Defect-Engineered Ba₄Ba_<i>x</i>Nb_2–<i>x</i>O_9−δ Ceramics: Oxygen Vacancy-Mediated Proton Transport for Low-Temperature Solid Oxide Fuel Cells

This study explores the structural, electrochemical, and proton transport properties of barium-doped barium niobate (Ba₄Ba_<i>x</i>Nb_2–<i>x</i>O_9−δ, BBN) ceramics for proton-conducting electrolytes. X-ray diffraction (XRD) confirmed single-phase orthorhombic perovskite structures for BBN (<i>x</i> = 0–0.4), whereas excessive doping (<i>x</i> = 0.5) led to cubic impurity phases. Rietveld refinement indicated lattice expansion (<i>V</i> = 779.29–791.33 ų) with increasing Ba²⁺ substitution, consistent with Vegard’s law. Electrochemical impedance spectroscopy (EIS) showed that BBN40 (<i>x</i> = 0.4) exhibited the highest total conductivity (5.21 mS·cm^–1 at 800 °C), which was 160 times higher than that of undoped BNO (<i>x</i> = 0), with a low activation energy of 0.41 eV. Proton conduction was dominant from 450 to 800 °C, with proton transference numbers reaching 0.88 at 550 °C, as validated by the defect equilibrium model and hydrogen concentration cell experiments (<i>Δ</i> < 10%). The increase in oxygen vacancy concentration from 39.9% (BNO) to 62.9% (BBN40), as demonstrated by X-ray photoelectron spectroscopy (XPS), underscores the role of Ba²⁺ doping in promoting ionic transport within the BBN40 structure. Optical bandgap analysis indicated a Burstein–Moss effect (3.78 eV in BNO to 4.08 eV in BBN40), leading to suppressed electronic conduction. Fuel cells utilizing BBN40 electrolytes achieved a peak power density of 514 mW·cm^–2 at 550 °C, which was 96% higher than that of BNO. These findings underscore the potential of Ba-doped BBN as a high-performance proton-conducting electrolyte for LT-SOFCs, offering a promising pathway for efficient and sustainable energy conversion technologies.