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

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Achieving fast ionic conductivity in the electrolyte at low operating temperatures while maintaining the stable and high electrochemical performance of solid oxide fuel cells (SOFCs) is challenging. Herein, we propose a new type of electrolyte based on perovskite Sr0.5Pr0.5Fe0.4Ti0.6O3−δ for low-temperature SOFCs. The ionic conducting behavior of the electrolyte is modulated using Mg doping, and three different Sr0.5Pr0.5Fe0.4–x Mg x Ti0.6O3−δ (x = 0, 0.1, and 0.2) samples are prepared. The synthesized Sr0.5Pr0.5Fe0.2Mg0.2Ti0.6O3−δ (SPFMg0.2T) proved to be an optimal electrolyte material, exhibiting a high ionic conductivity of 0.133 S cm–1 along with an attractive fuel cell performance of 0.83 W cm–2 at 520 °C. We proved that a proper amount of Mg doping (20%) contributes to the creation of an adequate number of oxygen vacancies, which facilitates the fast transport of the oxide ions. Considering its rapid oxide ion transport, the prepared SPFMg0.2T presented heterostructure characteristics in the form of an insulating core and superionic conduction via surface layers. In addition, the effect of Mg doping is intensively investigated to tune the band structure for the transport of charged species. Meanwhile, the concept of energy band alignment is employed to interpret the working principle of the proposed electrolyte. Moreover, the density functional theory is utilized to determine the perovskite structures of SrTiO3−δ and Sr0.5Pr0.5Fe0.4–x Mg x Ti0.6O3−δ (x = 0, 0.1, and 0.2) and their electronic states. Further, the SPFMg0.2T with 20% Mg doping exhibited low dissociation energy, which ensures the fast and high ionic conduction in the electrolyte. Inclusively, Sr0.5Pr0.5Fe0.4Ti0.6O3−δ is a promising electrolyte for SOFCs, and its performance can be efficiently boosted via Mg doping to modulate the energy band structure.

Tuning semiconductors as an electrolyte for advanced low-temperature fuel cells is an exciting but challenging research subject. To realize this, we develop the cobalt-doped SrSnO3 (SrCo0.1Sn0.9O3−δ and SrCo0.2Sn0.8O3−δ) toward an electrolyte, which only permits ions to pass but blocks the electrons simultaneously. The SrCo0.2Sn0.8O3−δ electrolyte fuel cell has achieved a remarkable performance with a maximum power density of 476 mW cm–2 and obtained a high ionic conductivity of 0.12 S cm–1 at a low temperature of 520 °C. This improved performance is accredited to the bandgap engineering and built-in electric field, which significantly enhanced the ionic transport while suppressing the electronic conduction. The doped SrCoSnO3−δ perovskite materials demonstrated a high potential for solving the low-temperature solid oxide fuel cell (LT-SOFC) material’s challenging problems.

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Introducing triple-charge (H+/O2–/e–) conducting materials is a promising alternative to modify a cathode as an electrolyte in advanced ceramic fuel cells (CFC). Herein, we designed a novel triple-charge conducting perovskite-structured semiconductor Co0.2/Fe0.2-codoped La0.5Ba0.5Zr0.3Y0.3O3−δ (CF-LBZY) and used as an electrolyte and an electrode. CF-LBZY perovskite as an electrolyte exhibited high ionic (O2–/H+) conductivity of 0.23 S/cm and achieved a remarkable power density of 656 mW/cm2 550 °C. X-ray photoelectron spectroscopy (XPS) analysis revealed that the Co/Fe codoping supports the formation of oxygen vacancies at the B-site of a perovskite structure. Besides, using CF-LBZY as a cathode, the fuel cell delivered 150 and 177 mW/cm2 at 550 °C, respectively, where Y-doped BaZrO3 and Sm-doped ceria (SDC) were used as electrolytes. During the fuel-cell operation, H+ injection into the CF-LBZY electrolyte may suppress electronic conduction. Furthermore, the metal–semiconductor junction (Schottky junction) has been proposed by considering the work function and electron affinity to interpret short-circuiting avoidance in our device. The current systematic study indicates that triple-charge conduction in CF-LBZYO3−δ has potential to boost the electrochemical performance in advanced low-temperature fuel-cell technology.

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