Specific Surface versus Electrochemically Active Area of the Carbon

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The specific surface versus electrochemically active area of the carbon/ polypyrrole capacitor: the correlation of ion dynamics studied by an electrochemical quartz crystal microbalance with BET surface Heike L.K.S. Mosch, Oluseun Akintola, Winfried Plass, Stephanie Hoeppener, Ulrich S. Schubert, and Anna Ignaszak Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00523 • Publication Date (Web): 15 Apr 2016 Downloaded from http://pubs.acs.org on April 19, 2016

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Fig 2 91x40mm (300 x 300 DPI)

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Fig 5 118x75mm (300 x 300 DPI)


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The specific surface versus electrochemically active area of the carbon/polypyrrole capacitor: the correlation of ion dynamics studied by an electrochemical quartz crystal microbalance with BET surface Heike L.K.S. Moscha, Oluseun Akintolab, Winfried Plassb, Stephanie Höppenera, Ulrich S. Schuberta, Anna Ignaszaka,c* a

Institute of Organic and Macromolecular Chemistry, Faculty of Chemical and Earth Sciences, Friedrich-Schiller University, D-07743 Jena, Germany. E-mail: [email protected], [email protected], [email protected] b Institute of Inorganic and Analytical Chemistry, Faculty of Chemical and Earth Sciences, Friedrich-Schiller University, D-07743 Jena, Germany E-mail: [email protected], [email protected] c Department of Chemistry, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3 Canada. Email: [email protected] *corresponding author: Anna Ignaszak

Abstract Carbon/polypyrrole (PPy) composites are promising electrode materials for energy storage applications such as light-weight capacitors. Although these materials are composed of relatively inexpensive components, there is a gap of knowledge regarding the correlation between surface, porosity, ion exchange dynamics, and the interplay of the double-layer and pseudo-capacitance. In this work we evaluate the specific surface area analyzed by the BET method and the area accessible for ions using electrochemical quartz crystal microbalance (EQCM) for SWCNT/PPy and carbon black Vulcan XC72-R/PPy composites. The study revealed that the polymer has significant influence on the pore size of the composites. Although the BET surface is low for the polypyrrole, the electrode mass change and thus the electrochemical area is large for the polymer-containing electrodes. This indicates that multiple redox active centers in the charged polymer chain are good ion scavengers. Also, for the composite electrodes, the effective charge storage occurs at the polypyrrole-carbon junctions, which are easy to design/multiply by a proper carbon-to-polymer weight ratio. The specific BET surface and electrochemically accessible surface area are both important parameters in calculation of the electrode capacitance. SWCNTs/PPy showed the highest 1 ACS Paragon Plus Environment

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capacitances normalized to the BET and electrochemical surface as compared to the polymercarbon black. TEM imaging revealed very homogenous distribution of the nano-sized polymer particles onto the CNTs, which facilitates the synergistic effect of the double layer (CNTs) and pseudo-capacitance (polymer). The trend in the electrode mass change in correlation with the capacitance suggest additional effects such as a solvent co-insertion into the polymer and the contribution of the charge associated with the redox activity of oxygencontaining functional groups on the carbon surface.

1. Introduction Carbon/polypyrrole composites are currently the most promising electrode components used in the flexible and light-weight capacitors. Various combinations of polypyrrole with popular carbon allotropes such as SWCNTs1, MWCNTs2-4, graphene5,

6

and also carbon black7 are

broadly discussed since numerous research aim to develop stable and cost-effective materials for energy storage – the capacitor technology has a significant contribution to this field. The combination of carbon with polypyrrole has several benefits as compared to the individual components (carbon or polymer). One of the most important is an improvement of electrochemical stability. This is triggered by carbon acting as a stabilizing backbone for the polymer. Also, the synergy of double layer capacitance (carbon) and pseudocapacitance (polypyrrole) results in superior capacitance of the composite electrode. This is generated by an additional charge accumulated/released at the carbon-polymer junctions.8 The capacitance acquired for such composites ranges between 144 and 200 F/g for SWCNT/PPy1, from 183 F/g8 or 427 F/g2 up to 890 F/g9 for MWCNT/PPy, from 335 F/g6 to 650 F/g5 for composites with graphene or 366 F/g7 for the carbon black/PPy combination. The quantification of the specific capacitance is usually carried out by electrochemical methods, however the correlation of capacitance with the surface area (also porosity) is not straightforward. As the basic principle, the capacitance of a classical capacitor is correlated with the specific surface area according to:10, 11

