Functionalized graphene for supercapacitor
Date Issued
2014
Date
2014
Author(s)
Chen, Yan-Ru
Abstract
Abstract
The high-rate performances of supercapacitors generally are limited by the polarization. As a result, the charge/discharge capacitances decay very dramatically with the increase of charge-discharge rates. Therefore, more effective additives with a much smaller mass fraction are needed in future supercapacitors, especially for the high-rate case where there must be a more efficient conducting network. Due to the fact that graphene exhibits exceptional electron transport properties and unique geometrical nature (a soft and ultrathin planar structure), graphene is introduced into supercapacitors as a conducting additive to improve the high rate charge/discharge performances. However, similar to other nanomaterials, a key challenge in synthesis and processing of graphenes sheets is aggregation. Due to the fact that graphenes possess high specific surface area, they tend to form irreversible agglomerates or even restack to form graphite through van der Waals interactions. In this study, a surface modification technology is used to change the surface of graphene and improve its dispersity. The dispersion stability of functionalized graphene is measured by zeta potential. Functionalized graphenes are used as conductive additive in the electrode of supercapacitor, and their electrochemical performances are compared by charge/discharge and AC impedence. With suitable functionalized graphene as conductive additive is important, therefore we will divide into two chapter to discussions.
In chapter 4, graphene with oxygen (M-rGO and H-rGO) and nitrogen (N-rGO) related functional groups have been fabricated. Reduce graphenes including H-rGO, M-rGO and N-rGO were mixed with activated carbons as the composite electrodes and characterized for supercapacitors. The effects of the functional groups on graphenes as the conductive additive have been investigated. It was found that a suitable content of functional groups can improve the stability of dispersion, and therefore reduce the internal resistance (IR drop) and charge transfer resistance (Rct) resulting in higher rate capability. The supercapacitor with M-rGO and KS6 as additive at the activated carbon electrode can be operated at a rate as high as 6 A/g and exhibits a capacitance of 208 F/g, whereas the supercapacitor using only KS6 as additive shows a capacitance of only 107 F/g. The graphene contained supercapacitor has been cycled over 2000 times at 4 A/g with almost no capacitance fading.
In the chapter 5, Sulfonated polyetheretherketone (SPEEK) has been synthesized by sulfonation process and used as the solid-state electrolyte, binder and surfactant for soild-state supercapacitors. The suspensions of M-rGO/SPEEK, H-rGO/SPEEK, N-rGO/SPEEK, M-rGO/PVDF, H-rGO/PVDF, and N-rGO/PVDF in organic solvents (DMSO) have been prepared and the surfactant effects of SPEEK and PVDF toward graphenes (M-rGO, H-rGO and N-rGO) have been investigated. Functionalized graphenes dispersed by SPEEK are used as high efficiency conducting additives in solid-state supercapacitors. It was found that SPEEK can dramatically improve the stability of graphene dispersion, and therefore the solid-state supercapacitors showed largely decrease of IR drop and charge transfer resistance (Rct), resulting in higher rate capability. The solid-state supercapacitors with M-rGO /SPEEK/activated carbon electrode can be operated from 1 to 8 A/g and exhibit capacity retention of 93%. The noteworthy is more than twice higher value for capacity retention by comparison with the solid-state supercapacitors using M-rGO/PVDF/activated carbon electrode (capacity retention is 36%). The cell of graphene with SPEEK has been cycled over 5000 times at 5 A/g with no capacitance fading.
Subjects
石墨烯
超級電容
磺化聚醚醚酮
分散性
電化學性能
Type
thesis
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