Enhancing Proton Conductivity in the Ce0.9Gd0.1O2–δ Electrolyte for Low-Temperature SOFCs via Surface Doping with a Cobalt Oxide Redox Mediator

Abstract Developing oxide electrolytes with high ionic conductivity (∼0.1 S cm–1) at temperatures below 600 °C is essential for next-generation solid oxide fuel cells. However, current electrolyte materials rarely meet this benchmark, primarily because of limitations inherent in bulk doping and high-temperature sintering. To overcome this challenge, we demonstrate a surface engineering strategy that involves controlled cobalt doping onto the acid-etched surface of gadolinium-doped ceria (Ce0.9Gd0.1O2–δ, GDC), thereby constructing a Co3+/Co2+ redox mediator. The 3 mol% cobalt-surface-doped GDC (GDC-Co3) electrolyte exhibits optimal ionic transport behavior, achieving an ionic conductivity of 0.153 S cm–1 at 500 °C and a 30% improvement over pristine GDC (0.118 S cm–1), while simultaneously reducing the activation energy from 0.64 eV to 0.36 eV. X-ray photoelectron spectroscopy (XPS) and multi-wavelength Raman spectroscopy reveal that Co2+/Co3+ redox pairs are introduced onto the surfaces of GDC particles through doping with different Co(II)/GDC molar ratios. The formation of a Schottky junction, as evidenced by a rectifying effect, effectively prevents electron leakage across the electrolyte. Electrochemical impedance spectroscopy (EIS), combined with distribution of relaxation times (DRT) analysis, indicates that ionic conductivity is strongly correlated with electronic conductivity. These results collectively support a mechanism where surface-localized Co2+/Co3+ redox pairs act as mediators for the Ce4+/Ce3+ couples in GDC, establishing favorable surface pathways for proton transport via an alternating interstitial proton-hydrogen hopping process. The accompanying reduction in electrode polarization resistance suggests that accelerated interfacial reaction kinetics may also contribute to the improved cell performance, alongside enhanced proton transport. This work presents an innovative surface engineering approach that significantly enhances proton transport in fluorite oxide materials, with important implications for low-temperature solid oxide electrochemical devices.