WO2015023193A1

WO2015023193A1 - A method for preparing mn02/oxidised carbon nanofibre composites for electrodes of an asymmetric electrochemical capacitor

Info

Publication number: WO2015023193A1
Application number: PCT/PL2014/050048
Authority: WO
Inventors: Agata ŚLIWAK; Grażyna GRYGLEWICZ
Original assignee: Wrocławskie Centrum Badań Eit+ Sp. Z O.O.
Priority date: 2013-08-14
Filing date: 2014-08-14
Publication date: 2015-02-19
Also published as: PL405072A1

Abstract

The present invention relates to a method for preparing MnO₂/carbon nanofibre composites for fabricating oxide electrodes and an asymmetric electrochemical capacitor built on the basis thereof.

Description

A METHOD FOR PREPARING MN02/OXIDISED CARBON NANOFIBRE COMPOSITES FOR ELECTRODES OF AN ASYMMETRIC ELECTROCHEMICAL CAPACITOR

The invention relates to a method for preparing MnC^/carbon nanofibre composites for fabricating oxide electrodes and an asymmetric electrochemical capacitor built on the basis thereof. The invention is applicable in the construction of electrochemical capacitors.

Carbon nanofibres (CNF), due to their unique morphology, structure, and properties, may find applications in electrochemical capacitors. Their use in electrode materials increases the efficiency of these devices, mainly by improving the electrical conductivity of the electrode. A prerequisite for the application of CNFs as an additive, is obtaining very good dispersion of the CNFs in the electrode material. CNFs, by their nature, form tightly tangled nanofibre bundles with a diameter of several tens of nm that are difficult to disentangle, even partially. Where CNF is used as component of a pseudo-capacitive composite with MnC>2 in the oxide electrode of a electrochemical capacitor, not only very good dispersion of CNF is desirable, but also good adhesion of MnC>2 particles to the surface of nanofibres. Enhancement of CNF dispersion in the electrode material and adhesion of MnC>2 particles can be achieved by modifying the chemical structure of the surface of nanofibres. The surface of the CNFs is hydrophobic, making them difficult to functionalise . Unlike carbon nanotubes, CNFs with a herringbone-like structure are a nanomaterial that is more easy to modify due to the presence of open graphene layers on the surface of fibres. The U.S. patent application US20110223480A1 presents decoration of carbon nanostructures (carbon nanotubes, carbon black) with metal and metal oxide nanoparticles . Carbon nanostructures (CNS) used in the invention have a diameter of less than 20 nm. CNS decorating nanoparticles may be metals and their alloys and metal oxides, including MnC>2. A method of depositing metal nanoparticles with the size ranging from 0.5 nm to 31 nm, depending on the conditions applied and the type of the metal, is discussed. Exemplary embodiments show the synthesis of composites containing nanoparticles of silver, gold, platinum, manganese oxide, and preparations of electrodes and electrochemistry of capacitors. And yet a composite allowing to improve the performance of electrochemical capacitors operating in neutral electrolytes, and in particular in a greater potential range, and to increase the capacitance of capacitors, their power and energy density, while maintaining cyclic stability, still has not been developed.

Symmetrical electrochemical capacitors operating in aqueous electrolytes are limited by the maximum operating voltage of about 0.8-1.0 V. This is due to the decomposition potential of water which is 1.23 V. An asymmetric arrangement where both electrodes of the capacitor are made of different materials may be used to greatly increase the operating voltage of capacitors operating in aqueous electrolytes. Commercially available capacitors are working in organic electrolytes ensuring stable performance at a voltage of about 2.5 V. Replacing a toxic organic electrolyte by an aqueous electrolyte is safe for the environment and may be achieved by using an asymmetric arrangement of electrodes in the electrochemical capacitor, where the positive and negative electrode are made of different materials. These problems have been solved by the present invention.

