Emulsion Electrospinning of Polytetrafluoroethylene (PTFE) Nanofibrous Membranes for High-Performance Triboelectric Nanogenerators

Electrospinning is a simple, versatile technique for fabricating fibrous nanomaterials with the desirable features of extremely high porosities and large surface areas. Using emulsion electrospinning, polytetrafluoro­ethylene/polyethene oxide (PTFE/PEO) membranes were fabricated, followed by a sintering process to obtain pure PTFE fibrous membranes, which were further utilized against a polyamide 6 (PA6) membrane for vertical contact-mode triboelectric nanogenerators (TENGs). Electrostatic force microscopy (EFM) measurements of the sintered electrospun PTFE membranes revealed the presence of both positive and negative surface charges owing to the transfer of positive charge from PEO which was further corroborated by FTIR measurements. To enhance the ensuing triboelectric surface charge, a facile negative charge-injection process was carried out onto the electrospun (ES) PTFE subsequently. The fabricated TENG gave a stabilized peak-to-peak open-circuit voltage (<i>V</i>_oc) of up to ∼900 V, a short-circuit current density (<i>J</i>_sc) of ∼20 mA m^–2, and a corresponding charge density of ∼149 μC m^–2, which are ∼12, 14, and 11 times higher than the corresponding values prior to the ion-injection treatment. This increase in the surface charge density is caused by the inversion of positive surface charges with the simultaneous increase in the negative surface charge on the PTFE surface, which was confirmed by using EFM measurements. The negative charge injection led to an enhanced power output density of ∼9 W m^–2 with high stability as confirmed from the continuous operation of the ion-injected PTFE/PA6 TENG for 30 000 operation cycles, without any significant reduction in the output. The work thus introduces a relatively simple, cost-effective, and environmentally friendly technique for fabricating fibrous fluoropolymer polymer membranes with high thermal/chemical resistance in TENG field and a direct ion-injection method which is able to dramatically improve the surface negative charge density of the PTFE fibrous membranes

Nanomaterials enabled advanced renewable energy technologies

The impending environmental crisis necessitates a pressing need to develop the next generation energy harvesting and generating technologies, ranging from the µW to W range. To this effect, this thesis focuses on the development of materials, measurement and analysis of devices for alternate energy devices ranging from energy harvesting nanogenerators (µW-mW range) to fuel cells (mW-W range). The work has been carried out in two parts, wherein part A encompasses seedless deposition of zinc oxide (ZnO) nanosheets via a facile electrochemical deposition method and study of its inherent piezoelectric and structural properties to fabricate piezoelectric and triboelectric effect based nanogenerators. In part B, an in-depth comparative study of platinum catalyst loaded on various two-dimensional (graphene nanoplatelets, graphene oxide) and one-dimensional (multi-walled carbon nanotubes) carbon nanostructures was carried out for enabling relatively inexpensive and stable materials for cathodic oxygen reduction reaction (ORR) in fuel cell applications. The ZnO nanosheet structures were synthesised on various conductive substrates using a low temperature electrochemical deposition process. The seedless deposition of ZnO nanosheets on Al substrates exhibited a honeycomb network like morphology with an additional plane of ZnO_Al.LDH with the layered double hydroxide (LDH) layer being highly conducive for the fabrication of piezoelectric nanogenerator (PENG) devices. The typical PENG devices developed using a metal/piezoelectric/metal structure with a ITO/ZnO_Al:LDH/Al configuration exhibited a maximum open circuit voltage (Voc) of ~0.7 V and a current density (JSC) of ~0.50 mAm-2 under an applied force of 80 N. To enhance the output power utilising the inherent piezoelectric properties of ZnO nanosheets, the ZnO:Al substrates were used as a charge injection interfacial layer in a triboelectric nanogenerator (TENG). A typical TENG comprising of a triboelectric negative composite of zinc stannate (ZnSnO3)/polyvinylidene fluoride (PVDF) blended with an interfacial ZnO nanosheet layer and a polyamide-6 (PA6) membrane as the triboelectric positive exhibited significantly higher power output than the PENG devices. The phase-inversion process for the synthesis of triboelectric membranes enabled a high piezoelectric coefficient for not only the pristine PVDF fluoropolymer membranes (d33 ~ -44 pmV-1) but also for the composite ZnSnO3/PVDF/ZnO:Al membranes (d33 ~ -74 pmV-1) as confirmed by piezo force microscopy (PFM) analysis. At a comparative 80 N force, the ZnSnO3/PVDF/ZnO:Al vs PA-6 TENG produced significantly higher Voc and Jsc of ~625 V and ~40 mAm-2 with a corresponding charge density of 100.6 μCm-2, respectively. The PFM analysis confirmed the interfacial dipole-dipole interactions occurring with the ferroelectric polarization of ZnSnO3-PVDF, promoting the alignment of polar axis of ZnO. Under the compressive stress during the TENG measurements, the piezoelectric potential produced in the ZnO nanosheets provides charge injection onto the surface of ZnSnO3-PVDF membrane, enhancing the charge density, which in-turn significantly enhances the power density from 0.11 to ~1.8 W/m2. Although the energy produced using this process is dynamic in nature, i.e. depends on the contact release cycles, the operation and the fabrication of energy harvesting devices is very economical and simple, thus making both the PENG and TENG devices excellent candidates for realising energy harvesting and self-powered electronic systems. In the area of fuel cells, the literature points to a significant on-going debate over the role of carbon supports in catalysing the ORR. The electrochemical performance of pristine carbon supports, including graphene nanoplatelets (GNP), graphene oxide (GO) and multi walled carbon nanotubes (MWCNTs) for catalysing ORR in an alkaline media (0.1M KOH), were studied and compared in detail. Amongst these supports, the pristine MWCNTs displayed a promising peak reduction current densities of ~3 mA/cm2, followed by GNP ~1.5 mA/cm2 and GO ~1.0 mA/cm2. The peak reduction potentials obtained by MWCNTs is comparable to heteroatom N- and B-doped MWCNTs. Further a variable metallic Pt (Pt0) loading values ranging from 20% to 50% was achived by using microwave-assisted polyol process on pristine carbon supports. A Pt loading-normalised comparison of ORR current densities shows that Pt/MWCNTs exhibited the highest linear sweep voltammetry (LSV) reduction current density of ~ 900 A/g, much higher than ~ 510 A/g of the commercial Pt carbon black supports, followed by Pt/GNP and Pt/GO that have the LSV value of ~500 A/g and ~200 A/g LSV, respectively. It is therefore suggested that the Pt/MWCNTs should be given a favourable consideration in ORR for the future development of fuel cell technologie

Exclusive self-aligned β-phase PVDF films with abnormal piezoelectric coefficient prepared via phase inversion

Self-polarised poly(vinylidene fluoride), (PVDF), films were prepared via a facile phase-inversion technique wherein the polymorphism of the films was controlled from exclusive α- (>90%) to β-phase (>98%) by simply varying the quenching temperature from 100 °C to -20 °C, respectively. At low temperatures, the β-phase crystallites were found to be self-aligned, with the PVDF thin films possessing a high piezoelectric coefficient of up to -49.6 pm V(-1). The extraordinarily high β-phase and piezoelectric coefficient of these PVDF films make them suitable for electroactive and energy harvesting applications