= ·

(1)

where C denotes as capacitance, A is the surface area of the electrode, d is the distance between capacitor plates and εr is the permittivity of the material. The relation between capacitance and the specific surface area of the electrode was investigated in respect to the porosity and the carbon surface functionalization only for the selected carbon-based double layer electrodes. For instance, micropores (pore size less than 2 nm) diminish the ion 2 ACS Paragon Plus Environment

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access,10,

12-16

while the oxygen-containing surface functionalities boost the wettability

resulting in the enhancement of a double-layer capacitance. In addition, they generate pseudocapacitive effects from the redox activity of the functional groups, such as quinones.10, 12

Following similar studies, the electrochemical test of a carbide-derived porous carbon

demonstrated that capacitance increases intensely with decreasing pore size. This occurs even when the size of pores is smaller than the diameter of solvated ion, indicating that ions can lose their solvation shell and still access ultra-fine pores.17 Recently, the influence of the pore size on the ion uptake has been demonstrated for organic electrolytes. The study revealed a partial desolvation where ions were adsorbed on very small pores (~ 1 nm), and the desolvation was more noticeable at size of 0.65 nm.18 In addition, the ion penetration strongly depends on the type and the concentration of electrolyte.19 The pore structure, size, chemical modification, hydrophilic/hydrophobic balances are critical for this process. Moreover, the interactions at the carbon-ion interphase are strictly dependent/can be modified by carbon synthesis method and the carbon post-synthesis treatment.20 Although, these studies employ an advanced physical characterization, the correlation of the specific surface with the ion dynamic at the polarized electrode require more specific instrumentation, such as the electrochemical quartz-crystal microbalance (EQCM, also called electro-gravimetry).21 The charge-discharge cycle generates the respective ion uptake-release at the electrode and thus its significant mass change. The quantification of the charge cumulated (released) is carried out based on the analysis of the resonance frequency of the electrode using appropriate mathematical models that are discussed in following section of this paper.22-25 The EQCM analysis of the ink-deposited resonators is complex and requires a special preparation of the electrode such as a casting of ultra-thin layers of the active material. In respect to this, the relevant published work is scarce and limited to very few capacitive systems such as a polyaniline26,

27

, polypyrrole22-25, carbon18 and only two articles dealing with the EQCM

analysis for the carbon/PPy composites.28, 29 The electro-gravimetric tests of pure polypyrrole were carried out only for electro-deposited films23, 30-33 and demonstrated that both mobility and valance of ion play a significant role in the charge exchange process,23 and are influenced by the pH of the electrolyte. For instance, it was possible to quantify the co-doped hydrated cations introduced simultaneously during an anion insertion. This co-insertion during the polymer doping is very sensitive to the pH and is rather difficult to identify and quantify by spectroscopic and electrochemical methods.22 Regarding the type of doping anion for polypyrroles, the complemented electro-gravimetric study showed that smaller ions with a high mobility are favored for the compensation of the charged polymer chain. Thus the ion 3 ACS Paragon Plus Environment

