Functionalized graphene for supercapacitor

超級電容在進行大電流充放電時，會受限於元件的內阻而造成極化現象，使其電容量明顯的衰退。為了改善電容量衰退的問題，需要一個更有效的導電網絡來降低內阻。由於石墨烯優良的導電特性與獨特的幾何結構，但是本身團聚的問題會限制他的應用。因此本研究將開發不同的功能性石墨烯，改善其團聚現象，並利用界達電位儀來分析。本研究將功能性石墨烯來當做超級電容的導電添加劑，探討石墨烯分散性與電化學性能的關係。 在本文第四章，完成不同官能基石墨烯粉末(M-rGO, H-rGO 和N-rGO)的製備，並將不同官能基石墨烯與 N-甲基吡咯酮(NMP)形成懸浮液。將其懸浮液來配製電極漿料。由結果可以明顯的發現，含有M-rGO型石墨烯助導劑的超級電容在不同電流密度的充放電操作下其元件的壓降（IR drop）明顯小於傳統型超級電容，並且在高電流密度（6 A/g）充放電量測時其電容量有208 F/g，相同條件下傳統型超級電容其電容量只有106 F/g。將含有M-rGO石墨烯助導劑的超級電容在電流密度4 A/g下進行2000次循環壽命的測試，其電容量沒有任何的衰退。 在本文第五章，將使用磺化聚醚醚酮（SPEEK）來當做固態電解質，黏著劑和石墨稀分散劑。將上面製備的石墨稀粉末（M-rGO, H-rGO 和N-rGO），分別利用磺化聚醚醚酮與傳統型黏著劑聚氟化二乙烯（PVDF）來分散石墨稀（M-rGO, H-rGO 和N-rGO），並且將其導入活性碳電極，其結果顯示使用磺化聚醚醚酮（SPEEK）分散石墨稀做為助導劑的固態超級電容，其阻抗和壓降比使用聚氟化二乙烯（PVDF）分散來的低，將其固態超級電容在電流密度由1 A/g到8 A/g下進行充放電來做為比較，結果顯示由磺化聚醚醚酮（SPEEK）分散石墨稀當做助導劑的固態超級電容電容維持率還有93% ，然而使用傳統聚氟化二乙烯（PVDF）分散的固態超級電容維持率只有36%。並且將固態超級電容在電流密度5 A/g下進行充放電5000次循環壽命的測試，其電容量維持率100%，可以證明使用磺化聚醚醚酮（SPEEK）來當做固態電解質，黏著劑和石墨稀分散劑相當穩定。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.Contents 致謝........... .......................................................................................................................1 摘要........... .......................................................................................................................2 Abstract ............................................................................................................................ 4 Contents ............................................................................................................................7 List of Figures .................................................................................................................10 List of Tables ................................................................................................................. 16 Chapter 1 Introduction................................................. .......... …...................................17 Chapter 2 Literature Review and Research Motivation .................................................19 2.1 Introduction to Supercapacitors…………….. ..........................................................19 2.2 Activated material for supercapacitors…………………………………………..…27 2.3 Introduction to Conductive additives…………………………………………..…..37 2.4 Introduction to Binders……………………………………………………………..39 2.5 Introduction to Electrolyte……………………………………………………….....39 2.6 Introduction to graphene………………………………………………………...….42 2.7 Motivation and Objectives……………………………………………………..…...46 Chapter 3 Experimental procedure ………………………………………………….....49 3.1 Material preparation …………………………………………………………..…...49 3.1.1 Fabrication of functionalized graphene …………………………………….....49 3.1.2 Preparation of SPEEK.………………………………………………………...50 3.2 Preparation of graphene suspensions………………………………………..……...50 3.3 Preparation of electrodes and Cell assembly…………………………………..…...51 3.4 Measurements………………………………………………………………...…….52 3.4.1 Structural Characterization of graphene………………………………………...52 3.4.2 Stability of graphene dispersion…………………………………………….......53 3.4.3 Electrochemical characterization…………………………………………...…...54 Chapter 4 The effect of dispersion status with functionalized graphenes for electric double-layer capacitors……………………………...………………………………….58 4.1 Morphology of graphite oxide and graphene…...……………………….…………58 4.2 XPS analysis of functionalized graphene…………………………………………..59 4.3 Dispersion stability of functionalized graphene with NMP………………………..60 4.4. Electrochemical performances for supercapacitor………………….