CN112542329B - High energy density super capacitor - Google Patents
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- 239000003990 capacitor Substances 0.000 title claims abstract description 26
- 239000002608 ionic liquid Substances 0.000 claims abstract description 100
- 239000003792 electrolyte Substances 0.000 claims abstract description 99
- RAXXELZNTBOGNW-UHFFFAOYSA-N imidazole Natural products C1=CNC=N1 RAXXELZNTBOGNW-UHFFFAOYSA-N 0.000 claims abstract description 76
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- QGZKDVFQNNGYKY-UHFFFAOYSA-O Ammonium Chemical compound [NH4+] QGZKDVFQNNGYKY-UHFFFAOYSA-O 0.000 claims abstract description 28
- 150000001450 anions Chemical class 0.000 claims abstract description 17
- 239000011148 porous material Substances 0.000 claims description 16
- XSGKJXQWZSFJEJ-UHFFFAOYSA-N bis(trifluoromethylsulfonyl)azanide;butyl(trimethyl)azanium Chemical compound CCCC[N+](C)(C)C.FC(F)(F)S(=O)(=O)[N-]S(=O)(=O)C(F)(F)F XSGKJXQWZSFJEJ-UHFFFAOYSA-N 0.000 claims description 4
- ZXMGHDIOOHOAAE-UHFFFAOYSA-N 1,1,1-trifluoro-n-(trifluoromethylsulfonyl)methanesulfonamide Chemical compound FC(F)(F)S(=O)(=O)NS(=O)(=O)C(F)(F)F ZXMGHDIOOHOAAE-UHFFFAOYSA-N 0.000 claims description 2
- IBZJNLWLRUHZIX-UHFFFAOYSA-N 1-ethyl-3-methyl-2h-imidazole Chemical compound CCN1CN(C)C=C1 IBZJNLWLRUHZIX-UHFFFAOYSA-N 0.000 claims 1
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- 238000001228 spectrum Methods 0.000 description 10
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- 238000000026 X-ray photoelectron spectrum Methods 0.000 description 4
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- 238000000157 electrochemical-induced impedance spectroscopy Methods 0.000 description 4
- 125000004433 nitrogen atom Chemical group N* 0.000 description 4
- LRESCJAINPKJTO-UHFFFAOYSA-N bis(trifluoromethylsulfonyl)azanide;1-ethyl-3-methylimidazol-3-ium Chemical compound CCN1C=C[N+](C)=C1.FC(F)(F)S(=O)(=O)[N-]S(=O)(=O)C(F)(F)F LRESCJAINPKJTO-UHFFFAOYSA-N 0.000 description 3
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- YCKRFDGAMUMZLT-UHFFFAOYSA-N Fluorine atom Chemical compound [F] YCKRFDGAMUMZLT-UHFFFAOYSA-N 0.000 description 2
- RAXXELZNTBOGNW-UHFFFAOYSA-O Imidazolium Chemical compound C1=C[NH+]=CN1 RAXXELZNTBOGNW-UHFFFAOYSA-O 0.000 description 2
- 238000005481 NMR spectroscopy Methods 0.000 description 2
- NQRYJNQNLNOLGT-UHFFFAOYSA-N Piperidine Chemical compound C1CCNCC1 NQRYJNQNLNOLGT-UHFFFAOYSA-N 0.000 description 2
- JUJWROOIHBZHMG-UHFFFAOYSA-N Pyridine Chemical compound C1=CC=NC=C1 JUJWROOIHBZHMG-UHFFFAOYSA-N 0.000 description 2
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- NJMWOUFKYKNWDW-UHFFFAOYSA-N 1-ethyl-3-methylimidazolium Chemical compound CCN1C=C[N+](C)=C1 NJMWOUFKYKNWDW-UHFFFAOYSA-N 0.000 description 1
- 238000005160 1H NMR spectroscopy Methods 0.000 description 1
- 229910002703 Al K Inorganic materials 0.000 description 1
- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 description 1
- GXVKHKJETWAWRR-UHFFFAOYSA-N a805143 Chemical compound C1CCNC1.C1CCNC1 GXVKHKJETWAWRR-UHFFFAOYSA-N 0.000 description 1
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- WEVYAHXRMPXWCK-FIBGUPNXSA-N acetonitrile-d3 Chemical compound [2H]C([2H])([2H])C#N WEVYAHXRMPXWCK-FIBGUPNXSA-N 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
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- 229910052782 aluminium Inorganic materials 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- XKMRRTOUMJRJIA-UHFFFAOYSA-N ammonia nh3 Chemical compound N.N XKMRRTOUMJRJIA-UHFFFAOYSA-N 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- IUNCEDRRUNZACO-UHFFFAOYSA-N butyl(trimethyl)azanium Chemical group CCCC[N+](C)(C)C IUNCEDRRUNZACO-UHFFFAOYSA-N 0.000 description 1
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- UMJSCPRVCHMLSP-UHFFFAOYSA-N pyridine Natural products COC1=CC=CN=C1 UMJSCPRVCHMLSP-UHFFFAOYSA-N 0.000 description 1
- JUJWROOIHBZHMG-UHFFFAOYSA-O pyridinium Chemical compound C1=CC=[NH+]C=C1 JUJWROOIHBZHMG-UHFFFAOYSA-O 0.000 description 1
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- ZSDSQXJSNMTJDA-UHFFFAOYSA-N trifluralin Chemical compound CCCN(CCC)C1=C([N+]([O-])=O)C=C(C(F)(F)F)C=C1[N+]([O-])=O ZSDSQXJSNMTJDA-UHFFFAOYSA-N 0.000 description 1
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Classifications
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-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 OR LIGHT-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
Abstract
The invention provides a high-energy-density supercapacitor, which comprises a reduced graphene oxide electrode and an ionic liquid electrolyte; electrolysis of the ionic liquidThe liquid comprises imidazole ionic liquid and/or fatty ammonium ionic liquid, and the imidazole ionic liquid and the fatty ammonium ionic liquid have the same anion. According to the invention, two ionic liquids with the same anions are designed as electrolytes, a reduced graphene oxide electrode is used in the super capacitor, and a high voltage window and a high specific capacitance are simultaneously realized by controlling the interaction of the electrolytes and the electrode, so that the high energy density of the super capacitor is further obtained. Experimental results show that the working voltage of the super capacitor can reach 4.7V, and the maximum specific capacitance is 293.1F g‑1The maximum energy density was 176.5Wh kg‑1。
Description
Technical Field
The invention belongs to the technical field of energy storage, and particularly relates to a high-energy-density super capacitor.