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dynamic (charge-discharge rate) can be well predicted/designed for this type of pseudocapacitive compound. EQCM investigations of polypyrrole electro-generated onto the resonating electrode demonstrated that the polymer thickness and ageing process are mostly controlled by the ion dynamics.23 In addition, for small anions such as Cl-, ClO4-, NO3- or SO42-, the electro-neutrality of the polymer is accomplished only by anion exchange since these species exhibit high mobility in the polypyrrole matrix.23, 34-36 The cation transfer occurs only for the polymer doped with large (immobile) anions, including polystyrene sulfonate23, polyvinylsulfonate37 or dodecylbenzene sulfonate38. The EQCM studies on SWCNTs39 and activated carbon40 correlated the cation uptake upon negative polarization with the microstructure, revealing the preferential ion doping on larger pores.18 A similar observation and relevant quantifications were made for the MWCNT/PPy composites, where potassium chloride and tetraethylammonium bromide were verified. This analysis demonstrated that smaller and mobile ions (Cl-) give better charge-discharge cycle efficiency.28 Also, bivalent systems with KNO3, Ca(NO3)2 and Mn(NO3)2 as the electrolyte showed similar trend.29 It has been proven that the mass change corresponding to the cation and anion exchange in carbon materials is independent of the potential scan rate and the scan direction. An important aspect is the potential amplitude (range) and its correlation to the pore structure. The electrode polarization features should be optimized so that the ion entering accessible, sufficiently wide pores can replace an equivalent number of water molecules. This solvent-ion exchange is less restricted in micropore centers than in the contact with pore walls. The effective pore width should be twice of the solvated Cl- that is 1.3 nm (pores smaller than 1.3 nm contain immobile water molecules).

41

In addition, in situ EQCM-voltammetry analysis carried out for the

carbon nanotube reinforced in polymer matrix (synthetized by electrochemical co-deposition) demonstrated that the exchange of chloride ions does not occur onto with the negatively charged carbon fibers.28 Thus, potassium cations balance the negative charge of the carbon fraction. In such case, the carbon acts as the cation scavenger while increasing the surface of the deposit and maintaining the electrical conductivity of the electrode when the polymer is reduced. The polypyrrole redox active centers are then responsible for anion uptake/release upon polarization. The EQCM-CV study on the electrodeposited PPy-carbon demonstrated by another group42 correlated the massogram with the mechanism of charge storage in both polypyrrole and carbon. The important observation is that the redox pseudocapacitor (polymer) showed significant mass increase as the doping ions are rigidly attached to the electrode due to the accommodation within the polymer matrix and the formation of an ionic 4 ACS Paragon Plus Environment

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bond between the polymer and counter ion. The increase of mass for the pure polymer is significant as it has multiple redox active centers. On the other hand, the bare carbon electrode did not show mass increase in the charging process (as compared to the polymer) since ions are only loosely attached to the surface.42 This result indeed demonstrates that in situ EQCMelectrochemical technique can be an excellent tool to separate faradaic (redox) response from pure double-layer capacitance of carbon. In this study we validate the EQCM method in estimating of the electrochemical active surface area (effective surface accessible for ions) and correlate it with the specific surface analyzed by the N2-adsorption method for the polypyrrole-carbon composites. In order to envisage the effect of carbon microstructure we generated two model compounds by combining polypyrrole with carbon black consisting of large pores (but lower BET surface) and the SWCNTs with a small pore diameter (but larger BET surface). In this work we adapt for the first time an ink-casted PPy-carbon EQCM electrode as the method is similar to the manufacturing of the commercial membranes (e.g., slurry deposition by a “doctor-blade” method or screen printing). Additional components are necessary to maintain the mechanical stability of the electrode, such as the polymeric binder (e.g., polyvinylidene fluoride). It generates other background effects that have to be deliberated during the EQCM quantifications. In respect to this, open questions are how the charge is stored in the presence of the binder? Will net stored charge correspond to a single or multiple charge adsorption? Our aim is to analyze and correlate the physical surface area (and microstructure) with the real accessible surface (EQCM surface) and to adapt the in situ electro-gravimetric tool for the screening, predicting and designing the capacitor electrode materials with optimum performance.

2. Experimental section 2.1 Chemicals All reagents and solvents used in this work are commercially available. Iron(II) chloride tetrahydrate was purchased from Alfa Aesar, the carbon black from Cabot (VULCAN® XC72R), the pyrrole (Py), isopropanol and perchloric acid (60%) were ordered from Merck. Hydrogen peroxide (30 wt%), iron(III) chloride, ammonium persulfate (APS), hydrochloric acid (37%), nitric acid (≥ 69%), the SWCNTs (Ø = 0.7 - 1.4 nm) and polyvinylidene fluoride (PVDF, average MW ≈ 534,000) were purchased from Sigma Aldrich, potassium chloride was obtained from VWR Prolabo. All chemicals were used without purification, except SWCNTs, 5 ACS Paragon Plus Environment

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the carbon black were washed in 3 M HNO3 and the pyrrole was distilled under N2 flow and kept in inert atmosphere at 4 °C.

