Graphene-Based Fiber Supercapacitors

ConspectusWearable electronics are smart devices that can be directly worn on the human body. Consumer-grade wearable devices (such as smart bracelets, watches, and glasses) are becoming increasingly popular. They provide continuous and reliable data analysis and guidance in our daily health monitoring and exercise activities. Meanwhile, professional medical-grade wearable devices (such as smart blood pressure monitors and heart rate and blood oxygen detectors) have been widely deployed in many medical institutions to assist doctors in diagnosing and treating patients. The expansion of their application scenarios has created a growing interest in wearable electronics, and the surge in demand for the Internet of Things further drives the market's growth. It is projected that the market size of global wearable devices will exceed US$200 billion by 2026. The major limiting factor to the development of wearable electronics is the absence of reliable energy storage systems that can be directly worn without compromising the devices' compactness, ease of use, and flexibility. One-dimensional (1D) fiber supercapacitors (SCs) have emerged as a promising solution due to their excellent mechanical properties, small size, and unique electrochemical characteristics inherited from SCs. As for fiber electrodes, graphene-based fibers (GFs) have attracted tremendous attention due to the desired properties originating from their main building block, i.e., graphene, including a large specific surface area, high electrical conductivity, and mechanical flexibility. In addition, wearable electronics move toward converging all and more units into one device for achieving multiple functions, creating increasing power demands. Therefore, research efforts are required to continuously enhance the energy storage performance of GFs and GFs-based SCs (GFSCs). This Account introduces our research on developing 1D GFSCs to power practical wearable electronics. We first discuss the assembly of GFs via hydrothermal methods to translate the properties of graphene into GFs and achieve scalable GF production, including mechanistic studies and process parameter optimization. We also discuss the critical roles of wet GFs' drying conditions in determining GFs' porous structures and electrochemical properties. Next, we present several strategies to enhance the electrochemical energy storage performance of GFSCs, including increasing the specific surface area of GFs and introducing pseudocapacitive materials to GFs. Then, we summarize several methods we used to improve the electrical conductivity of GFs, including adding conductive intercalators between adjacent graphene sheets, constructing core–sheath fibers with highly conductive carbon fiber cores, and tailoring GF hydrothermal conditions. Further, we introduce several device design methods for GFSCs to boost their energy storage capacity, including pairing distinctive electrodes in a single device and self-integrating multiple devices in a planar or three-dimensional (3D) manner. Last, the main challenges and future perspectives on the practical application of GFSCs are discussed. We hope this Account will inspire more research on exploring novel energy storage materials, fabrication of fiber electrodes, and novel electrode and device designs to realize a broad adoption of GFSCs in practical applications.