Effect of binder on the performance of carbon/carbon ...

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Apr 24, 2014 - The carbon SUPER C65 from. Timcal was used as percolator and the binders were either poly(tetrafluoroethylene) (PTFE - 60 wt. % dispersion ...

Electrochimica Acta 140 (2014) 132–138

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Effect of binder on the performance of carbon/carbon symmetric capacitors in salt aqueous electrolyte ˙ ˛ Qamar Abbas, Dorota Pajak, Elzbieta Frackowiak, Franc¸ois Béguin ∗,1 Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, Piotrowo 3, 60-965 Poznan, Poland

a r t i c l e

i n f o

Article history: Received 23 January 2014 Received in revised form 15 April 2014 Accepted 17 April 2014 Available online 24 April 2014 Keywords: Supercapacitor activated carbon salt aqueous electrolyte, PVDF binder, PTFE binder potentiostatic floating

a b s t r a c t The performance of AC/AC (AC = activated carbon) symmetric capacitors based on two commonly used binders–poly(vinylidene difluoride) (PVDF) and poly(tetrafluoroethylene) (PTFE)–introduced in the electrodes at same amount (10 wt%) has been compared in 1 mol·L−1 NaNO3 aqueous electrolyte. For capacitors charged up to 1.6 V, the PTFE and PVDF-based carbon electrodes exhibit capacitance values of 116 F·g−1 and 104 F·g−1 , respectively. This difference in capacitance is related to the presence of the binder which, in the case of the PVDF-based electrodes, causes a more pronounced reduction of pore volume as compared to the pristine powder. The positive electrode of the AC-PTFE/AC-PTFE cell operates under the thermodynamic potential limit of di-oxygen evolution (in 1 mol·L−1 NaNO3 , Eox = 0.834 V vs. SHE) up to 1.4 V voltage, while this limit is reached at a voltage of 1.2 V with the AC-PVDF/AC-PVDF cell. This shows that the porosity of the electrode strongly impacts the potential range of the positive electrode. Accelerated ageing performed by potentiostatic floating at 1.6 V for 120 h shows higher capacitance decrease and internal resistance increase in the case of the AC-PVDF/AC-PVDF cell due to a stronger oxidation of the positive electrode caused by the higher potential reached by this electrode. Hence, these results suggest that the performance of supercapacitors might differ depending on the selected binder, and discussions relating the capacitance performance to the textural parameters of the pristine carbon may be biased by a change of porosity. Therefore, it is highly recommended to take into account the gas adsorption data of the electrodes instead of the original powder. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Electrical double-layer capacitors (EDLCs), also known as supercapacitors, are energy storage devices based on activated carbon (AC) electrodes, which deliver high power in short periods of time with excellent cycle life [1,2]. They are used for both mobile and stationary applications - e.g., hybrid electric vehicles (HEVs), emergency systems in aircrafts, elevators, cranes and so on. In order to realize such devices, the use of low cost, environment friendly and industrially viable materials is preferred, while optimizing the energy density. The energy density (E) of electrochemical capacitors is expressed by equation (1): E = 1⁄2 CU 2

∗ Corresponding author. Tel.: +48 61 665 3632. E-mail address: [email protected] (F. Béguin). 1 ISE Member. http://dx.doi.org/10.1016/j.electacta.2014.04.096 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

(1)

where ‘C’ is the capacitance and ‘U’ the operating voltage; the maximum voltage depends on the electrochemical stability window of the electrolyte. Although H2 SO4 and KOH aqueous electrolytes display a high electrical conductivity, and consequently allow a low equivalent series resistance (ESR) and high capacitance values, they have a restricted industrial viability due to the limited voltage of AC/AC systems in these media, usually less than 1 V [3,4]. excellent cycle life under galvanostatic Recently, charge/discharge up to 1.6 V has been demonstrated for AC/AC symmetric capacitors in salt aqueous electrolyte (Na2 SO4 - 0.5 mol·L−1 ) [5,6]. Later, it has been even found that voltage around 2 V can be reached in aqueous Li2 SO4 [7–9]. Such high values are possible due to the high over-potential for di-hydrogen evolution at the negative electrode [7]. Taking into account the environmental friendly character and low corrosive properties of salt aqueous electrolytes, promising prospects can be expected from AC/AC systems built in these media. Besides, it is now well-accepted that capacitance can be enhanced by a good fitting between the pore size of AC and the size of electrolyte ions. In aqueous media, pores in the range of 0.7–1 nm demonstrate better efficiency for forming effectively the electrical

