Reversible Hydride Ion-Mediated Electrochemistry in Sodium Aluminum Hydride

Electrochemical devices using hydride ions (H^-) as charge carriers offer significant potential for hydrogen-based energy storage and conversion. However, only very few H^- based electrochemical devices have been demonstrated, lacking details regarding the hydride ion-mediated reaction processes. In this work, we find that the NaAlH₄ electrode can utilize at least 5.6 wt % H via an H^--mediated reaction under ambient conditions. Further, the differences between H^--mediated electrochemical reactions and traditional thermochemical processes are discussed from the perspectives of driving force, mass migration, and reaction kinetics. The thermally driven process is controlled by the enthalpy-entropy balance and the long-range mass transport. In contrast, the electrochemical pathway is driven by an electrochemical potential gradient and dominated by fast H^- migration. Analyses of the interface and phase spatial distribution show that the electrode/electrolyte interfaces exhibit good compatibility at mild reaction conditions and suppression of detrimental phase separation. These findings highlight the unique advantages of H^--mediated electrochemistry and provide a foundation for developing next-generation hydride-based energy storage and conversion technologies.