Graphene composites for fiber supercapacitors

Abstract

Fiber supercapacitors (FSCs) are promising energy storage devices for emerging wearable electronics due to their unique advantages, such as good flexibility, weaveability and integratablity. Graphene materials with high surface area and excellent conductivity have been explored as electrode materials for fabricating FSCs. However, there are some many challenges to be resolved before they can be used in practical devices. This thesis focuses on three critical issues. First, when graphene materials are assembled as graphene hydrogel fibers, they shrink significantly during drying accompanied by complex internal structural transforms, which affect their energy storage performance significantly. However, the vital drying process has been largely ignored in previous studies. Second, when assembling graphene nanosheets into graphene electrodes, they often stack together uncontrollably due to strong van der Waals interactions between adjacent nanosheets, which significantly compromise their energy storage performance, This phenomenon has limited the applications of graphene fibers, even though they have high theoretical specific capacitance. Third, although graphene material based electrodes often deliver high power and long cycle life, their energy storage capacity based on the electrochemical double-layer capacitance is often limited. How to efficiently incorporate pseudocapacitive materials into graphene fibers to increase energy storage density is still unclear. To address these three issues, first, a comprehensive study was conducted to investigate the effects of drying conditions of graphene fibers on their porous structures and electrochemical properties. Graphene fibers were dried systematically under five different representative drying conditions. It was found that (1) the d-spacing of graphene nanosheets is determined during their reduction in hydrothermal assembly; (2) pore structures of dried graphene fibers are significantly influenced by solvent removal rates during drying; (3) the interconnection of pores in graphene fibers can be retained if non-volatile solvents are trapped in hydrogel fibers and (4) the graphene fibers dried under different conditions show significantly different specific volumetric capacitance and rate capability in capacitive energy storage. These findings can guide the synthesis of 1D fibers from 2D materials for FSCs and beyond. Second, a 2D-covalent organic framework (2D-COF) with a thickness of around 2 nm was explored as a nano-spacer to prevent the stacking of reduced graphene oxide (rGO) nanosheets during their assembly. The 2D-COF was selected because its mesopores can serve as an efficient “highway” for ion diffusion. The rGO/COF hybrid delivered a high gravimetric capacitance of 321 F g–1, corresponding to an ultrahigh graphene utilization rate (74%) related to theoretical gravimetric capacitance of graphene. Further, its practical applications were demonstrated in both thin-film supercapacitors and FSCs. They delivered a high specific energy density of 10.3 Wh kg−1 (thin-film supercapacitors) or 7.9 mWh cm−3 (FSCs), respectively. The 2D-COF shows good potential to enhance the energy storage performance of graphene or other 2D materials. Third, a novel method was demonstrated to uniformly incorporate ruthenium oxide (RuO2) nanoparticles with an ultra-high mass loading of 42.5 wt.% into holey graphene oxide (HGO) fibers. The HGO fibers were first prepared by the hydrothermal assembly. Next, Ru3+ ions were incorporated into wet HGO fibers before drying. The resulting composite fibers exhibited an ultrahigh volumetric capacitance of 1054 F cm−3. Solid-state FSCs fabricated by these fibers showed an ultrahigh energy density of 27.3 mWh cm−3. This method has the general applicability to incorporate different pseudocapacitive materials into graphene fibers to increase their energy storage capacity. Forth, to further increase the electrical conductivity of hybrid fibers containing pseudocapacitive materials, a core-sheath fiber comprised of a graphite fiber core and a MoS2 nanosheet intercalated HGO sheath was designed and synthesized by the hydrothermal assembly. MoS2 was selected due to its high pseudocapacitance and conductivity. The graphite fiber core served as a faster electron transfer highway. The core-sheath fiber showed a high volumetric capacitance up to 421 F cm−3. It was found that more than half of the capacitance of the fiber can be retained when the scan rate increases from 2 to 100 mV s–1. The assembled solid-state FSC delivered a high energy density of 8.2 mWh cm−3 at the power density of 40 mW cm−3. Overall, this thesis has provided new fundamental understandings of the assembly of graphene materials. Several innovative methods were demonstrated to produce high-performance graphene-based electrodes for FSCs. These results will help to realize of various potential practical applications of FSCs based on graphene materials