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double-layer [10,11]. However, taking into account that the porosity of AC might be partly blocked by additives used in preparing electrodes, e.g., binder [12], it is important to consider their role in the discussion of relationship between the porous texture and capacitance of AC electrodes in salt aqueous electrolytes. So far, mainly two types of binders–poly(tetrafluoroethylene) (PTFE) [7,13,14] and poly(vinylidene difluoride) (PVDF) [5,6,8] have been employed indifferently in order to maintain the integrity of electrodes during operation of AC/AC capacitors in salt aqueous electrolytes. This work aims at comparing the performance of AC-based symmetric capacitors in 1 mol·L−1 sodium nitrate with electrodes based on either PVDF or PTFE binders. It has been observed that the PTFE-based capacitor performs better than the PVDF-based one and has ability to withstand floating at high voltage. We will demonstrate that different porosity of the electrodes, due to the presence of binder, is at the origin of different electrochemical performance.

2. Experimental 2.1. Electrode preparation The activated carbon (AC) used for the study was Norit DLC Supra 30 (SBET = 2066 m2 ·g−1 ). The carbon SUPER C65 from Timcal was used as percolator and the binders were either poly(tetrafluoroethylene) (PTFE - 60 wt. % dispersion in water, Sigma-Aldrich) or poly(vinylidene fluoride) (PVDF - Kynar HSV900 from Arkema, France). The PTFE-based electrodes (AC-PTFE) were made by mixing AC (80 wt. %), C65 (10 wt. %) and PTFE (10 wt. %) in 10 ml of ethanol. The dough obtained after mixing was rolled to a thickness of nearly 0.3 mm and was left overnight for ethanol to evaporate. Pellets were cut out from the rolled dough, pressed under 5 ton·cm−2 and finally dried under vacuum at 120 ◦ C for 12 h. The PVDF-based electrodes (AC-PVDF) were prepared by adding 10 ml of acetone to all components (AC 80 wt. %, C65 10 wt. % and PVDF 10 wt. %) in a mortar and by continuous mixing until its evaporation. The powder was pressed under 5 ton·cm−2 in order to make pellets. The later were dried under vacuum at 120 ◦ C for 3 h. The pellets had a diameter of 1 cm, a thickness of about 0.3 mm and a weight varying between 10 to 15 mg, whatever the preparation method. 2.2. Physicochemical characterization The porous texture of the materials was determined by nitrogen adsorption–desorption at 77 K using an Accelerated Surface Area and Porosimetry analyzer (ASAP 2020–Micromeritics, USA). Before the measurements, the samples were outgassed at 140 ◦ C for 24 h. The specific surface area was calculated by applying the Brunauer–Emmett–Teller (BET) equation to the N2 adsorption data, while the average pore size–L0 –was calculated by applying the Dubinin-Radushkevich (DR) and Stoeckli equations [15]. The Quenched Solid Density Functional Theory (QSDFT) was used to assess the pore size distribution, the micropore (< 2 nm) and mesopore (2-50 nm) volumes [16,17]. In order to allow a comparison in the case of electrodes, the amount of nitrogen adsorbed and the textural data were referred to the AC mass. 2.3. Electrochemical testing Electrochemical capacitors were built in a Teflon Swagelok® vessel, by sandwiching a glass microfiber separator (GF/A, WhatmanTM , 0.26 mm) between two pellet electrodes and using stainless steel (316L) current collectors and 1 mol·L−1 NaNO3 (pure

133

p.a., Sigma-Aldrich, conductivity = 74 mS·cm−1 , pH ∼ 6.7) as electrolyte. Taking into account equation (2): m+ · C+ · E+ = m− · C− · E−