Background
Supercapacitors (also referred to as "supercapacitors" or "double layer capacitors") are electrochemical capacitors with much higher capacitance values than other capacitors. Supercapacitors are widely used for energy storage and energy supply due to their high energy density, fast charge/discharge capability, long life over one million charge cycles and ability to operate over a wide temperature range of-40 ℃ to 70 ℃. In recent years, development of supercapacitors has been promoted by environmentally friendly materials in supercapacitors and their low maintenance costs, since the production and disposal of batteries can have adverse effects on environmental pollution and human health. Furthermore, supercapacitors are advantageous over batteries because they can provide higher power densities (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.
The energy density (E) of the super capacitor is represented by E-1/2 CV2It is given as proportional to the specific capacitance (C) and the square of the operating voltage window (V). Although the specific capacitance depends on the nature of the electrode and the electrolyte, the operating voltage window depends on the stability of the electrolyte and also on the nature of the electrode surface where surface oxygen functional groups are present. Since electrolytes are one of the determinants in determining specific capacitance and operating voltage window, the development of electrolytes with a wide electrochemical stability window and the ability to provide high capacitance is critical to increasing the energy density of supercapacitors. In this respect, room temperature ionic liquids, also called molten salts, are due to their wide electrochemistryThe stability window is typically greater than 4V, and is widely studied as a potential next generation supercapacitor electrolyte.
To date, the most common single-component ionic liquids containing cations of imidazole (imidazolium), pyridine (pyridinium), ammonium (ammonium) and pyrrolidine (pyrrolidinium) have been extensively studied. However, the existing electrolytic solution for super capacitor has a narrow or electrochemical stability window, or low conductivity, and also has the problem of low energy density, so that further development of a novel ionic liquid with required performance is required to enhance the energy density of super capacitor.
Disclosure of Invention
The invention aims to provide a high-energy-density super capacitor, which has a high voltage window, high capacitance and higher energy density.
The 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 fatty ammonium ionic liquid, and the imidazole ionic liquid and the fatty ammonium ionic liquid have the same anions.
Preferably, the anion in the ionic liquid electrolyte is bis (trifluoromethylsulfonyl) imide.
Preferably, the volume ratio of the imidazole ionic liquid to the fatty ammonium ionic liquid is (0.05-0.95): (0.95-0.05).
Preferably, the volume ratio of the imidazole ionic liquid to the fatty ammonium ionic liquid is 0.5: 0.5.
preferably, the imidazole ionic liquid is 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide ionic liquid; the fatty ammonium ionic liquid is butyl trimethyl ammonium bis (trifluoromethylsulfonyl) imide ionic liquid.
Preferably, the specific surface area of the reduced graphene oxide electrode is 225-308 m 2·g-1。
Preferably, the pore volume of the reduced graphene oxide electrode is 0.6-0.9 cm3·g-1。
Preferably, the aperture of the reduced graphene oxide electrode is 11-13 nm.
The 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 fatty ammonium ionic liquid, and the imidazole ionic liquid and the fatty ammonium ionic liquid have the same anions. According to the invention, two ionic liquids with the same anions are designed as electrolytes, a reduced graphene oxide electrode is used in the super capacitor, and a high voltage window and a high specific capacitance are simultaneously realized by controlling the interaction of the electrolytes and the electrode, so that the high energy density of the super capacitor is further obtained. Experimental results show that the working voltage of the super capacitor can reach 4.7V, and the maximum specific capacitance is 293.1F g-1The maximum energy density was 176.5Wh kg-1。
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.
