Linking Fast Sodium Conduction with Low-Temperature Hydrogen Release in Sodium Borohydride

Abstract

Complex hydrides, such as sodium borohydride (NaBH₄), are attractive materials for hydrogen storage because of their high hydrogen capacity. However, practical application of these materials is limited because of their unfavorable hydrogen thermodynamics and poor kinetics. Herein, it is demonstrated that the inclusion of BF₄⁻ in NaBH₄ results in remarkable Na⁺ conductivity of 1.5 × 10⁻³ S cm⁻¹, which is 10 000 times higher compared to pure NaBH₄ (7.0 × 10⁻⁸ S cm⁻¹) at 115 °C. The ionic conductivity is also comparable to values reported for some of the best borohydride-based conductors reported to date. More remarkably, this improvement of ionic conductivity is found to be correlated to lower hydrogen release temperatures for BF₄⁻-modified NaBH₄ releasing hydrogen at a temperature of 200 °C instead of 510 °C in the case of pristine NaBH₄. Nudged elastic band calculations based on density functional theory reveal that partial substitution of the [BH₄]⁻ groups in NaBH₄ by BF₄⁻ may lead to the formation of distortions within the NaBH₄ crystal lattice with favorable channels for Na⁺ mobility enabling the release of hydrogen at low temperatures.