Proton conductivity of Li+–H+ exchanged Li7La3Zr2O12 dense membranes prepared by molten long-chain saturated fatty acids

Abstract

Alkali-proton-exchanged oxides have broad potential as proton-conducting solid-state electrolytes for fuel cells and electrolyzers. However, materials in this class have not been widely explored because of the difficulty in preparing dense-body samples and investigating their electrical conductivities. In this study, using a Li₇La₃Zr₂O₁₂ (LLZ) membrane as a model material, alkali-proton exchange throughout dense-body samples was achieved using molten long-chain saturated fatty acids as proton sources. The 91% of Li⁺ in the LLZ dense bodies was exchanged with H⁺ within 15 h using the molten C₂₁H₄₃COOH. This Li⁺–H⁺ exchange decreased the conductivity of LLZ by more than 100-fold compared to its pristine state, whereas previous reports have suggested that protons in Li⁺–H⁺-exchanged LLZ are mobile. Experimental and theoretical investigations have indicated that most protons in Li⁺–H⁺-exchanged LLZ are bound to adjacent oxide ions, and that the proton transfer step of the Grotthuss mechanism is challenging for isolated ZrO₆ octahedra. Alkali-proton exchange using molten long-chain saturated fatty acids effectively explores new alkali-proton exchanged materials in which their exchange reactions are rate-determined by a thermally activated diffusion process.