Developing Core-Shell Borohydride Nanoarchitectures and Understanding their Structure-Hydrogen Release/Uptake Relationships

Abstract

One of the biggest challenges of implementing the future hydrogen (H2) economy is H2 storage with high density, compactness, and safety. Borohydrides have been extensively investigated as promising candidates for H2 storage; however, the high thermodynamic stability, sluggish H2 release kinetics, and limited H2 reversibility have hampered their practical use. Nanosizing is an attractive technique for improving the H2 release/uptake of borohydrides owing to the shorter H2 diffusion paths, new surface states, and/or high surface areas. The nanoconfinement of borohydrides in porous hosts (e.g. carbon, SiO2, and/or MOFs) has shown the possibility of improving their H2 release/uptake properties; however, this technique is impractical because of the hydroxyl/oxygen species, dead mass, and/or laborious synthetic procedures of the host materials. Alternatively, scalable synthesis of size and shape-controlled borohydrides as freestanding core-shell structures has surfaced as an appealing approach to enable H2 release/uptake at low temperatures. Through this technique, it is hypothesised that the shell would limit the loss of active species (e.g. boron), recrystallisation, side-reactions, and facilitate the H2 release/uptake from the borohydride core. However, the widespread implementation of this technique remains limited because of the lack of effective methods for controlling the borohydride nanoarchitectures. This work aims to synthesise controlled borohydride nanoarchitectures and explain their structure-H2 release/uptake relationships. Specifically, this work aims to (i) understand the paths to synthesise various NaBH4 nanoarchitectures (e.g. spheres, cubes, or bars) and potential nanostructure/H2 property interdependencies, (ii) rationalise the fabrication of core-shell NaBH4@Ni using wet-chemistry approaches to confine NaBH4 for reversible H2 storage, (iii) improve the H2 release of structurally controlled NaBH4 using catalysts in the core-shell framework, (iv) rationalise the fabrication of core-shell NaBH4@Ni nanoarchitectures using solvent-free approaches and their H2 release/uptake properties, and (v) understand the effect of the core-shell structure to mitigate the B2H6 release from NaZn(BH4)3. This work shows the effectiveness of wet-chemistry and solvent-free methods for designing core-shell borohydrides with improved H2 release/uptake properties. These methods will serve as a guide for designing practical storage materials with H2 release/uptake close to ambient conditions.