Interfacial Engineering for Enhanced Protonic Conduction in Na_<i>x</i>CoO_2−δ–Sm_0.2Ce_0.8O_2−δ Semiconductor Ionic Heterostructures for Low-Temperature Solid Oxide Fuel Cells

Interfacial engineering is pivotal in optimizing the ionic conductivity in semiconductor–ionic electrolytes for low-temperature solid oxide fuel cells (LT-SOFCs). In this study, we propose a semiconductor NaxCoO2−δ and ionic Sm0.2Ce0.8O2−δ (SDC) heterostructure as a functional membrane sandwiched between two symmetric porous electrodes LiNi0.8Co0.15Al0.05O2−δ (NCAL). The A-site non-stoichiometry in NaxCoO2−δ modifies the energy band structure by altering the Co3+/Co4+ concentration, thereby regulating the conduction properties. Structural and electrical characterization of the heterostructure material was conducted to investigate heterointerfaces, oxygen vacancies, and their influence on charge carrier transportation. Electrochemical impedance spectroscopy demonstrated remarkable performance for Na0.7CoO2–SDC (NCO7–SDC), which exhibited an ionic conductivity of 0.132 S/cm at 550 °C under 3% H2O humidified (4% H2 + 96% N2) conditions. Enhanced interfacial ionic transportation is attributed to the synergistic interplay of the Li+-rich space-charge layers, energy band alignment, and excess oxygen vacancies generated at the semiconductor–ionic interface along with the Schottky junction between the metallic Ni-electrode and heterostructure electrolyte. Our investigation further reveals that the optimal concentration of Na ions is crucial for inducing appropriate band bending and excess oxygen vacancy generation in Na0.7CoO2–SDC, to enhance the protonic conduction. XPS analysis of the hydrogen-exposed sample confirmed the dominant ionic conduction through the H+ and OH– charge species. These findings emphasize the potential of NaxCoO2–SDC as a high-performance electrolyte for LT-SOFC, even with low-concentration H2 fuel, paving the way for advancement in fuel cell technology.