The first object of the invention is a method for preparing a composite of MnC>2/oxidised carbon nanofibres characterized in that it involves: a) synthesis of carbon nanofibres, preferably of herringbone type with a diameter from 10 to 80 nm;

b) oxidation of carbon nanofibres from step a) ,

c) disentangling the nanofibres from step b) , preferably by ultrasonic treatment for 2 h;

d) adding H₂O₂ or MnS0₄ to the suspension of disentangled carbon nanofibres from step c) and mixing;

e) adding the KMn0₄ solution dropwise to the suspension from step d) ;

f) drying the product from step e), preferably at 110 °C for

12 h

In a preferred embodiment of the invention the method is characterized in that the oxidized carbon nanofibres from step b) comprise at least 70% of carbonyl/quinone and hydroxyl groups in the total content of surface oxygen groups.

The second object of the invention is an oxide composite for fabrication of electrodes characterized in that it comprises from 5% to 30%, by weight, of oxidized carbon nanofibres and from 95 % to 70%, by weight, of Mn0₂," where preferably 20 wt% are oxidized carbon nanofibres and 80 wt% is MnC>₂ of said combination.

The third object of the invention is an asymmetric electrochemical capacitor operating in neutral electrolyte, preferably in a metal sulphate solution, selected from the group of alkali metals, preferably K₂S0₄, Na₂S0₄, Li₂S0₄, at a concentration from 0.5 M to 2 M, preferably 0.5 M K₂S0₄, characterized in that at least one electrode is made of a composite defined in the second object of the invention, preferably this is the positive electrode. The composite according to the invention is advantageous in that it improves the performance of electrochemical capacitors operating in neutral electrolytes. The proposed solution permits operation with a wider potential window (2.3 V) and increases capacitance, power and energy performance of capacitors, while maintaining cyclic stability. Introduction of surface oxygen groups changes the surface properties of carbon nanofibres from hydrophobic to hydrophilic. The theoretical capacitance of MnC>2 is very high (1370 F/g) and results from using the charge generated by chemical reaction with the change of the degree of oxidation of manganese (pseudocapacitance ) , but its electrical resistance is a disadvantage. To address this problem, a composite has been developed, comprising, according to the invention, MnC>2 and carbon material exhibiting high electrical conductivity, just like oxidized carbon nanofibres (CNFox) . This combination ensures high capacitance and good conductivity of the material at relatively low cost. In the synthesis of a MnC^/CNF composite, the possibly greatest surface area of nanofibres was coated with a possibly uniform layer of MnC>2. The proposed embodiment relates to an MnC^/CNF composite with pseudocapacitive properties that can be used as a positive electrode material in asymmetric electrochemical capacitors, operating in aqueous electrolytes. The main feature distinguishing the discussed solution from those known in the prior art, is fabrication of a MnC^/CNF composite material using oxidized nanofibres (CNFox) . Electrochemical capacitor, with Mn02/CNFox composite used as the positive active electrode material, is characterised by high capacitance, long-term cycling stability and high power with a wide working voltage of 2.3 V in a neutral aqueous electrolyte. The proposed solution according to the invention allows to ensure better electrochemical performance by adding highly conductive CNFs to the poorly conductive manganese oxide. The distinguishing feature of the invention is the application of surface- modified CNFs by introducing oxygen containing groups to carbon nanofibre structure. Surface modification with the use of oxygen groups is extremely important for several reasons:

- oxidized carbon nanofibres (CNFox) show improved capacitive characteristics compared to the non-oxidized ones due to the pseudocapacitance mechanism of redox reactions involving oxygen groups. The greatest improvement of electrochemical capacity can be achieved by incorporating carbonyl/quinone or hydroxyl groups into the CNF structure, making at least 70% of the total content of surface oxygen groups;

CNF oxidation changes the characteristics of the material from hydrophobic to hydrophilic, which facilitates CNFox disentangling in aqueous solution;

CNF oxidation leads to improved adhesion of Μη0₂ particles to the surface of carbon nanofibres, thus improving the distribution of manganese oxide in the electrode material;

