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For efficient energy conversion and energy storage, developing highly efficient hydrogen permeable membranes is a crucial issue. Ceramic hydrogen permeable membranes, i.e., protonic electronic mixed conductors, work on bipolar transport of protons and electrons. Ceramic composite membranes meet this requirement with chemical and thermal stability, and they can be used not only as hydrogen permeable membranes but also as electrolytes in protonic ceramic fuel cells (PCFCs). Although improvement in protonic conductivity at heterointerfaces between constituting materials and/or space-charge relaxation around the resistive grain boundaries can be possible origins of the enhanced hydrogen permeability in the composite electrolyte membranes, its detailed mechanisms have not been clarified yet and the strategy of material design for composite membranes has not been established. In this study, the transport properties of composite electrolyte membranes consisting of protonic conductors and electronic conductors are investigated to realize highly efficient PCFC operating at low temperatures and to clarify the principles of carrier transport in the composite membranes. For the protonic conductor and the electronic conductor, we selected La2Ce2O7 (LDC) with a fluorite structure and La0.3Sr0.6TiO3 (LST) with a perovskite structure, respectively. The composite membrane composed of LDC and LST (LDC/LST) with various LDC to LST ratios was prepared by a solid-state reaction. Phase stability of LDC and LST were investigated by X-ray diffraction (XRD) measurements. Transport properties of hydrogen permeation cells using the LDC-LST composite membranes with Pt electrodes were assessed by electrochemical impedance spectroscopy (EIS) measurements and hydrogen permeation measurements. Hydrogen fluxes were measured by gas chromatography. The EIS measurements revealed that the grain-boundary resistance was reduced for the LDC/LST composite membrane in comparison with an LDC membrane. In particular, LDC/LST (weight ratio 1:2) showed the smallest grain-boundary resistance of 26 Ω cm at 600°C under 1%H2/Ar among the measured samples. Besides, the hydrogen permeation measurement showed that LDC/LST exhibited higher hydrogen permeability than the LDC membrane. In addition, the hydrogen flux of LDC/LST (1:2) was almost twice as high as that of LDC at 600°C. The low grain-boundary resistance and the high hydrogen permeability of LDC/LST composite membrane suggests that the introduction of LST accelerates the proton conduction in the membrane.

Solid oxide fuel cells (SOFCs) with proton-conducting solid electrolyte, instead of the oxide-ion conducting solid electrolyte have attracted attentions because of their high potential to reduce operating temperatures and to enhance the electrical efficiencies of SOFCs. In addition, the proton-conducting SOFCs with multistage electrochemical oxidation configuration will be promising technology for critically-high electric efficiencies. However, it is known that there are non-negligible charge -carriers other than protons in typical proton-conducting solid oxide electrolytes at relatively high temperatures. The existence of the partial conductivities of holes and/or electrons will cause the internal leakage current that consumes fuel but never generates any electrical power output. The higher ratio of the leakage current to external current will more deteriorate the electrical efficiency. In this study, the effects of blocking -layers formed on the air side surface of base electrolyte layer consisting of BaZr0.8Y0.2O3-δ (BZY82) for suppressing the leakage current have been investigated by using electrochemical parameters of the partial conduction of the materials. The chemical potential profile and leakage current showed large dependence on the material of the blocking -layer. Lanthanum tungstate was found to play a role as unique and strong blocking -layer against the leakage current.