(2)

where m+ and m- are the masses, C+ and C- the specific capacitance and E+ and E- the potential windows for positive and negative electrodes, respectively, we selected higher mass (by ∼15%) for the positive electrode, in order to reduce its maximum potential and consequently its irreversible oxidation. In some cases, an Hg/Hg2 SO4 in 1 mol.L−1 H2 SO4 (+0.674 V vs. Standard Hydrogen Electrode - SHE) reference electrode with 1 mol·L−1 NaNO3 salt bridge was incorporated in the capacitor cell in order to measure the potential of both electrodes. At pH ∼ 6.7, the thermodynamic di-hydrogen and di-oxygen evolution potentials calculated from the Nernst equation are EH2 ∼ -0.396 V vs. SHE and EO2 ∼ 0.834 V vs. SHE, respectively. The electrochemical performance of the cells was investigated by cyclic voltammetry (2 mV·s−1 ) and galvanostatic (200 mA·g−1 ) charge-discharge in a voltage range from 0 to 1.6 V using a VMP3 multichannel potentiostat/galvanostat (Biologic, France). The gravimetric capacitance (F·g−1 ) has been calculated from the galvanostatic discharge characteristics using equation (3): C = (2)I/[(dU/dt)mam ]

(3)

where I is the current (A), dU/dt the slope of the discharge curve (V·s−1 ), mam is the average mass (g) of AC in one electrode. Impedance spectra were recorded in the range of 1 mHz to 100 kHz at different voltages from 0 V to 1.6 V. Potentiostatic floating at 1.6 V has been applied to estimate the ageing of capacitors; in these experiments, the cell is charged at constant current up to 1.6 V and is maintained at this value for periods of 2 h. A total of 60 such floating periods were applied; each of them was followed by five galvanostatic charge/discharge cycles at 1 A·g−1 in order to estimate the discharge capacitance and the equivalent series resistance (ESR) from the voltage drop between 5th charge and discharge when reversing the sign of current. 3. Results and Discussion 3.1. Two-electrode cell performance Fig. 1 shows the cyclic voltammograms of AC/AC cells in 1 mol·L−1 NaNO3 when the electrodes are manufactured with the two binders. Whatever the voltage range, up to 0.8 V or 1.6 V, a higher capacitive current is observed with the PTFE binder. At low voltage range (Fig. 1a), both systems display a rectangular shape of CV which is typical for electrical double-layer charging; the slightly more rectangular curve in case of AC-PTFE electrodes reveals a better charge propagation. For the higher voltage range of 1.6 V (Fig. 1b), the current leap at high voltage is essentially attributed to hydrogen storage at the negative electrode. It has an associated hump below 0.4 V during the negative scan - more visible in the case of the AC-PTFE electrodes - suggesting that hydrogen stored in the negative electrode [18] is desorbed when the voltage decreases, it means when the potential of the negative electrode becomes higher than the thermodynamic limit for water reduction [7]. The capacitance and efficiency (discharge to charge time ratio) of the AC/AC capacitors in 1 mol·L−1 NaNO3 are plotted vs voltage for both kinds of binders in Fig. 2. The capacitance slightly increases with voltage being higher with AC-PTFE electrodes in all the voltage range. At 1.6 V, the capacitance values are 116 F·g−1 and 104 F·g−1 for the AC-PTFE and AC-PVDF electrodes, respectively. Whatever the binder, the efficiency decreases with increasing voltage, but values are higher for the AC-PTFE capacitor than for the ACPVDF one. Low efficiency values may indicate the occurrence of

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150

AC-PTFE

(a)

Capacitance / F g-1

Capacitance / F g-1

100 50 0 -50

-100 0.0

0.2

0.4

AC-PTFE

(b)

AC-PVDF

0.6

AC-PVDF

200

100

0

-100

0.8

0.0

U/V

0.4

0.8

1.2

1.6

U/V

Fig. 1. Cyclic voltammograms (2 mV·s−1 ) up to 0.8 V (a) and 1.6 V (b) of AC/AC capacitors in 1 mol·L−1 NaNO3 with PTFE and PVDF binders.

