Core–shell NaBH₄@Ni Nanoarchitectures: A Platform for Tunable Hydrogen Storage

Abstract

The core-shell approach has surfaced as an attractive strategy to make complex hydrides reversible for hydrogen storage; however, no synthetic method exists for taking advantage of this approach. Here, a detailed investigation was undertaken to effectively design freestanding core-shell NaBH 4 @Ni nanoarchitectures and correlate their hydrogen properties with structure and chemical composition. It was shown that the Ni shell growth on the surface of NaBH 4 particles could be kinetically and thermodynamically controlled. The latter led to varied hydrogen properties. Near-edge X-ray absorption fine structure analysis confirmed that control over the Ni 0 /Ni x B y concentrations upon Ni II reduction led to a destabilized hydride system. Hydrogen release from the sphere, cube, and bar-like core-shell nanoarchitectures occurred at around 50, 90, and 95 °C, respectively, compared to the bulk (> 500 °C). This core-shell approach, when extended to other hydrides, could open new avenues to decipher structure-property correlation in hydrogen storage/generation.

Doping and Structure-Promoted Destabilization of NaBH4 Nanocubes for Hydrogen Storage

“…Upon the evaporation of IPA, cube-like V-NaBH 4 particles were formed with ODA as the stabilizing ligand . The as-synthesized V-NaBH 4 and Ni–OAm (as the Ni precursor) were then used to synthesize core–shell V-NaBH 4 @Ni (Figure a) . By elemental mapping of V-NaBH 4 @Ni, it is apparent that Ni is uniformly distributed on the cube-like particles, suggesting the formation of a core–shell like structure (Figure c).…”

“…The decomposition of the borohydride core under prolonged electron beam exposure also confirmed the formation of a core–shell structure in agreement with our previous observations (Figure S1). , …”

“…For the synthesis of core–shell V-NaBH 4 @Ni, the nickel-oleylamine (Ni–OAm) complex was prepared according to our method reported earlier . Briefly, a set amount of anhydrous NiCl 2 (yellow color) was dissolved in DMSO at 70 °C and cooled down to room temperature (appearance of pale green color due to Ni-DMSO complex formation).…”

“…For example, the sphere, cube, and bar-like structures of NaBH 4 started releasing hydrogen at 50 °C compared to bulk NaBH 4 . Likewise, the core–shell NaBH 4 @Ni nanoarchitectures showed improved H 2 release properties (onset 50 °C and major hydrogen release between 300 and 450 °C) compared to bulk NaBH 4 (onset 500 °C), and this was correlated with a combination of factors such as particle size, particle morphology, and the in situ formed Ni x B y species at the Ni/NaBH 4 interface …”

“…To facilitate rehydrogenation at lower temperatures, the decomposition of NaBH 4 should occur along the thermodynamic paths that are more favorable, and elemental (e.g., Na and B) loss must be minimized. To this aim, several strategies have been proposed to modify the B–H bond and lower the hydrogen release temperature of NaBH 4 such as anion/cation substitution, destabilization via transition metal doping, formation of reactive hydride composites, nanoconfinement in porous scaffolds, and nanosizing/nanostructuring. − Among these approaches, destabilization through the use of transition metals is considered to be the easiest way to trigger hydrogen release at lower temperatures. For example, the main hydrogen release from NiCl 2 or NiF 2 -doped NaBH 4 occurs at 450 °C instead of 500 °C required for pure NaBH 4 .…”