Hydrogenation properties of lithium and sodium hydride – closo-borate, [B10H10]2− and [B12H12]2−, composites

Abstract

The hydrogen absorption properties of metal closo-borate/metal hydride composites, M₂B₁₀H₁₀–8MH and M₂B₁₂H₁₂–10MH, M = Li or Na, are studied under high hydrogen pressures to understand the formation mechanism of metal borohydrides. The hydrogen storage properties of the composites have been investigated by in situ synchrotron radiation powder X-ray diffraction at p(H₂) = 400 bar and by ex situ hydrogen absorption measurements at p(H₂) = 526 to 998 bar. The in situ experiments reveal the formation of crystalline intermediates before metal borohydrides (MBH₄) are formed. On the contrary, the M₂B₁₂H₁₂–10MH (M = Li and Na) systems show no formation of the metal borohydride at T = 400 °C and p(H₂) = 537 to 970 bar. ¹¹B MAS NMR of the M₂B₁₀H₁₀–8MH composites reveal that the molar ratio of LiBH₄ or NaBH₄ and the remaining B species is 1 : 0.63 and 1 : 0.21, respectively. Solution and solid-state ¹¹B NMR spectra reveal new intermediates with a B : H ratio close to 1 : 1. Our results indicate that the M₂B₁₀H₁₀ (M = Li, Na) salts display a higher reactivity towards hydrogen in the presence of metal hydrides compared to the corresponding [B₁₂H₁₂]²⁻ composites, which represents an important step towards understanding the factors that determine the stability and reversibility of high hydrogen capacity metal borohydrides for hydrogen storage.