The thermodynamical stabilities for the series of metal borohydrides M͑BH 4 ͒ n ͑M = Li, Na, K, Cu, Mg, Zn, Sc, Zr, and Hf; n =1-4͒ have been systematically investigated by first-principles calculations. The results indicated that an ionic bonding between M n+ cations and ͓BH 4 ͔ − anions exists in M͑BH 4 ͒ n , and the charge transfer from M n+ cations to ͓BH 4 ͔ − anions is a key feature for the stability of M͑BH 4 ͒ n . A good correlation between the heat of formation ⌬H boro of M͑BH 4 ͒ n and the Pauling electronegativity of the cation P can be found, which is represented by the linear relation, ⌬H boro = 248.7 P − 390.8 in the unit of kJ/mol BH 4 . In order to confirm the predicted correlation experimentally, the hydrogen desorption reactions were studied for M͑BH 4 ͒ n ͑M = Li, Na, K, Mg, Zn, Sc, Zr, and Hf͒, where the samples of the later five borohydrides were mechanochemically synthesized. The thermal desorption analyses indicate that LiBH 4 , NaBH 4 , and KBH 4 desorb hydrogen to hydride phases. Mg͑BH 4 ͒ 2 , Sc͑BH 4 ͒ 3 , and Zr͑BH 4 ͒ 4 show multistep desorption reactions through the intermediate phases of hydrides and/or borides. On the other hand, Zn͑BH 4 ͒ 2 desorbs hydrogen and borane to elemental Zn due to instabilities of Zn hydride and boride. A correlation between the desorption temperature T d and the Pauling electronegativity P is observed experimentally and so P is an indicator to approximately estimate the stability of M͑BH 4 ͒ n . The enthalpy change for the desorption reaction, ⌬H des , is estimated using the predicted ⌬H boro and the reported data for decomposed product, ⌬H hyd/boride . The estimated ⌬H des show a good correlation with the observed T d , indicating that the predicted stability of borohydride is experimentally supported. These results are useful for exploring M͑BH 4 ͒ n with appropriate stability as hydrogen storage materials.

Abstract:The prerequisite for widespread use of hydrogen as an energy carrier is the development of new materials that can safely store it at high gravimetric and volumetric densities. Metal borohydrides M(BH 4 ) n (n is the valence of metal M), in particular, have high hydrogen density, and are therefore regarded as one such potential hydrogen storage material. For fuel cell vehicles, the goal for on-board storage systems is to achieve reversible store at high density but moderate temperature and hydrogen pressure. To this end, a large amount of effort has been devoted to improvements in their thermodynamic and kinetic aspects. This review provides an overview of recent research activity on various M(BH 4 ) n , with a focus on the fundamental dehydrogenation and rehydrogenation properties and on providing guidance for material design in terms of tailoring thermodynamics and promoting kinetics for hydrogen storage.

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Developing nonprecious oxygen evolution electrocatalysts that can work well at large current densities is of primary importance in a viable water-splitting technology. Herein, a facile ultrafast (5 s) synthetic approach is reported that produces a novel, efficient, non-noble metal oxygen-evolution nano-electrocatalyst that is composed of amorphous Ni-Fe bimetallic hydroxide film-coated, nickel foam (NF)-supported, Ni S nanosheet arrays. The composite nanomaterial (denoted as Ni-Fe-OH@Ni S /NF) shows highly efficient electrocatalytic activity toward oxygen evolution reaction (OER) at large current densities, even in the order of 1000 mA cm . Ni-Fe-OH@Ni S /NF also gives an excellent catalytic stability toward OER both in 1 m KOH solution and in 30 wt% KOH solution. Further experimental results indicate that the effective integration of high catalytic reactivity, high structural stability, and high electronic conductivity into a single material system makes Ni-Fe-OH@Ni S /NF a remarkable catalytic ability for OER at large current densities.