WO2022105680A1 - 一种高能量密度超级电容器 - Google Patents

一种高能量密度超级电容器 Download PDF

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WO2022105680A1
WO2022105680A1 PCT/CN2021/130255 CN2021130255W WO2022105680A1 WO 2022105680 A1 WO2022105680 A1 WO 2022105680A1 CN 2021130255 W CN2021130255 W CN 2021130255W WO 2022105680 A1 WO2022105680 A1 WO 2022105680A1
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ionic liquid
electrolyte
n1114tfsi
emimtfsi
energy density
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French (fr)
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林瀚
贾宝华
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伊诺福科光学技术有限公司
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/22—Electrodes
    • H01G11/30—Electrodes characterised by their material
    • H01G11/32—Carbon-based
    • H01G11/36—Nanostructures, e.g. nanofibres, nanotubes or fullerenes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/54—Electrolytes
    • H01G11/58—Liquid electrolytes
    • H01G11/62—Liquid electrolytes characterised by the solute, e.g. salts, anions or cations therein
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/13—Energy storage using capacitors

Definitions

  • the invention belongs to the technical field of energy storage, and in particular relates to a high-energy-density supercapacitor.
  • Supercapacitors are electrochemical capacitors that have a much higher capacitance value than other capacitors.
  • Supercapacitors are widely used in energy storage and energy supply due to their high energy density, fast charge/discharge capability, long life of over one million charge cycles, and ability to operate in a wide temperature range from -40°C to 70°C . Since the production and disposal of batteries can adversely affect environmental pollution and human health, the environmentally friendly materials in supercapacitors and their low maintenance costs have promoted the development of supercapacitors in recent years.
  • supercapacitors are superior to batteries because they can provide higher power density (up to 45kW kg -1 ) and longer cycle life (one million cycles). Nevertheless, the energy density of supercapacitors is about an order of magnitude lower than that of batteries, which limits the use of supercapacitors in practical applications.
  • C specific capacitance
  • V operating voltage window
  • the electrolyte is one of the determinants in determining the specific capacitance and operating voltage window, developing electrolytes with a wide electrochemical stability window and the ability to provide high capacitance is crucial for improving the energy density of supercapacitors.
  • room-temperature ionic liquids also known as molten salts, have been extensively investigated as potential next-generation supercapacitor electrolytes due to their wide electrochemical stability windows typically greater than 4 V.
  • the purpose of the present invention is to provide a high energy density supercapacitor, and the supercapacitor in the present invention has a high voltage window, high capacitance and high energy density at the same time.
  • the present invention provides a high energy density supercapacitor, which is characterized by comprising a reduced graphene oxide electrode and an ionic liquid electrolyte;
  • the ionic liquid electrolyte includes imidazole-based ionic liquid and/or aliphatic ammonium-based ionic liquid, and the imidazole-based ionic liquid and aliphatic ammonium-based ionic liquid have the same anion.
  • the anion in the ionic liquid electrolyte is bis(trifluoromethylsulfonyl)imide.
  • the volume ratio of the imidazole-based ionic liquid to the aliphatic ammonium-based ionic liquid is (0.05-0.95): (0.95-0.05).
  • the volume ratio of the imidazole ionic liquid to the aliphatic ammonium ionic liquid is 0.5:0.5.
  • the imidazole ionic liquid is 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid; the aliphatic ammonium ionic liquid is butyltrimethylammonium bis(trifluoromethylsulfonyl)imide ionic liquid (Trifluoromethylsulfonyl)imide ionic liquid.
  • the specific surface area of the reduced graphene oxide electrode is 225-308 m 2 ⁇ g -1 .
  • the pore volume of the reduced graphene oxide electrode is 0.6-0.9 cm 3 ⁇ g -1 .
  • the pore size of the reduced graphene oxide electrode is 11-13 nm.
  • the present invention provides a high energy density supercapacitor, which is characterized by comprising a reduced graphene oxide electrode and an ionic liquid electrolyte; the ionic liquid electrolyte comprises imidazole ionic liquid and/or aliphatic ammonium ionic liquid, and the The imidazole-based ionic liquid and the aliphatic ammonium-based ionic liquid have the same anion.
  • the invention designs two ionic liquids with the same anion as the electrolyte, and uses the reduced graphene oxide electrode in the supercapacitor. By controlling the interaction between the electrolyte and the electrode, a high voltage window and a high specific capacitance are simultaneously achieved, Further obtain the high energy density of supercapacitors.
  • the experimental results show that the working voltage of the supercapacitor in the present invention can reach 4.7V, the maximum specific capacitance is 293.1F ⁇ g -1 , and the maximum energy density is 176.5Wh ⁇ kg -1 .
  • Figure 1 is a schematic diagram of a reduced graphene oxide-based supercapacitor.
  • the ionic motions of pure N1114TFSI (upper row), pure EMIMTFSI (lower row), and N1114TFSI-EMIMTFSI electrolyte mixture (middle row) in the anode pores are illustrated during the charge-discharge process.
  • the chemical structures of the ammonium cation in N1114TFSI, the imidazolium cation in EMIMTFSI and the bis(trifluoromethylsulfonyl)imide anion are also described;
  • Figure 2 shows the wide-scan XPS spectra of (a) RGO_0, (b) RGO_0.5 and (c) RGO_1 charged/discharged in pure N1114TFSI, [EMIMTFSI] 0.5 [N1114TFSI] 0.5 and pure EMIMTFSI, respectively;
  • Fig. 3 step-by-step CV test chart and GCD test chart of graphene-based supercapacitors with different binary electrolyte components
  • Figure 4 shows (a) the highest operating voltage curve of the electrolyte as a function of x; (b) the CV curve of the supercapacitor based on the electrolyte system as a function of x at 4.1 V; (c) the supercapacitor based on the electrolyte system The GCD curve of RGO as a function of x at 4.1V; (d) the specific capacitance and supercapacitor energy density of RGO as a function of x at 4.1V;
  • Figure 5 shows the CV curves of the supercapacitors at their respective highest operating voltages for electrolytes with different mixing ratios when the scan rates are (a) 5mVs -1 and (b) 200mVs -1 ; when the current density is (c ) GCD curves of supercapacitors with electrolytes with different mixing ratios at 0.5 A ⁇ g -1 and (d) 10 A ⁇ g -1 ; (e) the difference between electrode specific capacitance and current density at each highest working voltage relationship, and (f) Ragone plots for x varying from 0 to 1; (g) curves of how the maximum supercapacitor operating voltage and maximum electrode specific capacitance affect the maximum supercapacitor energy density;
  • FIG. 7 Wide scan XPS of (a) fresh RGO and charged/discharged (b) RGO_0.05 in [EMIMTFSI] 0.05 [N1114TFSI] 0.95 and (c) RGO_0.95 in [EMIMTFSI] 0.95 [N1114TFSI] 0.05 Spectra 0.05; (d) high-resolution F 1s spectra of fresh RGO and charged/discharged (e) RGO_0.05 and (f) RGO_0.95; charged/discharged (g) fresh RGO and charged/discharged (h ) high-resolution N 1s spectra of RGO_0.05 and (i) RGO_0.95;
  • Figure 8 (a) High resolution C 1s spectra of fresh RGO and charged/discharged (b) RGO_0, (c) RGO_0.05, (d) RGO_0.5, (e) RGO_0.95 and (f) RGO_1. (g) High-resolution S 2p spectra of fresh RGO and charged/discharged (h) RGO_0, (i) RGO_0.05, (j) RGO_0.5, (k) RGO_0.95 and (l) RGO_1;
  • the present invention provides a high energy density supercapacitor, which is characterized by comprising a reduced graphene oxide electrode and an ionic liquid electrolyte;
  • the ionic liquid electrolyte includes imidazole-based ionic liquid and/or aliphatic ammonium-based ionic liquid, and the imidazole-based ionic liquid and aliphatic ammonium-based ionic liquid have the same anion.
