[N2H7+][B2H7–] supersalt as an alternative of [NH4+][BH4–] for efficient hydrogen storage

Abstract

Context

Supersalts are charge transfer salts, which differ from traditional salts due to preferred dissociation into ionic fragments. These can be formed by the interaction of superalkalis with superhalogens. The inherent instability of [NH₄⁺][BH₄^–] against dissociation to ammonia borane restricts its practical application in hydrogen storage. In this work, we design [N₂H₇⁺][B₂H₇^–] by using binuclear superalkali cation (N₂H₇⁺) and superhalogen anion (B₂H₇^–) using DFT and MP2 methods. Although its gravimetric hydrogen density (22%) is slightly smaller than that of [NH₄⁺][BH₄^–] (24%), its dissociation energy and enthalpy are large enough to confirm its stability. The enhanced stability of [N₂H₇⁺][B₂H₇^–] can be attributed to its supersalt behavior, which makes it a possible candidate for chemical hydrogen storage.

Methods

DFT calculations were performed using a long-range dispersion corrected ωB97xD functional with a 6–311 +  + G(d,p) basis set in the Gaussian 16 program. The results were recalculated using the second-order Moller–Plesset perturbation theory (MP2) with the same basis set.