Screen-printed textile supercapacitors

The ongoing revolution in electronics gave rise to advancements in the field of electronic textiles. Electronic textiles have gained tremendous interest in recent years and found widespread applications in wearable medical monitors, sports, personal electronics and in military field. However, existing prototypes employ detachable energy storage units, which limit the widespread use of electronic garments, requiring leak-free textile powering configurations. Supercapacitors are electrochemical energy storing systems that possess high power densities and moderate energy densities, bridging the gap between batteries and capacitors. The demand for textile electrochemical energy storing technologies requires scalable manufacturing techniques.

In this doctoral research, the combination of electrochemical enhancing strategies through ink formulation and optimisation, and the development of suitable textile composite architectures that allow the integration of a multi-component electrode design to maximise the performance of supercapacitors were investigated. To fulfil the targeted aims, three research objectives were established, which include (i) the selection of electrochemical energy storing textile architecture, suitable electrode materials and manufacturing process, (ii) formulation and optimisation of inks compatible with the printing technique, but also demonstrate the mechanical, electrical and electrochemical properties required in textile supercapacitors, and (iii) the development of all-printed textile architectures able to accommodate the multicomponent electrode design, but also endure external mechanical stresses during wear. According to the literature review, planar-type textile architecture delivers the best properties regarding textile supercapacitors. Regarding the electrode materials, a range of carbon allotropes was selected. As for the transferring technique, screen printing was chosen due to its technological robustness and sustainability.

Due to the rheological requirements of the printing technique, a novel ink system was investigated. It was found that the rheological stability of the ink was mostly influenced by the solvent and binder contents, requiring fine-tuning upon formulation. Likewise, the electrochemical properties were adversely affected by high binder and solvent contents. To increase the electrical conductivity of printed electrodes, a combination of conductive carbon additives, such as carbon black Ketjenblack EC-600JD and carboxylic-functionalised multiwalled carbon nanotubes, was introduced. The proposed strategy not only allowed a decrease in electrical resistivity by half, but also an increase of activated carbon content in the electrode. It was also found that the utilisation of functionalised multiwalled carbon nanotubes provided an increase in flexural strength due to the mechanical properties of the material, but also due to the additional bonds formed between the binder and the carboxylic groups attached to the nanotubes. Following studies focused on the decrease of self-discharge in supercapacitors by introducing surfactants in the ink formulation. It was found that small quantities of the anionic surfactant sodium dodecyl sulfate provided a decrease in self-discharge.

To accommodate printed materials on fabrics, an interfacial layer was deposited on the textile substrate. As for the current collector, printed silver was selected since it possesses low sheet resistances (16.5 mΩ □^-1 for two printed layers). However, the imminent exposure of silver in an electrochemical medium result in corrosive reactions, ruining the long-term stability. To mitigate this issue, carbon conductive paste was deposited on the printed silver current collectors. To minimise the content of silver, the current collector was patterned, allowing a decrease of 50% in silver content in the textile electrodes. The proposed textile electrode architecture demonstrated electrochemical performances as good as in textile storing units without patterned current collectors. With a stable textile architecture, able to withstand high flexural stresses (10,000 bending cycles), and with the lowest equivalent series resistance reported in flexible textile supercapacitors (0.2 Ω), the textile composite electrodes were used to evaluate the electrochemical performance of different active layer formulations. It was found that a formulation comprised of activated carbons, carbon black Ketjenblack EC-600JD, functionalised multiwalled carbon nanotubes, and sodium dodecyl sulfate, demonstrated the best electrochemical performance, depicting an areal capacitance, energy and power densities of 116.9 mF cm^-2, 7.91 μWh cm^-2, 3.45 mW cm^-2, respectively, and also low self-discharge rates.

The strategies herein reported allow the manufacture of textile supercapacitors with improved mechanical properties and high-power densities, to meet the demand for textile electrochemical storing systems.