higher capacitance by ∼15 F·g−1 at low frequency (1 mHz), whereas at higher frequency (around 1 Hz), the capacitance is higher for the AC-PVDF based system. It suggests that, at open circuit voltage, deeper pores are more accessible in AC-PVDF electrodes than in AC-PTFE ones, due to better initial wetting. This lesser accessible porosity for AC-PTFE electrodes in 1 mol·L−1 NaNO3 correlates with their more hydrophobic nature demonstrated by the contact angle measurements realized (in absence of polarization) at room temperature, with values decreasing from 156o to 129o and 64o to 24o

(a)

0V 0.8V 1.2V 1.6V

300 5 4

200

-Z" / ohm

-Z" / ohm

irreversible redox side reactions during charging. However, with AC-PTFE electrodes, the efficiency is higher than 98% in all voltage range, revealing an excellent reversibility of these electrodes. The Nyquist plots realized at different voltages in two-electrode cells in 1 mol·L−1 NaNO3 are presented in Fig. 3 for both binders. At 0 V, the two systems exhibit comparable internal resistance values (ESR = 0.461  and 0.427  for AC-PTFE and AC-PVDF, respectively) originating mainly from the bulk electrolyte; at all voltages, the ESR values remain unchanged. Besides, the charge transfer resistance (semi-circle region), which is believed to be related to the resistances of the electrode/electrolyte and electrode/current collector interfaces, is smaller with the AC-PTFE electrodes than with the AC-PVDF ones indicating a good contact between AC and current collectors and between activated carbon particles when using PTFE. Next to the semi-circle is the Warburg region, with a phase angle of 45o , up to the knee frequency; this so-called mass transport region is essentially governed by the diffusion of the charged species within the porosity. Hence, the AC-PTFE based capacitor displays a more prominent Warburg diffusion region than the AC-PVDF one, which could be related to a higher porosity available for the transport of ions in the case of the former. The knee frequency for the AC-PTFE capacitor shifts to higher values when the voltage increases. This shift results in almost linear decrease of the EDR values (Table 1), revealing a more accessible porosity of the AC-PTFE electrode(s) at higher voltage. By contrast, the negligible shift with increasing voltage in case of AC-PVDF seems to reveal good initial wetting of the electrodes. The near vertical line in the Nyquist plot at low frequency (10 mHz to 1 mHz) characterizes the capacitive behavior of the system [19]. The frequency dependence of capacitance at open circuit voltage presented in Fig. 4 confirms that the PTFE-based capacitor displays

3 2 1

100

Knee frequency

0 0

0

0

100

1

2 3 Z' / ohm

200

4

5

300

Z' / ohm

(b)

0V 0.8V 1.2V 1.6V

300 5

100

140

80 60

94

AC-PVDF AC-PTFE

40

92

20

90

0 0.8

1.0

1.2

1.4

1.6

U/V Fig. 2. Discharge (200 mA·g−1 ) capacitance and efficiency of AC/AC capacitors in 1 mol·L−1 NaNO3 vs. maximum voltage.

200 -Z" / ohm

96

-Z" / ohm

100

Capacitance / F g-1

Efficiency (%)

4

120

98

100

3 2 Knee frequency

1 0 0

1

0 0

100

2 3 Z' / ohm

200

4

5

300

Z' / ohm Fig. 3. Nyquist plots of two-electrode cells in 1 mol·L−1 NaNO3 at 0 V, 0.8 V, 1.2 V and 1.6 V: a) AC-PTFE electrodes, b) AC-PVDF electrodes.

