Water- and acid-stable self-passivated dihafnium sulfide electride and its persistent electrocatalytic reaction.

Electrides have emerged as promising materials with exotic properties, such as extraordinary electron-donating ability. However, the inevitable instability of electrides, which is caused by inherent excess electrons, has hampered their widespread applications. We report that a self-passivated dihafnium sulfide electride ([Hf2S]2+∙2e-) by double amorphous layers exhibits a strong oxidation resistance in water and acid solutions, enabling a persistent electrocatalytic hydrogen evolution reaction. The naturally formed amorphous Hf2S layer on the cleaved [Hf2S]2+∙2e- surface reacts with oxygen to form an outermost amorphous HfO2 layer with ~10-nm thickness, passivating the [Hf2S]2+∙2e- electride. The excess electrons in the [Hf2S]2+∙2e- electride are transferred through the thin HfO2 passivation layer to water molecules under applied electric fields, demonstrating the first electrocatalytic reaction with excellent long-term sustainability and no degradation in performance. This self-passivation mechanism in reactive conditions can advance the development of stable electrides for energy-efficient applications.