Exemplary embodiments of the invention are presented in the drawing, where: Fig. 1 shows SEM and TEM images of carbon nanofibres; Fig.2 illustrates graphs of potentiodynamic curves obtained for Mn0₂/CNF and Mn0₂/CNFox composites with component mass ratio of 1:1 in 0.5 M K₂S0₄, at scan rates of 10 and 100 mV/s, Fig. 2a and 2b respectively; Fig.3 illustrates results of galvanostatic measurements for Mn0₂/CNF and Mn0₂/CNFox composites with component mass ratio of 1:1 in 0.5 M K₂S0₄; Fig.4 shows potentiodynamic curves of Mn0₂/CNFox composites with different shares of components, obtained at various scan rates in 0.5 M K₂S0₄; Fig.5 graphically illustrates relation between capacitance of Ml, M2 and M3 capacitors and current loads, Fig.6 shows TEM images of M2 composites, Fig. 6a, b and M3 , Fig. 6c, d; Fig.7 illustrates the results of measurements of cyclic stability of an asymmetric capacitor with M2 composite as a positive electrode; Fig.8 shows measurement results of cyclic stability of an asymmetric capacitor with M3 composite as a positive electrode; Fig.9 illustrates Ragone plots obtained for asymmetric capacitors with M2 and M3 as positive electrodes. Embodiment 1 Synthesis and oxidation of herringbone-type carbon nanofibres

Carbon nanofibres were synthesized by catalytic chemical vapour deposition (CCVD) , using a nickel catalyst, and methane as a a source of carbon. A nickel catalyst, containing 14% Ni, by weight, deposited on AI₂O₃ as a support, was used for the synthesis of CNF. The catalyst was prepared by wet impregnation using an aqueous solution of nickel nitrate Ni (NC>3)2_A as the precursor of the active phase. The synthesis of nanofibres was carried out at 650°C, using a mixture of CH₄ and ¾ gases, in a volume ratio of 1:1 (9/9 1/h) for 1 h. Synthesis was preceded by a reduction of the catalyst in the flow of ¾ (9 1/h) at 700 ⁰ C for 2 h.

Nanofibres were separated from the catalyst in concentrated HF with sonication-assisted mixing for 1 h. SEM and TEM images of herringbone type carbon nanofibres obtained are shown in Fig. 1, where a) is an SEM image, and b) and c) are TEM images of CNFs obtained.

Oxidation of carbon nanofibres was performed using concentrated HNO₃ for 12 h at 45°C on a magnetic stirrer under a return cooler to introduce surface carbonyl/quinone groups exhibiting pseudocapacitance . After oxidation, the sample was separated on a Teflon suction filter, washed to achieve a neutral pH of the filtrate, and then dried for 12 h at 110 °C. Embodiment 2 Synthesis of manganese oxide on oxidized carbon nanofibres

Oxidized CNFs were wetted with alcohol, Milli-Q water was added, and then sonicated for 2 h. As a result of ultrasonic treatment, carbon nanofibres had been disentangled which increased the CNF surface area for Mn0₂ deposition.

To a suspension of oxidized disentangled CNFs, an adequate quantity of Mn(N03)2 was added, so that the CNFox share in the composite is 50 wt%. The mixture was stirred on a magnetic stirrer and a hot ΚΜη0₄ solution was added dropwise. The reaction was carried out until disappearance of the characteristic purple colour. The resulting MnC^/CNF composite was washed with Milli-Q water, filtered, and then dried at 110°C for 12 h. The Mn0₂/CNF mass ratio was 1:1. Electrochemical measurements of these composites were carried out in a symmetrical two-electrode system, using 0.5 M K₂SO₄ as electrolyte .

Potentiodynamic measurements

Graphs of potentiodynamic curves obtained for capacitors made of Mn0₂/CNF and MnC^/CNFox composites containing non-oxidized and oxidized CNFs in 0.5 M K₂SO₄, at scan rates of 10 and 100 mV/s, are shown in Fig. 2a and b. Use of CNFox in the oxide composite of the electrode material has increased the capacitance of capacitors compared to a composite comprising a non-oxidized CNFs. This is due to the hydrophilic characteristics of the material, which favours disentangling of CNFs and improves Mn0₂ adhesion to oxidized carbon nanofibres compared to non-oxidized CNFs, and to pseudocapacitance of oxidized CNFs.