2.2 Methods The BET and BJH measurements were carried out on a Quantachrome Auto sorb-IQ-MP gas sorption analyzer. 80 – 120 mg per sample was degassed and dried for 1 h in the temperature range from 150 to 200 ºC prior to the N2 adsorption measurement at 77 K. The Brunauer– Emmett–Teller (BET) surface areas were determined from the adsorption data in the range of relative pressure between 0.01–0.28 and with the consistency criteria (table S1 in supporting information).43 The total pore volume was calculated at a relative pressure of 0.99. The pore widths were estimated by fitting the data using QSDFT method.44 Best fits were obtained with N2 at 77 K on carbon adsorbent with slit/cylindrical pores, QSDFT adsorption branch model. Electrochemical quartz-crystal microbalance (EQCM) tests were carried out using a Metrohm Autolab B.V. PGSTAT204 potentiostat connected with a Metrohm Autolab B.V. EQCM oscillator. The three-electrode measuring cell consisting of an Ag/AgCl reference electrode in 3 M KCl gel (electrode potential E = 0.197 V), a gold counter electrode and the Au-spattered quartz-crystal coated with the composite material was used as a working electrode (5 mm diameter). The quartz crystal has a nominal frequency of 6 MHz and a sensitivity factor of 0.0815 Hz/ng·cm2.45 Each test was carried out in 2 mL of the 0.1 M KCl electrolyte and repeated ten times (the average values are presented in all figures and tables). The EQCM and the cyclic voltammetry were recorded simultaneously in a potential range from -0.5 to 0.5 V and at the scan rate of 50 mV/s. All measurements were carried out at room temperature.

2.3 Synthesis of carbon/PPy composites For the SWCNTs-PPy synthesis, 100 mg of purified SWCNTs were dispersed in 100 mL dist. water and sonicated for 30 min. Subsequently, 25 or 400 mL of the freshly distilled pyrrole was added in order to generate 1:4 and 4:1 of the PPy/C weight ratio, respectively. The iron(II) chloride tetrahydrate / hydrogen peroxide mixture (4 mg / 125 µL and 64 mg / 2 mL with respect to the pyrrole amount) was introduced in order to initiate polymerization, and the suspension was stirred for 6 h at room temperature.46 For the carbon black-PPy composites, 100 mg of carbon black dispersed in 100 mL of an ethanol / 1 M perchloric acid mixture (Vethanol : VHClO4 = 1:5) was sonicated for 10 min. An appropriate amount of Py was added (compositions as above) and the suspension was stirred for 30 min. Furthermore, APS 6 ACS Paragon Plus Environment

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dissolved in 1 M perchloric acid was used as an initiator (98 mg in 10 mL and 1.46 g in 160 mL, respectively to the pyrrole content). The suspension was stirred for 24 h and the reaction was interrupted through the addition of 1 mL acetone.47 The product was washed with dist. water under vacuum filtration until the filtrate became colorless, dried and homogenized in a mortar. 2.4 Preparation of the working electrode PVDF (2 mg) dissolved in isopropanol (2 mL) was sonicated for 0.5 h, mixed with the carbon-polypyrrole composite (19 mg) and homogenized under ultrasound for another 30 min. The resulting ink (5 µL, 4.75 × 10-5 g) was then casted on the quartz-crystal working electrode and dried for 10 min under an ordinary desk lamp. The working electrode was kept in the electrolyte (0.1 M KCl) overnight in order to activate the PVDF binder. Additionally, pure PVDF (2 mg) in isopropanol (2 mL) was casted onto the resonator electrode and the EQCM spectra (background) was taken and subtracted from all measurements carried out for the composite electrodes.