  • the supercapacitor in the present invention has The operating voltage depends on the electrochemical stability window of the electrolyte and the interaction of its electrodes to achieve high energy density of supercapacitors.
  • the supercapacitor uses reduced graphene oxide as an electrode
  • the reduced graphene oxide electrode has a pore structure, and the pore volume of the reduced graphene oxide electrode is preferably 0.6-0.9 cm 3 ⁇ g -1 ;
  • the pore size distribution of the reduced graphene oxide electrode is preferably 11-13 nm, and the specific surface area of the reduced graphene oxide electrode is preferably 225-308 m 2 ⁇ g -1 .
  • the reduced graphene oxide is preferably prepared according to the following steps:
  • the reduced graphene oxide electrode in the present invention can weaken the interaction with the electrolyte, Obtain higher energy density of supercapacitors.
  • the electrolyte is preferably a mixture of two ionic liquids with the same anion, preferably an imidazole ionic liquid and aliphatic ammonium ionic liquid.
  • the imidazole ionic liquid and aliphatic ammonium ion are mixed.
  • the liquids can be mixed in any proportion, and the two extremes of mixing are pure imidazole-based ionic liquids and pure aliphatic ammonium-based ionic liquids.
  • the aliphatic ammonium ionic liquid preferably does not contain cyclic structures and double bonds; preferably butyltrimethylammonium bis(trifluoromethylsulfonyl)imide ionic liquid (N1114TFSI) .
  • the imidazole-based ionic liquid is preferably 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid (EMIMTFSI).
  • the imidazole ionic liquid is mixed with aliphatic ammonium ionic liquid in a glove box to obtain an ionic liquid electrolyte, which is expressed as [EMIMTFSI] x [N1114TFSI] (1-x) , wherein x is an imidazole ionic liquid
  • the volume fraction of is selected from any value from 0 to 1. Specifically, in the embodiment of the present invention, it may be 0, 0.05, 0.25, 0.5, 0.75, 0.95 or 1, and more preferably 0.5.
  • the main reason for the small voltage window of ionic liquids in supercapacitors is parasitic reactions caused by electrode impurities and/or oxygen-containing functional groups.
  • the ionic liquid electrolyte and the graphene-based electrode exceed their electrochemical stability window, the electrochemical reaction between them occurs before the decomposition of the ionic liquid. Therefore, in the present invention, the highest operating voltage provided by binary ionic liquid electrolytes with different concentrations can be directly measured using a symmetric two-electrode supercapacitor comprising a reduced graphene oxide electrode.
  • the reduced graphene oxide electrode has a specific surface area of 240.8 m 2 ⁇ g -1 and a pore volume of 0.7 cm 3 ⁇ g -1 , which indicates that it has a highly porous ion adsorption structure.
  • the reduced graphene oxide electrode consists of a large number of mesopores with an average pore size of 12 nm.
  • the reduced graphene oxide electrode also showed an atomic ratio of carbon to oxygen (C/O) of 15, with 5 at% of oxygen-containing functional groups, which could react with electrolyte ions. Helps to improve specific capacitance by pseudocapacitance.
  • the butyltrimethylammonium group in the aliphatic ammonium-based ionic liquids of the present invention does not have a ring structure, which can weaken the interaction between the electrolyte and the electrode.
  • the interaction, applied in supercapacitors, can achieve higher energy densities than other ionic liquids.
  • the present invention provides a high energy density supercapacitor, which is characterized by comprising a reduced graphene oxide electrode and an ionic liquid electrolyte; the ionic liquid electrolyte comprises imidazole ionic liquid and/or aliphatic ammonium ionic liquid, and the The imidazole-based ionic liquid and the aliphatic ammonium-based ionic liquid have the same anion.
  • the present invention designs two ionic liquids with the same anion as the electrolyte, and uses the reduced graphene oxide electrode in the supercapacitor. to obtain high energy density of supercapacitors.
  • the experimental results show that the working voltage of the supercapacitor in the present invention can reach 4.7V, the maximum specific capacitance is 293.1F ⁇ g -1 , and the maximum energy density is 176.5Wh ⁇ kg -1 .
  • Free-standing graphene oxide films were prepared by a simple laboratory-scale doctor blade machine, which was then cut into 15 mm diameter discs.
  • the reduced graphene oxide was obtained by reducing the prepared graphene oxide discs by a flash device with a power of 171.5 Ws in a glove box with a power of less than 0.5 ppm H2O and less than 0.5 ppm O2 .
  • the mass of each reduced graphene oxide was measured to be 0.3 mg.
  • the prepared binary ionic liquid electrolyte is hereinafter referred to as [EMIMTFSI] x [N1114TFSI] (1-x) .
  • the morphology and physical structure of the reduced graphene oxide samples were examined using a Zeiss Supra 40VP scanning electron microscope (SEM) at an accelerating voltage of 15 kV and a magnification of 1000 times.
  • SEM Zeiss Supra 40VP scanning electron microscope
  • the porous structure of the reduced graphene oxide samples was investigated using the Micromeritics TriStar II Plus for nitrogen (N 2 ) adsorption/desorption isotherms.
  • the samples were degassed under nitrogen at 70°C for 12 hours before measurement.
  • the specific surface area (SSA) was obtained by Brunauer-Emmett-Teller (BET) analysis of adsorption isotherms.
  • the surface chemistry of fresh and used reduced graphene oxide samples was characterized using X-ray photoelectron spectroscopy (XPS) spectroscopy.
  • XPS X-ray photoelectron spectroscopy
  • XPS uses a Krato Axis Nova surface analysis spectrometer with Al-K (1486.6 eV) as the X-ray source, two pass energies of 160 eV (measurement scan) and 20 eV (high resolution scan), and a 700 ⁇ m beam size.
  • the spent reduced graphene oxide was removed from the supercapacitor and rinsed several times with acetone, ethanol and distilled water before being used for characterization. The washed reduced graphene oxide was then dried in an oven.
  • the viscosity and ionic conductivity of each binary ionic liquid electrolyte system at 23°C and 25°C were measured using a microviscosity meter and a conductivity meter (equipped with a conductivity sensor) placed in a glove box, respectively.
  • the cell constant of the conductivity sensor was determined by calibration after each sample measurement using a 0.01 M aqueous KCl solution.
  • Two-electrode symmetric supercapacitors were assembled to evaluate the electrochemical performance of reduced graphene oxide samples and their resulting supercapacitors in various concentrations of binary ionic liquid electrolytes.
  • the two-electrode cell was assembled in a nitrogen glove box.
