Atomic-scale defects in thin sheets of graphene oxide (red) allow hydrogen gas molecules to pass through while blocking oxygen and water. Encapsulating nanoscale magnesium crystals (yellow) with graphene oxide sheets produces a new formula for metal hydride fuel cells.

Hydrogen, the lightest and most plentiful element in our universe, could become a clean, carbon-free, virtually limitless energy source for cars, portable generators, telecommunications towers, and other products, with water as the only combustion byproduct.

Scientific challenges remain in making hydrogen-based energy sources more competitive with current automotive propulsion systems but researchers at the U.S. Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a new materials recipe for battery-like hydrogen fuel cells.

Surrounding hydrogen-absorbing magnesium nanocrystals with atomically thin graphene sheets pushes performance in key areas.

Graphene shields the nanocrystals from oxygen, moisture, and contaminants, while tiny, natural holes allow hydrogen molecules to pass through. This filtering process overcomes common hydrogen storage problems degrading metal hydrides.

These graphene-encapsulated magnesium crystals act as sponges for hydrogen, offering a compact and safe way to take in and store the fuel. The nanocrystals also permit faster fueling while reducing the tank size.

“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,” says Eun Seon Cho, a postdoctoral researcher at Berkeley Lab and lead author of a study related to the new fuel cell formula, published recently in Nature Communications (http://goo.gl/KjikSb).

In a hydrogen fuel cell-powered vehicle using these materials, hydrogen gas pumped into a vehicle would be chemically absorbed by the magnesium nanocrystaline powder and rendered safe at low pressures.

The research, conducted at Berkeley Lab’s Molecular Foundry and Advanced Light Source, is part of a National Lab Consortium dubbed Hydrogen Materials-Advanced Research Consortium (HyMARC) that seeks safer and more cost-effective hydrogen storage. Jeff Urban, Berkeley Lab staff scientist, is lead scientist for that effort.

A measure of the energy storage capacity per weight of hydrogen fuel cells, known as the gravimetric energy density, is roughly 3x that of gasoline. The tiny size of the graphene-encapsulated nanocrystals created at Berkeley Lab, which measure about 3nm to 4nm, is a key in the new fuel cell materials’ fast capture and release of hydrogen, as they have more surface area available for reactions than the same material at larger sizes. Another key is protecting the magnesium from exposure to air, which renders it unusable for the fuel cell.

Next steps in the research will focus on using different types of catalysts to improve the speed and efficiency of chemical reactions to further improve the fuel cell’s conversion of electrical current. Researchers will also study whether different types of material can also improve the fuel cell’s capacity.

Lawrence Berkeley National Laboratory

www.lbl.gov