……………..61 4.5 summary……………………………………………………………………………69 Chapter 5 Graphene/activated carbon supercapacitors with sulfonated-polyetheretherketone as solid-state electrolyte and multifunctional binder …………………………..………………………………………………...…..88 5.1 Molecular structure of SPEEK…………………………..……………………..…..88 5.2 Morphologies of activated material, conductive additive and electrode………………………………………………………………………………...88 5.3 Stability of functionalized graphene dispersion with SPEEK…………….….….....89 5.4 Electrochemical performances for solid state supercapacitor……………...............90 5.5 summary ………………………………………………………………….………..98 Chapter 6 Conclusion………………………………………………………….……...114 reference……………………………………………………………..…………...……116 List of Figure Fiqure 2.1 simplified Ragone plot of various type of energy storage devices…….….. 22 Figure 2.2 the storage mechanism of electric double layer capacitor……………….... 23 Figure 2.3 A scheme of the structure of the electric double layers showing the IHP, OHP, and diffusion regions ……………………………………………………..….….24 Figure 2.4 Cell configurations and mechanisms for Li-ion battery (LIB), LIC, and EDLC.………………………………………………………………………….……… 30 Figure 2.5 A scheme of double injection/expel of protons and electrons during the redox reaction in various oxidation state of hydrous RuO2 at the potential window of water decomposition (1.229 V at 25 oC). Note that various water contents (x, y, z, w) ………………………………………………………………………………...…….36 Figure2.6 Schematic of a charged asymmetric capacitor……………………….……...37 Figure 2.7 the model of a graphene layer………………………………………...…….45 Figure 2.8 the ambipolar electric-field effect in monolayer graphene. The insets show the changes in the position of the Fermi energy E F with changing gate voltage Vg……………………………………………………………………………………….46 Figure 3.1 the supercapacitor cells (2 × 2 cm) were packed using laminated Al foils………………………………………………………………………………….…57 Figure 3.2 the particle velocity is proportional to the electrical potential of the particle at the shear plane, which is the zeta potential………………………………………….…57 Figure 4.1 (a) SEM image of graphite oxide. ……………………...…………….…….70 Figure 4.1 (b) SEM image of H-rGO. ………………………………..…………….….70 Figure 4.1 (c) SEM image of M-rGO. …………………………………….………..….71 Figure 4.1 (d) SEM image of N-rGO. …………………………………..………….….71 Figure 4.2(a) TEM images of graphene oxide……………………………………....….72 Figure 4.2(b) TEM images of H-rGO………………………………………………..…72 Figure 4.2(c) TEM images of M-rGO……………………………………………..…...73 Figure 4.2(d) TEM images of N-rGO………………………………………………..…73 Figure 4.3(a) C1s spectra spectra of the graphite oxide. ………….……………….…. 74 Figure 4.3 (b) C1s spectra of the M-rGO. …………………………..……………..…..74 Figure 4.3(c) C1s spectra of the H-rGO……………………………………………..…75 Figure 4.3(d) C1s spectra of the N-rGO……………………………………………..…75 Figure 4.3(e) N1s spectra of the N-rGO and H-rGO…………………………………...76 Figure 4.4 Zeta potential values of functionalized graphene dispersed in NMP.………………………………………………………………………………...… 77 Figure 4.5(a) Figure 4.5(a) Charge–discharge curves of sample 1 in 6M KOH aqueous electrolyte at a current load from 0.5 to 6 A/g.………………..………………….….…78 Figure 4.5(b) Charge–discharge curves of sample 2 in 6M KOH aqueous electrolyte at a current load from 0.5 to 6 A/g.………………………………………………………... 78 Figure 4.5(c) Charge–discharge curves of sample 3 in 6M KOH aqueous electrolyte at a current load from 0.5 to 6 A/g………………………………………………………….79 Figure 4.5(d) Charge–discharge curves of sample 4 in 6M KOH aqueous electrolyte at a current load from 0.5 to 6 A/g………………………………………………………….79 Figure 4.6 Specific capacitances of different samples as a function of current densities……………………………..