Tuning kinetics and thermodynamics of hydrogen storage in light metal element based systems – A review of recent progress

Abstract

High capacity hydrogen storage is a key issue for future hydrogen energy. The hydrides, constituted of light elements such as Li, B, C, N, Na, Mg, Al, Si, etc., are excellent candidates for high gravimetric and volumetric density of hydrogen storage. However, these light-weight hydrides generally suffer from poor reversibility under moderate temperature and pressure conditions due to over high thermodynamic stability and/or sluggish kinetics. This review summarizes recent advances in the effort of overcoming thermodynamic and kinetic challenges for Mg based hydrides and complex hydrides. With respect to kinetic enhancement, great achievements have been achieved through several successful approaches such as nanoscaling, catalyzing, and compositing. In particular, a simple and massive fabrication method was developed to in-situ form multi-valence nano-catalyst in Mg based hydrides by conventional melting and hydrogenation treatment. Also, the reversibility of complex hydrides can be greatly improved through nanoscaling and doping catalysts. Comparatively, the thermodynamic destabilization effect is limited by applying the strategies of nanoscaling, ionic substituting for complex hydrides, alloying for Mg based hydrides, and altering the reaction route. For the approach of nanoscaling, both theoretical calculation and experimental results prove that thermodynamic destabilization can only be achieved by reducing the size of hydrides down to several nanometers, which is extremely difficult to realize and maintain, and thus, nano-confinement has been adopted. On the other hand, although the reversibility of most of Mg based alloys is generally poor, fully reversible hydriding has been achieved in some Mg based solid solution alloys with reduced reaction enthalpy. On the whole, based on the current status of the research progress, the existing Mg based and complex hydrides could not fully satisfy the application requirements of hydrogen energy storage, and the sustainable research effort are still making to solve their intrinsic thermodynamic and kinetic challenges.