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

Abstract

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.