Proton Shuttles in CeO2/CeO2−δ Core–Shell Structure

A core-shell structure with a CeO₂ core and a non-stoichiometry CeO_2-d surface layer built up proton shuttles, leading to a super proton conductivity of 0.15 S cm^-1 and advanced fuel cell performance, 661 mW cm^-2 at 520 ºC. The surface induced conduction process was investigated through a heat-treatment procedure for CeO₂ pellets at different temperatures. The surface layer was observed to experience ordering and disappearing after high temperature treatments. Proton conduction was verified via electrochemical impedance spectra, proton conducting isotopic effect and fuel cell measurement. The isotopic effect provided a direct evidence on proton conduction and proton conduction mechanism was further investigated based on the semiconductor nature with the intrinsic CeO₂ (i-type) core and n-type CeO_2-δ shell. The charged layers were formed at the interface of CeO_2-δ/CeO₂ core-shell heterostructure with a positively charged layer located at CeO_2-δ side of the interface and negative charged layer located at the intrinsic CeO₂. Due to the electrostatic repulsion between protons and the positively charged layer, protons’ transportation is limited at the surface region of the CeO_2-δ shell. Thus, continuous “proton shuttles” are formed, resulting in super conductivity. This work presents a new methodology and scientific understanding for fast proton transport in general oxides and advanced proton ceramic fuel cells (PCFCs).