Effects of LnF3 on reversible and cyclic hydrogen sorption behaviors in NaBH4: electronic nature of Ln versus crystallographic factors
In the present work, hydrogen sorption behaviors of some of the 3NaBH4–LnF3 (Ln = Ce, Sm, Gd and Yb) composites were investigated and the mechanisms associated with different effects of LnF3 (Ln = La, Ce, Pr, Nd, Sm, Gd, Ho, Er and Yb) on reversible hydrogen sorption in NaBH4 were proposed based on careful comparisons. The key factors controlling the properties of 3NaBH4–LnF3 can be summarized as follows: (i) electronegativity χp of Ln3+ determines the thermodynamic stability of 3NaBH4–LnF3 composites with their χp in the range of 1.23–1.54 being suitable for reversible hydrogen storage; (ii) the electronic configuration of Ln3+ influences rehydrogenation behaviors: more stable the oxidation state of the Ln3+ is, the better is the rehydrogenation performance of NaBH4; (iii) the unique crystal structure of the Ln–B phase formed during dehydrogenation, and geometrical configuration of B in Ln–B, provide dangling bonds for hydrogen atoms to embed in, consequently modifying the rehydrogenation kinetics. Because Gd3+ possesses a combination of suitable electronegativity, stable oxidation state and favorable geometric structure in GdB4, the 3NaBH4–GdF3 composite exhibits the best overall hydrogen storage properties among all the studied 3NaBH4–LnF3 composites, with high cycling stability up to 51 cycles along with fast kinetics. This understanding provides us with criterions to design new borohydride-based hydrogen storage systems and to optimize their hydrogen storage properties.
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