polypyrrole composites for electrochemical capacitors

Synthetic Metals 203 (2015) 44–48

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Carbon/polypyrrole composites for electrochemical capacitors Katarzyna Lota, Grzegorz Lota, Agnieszka Sierczynska, Ilona Acznik * Institute of Non-Ferrous Metals Division in Poznan, Central Laboratory of Batteries and Cells, Forteczna 12, 61-362 Poznan, Poland

A R T I C L E I N F O

A B S T R A C T

Article history: Received 7 January 2015 Received in revised form 9 February 2015 Accepted 12 February 2015 Available online xxx

Composite materials made of electrically conductive polymer–polypyrrole (PPy) and carbon materials have been prepared and characterized. Oxidative chemical polymerization has been selected as the synthesis method. FeCl3 was used as oxidizing agent. Process was carried out in aqueous acidic medium. Composites were prepared by immersion of selected carbon materials in the solution of monomer and followed by addition of oxidant specimen to subjected solution under vigorous stirring. Obtained composite materials were characterized by three electrochemical methods (cyclic voltammetry, galvanostatic charging/discharging, electrochemical impedance spectroscopy) to determine capacitive parameters for further applications in the electrochemical capacitors. Capacitance of the materials varied in the range of 90–135 F g 1 and has been retained during 5000 cycles of galvanostatic charging/ discharging, working in the acidic medium (1 mol L 1 H2SO4 aqueous solutions). Moreover, the measurements confirmed a very high electrochemical stability of polypyrrole supported by carbon materials with a current load of up to 50 A g 1. ã 2015 Published by Elsevier B.V.

Keywords: Electrochemical capacitor Conducting polymer Polypyrrole/carbon material composite Performance enhancement

1. Introduction Development of energy storage systems designed for supplying portable electrical devices requires enhancement of the energy and/or power release, indeed. Devices which presently focused scientific attention as a promising solution for efficient energy storage are electrochemical capacitors. Mechanism of energy storage in these devices can be considered as two phenomena: fundamental one utilizes electrostatic accumulation of charge in electrical double layer and involves carbon materials with a well-developed surface area; thus, charge storage takes place at the electrode/electrolyte interface. The second mechanism is based on reversible redox reactions occurring on electrode materials. Example of such materials are metal oxides and conducting polymers [1–4]. Different carbon materials with their microtexture, degree of graphitization, dimensionality (from 0 to 3D) or form of occurrence (powder, fibers, foams) have been studied as a potential electrode materials in electrochemical capacitors [5]. Much attention has been focused also on conducting polymers such as polypyrrole (PPy), polyaniline, polythiophene, and their derivatives [6]. Most studied among the conducting polymers is polypyrrole because of its high conductivity, high storage ability, good thermal and environmental stability, high doping/dedoping rates and biocompatibility [7–10].

* Corresponding author. Tel.: +48 61 2797808; fax: +48 61 2797897. E-mail address: [email protected] (I. Acznik). http://dx.doi.org/10.1016/j.synthmet.2015.02.014 0379-6779/ ã 2015 Published by Elsevier B.V.

Conductive polymers can be synthesized by a number of methods, for example by chemical polymerization, chemical vapor deposition or electrochemical polymerization [11]. However, regardless of the method of preparation, in comparison to the carbon materials, their main disadvantages include reduced mechanical strength and a shorter cycle life caused by gradual deterioration of the conductive properties originating from volumetric changes during the process of doping/dedoping [12,13]. In order to improve a cycle performance and to enhance the electrical conductivity, use of PPy composites with carbon materials, which will provide a flexible skeleton adaptable to any mechanical stress seems to be a reasonable approach. Several studies reporting on the use of PPy/carbon material composites as an electrochemical capacitor electrodes can be found in the literature [12–20]. Mainly, authors reports different electrochemical characteristic of the composites originating from the different routes of synthesis and type of carbon material used. Briefly, Lee et al. [21] fabricated PPy/carbon nanotubes (CNTs) on a ceramic fabric; the specific capacitance of this composite was 152 F g 1 at 1 mA cm 2. 1 mol L 1 of LiClO4 in propylene carbonate solution was selected as electrolyte, giving operating voltage of 2 V. The ceramic material was a substrate for the growth of carbon nanotubes and chemical polymerization. Good stability even after 5000 cycles was also achieved. Ternary composites like CNTs/PPy nanofibers core shell decorated with titanium dioxide nanoparticles demonstrated the capacitance values of 282 F g 1 in 1 mol L 1 KCl electrolyte solution after few cycles. Unfortunately, the same material charged/discharged with the current load of 0.5 A g 1 after 1000 cycles showed capacitance

