Proton conducting solid oxide electrolyzer cell (SOEC) enables economic production of hydrogen gas at intermediate temperatures (400- 700 °C). Lithium super ionic conductors are solid state electrolytes with very high lithium ion conductivity in the order of 10^-2 to 10^-4 S/cm at room temperature¹. At temperature range 60 – 300 °C, the lithium ion conductivity reaches around 1 to 10^-2 S cm^-1. Mechanism of the lithium ion transport starts with ions at local sites being excited to neighboring sites and collectively diffusing on a macroscopic scale. Under electrical field, long distance transport is realized by continuous hopping of Li⁺ ions². However, in moisture containing ambient atmosphere, proton−lithium ion (H⁺/Li⁺) exchange takes place on the surface of the electrolyte making it unsuitable for Li-ion batteries. It has been shown that the exchange occurs readily when Li stoichiometry is greater than 3 atoms per formula^{3, 4}. One such reaction is given below⁵:

Li_6.4La₃Zr_1.4Ta_0.6O₁₂ + xH₂O → Li_6.4-xH_xLa₃Zr_1.4Ta_0.6O₁₂ + xLiOH

After fully replacing the mobile lithium ions by protons, the solid electrolyte acts as a proton conductor. Very few works reported such lithium compounds for proton conduction and proton conductivity is several orders higher when compared to other standard electrolytes⁶. In this work, we demonstrate for the first time the proton conduction behavior of garnet type ceramic Li_6.4La₃Zr_1.4Ta_0.6O₁₂ electrolyte with a completely new mechanism of conduction of protons demonstrating the highest protonic conductivity of 8.1 x 10^-2 S cm^-1 and chemically stable upto 200hrs in 50% moisture/Argon atmosphere at 600 °C. Ceramic type lithium super ionic conductors which are stable at very high temperatures (up to 1100 °C) and are suitable for intermediate temperature (400- 600 °C) solid oxide electrolyzer cells (SOEC). The mechanism of proton conduction in lithium super ionic conductor is entirely different from the standard electrolytes. While proton conduction in the latter is through oxygen vacancies in the crystal lattice, the proton conduction in the former is by diffusion and migration of protons through lithium ions sites in the crystal lattice⁶. The proton – lithium exchange is also reversible when the electrolyte is flushed with 1M LiOH³. The high proton conductivity suggests that garnet-based ceramics represents a new class of proton conductors that could be effectively used in SOEC’s for hydrogen generation at intermediate operating temperatures.

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