P-toluenesulfonate doped polypyrrole/carbon composite electrode and a process for the preparation thereof

Abstract

Polypyrrole/carbon (PPy/C) composite doped with organic anion p-toluenesulfonate (pTS) is utilized as an electrode in supercapacitor for energy storage application. The surface initiated in-situ chemical oxidative polymerization yields a composite material PPy/C in the presence of varying concentrations of pTS. The novelty of the present invention lies in the doping of PPy/C composite with organic anion pTS and consequent enhancement of its electrochemical activity and stability. The conjugation length and electrical conductivity of pTS doped PPy/C composites increase with the increase in dopant concentration. The pTS doped PPy/C composite synthesized using equimolar concentration (0.1 M) of pTS to pyrrole shows the maximum specific capacitance of 395 F/g in 0.5 M Na.sub.2SO.sub.4 aqueous solution with significant stability 95% capacitance retention after 500 cycles.

Claims

1. A process for preparing a p-toluenesulfonate (pTS) doped conducting polypyrrole/acid-treated carbon black composite comprising the steps of: i) treating carbon black with 6 M HNO.sub.3 for 2-4 h; ii) ultrasonic dispersion of treated carbon black as obtained in step (i) in water for 50 to 70 min to form a uniform suspension; iii) adding 0.1 M distilled pyrrole monomer to the suspension as obtained in step (ii) followed by adding 0.06-0.1 M pTS at temperature in the range of 273-280 K to obtain a solution; iv) constantly stirring the solution of step (iii) under inert atmosphere for 30-45 min at 273-280 K to obtain a stirred solution; v) adding 10-30 ml of ammonium persulfate (APS) solution comprising a molar ratio of ammonium persulfate to pyrrole of 1:1 dropwise to the stirred solution of (iv) with constant stirring for period in the range of 8-12 h to obtain a precipitate; vi) filtering and washing of the precipitate obtained from (v) and keeping it in an oven at 313-353 K for 10-14 h to obtain a p-toluenesulfonate (pTS) doped conducting polypyrrole/acid-treated carbon black composite.

2. The process for preparing a p-toluenesulfonate (pTS) doped conducting polypyrrole/acid-treated carbon black composite according to claim 1, wherein the washing of the precipitate obtained from (v) comprises washing with methanol and water.

3. The process for preparing a p-toluenesulfonate (pTS) doped conducting polypyrrole/acid-treated carbon black composite according to claim 1, wherein the inert atmosphere comprises a gas selected from the group consisting of nitrogen, argon and helium.

4. A process for preparing an electrode for a supercapacitor comprising the steps of: i) treating carbon black with 6 M HNO.sub.3 for 2-4 h; ii) ultrasonic dispersion of treated carbon black as obtained in step (i) in water for 50 to 70 min to form a uniform suspension; iii) adding 0.1 M distilled pyrrole monomer to the suspension as obtained in step (ii) followed by adding 0.06-0.1 M pTS at temperature in the range of 273-280 K to obtain a solution; iv) constantly stirring the solution of step (iii) under inert atmosphere for 30-45 min at 273-280 K to obtain a stirred solution; v) adding 10-30 ml of ammonium persulfate (APS) solution comprising a molar ratio of ammonium persulfate to pyrrole of 1:1 dropwise to the stirred solution of (iv) with constant stirring for period in the range of 8-12 h to obtain a precipitate; vi) filtering and washing of the precipitate obtained from (v) and keeping it in an oven at 313-353 K for 10-14 h to obtain a p-toluenesulfonate (pTS) doped conducting polypyrrole/acid-treated carbon black composite, a) adding 80-90 wt. % of the p-toluenesulfonate (pTS) doped conducting polypyrrole/acid-treated carbon black composite and 10-20 wt. % of a polyvinylidene fluoride (PVDF) in N,N-dimethylformamide (DMF) followed by ultrasonicating for 50 to 60 min to form a uniform slurry; b) coating a platinum (Pt) disk working electrode with the slurry obtained in step a to cover only a top active surface of the electrode; c) drying the electrode in an oven at 313-353 K for 30-45 min to obtain the electrode.