3. Results and discussion 3.1 BET measurements Table 1 shows the specific surface area together with the pore width, the total pore volume and total adsorbed volume for SWCNTs, carbon black, polypyrrole and their composites for the polymer-to-carbon weight ratio of 1:4 and 4:1. Table 1. BET data carbons, polypyrrole and their composites.

Sample

2

ABET [m /g]

Total pore

Total ads.

Ø pore width

volume

volume

[nm]

[cc/g]

[cc/g]

p-SWCNTS

969

1.43

927

5.9

p-CB

70

0.31

198

17.6

PPy

32

0.19

124

24.3

CNTPPy41

542

0.87

561

6.4

CNTPPy14

38

0.14

93

15.3

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CBPPy41

74

0.47

305

25.5

CBPPy14

47

0.59

378

29.9

p-SWCNTS (purified single-walled carbon nanotubes); p-CB (purified carbon black Vulcan XC-72R); PPy (polypyrrole); CNTPPy41 (single wall carbon nanotubes : polypyrrole composite with weight ratio of 4:1); CNTPPy14 (single wall carbon nanotubes : polypyrrole composite with weight ratio of 1:4); CBPPy41 (carbon black Vulcan : polypyrrole composite with weight ratio of 4:1); CBPPy14 (carbon black Vulcan : polypyrrole composite with weight ratio of 1:4). The purified SWCNTs showed large surface area of 969 m2/g, whereas the purified carbon black significantly less that is 70 m2/g. Such difference corresponds to the variation of the pore size. The ultra-small pores contribute to the enlarged surface of the CNTs (Table 1). The bare polypyrrole shows only 32 m2/g since the total pore volume is the smallest and macropores (pores larger than 50 nm) are predominant. The BET area of corresponding composites is lower, especially for the SWCNTs systems as the smallest pores are blocked by the polymer. Accordingly, the average pore diameter increases with an increasing polymer content, and for the 4:1 composition the polypyrrole controls the composite microstructure. For the carbon black/polypyrrole composites, the measured pore width exceeds the pore size of pure polypyrrole. This is also in agreement with the TEM imaging that demonstrates the formation of macropores in the composites with a high polymer content (Figure 1). Herein the difference in the porosity and structure can be clearly seen for both carbon types and their composites with PPy. As shown in Figure 1A, the smallest pores are identified in pure SWCNTs, which are filled when combined with PPy (Figure 1C and D). For the carbon black the particle shape, size and thus the pore size (also the presence of open porosity) is different as compared to the SWCNTs (Figure 1B). The TEM images for the C:PPy = 4:1 (Figure 1E) and C:PPy = 1:4 (Figure 1F) reveal the presence of porous polymer islands or semicontinuous layers, indicating that the polymer has a significant contribution to the average pore size in the composites. Also, at the higher polymer content the open porosity is more noticeable.


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Figure 1. TEM images of the purified SWCNTs (A) and carbon black (B) and the SWCNT/PPy composites with carbon:PPy = 4:1 (C) and 1:4 (D); the CB/PPy at the 4:1 (E) and 1:4 (F) weight ratio, respectively.

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The electrode mass change upon polarization can be investigated using an in situ electrochemical quartz crystal microbalance (EQCM) coupled with cyclic voltammetry. For the gravimetric analysis, the following processes occurring in a bulk of the electroactive material and at the electrode/electrolyte interface were considered. One is the oxidation of polypyrrole associated with the insertion of Cl- ions from the electrolyte, which balance the positively charged polymer chain (oxidized polypyrrole), and the subsequent release chloride anions when polypyrrole is reduced (Figure 2, top). The second major contribution to the electrode mass change is the intercalation of Cl- and K+ ions into the pores of the carbon upon positive or negative polarization, respectively. This mass fluctuation is associated with the formation of a Helmholtz double layer (Figure 2, bottom).