  • a separator soaked with a certain amount of the prepared electrolyte was placed between two reduced graphene oxide electrodes.
  • a carbon-coated aluminum foil was used as a current collector, on which the reduced graphene oxide electrode was pressed.
  • Electrochemical measurements of the assembled supercapacitors were performed at room temperature using a multi-channel electrochemical workstation (EC-Lab, VMP-300).
  • the highest operating voltages of supercapacitors with different concentrations in binary ionic liquid electrolytes were first measured by cyclic voltammetry (CV), which was performed at a scan rate of 5 mV s in the range of 3.5 to 5.5 V of 1 .
  • CV cyclic voltammetry
  • the same procedure was repeated at a current density of 0.5 A ⁇ g ⁇ 1 using a galvanostatic charge/discharge (GCD) test.
  • GCD galvanostatic charge/discharge
  • C S (F ⁇ g -1 ) is the specific capacitance of a single electrode
  • m(g) is the total mass of both electrodes
  • I/m(A ⁇ g -1 ) is the current density
  • ⁇ t(s) is the discharge time
  • ⁇ V(V) is the voltage during discharge after IR drops.
  • E S (Wh ⁇ kg -1 ) is the gravimetric energy density of the supercapacitor
  • P S (W ⁇ kg -1 ) is the gravimetric power density of the supercapacitor
  • C S (F ⁇ g -1 ) is normalized to a
  • the specific capacitance of the electrode, ⁇ V(V), is the cell voltage during discharge after the IR drops
  • ⁇ t(s) is the discharge time.
  • the cycling stability of the supercapacitors based on pure N1114TFSI, [EMIMTFSI] 0.5 [N1114TFSI] 0.5 , EMIMTFSI, can also be determined by charging and discharging the supercapacitors at a current density of 1 A g for 1000 cycles. Then create a graph to illustrate the trend.
  • the charge storage of ionic liquid-based supercapacitors is not a pure adsorption process. Instead, it involves ion exchange, where ion adsorption and ion desorption occur simultaneously. The counterpart ion is the oppositely charged ion to the electrode surface, while the co-ion has the same charge as the electrode surface.
  • the schematic in Figure 1 shows the charge/discharge process of negative supercapacitor electrodes with different electrolyte systems including N1114TFSI and/or EMIMTFSI. Charging occurs by the simultaneous introduction of counter ions (cations in this case) into the pores and the release of co-ions (anions in this case) from the pores, while discharge occurs by adsorption of co-ions to counter ions and desorb.
  • the interaction in the liquid system is weak.
  • the results indicate that a large amount of [N1114] cations are required to weaken the electrolyte/electrode interaction.
  • the MW and GCD curves of reduced graphene oxide from individual supercapacitors are shown in Fig. 5(a) and (c), respectively.
  • the CV and GCD curves showed quasi-rectangular (Fig. 5(a)) and symmetrical triangles (Fig. 5(c))) , respectively, demonstrating the capacitive properties of supercapacitors when assembled with binary ionic liquid mixtures.
  • the CV curve was inclined (Fig. 5(b)), which was caused by the high ohmic resistance of the electrolyte in the pores during the faster process.
  • the electrode shows a lower maximum specific capacitance in [EMIMTFSI] 0.5 [N1114TFSI] 0.5 than pure EMIMTFSI at 4.1V, it only increases by 0.1V in voltage, i.e. the highest operating voltage is 4.2V [EMIMTFSI] 0.5 [N1114TFSI] 0.5
