Although complex chemical hydrides exist that store hydrogen in concentrations that are well above 10 wt%, these materials have slow desorption kinetics and can release other chemicals such as ammonia or borazine which could poison a fuel cell. Many are also difficult to handle in that they degrade rapidly in air.

Cella Energy scientists, working with the London Centre for Nanotechnology at University College London and University of Oxford, developed a method using a low-cost process called coaxial electrospinning or electrospraying that can trap a complex chemical hydride inside a nano-porous polymer that speeds up the kinetics of hydrogen desorption, reduces the temperature at which the desorption occurs and filters out many if not all of the damaging chemicals. It also protects the hydrides from oxygen and water, making it possible to handle it in air.

The coaxial electrospinning process that Cella uses is simple and industrially scalable, it can be used to create micron-scale micro-fibers or micro-beads nano-porous polymers filled with the chemical hydride.

Cella’s current composite material uses ammonia borane (NH₃BH₃) as the hydride and polystyrene as the polymer nano-scaffold. Ammonia borane in its normal state releases 12 wt% of hydrogen at temperatures between 110 °C and 150 °C, but with very slow kinetics.

In Cella’s materials the accessible hydrogen content is reduced to 6 wt% but the temperature of operation is reduced so that it starts releasing hydrogen below 80 °C and the kinetics are an order of magnitude faster. Cella notes that the current material, while suitable for proof-of-concept work and potentially useful for the initial demonstrator projects, is not a viable commercial material: it is expensive to make and cannot be easily re-hydrided or chemically recycled.

Cella is working on other hydride materials with slightly lower hydrogen contents but with faster cycle time; these are being encapsulated in hydrogen permeable high-temperature polymers based on polyimide.

The polymer micro-beads can be moved like a fluid; this fluidized hydride offers several opportunities for transportation storage:

• It is no longer necessary to try and rehydrogenate the material within the vehicle. For most hydrogen storage materials this releases megajoules of energy. If the refuelling is to be done in a few minutes, this requires cooling to remove several hundred kilowatts of power. To facilitate rehydrogenation in the 3-4 minutes that the DOE targets stipulate, the thermodynamics require high temperatures and pressures of around 100 bar.

  With a fluidized hydride, it is possible to quickly fill or remove the material from the vehicle so that it can be recycled or rehydrided elsewhere.

• It is possible to move the material within the vehicle making it possible to separate the storage unit from thermolysis. One implementation of this concept would be storing the beads are stored in a fuel tank, which, because it does not need to contain high pressures or be heated and cooled, could be a simple lightweight plastic tank of complex shape similar to that used in current vehicles.

  The hydride beads would then pumped to a hot cell where waste heat from the engine exhaust is used to drive the hydrogen into a small buffer volume. The hydrogen buffer is maintained at a pressure suitable for the internal combustion engine ICE or fuel cell and which is sufficient in volume to be able to restart the vehicle. Once the hydride has been heated and the hydrogen driven off, the waste beads are moved to and stored in another lightweight plastic tank.

In some senses hydrogen is the perfect fuel; it has three times more energy than petrol per unit of weight, and when it burns it produces nothing but water. But the only way to pack it into a vehicle is to use very high pressures or very low temperatures, both of which are expensive to do. Our new hydrogen storage materials offer real potential for running cars, planes and other vehicles that currently use hydrocarbons on hydrogen, with little extra cost and no extra inconvenience to the driver.

—Professor Stephen Bennington, lead scientist on the project for STFC and Chief Scientist at Cella

Cella Energy has received investment from Thomas-Swan & Co Ltd.; a specialist chemical company based in the North-East of England.

Resources

• Zeynep Kurban, Arthur Lovell, Stephen M. Bennington, Derek W. K. Jenkins, Kate R. Ryan, Martin O. Jones, Neal T. Skipper, and William I. F. David (2010) A Solution Selection Model for Coaxial Electrospinning and Its Application to Nanostructured Hydrogen Storage Materials. J. Phys. Chem. C 114, 21201–21213 doi: 10.1021/jp107871v