Electrospun piezoelectric polymer 3D structures for wearable energy harvesters
Diaz Sanchez, Francisco Javier
Wearable devices have emerged as one of the most rapidly growing branches of the consumer electronics industry in recent years. Having a wide breadth of applications, ranging from leisure and fitness tracking to therapeutics and diagnostics, their development has become a critical driving force in the field of personalised medicine and point-of-care technologies. With the availability of more powerful processing techniques, efficient design approaches, and the miniaturisation of the basic building blocks that conform them, the capabilities of wearable devices have great potential for growth. Energy sources are one of the critical challenges associated with the design of wearable electronics. Renewable sources such as piezoelectric energy harvesters are of great interest, offering a viable alternative that can help tackle the problem of e-waste by enhancing the lifespan of a primary power source or as an independent power source. The piezoelectric active core materials of energy harvesters are the elements that allow for the conversion of mechanical energy to electrical energy. Contrary to the case of using piezoelectric ceramics, polymer based active cores offer superior flexibility, low manufacturing costs, and are non-toxic. However, their piezoelectric properties are comparatively lower than those of ceramics. Micro and nanofabrication methods for the manufacture of polymer based piezoelectric structures are of great interest in the field of energy harvesting because they allow for the tuning of specific morphological properties of these materials, offering the possibility of tailoring the material to the intended application and for the enhancement of the piezoelectric properties of the manufactured structures in some cases, which can bring the piezoelectric performance of polymer based materials closer to that of ceramics Electrospinning is a technique for the fabrication of nano and microfibrous structures based on the principles of electrohydrodynamics. This versatile manufacturing method not only allows for the fabrication of diverse morphologies of a material depending on the working parameters, ambient conditions and reagents, but can also intrinsically enhance the properties of the product. In this thesis, electrospinning will be used for the fabrication of polymer based piezoelectric materials. The work presented in the following chapters will focus firstly on the optimisation of the working parameters and on the composition of the polymer solutions for the fabrication of morphologically stable fibres and consequently will deal with improving the electrical response of these structures when they are used as the active core of a piezoelectric generator. Initial experimental work deals with the optimisation of polymer solutions containing the ferroelectric polymer poly(vinylidene fluoride) (PVDF). Favourable conditions for the fabrication of PVDF nanofibres were identified, and the resulting 2D fibrous mats were used for assembling a first iteration of piezoelectric generators. The findings indicated that the electrospun PVDF product had a favourable electrical response in spite of the morphology of the fibrous product not being ideal. Thus, improving the quality of the electrospun products would certainly allow for the fabrication of better performing generators. The use of chemical additives, solvent systems, and the combination of polymers for electrospinning can heavily influence the quality of the product. This thesis proceeds with the exploration of this premise, using combinations of PVDF with poly(ethylene oxide) (PEO) and lithium chloride (LiCl) for improving the quality of the material. Fibre morphology improved dramatically with the use of these additives, and it was observed that the fabricated fibrous structures could now transition to 3D materials under specific conditions, with variants ranging from a cloud-like structure to thick sponge-like fibrous mats. The conditions required for the production of 3D structures were found to be compatible with poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE), a copolymer known to have intrinsically superior piezoelectric properties than PVDF. The fabricated structures were used for assembling piezoelectric generators, and their electrical properties were shown to be comparable or to outperform similar state-of-the-art devices. Design opportunities were identified while working on the proposed piezoelectric generator architecture and the interfacing methods used for bonding the active core to the electrode materials. The thesis finalises with an exploration of additional methods that can be used to further increase the electrical response of generators with thick sponge-like fibrous PVDF-TrFE/PEO active cores. The findings of this final study revealed that electrode placement and design that conforms to the characteristics of the electrospun fibrous core and the use of electrode materials that can interface with both the surface of the active core and the fibrous network within the core material can improve the electrical output of the generators dramatically. The multidisciplinary work presented in this thesis explored fields ranging from chemistry and materials science to electronics and electrical engineering, laying the ground work upon which new research opportunities for the development of portable renewable energy sources can develop.