  • the maximum specific capacitance ratio displayed by the electrode was 12.4 F ⁇ g -1 higher at a voltage of 4.1 V. This suggests that a slightly higher maximum operating voltage can induce considerable specific capacitance enhancement. However, this assumption only holds true if at the same time the electrolyte viscosity is sufficiently low.
  • the supercapacitors based on pure N1114TFSI exhibit the same high energy density as those based on [EMIMTFSI] 0.5 [N1114TFSI] 0.5 , despite exhibiting a lower electrode specific capacitance of 50 F g-1 in pure N1114TFSI. Furthermore, when the maximum operating voltage is widened by 0.6 V, the energy density of the supercapacitors containing pure N1114TFSI is higher than that of the supercapacitors containing pure EMIMTFSI even though the specific capacitance of the electrodes in pure N11114TFSI is 38 F ⁇ g ⁇ 1 lower than that in pure EMIMTFSI Capacitors are much larger.
  • This low cycling stability may be due to the increased aggregation of ionic liquids in the pores and electrode electroactive regions, thereby limiting ion adsorption and ion/electrode processes, especially when mixing two or more ionic liquids.
  • the interaction between electrolyte and electrodes also reduces the availability of ions for adsorption after successive charge/discharge cycles.
  • the aging of the electrolyte also greatly reduces the ionic conductivity of the electrolyte, which in turn reduces the movement of ions in the electrolyte.

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Abstract

本发明提供了一种高能量密度超级电容器，包括还原氧化石墨烯电极和离子液体电解液；所述离子液体电解液包括咪唑类离子液体和/或脂肪铵类离子液体，且所述咪唑类离子液体和脂肪铵类离子液体具有相同的阴离子。本发明设计了具有相同阴离子的两种离子液体作为电解液，并在超级电容器中使用了还原氧化石墨烯电极，通过控制电解液与电极的相互作用，同时实现高电压窗口和高的比电容，进一步的获得超级电容器的高能量密度。实验结果表明，本发明中的超级电容器工作电压可以达到4.7V，最大比电容为293.1F·g -1，最大的能量密度为176.5Wh·kg -1。

Description

一种高能量密度超级电容器
本申请要求于2020年11月17日提交中国专利局、申请号为202011287210.5、发明名称为“一种高能量密度超级电容器”的中国专利申请的优先权，其全部内容通过引用结合在本申请中。
技术领域
本发明属于能量存储技术领域，尤其涉及一种高能量密度超级电容器。
背景技术
超级电容器(也称为“超级电容器”或“双电层电容器”)是电化学电容器，其电容值远高于其他电容器。由于其高能量密度，快速充电/放电能力，超过一百万次充电循环的长寿命以及在-40℃至70℃的宽温度范围内工作的能力，超级电容器被广泛用于储能和能源供应。由于电池的生产和处置会对环境污染和人体健康造成不利影响，近年来，超级电容器中的环保材料及其低维护成本促进了超级电容器的发展。此外，超级电容器比电池优越，因为它们可以提供更高的功率密度(最高45kW kg -1)和更长的循环寿命(一百万次循环)。尽管如此，超级电容器的能量密度比电池的能量密度低约一个数量级，这限制了超级电容器在实际应用中的使用。
超级电容器的能量密度(E)由E＝1/2CV 2给出，它与比电容(C)和工作电压窗口(V)的平方成正比。尽管比电容取决于电极和电解液的性质，但工作电压窗口取决于电解液的稳定性，并且还取决于存在表面氧官能团的电极表面的性质。由于电解液是确定比电容和工作电压窗口的决定因素之一，因此开发具有宽电化学稳定性窗口和提供高电容能力的电解液对于提高超级电容器能量密度至关重要。在这方面，室温离子液体，也称为熔融盐，由于其宽的电化学稳定性窗口通常大于4V，作为潜在的下一代超级电容器电解液被广泛研究。
迄今为止，最常见的单一组分的含有阳离子为咪唑(imidazolium)，吡啶(pyridinium)，铵(ammonium)和吡咯烷(pyrrolidinium)的离子液体已经被广泛研究。但现有的超级电容器用电解液的或者电化学稳定性窗口较窄，或者电导率较低，而且，均存在能量密度较低的问题，因此，需要进一步研发具有需要的性能的新型离子液体以增强超级电容器的能量密度。
发明内容
本发明的目的在于提供一种高能量密度超级电容器，本发明中的超级电容器同时具有高电压窗口、高电容量和较高的能量密度。
本发明提供一种高能量密度超级电容器，其特征在于，包括还原氧化石墨烯电极和离子液体电解液；
所述离子液体电解液包括咪唑类离子液体和/或脂肪铵类离子液体，且所述咪唑类离子液体和脂肪铵类离子液体具有相同的阴离子。
优选的，所述离子液体电解液中的阴离子为双(三氟甲基磺酰基)酰亚胺根。
优选的，所述咪唑类离子液体和脂肪铵类离子液体的体积比为(0.05～0.95)：(0.95～0.05)。
优选的，所述咪唑类离子液体和脂肪铵类离子液体的体积比为0.5：0.5。
优选的，所述咪唑类离子液体为1-乙基-3-甲基咪唑双(三氟甲基磺酰基)酰亚胺离子液体；所述脂肪铵类离子液体为丁基三甲基铵双(三氟甲基磺酰基)酰亚胺离子液体。
优选的，所述还原氧化石墨烯电极的比表面积为225～308m 2·g -1。
优选的，所述还原氧化石墨烯电极的孔体积为0.6～0.9cm 3·g -1。
优选的，所述还原氧化石墨烯电极的孔径为11～13nm。
本发明提供了一种高能量密度超级电容器，其特征在于，包括还原氧化石墨烯电极和离子液体电解液；所述离子液体电解液包括咪唑类离子液体和/或脂肪铵类离子液体，且所述咪唑类离子液体和脂肪铵类离子液体具有相同的阴离子。本发明设计了具有相同阴离子的两种离子液体作为电解液，并在超级电容器中使用了还原氧化石墨烯电极，通过控制电解液与电极的相互作用，同时实现高电压窗口和高的比电容，进一步的获得超级电容器的高能量密度。实验结果表明，本发明中的超级电容器工作电压可以达到4.7V，最大比电容为293.1F·g -1，最大的能量密度为176.5Wh·kg -1。
