Researchers from the Materials Energy Research Laboratory in nanoscale (MERLin) at UNSW have synthesised nanoparticles of a commonly overlooked chemical compound called sodium borohydride (NaBH4) and encased these inside nickel shells. Their unique nanostructure has demonstrated remarkable hydrogen storage properties.
"No one has ever tried to synthesise these particles at the nanoscale because they thought it was too difficult, and couldn't be done. We're the first to do so, and demonstrate that energy in the form of hydrogen can be stored with sodium borohydride at practical temperatures and pressures," says Dr Kondo-Francois Aguey-Zinsou from the School of Chemical Engineering at UNSW.
Considered a major a fuel of the future, able to bridge the gap between renewables and fossil fuels, hydrogen could be used to power buildings, portable electronics and vehicles – but this application hinges on practical storage technology.
Lightweight compounds known as borohydrides (including lithium and sodium compounds) are known to be effective storage materials but it was believed that once the energy was released it could not be reabsorbed – a critical limitation. This perceived "irreversibility" means there has been little focus on sodium borohydride.
The core-shell NaBH4@Ni nanoparticles show high reversible hydrogen storage under reasonable conditions. Credit: ACS, Christian and Aguey-Zinsou
ACS Nano - A Core-Shell Strategy Leading to High Reversible Hydrogen Storage Capacity for NaBH4
Owing to its high storage capacity (10.8 mass %), sodium borohydride (NaBH4) is a promising hydrogen storage material. However, the temperature for hydrogen release is high (over 500 C) and reversibility of the release is unachievable under reasonable conditions. Herein, we demonstrate the potential of a novel strategy leading to high and stable hydrogen absorption/desorption cycling for NaBH4 under mild pressure conditions (4 MPa). By an antisolvent precipitation method, the size of NaBH4 particles was restricted to a few nanometers (< 30 nm) resulting in a decrease of the melting point and an initial release of hydrogen at 400C. Further encapsulation of these nanoparticles upon reaction of nickel (Ni) chloride at their surface allowed the synthesis of a core-shell nanostructure, NaBH4@Ni, and this provided a route for: a) the effective nanoconfinement of the melted NaBH4 core and its dehydrogenation products, and b) reversibility and fast kinetics owing to short diffusion lengths, the unstable nature of nickel borohydride and possible modification of reaction paths. Hence at 350C, a reversible and steady hydrogen capacity of 5 mass % was achieved for NaBH4@Ni. 80 % of the hydrogen could be desorbed or absorbed in less than 60 min and full capacity was reached within 5 h. To the best of our knowledge this is the first time that such performances have been achieved with NaBH4. This demonstrates the potential of the strategy in leading to major advancements in the design of effective hydrogen storage materials from pristine borohydrides
Greencarcongress - Using an antisolvent precipitation method, they synthesized NaBH4 particles with a size restricted to a few nanometers (less than 30 nm), which resulted in a decrease of the melting point and an initial release of hydrogen at 400 °C. Further encapsulating these nanoparticles upon reaction of nickel chloride at their surface enabled the synthesis of a core-shell nanostructure. This NaBH4@Ni structure provided a route for:
* the effective nanoconfinement of the melted NaBH4 core and its dehydrogenation products, and
* reversibility and fast kinetics owing to short diffusion lengths, the unstable nature of nickel borohydride and possible modification of reaction paths.
With the core-shell structure, the release of hydrogen began from only 50 °C with significant desorption from 350 °C; “more remarkably”, the team found, NaBH4 became fully reversible for the first time with hydrogen desorption/absorption occurring under relatively mild conditions of pressure (4 MPa) and temperature (350 °C).
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