Fig. 1 is a schematic diagram of a reduced graphene oxide based supercapacitor. The ionic movement of pure N1114TFSI (upper row), pure EMIMTFSI (lower row) and N1114TFSI-EMIMTFSI electrolyte mixture (middle row) in the pores of the negative electrode during charging and discharging is illustrated. The chemical structures of the ammonium cation in N1114TFSI, the imidazolium cation in EMIMTFSI and the bis (trifluoromethylsulfonyl) imide anion are also described;
FIG. 2 shows the results of the measurements on pure N1114TFSI, [ EMIMTFSI]0.5[N1114TFSI]0.5And pure EMIMTFSI for (a) RGO _0, (b) RGO _0.5 and (c) RGO _1, respectivelyWide scan XPS spectra of charge/discharge;
(d) high resolution F1s spectra of RGO _0, (e) RGO _0.5, and (F) RGO _ 1;
charged/discharged high resolution N1s spectra of (g) RGO _0, (h) RGO _0.5, and (i) RGO _ 1;
(j)[EMIMTFSI]x[N1114TFSI](1-x)(k) the ionic conductivity and viscosity at 23 ℃ and 25 ℃ respectively are a function of x;
fig. 3, step-by-step CV test plots and GCD test plots for graphene-based supercapacitors of different binary electrolyte compositions;
(a) pure 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[N 1114TFSI]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) and (g) pure EMIMTFSI (x ═ 1). (h) Determining a condition for a maximum operating voltage using (i) the CV and (ii) the GCD;
FIG. 4 is a plot of (a) the maximum operating voltage of the electrolyte as a function of x; (b) the CV curve of the supercapacitor based on the electrolyte system changes with x at 4.1V; (c) the GCD curve of the supercapacitor based on the electrolyte system changes with x at 4.1V; (d) the specific capacitance of RGO and the supercapacitor energy density at 4.1V as a function of x;
FIG. 5 shows the scanning rate at (a)5mVs-1And (b)200mVs-1CV curves of supercapacitors at their respective highest operating voltages with electrolytes having different mixing ratios; when the current density is (c) 0.5A · g-1And (d)10 A.g-1GCD curves of supercapacitors with electrolytes of different mixing ratios; (e) the relationship between electrode specific capacitance and current density at each highest operating voltage, and (f) the Ragone plot for x varying between 0 and 1; (g) how the maximum working voltage and the maximum electrode specific capacitance of the supercapacitor affect the curve of the maximum supercapacitor energy density;
(h) nyquist fit plot for supercapacitors with different mixing ratios of electrolytes, plot (h) right inset: an amplified high frequency region. Insert at bottom of figure (h): an equivalent circuit for fitting a nyquist plot;
FIG. 6(a) scanning electron microscope top view of the RGO electrode (magnification: 1 kX). Illustration is shown: pictures of RGO electrodes. (b) Nitrogen adsorption/desorption isotherms and (c) pore size distribution of the RGO electrode. (d) XPS wide scan spectra of RGO electrodes.
FIG. 7(a) fresh RGO and charged/discharged (b) [ EMIMTFSI]0.05[N1114TFSI]0.95RGO-0.05 and (c) [ EMIMTFSI ] in (1)]0.95[N1114TFSI]0.05Wide scan XPS spectrum 0.05 for RGO — 0.95 in (a); (d) high resolution F1s spectra of fresh RGO and charged/discharged (e) RGO-0.05 and (F) RGO-0.95; high resolution N1s spectra charged/discharged over (g) fresh RGO and charged/discharged (h) RGO-0.05 and (i) RGO-0.95;
FIG. 8(a) fresh RGO and charged/discharged (b) RGO _0, (C) RGO _0.05, (d) RGO _0.5, (e) high resolution C1s spectra of RGO _0.95 and (f) RGO _ 1. (g) High resolution S2 p 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 complete step CV and GCD test of fig. 9-1 is for the assembled (a) pure N11114TFSI (x ═ 0), (b) [ emitfsi]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)；
The complete step CV and GCD test of FIG. 9-2 is for an assembly with (e) [ EMIMTFSI]0.75[N1114TFSI]0.25(x＝0.75)，(f)[EMIMTFSI]0.95[N1114TFSI]0.05(x ═ 0.95) and (g) pure EMIMTFSI (x ═ 1);
figure 10 fresh when x is 0, 0.05, 0.50, 0.95 and 1 [ EMIMTFSI ═ EMIMTFSI]x[N1114TFSI](1-x) 1H and (ii)19F NMR spectra of the electrolyte;
fig. 11(a) x is 0, (b) x is 0.05, (c) x is 0.50, (d) x is 0.95, and (e) x is 1. The electrolytes used were tested after charging the corresponding maximum operating voltages, which can be provided in the supercapacitors. Yellow asterisks indicate intrinsic composition of N1114TFSI, blue triangles indicate intrinsic composition of EMIMTFSI;
FIG. 12: assembled with pure N1114TFSI (x ═ 0) [ EMIMTFSI]0.5[N1114TFSI]Cycle stability of supercapacitors of 0.5(x ═ 0.5) and pure EMIMTFSI (x ═ 1), cycled through 1500GCD and at 1A g-1And (4) measuring. Illustration is shown: by [ EMIMTFSI ] ]0.5[N1114TFSI]0.5GCD curves for the first and 1000 th cycles of the assembled supercapacitor.
Detailed Description
The 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 fatty ammonium ionic liquid, and the imidazole ionic liquid and the fatty ammonium ionic liquid have the same anions.
Different from the working principle of a lithium battery based on chemical reaction, the super capacitor in the invention has no chemical reaction, and realizes electric energy storage by adsorbing positive and negative ions in electrolyte on different electrodes, so the working voltage of the super capacitor in the invention depends on the electrochemical stability window of the electrolyte and the interaction of the electrodes thereof, thereby realizing high energy density of the super capacitor.