附图说明
为了更清楚地说明本发明实施例或现有技术中的技术方案，下面将对实施例或现有技术描述中所需要使用的附图作简单地介绍，显而易见地，下面描述 中的附图仅仅是本发明的实施例，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据提供的附图获得其他的附图。
图1为还原氧化石墨烯基超级电容器的示意图。说明了在充放电过程中，纯N1114TFSI(上排)，纯EMIMTFSI(下排)和N1114TFSI-EMIMTFSI电解液混合物(中排)在负极孔内的离子运动。还描述了N1114TFSI中的铵阳离子，EMIMTFSI中的咪唑阳离子和双(三氟甲基磺酰基)酰亚胺阴离子的化学结构；
图2为分别在纯N1114TFSI，[EMIMTFSI] 0.5[N1114TFSI] 0.5和纯EMIMTFSI中分别对(a)RGO_0，(b)RGO_0.5和(c)RGO_1进行充电/放电的宽扫描XPS光谱；
(d)RGO_0，(e)RGO_0.5和(f)RGO_1的高分辨率F 1s光谱；
充电/放电的(g)RGO_0，(h)RGO_0.5和(i)RGO_1的高分辨率N 1s光谱；
(j)[EMIMTFSI] x[N1114TFSI] (1-x)在23℃和25℃的粘稠度和(k)离子电导率分别是x的函数；
图3、不同二元电解液组分的石墨烯基超级电容器进行的逐步CV测试图和GCD测试图；
(a)纯N1114TFSI(x＝0)，(b)[EMIMTFSI] 0.05[N1114TFSI] 0.95(x＝0.05)，(c)[EMIMTFSI] 0.25[N1114TFSI] 0.75(x＝0.25)，(d)[EMIMTFSI] 0.5[N1114TFSI] 0.5(x＝0.5)，(e)[EMIMTFSI] 0.75[N1114TFSI] 0.25(x＝0.75)，(f)[EMIMTFSI] 0.95[N1114TFSI] 0.05(x＝0.95)和(g)纯EMIMTFSI(x＝1)。(h)使用(i)CV和(ii)GCD确定最高工作电压的条件；
图4为(a)随x变化的电解液的最高工作电压曲线；(b)基于电解液系统的超级电容器的CV曲线在4.1V时随x变化曲线；(c)基于电解液系统的超级电容器的GCD曲线在4.1V时随x变化曲线；(d)在4.1V时RGO的比电容和超级电容器能量密度随x变化的函数；
图5为当扫描速率为(a)5mVs -1和(b)200mVs -1时，具有不同混合比的电解液在其各自的最高工作电压处的超级电容器的CV曲线；当电流密度为(c)0.5A·g -1和(d)10A·g -1时，具有不同混合比的电解液的超级电容器的GCD曲线；(e)各个最高工作电压处的电极比电容与电流密度之间的关系，以及(f)x 在0到1之间变化时的Ragone图；(g)超级电容器最高工作电压和最大电极比电容如何影响最大超级电容器能量密度的曲线；
(h)具有不同混合比的电解液的超级电容器的奈奎斯特拟合图，图(h)右插图：放大的高频区域。图(h)底部插图：用于拟合奈奎斯特图的等效电路；
图6(a)RGO电极的扫描电子显微镜俯视图(放大倍数：1k×)。插图：RGO电极的照片。(b)氮吸附/解吸等温线和(c)RGO电极的孔径分布。(d)RGO电极的XPS宽扫描光谱；
图7(a)新鲜RGO和已充电/已放电(b)[EMIMTFSI] 0.05[N1114TFSI] 0.95中的RGO_0.05和(c)[EMIMTFSI] 0.95[N1114TFSI] 0.05中的RGO_0.95的宽扫描XPS光谱0.05；(d)新鲜RGO和已充电/已放电(e)RGO_0.05和(f)RGO_0.95的高分辨率F 1s光谱；充电/放电过(g)新鲜RGO和充电/放电(h)RGO_0.05和(i)RGO_0.95的高分辨率N 1s光谱；
图8(a)新鲜RGO和已充电/已放电(b)RGO_0，(c)RGO_0.05，(d)RGO_0.5，(e)RGO_0.95和(f)的高分辨率C 1s光谱)RGO_1。(g)新鲜RGO和已充电/已放电(h)RGO_0，(i)RGO_0.05，(j)RGO_0.5，(k)RGO_0.95和(l)RGO_1的高分辨率S 2p光谱；
图9-1完整的分步CV和GCD测试是针对组装有(a)纯N11114TFSI(x＝0)，(b)[EMIMTFSI] 0.05[N1114TFSI] 0.95(x＝0.05)，(c)[EMIMTFSI] 0.25[N1114TFSI] 0.75(x＝0.25)，(d)[EMIMTFSI] 0.5[N1114TFSI] 0.5(x＝0.5)；
图9-2完整的分步CV和GCD测试是针对组装有(e)[EMIMTFSI] 0.75[N1114TFSI] 0.25(x＝0.75)，(f)[EMIMTFSI] 0.95[N1114TFSI] 0.05(x＝0.95)和(g)纯EMIMTFSI(x＝1)；
图10当x＝0、0.05、0.50、0.95和1时，新鲜[EMIMTFSI] x[N1114TFSI] (1-x)电解液的1H和(ii)19F NMR光谱；
图11(i)(a)x＝0，(b)x＝0.05，(c)x＝0.50，(d)x＝0.95，(e)x＝1。所用的电解液在充入相应的最高工作电压后进行测试，这些最高工作电压可在超级电容器中提供。黄色星号表示N1114TFSI的固有成分，蓝色三角形表示 EMIMTFSI的固有成分；
图12：组装有纯N1114TFSI(x＝0)，[EMIMTFSI]0.5[N1114TFSI]0.5(x＝0.5)和纯EMIMTFSI(x＝1)的超级电容器的循环稳定性，经过1500GCD循环并在1A g -1下测量。插图：用[EMIMTFSI] 0.5[N1114TFSI] 0.5组装的超级电容器的第一个和第1000个循环的GCD曲线。
具体实施方式
本发明提供了一种高能量密度超级电容器，其特征在于，包括还原氧化石墨烯电极和离子液体电解液；
所述离子液体电解液包括咪唑类离子液体和/或脂肪铵类离子液体，且所述咪唑类离子液体和脂肪铵类离子液体具有相同的阴离子。
与锂电池基于化学反应的工作原理不同，本发明中的超级电容器中没有化学反应的存在，是通过电解液中的正负离子吸附在不同电极上来实现电能存储，因此，本发明中的超级电容器的工作电压取决于电解液的电化学稳定窗口和其电极的相互作用，以实现超级电容器高的能量密度。
在本发明中，所述超级电容器以还原氧化石墨烯为电极，所述还原氧化石墨烯电极具有孔结构，所述还原氧化石墨烯电极的孔体积优选为0.6～0.9cm 3·g -1；所述还原氧化石墨烯电极的孔径分布优选为11～13nm，所述还原氧化石墨烯电极的比表面积优选为225～308m 2·g -1。
在本发明中，所述还原氧化石墨烯优选按照以下步骤制备得到：
采用浓度为10mg/mL的氧化石墨烯水溶液，通过简单的实验室规模的刮刀机制备独立的氧化石墨烯膜，然后将其切成直径为15mm的圆盘。还原氧化石墨烯是通过在功率小于0.5ppm H 2O和小于0.5ppm O 2的手套箱中通过功率为171.5Ws的闪光装置还原准备好的氧化石墨烯圆盘而获得的。
相比于其他材料的电极，如金属氧化物电极、有机聚合物电极等，甚至与氧化石墨烯电极和石墨烯电极相比，本发明采用还原氧化石墨烯电极能够减弱与电解液的相互作用，获得更高的超级电容器的能量密度。
在本发明中，所述电解液优选为两种具有相同阴离子的离子液体的混合，优选为咪唑类离子液体和脂肪铵类离子液体混合，原理上，所述咪唑类离子液体和脂肪铵类离子液体可以按照任意比例进行混合，其混合的两个极端分别为 纯的咪唑类离子液体和纯的脂肪铵类离子液体。
