Designing high-performance Eu3+-doped CeO₂ electrolytes for low-temperature semiconductor ionic fuel cells

Achieving high-performance operation in ceramic fuel cells (CFCs) at reduced temperatures (<600 °C) remains a significant challenge in pursuing next-generation clean energy technologies. In this study, we introduce a fluorite-structured electrolyte, Europium-doped ceria (EDC), which enables high performance in semiconductor-ionic membrane fuel cells (SIMFCs), particularly at low operating temperatures (<520 °C). Incorporating Eu3+ into the ceria lattice induces a high density of oxygen vacancies (O_VS) and local lattice strain, which together foster electroactive grain boundaries due to a dynamic Ce4+/Ce3+ redox interplay. These modifications establish a dual-ion conduction mechanism for both protons (H+) and oxide ions (O2−), supported by extensive spectroscopic evidence from Raman, EPR, and XPS studies. This multifaceted design yields effective ionic conductivity of 0.25 S cm−1 and a peak power density of 1316 mW/cm2 at 520 °C, outperforming traditional fluorite-based systems. Additionally, various experiments were conducted to confirm proton conduction in EDC, including hydrogen concentration tests, proton filtering layer approaches, isotopic effect studies, and electrochemical impedance spectroscopy (EIS) combined with distribution of relaxation time (DRT) analysis. The fuel cell demonstrated good durability, maintaining stable performance for 80 h at 520 °C. The findings establish a new design framework where tailored interfacial chemistry, controlled defect structures, and synergistic ion transport mechanisms converge to enable efficient low-temperature operation in advanced SIMFCs.