In the invention, the supercapacitor takes 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 cm3·g-1(ii) a The preferred pore size distribution of the reduced graphene oxide electrode is 11-13 nm, and the preferred specific surface area of the reduced graphene oxide electrode is 225-308 m 2·g-1。
In the present invention, the reduced graphene oxide is preferably prepared according to the following steps:
preparing an independent graphene oxide film by using a graphene oxide aqueous solution with the concentration of 10mg/mL through a simple laboratory-scale scraper machine, and then cutting the independent graphene oxide film into films with the diameters ofA 15mm disk. Reducing the graphene oxide by reducing the graphene oxide at a power of less than 0.5ppmH2O and less than 0.5ppmO2Obtained by reduction of a prepared graphene oxide disc by means of a flash device with a power of 171.5 Ws.
Compared with electrodes made of other materials, such as metal oxide electrodes, organic polymer electrodes and the like, and even compared with graphene oxide electrodes and graphene electrodes, the reduced graphene oxide electrode provided by the invention can weaken the interaction with electrolyte, and higher energy density of the supercapacitor is obtained.
In the invention, the electrolyte is preferably a mixture of two ionic liquids with the same anion, preferably an imidazole ionic liquid and a fatty ammonium ionic liquid, and in principle, the imidazole ionic liquid and the fatty ammonium ionic liquid can be mixed according to any proportion, and the two extremes of the mixture are respectively a pure imidazole ionic liquid and a pure fatty ammonium ionic liquid.
In the present invention, the fatty ammonium ionic liquid preferably contains no cyclic structure and no double bond; preferably butyltrimethylammonium bis (trifluoromethylsulfonyl) imide ionic liquid (N1114 TFSI).
The imidazole ionic liquid is preferably 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide ionic liquid (EMIMTFSI).
The imidazole ionic liquid and the fatty ammonium ionic liquid are mixed in a glove box to obtain ionic liquid electrolyte, which is expressed as [ EMIMTFSI ]]x[N1114TFSI](1-x)Wherein x is a volume fraction of the imidazole-based ionic liquid, and is any one selected from 0 to 1, specifically, in the embodiment of the present invention, x may be 0, 0.05, 0.25, 0.5, 0.75, 0.95, or 1, and more preferably 0.5.
In the literature, the operating voltage of a supercapacitor is typically reported as the electrochemical stability window of the electrolyte, measured using a three-electrode configuration. However, this is inaccurate because the actual operating voltage of a supercapacitor is always lower than the corresponding electrolyte voltage due to limitations of current collectors and electrode materials in the supercapacitorA chemically stable window, both of which may interact with the electrolyte. This is particularly true for ionic liquid electrolytes, which are very stable over a potential window of 5 to 6V when tested with a glassy carbon electrode in a three electrode configuration, but the final supercapacitor rarely can exceed an operating voltage of 4V. The main reason for the smaller voltage window of ionic liquids in supercapacitors is parasitic reactions caused by electrode impurities and/or oxygen-containing functional groups. When the ionic liquid electrolyte and graphene-based electrode exceed their electrochemical stability windows, the electrochemical reaction between them occurs before the ionic liquid decomposes. Thus, 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 containing reduced graphene oxide electrodes. The specific surface area of the reduced graphene oxide electrode was 240.8m 2·g-1Pore volume of 0.7cm3·g-1This 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, having 5 at% of oxygen-containing functional groups, which can react with electrolyte ions. Which contributes to the improvement of the specific capacitance by the pseudocapacitance.
Compared with other common ionic liquids, such as piperidine ionic liquids, the butyl trimethyl ammonium group in the aliphatic ammonium ionic liquid has no ring structure, so that the interaction between the electrolyte and the electrode can be weakened, and the energy density can be higher than that of other ionic liquids when the aliphatic ammonium ionic liquid is applied to a super capacitor.
The 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 fatty ammonium ionic liquid, and the imidazole ionic liquid and the fatty ammonium ionic liquid have the same anions. The invention designs two ionic liquids with the same anions as electrolyte, and uses a reduced graphene oxide electrode in a super capacitor by electricity The interaction of the electrolyte and the electrode simultaneously realizes a high voltage window and high specific capacitance, and further obtains the high energy density of the super capacitor. Experimental results show that the working voltage of the super capacitor can reach 4.7V, and the maximum specific capacitance is 293.1F g-1The maximum energy density was 176.5Wh kg-1。
In order to further illustrate the present invention, the following detailed description is made for a high energy density super capacitor provided by the present invention with reference to the examples, but it should not be construed as limiting the scope of the present invention.
Unless otherwise indicated, all chemicals were commercially available and used as received. Graphene oxide aqueous solution (concentration: 10 mg. ml)-1) From SuperG Energy. Ionic liquids 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide (EMIMTFSI, 99.5%) and butyltrimethylammonium bis (trifluoromethylsulfonyl) imide (N1114TFSI, 99%) were obtained from Iolitec.
Individual graphene oxide films were prepared by a simple laboratory scale doctor blade machine and then cut into disks 15mm in diameter. Reducing the graphene oxide by reducing the graphene oxide to a power of less than 0.5ppm H2O and less than 0.5ppm O2Obtained by reducing a prepared graphene oxide disc by a flash device with a power of 171.5 Ws. The mass of each reduced graphene oxide was measured to be 0.3 mg.