Galvanostatic measurements

Compared to MnC^/CNF, the MnC^/CNFox composite features higher capacitance value in the whole current density range tested. The results of galvanostatic measurements of capacitors carried out in 0.5 M K₂S0₄ are shown in Fig. 3. The Mn0₂/CNF composite has lower capacitance values and becomes less stable as the current load increases, resulting in a greater decrease in capacitance, see Fig. 3, showing a correlation between capacitance and current loads of oxide composites.

Impedance measurements

The equivalent series resistance (ESR) measured by impedance spectroscopy is shown in Table 1. Capacitor resistance dropped from 0.69 to 0.46 Ω through the use of oxidised and disentangled herringbone type nanofibres.

Table 1. Equivalent series resistance (ESR) measured by

impedance spectroscopy, Ω

Mn0₂/CNF Mn0₂/CNFox

0.5 M K₂S0₄

0.69 0.46

Embodiment 3 Optimization of Mn0₂ and CNFox shares in the composite

In order to determine the optimal share of Mn0₂ in the Mn0₂/CNFox composite, a series of materials with different CNFox contents were synthesized. Mn0₂:CNFox share varied from 10:0 to 5:5. Mn0₂ was deposited by a chemical suspension method using KMn0₄ and MnS0₄ on the surface of CNFox, pre-dispersed by ultrasonic treatment for 2 h.

CNFox was weighed into a flask, thoroughly wetted with alcohol, and then adequate quantities of Milli-Q water and MnS0₄ were added. The mixture was dispersed by ultrasonic treatment for 2 h, and then stirred with magnetic stirrer. The adequate amount of KMn0₄ was dissolved in Milli-Q water and heated to 95°C. The hot solution of potassium permanganate was added dropwise to a mixture containing CNFox and dissolved MnS0₄. The reaction was carried out until KMn0₄ was fully reduced, i.e. the characteristic purple colour disappeared.

Potentiodynamic measurements

Fig. 4 presents potentiodynamic curves of capacitors made of Mn0₂/CNFox composites containing various shares of components and tested in 0.5 M K₂S0₄. Mn0₂ demonstrated the worst electrochemical performance due to its poor conductive properties. Among the materials containing CNFox, at a scan rate of 10 mV/s, the lowest capacitance values were achieved for Mn0₂/CNFox 5:5 (66 F/g) and the highest for Mn0₂/CNFox 9:1

(107 F/g) . At a scan rate of 100 mV/s, capacitors made of Mn0₂/CNFox 8:2, Mn0₂/CNFox 7:3 and Mn0₂/CNFox 6:4 composites performed very similarly, both in terms of capacitance and potentiodynamic curve shape. This can be explained by the presence of conductive CNFox in the composite material, ensuring more stable operation at high scan rates. The decrease in capacitance of Mn0₂/CNFox 9:1 capacitor at a scan rate of 100 mV/s results from the poor conductivity of the material caused by the high Mn0₂ content in the composite. A 20% share of the oxidized carbon nanofibres in the oxide composite seems to be an optimum share.

Impedance measurements

Impedance measurement results are shown in Table 2. The capacitor built only of Mn0₂ was found to have the greatest resistance - 1.34 Ω. Use of CNFox in the composite significantly reduced the resistance of the system, which varied from 0,92 to 1,34 Ω. In the case of composite Mn0₂/CNFox 8:2, capacitor resistance was 1.08 Ω in 0.5 M K₂S0₄.

Table 2. Equivalent series resistance (ESR) of the composite depending on the composition of ΜηΟ_∑/ CNFox composite, Ω

0.5 M K₂S0₄

5 : 5 6 : 4 7:3 8:2 9 : 1 10 : 0

1.01 0. 92 1.14 1.08 1.09 1.34

In view of the results of electrochemical measurements, system stability at high scan rates and high current densities, and bearing in mind the aspects of potential commercial use of the composite, the Mn0₂/CNFox 8:2 composite was selected for further research. Manganese oxide has a lower price than the carbon nanofibres, hence minimisation of their share in the composite while maintaining good conductivity of the composite material .

