Effects of LnF3 on reversible and cyclic hydrogen sorption behaviors in NaBH4: electronic nature of Ln versus crystallographic factors

Abstract

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 χ_p of Ln³⁺ determines the thermodynamic stability of 3NaBH₄–LnF₃ composites with their χ_p 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.