A binary ionic liquid electrolyte was obtained by mixing together (x) emitfsi and (1-x) N1114TFSI in a glove box, where x represents the volume fraction of emitfsi (x ═ 0, 0.05, 0.25, 0.5, 0.75, 0.95, or 1). 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 15kV and 1000 times magnification.
Nitrogen (N) using Micromeritics TriStar II Plus2) The adsorption/desorption isotherm studied the porous structure of the reduced graphene oxide sample. Under testBefore dosing, the samples were degassed under nitrogen at 70 ℃ for 12 hours. Specific Surface Area (SSA) was obtained by Brunauer-Emmett-Teller (BET) analysis of the adsorption isotherm. N under the condition that P/Po is 0.992The amount of adsorption was calculated as the total pore volume. The pore size distribution was determined using the Barrett-Joyner-Halenda (BJH) method.
The surface chemistry of fresh and used reduced graphene oxide samples was characterized using X-ray photoelectron spectroscopy (XPS) spectroscopy. XPS uses a Krato Axis Nova surface analysis spectrometer with Al-K (1486.6eV) as the X-ray source, two passes at 160eV (survey scan) and 20eV (high resolution scan), respectively, and a 700 μm beam size. The spent reduced graphene oxide was removed from the supercapacitor and rinsed several times with acetone, ethanol and distilled water before use for characterization. The washed reduced graphene oxide is then dried in an oven.
The viscosity and ionic conductivity at 23 ℃ and 25 ℃ were measured for each binary ionic liquid electrolyte system using a microadensitometer and conductivity meter (equipped with conductivity sensors) placed in a glove box, respectively. After each sample measurement using 0.01M aqueous KCl, the cell constant of the conductivity sensor was determined by calibration.
Of fresh and used electrolytes of binary ionic liquids1H NMR experiments were performed on a Bruker Avance 300 spectrometer equipped with a water-cooled Bruker Diff30 diffusion probe, with an observation frequency of 400 MHz. Also recorded at 376MHz on the same instrument19F NMR spectrum. All spectral measurements were performed in deuterated acetonitrile CD3And (C) in CN. The volume of electrolyte per sample in solution was constant at 0.6ml CD3CN was 0.1ml each.
And assembling the two-electrode symmetrical supercapacitor to evaluate the electrochemical performance of the reduced graphene oxide sample and the supercapacitor obtained from the reduced graphene oxide sample in binary ionic liquid electrolytes with different concentrations. The two electrode cell was assembled in a nitrogen glove box. A separator soaked with a quantity of prepared electrolyte is placed between two reduced graphene oxide electrodes. Carbon-coated aluminum foil was used as a current collector, and a reduced graphene oxide electrode was pressed thereon.
Electrochemical measurements were performed on the assembled supercapacitor at room temperature using a multichannel electrochemical workstation (EC-Lab, VMP-300). The maximum operating voltage of supercapacitors of different concentrations in the binary ionic liquid electrolyte was first measured by Cyclic Voltammetry (CV) at 5mV s over a range of 3.5 to 5.5V of 1-1Is performed at the scanning rate of (a). Using a constant current charge/discharge (GCD) test at 0.5A g-1The same process is repeated.
In order to evaluate the electrochemical performance at the respective highest operating voltages, s was measured at 5, 10, 20, 50 and 100mV-1CV tests at scan rate and GCD tests at current densities of 0.5, 1, 2, 5 and 10A · g were then performed from 0V to the highest operating voltage of each supercapacitor, depending on the composition of the different binary ionic liquid electrolytes. Electrochemical Impedance Spectroscopy (EIS) plots were also obtained from the EC-Lab electrochemical workstation at an open circuit potential of 5mV ac voltage in the frequency range of 100kHz to 10 mHz. The gravimetric capacitance of one electrode was calculated from the GCD curve using the following formula:
wherein C isS(F·g-1) Is the specific capacitance of a single electrode, m (g) is the total mass of the two electrodes, I/m (A.g.)-1) Is the current density, Δ t(s) is the discharge time, and Δ v (v) is the voltage during discharge after IR drop.
The gravimetric energy density and power density of the resulting supercapacitor were evaluated according to the following formulas:
wherein ES(Wh·kg-1) Is the gravimetric energy density, P, of the supercapacitorS(W·kg-1) Is the gravimetric power density, C, of the supercapacitorS(F·g-1) Is normalized to the specific capacitance Δ V (V) of one electrode, is discharged after IR dropCell voltage in process, and Δ t(s) is discharge time.
Based on pure N1114TFSI, [ EMIMTFSI]0.5[N1114TFSI]0.5The cycling stability of the supercapacitor of EMIMTFSI can also be improved by adding 1 A.g-1Is determined by charging and discharging the supercapacitor within 1000 cycles. A graph is then created to illustrate the trend.
Charge storage of ionic liquid based supercapacitors is not a pure adsorption process. Rather, it involves ion exchange, in which ion adsorption and ion desorption occur simultaneously. The counter ion is an ion that is oppositely charged to the electrode surface, while the co-ion has the same charge as the electrode surface. The schematic in fig. 1 shows the charge/discharge process of negative supercapacitor electrodes with different electrolyte systems (including N1114TFSI and/or EMIMTFSI). Charging occurs by simultaneously admitting and releasing a counter ion (in this case a cation) into and out of the pore (in this case an anion), while discharging is by adsorbing and desorbing the co-ion to the counter ion. To experimentally demonstrate the effect of this behavior and ionic liquid mixture on the supercapacitor charge storage mechanism, we investigated the ionic composition and distribution on charged anions due to differences in cations in the mixed electrolyte system. A supercapacitor consisting of the same reduced graphene oxide electrode (fig. 6) and a different electrolyte system was charged to 4.1V, which is considered to be the lowest highest operating voltage achievable in all electrolyte systems, and then discharged to 0V in multiple cycles. Characterization of the dried charge-discharge negative electrode was performed using X-ray photoelectron spectroscopy (XPS) to quantify the surface chemical composition.