在本发明中，所述脂肪铵类离子液体中优选不含环状结构、不含双键；优选为丁基三甲基铵双(三氟甲基磺酰基)酰亚胺离子液体(N1114TFSI)。
所述咪唑类离子液体优选为1-乙基-3-甲基咪唑双(三氟甲基磺酰基)酰亚胺离子液体(EMIMTFSI)。
本发明将所述咪唑类离子液体与脂肪铵类离子液体在手套箱中混合，得到离子液体电解液，表示为[EMIMTFSI] x[N1114TFSI] (1-x)，其中，x为咪唑类离子液体的体积分数，选自0～1的任意一个数值，具体的，在本发明的实施例中，可以是0、0.05、0.25、0.5、0.75、0.95或1，更优选为0.5。
在文献中，超级电容的工作电压通常被报告为电解质的电化学稳定窗口，是使用三电极配置测量的。但是，这是不准确的，因为由于超级电容器中集电器和电极材料的局限性，超级电容器的实际工作电压始终低于相应的电解液电化学稳定窗口，两者都可能与电解液相互作用。这种情况对于离子液体电解液尤为显着，当用三电极配置的玻璃碳电极测试时，离子液体电解液在5至6V的电位窗口下非常稳定，但最终的超级电容器很少能超过4V的工作电压。超级电容中离子液体的电压窗口较小的主要原因是由电极杂质和/或含氧官能团引起的寄生反应。当离子液体电解液和石墨烯基电极超过其电化学稳定窗口时，它们之间的电化学反应发生在离子液体分解之前。因此，在本发明中，可以使用包含还原的氧化石墨烯电极的对称两电极超级电容直接测量具有不同浓度的二元离子液体电解液所提供的最高工作电压。还原的氧化石墨烯电极的比表面积为240.8m 2·g -1，孔容为0.7cm 3·g -1，这表明它具有高度多孔的离子吸附结构。还原氧化石墨烯电极由大量的中孔组成，平均孔径为12nm。还原氧化石墨烯电极还显示出碳与氧(C/O)的原子比为15，具有5at％的含氧官能团，可以与电解质离子发生反应。有助于通过赝电容提高比电容。
与其他常用的离子液体相比，如哌啶类离子液体，本发明中的脂肪铵类的离子液体中的丁基三甲基铵基团不具有环结构，能够减弱电解液与电极之间的相互作用，应用在超级电容器中，能够获得比其他的离子液体更高的能量密度。
本发明提供了一种高能量密度超级电容器，其特征在于，包括还原氧化石墨烯电极和离子液体电解液；所述离子液体电解液包括咪唑类离子液体和/或 脂肪铵类离子液体，且所述咪唑类离子液体和脂肪铵类离子液体具有相同的阴离子。本发明设计了具有相同阴离子的两种离子液体作为电解液，并在超级电容器中使用了还原氧化石墨烯电极，通过电解液与电极的相互作用，同时实现高电压窗口和高的比电容，进一步的获得超级电容器的高能量密度。实验结果表明，本发明中的超级电容器工作电压可以达到4.7V，最大比电容为293.1F·g -1，最大的能量密度为176.5Wh·kg -1。
为了进一步说明本发明，以下结合实施例对本发明提供的一种高能量密度超级电容器进行详细描述，但不能将其理解为对本发明保护范围的限定。
除非另有说明，否则所有化学药品均可以商购获得，并按原样使用。氧化石墨烯水溶液(浓度：10mg·ml -1)购自SupraG Energy。从Iolitec获得离子液体1-乙基-3-甲基咪唑双(三氟甲基磺酰基)酰亚胺(EMIMTFSI，99.5％)和丁基三甲基铵双(三氟甲基磺酰基)酰亚胺(N1114TFSI，99％)。
通过简单的实验室规模的刮刀机制备独立的氧化石墨烯膜，然后将其切成直径为15mm的圆盘。还原氧化石墨烯是通过在功率小于0.5ppm H 2O和小于0.5ppm O 2的手套箱中通过功率为171.5Ws的闪光装置还原准备好的氧化石墨烯圆盘而获得的。每个还原氧化石墨烯的质量经测量为0.3mg。
通过在手套箱中将(x)EMIMTFSI和(1-x)N1114TFSI混合在一起获得二元离子液体电解液，其中x表示EMIMTFSI的体积分数(x＝0、0.05、0.25、0.5、0.75、0.95或1)。以下将所制备的二元离子液体电解液称为[EMIMTFSI] x[N1114TFSI] (1-x)。
使用Zeiss Supra 40VP扫描电子显微镜(SEM)在15kV的加速电压和1000倍的放大倍数下检查了还原氧化石墨烯样品的形貌和物理结构。
使用Micromeritics TriStar II Plus对氮气(N 2)吸附/解吸等温线研究了还原氧化石墨烯样品的多孔结构。在测量之前，将样品在70℃的氮气下脱气12小时。通过吸附等温线的Brunauer-Emmett-Teller(BET)分析获得比表面积(SSA)。由P/Po＝0.99下的N 2吸附量计算总孔体积。使用Barrett-Joyner-Halenda(BJH)方法确定孔径分布。
使用X射线光电子能谱(XPS)光谱对新鲜和使用过的还原氧化石墨烯样品的表面化学特性进行表征。XPS使用Krato Axis Nova表面分析光谱仪以 Al-K(1486.6eV)作为X射线源，两次通过能量分别为160eV(测量扫描)和20eV(高分辨率扫描)，以及700μm光束尺寸。在用于表征之前，将用过的还原氧化石墨烯从超级电容器中取出，并用丙酮，乙醇和蒸馏水冲洗几次。然后将洗涤过的还原氧化石墨烯在烘箱中干燥。
使用置于手套箱中的微粘稠度计和电导率仪(配备电导率传感器)分别测量了每个二元离子液体电解液系统在23℃和25℃时的粘稠度和离子电导率。在使用0.01M KCl水溶液进行每次样品测量后，通过校准确定电导率传感器的电池常数。
新鲜和使用过的二元离子液体电解液的 1H NMR实验在装有水冷Bruker Diff30扩散探针的Bruker Avance 300光谱仪上进行，观察频率为400MHz。在同一仪器上在376MHz处也记录了 19F NMR光谱。所有光谱测量均在氘代乙腈CD 3CN中进行。溶液中电解液的每个样品体积始终恒定，在0.6ml CD 3CN中每个为0.1ml。
组装两电极对称超级电容器，以评估还原氧化石墨烯样品及其在不同浓度的二元离子液体电解液中所得超级电容器的电化学性能。将两电极电池组装在氮气手套箱中。将用一定量的准备好的电解液浸泡的隔板放在两个还原氧化石墨烯电极之间。将碳涂覆的铝箔用作集电器，将还原氧化石墨烯电极压在其上。
使用多通道电化学工作站(EC-Lab，VMP-300)在室温下对组装后的超级电容器进行电化学测量。首先通过循环伏安法(CV)测量二元离子液体电解质中不同浓度的超级电容器的最高工作电压，该循环伏安法在1的3.5至5.5V范围内以5mV·s -1的扫描速率进行。使用恒电流充电/放电(GCD)测试以0.5A·g -1的电流密度重复相同的过程。
为了评估相应最高工作电压的电化学性能，在5、10、20、50和100mV s -1扫描速率下的CV测试和在0.5、1、2、5和10A·g电流密度下的GCD测试然后从0V到每个超级电容器的最高工作电压进行，这取决于不同的二元离子液体电解质的组成。还从EC-Lab电化学工作站在100kHz至10mHz的频率范围内，交流电压为5mV的开路电势下获得了电化学阻抗谱(EIS)图。使用以下公式由GCD曲线计算一个电极的重量电容：
Figure PCTCN2021130255-appb-000001
其中C S(F·g -1)是单个电极的比电容，m(g)是两个电极的总质量，I/m(A·g -1)是电流密度，Δt(s)是放电时间，ΔV(V)是IR下降后放电过程中的电压。
根据以下公式评估所得超级电容器的重量能量密度和功率密度：
Figure PCTCN2021130255-appb-000002
其中E S(Wh·kg -1)是超级电容器的重量能量密度，P S(W·kg -1)是超级电容器的重量功率密度，C S(F·g -1)是归一化到一个电极的比电容ΔV(V)是IR下降后放电过程中的电池电压，而Δt(s)是放电时间。
基于纯N1114TFSI，[EMIMTFSI] 0.5[N1114TFSI] 0.5，EMIMTFSI的超级电容器的循环稳定性，还可以通过以1A·g -1的电流密度在1000个循环内对超级电容器进行充电和放电来确定。然后创建一个图以说明趋势。
