Closed loops for hydrogen storage: Hydrolysis and regeneration of metal borohydrides

Metal borohydrides (M(BH₄)_n) are extensively studied due to their high hydrogen storage capacities via hydrolysis. However, it is challenging to develop a closed loop of hydrolysis and regeneration of M(BH₄)_n that is of low-cost and high-efficiency. In this work, we overview the state-of-the-art advances in both hydrolysis and regeneration processes of Li(Na)BH₄ and Mg(BH₄)₂. In particular, a special focus is placed on NaBH₄ hydrolysis with respect to catalysts, capacity, kinetics and thermodynamics. NaBH₄ regeneration is also discussed in terms of synthesis techniques, the sources of boron and hydrogen, and the reducing agents. There are great advances in the development of catalysts in facilitating hydrogen evolution. Critically, low-cost transition metal-based catalysts is effective in catalyzing the hydrolysis. Catalyzed hydrogen evolution can meet the requirement of PEMFC in small devices. For future applications in large scale, more research is needed to optimize the performance to deliver the required rates and quantities. Cost becomes more paramount for large-scale applications and effective regeneration of these borohydrides needs to be developed. Most recent development in mechanochemical synthesis of borohydrides features high efficiency and low cost and therefore deserves follow-up research. The review sheds light on the future development and deployment of M(BH₄)_n for hydrogen storage.