F1s and N1s peaks that were not present in the broad scan XPS spectrum of fresh reduced graphene oxide were found in the charged and discharged reduced graphene oxide negative electrode (fig. 7(a), (d), (g)) (fig. 2 (a-c) and fig. 7 (b-c)). In the high-resolution F1s spectra of most charged electrodes except for RGO-0.5 and RGO-0.95 (indicating that the concentration of x is 0.5 and 0.95, respectively), peaks 685.7-686.1 eV correspond to the interaction between the fluorine component in the anion and the oxygen-containing functional groups remaining on the electrodeWith, 689 eV representing the CF of the anion3Components (FIG. 2(d-f) and FIG. 7 (e-f)). In the high resolution N1s spectra of all charged electrodes, two distinct peaks were also observed, peaks at 399.5-399.6 eV, which are characterized as anionic imides, while peaks at 401.9-402.7 eV represent nitrogen atoms [ EMIM ] in imidazole]Cation or ammonium [ N1114 ]]Cation of cation (FIG. 2(g-i) and FIG. 7 (h-i)). Despite the fact that nitrogen atoms can be derived from either cations or anions, and fluorine atoms can be derived from only anions, the intensity of F1s is higher than the intensity of N1s as depicted in all the broad scan XPS spectra of charged/discharged negative electrodes, consistent with previous work. This confirms the concept that anion absorption/cation desorption takes place on the negative electrode of the electrical discharge in the ionic liquid.
More specifically, the relative atomic concentration of nitrogen atoms from the cations decreases as the concentration of N1114TFSI in the electrolyte system increases. This is mainly due to the absence of double bonds in the ammonium cation of N1114TFSI (fig. 1). Without the double bond, the electron density would be more localized, localizing the net positive charge on the nitrogen atom and resulting in strong cation-anion interactions in pure N1114 TFSI. In addition to the larger molecular size of the [ N1114] cation compared to the [ EMIM ] cation, the higher charge localization in the [ N1114] cation eventually leads to a relatively lower ion mobility, resulting in higher viscosity and lower ionic conductivity in electrolyte systems with higher N1114TFSI concentrations (fig. 2 (j-k)). Thus, in electrolyte systems consisting of higher N1114TFSI concentrations, ion transport from the bulk electrolyte to the electrode pores was much slower, suggesting that earlier XPS observations indicate that relatively less fluorine and nitrogen components were found on the electrodes during charge/discharge.
Since the transmission characteristics of the electrolyte ions and the interaction between the electrodes and the electrolyte affect the maximum operating voltage, it is important to determine the maximum operating voltage of each supercapacitor containing a different electrolyte system. Stepwise Cyclic Voltammetry (CV) (FIG. 3(a-g) (i)) and galvanostatic charge-discharge (GCD) tests (FIG. 3(a-g) (ii)) of reduced graphene oxide based supercapacitors were performed to evaluate each The highest operating voltage. Figure 9 shows the complete stepped CV and GCD tests. CV and GCD testing at Low Scan rates (5mV s)-1) And low current density (0.5A g)-1) To ensure that any reactions occur, particularly those that are time consuming. The determination of the highest working voltage can be very random, similar to the inconsistency of the electrochemical stability window of electrolytes reported in the literature, due to the differences in the working and reference electrodes used and the random choice of the cutoff current density. In this work, the supercapacitor is considered to reach its maximum operating voltage before: (i) the peak value of the CV becomes large, wherein the difference in current density between the peak value and the baseline
Greater than 50% (fig. 3(h) (i)), and (ii) the GCD deviates significantly from its original symmetrical triangular shape by less than 30% (difference between charge and discharge times,
) (FIG. 3(h) (ii)). XPS spectroscopy demonstrated that the interaction between the ionic liquid and the oxygen-containing functional groups is well-accepted for the light microspeaks in CV, which contributes to the useful pseudocapacitance. However, the violent reaction represented by a large peak is disadvantageous because it rapidly deteriorates the cycle stability of the electrolyte and the supercapacitor. The heavily distorted GCD curve indicates a high resistance across the electrode/electrolyte interface, which may be a new product due to excessive oxygen-containing functional group-electrolyte interaction and/or electrolyte decomposition. Stable operation of the various electrolyte systems at the respective highest operating voltages was confirmed by Nuclear Magnetic Resonance (NMR).
