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. For the TENG assembly, instead of the conventional system of contact/separation or sliding modes, Tabassian et al (2024) introduced a rolling mechanism which is expected to have less mechanical stress on the component due to its smoother and continuous motion. Tabassian et al (2024) blended a metal-organic framework, zeolitic imidazolate framework (ZIF-8) into polyacrylonitrile (PAN) solution to electrospin ZIF/PAN nanofibrous mat. ZIF-8 nanocrystals have high surface potential, good temperature stability and are resistant to environmental degradation. The resultant electrospun ZIF/PAN nanofibers had a rough surface due to the cubic ZIF nanocrystals which increased triboelectrification. The voltage output of triboelectric nanogenerator (TENG) with ZIF/PAN nanofibrous mat at optimum ZIF loading of 1.5 wt% was 178 V and current of 7.5 µA which is significantly greater than pure PAN electrospun nanofibrous mat with voltage output of 100 V and current of 4.1 µA. The higher output can be attributed to the enhanced surface charges of the ZIF/PAN nanofibers due to ZIF. Greater surface roughness which increases the effective contact area would also increase the output. Greater addition of ZIF may cause aggregation of the nanoparticles and cause uneven distribution. A maximum power density of 204.8 mW/m² was obtained with a 100 MΩ resistor.

Experimental steps and setups. (A) Synthesis process of zeolitic imidazolate framework (ZIF)-8 nanocrystal and electrospun ZIF/polyacrylonitrile (PAN) nanofibrous mat. (B) Contact-separation triboelectric nanogenerator (TENG) device made by the ZIF/PAN nanofibrous mat as electropositive triboelectric material. (C) Layout of the proposed rotary TENG device works based on rolling mode [Tabassian et al 2024].

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. Wu et al (2023) prepared an electrospun Mo-doped carbon nanofibers scaffold as the anode in a H-type double-chamber microbial fuel cells (MFCs) inoculated with Shewanella putrefaciens CN32 (S. putrefaciens CN32) cell suspension. The resultant MFC showed an output power density of about 1287.38 mW·m^-2 with a peak current density of 306.1 µAcm^-2. S. putrefaciens CN32) formed a good biofilm with a large number of bacteria on the surface of Mo₂C nanofiber scaffold compared to scarce bacteria count on the surface of pure carbon nanofiber anode. This demonstrated the improved biocompatibility of Mo₂C which led to better MFC performance. At the third discharge cycle, the Mo₂C nanofiber anode produced an output current density of 0.20 mA cm^-2 compared to 0.14 mA cm^-2 from the carbon nanofiber anode. Further, the rough surface of Mo₂C nanofibers scaffold increases the number of active sites and encourages the adsorption of extracellular bacterial proteins.

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. Jia et al (2024) constructed a Ni₂P-MoC/coal-based carbon fiber (Ni₂P-MoC/C-CF) self-supporting catalysts for hydrogen production with the use of electrospinning to form the carbon fiber support. The electrospinning solution was prepared using polyacrylonitrile (PAN), oxidized coal and molybdenyl acetylacetonate. The resultant membrane is then soaked in Ni(NO₃)₂.6H2O to load the Ni source followed by phosphorization and carbonisation to give Ni₂P-MoC/C-CF. The distribution of Mo and Ni₂P nanoparticles were uniform on the surface of C-CFs. The presence of Ni₂P nanoparticles were found to increase the hydrophilicity of the membrane which increases the contact area of the catalyst with the electrolyte hence facilitating the evolution of H₂. The catalytic production of H₂ showed a low overpotential of 112 mV at 10 mA.cm². Al-Dhubhani et al (2023) constructed bipolar membranes (BPMs), an ion exchange membrane that combines cation exchange and anion exchange layers for hydrogen gas formation. The anion exchange layer was made from electrospun Fumasep (FAA-3)/polymeric catalysts poly(4-vinylpyrrolidone) (P4VP) and the cation exchange polymer layer made of electrospun sulfonated poly(ether ether ketone) (SPEEK). A water dissociating catalyst for BPMs, MCM-41 with mesoporous structure of a zeolitic-like framework of amorphous silica, was electrosprayed in the form of nanoparticles onto the electrospun membrane. The resultant BPMs/MCM-41 demonstrated a water dissociation overpotential of only 280 mV (at 1000 A/m²) above the theoretical water dissociation limit of 0.83 V. In the absence of MCM-41, the water dissociation overpotential was greater than 5 V.

[+]

[-] comments powered by Disqus

Content [Power Generation]