Figure 2. Oxidation (doping) and reduction (dedoping) of polypyrrole in presence of Cl(top)48 and the formation of Helmholtz double layer at carbon surface upon positive and negative polarization in presence of K+ and Cl- (bottom).10 Figure 3 represents the frequency change upon the ion insertion and release together with simultaneously recorded cyclic voltammograms for individual components (Figure 3 A-C) and the respective carbon and polypyrrole composites (Figure 4 A-D). The square-like voltammogram for the p-SWCNTs has typical features of the double-layer capacitor (Figure 3 A). The corresponding EQCM scan demonstrates the frequency variation between -0.5 V and 0.5 V. Starting from the negative polarization of the SWCNTs electrode at -0.5 V (here K+ accumulate at the electrode and balance its negative charge) towards the positive polarization, we observe the frequency increase (mass decrease according Eq. 3) associated with release of cations to the electrolyte. During the reverse scan when the positive potential is applied the frequency decreases due to the anion uptake. The CV scan for carbon black is different 10 ACS Paragon Plus Environment

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(Figure 3 B) as this material exhibits pseudocapacitive properties generated by the redox activity of the surface oxygen-containing functional groups.49 In first attempt, we observe the gold reduction and oxidation at the -0.3 V and +0.2 V, respectively. As the current magnitude is smaller for carbon black, the peaks associated with gold activity are better pronounced as compared to the SWCNTs electrode in Figure 3A. The gold contribution to the electrode mass change is deliberated in the calculations presented in following section of this paper. The relevant frequency response of the carbon black is weaker in comparison to the SWCNTs. This corresponds to the BET surface area that is much larger for carbon nanotubes, providing enormous capacity for the scavenging of ions. The EQCM changes of the carbon black are very weak and correspond to the cation insertion at the beginning of the scan when the electrode is negatively polarized (frequency increase in the range from -0.5 to -0.42 V), followed by the frequency decrease due to the formation of gold oxide, and the electrode mass is constant almost in whole potential range up to 0.3 V. At higher potential the frequency decrease is associated with an additional gold activity (gold oxidation as indicated by the current increase on CV, Figure 3 B). For the reverse scan the rapid frequency drop corresponds to the detachment of chloride ions adsorbed on the positively polarized carbon. The polarization towards negative potentials leads to the minor frequency changes since two processes compete in this potential range. One is the reduction of gold oxide (frequency increase) and the second refers to the cation uptake (frequency decrease). For pure PPy (Figure 3 C), the gold oxidation and reduction peaks are identified at +0.2 and -0.3 V, respectively. Overall, the shape of the CV curve is similar to the pseudo-capacitive carbon. Since the BET surface area of the polymer is low, its contribution to the frequency changes from the ion uptake/release during polymer oxidation/reduction is weak. The Cl- that compensate the positive charge of the oxidized polymer chain are released at low potentials as indicated by the frequency increase. For the composites (Figure 4 A-D), the general observations is that all EQCM scans follow a trend observed for the pure polymer (Figure 3 C). This indicates that the Cl- doping/release upon oxidation and reduction of the polymer has significant contribution to the electrode mass changes, regardless the carbon type and a low BET surface of the polymer. For the SWCNTsbased composite, the CV scans show features typical for the double-layer capacitor (as for pure SWCNTs, Figure 3 A), even for the high polymer content (Figure 4 B). This demonstrates that double layer current is a direct function of the BET surface area, which is common for the porous carbons.10, 12-16 For the carbon black-polypyrrole (Figure 4 C-D) the shape of voltammograms is almost identical as recorded for the pure polymer (Figure 3 C), 11 ACS Paragon Plus Environment