Thus, the maximum operating voltages of supercapacitors based on ionic liquid electrolyte systems with x ═ 0, 0.05, 0.25, 0.5, 0.75, 0.95 and 1 were 4.7, 4.3, 4.2, 4.1 and 4.1V, respectively (fig. 4 (a))). The measured maximum operating voltage of the supercapacitor with pure emitfsi and pure N1114TFSI is much lower than the electrochemical stability window reported by the manufacturer (emitfsi 4.7V and N1114TFSI 6.1V). This further verifies that the ionic liquid can provide a practical supercapacitor operating voltage that is significantly less than the electrochemical stability window of the electrolyte itself. As expected, the higher the highest operating voltage in the supercapacitor, the higher the concentration of N1114TFSI in the electrolyte (i.e., the lower the value of x), due to the weaker interaction of electrolyte ions with oxygen-containing functional groups in electrolyte systems containing more N1114 TFSI. The maximum operating voltage of the supercapacitor in the electrolyte is not further reduced to more than 75 vol% EMIMTFSI (i.e. x ═ 0.75). The results indicate that large quantities of N1114 cations are required to weaken the electrolyte/electrode interaction. In addition to the reduced ion mobility in the electrolyte system, another factor contributing to this weaker interaction is the inhibition of pi-pi stacking between cations and sp 2-pi electrons in the reduced graphene oxide due to the lack of a ring structure in the aliphatic ammonium cations. This is evidenced by the absence of pi-pi satellite peaks in XPS C1 s spectra of charged electrodes in pure N1114TFSI and [ EMIMTFSI ]0.05[ N1114TFSI ]0.95 (fig. 8 (b-C)). The adsorption of ions on the surface of the reduced graphene oxide is inhibited without pi-pi interaction.
The electrochemical performance of the reduced graphene oxide was also evaluated, as well as the performance of the resulting supercapacitors with different electrolyte systems of x at 4.1V and their respective highest operating voltages. Fig. 4(b) and 4 (c) show CV and GCD curves for reduced graphene oxide at 4.1V, respectively, for all cases. At 4.1V, the specific capacitance and energy density in the electrolyte was found to increase with increasing volume fraction of EMIMTFSI (fig. 4 (d)). In the case where the operating voltage window is not the determining factor, viscosity is the main factor affecting specific capacitance and thus energy density. With the increase of EMIMTFSI concentration, the ion mobility is increased due to the reduction of viscosity, and the diffusion of ions from the electrolyte to the electrode is promoted, so that the following effects are realized: (1) higher Electric Double Layer (EDL) capacitance supports the GCD curve at higher EMIMTFSI concentrations with lower IR drop; (2) the pseudocapacitance is higher, which becomes more and more pronounced at the peak of 1.3V to 2.5V in the CV, indicating a more vigorous interaction between the electrolyte and the electrode (fig. 4 (b)).
On the other hand, the MW and GCD curves for reduced graphene oxide from each supercapacitor are shown in fig. 5(a) and (c), respectively. At 5 mV. s -1Low scan rate and 0.5A · g-1The CV and GCD curves are shown as quasi-rectangular (fig. 5(a)) and symmetrical triangular (fig. 5(c)), respectively, showing the capacitive characteristics of the supercapacitor when assembled with a binary ionic liquid mixture. At 200mV · s-1The CV curve slopes at high scan rates (fig. 5(b)), due to the high ohmic resistance of the electrolyte in the pores in the faster process. Through the reaction at 10 A.g-1Further confirmed the situation with higher IR drop and distorted GCD curves at high current densities (fig. 5 (d)). When measured at the corresponding highest operating voltage, at [ EMIMTFSI ]]0.5[N1114TFSI]0.5(293.1 F g-1) The highest maximum electrode specific capacitance is obtained, followed by pure 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) Pure N1114TFSI (242.8F g)-1))，[EMIMTFSI]0.05[N1114TFSI]0.95(242.3F g-1) (FIG. 5 (e)). Based on this result, a balance between maximum operating voltage and viscosity is necessary to achieve optimal electrochemical performance, although capacitance increases with increasing cell voltage due to a decrease in charge separation distance at the electrode/electrolyte interface and/or an increase in capacitance. The space charge due to the charge displacement results in a change in the internal series capacitance of the electrodes. When the electrode is in [ EMIMTFSI ] ]0.5[N1114TFSI]0.5Shows a ratio of (1) pure EMIMTFSI and [ EMIMTFSI]0.95[N1114TFSI]0.05The highest working voltage/viscosity balance can be observed obviously when the specific capacitance is higher than the maximum specific capacitance; (2) pure N1114TFSI and [ EMIMTFSI]0.05[N1114TFSI]0.95Both have high viscosity.
Although the electrode was at [ EMIMTFSI ] at 4.1V]0.5[N1114TFSI]0.5The maximum specific capacitance shown in (A) is lower than that of pure EMIMTFSI, but only 0.1V is increased in voltage, namely the maximum working voltage is 4.2V [ EMIMTFSI ]]0.5[N1114TFSI]0.5The maximum specific capacitance of the middle electrode is 12.4 F.g higher than that of the electrode at a voltage of 4.1V-1. This indicates that a slightly higher maximum operating voltage may cause a considerable increase in specific capacitance. However, this assumption only applies if at the same time the electrolyte viscosity is sufficiently low. Though [ EMIMTFSI]0.5[N1114TFSI]0.5The highest working voltage is 0.5V lower than that of pure N1114TFSI, but the specific capacitance of the electrode is found to be 50.3 F.g higher-1This is due to its improved ion mobility.
