Mixed hydride-electronic conductivity in Rb2CaH4 and Cs2CaH4

Hydride-ion conductors and mixed hydride-electronic conductors are promising materials for various applications, especially in (electro)chemical energy conversion and storage. Many of the hydride-ion conductors discovered to date are oxyhydrides with the K2NiF4-type structure. In this work, Cs2CaH4 and Rb2CaH4, which crystallize in the K2NiF4-type structure, were synthesized and electrochemically characterized. By employing electrochemical impedance spectroscopy (EIS) and single-step chronoamperometry measurements, it is found that both materials show mixed ionic-electronic conductivity at moderate (100-200 degrees C) temperatures. The overall conductivity of both materials is increased by the release of hydrogen at elevated temperatures, indicating an effect of hydride vacancy concentration on the conductivity. This suggests that the ionic conductivity is due to hydride-ion transport, which is further supported by Climbing Imaged Nudged Elastic Band (CINEB) calculations. Cs2CaH4 shows approximately equal ionic and electronic conductivity at 190 degrees C (total conductivity sigma = 2.1 x 10-6 S cm -1), while Rb2CaH4 (sigma = 8.8 x 10-7 S cm-1 at 190 degrees C) is primarily an ionic conductor. As mixed hydride-electronic conductors, both materials show promise in chemical conversion and energy conversion applications.