……………………………………………….... 80 Figure 4.7 IR drop of different samples as a function of current densities.…...….……81 Figure 4.8 Nyquist impedance plots for different electrodes…………………...……...82 Figure 4.9(a) Evolution of the real and imaginary part (C′ and C′′) of the stack capacitance of sample 1………………………………………………………….……..83 Figure 4.9(b) Evolution of the real and imaginary part (C′ and C′′) of the stack capacitance of sample 2……………………………………………………….………83 Figure 4.9(c) Evolution of the real and imaginary part (C′ and C′′) of the stack capacitance of sample 3…………………………………………………………..…..84 Figure 4.9(d) Evolution of the real and imaginary part (C′ and C′′) of the stack capacitance of sample 4………………………………………………………………...84 Figure 4.10 the relaxation time constatnt of different samples in the supercapacitor…………………………………………………………………………..85 Figure 4.11 Ragone plots of different cells in 6M KOH electrolyte………………..….86 Figure 4.12 Specific capacitance as a function of cycle numbers for the electrodes with and without reduce graphene under a current density of 4 A/g………………………...87 Fiqure.5.1 Nomenclature of the aromatic protons for the PEEK and SPEEK repeat unit………………………………………………………..………………………….... 99 Figure 5.2(a) TEM image of the activated carbon………………………………..….....99 Figure 5.2(b) TEM image of the KS6………………………………………………....100 Figure 5.2(c) TEM image of the M-rGO……………………………………………...100 Figure 5.2(d) TEM image of AC/KS6/SPEEK electrode………………………….….101 Figure 5.2(e) TEM image of AC/M-rGO/SPEEK electrode……………………..…...101 Figure 5.3 Zeta potential values of functionalized graphene dispersed in SPEEK or PVDF solutions……………………………………………………………………….102. Figure 5.4 (a) Charge–discharge curves of sample A at a current load from 1 to 8 A/g………………………………………………………………………………..…...103 Figure 5.4 (b) Charge–discharge curves of sample B at a current load from 1 to 8 A/g………………………………………………………………………………….…103 Figure 5.4 (c) Charge–discharge curves of sample C at a current load from 1 to 8 A/g………………………………………………………………………………..…...104 Figure 5.4 (d) Charge–discharge curves of sample D at a current load from 1 to 8 A/g………………………………………………………………………………...…..104 Figure 5.4 (e) Charge–discharge curves of sample E at a current load from 1 to 8 A/g…………………………………………………………………………..….…….105 Figure 5.5 Specific capacitances of different cells as a function of current densities…………………………………………………………………..……..……106 Figure 5.6 IR drop of different cells as a function of current densities……………………………………………………………………………….107 Figure 5.7 Nyquist impedance plots for different samples…………………………...108 Figure 5.8(a) Evolution of the real and imaginary part (C′ and C′′) of the stack capacitance of sample A……………………………………………………………...109 Figure 5.8(b) Evolution of the real and imaginary part (C′ and C′′) of the stack capacitance of sample B……………………………………………………………....109 Figure 5.8(c) Evolution of the real and imaginary part (C′ and C′′) of the stack capacitance of sample C………………………………………………………….…..110 Figure 5.8(d) Evolution of the real and imaginary part (C′ and C′′) of the stack capacitance of sample D…………………………………………………………...….110 Figure 5.8(e) Evolution of the real and imaginary part (C′ and C′′) of the stack capacitance of sample E……………………………………………………………....111 Figure 5.9 show the relaxation time constatnt of different samples in the supercapacitor………………………………………………………………………....112 Figure 5.10 Ragone plots of cell A-E………………………………………………....112 Figure 5.11 Specific capacitance as a function of cycle numbers for the sample C under a current density of 5 A/g……………………………………………………………...11