Electrospun nanofibers in energy applications

V. Thavasi, G. Singh and S. Ramakrishna of the National University of Singapore write in Energy and Environmental Science that materials of nanofibrous morphology are attractive to solve numerous energy and environmental issues. Nanofibers can be effectively produced by electrospinning, which is a simple and low cost technique for both organics and inorganics in different configurations. This is for example, inorganic materials, notably metal oxides, can be synthesized and electrospun for energy devices, improving conducting and ceramic properties. Excitonic solar cells fabricated with aligned nanofibrous metal oxide electrodes can provide higher solar-electric energy conversion efficiency. Fuel cells made with nanofibrous electrodes facilitate uniform dispersion of catalysts, which increases electrocatalytic activity leading to higher chemical-electric energy conversion efficiency.

They summarise the potential as follows:

Table 1: Foresights on the broad applications of electrospinning

Source: National University of Singapore

Uniform nanofibers of conjugated polymers have been prepared via a coaxial electrospinning technique.

Scanning electron micrographs of electrospun (a) random nanofibers, (b) aligned fibers at an angle, (c) aligned fibers.

Source: National University of Singapore

Dye Sensitised Solar Cells conventionally use titanium dioxide semiconductor in the form of nanoparticles to maximise the area of electron generating dye on them and to give narrow angle of operation where needed. However, the Singapore team have made DSSCs using titanium dioxide nanorods. Under air mass 1.5 global filter (AM1.5 G) condition, these delivered a current density 13.6 mA cm−2, open circuit voltage 0.8 V, fill factor 51% and energy conversion efficiency 5.8%. This set no records but it gives a new route forward in improving the properties of DSSC. The group then introduced an ultra-thin surface treatment layer (STL) on the conducting substrate before the deposition of the electrospun TiO2 nanofibers, which retained adhesion of nanofibers on the conductive substrate even after calcination. After calcination, the STL acted as an adhesive and thus improved the adhesion of TiO2 nanofibers. Doping may further improve the parameters.

Air spraying of TiO2 nanorods onto the surface of FTO glass to make a photoanode for a DSSC

The team also carried out work that may lead to the improvement of organic and hybrid photovoltaics.

The team conclude that, "One of the drawbacks of electrospinning is that, it has been difficult to obtain uniform nanofibers with diameters below 50 nm using electrospinning. Another drawback is the relatively low production rate. In the near future, it is likely that research efforts will be focused on engineering the electrospinning process, with the ultimate goal of producing nanofibers with diameters below 50 nm, and at a faster rate. In the long term, it is expected that vertical patterned nanofibers, an ideal morphology using electrospinning, should be possible to produce in order to achieve maximum electron transport in energy and electronic devices and controlled pore sizes for environmental filtration."

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