Synergistic Proton and Oxygen Ion Transport in Fluorite Oxide-Ion Conductor

Current perovskite oxide electrolytes, i.e., acceptor-doped Ba(Ce,Zr)O3-δ, exhibit proton conductivity ranging from 10−3 to 10−2 S cm−1 at 600 °C for protonic ceramic fuel cells (PCFCs), which rely on the structural defects. However, bulk doping and sintering restrict these oxides to possess higher ionic conductivity. New-generation PCFCs with alternative ion conduction mechanism need to be developed. This study presents a novel approach to realize high proton conduction along a fluorite oxide-ion conductor gadolinium-doped ceria (GDC: Gd0.1Ce0.9O2-δ) by electrochemical proton injection via a fuel cell process. A high protonic conductivity of 0.158 S cm−1 has been achieved. This fuel cell employing a 400-μm-thick GDC electrolyte delivered a peak power output close to 1,000 mW cm−2 at 500 °C. Proton conduction is verified by electrochemical impedance spectroscopy, proton filtering cell and isotopic effect, and so on. Proton injection into GDC after fuel cell testing is clarified by x-ray photoelectron spectroscopy, Raman spectra, 1H solid-state nuclear magnetic resonance spectra, and so on. Furthermore, a synergistic mechanism involving both surface proton conduction and bulk oxygen-ion migration is proposed by comparing electrochemical impedance spectroscopy with distribution of relaxation time results of GDC and pure ceria. This finding may provide new insights into the ion transport mechanism on fluorite oxides and open new avenues for advanced low-temperature PCFCs.