K. Lota et al. / Synthetic Metals 203 (2015) 44–48

retention at the level of 64% [22]. CNTs/PPy/hydrous MnO2 gave 88% of the initial capacitance after 10,000 voltammetry cycles. For the galvanostatic charging/discharging method, specific capacitance value was of 146 F g 1 during initial cycles at current load of 1 mA cm 2 and decrease rapidly after 2000 cycles reaching 35 F g 1. Applied electrolyte was 1 mol L 1 aqueous solution of Na2SO4 [23]. Zhu et al. [24] investigate materials composed of PPy and multiwalled carbon nanotubes (MWCNT) combined in various proportions. Obtained composites were examined as electrodes in coin- and pouch-type cells at various current regimes. During galvanostatic charging/discharging of coin cell an increase of capacitance was observed and were attributed by authors to microstructure changes of the active material. Capacitance retention after 5000 cycles was close to 100%. In case of pouch cell assembly, capacitance retention was 90% after 5000 cycles with values in the range of 30–40 F g 1 at current load of 1–33 mA cm 2. On the other hand, all aforementioned materials provide good capacitance values but their cycle life as well charge propagation (power performance) is rather moderate. Hence, the aim of the present study was to obtain a composite material (PPyC) composed of polypyrrole and carbon materials such as carbon nanotubes (CNTs) and carbon black (CB). Such combination allowed us to benefit from good pseudocapacitive properties of polypyrrole and enhancement of charge propagation due to carbon nanotubes or carbon black addition. Composites were obtained by chemical polymerization of pyrrole in suspension of carbon material and tested as electrodes for the electrochemical capacitor. For comparison, pure PPy was produced and subjected to electrochemical analysis. Performed electrochemical tests showed enhancement of the charge propagation in composite materials with a current load above 10 A g 1. 2. Experimental 2.1. Materials preparation and characterization Pyrrole (Py) (Aldrich), hydrochloric acid (HCl) (POCh), iron(III) chloride (FeCl3) (POCh), carbon black P 1042 (Kohlenstoff), multiwalled carbon nanotubes of two different size, O.D.  L 110–170 nm  5–9 mm and O.D.  I.D.  L 10–15 nm  2–6 nm  0.1 –10 mm (Sigma–Aldrich) were used as received from suppliers. The polymerization of pyrrole was carried out by a chemical reaction at 0  C directly on the surface of the carbon material. The specified amount of carbon material (carbon nanotubes, carbon black) was suspended in 0.1 mol L 1 aqueous solution of HCl. Subsequently, cooled liquid pyrrole was added to the suspension, kept all time under intensive stirring. An aqueous solution of FeCl3 was used as oxidizing agent; the solution was gradually added in the initial ratio of 1:2.33 (pyrrole:FeCl3) to ensure an excess of oxidant. The

45

synthesis was carried out for 1 h. Obtained composite materials were filtered, thoroughly washed with distilled water and dried at 65  C for 24 h. For comparison, pure polypyrrole was synthesized the same way but without the addition of carbon material. Finally, 9 composite materials from three carbon materials (multiwalled carbon nanotubes with two outer diameters and carbon black) in the three different percentages was obtained. The content of carbon in the composite material was adjusted to be approximately 15, 20 and 40%. Detailed composition and sample labeling are given in Table 1. The proportion of components was estimated by weighing composite materials in the dried state. Morphology of the obtained materials was observed by scanning electron microscope (SEM EVO140 ZEISS). 2.2. Electrochemical measurements The electrochemical performance of obtained materials in symmetric capacitors were studied in two and three electrode Swagelok1 systems using 1 mol L 1 H2SO4 aqueous solutions as electrolyte. The type of electrolyte has been chosen taking into account the properties of selected polymers; in case of neutral or alkaline electrolytes, conductive polymers can undergo irreversible change to the non-conductive state. In this case, the acidic environment preserves good structural stability. The capacitor electrodes were formed as pellets with 85% of active material, 10% of binder (PVDF, Kynar Flex 2801) and 5% of carbon black (to preserve good conductivity). Average mass of the electrodes in the form of pressed pellets was in range 10–12 mg. Glass microfibre paper GF/C (WhatmanTM) has been selected as a separator. All electrochemical measurements were carried out at room temperature in a voltage range from 0 to 0.8 V. The specific capacitance of electrode materials was investigated by three electrochemical techniques: cyclic voltammetry at scan rates from 1 to 1000 mV s 1, galvanostatic charging/discharging with current load ranged between 100 mA g 1 and 50 A g 1 and electrochemical impedance spectroscopy in the frequency range of 100 kHz–1 mHz using potentiostat/galvanostat VMP3 (Biologic, France). The capacitance values were expressed per active mass of one electrode. 3. Results and discussion 3.1. Structural characterization SEM images of composites with the lowest content of the carbon materials (c.a. 15%), and pure polypyrrole are shown in Fig. 1. From structural observations one can see the presence of PPy particles with spherical shape distributed on carbon surface. Carbon black and carbon nanotubes with O.D. 10–15 nm form a compact skeleton covered with aggregates of polypyrrole. Carbon