Q. Abbas et al. / Electrochimica Acta 140 (2014) 132–138 Table 1 EDR values () estimated from the Nyquist plots (as described in [3]) at different cell potentials for AC/AC capacitors with PTFE and PVDF binders. 0V

0.8 V

1.2 V

1.6 V

AC-PTFE AC-PVDF

3.72 3.0

3.49 2.98

3.28 2.96

2.96 2.73

120

Capacitance / F g-1

Cell potential

135

in 10 minutes for AC-PTFE and AC-PVDF electrodes, respectively. The shift of Knee frequency to lower values, when voltage decreases for AC-PTFE electrodes, is also a proof of a lesser accessible porosity. Overall, the AC-PTFE based capacitor performs better under polarization conditions where ionic species are forced into the carbon porosity.

AC-PVDF AC-PTFE

100 80 60 40 20 0 1E-3 0.01

0.1

1

10

100 1000 10000100000

Frequency / Hz 3.2. Porous texture of pristine carbon and electrodes In order to understand the differences in capacitance values and Nyquist plots displayed with electrodes made from the two binders, and taking into account that it has been already shown that the binder may block a part of carbon porosity [12], the porous texture of the electrodes and of the pristine carbon powder has been investigated by nitrogen adsorption at 77 K. Fig. 5a shows a remarkable decrease of the amount of gas adsorbed from the pristine AC carbon to the AC-PTFE and AC-PVDF electrodes. As shown in Table 2, the BET specific surface area drops from 2066 m2 ·g−1 to 1835 m2 ·g−1 and 1544 m2 ·g−1 , respectively. Hence, the PVDF binder blocks more the porosity than the PTFE one. The plot of pore size distribution (PSD, Fig. 5b) demonstrates that all pores are affected by the presence of the binder. In Table 2, it is seen that the Vmicro /Vmeso ratio is comparable for the electrodes and the pristine carbon, meaning that micropores and mesopores are almost affected equally by the presence of the binder. However, the slightly higher value of average micropore size L0 in the case of the AC-PVDF electrodes reveals that small micropores are more affected by this binder, as proved by a lower contribution below 0.8 nm in the PSD (Fig. 5b). Hence, the different change of porous texture provoked by the binder is the reason for the differences in electrochemical performance of both types of electrodes.

Fig. 4. Capacitance vs. frequency at open circuit voltage for AC/AC cells with PTFE and PVDF-based electrodes operating in 1 mol·L−1 NaNO3 .

(200 mA·g−1 ) charge/discharge are shown in Fig. 6. In the PTFEbased symmetric capacitor, the maximum potential of the positive electrode exceeds the thermodynamic limit for di-oxygen evolution (Eox = 0.834 V vs. SHE) at a voltage of 1.4 V (Fig. 6a), while this limit is reached at 1.2 V for the cell with PVDF-based electrodes (Fig. 6b), that is the positive electrode in the PVDF-based system undergoes irreversible carbon oxidation at voltage values lower by ∼200 mV than in the PTFE-based system. By contrast, in both AC cells, the operating potential of the negative electrode is well above the potential for practical di-hydrogen evolution (EH2 ∼ 0.8 V vs SHE) at all voltage values. This potential was determined by cyclic voltammetry in three electrode cell while polarizing AC to more negative potentials. Below the thermodynamic potential of water reduction, hydrogen is first chemisorbed in the porosity of carbon, and polarizing to more negative potentials results in di-hydrogen production producing oscillations in the cyclic voltammograms [7,18]. Below the thermodynamic potential for water reduction, OH− ions are generated in the porosity of AC according to equation (4), provoking a local increase of pH and consequently a decrease of practical di-hydrogen evolution potential (as explained by application of the Nernst relation E = -0.06 pH) [20]. AC + xH2 O + xe− →< ACHx > + xOH−

The effect of the kind of binder on the potential range of both electrodes at increasing voltage values has been investigated in a two-electrode cell equipped with a reference electrode. The values of potential extrema vs voltage determined from galvanostatic

Fig. 7 shows the capacitance values of positive and negative electrodes obtained from galvanostatic (200 mA.g−1 ) discharge when AC-PTFE/AC-PTFE and AC-PVDF/AC-PVDF cells with reference electrode are charged up to 1.6 V. Whatever the binder, the capacitance

(a) 700 600

Nitrogen Adsorption Isotherm AC AC-PTFE

Pore Size Distribution - PSD

(b)

