Engineers have demonstrated that hydrogen can be released and reabsorbed from a promising new storage material.
Hydrogen energy could be used to power buildings, vehicles and portable electronic devices, if scientists find a practical way to store it. Lightweight compounds known as borohydrides are known to be effective storage materials, but it has been thought that once the energy was released from these materials it couldn’t be reabsorbed.
“Research groups who have tried before have measured the amount of heat required to come back to NaBH4 (sodium borohydride) from the elements and the heat value they found is too high to be feasible,” says Dr Kondo-Francois Aguey-Zinsou, a chemical engineer from the University of New South Wales. “So these materials were classified as irreversible for hydrogen storage purposes.”
Aguey-Zinsou and his team have demonstrated that reversibility is actually possible using borohydride by itself in a study published in ACS Nano. “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.”
After some preliminary investigations, they felt that if could prepare very small particles of borohydrides they would be able to reversibly store hydrogen with these materials. The problem was how to do it. NaBH4 in its bulk form requires temperatures above 550 degrees Celsius to release hydrogen and there were minimal improvements even on the nanoscale.
“After several attempts, we realised that borohydrides nanoparticles were melting as soon as we were applying heat so we had to find a method to encase the melt,” Aguey-Zinsou explains. “We then (realised) that a clever way to do this was to use the borohydride core itself and reduce a metal at the surface of borohydride nanoparticles.”
This method not only created a metallic casing, but also improved the borohydride material’s properties for hydrogen storage purposes. The coreshell nanostructure allowed the initial energy release to take place at just 50°C, with significant energy release occurring at 350 °C.
These lower temperatures and faster reactions times have made applying this technology more practical and opened the doorway for solutions to meet the hydrogen energy targets. “If we can control borohydride we then have (the means) to meet the practical targets for vehicle application,” Aguey-Zinsou says. “These materials will also have (applications) in portable electronic and energy storage from renewables, e.g. solar and wind.”