Carbon-based Flexible Composite Electrodes for Supercapacitor and Sensor Applications

Abstract

Wearable biomedical electronic devices (WBEDs) can be attached to human epidermal tissues and used to analyze diverse health-related chemical/biological signals. WBEDs can serve as a key node of a future internet of medical things for significantly improving personal health management. Wearable sensors and power devices, which are mainly constituted by flexible composite electrodes (FCEs), are generally regarded as two functional core components of WBEDs. Therefore, developing advanced FCEs capable of realizing high-performance physiological index monitoring, energy storage or even both of them has attracted considerable research attention. In recent years, owing to their multiple function and performance superiorities, carbon-based paper-like films have been preliminarily exploited as advanced substrates of FCEs. By rational structural optimization, compositional regulation and surface functionalization, they can be further fabricated into various types of advanced FCEs for diverse wearable energy and sensing applications. This Ph.D. project is devoted to the design, fabrication, characterization and application of novel carbon-based FCEs. Employing a series of facile, efficient and scalable fabrication techniques, we first explore the engineering of three types of carbon-based paper-like films. Afterwards, these carbon-based paper-like films are either directly developed into high-performance flexible solid-state supercapacitor (FSSSC) electrodes, or utilized as flexible substrates to further load redox-active transition metal oxides (TMOs) or chalcogenides (TMCs) and construct bifunctional FCEs for both supercapacitive energy storage and biomolecule sensing. The first case introduces a novel strategy for the fabrication of nitrogen-doped hybriddimensional nanocarbons (N-RGO-CNT-CBNP) based paper electrodes (N-RGO-CNTCBNP-Ps) for FSSSCs. Three types of representative nanocarbons including reduced graphene oxide (RGO) nanosheets, carbon nanotubes and carbon black nanoparticles are used as building-block materials to synergistically construct N-RGO-CNT-CBNP via facile and low-cost solution processing. With melamine as a multifunctional additive, both a three-dimensional highly-porous structure and a high nitrogen-doping level can be achieved by N-RGO-CNT-CBNP. A facile multi-step filtration method is further adopted to fabricate sandwich-structured N-RGO-CNT-CBNP-Ps. Thanks to the significant structural and compositional merits of N-RGO-CNT-CBNP-Ps, an N-RGOCNT-CBNP-P based FSSSC could exhibit ultrahigh areal specific capacitance, excellent cycling stability and remarkable rate capability, while meeting operational reliability/durability requirements of practical WBED applications. In the second exploration, for improving the electrochemical behaviors of TMCs, gold nanoparticles (AuNPs) decorated RGO papers (RGOPs) are developed as flexible substrates (AuNPs@RGOPs), which are further used for loading tin disulfide nanoflakes anchored RGO nanosheets (RGO@SnS2). The two-dimensional self-assembly of AuNPs on the surface of RGO papers endows the as-obtained AuNPs@RGOPs with enhanced electrical and mechanical performances. Besides, by forming stable gold-sulfur bonds, the electron transfer between AuNPs@RGOPs and RGO@SnS2 is greatly enhanced. Owing to these merits, the as-fabricated FCEs (AuNPs@RGOPs//RGO@SnS2) exhibit a remarkable bifunctional characteristic for both supercapacitive energy storage and glucose sensing. In the third study, a microwave-assisted solvothermal method combined with thermal annealing is employed to synthesize copper cobaltate nanowires (CCONWs) that are loaded on activated graphite papers (AGPs). Mixed-acid preactivation on commercial flexible graphite papers effectively enriches and homogenizes their surface active sites for the in-situ growth of CCONWs. The as-synthesized CCONWs are highly-porous and uniformly grown on the surface of AGPs, and their unique micro-/nanostructure is capable of realizing multidimensional electron transport and rapid electrolyte ion diffusion. The as-fabricated FCEs (CCONWs@AGPs) are employed as supercapacitor electrodes, which exhibit excellent specific capacitance, as well as good rate capability and long-term cycling stability. Besides, they can be further used as electrochemical dopamine sensors, which exhibit high sensitivity, remarkable linear response and good anti-interference performance.