1.0 AC

0.8

AC-PTFE

AC-PVDF

500

(4)

0.6

AC-PVDF

400

0.4

300

0.2 0.0

200

Differential pore volume / cm3 g-1 nm-1

Adsorbed amount (STP) / cm3 g-1

3.3. Potential range of the electrodes

Pristine

100

PTFE

0

PVDF

0.0

0.2 0.4 0.6 0.8 1.0 Relative pressure, (P/P0)

0

1

2 3 4 Pore width / nm

5

Fig. 5. Nitrogen adsorption/desorption isotherms obtained at 77 K (a) and QSDFT pore size distribution (b) of AC, AC-PTFE and AC-PVDF electrodes. For the electrodes, the amount of nitrogen adsorbed is referred to the mass of AC.

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(a)

1.2 Thermodynamic oxygen

1.2 Thermodynamic oxygen

evolution potential

0.0 Thermodynamic hydrogen evolution potential

-0.8

EH

Practical di-hydrogen evolution potential

Eox

0.8

E vs. SHE / V

E vs. SHE / V

E0 E+ E-

0.4

-0.4

evolution potential

Eox

0.8

(b)

0.4 E0 E+ E-

0.0 -0.4 -0.8

Thermodynamic hydrogen evolution potential Practical di-hydrogen evolution potential

EH

-1.2

-1.2

0.8

1.0

1.2

1.4

0.8

1.6

1.0

1.2

1.4

1.6

U/V

U/V

Fig. 6. Potential extrema vs SHE reached by each electrode during galvanostatic (200 mA·g−1 ) charge/discharge of AC-PTFE/AC-PTFE (a) and AC-PVDF/AC-PVDF (b) cells with reference electrode in 1 mol·L−1 NaNO3 up to different voltage values. Hg/Hg2 SO4 reference electrode with 1 mol·L−1 NaNO3 salt bridge.

values of the positive and negative electrodes are comparable at all voltages. By contrast, the AC-PTFE positive electrode exhibits much higher capacitance than the AC-PVDF one, due to a higher electrosorption of the bulky NO3 − anions favored by the higher pore volume of the former compared to the later. According to equation (2), the AC-PTFE positive electrode then operates in a lower potential range, and for this reason reaches a lower maximum potential than the AC-PVDF positive one when the voltage is increased up to 1.6 V. One might expect the generation of oxygenated functionalities on the surface of the AC-PVDF electrode which should affect its long-term performance.

Capacitance / F g-1

140 120 100 80 60

AC-PVDF (+) AC-PVDF (-) AC-PTFE (+) AC-PTFE (-)

40 20 0 0.8

1.0

1.2

1.4

3.4. Accelerated ageing by potentiostatic floating

1.6

U/V Fig. 7. Discharge capacitance of electrodes measured by galvanostatic (200 mA·g−1 ) cycling of AC-PTFE/AC-PTFE and AC-PVDF/AC-PVDF cells with reference electrode in 1 mol·L−1 NaNO3 up to different voltage values. Hg/Hg2 SO4 reference electrode with 1 mol·L−1 NaNO3 salt bridge.

Accelerated ageing by floating at 1.6 V has been performed on AC/AC capacitors with electrodes made from the two binders, in order to detect the impact of different porosity and different operating potential range of the positive electrode on the cycle life. Fig. 8 displays, for the two binders, the evolution of cell capacitance and

Table 2 Textural data of AC, AC-PVDF and AC-PTFE electrodes. For the electrodes, the specific surface area and the pore volumes are referred to the mass of AC. Material

SBET [m2 ·g−1 ]

Vtotal a [cm3 ·g−1 ]

Vmicro b [cm3 ·g−1 ]

Vmeso c [cm3 ·g−1 ]

Vmicro /Vmeso

L0 d [nm]

AC AC-PVDF AC-PTFE

2066 1544 1835

1.100 0.855 1.003

0.908 0.675 0.807

0.172 0.130 0.156

5.28 5.19 5.17

1.54 1.61 1.51

a, b, c d

Calculated from the cumulative pore volume using the QSDFT method [16,17]. The average micropore size L0 is calculated by applying the Dubinin-Radushkevich (DR) and Stoeckli equations [15] to the adsorption data.