基于离子液体的超级电容器的电荷存储不是纯粹的吸附过程。相反，它涉及离子交换，其中离子吸附和离子解吸同时发生。对应离子是与电极表面带相反电荷的离子，而共离子具有与电极表面相同的电荷。图1中的示意图显示了具有不同电解液体系(包括N1114TFSI和/或EMIMTFSI)的负超级电容器电极的充电/放电过程。通过同时将对应离子(在这种情况下为阳离子)进入孔中和从孔中放出共离子(在这种情况下为阴离子)而发生充电，而放电则是通过将共离子吸附到对应离子并进行解吸。为了通过实验说明这种行为和离子液体混合物对超级电容器电荷存储机制的影响，我们研究了由于混合电解液系统中阳离子不同而导致的带电负离子上的离子组成和分布。由相同的还原氧化石墨烯电极(图6)和不同的电解液系统组成的超级电容器充电至4.1V，这被认为是所有电解液系统中可达到的最低最高工作电压，然后在多个循环中放电至0V。使用X射线光电子能谱(XPS)对干燥的充放电负电极进行表征，以定量分析表面化学成分。
在充放电的还原氧化石墨烯负电极中发现了新鲜的还原氧化石墨烯的宽扫描XPS光谱中不存在的F 1s和N 1s峰(图7(a)，(d)，(g))(图2(a-c) 和图7(b-c))。在除RGO_0.5和RGO_0.95(表示x的浓度分别为0.5和0.95)以外的大多数带电电极的高分辨率F 1s光谱中，峰685.7～686.1eV对应于阴离子中的氟组分与电极上残留的含氧官能团之间的相互作用，而689eV代表阴离子的CF 3成分(图2(d-f)和图7(e-f))。在所有带电电极的高分辨率N1s光谱中，还观察到两个不同的峰，即在399.5～399.6eV处的峰，该峰表征为阴离子的酰亚胺，而在401.9～402.7eV处的峰代表咪唑中的氮原子[EMIM]阳离子或铵[N1114]阳离子的阳离子(图2(g-i)和图7(h-i))。尽管事实上氮原子既可以来自阳离子也可以来自阴离子，而氟原子只能来自阴离子，但F 1s的强度要比带电/放电负电极的所有宽扫描XPS光谱中所描绘的N 1s强度都要高，这与以前的工作一致。这证实了阴离子吸收/阳离子解吸发生在离子液体中放电的负电极上的概念。
更具体地，来自阳离子的氮原子的相对原子浓度随电解液体系中N1114TFSI浓度的增加而降低。这种情况主要是由于N1114TFSI的铵阳离子中不存在双键(图1)。如果没有双键，电子密度将更加局限，从而使净正电荷局域在氮原子上，并在纯净的N1114TFSI中导致强的阳离子-阴离子相互作用。除了[N1114]阳离子的分子大小比[EMIM]阳离子大之外，[N1114]阳离子中更高的电荷局部化最终会导致离子迁移率相对较低，从而在具有较高N1114TFSI浓度的电解液系统中产生较高的粘稠度和较低的离子电导率(图2(j～k))。因此，在由较高N1114TFSI浓度组成的电解液系统中，从本体电解液到电极孔的离子传输要慢得多，这说明了较早的XPS观察表明，在充电/放电时在电极上发现了相对较少的氟和氮成分。
由于电解液离子的传输特性以及电极与电解液之间的相互作用会影响最高工作电压，因此，确定装有不同电解液系统的各个超级电容器的最高工作电压非常重要。进行了基于还原氧化石墨烯的超级电容器的逐步循环伏安法(CV)(图3(a-g)(i))和恒电流充放电(GCD)测试(图3(a-g)(ii))，以评估各自的最高工作电压。图9显示了完整的分步CV和GCD测试。CV和GCD测试在低扫描速率(5mV s -1)和低电流密度(0.5A g -1)下进行，以 确保任何反应的发生，特别是那些耗时的反应。确定最高工作电压时可以是非常随机的，由于使用的工作电极和参比电极的差异以及截止电流密度的随机选择，其情况类似于文献中报道的电解液电化学稳定窗口的不一致。在这项工作中，认为超级电容器在以下情况之前达到其最高工作电压：(i)CV的峰值变得巨大，其中峰值和基线之间的电流密度差
Figure PCTCN2021130255-appb-000003
大于50％(图3(h)(i))，并且(ii)GCD明显偏离了其最初的对称三角形形状小于30％(充电和放电时间之间的差异，
Figure PCTCN2021130255-appb-000004
)(图3(h)(ii))。XPS光谱证明，离子液体和含氧官能团之间的相互作用完全可以接受CV中的轻微峰，这有助于有用的赝电容。然而，以大峰表示的剧烈反应是不利的，因为它会使电解液和超级电容器循环稳定性迅速劣化。严重扭曲的GCD曲线表明，跨电极/电解液界面的电阻较高，这可能是由于过剩的含氧官能团-电解液相互作用和/或电解液分解而导致的新产品。通过核磁共振(NMR)确认了各种电解液系统在相应的最高工作电压处的稳定运行。
因此，基于x＝0、0.05、0.25、0.5、0.75、0.95和1的离子液体电解液系统的超级电容器的最高工作电压分别为4.7、4.3、4.2、4.1、4.1和4.1V(图4(a)))。带有纯EMIMTFSI和纯N1114TFSI的超级电容器的测得的最高工作电压比制造商报告的电化学稳定窗口低得多(EMIMTFSI＝4.7V和N1114TFSI＝6.1V)。这进一步验证了离子液体可以提供的实际超级电容器工作电压明显小于电解液本身的电化学稳定窗口。正如预期的那样，超级电容器中的最高工作电压越高，则电解液中的N1114TFSI浓度越高(即，x值越低)，这是由于电解液离子与含氧官能团在含有更多N1114TFSI的电解液系统中的相互作用较弱。电解液中超级电容器的最高工作电压不会进一步降低到超过75vol％EMIMTFSI(即x＝0.75)。结果表明需要大量的[N1114]阳离子来减弱电 解液/电极的相互作用。除了电解液系统中离子迁移率降低外，另一个导致这种较弱相互作用的因素是，由于脂肪族铵阳离子中缺乏环结构，还原氧化石墨烯中阳离子与sp2-π电子之间的π-π堆积受到抑制。这可以通过纯N1114TFSI和[EMIMTFSI]0.05[N1114TFSI]0.95中带电电极的XPS C 1s光谱中不存在π-π*卫星峰来证明(图8(b-c))。离子在还原氧化石墨烯表面的吸附被抑制，而没有π-π相互作用。
还评估了还原氧化石墨烯的电化学性能以及带有x的不同电解液体系的所得超级电容器在4.1V及其各自的最高工作电压下的性能。图4(b)和图4(c)分别显示了在4.1V下所有情况下还原氧化石墨烯的CV和GCD曲线。在4.1V时，发现电解液中的比电容和能量密度随EMIMTFSI体积分数的增加而增加(图4(d))。在工作电压窗口不是决定性因素的情况下，粘稠度是影响比电容进而影响能量密度的主要因素。随着EMIMTFSI浓度的增加，由于粘稠度降低而导致的离子迁移率增加，促进了离子从电解液到电极的扩散，从而实现了：(1)较高的双电层(EDL)电容受到较低IR压降的支持EMIMTFSI浓度较高时的GCD曲线；(2)赝电容较高，这在CV中1.3V至2.5V的峰值处变得越来越明显，这表明电解液与电极之间的相互作用更加剧烈(图4(b))。
