stream to develop advanced low-temperature (300-600 °C) solid oxide fuel cells (LTSOFCs) with excellent power outputs at 1000 mW cm −2 level. The state-ofthe-art ceria-carbonate or ceria-semiconductor heterostructure composites have made the CHC systems significantly contribute to both fundamental and applied science researches of LTSOFCs; however, a deep scientific understanding to achieve excellent fuel cell performance and high superionic conduction is still missing, which may hinder its wide application and commercialization. This review aims to establish a new fundamental strategy for superionic conduction of the CHC materials and relevant LTSOFCs. This involves energy band and built-in-field assisting superionic conduction, highlighting coupling effect among the ionic transfer, band structure and alignment impact. Furthermore, theories of ceria-carbonate, e.g., space charge and multi-ion conduction, as well as new scientific understanding are discussed and presented for functional CHC materials.

The p-i-n junction structure has garnered great interest from researchers due to its increased photoactive area and wider space charger region compared with that of the p-n junction. Herein the ultrawide bandgap (∼6.7 eV) semiconductor alumina (Al2O3) has been introduced to construct a p-i-n junction with ZnO and NiO. ZnO–Al2O3–NiO powders are suitable for electrolyte membrane material of low-temperature solid oxide fuel cells (LT-SOFCs). The insulating Al2O3 brings a doubled depletion region and more effective charge separation compared with the p-n junction consisting of ZnO and NiO. The fuel cell based on the p-i-n junction membrane material delivers an enhanced maximum power density (MPD) of 917 mW cm–2 along with a high open-circuit voltage (OCV) of 1.013 V at 550 °C. An analysis based on the space charge region and energy band alignment in the p-i-n junction is proposed to account for the appreciable cell performance and ionic conductivity, as well as the electron blocking and charge separation ability. This study presents a practical p-i-n junction design based on ZnO–Al2O3–NiO via a solution process for LT-SOFC electrolyte material and sheds light on the extensive exploration of p-i-n junctions applied in LT-SOFCs.