5. The process for preparing an electrode according to claim 4, wherein specific capacitance of the electrode is 395 F/g in 0.5 M Na.sub.2SO.sub.4 aqueous solution with 95% capacitance retention after 500 cycles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: FTIR of PPy/C composite samples; E1, E2, E3, E4 and E5.

(2) FIG. 2: Raman spectra of PPy/C composite samples; E1, E2, E3, E4 and E5.

(3) FIG. 3: Room temperature (300K) dc conductivity (.sub.dc) of PPy/C composites as a function of dopant (pTS) concentration.

(4) FIG. 4:a) Cyclic voltammogram (CV) curves of (a) synthesized PPy/C composite (E1, E2, E3, E4 and E5) electrodes in 0.5 M Na.sub.2SO.sub.4 aqueous solution at 50 mV s.sup.1 and (b) sample E5 with increasing scan rate (5-200 mV/s) in 0.5 M Na.sub.2SO.sub.4 aqueous solution.

(5) FIG. 5: Specific capacitance of PPy/C composite electrodes; E1, E,2, E3, E4 and E5 (a) at various scan rates (5-200 mV/s) in 0.5 M Na.sub.2SO.sub.4 aqueous solution and (b) during the cycle test at 100 mV/s in 0.5 M Na.sub.2SO.sub.4 aqueous solution.

(6) FIG. 6: (a) Nyquist plot and (b) the specific capacitance as a function of frequency for PPy/C composite electrodes; E1, E2, E3, E4 and E5.

DETAILED DESCRIPTION OF THE INVENTION

(7) The primary basis of the present invention is to provide a composite of polypyrrole/carbon (PPy/C) doped with an organic dopant anion (p-toluenesulfonate) having the enhanced specific capacitance and capacitance retention ability, which will be useful in the development of composite electrode for supercapacitor.

(8) Accordingly in the present invention the polypyrrole/carbon (PPy/C) composites have been doped with varying concentration of p-toluenesulfonate (pTS) by surface initiated in-situ chemical oxidative polymerization with a purpose to develop an electrode material for supercapacitors.

(9) The influence of pTS on the structure of the composite is observed through Fourier transform infrared (FT-IR) and Raman spectroscopy. EDAX was performed to estimate the S/N ratio. The performance of PPy/C composite electrode for charge storage has been analyzed using electrochemical tools such as cyclic voltammetry and electrochemical impedance spectroscopy. The maximum specific capacitance 395 F/g in 0.5 M Na.sub.2SO.sub.4 aqueous solution with significant stability over 500 cycles is obtained for the material synthesized using equimolar concentration (0.1 M) of pTS to pyrrole (Py).

(10) The method generally used to prepare conducting polymers are i) chemical polymerization in solution, ii) chemical vapour deposition and iii) electrochemical polymerization.

(11) The general scheme for preparation of conducting polymers is oxidative coupling, which involves the oxidation of monomers to form cation radical followed by coupling to form dications and repetition of process to produce a polymer. Oxidative coupling can be done by electrochemical or chemical polymerization process. By selection of suitable oxidant, synthetic medium and monomer, one can perform polymerization process to obtain a desired polymer or polymer composite.