Embodiment 4 Optimization of reagents for MnC>2 synthesis in Mn02/CNFox composites

Three different combinations of reagents used in the synthesis of MnC>2, as summarized in Table 3, have been tested. Quantities of reagents were selected to suite the reactions. The resulting composite contained 80 wt% of MnC>2 and 20 wt% of CNFox. Synthesis of MnC>2 was carried out on carbon nanofibres pre-dispersed with ultrasonic treatment for 2 h.

Table 3. Methods for Mn0₂ synthesis on carbon nanofibres

Symbol Reagents Description of the method

A hot KMn0₄ solution was added

Ml Mn(N0₃)₂ + KMn0₄ dropwise to Mn(N03)2 solution

containing dispersed CNFox

A hot KMn0₄ solution was added

M2 MnS0₄ + KMn0₄ dropwise to MnS0₄ solution

containing dispersed CNFox

A hot KMn0₄ solution was added

M3 KMn0₄ + H₂0₂ dropwise to H2O2 solution

containing dispersed CNFox

Reactions with KMn0₄ proceeded until the purple colour characteristic of permanganate disappeared. All composites were washed to achieve a neutral pH of the filtrate and dried at 110 °C for 12 h.

Galvanostatic measurements

Table 4 compares results of galvanostatic measurements of Ml, M2 and M3 composites. The M3 composite was found to have the best electrochemical performance across the whole range of current loads tested. Capacitance of this composite at charging current of 0.2 A/g is 128 F/g, just like that of M2.

Capacitance values obtained for both capacitors are similar, only at charging current of 2 A/g differences can be observed. The M3 composite exhibits higher capacitance values compared to M2. At charging current of 5 A/g, capacitance values of Ml, M2 and M3 capacitors were 3, 15 and 23 F/g, respectively. The Ml composite shows the worst electrochemical performance of the three composites, indicating a significant effect of reagents used for the synthesis of manganese oxide. Table 4. Capacitance of MnO_∑/ CNFox synthesized with a variety of reagents at different charging currents

Charging Capacitance, F/g

current, A/g Ml M2 M3

0.2 102 128 128

0.5 80 87 89

1 52 70 2

2 19 47 54

5 3 15 23

1 2 4 7

20 0 1 1

At higher current densities, the differences between composites were more pronounced. For 10 A/g, capacitance of M3 capacitor is about two-fold higher compared to M2. Fig.5 shows capacitance graphs of MnC>2/CNFox capacitors relative to current loads in 0.5 M K₂S0₄. The obtained results allow to rank the composites by decreasing capacitance with M3>M2>M1. The increase in capacitance is probably due to a different structure of M3 composite resulting from the use of KMn0₄ and H2O2 as reagents for the synthesis of MnC>2. TEM images of M2 composites are shown in Fig. 6a, b, and TEM images of M3 composite in Fig. 6c, d. Impedance measurements

Impedance measurement results are shown in Table 5. Composite resistance ranged from 0. 86 to 1.08 Ω. The lowest resistance value was obtained for the M3 composite. Differences between composites are minor, indicating that the use of various reactants in MnC>2 synthesis greatly affects the capacitive characteristics of capacitors, but has only a minor effect on resistance .

Table 5. Equivalent series resistance (ESR) of MnO_∑/ CNFox systems synthesized with various reagents in 0.5 M K₂SO₄, Ω

Ml M2 M3

1.01 1.08 0.86

Embodiment 5 Asymmetric system

In the present invention, the MnC^/CNFox composite is the active material of the positive electrode, while the active carbon - obtained in the activation process of mesophase pitch-based carbonization products with potassium hydroxide - serves as the negative electrode.

Results of galvanostatic measurements of M2 composite during 5000 charge/discharge cycles with 1 A/g current in voltage ranges of 2.05; 2.3 and 2.6 V are shown in Fig. -8- 7. Capacitance of an asymmetric capacitor with the working voltage of 2.3 V, with M2 composite serving as the positive electrode, is about 150 F/g, which is 20% higher than in the case of measurements conducted with the voltage of 2.05 V. The material working in a wider potential window (2.6 V) exhibits lower capacitance of about 100 F/g.

