Study of high-performance flexible and wearable supercapacitors for energy storage

Abstract

The advent of smart textiles and functional garments gives rise to an unprecedented demand for energy storage in a flexible manner. The flexible and wearable supercapacitors (SCs) with lighter weight, smaller size, non-toxic materials and non-flammable solid-state electrolyte have thus attracted great interests owing to their unique electrochemical performances and excellent wearability, as compared to lithium-ion batteries and conventional capacitors that are normally bulky and rigid. In particular, the textile-based SCs have great potential and feasibility in the practical application in storing electrical energy for the smart textiles and wearable electronic devices. However, challenges still remain in both electrochemical performances and flexibilities. The significant bonds between material synthesis and product property motivate us to design and fabricate novel textile-based SCs by utilizing more common textiles materials and processing technologies in combination with the high-performance pseudocapacitive materials. Garments exhibit a hierarchical structure which is normally composed of fiber, yarn and fabric, and thus the wearable SCs should also be designed in the same way. Therefore this work is carried out by designing, exploring and developing the flexible textile-based SCs in different dimensions, including one-dimensional (1D) substrate based SCs, two-dimensional (2D) embroidered fabric SCs, three dimensional (3D) porous fiber-network and 3D spacer fabric SCs. Firstly, the one-dimensional (1D) flexible SC was designed and fabricated via a soft and highly conductive stainless steel yarn (SSY). By using the strategy of electrodepositing highly pseudocapacitive nickel and cobalt mixed oxides and th conductive polymer of polypyrrole (PPy), the 1D-configuration all-solid-state asymmetric supercapacitors (AASs) were finally obtained. By performing the measurements of mechanical flexibility and the electrochemical tests, the highest specific capacitance of about 14.69 F·cm⁻³ was achieved at a current density of 25 mA·cm⁻³, and the maximum volumetric energy density reached 3.83 mWh·cm⁻³ (0.032 mWh·cm-2) at a corresponding power density of 18.75 mW·cm⁻³ (0.284 mW·cm⁻²). The high electrochemical property lies in both the morphology merit of the electroactive materials and the highly conductive 1D current collector. The AASs were also sewn into a widely used cloth, exhibiting the remarkable stability after 6,000 charging/discharging cycles. Subsequently, by rendering a 2D interdigitally-patterned conductive embroidery from the silver-plated nylon fibers, a novel electroactive material of cobalt phosphides was successfully designed and electrodeposited, creating the very first all-textile wearable embroidery SCs. The electrochemical evaluation demonstrated that a high volumetric capacitance of 5.98 F·cm⁻³ was achieved at a current density of 0.025 A·cm⁻³, and the highest energy density was about 0.51 mWh·cm⁻³ at the power density of 9.1 mW·cm⁻³ . To apply it in the real garment, a letter-patterned device was embroidered on the coat as a truly flexible planar SC. To further develop ultra-flexible SCs, the computerized textile processing technologies were adopted for knitting the truly flexible and conductive 3D spacer fabric (3DSF), which was designed for serving as the skeleton of the flexible SC similar to the configuration of the conventional flat-plate capacitor. By electrodepositing MnO2 and electrochemically reduced graphene oxide (ErGO) on the top and bottom layer of the 3DF respectively, the 3D all-textile asymmetric SCs (3DASs) were successfully fabricated. And the fusible interlinings were further adopted for packaging the 3DAS in a flexible manner. The device showed a high capacitance of 1.02 F·cmˉ² (36.59 F·g⁻¹) at the current density of 7.5 mA·cmˉ² (0.275 A·g⁻¹), and the highest areal energy density was nearly 1.02 mWh·cmˉ² at the areal power density of 5.27 mW·cmˉ², showing a promising prospect for the general use of the textile techniques in developing the wearable energy storage devices. Lastly, to satisfy the need from the 3D wearables, the flexible porous graphite felt was used as the substrate for fabricating 3D patternable supercapacitors. By depositing polypyrrole nanowires arrays (PPy NAs) and Ni/Co selenide materials on the GF fibers in order, the 3D flexible SCs were readily obtained, which exhibited both stable electrochemical performance and effective waterproof property, with the highest specific capacitance of 5.21 F·cm⁻³ (113.36 F·g⁻¹) at the current density of 0.025 A·cm⁻³ (0.5 F·g⁻¹ ), and the highest energy density of 1.09 mWh·cm⁻³ (22.14 Wh·kg⁻¹) at a power density of 16.5 mW·cm⁻³ (358.7 W·kg⁻¹). Then, the laser engraving and silicone sealing techniques were successfully employed, endowing the 3D device with diverse patterns and excellent waterproofness. In summary, this thesis carried out a systematic study on designing, exploring and developing truly flexible and wearable textile-based SCs in different dimensions. By combining the highly pseudocapacitive electroactive materials and truly flexible textile substrates, electrodes with both high electrochemical and flexibility were successfully achieved. Moreover, methods for synthesizing the electroactive materials were carefully designed and adopted to be compatible with the common textile substrates that usually cannot withstand high temperature and strong acid-base condition. The series of flexible and wearable SCs designed and developed in this research study, including 1D fiber-shaped SCs, 2D planar embroidered SCs, 3D porous and 3D fabric SCs, have inspired a promising perspective for the development and innovation of textile-based flexible SCs for wearable energy storage.