Development of Proton Conductors with Skelton of Lithium-Ion Conducting Oxides
Authors: Ishii Akihiro , Matsumoto Hiroshi , Kato Hisashi , Oikawa Itaru , Takamura Hitoshi 
Published: December 2024

Efficient solid-oxide fuel cells and electrolyzers require proton-conducting electrolyte materials that exhibit high proton conductivity at intermediate temperatures (300–600 °C), chemical stability against CO2 and H2O, and compatibility with electrode materials. Two well-known classes of proton-conducting inorganic materials exist: solid oxyacids that show high proton conductivity but decompose at relatively low temperatures (≈200 °C) when dehydrated, and perovskite-type oxide refractories that remain stable even in the dehydrated state but exhibit high conductivity only at high temperatures above 600 °C. Significant efforts have been made to develop thermostable oxyacids or perovskite-type oxides showing high proton conductivity at intermediate temperatures.

As a new class of proton-conducting materials, alkali–proton-exchanged oxides have gradually gained attention. Unlike perovskite-type oxides, these oxides are likely to experience less electronic current leakage under high oxygen potentials due to the proton being incorporated without oxygen vacancy hydration. This makes them especially suitable for use as solid-state electrolytes in electrolyzers. Recent studies on their proton conductivity have emerged due to advances in bulk preparation methods, such as gigapascal-level compaction, electrochemical ion exchange, or thermally-assisted ion exchange using fatty acids. Several materials with crystal structural frameworks of lithium-ion conducting oxides (e.g., LixAl0.5Co0.5O2, LISICON-type Li14Zn(GeO4)4, NASICON-type LiZr2(PO4)3) have been reported to exhibit high proton conductivity on the order of 10−3 S·cm−1 or higher at intermediate temperatures. [1–5] However, the thermal decomposition of these oxides due to thermodynamic metastability poses a significant issue when used as solid electrolytes in electrolyzers.

In this study, proton forms of cubic garnet-type Li7La3Zr2O7 and NASICON-type LiZr2(PO4)3 are synthesized, and their preparation difficulty, proton conductivity, and thermal stability are examined and their differences are discussed. The thermally-assisted ion exchange using fatty acids achieved 91% lithium-proton exchange in Li7La3Zr2O7. However, proton-exchanged Li7La3Zr2O7 decomposed at 325 °C by the dehydration reaction. The conductivity of the proton-exchanged Li7La3Zr2O7 was on the order of 10−4 S·cm−1. Using theoretical calculation, the low conductivity was considered to be due to the isolated configuration of ZrO6, which hinders proton hopping. [6] Conversely, NASICON-type LiZr2(PO4)3 showed only 11% lithium-proton exchange. Instead, HZr2(PO4)3 was synthesized via NH3 desorption reaction from NH4Zr2(PO4)3. While maintaining NASICON-type crystal structure stability up to 600 °C, proton desorption occurred above 500 °C, as indicated by magic-angle-spinning nuclear magnetic resonance (MAS NMR) analysis. MAS NMR also suggested the introduction of mobile protons by aliovalent doping to HZr2(PO4)3. Proton hopping along with corner-shared ZrO6–PO4 chains is being considered in this material.

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