Mixed Ionic-Electronic Conductivity in Oxygen-Free K2NiF4-Type Hydrides

Abstract

Hydride-ion conductors and mixed hydride-electronic conductors are promising materials for numerous applications, for example as catalysts, catalyst supports or promoters for hydrogenation reactions, especially ammonia or methanol production. They could also be used in (electro)chemical energy conversion and storage devices such as batteries, fuel cells and reversible hydrogen storage systems. Therefore, there is a major interest in hydride-ion conductors with high conductivity at moderate temperatures.Many presently known hydride-ion conductors are oxyhydrides, of which a large number have the K2NiF4-type structure.[1-5] As oxide ions occupy part of an oxyhydride’s anion sublattice, not all anions participate in hydride-ion transport, thereby potentially limiting the ionic conductivity at moderate temperatures. In this presentation, the synthesis and electrochemical characterization of several different oxygen-free K2NiF4-type structured hydride-ion conductors, such as Rb2CaH4 and Cs2CaH4, will be discussed.By electrochemical analysis, the materials are found to exhibit mixed ionic-electronic conductivity at moderate temperatures. Small instabilities in the conductivity and TPD results which show that the materials release hydrogen gas indicate that the ionic conductivity is related to the presence of hydride-ion vacancies. In addition, computational results show that the measured activation energies are close to the expected values for hydride-ion transport.The total conductivities at 200 °C are around 10-6 S cm-1, a significant portion of this being due to hydride-ion transport. The discovery of mixed hydride-electronic conductivity in these materials opens up new possibilities to explore mixed hydride-electronic conductors in catalysis and low-temperature energy conversion and storage applications. G. Kobayashi et al., Science 2016, 351, 1314-1317.A. Watanabe et al., Electrochemistry 2017, 85, 88-92.Y. Iwasaki et al., J. Mater. Chem. A 2018, 6, 23457-23463.F. Takeiri et al., Inorg. Chem. 2019, 58, 4431-4436.F. Takeiri et al., Nat. Mater. 2022, 21, 325-330. Figure: Illustration of one transport pathway that hydride-ions can take through K2NiF4-type structured materials. The path taken is informed by computational results. Figure 1