Graphene-wrapped nanocrystals improve fuel cell

US RESEARCHERS have developed hydrogen-absorbing magnesium nanocrystals wrapped in graphene sheets that can improve the performance of hydrogen-powered vehicles.

A team from the Department of Energy’s Lawrence Berkeley National Laboratory designed magnesium nanocrystals, measuring 3–4 nm in length, can act as ‘sponges’ for hydrogen, which can lead to a new compact and safe method to absorb and store hydrogen.

Thin sheets of graphene oxide that contain natural, atomic-scale defects can allow hydrogen molecules to pass through while blocking larger molecules that can corrode magnesium, such as oxygen and water, from reaching the nanocrystal.

The graphene-surrounded magnesium nanocrystal, also known as a “metal hydride” fuel cell, acts as a battery for the cell as hydrogen gas pumped into a vehicle would be chemically absorbed by the magnesium nanocrystaline powder and rendered safe at low pressures.

The nanocrystals would also permit faster fuelling, which would reduce the size of the storage tank needed for the fuel cell.

Eun Seon Cho, a postdoctoral researcher at Berkeley, said, “Among metal hydride-based materials for hydrogen storage for fuel-cell vehicle applications, our materials have good performance in terms of capacity, reversibility, kinetics and stability.”

The team admits more research is needed before hydrogen fuel cells can be used widely by the commercial auto industry. However, manufacturers including Toyota, Honda, and General Motors have invested in developing hydrogen fuel-cell vehicles.

Cho said that the primary problem with the metal hydride storage has been relatively slow absorption and desorption rates of hydrogen during the cycling of the units, leading to intermittent electricity production, resulting in stalling. This would be unsuitable for vehicles on the open road.

She also said higher-capacity hydrogen storage would be needed in order to exceed the range performance of existing electric-vehicle batteries. Maintaining perfect seals on the magnesium nanocrystal core would be crucial, as any damage leading to air or moisture exposure would render it unusable.

Cho suggested other applications may be better suited for hydrogen fuel cells in the short term, including indoor vehicles such as forklifts and airport vehicles, water and sewage pumps, portable power sources like laptop battery chargers, portable lighting, and emergency services equipment.

The team has been recently tackling the exposure problem by using a “one pan” technique to mix up the graphene sheets and magnesium oxide nanocrystals in the same batch. X-ray studies showed how hydrogen gas in the fuel cell mixture reacted with the magnesium nanocrystals to form a more stable molecule called magnesium hydride while keeping the oxygen from reaching the magnesium.

The next stage for the team will include researching catalysts that will help solve the absorption speed and efficiency problem to improve the fuel cell’s conversion to electrical current, and will investigate whether different materials can improve the fuel cell’s capacity.

Nature Communications, DOI: 10.1038/ncomms10804