Table 1 Composition and labeling of composite materials. Designation

Type of carbon material

Content of carbon material (%)

Content of PPY (%)

PPyC1-1

Carbon black P 1042

39

61

20 15

80 85

PPyC1-2 PPyC1-3 PPyC2-1 PPyC2-2 PPyC2-3

Multiwalled carbon nanotubes OD 10–15 nm

36 19 14

64 81 86

PPyC3-1 PPyC3-2 PPyC3-3

Multiwalled carbon nanotubes OD 110–170 nm

38 23 16

62 77 87

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K. Lota et al. / Synthetic Metals 203 (2015) 44–48

Fig. 1. SEM images of (a) PPy (b) PPyC1-3 (c) PPyC2-3 (d) PPyC3-3.

nanotubes with O.D. 110–170 nm formed cylinders covered by single or multiple PPy particles. 3.2. Capacitance properties Composite materials were examined by three electrochemical techniques. The results obtained by various methods are in good accordance with each other. It has been demonstrated that the addition of carbon material improves charge propagation in the electrode material. Moreover, no significant decrease of capacitance has been observed. It should be taken into account that the capacitance of composite materials was calculated for total mass of the electrode material, not for the individual components. It has to be noticed that the capacitance of used CB or CNTs does not exceed

PPyC2-1

PPyC2-2

PPyC2-3

200

C / F g-1

150

C / F g-1

150

PPy

PPy PPyC1-3 PPyC2-3 PPyC3-3

200

0

PPyC2-3

PPyC3-3

0.5

0.7

100

100

50

50

100

PPyC1-3

150

0

200

400 600 800 Scan rate / mV s-1

1000

C / F g-1

PPy

10 F g 1, therefore, one can assume that recorded capacitance originates from polypyrrole. Fig. 2 shows the capacitance vs. scan rate dependence in the voltage range 0–0.8 V. In case of PPYC2 composites with the CNTs (10–15 nm) content of c.a. 15, 20 and 40%, a significant increase of the capacitance values of the material in relation to the pure polypyrrole is shown. The highest capacitance values were obtained for PPyC2-3 with the 14% CNTs content. The inset on Fig. 2 presents the same dependence for all used carbon materials and shows PPyC2-3 predominance over the other composites. With a maximum speed of 1000 mV s 1 capacitance of this material was of ca. 50 F g 1, where for pure PPy was negligibly low. The increase in capacitance in relation to the starting material demonstrates improved properties of the produced electrode materials, also confirmed by cyclic voltammetry (Fig. 3).

0 -50 -100

50

-150 -200

0 0

200

400

600

800

1000

Scan rate / mV s-1

0.0

0.1

0.2

0.3

0.4

0.6

0.8

U/V Fig. 2. The relationship between capacitance and scan rate for composite materials with multiwalled carbon nanotubes (O.D. 10–15 nm) and for all carbonaceous materials applied in the study in an amount of 15% (inset).

Fig. 3. Cyclic voltammogram with current values recalculated prior to scan rate of 100 mV s 1.

K. Lota et al. / Synthetic Metals 203 (2015) 44–48

PPy

175

PPyC1-3

PPyC2-3

(+) initial state (+) after 5000 cycles

5

0

-5

-10 -0.6

-0.5

-0.4

-0.3 -0.2 -0.1 0.0 E / V vs. Hg/Hg2SO4

0.1

0.2

0.3

Fig. 5. Cyclic voltammetry curves for PPyC2-3 electrode material in three-electrode system recorded at 5 mV s 1.

shown in Fig. 6. Based on this measurement method, calculated capacitance of the electric double layer for all composite materials was below 1 F/g which means that all obtained capacitance was from pseudocapacitive reactions. At lower frequencies, a difference between capacitance of fresh cell and after cycling indicate small changes in electrode material properties. Decrease of capacitance values was approximately 20 F g 1 but capacitance of 92 F g 1 has been retained until frequency of 1 Hz, giving a satisfactory result. Most important influence of CNTs or AB added as a performance support of polypyrrole working as electrode material is shown in Fig. 7. Ragone plot shows the relationship between power and energy of the system. In lower range of current densities (0.1–2 A g 1) P = f (E) dependence shows the same tendency, regardless the type of electrode material (pure polypyrrole or in composite). It has to be pointed out that energy of the system is maintained between 3–4 Wh kg 1 with power increase from 20 to 400 W kg 1. Exceeding the value of 10 A g 1 for pure polypyrrole causes dramatic power drop, whereas for composite materials a considerable improvement in operating parameters can be seen,

