Coupled carrier highways enabled by vacancy-band co-engineering in fluorite ceria for developing superionic conductors

Enabling high protonic conductivity in single-phase ceramic electrolytes below 600 °C remains a central challenge for electrochemical energy conversion. In fluorite oxides, proton transport is typically constrained by the energetic of oxygen-vacancy formation and high migration barriers. Here, we demonstrate that vacancy-band co-engineering activates bulk proton transport in a single-phase fluorite lattice, using cobalt-aluminium co-doped ceria (Ce_0.8Co_0.1Al_0.1O_2-δ, 10-ACCO) as a model system. Cobalt (Co) increases oxygen-vacancy concentration and introduces near-edge defect states, whereas aluminium (Al) tunes the band-edge positions. These combined modifications correlate with enhanced proton conductivity and reduced total resistance (R_t), establishing cooperative "coupled carrier highways." Density functional theory calculations reveal a reduced proton migration barrier (∼0.26 eV) along with dopant-induced modifications in the electronic structure. These results are supported by XPS, UPS, UV-Vis spectroscopy, and structural analyses obtained from XRD and HRTEM. Electrochemical measurements demonstrate high performance, with a peak power density of 1245 mW cm−2 at 550 °C and an ionic conductivity of 0.223 S cm−1. Furthermore, impedance spectroscopy and distribution-of-relaxation-time analysis confirm suppressed polarization losses and reduced activation barriers. This work establishes vacancy-band coupling as an effective design principle for proton-conducting fluorite electrolytes, enabling lower-temperature ceramic electrochemical technologies.