Since colossal ionic conductivity was detected in the planar heterostructures consisting of fluorite and perovskite, heterostructures have drawn great research interest as potential electrolytes for solid oxide fuel cells (SOFCs). However, so far, the practical uses of such promising material have failed to materialize in SOFCs due to the short circuit risk caused by SrTiO3. In this study, a series of fluorite/perovskite heterostructures made of Sm-doped CeO2 and SrTiO3 (SDC–STO) are developed in a new bulk-heterostructure form and evaluated as electrolytes. The prepared cells exhibit a peak power density of 892 mW cm−2 along with open circuit voltage of 1.1 V at 550 °C for the optimal composition of 4SDC–6STO. Further electrical studies reveal a high ionic conductivity of 0.05–0.14 S cm−1 at 450–550 °C, which shows remarkable enhancement compared to that of simplex SDC. Via AC impedance analysis, it has been shown that the small grain-boundary and electrode polarization resistances play the major roles in resulting in the superior performance. Furthermore, a Schottky junction effect is proposed by considering the work functions and electronic affinities to interpret the avoidance of short circuit in the SDC–STO cell. Our findings thus indicate a new insight to design electrolytes for low-temperature SOFCs.

Interface engineering holds huge potential for enabling exceptional physical properties in heterostructure materials via tuning properties at the atomic level. In this study, a heterostructure built by a new redox stable semiconductor SrFe 0.75 Ti 0.25 O 3−δ (SFT) and an ionic conductor Sm 0.25 Ce 0.75 O 2 (SDC) is reported. The SFT−SDC heterostructure exhibits a high ionic conductivity >0.1 S/cm at 520 °C, which is 1 order of magnitude higher than that of bulk SDC. When it was applied into the fuel cell, the SFT−SDC can realize favorable electrolyte functionality and result in an excellent power density of 920 mW cm −2 at 520 °C. The prepared SFT−SDC heterostructure materials possess both electronic and ionic conduction, where electron states modulate local electrical field to facilitate ion transport. Further investigations to calculate the structure and electronic structure/state of SFT and SDC are done using density functional theory (DFT). It is found that the reconstruction of the energy band at interfaces is responsible for such enhanced ionic conductivity and cell power output. The current study about the perovskite-based heterostructure presents a novel strategy for developing advanced ceramic fuel cells. KEYWORDS: heterostructure, SrFe 0.75 Ti 0.25 O 3-δ -Sm 0.25 Ce 0.75 O 2−δ (SFT−SDC), ionic conduction, band structure, built-in field

Semiconductors and the associated methodologies applied to electrochemistry have recently grown as an emerging field in energy materials and technologies. For example, semiconductor membranes and heterostructure fuel cells are new technological trend, which differ from the traditional fuel cell electrochemistry principle employing three basic functional components: anode, electrolyte, and cathode. The electrolyte is key to the device performance by providing an ionic charge flow pathway between the anode and cathode while preventing electron passage. In contrast, semiconductors and derived heterostructures with electron (hole) conducting materials have demonstrated to be much better ionic conductors than the conventional ionic electrolytes. The energy band structure and alignment, band bending and built-in electric field are all important elements in this context to realize the necessary fuel cell functionalities. This review further extends to semiconductor-based electrochemical energy conversion and storage, describing their fundamentals and working principles, with the intention of advancing the understanding of the roles of semiconductors and energy bands in electrochemical devices for energy conversion and storage, as well as applications to meet emerging demands widely involved in energy applications, such as photocatalysis/water splitting devices, batteries and solar cells. This review provides new ideas and new solutions to problems beyond the conventional electrochemistry and presents new interdisciplinary approaches to develop clean energy conversion and storage technologies. Graphic Abstract

Dual-ion electrolytes with oxygen ion and proton-conducting properties are among the innovative solid oxide electrolytes, which exhibit a low Ohmic resistance at temperatures below 550 °C. Ba-Co 0.4 Fe 0.4 Zr 0.1 Y 0.1 O 3−δ with a perovskite-phase cathode has demonstrated efficient triple-charge conduction (H + /O 2− /e − ) in a high-performance lowtemperature solid oxide fuel cell (LT-SOFC). Here, we designed another type of triple-charge conducting perovskite oxide based on Ba 0.5 Sr 0.5 Co 0.1 Fe 0.7 Zr 0.1 Y 0.1 O 3−δ (BSCFZY), which formed a heterostructure with ionic conductor Ca 0.04 Ce 0.80 Sm 0.16 O 2−δ (SCDC), showing both a high ionic conductivity of 0.22 S cm −1 and an excellent power output of 900 mW cm −2 in a hybrid-ion LT-SOFC. In addition to demonstrating that a heterostructure BSCFZY−SCDC can be a good functional electrolyte, the existence of hybrid H + /O 2− conducting species in BSCFZY−SCDC was confirmed. The heterointerface formation between BSCFZY and SCDC can be explained by energy band alignment, which was verified through UV−vis spectroscopy and UV photoelectron spectroscopy (UPS). The interface may help in providing a pathway to enhance the ionic conductivities and to avoid short-circuiting. Various characterization techniques are used to probe the electrochemical and physical properties of the material containing dual-ion characteristics. The results indicate that the triple-charge conducting electrolyte is a potential candidate to further reduce the operating temperature of SOFC while simultaneously maintaining high performance. KEYWORDS: triple-charge conduction, Ba 0.5 Sr 0.5 Co 0.1 Fe 0.7 Zr 0.1 Y 0.1 O 3−δ (BSCFZY) perovskite, semiconductor−ion heterostructure, Schottky junction, dual-ion conductivity, band alignment