UNSW team demonstrates high reversible hydrogen storage capacity under mild conditions for sodium borohydride using novel core-shell nanostructure; potential for vehicles
|The core-shell NaBH4@Ni nanoparticles show high reversible hydrogen storage under reasonable conditions. Credit: ACS, Christian and Aguey-Zinsou. Click to enlarge.|
A team from the University of New South Wales (Australia) reports on a novel core-shell strategy leading to high and stable hydrogen absorption/desorption cycling for sodium borohydride (NaBH4) under mild pressure conditions (4 MPa) in an open-access paper in the journal ACS Nano. The results could create an opportunity for the use of borohydride materials for hydrogen storage in vehicles.
With a high storage capacity (10.8 mass %), sodium borohydride is a promising hydrogen storage material. However, the temperature for hydrogen release is high (> 500 °C) and reversibility of the release is unachievable under reasonable conditions. Most hydrogen storage research using NaBH4 has focused on its hydrolysis rather than thermolysis for hydrogen generation; the hydrolysis route has been put aside for automotive applications because of the non-reversibility of the process, Meganne Christian and Kondo-Francois Aguey-Zinsou noted in their paper.
In this work, a novel route leading to high reversible hydrogen storage capacity with NaBH4 is reported. Our strategy was to use a core-shell approach with the aim of achieving the synthesis and stabilization of isolated NaBH4 nanoparticles. Realizing the stabilization of discrete NaBH4 nanoparticles would provide a path for high hydrogen storage capacity and the effective exploitation of any nanoscale effects.—Christian and Aguey-Zinsou
By synthesizing nanoparticles of sodium borohydride and encasing these inside nickel shells (NaBH4@Ni), the UNSW team demonstrated a reversible and steady hydrogen capacity of 5 mass % at 350 °C. 80 % of the hydrogen could be desorbed or absorbed in less than 60 minutes and full capacity was reached within 5 h.
Using an antisolvent precipitation method, they synthesized NaBH4 particles with a size restricted to a few nanometers (< 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).
In comparison to current nanoconfinement approaches using porous carbon or inorganic materials, our core-shell strategy provided a platform for maximizing the hydrogen storage capacity as the shell can also store hydrogen, and for additional modification of the properties of NaBH4 since an appropriate metallic coating would provide a means for a ‘localized’ adjustment of the properties of NaBH4. The new materials that could be generated by this exciting strategy could provide practical solutions to meet many of the targets set forward by the US Department of Energy. Further investigations are underway into the structure of these core-shell nanoparticles and their applicability.—Christian and Aguey-Zinsou
No one has ever tried to synthesize 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.
By controlling the size and architecture of these structures we can tune their properties and make them reversible—this means they can release and reabsorb hydrogen. We now have a way to tap into all these borohydride materials, which are particularly exciting for application on vehicles because of their high hydrogen storage capacity.—Dr. Kondo-Francois Aguey-Zinsou, UNSW School of Chemical Engineering
Meganne L. Christian and Kondo-Francois Aguey-Zinsou (2012) A Core-Shell Strategy Leading to High Reversible Hydrogen Storage Capacity for NaBH4. ACS Nano doi: 10.1021/nn3030018