Coaxial electrospinning was used to encapsulate the complex hydride ammonia borane in polystyrene to improve its properties as a hydrogen storage material. A solvent selection system was developed by using the Hansen solubility parameters to facilitate the choice of compatible solvents for core and shell. This enabled systematic optimization of the parameters needed for successful coelectrospinning. This approach has general application for any multiphase electrospinning system, including ones where the core is highly conducting or nonpolymeric. The resulting fiber morphologies depend strongly on the degree of miscibility of core and shell solutions. Fibers spun from immiscible core−shell solutions had a classic coaxial structure. Fibers produced from semimiscible core−shell solutions were highly porous, with inclusions extending through the fiber and an ordered radial and longitudinal distribution of nanoscale pores on the fiber surface. We suggest that this type of porosity may be due to an instability created in the nonaxisymmetric modes at the core−shell interface, resulting in intrusion of the core into the shell polymer. These controllably porous structures have numerous potential applications including materials templating or drug delivery. In the porous fibers, the temperature of the first hydrogen release of ammonia borane is reduced to 85 °C. This result suggests a nanostructured hydride, but a large mass loss indicates that much of the ammonia borane is expelled on heating. The coaxial fibers, in contrast, appear to encapsulate the hydride successfully. The coaxial and porous fibers alike showed no significant release of borazine, suggesting two different suppression mechanisms for this impurity.
Example of micro fibers produced with 20 wt % AB (ammonia borane) in water as core solution, showing smooth (nonporous) and cylindrical (noncollapsed) fibers; from a 2010 paper by the scientific team. Credit: ACS, Kurban et al.
Cella Energy’s final product can be either a fine micro-fibrous polymer mat that resembles white tissue paper, or polymer micro-beads with a diameter of ~ 0.5 – 5µm, with the hydride material entrained in ~50 – 200nm pores within the polymer. The material could allow hydrogen to be stored in a cheap and practical way for transport applications.
They are 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 Energy has received investment from Thomas-Swan & Co Ltd.; a specialist chemical company based in the North-East of England.