Alternatives to Fluoropolymers for Motion-Based Energy Harvesting: Perspectives on Piezoelectricity, Triboelectricity, Ferroelectrets, and Flexoelectricity.

Fluoropolymers, including polytetrafluoroethylene (PTFE, Teflon), polyvinylidene difluoride (PVDF), and fluorine kautschuk materials (FKMs, Viton) are critical polymers for applications ranging from non-stick coatings, corrosion resistant seals, semiconductor manufacturing, membranes, and energy harvesting technologies. However, the synthesis of these fluoropolymers requires the use of per- and polyfluorinated alkyl substances (PFAS) known colloquially as "forever chemicals," and as such there is a pressing need to develop alternative technologies that can serve the end-use of fluoropolymers without the environmental cost of using PFAS. Further, fluoropolymers themselves fall under the PFAS umbrella. Here, alternative mechanical-to-electrical energy harvesting polymers are reviewed and benchmarked against the leading fluoropolymer energy harvesters. These alternative technologies include nonfluoropolymer piezoelectric polymers, triboelectric nanogenerators (TENGs), ferroelectric elastomers, and flexoelectric polymers. A vision towards sustainable, non-fluoropolymer-based energy harvesting is provided.

Low-density PDMS foams by controlled destabilization of thixotropic emulsions.

In the current study we demonstrate a method of preparation of low-density polydimethylsiloxane (PDMS) foams from emulsions by using water-based thixotropic fluids as porogens. Aqueous dispersions of synthetic hectorite clay and nanocellulose were used as thixotropic fluids, enabling the preparation of fine emulsions in bulk form with the droplet size down to few tens of microns by simple hand mixing. Contrary to conventional emulsion templating where stabilization of emulsion is required, a strategy was developed for obtaining foams by using controlled destabilization of an emulsion, induced during the curing of the PDMS matrix phase by adding a carefully selected surfactant in optimized concentration. This strategy enables the preparation of bulk PDMS foams with interconnected porosity in a range of density values, fast and deformation-free drying and uniform porous structure with a range of mechanical properties. Clay microplatelet with clearly defined shape and with mass in the nanogram range is retained in spherical pores as the porogen is removed by evaporation. Foams with density down to 0.353 g/cm3 and thermal conductivity of 0.0745 W/m * K were prepared. Elastic modulus of the prepared foams ranged from 0.156 to 0.379 MPa, a reduction of 94.3-86.3% as compared to pure nonporous PDMS.

Thermal, Mechanical, and Acoustic Properties of Polydimethylsiloxane Filled with Hollow Glass Microspheres.

Polydimethylsiloxane (PDMS) is the most widely used silicon-based polymer due to its versatility and its various attractive properties. The fabrication of PDMS involves liquid phase cross-linking to obtain hydrophobic and mechanically flexible material in the final solid form. This allows to add various fillers to affect the properties of the resulting material. PDMS has a relatively low Thermal Conductivity (TC), in the order of 0.2 W/mK, which makes it attractive for thermal insulation applications such as sealing in construction. Although a further decrease in the TC of PDMS can be highly beneficial for such applications, most research on the thermal properties of PDMS composites have focused on fillers that increase the TC rather than decrease it. In the present work, we propose a simple and reliable method for making a PDMS-based composite material with significantly improved thermal insulation properties, by adding hollow glass microspheres (HGMs) to the mixture of the liquid base and the cross-linker (10:1 ratio), followed by degassing and heat-assisted crosslinking. We obtained a 31% reduction of thermal conductivity and a 60% increase in the elastic modulus of samples with HGM content of 17% by weight. At the same time, the sound insulation capacity of the PDMS-HGM composite is slightly decreased in comparison to pure PDMS, as a result of its lower density. Finally, the wettability of the samples had no dependence on HGM content.

Probing Contact Electrification: A Cohesively Sticky Problem.