另一方面，来自各个超级电容器的还原氧化石墨烯的MW和GCD曲线分别在图5(a)和(c)中显示。在5mV·s -1的低扫描速率和0.5A·g -1的低电流密度下，CV和GCD曲线显示为准矩形(图5(a))和对称的三角形(图5(c)))，分别展示了与二元离子液体混合物组装时超级电容器的电容特性。在200mV·s -1的高扫描速率下，CV曲线倾斜(图5(b))，这是由于在较快的过程中孔中电解液的高欧姆电阻引起的。通过在10A·g -1的高电流密度下具有较高的IR压降和扭曲的GCD曲线进一步证实了这种情况(图5(d))。当以相应的最高工作电压进行测量时，在[EMIMTFSI] 0.5[N1114TFSI] 0.5(293.1F g -1)中获得最高的最大电极比电容，其次是纯EMIMTFSI(280.7F g -1)，[EMIMTFSI] 0.95[N1114TFSI] 0.05(268.3F g -1)，[EMIMTFSI] 0.75[N1114TFSI] 0.25(254.3F g -1)，[EMIMTFSI] 0.25[N1114TFSI] 0.75(253.2F g -1)，纯N1114TFSI(242.8F g -1))，[EMIMTFSI] 0.05[N1114TFSI] 0.95(242.3F g -1)(图5(e))。 基于该结果，尽管由于电极/电解液界面处的电荷分离距离的减小和/或电容量的增加而导致电容随电池电压的增加而增加，但为了实现最佳的电化学性能，最高工作电压和粘稠度之间的平衡是必需的。由于电荷位移产生的空间电荷，导致电极内部串联电容的变化。当电极在[EMIMTFSI] 0.5[N1114TFSI] 0.5中显示出比(1)纯EMIMTFSI和[EMIMTFSI] 0.95[N1114TFSI] 0.05中都更高的最大比电容时，显然可以观察到这种最高工作电压/粘稠度平衡；(2)纯N1114TFSI和[EMIMTFSI] 0.05[N1114TFSI] 0.95，两者均具有较高的粘稠度。
尽管在4.1V下电极在[EMIMTFSI] 0.5[N1114TFSI] 0.5中显示的最大比电容比纯EMIMTFSI低，但在电压仅增加0.1V，即最高工作电压为4.2V[EMIMTFSI] 0.5[N1114TFSI] 0.5中电极显示的最大比电容比的在电压为4.1V时最大比电容高12.4F·g -1。这表明稍高的最高工作电压可以引起相当大的比电容增强。然而，这种假设仅在同时电解液粘稠度足够低的情况下适用。尽管[EMIMTFSI] 0.5[N1114TFSI] 0.5的最高工作电压比纯N1114TFSI低0.5V，但发现该电极的比电容高50.3F·g -1，这归因于其改善的离子迁移率。
当使用[EMIMTFSI] 0.5[N1114TFSI] 0.5时，超级电容器的最大能量密度最高(177Wh·kg -1)，其次是纯N1114TFSI(176.5Wh·kg -1)，纯EMIMTFSI(161.1Wh·kg -1)，[EMIMTFSI] 0.95[N1114TFSI] 0.05(155.5Wh·kg -1)，[EMIMTFSI] 0.25[N1114TFSI] 0.75(154.2Wh·kg -1)，[EMIMTFSI] 0.05[N1114TFSI] 0.95(153.3Wh·kg -1)，和[EMIMTFSI] 0.75[N1114TFSI] 0.25(146.5Wh·kg -1)(图5(f))。出乎意料的是，基于[EMIMTFSI] 0.5[N1114TFSI] 0.5的超级电容器显示出最高的最大能量密度，尽管其最高工作电压并不是最大。这暗示了电容和最高工作电压在最大化能量密度方面的同等重要性。然而，考虑到最高工作电压与能量密度的二次关系，实质上更高的最高工作电压仍然有利于高能量密度，这可以帮助补偿较低的比电容(图5(g))。例如，尽管在纯N1114TFSI中表现出50F·g-1的较低电极比电容，基于纯净的N1114TFSI的超级电容器表现出与基于[EMIMTFSI] 0.5[N1114TFSI] 0.5的超级电容器一样高的能量密度。此外，当最高工作电压加宽0.6V时，即使纯N11114TFSI中的电极的比电容比纯EMIMTFSI中的电极低38F·g -1，含有纯N1114TFSI的超级电容器的能量密度也比含有纯EMIMTFSI的超级电容器大得多。当比较基于[EMIMTFSI] 0.05[N1114TFSI] 0.95的超级电容器 和基于[EMIMTFSI] 0.75[N1114TFSI] 0.25的超级电容器的能量密度时，也观察到相同的情况。
使用电化学阻抗谱(EIS)在开路电势下从1MHz到10mHz的频率范围内，进一步研究了具有变化x的电解液系统的超级电容器的传输特性。图5(h)中的奈奎斯特图由等效电路拟合，该电路演示了电极/电解液系统中的频率响应。发现低频区域中的垂直线随着x值的增加而更接近虚轴，这表明当与含有更多EMIMTFSI浓度的电解液一起使用时，还原氧化石墨烯中的离子扩散更快。结果分别与图2(j)和(g～k)所示的粘稠度和离子电导率数据一致。此外，超级电容器s的ESR随着x值的增加而降低，这意味着当EMIMTFSI浓度较高时，电极/电解液界面的电荷转移电阻(R ct)较低，电解液电阻(R s的一部分)较低。在电解液系统中。但是，在纯N1114TFSI，[EMIMTFSI] 0.5[N1114TFSI] 0.5和纯EMIMTFSI中，在相应的最高工作电压下经过1500个连续的GCD循环后，还原氧化石墨烯中仅保留了33.4％至57.1％的初始比电容。(图12)。这种低循环稳定性可能是由于孔隙和电极电活性区域中离子液体聚集的增加所致，从而限制了离子吸附和离子/电极过程，特别是在混合两个或多个离子液体时。电解液和电极之间的相互作用还降低了在连续充电/放电循环后用于吸附的离子的可用性。电解液的老化还会大大降低电解液的离子电导率，进而降低离子在电解液中的移动。为了减轻循环稳定性问题，可以在将来的工作中采取例如在电解液体系中添加有机溶剂以减少离子聚集和/或尽可能多地去除碳基电极上的含氧官能团以减少电解液/电极相互作用的策略。
在本发明中，单数形式“一个”，“一种”和“该”既表示单数也表示复数，除非明确指出仅表示单数。
术语“大约”和通常使用的范围，无论是否由术语“大约”限定，都意味着所理解的数字不限于本文所述的确切数字，并且旨在指代基本上在所引用范围内的范围。而不脱离本发明的范围。如本文中所使用的，“约”将被本领域普通技术人员理解，并且将在使用它的上下文中在某种程度上变化。如果给定使用上下文的术语的使用对本领域普通技术人员而言尚不清楚，则“约”表示特定术语的正负10％。
除非另有说明，否则本发明所指的百分比(％)基于重量百分比(w/w或w/v)。
在本发明中，除非上下文另有要求，否则词语“包括”以及诸如“包括”和“包含”之类的变体将被理解为暗示包括所述整数或步骤或整数组。或步骤，但不排除任何其他整数或步骤或一组整数或步骤。
以上所述仅是本发明的优选实施方式，应当指出，对于本技术领域的普通技术人员来说，在不脱离本发明原理的前提下，还可以做出若干改进和润饰，这些改进和润饰也应视为本发明的保护范围。

Claims (8)

  1. 一种高能量密度超级电容器，其特征在于，包括还原氧化石墨烯电极和离子液体电解液；
    所述离子液体电解液包括咪唑类离子液体和/或脂肪铵类离子液体，且所述咪唑类离子液体和脂肪铵类离子液体具有相同的阴离子。
  2. 根据权利要求1所述的高能量密度超级电容器，其特征在于，所述离子液体电解液中的阴离子为双(三氟甲基磺酰基)酰亚胺根。
  3. 根据权利要求1所述的高能量密度超级电容器，其特征在于，所述咪唑类离子液体和脂肪铵类离子液体的体积比为(0.05～0.95)：(0.95～0.05)。
  4. 根据权利要求3所述的高能量密度超级电容器，其特征在于，所述咪唑类离子液体和脂肪铵类离子液体的体积比为0.5：0.5。
  5. 根据权利要求1～4任意一项所述的高能量密度超级电容器，其特征在于，所述咪唑类离子液体为1-乙基-3-甲基咪唑双(三氟甲基磺酰基)酰亚胺离子液体；所述脂肪铵类离子液体为丁基三甲基铵双(三氟甲基磺酰基)酰亚胺离子液体。
  6. 根据权利要求1所述的高能量密度超级电容器，其特征在于，所述还原氧化石墨烯电极的比表面积为225～308m 2·g -1。
  7. 根据权利要求1所述的高能量密度超级电容器，其特征在于，所述还原氧化石墨烯电极的孔体积为0.6～0.9cm 3·g -1。
  8. 根据权利要求1所述的高能量密度超级电容器，其特征在于，所述还原氧化石墨烯电极的孔径为11～13nm。
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