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Once a niche technique, the art of using electrospinning methods to create nanoscale fibers is becoming a widely used method, both in the production of various nanofibers as raw materials, and in producing fibers for specific applications. In this article, we look at how electrospinning is used for energy storage applications.
What is Electrospinning?
Electrospinning is an effective way of producing nanofibers. Threads of polymer fibers are drawn out from a polymer melt solution using an electrostatic force. While polymers make up the majority of the fibers produced using this method, carbon fibers have also emerged as a fiber that can be created using this method. Carbon fibers can be produced from a carbonizing polymer, so the method can be adapted to create carbon fibers, and the fibers produced can also be incorporated with other constituents (such as nanoparticles) during the electrospinning process.
As a general method, a melt is used as the starting material. Commonly this is a polymer melt, but it can be a different carbonaceous melt for producing non-polymeric carbon fibers. In short, the melt is subjected to an applied electrical field where it becomes removed and 'whipped' from the source in fine needle-like structures to create nanofibers.
If you look at the method in more detail, it is a bit more complex. The melt moves towards an opening of a needle tip/capillary and melt droplets are formed. A high voltage is then applied to the material, where it becomes charged. When this happens, the electrostatic repulsion of like electronic charges counteracts the change in surface tension, and the droplets become elongated.
The stretching reaches a critical point and becomes a charged jet of liquid. The liquid jet dries while in flight, which in turn causes both the flow and rheology of the liquid to change. This rheological change causes the charges to migrate and stay at the surface of the polymer jet. The charge migration also causes the jet to elongate and cause the jet to 'whip'.
A ground collector is then used to collect the now dried and solid fibers. The ground collector can be either stationary or moving, and typically yields aligned and randomly orientated nanofibers, respectively. More than one jet can also be applied to produce fibers with multiple constituents or more complex fiber meshes.
Applying Electrospinning to Energy Storage Applications
Electrospinning is now used to create structures that can be used in a wide variety of energy storage applications, including various types of batteries and supercapacitors. Both polymeric and carbon-based fibers are being trialed in these areas.
Electrospinning can also be used in the energy harvesting, for example in nanogenerators.
Lithium-Ion Batteries
Lithium-ion (Li-ion) batteries are the most widely developed and used class of batteries, but people (be it industry or academic researchers) are always trying to improve their efficiency and safety further.
Electrospinning carbon membranes into electrodes have emerged as a way of improving the efficiency of electrodes, by removing the need for inactive materials—such as the binder molecules—within the electrode, allowing the molecular density to have a higher proportion of active materials that contribute to the energy density of the battery. These membranes have also been shown to increase the flexibility of the electrodes and provide higher resistance to electrode volume changes.
Redox Flow Batteries
Due to reservations regarding long-term lithium supplies, many other batteries are being trialed, and redox flow batteries are one class that shows a lot of promise due to their high efficiencies, life cycle stability, and greater environmental friendliness. However, many of these batteries need large chemical tanks if they are to provide a large amount of energy, so new ways of improving the efficiency of smaller tanks have been sought.
One option that is currently being explored is the use of electrospun cation-exchange membranes with specific channels that can better facilitate the movement of ions. These membranes, so far, have shown to have an improved permeability, efficiency, and stability to chemicals (in a continuous chemical environment) compared to other membranes trialed in redox flow batteries.
Metal-Air Batteries
Metal-air batteries are not as widely developed as other batteries, and commercialization efforts have been stunted due to the large overpotentials which are generated during the cycle and discharge cycles. But their lower cost and high theoretical energy density still mean that they have promise for the future, especially if lithium-ion batteries are to be reduced in the future when fewer natural resources are available.
Electrospinning is helping in this effort to produce more efficient metal-air batteries through the creation of porous graphite-like carbon fibers. These fibers can have metal nanoparticles embedded in them and have been used as effective bi-functional catalysts within these batteries to better facilitate oxygen evolution reactions (OERs) and oxygen reduction reactions (ORRs). It's a small step but one that shows promise for the future of metal-air batteries.
Supercapacitors
Aside from the various batteries that can be enhanced with electrospun meshes, supercapacitors are another area where electrospinning has potential. To work, supercapacitors need an efficient electric double layer, and tailored electrospun meshes composed of conductive carbon fibers have shown a lot of promise.
Electrospinning has been beneficial because the process has been able to create meshes which have a specific pore size, short diffusion pathway, high flexibility, high electrical conductivity, and a high electrochemically accessible surface area, all of which have helped to yield efficient devices so far.
Sources and Further Reading
- Electrospin Tech: http://electrospintech.com/carbonfibers.html#.XWrcKihKjIU
- “Nanofiber-Based Membrane Separators for Lithium-ion Batteries”- Alcoutlabi M. et al, Mater. Res. Soc. Symp. Proc. Vol. 1718, 2015, DOI: 10.1557/opl.2015.556
- “Electrospun Nanomaterials for Energy Applications: Recent Advances”- Santangelo S., Applied Sciences, 2019, DOI: 10.3390/app9061049
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