Synthesis, Hydrogen Sorption Properties, and Hydride-Ion Conductivity of K₂MgH₄

Metal hydrides capable of transporting hydride-ions have recently attracted attention due to their potential for applications in a wide range of electrochemical conversion processes that involve hydrogen. Here, we report on the synthesis and hydride-ion conductivity of K₂MgH₄, a mixed hydride-electronic conductor. K₂MgH₄ is synthesized via thermal, mechanochemical, and combined thermal-mechanochemical methods. We show that the synthesis route strongly influences the material’s structure, hydrogen content, and thus, its conductivity. The K₂MgH₄ samples with lower hydrogen contents exhibit higher ionic conductivity than samples with higher hydrogen contents. At 190 °C, the highest conductivity (σ = 2.1 × 10^-6 S cm^-1, of which 47% is ionic) is obtained in a mechanochemically prepared sample that contains only 63% of the stoichiometric hydrogen content. The good thermal stability and tuneability of the (ionic) conductivity by adjusting the hydrogen content render K₂MgH₄ an interesting mixed ionic-electronic conductor for hydrogen-based electrochemical energy conversion/storage applications in the intermediate-temperature range.