MXenes as catalysts for lightweight hydrogen storage materials: A review

Hydrogen can serve as a clean storage medium for large-scale renewable energy due to its characteristics of cleanness, high gravimetric energy density, abundant resources, and flexible applications. However, storing hydrogen in a manner both compactly and safely is still a thorny issue currently. Hydrogen storage in materials provides a feasible solution for such tough issue. Unfortunately, most of the light-weight hydrogen storage materials such as complex metal hydrides (LiBH4, Mg(BH4)2, LiAlH4, NaAlH4, etc.), binary light-weight metal hydrides (MgH2, AlH3, etc.) are currently facing the problems of high thermal stability, slow desorption and absorption kinetics, or poor reversibility. Introduction of catalysts or constructing nanostructures are two of the feasible methods that can efficiently tailor the hydrogen storage properties of the materials. Recently two-dimensional (2D) transition metal carbides, nitrides, or carbonitrides (called MXenes) have shown great development potential as catalysts to regulate the performances of hydrogen storage materials due to their unique electronic properties, layered structures and catalytic activity of the transition metals contained. It is possible to simultaneously nanoconfine and catalyze the hydrogen storage materials by layered MXenes. In this review, the synthesis methods and application situation of MXenes are first briefly introduced. Then, the emphasis is placed on the research progress and recent advances of MXenes as catalysts for regulating the hydrogen storage properties of light materials such as MgH2, AlH3, LiAlH4, NaAlH4, LiBH4, Mg(BH4)2 or multicomponent hydrogen storage composites such as LiBH4−MgH2, MgH2−LiAlH4, LiBH4−Mg(BH4)2, etc. This review demonstrates that MXenes have exhibited very good catalytic activity on the dehydrogenation and rehydrogenation of various hydrogen storage materials. Since there is barely review focused on the various kinds of hydrogen storage materials, this review will close this gap and aims at making a comprehensive discussion and prospect on the studies of MXenes for regulating the properties of various kinds of hydrogen storage materials.