4

100

Resistance / ohm

Capacitance / F g-1

120

80 60 AC-PVDF

(a)

AC-PTFE

40 20

AC-PTFE

(b)

AC-PVDF

3

2

1

0

0 0

20

40

60

80

Floating time / h

100

120

0

20

40

60

80

100 120

Floating time / h

Fig. 8. Capacitance (a) and internal resistance (b) of AC/AC capacitors with PTFE and PVDF binders plotted versus floating time at 1.6 V and room temperature in 1 mol·L−1 NaNO3 .

(a)

(b)

AC-PTFE

1600

Specific current / mA g-1

Specific current / mA g-1

Q. Abbas et al. / Electrochimica Acta 140 (2014) 132–138

1200

800

400

137

AC-PVDF

1600

1200

0

800

400

0 0

20

40

60

80

100

120

140

0

20

Floating time / h

40

60

80

100

120

140

Floating time / h

Fig. 9. Leakage current profile of AC/AC capacitors with PTFE (a) and PVDF (b) binders plotted versus floating time at 1.6 V and room temperature in 1 mol·L−1 NaNO3 .

resistance with floating time. For the freshly prepared devices, the capacitance values are higher by ∼15 F·g−1 when using AC-PTFE electrodes (Fig. 8a). After 120 h of floating, the capacitance of the AC-PTFE/AC-PTFE capacitor has slightly decayed by about 14%. By contrast, the capacitance decay is much more pronounced for the capacitor containing AC-PVDF electrodes. A difference between the two binders is also found when the internal resistance is plotted versus the floating time (Fig. 8b); the resistance increases faster for the system operating with AC-PVDF electrodes. Such dramatic ESR increase and capacitance fading for the AC-PVDF electrodes is related to the higher potential reached by the positive electrode, as shown in Fig. 6b, giving rise to a larger irreversible di-oxygen evolution together with carbon oxidation [21]. Considering the generally accepted end-of-life criteria of supercapacitors, i.e. capacitance decay by 20% or resistance increase by 100% [21], the PTFE and PVDF-based capacitors are out of usage after approximately 60 and 40 h of floating at 1.6 V, respectively. Even if the AC-PTFE/ACPTFE capacitor performs better than the AC-PVDF/AC-PVDF one, it is obvious that for keeping a good state-of-health it should not be cycled at voltage higher than 1.5 V. Since redox phenomena may be involved in the ageing of the AC/AC capacitors in 1 mol·L−1 NaNO3 , it is interesting to plot the leakage current to discriminate between the two binders. The leakage current recorded on the two kinds of capacitors during the repeated two-hour sequences of potentiostatic floating at 1.6 V is shown in Fig. 9. When the capacitors are charged up to 1.6 V, there is a steep increase of current, and then the current drops rapidly within the initial few minutes of potentiostatic floating at 1.6 V.

AC-PTFE AC-PVDF

(a)

200

AC-PVDF AC-PTFE

(b)

100

Capacitance / F g-1

Capacitance / F g-1

150

Such rapid decay in current is generally attributed to the reorganization of the ionic species in the electrical double-layer and within the diffusion layer until reaching the equilibrium state [19,21]. It is also described as a transition from stronger to weaker interaction of the electrode with i) the double layer and ii) the diffusion layer, resulting in regress of the ionic species to the bulk solution [22]. For the capacitor with AC-PTFE electrodes (Fig. 9a), the leakage current traces remain nearly identical during the successive floating periods, suggesting a negligible contribution of side reactions, such as oxidation of positive carbon electrode, evolution of gases and corrosion of current collectors. By contrast, with the AC-PVDF based capacitor (Fig. 9b), the initial current is higher than with AC-PTFE and it continuously increases up to 90 h. This increase is attributed to surface reactions requiring higher current at the positive electrode, due to the higher maximum potential reached by this electrode. After 90 h, a sudden increase in current can be seen with the AC-PVDF based capacitor, and can be correlated to the capacitance drop (Fig. 8a) and resistance increase (Fig. 8b) during the same period of time. This can be due either to the creation of heterogeneous oxygenated functionalities on the positive carbon electrode [23] or to the corrosion of the positive current collector, both phenomena leading to a significant increase in resistance values. However, after opening the cells which operated at 1.6 V with AC-PVDF electrodes, we could not detect any corrosion of the stainless steel collectors. Hence, it is likely that the high potential reached by the positive electrode in this system causes essentially oxidation of the carbon surface.