(12) Accordingly the present invention provides a porous conducting polymer encapsulated carbon composite having dc electrical conductivity ranging between 0.98-6.85 S/cm (See the nomenclature of the samples in Table 1), and a process for the preparation of the said conducting polymer/carbon composite which comprises i) distilling pyrrole by known methods; ii) treatment of Vulcan carbon with 6 M HNO.sub.3 for 2-4 h; iii) ultrasonic dispersion of treated Vulcan carbon in 100 ml distilled water for 60 min to form a uniform suspension; iv) addition of distilled pyrrole monomer (0.1 M) from (i) to the above mentioned suspension of (iii); v) temperature of the reaction solution of (iv) was maintained at 275 K using Julabo low temperature bath FP-50; vi) constant stirring of reaction solution under inert atmosphere (N.sub.2) for 30 min at 275 K; vii) addition of 10-30 ml of ammonium persulfate (APS) (oxidant) solution (molar ratio of APS:Py 1:1) dropwise to the reaction solution of (iv); viii) constant stirring of the reaction solution of (vii) for 8-12 h; ix) washing the resultant precipitate from (viii) with copious amount of methanol and deionized water to remove any trace amount of impurities; x) filtration and washing of the precipitate obtained from (ix) and keeping it in an oven at 313-353 K overnight; xi) for the synthesis of pTS doped PPy/C composite samples, varying concentration of pTS (0.00-0.15 M) were added with pyrrole monomer prior to addition of oxidant (APS) i.e. before step (vii), keeping the rest of polymerization procedure same. The nomenclature of these synthesized samples is described in Table 1; xii) PPy/C slurry was made using 80-90 wt. % active material (10 mg) and 10-20 wt. % polyvinylidene fluoride (PVDF) in N,N-dimethylformamide (DMF) and was ultrasonicated for 60 min to form a uniform suspension; xiii) the platinum (Pt) disk working electrode was coated with the active material, wherein, a drop of this slurry from (xii) was carefully released on the disk electrode so that it covers only the top active surface of the electrode; xiv) this electrode was then left to dry in an oven at 313-353 K for 30 min; xv) similar procedure was adopted for making electrodes for other pTS doped PPy/C composites.
Characterization of P-Toluenesulfonate Doped Polypyrrole/Carbon Composites

(13) Fourier transform infrared (FT-IR) of PPy/C composites was performed by IR spectrometer (Cary 630, Agilent Technologies) using Diamond ATR accessory. Raman spectra were recorded using Renishaw Raman Spectrometer, Germany with laser excitation source of 714 nm. The laser power was reduced to 2 mW to avoid destruction of the samples. The dc electrical conductivity (.sub.dc) of these PPy/C composites was measured on pressed pellets using collinear four-point probe method. EDAX of the sample was carried out using Zeiss microscope model EVO MA-10 equipped with Oxford INCA EDX microprobe.

EXAMPLES

(14) The following examples are given by way of illustration of the present invention and should not be construed to limit the scope of the present invention:

Example 1

(15) The PPy/C composites were prepared using in-situ chemical oxidative polymerization technique in aqueous medium. The temperature of the reaction solution was maintained at 2 C. using Julabo low temperature bath FP-50. The Vulcan-carbon was pre-treated with 6 M HNO.sub.3 for 2 h prior to PPy/C synthesis. Firstly, 20 wt. % of acid activated Vulcan-carbon was dispersed ultrasonically in 100 ml D.I. water for 60 min to form a suspension. Subsequently, pyrrole monomer (0.1 M) was added to this reaction solution and was stirred under inert atmosphere (N.sub.2) for 30 min. Then, APS (oxidant) was dissolved in 20 ml D.I. water (molar ratio of APS:Py, 1:1) and added to the reaction solution drop-wise under constant stirring. The polymerization was made to continue under constant stirring for 8 h. This resultant PPy/C composites was then washed with copious amount of methanol and deionized water to remove any trace amount of impurities. Afterwards, the sample was successively filtered and kept overnight (12 h) at 60 C. in oven. The material thus prepared was named as sample E1.