When using [ EMIMTFSI ]]0.5[N1114TFSI]0.5The maximum energy density of the supercapacitor is highest (177Wh kg)-1) Followed by pure N1114TFSI (176.5 Wh. kg)-1) Pure EMIMTFSI (161.1 Wh. 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(153.3 Wh·kg-1) And [ EMIMTFSI ]]0.75 [N1114TFSI]0.25(146.5Wh·kg-1) (FIG. 5 (f)). Unexpectedly, based on [ EMIMTFSI ]]0.5[N1114TFSI]0.5The supercapacitor of (a) shows the highest maximum energy density, although its highest operating voltage is not the maximum. This implies that the capacitance and the highest operating voltage are equally important in maximizing the energy density. However, given the quadratic relationship of the highest operating voltage to energy density, a substantially higher highest operating voltage still favors high energy density, which may help compensate for the lower specific capacitance (fig. 5 (g)). For example, while exhibiting a lower specific electrode capacitance of 50F g-1 in pure N1114TFSI, supercapacitors based on pure N1114TFSI exhibit a similar behavior to that based on [ EMIMTFSI ] ]0.5[N1114TFSI]0.5As high as the super capacitor. Further, when the maximum operating voltage is widened by 0.6V, even though the specific capacitance of the electrode in the pure N11114TFSI is lower by 38F · g than that of the electrode in the pure EMIMTFSI-1The energy density of the supercapacitor containing pure N1114TFSI is also much greater than that of the supercapacitor containing pure EMIMTFSI. When the comparison is based on [ EMIMTFSI ]]0.05[N1114TFSI]0.95And based on [ EMIMTFSI ]]0.75[N1114TFSI]0.25The same is observed for the energy density of the supercapacitor of (1).
The transmission characteristics of supercapacitors with electrolyte systems varying x were further investigated using Electrochemical Impedance Spectroscopy (EIS) at open circuit potentials in the frequency range from 1MHz to 10 MHz. The nyquist plot in fig. 5 (h) was fitted by an equivalent circuit demonstrating the frequency response in the electrode/electrolyte system. The vertical lines in the low frequency region were found to be closer to the imaginary axis as the value of x increased, indicating that the ions in the reduced graphene oxide diffused faster when used with electrolytes containing more EMIMTFSI concentrations. The results were consistent with the viscosity and ionic conductivity data shown in FIGS. 2(j) and (g-k), respectively. Furthermore, the ESR of the supercapacitor s decreases with increasing value of x, which means the charge transfer resistance (R) of the electrode/electrolyte interface when the EMIMTFSI concentration is higher ct) Low, electrolyte resistance (R)sA fraction of) is lower. In an electrolyte system. However, in pure N1114TFSI, [ EMIMTFSI]0.5[N1114TFSI]0.5And pure EMIMTFSI, only 33.4% to 57.1% of the initial specific capacitance remains in the reduced graphene oxide after 1500 consecutive GCD cycles at the corresponding maximum operating voltage. (FIG. 12). This low cycle stability may be due to increased aggregation of the ionic liquid in the pores and electrode electroactive zone, limiting the ionic adsorption and ion/electrode processes, particularly when mixing two or more ionic liquids. The interaction between the electrolyte and the electrode 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, thereby reducing the movement of ions in the electrolyte. To alleviate circulationStability issues, strategies such as adding organic solvents to the electrolyte system to reduce ion aggregation and/or removing as much oxygen-containing functional groups on the carbon-based electrode as possible to reduce electrolyte/electrode interactions may be undertaken in future work.
In the present invention, the singular forms "a", "an" and "the" mean both the singular and the plural, unless specifically stated otherwise.
The term "about" and the ranges generally used, whether or not defined by the term "about," means that the numbers understood are not limited to the exact numbers described herein, and are intended to refer to ranges substantially within the recited ranges. Without departing from the scope of the invention. As used herein, "about" will be understood by one of ordinary skill in the art and will vary to some extent in the context in which it is used. If the use of a term given the context of use is not clear to one of ordinary skill in the art, "about" means plus or minus 10% of the particular term.
Percentages (%) referred to herein are based on weight percent (w/w or w/v), unless otherwise indicated.
In the present invention, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers. Or a step, but does not exclude any other integer or step or group of integers or steps.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (5)
1. A high energy density super capacitor is characterized by comprising a reduced graphene oxide electrode and an ionic liquid electrolyte;
the ionic liquid electrolyte is imidazole ionic liquid and fatty ammonium ionic liquid, and the imidazole ionic liquid and the fatty ammonium ionic liquid have the same anion;
the imidazole ionic liquid is 1-ethyl-3-methylimidazole bis (trifluoromethylsulfonyl) imide ionic liquid; the fatty ammonium ionic liquid is butyl trimethyl ammonium bis (trifluoromethylsulfonyl) imide ionic liquid;
the volume ratio of the imidazole ionic liquid to the fatty ammonium ionic liquid is (0.05-0.95): (0.95-0.05).
2. The high energy density supercapacitor according to claim 1, wherein the volume ratio of the imidazole-based ionic liquid to the fatty ammonium-based ionic liquid is 0.5: 0.5.
3. the high energy density supercapacitor according to claim 1, wherein the reduced graphene oxide electrode has a specific surface area of 225-308 m2·g-1。
4. The high energy density supercapacitor according to claim 1, wherein the reduced graphene oxide electrode has a pore volume of 0.6-0.9 cm 3·g-1。
5. The high energy density supercapacitor according to claim 1, wherein the reduced graphene oxide electrode has a pore size of 11-13 nm.
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