Contact electrification and the triboelectric effect are complex processes for mechanical-to-electrical energy conversion, particularly for highly deformable polymers. While generating relatively low power density, contact electrification can occur at the contact-separation interface between nearly any two polymer surfaces. This ubiquitousness of surfaces enables contact electrification to be an important phenomenon to understand energy conversion and harvesting applications. The mechanism of charge generation between polymeric materials remains ambiguous, with electron transfer, material (also known as mass) transfer, and adsorbed chemical species transfer (including induced ionization of water and other molecules) all being proposed as the primary source of the measured charge. Often, all sources of charge, except electron transfer, are dismissed in the case of triboelectric energy harvesters, leading to the generation of the "triboelectric series", governed by the ability of a polymer to lose, or accept, an electron. Here, this sole focus on electron transfer is challenged through rigorous experiments, measuring charge density in polymer-polymer (196 polymer combinations), polymer-glass (14 polymers), and polymer-liquid metal (14 polymers) systems. Through the investigation of these interfaces, clear evidence of material transfer via heterolytic bond cleavage is provided. Based on these results, a generalized model considering the cohesive energy density of polymers as the critical parameter for polymer contact electrification is discussed. This discussion clearly shows that material transfer must be accounted for when discussing the source of charge generated by polymeric mechanical energy harvesters. Thus, a correlated physical property to understand the triboelectric series is provided.

Deposition of low-density thick silica films from burning sol-gel derived alcogels.

In the current study we show that the combustion of sol-gel derived alcogels with specifically tailored composition leads to the release of silica nanoparticles from the burning alcogel in a controlled manner which enables direct deposition of the released nanoparticles into low-density silica thick films. The process has some similarities to flame spray pyrolysis but requires no aerosol generator or other sophisticated instrumental setup. By the proper choice of catalysts and mixture of silicon alkoxides for the synthesis of the alcogel, preferential hydrolysis and polycondensation of one of the alkoxides is achieved. This leads to the formation of an alcogel with volatile silica precursor trapped in the gel pores. Resulting alcogels were burned to deposit uniform porous silica films with density of ~0.1 g/cm3 and primary particle size of ~10 nm. Demonstrated method yields silanol-free silica directly, without additional treatment steps and enables straightforward control over the deposition rate and coarseness of the layer by simple adjustment of the composition of the silica alcogel. The maximum layer thickness is limited only by the deposition time (in the current work up to 134 µm). Such technique of porous oxide film preparation could potentially be extended to the preparation of porous films from other oxides by using respective metal alkoxides as precursors.

Contact electrification between identical polymers as the basis for triboelectric/flexoelectric materials.

Polymer contact electrification offers the possibility to harvest mechanical energy using lightweight, flexible and low-cost materials, but the mechanism itself is still unresolved. Several recent studies confirm heterolytic covalent bond breaking as the mechanism for surface charge formation. Here it is shown that the reason for the formation of surface charge by contacting two identical polymers results from the fluctuation in the surface irregularities, and that contacted materials with a greater porosity or surface roughness differential result in a greater generation of surface charge. Porosity and surface roughness create uneven surface length percentage changes in the lateral direction during deformation, which changes the charge density across the surface during relaxation. Multilayered membranes exhibit flexoelectric properties upon pressing and releasing by generating charge without separating individual membrane layers. This new insight deepens the understanding of polymer contact electrification and highlights better ways to prepare triboelectric or flexoelectric nanogenerator devices.

Matching the Directions of Electric Fields from Triboelectric and Ferroelectric Charges in Nanogenerator Devices for Boosted Performance.

Embedding additional ferroelectric dipoles in contacting polymer layers is known to enhance the performance of triboelectricnanogenerator (TENG) devices. However, the influence of dipoles formed between the triboelectric surface charges on two contacting ferroelectric films has been ignored in all relevant studies. We demonstrate that proper attention to the alignment of the distinct dipoles present between two contacting surfaces and in composite polymer/BaTiO3 ferroelectric films can lead to up to four times higher energy and power density output compared with cases when dipole arrangement is mismatched. For example, TENG device based on PVAc/BaTiO3 shows energy density increase from 32.4 µJ m-2 to 132.9 µJ m-2 when comparing devices with matched and mismatched dipoles. The presented strategy and understanding of resulting stronger electrostatic induction in the contacting layers enable the development of TENG devices with greatly enhanced properties.

TiO2 nanowire dispersions in viscous polymer matrix: electrophoretic alignment and optical properties.

The changes in optical properties during TiO2 nanowire orientation in polydimethylsiloxane (PDMS) matrix under the influence of an electric field are strongly influenced by nanowire (NW) diameter. It was demonstrated for the first time that either positive or negative change in transmittance can be induced by NW alignment parallel to the electric field depending on the NW diameter. These effects can be explained by the interplay between scattering and reflectance. Experimental findings reported could be important for smart window applications for the regulation of visible or even infrared transparency, thus reducing the energy consumption by air conditioning systems in buildings and automobiles in the future.