Example 2

(16) The PPy/C composites were prepared using in-situ chemical oxidative polymerization technique in aqueous medium. The temperature of the reaction solution was maintained at 2 C. using Julabo low temperature bath FP-50. The Vulcan-carbon was pre-treated with 6 M HNO.sub.3 for 2 h prior to PPy/C synthesis. Firstly, 20 wt. % of acid activated Vulcan-carbon was dispersed ultrasonically in 100 ml D.I. water for 60 min to form a suspension. Subsequently, pyrrole monomer (0.1 M) was added to this reaction solution along with p-toluenesulfonate (0.01 M) and was stirred under inert atmosphere (N.sub.2) for 30 min. Then, APS (oxidant) was dissolved in 20 ml D.I. water (molar ratio of APS:Py, 1:1) and added to the reaction solution drop-wise under constant stirring. The polymerization was made to continue under constant stirring for 8 h. This resultant PPy/C composites was then washed with copious amount of methanol and deionized water to remove any trace amount of impurities. Afterwards, the sample was successively filtered and kept overnight (11 h) at 60 C. in oven. The material thus prepared was named as sample E2.

Example 3

(17) The PPy/C composites were prepared using in-situ chemical oxidative polymerization technique in aqueous medium. The temperature of the reaction solution was maintained at 2 C. using Julabo low temperature bath FP-50. The Vulcan-carbon was pre-treated with 6 M HNO.sub.3 for 2 h prior to PPy/C synthesis. Firstly, 20 wt. % of acid activated Vulcan-carbon was dispersed ultrasonically in 100 ml D.I. water for 60 min to form a suspension. Subsequently, pyrrole monomer (0.1 M) was added to this reaction solution along with p-toluenesulfonate (0.03 M) and was stirred under inert atmosphere (N.sub.2) for 30 min. Then, APS (oxidant) was dissolved in 20 ml D.I. water (molar ratio of APS:Py, 1:1) and added to the reaction solution drop-wise under constant stirring. The polymerization was made to continue under constant stirring for 8 h. This resultant PPy/C composites was then washed with copious amount of methanol and deionized water to remove any trace amount of impurities. Afterwards, the sample was successively filtered and kept overnight (12 h) at 60 C. in oven. The material thus prepared was named as sample E3.

Example 4

(18) The PPy/C composites were prepared using in-situ chemical oxidative polymerization technique in aqueous medium. The temperature of the reaction solution was maintained at 2 C. using Julabo low temperature bath FP-50. The Vulcan-carbon was pre-treated with 6 M HNO.sub.3 for 2 h prior to PPy/C synthesis. Firstly, 20 wt. % of acid activated Vulcan-carbon was dispersed ultrasonically in 100 ml D.I. water for 60 min to form a suspension. Subsequently, pyrrole monomer (0.1 M) was added to this reaction solution along with p-toluenesulfonate (0.06 M) and was stirred under inert atmosphere (N.sub.2) for 30 min. Then, APS (oxidant) was dissolved in 20 ml D.I. water (molar ratio of APS:Py, 1:1) and added to the reaction solution drop-wise under constant stirring. The polymerization was made to continue under constant stirring for 8 h. This resultant PPy/C composites was then washed with copious amount of methanol and deionized water to remove any trace amount of impurities. Afterwards, the sample was successively filtered and kept overnight (12 h) at 60 C. in oven. The material thus prepared was named as sample E4.

Example 5

(19) The PPy/C composites were prepared using in-situ chemical oxidative polymerization technique in aqueous medium. The temperature of the reaction solution was maintained at 2 C. using Julabo low temperature bath FP-50. The Vulcan-carbon was pre-treated with 6 M HNO.sub.3 for 2 h prior to PPy/C synthesis. Firstly, 20 wt. % of acid activated Vulcan-carbon was dispersed ultrasonically in 100 ml D.I. water for 60 min to form a suspension. Subsequently, pyrrole monomer (0.1 M) was added to this reaction solution along with p-toluenesulfonate (0.1 M) and was stirred under inert atmosphere (N.sub.2) for 30 min. Then, APS (oxidant) was dissolved in 20 ml D.I. water (molar ratio of APS:Py, 1:1) and added to the reaction solution drop-wise under constant stirring. The polymerization was made to continue under constant stirring for 8 h. This resultant PPy/C composites was then washed with copious amount of methanol and deionized water to remove any trace amount of impurities. Afterwards, the sample was successively filtered and kept overnight (12 h) at 60 C. in oven. The material thus prepared was named as sample E5.

