Polypyrrole Hydrogels for Conductive, Energy-Autonomous Textile Based Electronic Skin

Abstract

This thesis aims to progress the understanding and development of hydrogels made from conducting polypyrrole. In particular, it examines their suitability as gel electrodes for stretchable, wearable energy storage that can mimic the elastic properties and enhance the electronic properties of skin. This research is vital due to the increasing developments of wearable electronics, whose applications are currently limited by their dependence on conventional energy storage. Creation of wearable, elastic devices will enable full energy autonomy of e-skin, eliminating bulky batteries and furthering full integration of soft electronics. Whilst research has addressed non-metallic energy storage, knowledge is still sparse regarding pure polymer electrodes and how their operation (both electrochemical and mechanical) can be improved. Here, research focuses on the optimisation of polypyrrole hydrogels for wearable supercapacitor electrodes. A fully non-metallic, gel device can be made highly elastic, and the pseudocapacitive contribution from redox reactions of the polypyrrole can help produce supercapacitors for high energy storage. Polypyrrole hydrogels are here made using simple and rapid creation methods without the use of high energies or harsh chemicals, to enable a highly scalable production method and maintain environmental compatibility. Investigation of several methods of incorporating these electrodes into textiles is then performed, to produce wearable energy storage devices which are highly stretchy and therefore comfortable to wear. This research first optimises the one-pot creation of polypyrrole hydrogels, achieving a conductivity of 8.13 S /cm which is comparable to polypyrrole films. The mechanical and electrochemical properties of the gels are improved by additional physical and chemical bonding: in particular, the adhesion and conductivity of these gels can be increased by 20x and 100x respectively through electropolymerising a phase of covalently-bonded polydopamine within the gel. Separately, the research performed here shows that electropolymerisation of dopamine can form a conductive film in highly acidic Fe(III) solution. Evidence of induced amide bonding within modified gels is also seen, presenting new avenues for double-network hydrogel research whilst increasing the stiffness of the gels by 40%. Additionally, more in-depth research into the secondary polymerisation step to form polypyrrole hydrogels is performed to show the influence of nitrate dopants, the ageing process, and the addition of hydrogen-bonding polyphenols on the gels' resultant mechanical and electrochemical properties. Primarily, whilst additional gel interconnectivity improved the stiffness and toughness of gels, the decrease is accessible surface area led to poorer electrochemical results. The highest capacitance of a coated polypyrrole layer is achieved with knitted substrates of Lycra, when infiltrated with a high-molecular weight PVA gel electrolyte, and reaches over 400 F /m. These can experience repeated extension to 100% without significant degradation of their properties. Cotton and wool fibres and knit substrates result in the most porous polypyrrole coatings when dipcoated due to their high number of surface hydroxyl groups enabling better pyrrole infiltration, but also exhibit hysteresis making them less suitable for performance textiles. The additional redox reactions provided by Lycra's polyurea component enable this unexpectedly high capacitance. These results are highly promising for the development of polymer electrodes, enabling better electrochemical and mechanical performance for the next generation of all-polymer, energy autonomous electronic skin. In particular, electropolymerisation of polydopamine results in a previously-unreported conductivity, whilst its addition to polypyrrole hydrogels increases their suitability for e-skin electrodes. Their simple incorporation into stretchable textile substrates means this method

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Xuqing Liu (Supervisor) & Yi Li (Supervisor)