Abstract:

Significance Corresponding to the increasing demand for the electronics with higher portability, intelligence, and conformability, wearable devices present a fast-growing market and application prospect. The application scenarios of these wearable devices normally present features of high flexibility, deformability and complexity that restrict the service of traditional rigid energy storage systems. In this context, fiber shaped flexible supercapacitors have aroused vast interests for their distinctive advances in flexibility, power density, operation safety and cycling life. Thereinto, graphene fiber-based supercapacitors show the characteristics of high energy density, good flexibility and high safety, presenting great potential to power wearable devices. To date, remarkable progress has been achieved in designing and fabricating graphene-based fiber via different spinning methods such as wet spinning, dry spinning and hydrothermal spinning. However, to accommodate simultaneously the requirements of high electrochemical property and mechanical robustness of electrode materials that originate from the complex application scenarios, it is still highly demanded to develop new spinning approaches to further control the chemical component and structure of graphene-based fiber.
Progress Microfluidic spinning method, as a new generation of spinning approach evolving from microfluidic chip technology, has aroused wide attention for its advantages in preparing refined hetero-structured fibers, and a number of studies have been reported. Precise control of primitive structure and chemical component can be facilitated on graphene fiber through simulating fluid flow of graphene oxide (GO) dispersion, through designing spinneret with different structures and adjusting spinning solution composition and fluid flow state. Typically, for fiber structure adjustment, converging, expanding and coaxial spinning channels have been developed to prepare graphene fibers with high axial order degree, vertical sheet alignment and core-shell structure, respectively. Correspondingly, the as-prepared graphene fibers present high mechanical strength, short ion transport pathway and multi-scenario application capability. For chemical component adjustment, coagulation methods including chemical crosslinking, ionic crosslinking, solvent exchange and solvent vaporing are selected to fabricate graphene based composite fiber with capacitance reinforcement phase. On this basis, heteroatom doping, porous structure and core-shell structured graphene-based composite fibers have been subsequently developed. Through structure and component adjustments, graphene-based fibers exhibit large ion accessible surface, improved ion/electron transport ability, high electrochemical activity and great mechanical stability. Accordingly, the graphene fiber-based supercapacitor prepared by microfluidic spinning technology present high energy density and desirable dynamic output, which can steadily drive multi-color display.
Conclusion and Prospect Predominant progresses have been acquired in preparing hetero-structured graphene based composite fibers that show ideal mechanical-electrochemical performance via microfluidic spinning technology. Basing on the computational fluid dynamics simulation, structure design of spinneret, and spinning solution compound regulation, microfluidic spinning technology can effectively control the flow behavior and composition. The resultant programmable chemical component adjustments combined with accurate regulation in fiber structure endow graphene-based fiber one of the best candidates in flexible energy storage application. In this regard, with the persistent growth of wearable device, microfluidic spinning technology will become an indispensable method for preparing high performance graphene fiber-based supercapacitor. In this process, it would be helpful to consider the following issues. Exploration of appropriate methods and materials for microfluidic channel fabrication. To date, numerous materials and fabrication methods have been developed to prepare spinning channel. Nevertheless, with the consideration that structure design of the spinning channel is the primary approach to control the fluid flow, and it is still necessary to further explore relevant preparation methods and materials to realize the refined channel structure. Simulation of the flow behavior of GO dispersion in microchannels. The complex rheological behaviors of GO dispersion led to difficulty in formulating flow equations to describe solution process, and thus numerical simulation remains the fundamental method for investigation, where, unfortunately, the research in this regard appears to be limited. It is hence highly necessary to simulate and predict the flow behavior of GO dispersion in microchannels based on the experiences of computational fluid dynamics in other fields. Development of graphene fiber-based electrode with novel structure and chemical composition. Although graphene presents the advantages of high conductivity, high strength, and large specific surface area, its assemblies suffer from the severe restacking structure and inferior electrochemical activity. To this end, exploration of graphene fiber-based electrode with novel structure and electrochemical active composition is of profound significance to narrow the Laboratory-Factory gap in the area of flexible energy storage.

Key words: microchannel, fluid mechanic, graphene fiber, flexible supercapacitor, flexible energy storage, microfluidic spinning technology