Example 6

(20) Preparation of the Electrode

(21) PPy/C slurry was made using 90 wt. % active material (E1 to E5 as prepared in example 1 to 5 respectively) (10 mg) and 10 wt. % polyvinylidene fluoride (PVDF) in N,N-dimethylformamide (DMF) and was ultrasonicated for 60 min to form a uniform suspension. To coat platinum (Pt) disk working electrode with the active material, a drop of this slurry was carefully released on the disk electrode so that it covers only the top active surface of the electrode. This electrode was then left to dry in oven at 323 K for 30 min. Similar procedure was adopted in making electrode for other pTS doped samples of PPy/C.

Example 7

(22) The electrochemical performance of the synthesized electrodes were analyzed using Autolab PGSTAT 302N (Eco Chemie, The Netherlands) operating with computer controlled software NOVA 1.9 at room temperature. The electrolytic cell consists of three electrode one compartment cell having PPy/C modified Platinum (Pt) (area 0.07 cm.sup.2), Pt sheet and Ag/AgCl (3 M KCl) as working, counter and reference electrodes, respectively. The electrochemical behavior of the composite samples (E1 to E5) was investigated using cyclic voltammetry in the potential window 0.2 to 0.8 V (vs Ag/AgCl) at various successive scan rates (5-200 mV/s). The electrolyte used for the electrochemical investigation contains 0.5 M Na.sub.2SO.sub.4 aqueous solution. The electrochemical impedance spectroscopy was performed in the range 100 kHz to 0.1 Hz at an open circuit potential (OCP) with an equilibrating time of 15 min. Prior to any electrochemical investigation the solution was purged with inert gas (N.sub.2) for 30 min and a slight overpressure of the same was attained during the experiment. It can be seen from FIG. 5(b) that the sample E1 shows decrease in specific capacitance (Cs) from 187 F/g initially to 70 F/g at 500th cycle, however, with increase in dopant concentration (pTS) the specific capacitance of the material is found to improve and is stable for a higher number of cycles. The sample E5 synthesized with an equi-molar concentration (0.1 M) of pTS to pyrrole not only possess high specific capacitance (395 F g.sup.1) than other samples but also shows better retentivity of the same, even after 500 cycle at a high scan rate of 100 mV s.sup.1.

(23) TABLE-US-00001 TABLE 1 Samples nomenclature of polypyrrole/carbon (PPy/C) composites with their respective dopant concentration, room temperature (300K) dc conductivity (.sub.dc), conjugation length and S/N ratio. dc electri- Specific Specific cal con- Capaci- Capaci- ductivity tance tance Sam- at 300K Conju- (1.sup.st (500.sup.th ple Pyrrole pTS (.sub.dc) gation S/N cycle) cycle) Name (M) (M) (S/cm) length ratio F/g F/g E1 0.10 0.00 0.98 1.38 187 70 E2 0.10 0.01 1.30 1.56 0.3814 226 150 E3 0.10 0.03 2.50 1.75 0.3992 305 208 E4 0.10 0.06 4.63 1.96 0.4201 318 250 E5 0.10 0.10 6.85 2.00 0.4336 395 386

ADVANTAGES OF THE INVENTION

(24) There has been an ever increasing demand for environment friendly and efficient energy storage systems. Among all the energy storage systems available, supercapacitors are high in demand due to their distinctively high power density, reasonable energy density and longer cycle life.

(25) The main advantages of the present invention are:

(26) The process for the in-situ synthesis of pTS doped PPy/C composite having substantial charge storage ability.

(27) It possesses unique features such as high conductivity, environment friendliness, fast charge-discharge kinetics and can be prepared at low cost.

(28) Moreover, it's characteristic redox doping-undoping process can be exploited in the charge storage systems, utilizing both the electrochemical double layer at the interface and pseudo-capacitive behavior.

(29) An aromatic dopant such as p-toluenesulfonate (pTS) has been found to resist overoxidation and also imparts electrochemical stability to the present PPy/C composites.