Similar electrochemical measurements were performed with a M3 composite serving as the positive electrode. The measurement results for capacitors containing M3 material for the three selected voltage ranges are shown in Fig. 8, i.e. cyclability of asymmetric capacitors with M3 as a positive electrode during the charge/discharge process with the current of 1 A/g in 0.5 M K₂SO₄ at working voltages of 2.05; 2.3 and 2.6 V at 5000 cycles. A similar tendency was observed as in the case of capacitor with M2 composite (Fig. 7) . The obtained capacitance values are about 10% higher compared to M2. In addition, the capacitor with M3 exhibits a shorter formatting time, with the exception of 2.6 V potential window. The operating voltage of 2.6 V was found to be too high to ensure the desired stable operation of tested capacitors with M2 and M3.

Embodiment 6 Energy/power relation of asymmetric capacitors For the asymmetric capacitor in which M2 and M3 serves as a positive electrode, the characteristics of energy/power relation were determined. Fig. 9 shows Ragone plots, i.e. the energy/power relation of asymmetric systems in 0.5 M K₂S0₄ operating at different voltage limits with positive electrodes M2 and M3, and Table 6 shows the energy density values determined for the relevant power densities of 20, 100, 1000, and 10 000 W/kg obtained for capacitors with M2 and M3 composite, respectively. Analysis of Ragone plots suggests that extending the potential window to 2.3 V has a beneficial effect on the amount of energy recovered, irrespectively of power density for both M3 and M2. With the minimum power density (20 W/kg), the energy density is 28 and 31 Wh/kg, for M2 and M3 respectively. In the case of M2, the amount of energy recovered increased from 19 Wh/kg for U=2.05 V to 28 Wh/kg for U=2.3 V. Increasing the operating voltage of the capacitor caused a ca. 50% increase in energy for similar power density. Capacitors made of both M2 and M3 exhibit high energy density in excess of 10 Wh/kg achieved for power density of 1000 W/kg. Where U=2.6 V, a drastic decrease in energy density, even at power densities in excess of 100 W/kg, can be observed. This is caused by the excessive operating voltage of the capacitor.

Table 6. Energy density values of the capacitor for input power densities of 20, 100, 1000 and 10000 W/kg for asymmetric systems operating at different voltage limits.

M2

2. 05 V 2.3 V 2.6 V

E, Wh/kg P, W/kg E, Wh/kg P, W/kg E, Wh/kg P, W/kg

19 19 28 21 24 19

14 97 25 104 16 93

4 971 12 1036 0 925

0 9709 2 10363 0 9251

M3

2.3 V 2.6 V

E, Wh/kg P, W/kg E, Wh/kg P, W/kg

31 22 25 17

26 109 17 83

8 1089 0 826

3 10893 0 E 5258

Claims

1. A method for preparing MnC^/carbon nanofibre composites wherein it comprises: a) synthesis of carbon nanofibres, preferably herringbone type with a diameter of 10 to 80 nm;

b) oxidation of carbon nanofibres from step a) ,

c) disentangling nanofibres from step b) , preferably by ultrasonic treatment for 2 h;

d) adding H₂O₂ or MnSC>4 to the suspension of disentangled carbon nanofibres from step c) and mixing;

e) adding the KMnC>4 solution dropwise to the suspension from step d) ;

f) drying the product from step e), preferably at 110 °C for 12 h.

2. A method according to claim 1 wherein the oxidized carbon nanofibres from step b) comprise at least 70% of carbonyl /quinone and hydroxyl groups in the total content of surface oxygen groups;

3. An oxide composite for fabrication of electrodes, wherein it comprises from 5% to 30%, by weight, of oxidized carbon nanofibres and from 95% to 70%, by weight, of Mn0₂," where preferably 20 wt% are oxidized carbon nanofibres and 80 wt% is MnC>₂ of said combination.

4. An asymmetric electrochemical capacitor operating in neutral electrolyte, preferably in a metal sulphate solution, selected from the group of alkali metals, preferably K₂S0₄, Na₂S0₄, Li₂S0₄, at a concentration from 0.5 M to 2 M, preferably 0.5 M K₂S0₄, wherein at least one electrode is made of a composite defined in the third object of the invention, preferably it is the positive electrode.