Electrospun fibers for power generation

Electrospun fibers have been investigated for use as a power generation substrate for over a decade. Early adoption of electrospun fiber for clean energy generation is to use it in dye sensitized solar cell. Later efforts have been shifted to using electrospun polymers to produce electrical energy in which two methods are been investigated. The first is the use of piezoelectric polymers and the second is through triboelectric generation. Although both methods convert mechanical movement to electrical output, the mechanisms for electrical charge generation are very different and their implementation for power generation will also differ as a result. Electrospun fibers have also been tested for the production of clean fuel such as hydrogen.

Dye sensitized solar cell (DSSC) comprise of a transparent electrode, a photo-sensitized anode, a redox electrolyte and a cathode. Where electrosun fibers are used in the setup, it is normally used as the photo-sensitized anode. Although more commonly used for producing polymer nanofibers, electrospinning can also be used to manufacture inorganic nanofibers by electrospinning of their precursors followed by annealing. This makes it possible to produce TiO₂ nanofibers which are commonly used as the photo-sensitized anode in DSSC. It is thought that a network of inorganic nanorods allow higher rate of electron transfer [Adachi et al 2004] and better electrolyte dye penetration [Song et al 2005]. Various modifications have been made in terms of choice of anode [Zhang et al 2009], doping [Jin et al 2012] and structure [Hamadanian and Jabbari 2014]. Current energy efficiency from DSSC using electrospun component is limited to about 6.2% [Sibg et ak 2005].

Piezoelectric materials are materials that are able to generate electric charges in response to mechanical stress. Piezoelectric polymers are attractive for many potential applications due to its availability as flexible thin film. Although their piezoelectric charge constants are much smaller than piezoelectric ceramic materials, their benefits lie in their flexibility, durability and sensitivity to smaller force. These allow them to be used in areas which are not possible for the more fragile ceramic materials.

Due to the low power output, piezoelectric polymer materials are commonly used as sensors. A study by Chang (2009) found that the average energy conversion efficiency for a single PVDF nanofiber was 12.5%, going as high as 21.8%, which is much higher than the energy conversion efficiency of PVDF thin film which average about 1.3%. This gives electrospun fibers the potential for use as power generator. So far, the output from the electrospun piezoelectric material is only suitable as pressure sensors as the current generated are very low. Persano et al (2013) constructed highly aligned poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) nanofibers using a rotating drum. They were able to generate a maximum short-circuit current and voltage output of 40 nA and 1.5V respectively. Shafii (2014) electrospun randomly oriented PVDF fiber membrane and obtained a maximum power output of 2200 pW/cm² while Liu et al (2014) recorded a maximum power output of 577.6 pW/cm² from electrospun aligned PVDF fibers. For electrospun piezoelectric fibers to be used as power generator, a significant improvement power output is necessary.

Another method of generating electrical power using polymers is through triboelectric generation. This involves selection of materials that are far away from each other in the triboelectric series to obtain a large difference in electronegativity. Electric power is generated during periodic switching between contact and separation of the charged surfaces. Unlike piezoelectric materials, this method requires at least two different materials to generate charges.

Preliminary studies have shown that the power output using triboelectric generation is much higher than the power generated from electrospun piezoelectric fibers. Ye et al (2015) selected poly(vinylidene fluoride-cohexafluoropropylene) (P(VDF-HFP)) and Kapton for the construction of a triboelectric nanogenerator device. Comparing the performance of spin coated poly(vinylidene fluoride-cohexafluoropropylene) (P(VDF-HFP)) and electrospun P(VDF-HFP) nanofiber membrane, they found that the power output of the electrospun membrane was higher at 20 V and 19 µAcm^-2 to 3 V and 5 µAcm^-2 of spin coated P(VDF-HFP). By doping ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsufonyl)imide, [EMIM][TFSI] into P(VDF-HFP), they were able to fabricate ion gel nanofibers and further improve the output of the triboelectric nanogenerator device to 45 V and 49 µAcm^-2. Gu et al (2015) constructed an enclosed triboelectric nanogenerator with electrospun PVDF fibers and polyethylene (PE) film to generate negative triboelectric charges in a 4-parts setup. This setup was able to generate a maximum output voltage of 72 V and current of 0.66 mA when sound waves passed through it, sufficient to light up 24 red commercial LEDs without energy storage.

Electrospun membrane may be used indirectly for power generation in the form of a supporting and conductive substrate for inoculation of electrochemically active bacteria. Guzman et al (2017) explored such a bioelectrochemical systems (BESs) using electrospun polyacrylonitrile(PAN)-derived carbon nanofiber (CNF) coated with the conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and inoculated with Geobacter sulfurreducens. Using PEDOT coated carbon nanofiber and inoculated with Geobacter sulfurreducens, a current of 10.66 A m^-2 was recorded. However, this was lower than PEDOT coated carbon cloth with the bacteria which gave a current of 15.22 A m^-2. In terms of output normalized by mass, high porosity of carbon nanofiber outperforms carbon cloth.

Electrospun fibers may be used in the facilitation of power generation. For thermoelectric generators (TEGs), electrospun film incorporated thermal management devices have been shown to significantly increase power output. Since the power output of the TEG depends on the temperature difference between the cold and hot surfaces, phase change materials (PCMs) which absorb heat from the cold surface are able to increase the temperature difference. Lin et al (2023) constructed a phase change nanocomposite (PCN) film with flame retardant property using electrospinning, electrospraying and mold-pressing. Boron nitride nanosheets (BNNS) were loaded into the phase change materials (PCM) for thermal management applications. Using coaxial electrospinning, the PCM material, polyethylene glycol (PEG) forms the core and the sheath consists of boron nitride and thermoplastic polyurethane (TPU). The composite fibers were aligned on a rotating drum during electrospinning and an additional layer of boron nitride was electrosprayed on the nanofiber layer. A final step of mold-pressing was used to bind the electrosprayed boron nitride nanosheet (BNNS-es) to the nanofiber layer. The constructed PCM thermal management device fitted to the TEG was able to enhance the energy conversion by 100% power output at 70 °C heating temperature compared to TEG exposed to air.

Scheme of the preparation process of the PEG@TPU/BNNS-es nanocomposite films [Lin et al 2023]

Electrospinning derived photocatalytic nanofibers have also been tested in the production of hydrogen from water splitting. Al-Enizi et al (2023) constructed ZnO/CdS nanofibers by electrospinning precursors of ZnO mixed with CdS nanoparticles (NPs). As ZnO is inactive in the visible light region, the addition of CdS which has a narrow-band gap was able to tune the ZnO band gap to harvest visible light. Production of H₂ was carried out by placing the membrane in an aqueous solution containing Na₂SO₃/Na₂S under light irradiation. Comparison was made with ZnO only nanofibrous membrane and CdS NPs. The ZnO/CdS composite nanofibrous membrane was able to produce up to 820 µmolh^-1 g^-1. This is much greater than ZnO nanofibrous membrane and CdS NPs which produced 115 µmolh^-1 g^-1, and 180 µmolh^-1 g^-1 respectively. Superior performance of the ZnO/CdS composite nanofibrous membrane was probably due to the high surface area of the nanofibrous membrane, absorption efficiency of visible light and the low photogenerated electron-hole recombination rate.