50 0 -50 -100

100

0

-100

-150 -200 0.0

0.2

0.4

U/V

0.6

0.8

0.0

0.4

0.8

1.2

1.6

U/V

Fig. 10. Cyclic voltammograms (2 mV·s−1 ) up to 0.8 V (a) and 1.6 V (b) for AC/AC capacitors with AC-PTFE and AC-PVDF electrodes in 1 mol·L−1 NaNO3 realized after floating at 1.6 V for 120 h.

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Cyclic voltammograms (CVs) were recorded after floating the AC-PTFE and AC-PVDF based capacitors at 1.6 V for a total of 120 h (Fig. 10). The AC-PTFE/AC-PTFE capacitor displays a nearly rectangular shape of CV up to 0.8 V, while the capacitive current of the AC-PVDF/AC-PVDF starts to decrease from 0.4 V (Fig. 10a). When comparing the two systems, this decrease of capacitive current is even more marked when the voltage is raised up to 1.6 V. Once more, this is an indication that the electrodes manufactured with PTFE undergo lesser ageing than the PVDF-based ones. Knowing that, at voltage of 1.6 V, the AC-PVDF positive electrode is more oxidized than the AC-PTFE one (Fig. 6), one might anticipate that the pore volume is more reduced in the case of AC-PVDF electrodes due to the blockage of pore entrances by oxygenated groups. Hence, if the pore volume of AC-PVDF is more reduced by these groups than in the case of AC-PTFE, one can attribute the important narrowing of the AC-PVDF/AC-PVDF voltammogram to the saturation of porosity by the ions of the electrolyte [24]. The comparison of Figs. 1 and 10 for the AC-PTFE/AC-PTFE capacitor indicates clearly a little narrowing of the CV after floating for 120 h at voltage as high as 1.6 V, suggesting that the electrodes are not much aged by the use of such conditions. 4. Conclusion We have demonstrated that the performance of AC/AC symmetric capacitors in 1 mol L−1 NaNO3 depends on the nature of the binder, although using the same mass of PTFE and PVDF in the manufacture of electrodes. Electrodes are less microporous than the pristine carbon powder, and the loss of porosity is more important with PVDF which seems to block more the small micropores than does PTFE. Consequently, higher capacitance is achieved with AC-PTFE electrodes due to their higher microporous volume as compared to AC-PVDF ones. The difference of porosity between the two types of electrodes has a noticeable impact on the potential window of the positive electrode, and the latter operates at lower potentials when PTFE is used, limiting by this way the oxidation of carbon even when the capacitor operates at voltage as high as 1.6 V. Accelerated ageing together with the plot of leakage current measured during floating confirm a better state-of-health of the ACPTFE/AC-PTFE capacitor. Overall, PTFE seems to be a better binder for electrodes of capacitors operating in salt aqueous electrolytes. Hence, these results confirm that the performance of supercapacitors might differ depending on the selected binder, and discussions relating the capacitance performance to the textural parameters of the pristine carbon may be biased by a change of porosity when realizing electrodes. Therefore, it is highly recommended to take into account the gas adsorption data of the electrodes instead of the original powder. However, it should not be systematically extrapolated that PTFE is a better binder than PVDF. Indeed, the change of porosity by the binder and the diffusion of ions in the pores may depend on the physicochemical characteristics of the carbon material and nature of the electrolyte. Acknowledgments The Foundation for Polish Science is acknowledged for supporting the ECOLCAP project realized within the WELCOME program,

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