Cella Energy have 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-fibres or micro-beads nano-porous polymers filled with the chemical hydride. Cella believes that this technology can produce an inexpensive, compound material that can be handled safely in air, operates at low pressures and temperatures and has sufficiently high hydrogen concentration and rapid desorption kinetics to be useful for transport applications.
Our current composite material uses ammonia borane NH3BH3 as the hydride and polystyrene as the polymer nano-scaffold. Ammonia borane in its normal state releases 12wt% of hydrogen at temperatures between 110Â°C and 150Â°C, but with very slow kinetics. In our materials the accessible hydrogen content is reduced to 6wt% but the temperature of operation is reduced so that it starts releasing hydrogen below 80Â°C and the kinetics are an order of magnitude faster. Although ideal for our proof-of-concept work and potentially useful for the initial demonstrator projects it is not currently a viable commercial material: it is expensive to make and cannot be easily re-hydrided or chemically recycled.
Cella is now working on other hydride materials, these have slightly lower hydrogen contents but it is possible to cycle them into the hydride phase many hundreds of times and we are encapsulating these in hydrogen permeable high-temperature polymers based on polyimide.
Use of the technology
There are two ways to use these materials:
Pure hydrogen solution Zero carbon emissions
as a way of storing and delivering hydrogen safely for use in an internal combustion engine or a fuel cell
Fuel additive Lower carbon emissions
For use as a fuel additive to reduce the carbon emissions from a hydrocarbon fuel such as gasoline, diesel, JP-8, jet-fuel or kerosene
Pure hydrogen solution, how it would work in a vehicle
Cella can manufacture the materials in the form of micron-sized beads. This makes it possible to move the beads like a fluid. This opens up a number of opportunities:
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 100bar. This requires substantial engineering and as such we don’t believe that on-car rehydrogenation is reasonable. 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 from thermolysis. A schematic for the kind of idea that we have to achieve this is shown in figure 1. The beads are stored in a fuel tank, which does not need to contain high pressures or be heated and cooled, therefore it can be a simple lightweight plastic tank of complex shape similar to that used in current vehicles. The hydride beads are 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 stored in another lightweight plastic tank.
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