Current perovskite oxide electrolytes, i.e., acceptor-doped Ba(Ce,Zr)O_3-δ, exhibit proton conductivity ranging from 10⁻³ to 10⁻² S cm⁻¹ at 600 ℃ 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: Gd_0.1Ce_0.9O_2-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 ℃. 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.

Extending the ionic conductivity is the pre-requisite of electrolytes in fuel cell technology for high-electrochemical performance. In this regard, the introduction of semiconductor-oxide materials and the approach of heterostructure formation by modulating energy bands to enhance ionic conduction acting as an electrolyte in fuel cell-device. Semiconductor (n-type; SnO₂) plays a key role by introducing into p-type SrFe_0.2Ti_0.8O_3-δ (SFT) semiconductor perovskite materials to construct p-n heterojunction for high ionic conductivity. Therefore, two different composites of SFT and SnO₂ are constructed by gluing p- and n-type SFT-SnO₂, where the optimal composition of SFT-SnO₂ (6:4) heterostructure electrolyte-based fuel cell achieved excellent ionic conductivity 0.24 S cm⁻¹ with power-output of 1004 mW cm⁻² and high OCV 1.12 V at a low operational temperature of 500 °C. The high power-output and significant ionic conductivity with durable operation of 54 h are accredited to SFT-SnO₂ heterojunction formation including interfacial conduction assisted by a built-in electric field in fuel cell device. Moreover, the fuel conversion efficiency and considerable Faradaic efficiency reveal the compatibility of SFT-SnO₂ heterostructure electrolyte and ruled-out short-circuiting issue. Further, the first principle calculation provides sufficient information on structure optimization and energy-band structure modulation of SFT-SnO₂. This strategy will provide new insight into semiconductor-based fuel cell technology to design novel electrolytes.