PPyC3-3

initial state

200

after 5000 cycles

175

150

150

125

125

100

C / F g -1

C / F g-1

(-) initial state (-) after 5000 cycles

10

I / mA

For ideal capacitor voltammetry profile should reflect a rectangular shape but such system does not exist. Any deviation is associated with the internal system disorders. This may be introduced by material resistance (low conductivity), low mechanical strength, limited access of the electrolyte into the pores of the material, irreversible charge transfer reactions, etc. All mentioned features affect propagation of electric charge in the electrode. In Fig. 3 one can observe a significant increase in the area of voltammetry curve for materials containing CNTs and AB, mostly for the PPYC2-3 composite. The use of conductive carbon skeleton has a positive effect on the propagation of electric charge by increasing the capacitance of the capacitor. Cell performance during 5000 cycles of galvanostatic charging/discharging with current load 2 A g 1 is shown in Fig. 4. All samples demonstrated good cycle stability, i.e., no significant decrease of capacitance has been observed. At the same time all electrode materials showed good efficiency during cyclic work at approximately 99.6%. The lowest capacitance values were achieved for the PPyC3-3 composite, approximately 92 F g 1, whereas the highest capacitance (c.a. 135 F g 1) was achieved for pure PPy. Materials with CNTs and AB additives give a response at similar level of about 115 F g 1. In case of pure polypyrrole and composite containing carbon nanotubes with O.D. 110–170 nm, an initial increase in the value of capacitance can be observed, which is caused by poorer wettability of electrodes in comparison with other materials. Fig. 5 shows cyclic voltammetry curves recorded at 5 mV s 1 before and after 5000 cycles of galvanostatic charging/discharging. For more in depth analysis, measurement was performed in a three-electrode system using Hg/Hg2SO4 reference electrode. This method allows monitoring the work of the individual electrodes. One can observe the small difference between registered curves as a negligible shift of the potential toward positive direction. Capacitance of mentioned electrodes changed by ca. 5 F g 1, reaching 162 F g 1 for negative electrode and 127 F g 1 for positive electrode, after charging/discharging processes. The charge of positive and negative electrode was approximately 0.128 mA h (16 mA h/g) and 0.123 mA h (15 mA h/g), respectively. Stability of electrode materials during cycle work was also observed by electrochemical impedance spectroscopy. Capacitance vs. frequency dependence for PPyC2-3 composite material before and after 5000 cycles of galvanostatic charging/discharging is

47

75 50

100 75 50

25

25 0

0 0

1000

2000

3000 Cycle

4000

5000

Fig. 4. Cycle performance of capacitors made of composite materials containing 15% of carbon materials. Current load: 2 A g 1.

1E-3

0.01

0.1

1

10

100

1000

10000 100000

f / Hz Fig. 6. Capacitance vs. frequency dependence for PPyC2-3 composite material before and after 5000 cycles of galvanostatic charging/discharging.

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K. Lota et al. / Synthetic Metals 203 (2015) 44–48

PPy

100

PPyC1-3

PPyC2-3

nanotubes demonstrated the best charge propagation as well as the highest capacitance values. Moreover, the type of carbon material used as well as the carbon material content has significant influence on the electrochemical performance of the materials obtained. It has been presented that the carbon nanotubes with smaller outer diameter are the most suitable ones; moreover, the carbon content cannot be of any value – it seems that 20% of carbon nanotubes in the composite is the limit for good electrochemical performance as well as acceptable cyclability.

PPyC3-3

10

1

PPy PPyC1-3 PPyC2-3 PPyC3-3

200

0.01

1E-3

150 C / F g -1

E / Wh kg-1

0.1

Acknowledgment This work was financially supported by the National Science Centre of Poland (Grant no. DEC-2012/07/D/ST5/02283).

100 50

References

1E-4 0 0

1E-5

10

20

30

40

50

j / A g-1

1

10

100

1000

10000

P / W kg-1 Fig. 7. Ragone plot and the relationship between capacitance and current density (inset).

even at maximum load of 50 A g 1. Relationship between capacitance and current density is shown in the inset. Higher values of capacitance (more than 100 F g 1) can be achieved below 5 A g 1, nevertheless, in the case of the composite structure, value of c.a. 75 F g 1 at a load of 50 A g 1 was obtained (for PPyC1-3 and PPyC23 materials) and represents a very satisfactory outcome. 4. Conclusions Composites of PPy and various carbon materials such as carbon nanotubes with different outer diameters as well as with carbon black have been synthesized. In mild current regimes, PPy demonstrated the highest capacitance values; at higher current loads as well as at higher scan rates, the composite with carbon

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