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indicating that the polymer redox activity (pseudo-capacitance) has a significant contribution to the total measured capacitance of the system. Also, the BET surface area of the carbon black and the polymer are in the same range of magnitude. This demonstrates that the doublelayer capacitance is negligible, as the relevant current is one order of magnitude lower and the frequency changes weaker when compared to SWCNTs-PPy samples (Fig. 4 A and B). The composites with a significant difference in BET due to the polymer content as for the SWCNTs-PPy 1:4 and 4:1 (Table 1), demonstrate almost the same frequency change (∆f = 23 Hz and 25 Hz, respectively). Also, for the carbon black-PPy at 1:4 ∆f = 20 Hz and for the 4:1 formulation it is 13 Hz. This result suggests that both BET and the polymer content are not discriminating parameters and the total amount of ion incorporated is similar and balanced by the double-layer or pseudo-capacitance effects (these vary with the composition). Regarding the frequency change for the SWCNTs-PPy, at low polymer content (Figure 4 A) the frequency drops rapidly upon polarization at -0.5 V, followed by slow frequency decay up to 0.1 V, as anions are expelled from both the carbon and polymer. A small increase in the resonance frequency is observed between 0.5 V and 0.45 V where all anions are released, followed by the frequency increase in the potential range from 0.4 to 0.2 V due to the absorption of K+. For the SWCNTs-PPy 1:4 (Figure 4 B), the trend in frequency indicates that the ion exchange is slower when compared to the 4:1 formulation. For the sample containing more polypyrrole, the ion dynamic is governed by the polymer, which is different as for pure carbon. The second reason can be the modification of the electrical conductivity of composite electrode that can be improved by increase of the carbon content, which in our case, facilitates fast responses towards the ion exchange. Similar trend is observed for the carbon black-PPy (Fig. 4 C and D), with more significant frequency change for the 4:1 composition.


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Figure 3. Cyclic voltammograms and the frequency change measured by EQCM for p-SWCNTs (A), p-CB (B), PPy (C).


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Figure 4. Cyclic voltammograms and the frequency change measured by EQCM for SWCNT/PPy composites in mass ratio C:PPy = 4:1 (A) and 1:4 (B), as well as for CB/PPy composites in mass ratio C:PPy = 4:1 (C) and 1:4 (D).

3.3 Determination of electrochemically accessible surface area The frequency change corresponding to the ion dynamics in the active materials was calculated by subtracting the frequency response of the bare gold electrode and the signal of the pure binder (PVDF) according to the equation: ∆ = ∆

∆ ∆

(2)

where ∆ corresponds to the electrode frequency change of PVDF binder, and the

∆ is the response of the bare gold electrode, ∆

the total frequency change of

the sample with the binder casted on the gold resonator. The mass change related to the ion exchange in the active material is calculated using the Sauerbrey’s equation:50 ∆ =

∙!"#

$%& ∙ '&

∙ ∆(

= ! ∙ ∆(

(3)

where Cf is the sensitivity factor for the type of resonator applied in our work and is equal 0.0815 Hz/ng·cm2 at 20 °C.45 The mass change, ∆m [g] is further used to calculate the electrochemically accessible surface area, AEQCM according to the Eq. 4:51 )*+,- =

│∆│ -/

∙ 01 ∙ 2 ∙ 3

(4) 14


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where Δm [g] is the mass change, MW (molecular weight) of potassium cations [39.09 g/mol] for the p-SWCNTs and carbon black, and MW of chloride anions [35.45 g/mol] for the pure polypyrrole and the respective composites (calculated after the background correction according to Eq. 2 and 3). N is the Avogadro number and R the ionic radius for potassium cations [0.138 nm] and chloride anions [0.181 nm], respectively.52 Important notice is that the EQCM curves in Fig. 3 and 4 represent the total frequency fluctuation of the active coating without PVDF and gold correction. These scans demonstrate only the general trend for the composites and the individual components, and were used for quantification after the necessary baseline subtraction. Table 2 summarizes the frequency changes, electrode mass change upon ion uptake/release and the electrochemically active surface area calculated from Eq. 4. Table 2. Frequency change upon ion exchange in active materials (∆f), mass change (∆m) and the electrochemically active surface area of the capacitor electrode made from pure SWCNTs, carbon black, polypyrrole and their respective composites.

Sample

∆fsample [Hz]

∆m [10-7 g]

AEQCM (ions) [m2/g]

p-SWCNTS

-21.91

1.854

3.588 (K+)

p-CB

-16.43

1.391

2.691 (K+)

PPy (50 mV/s)

-30.00

2.538

9.345 (Cl-)

PPy (25 mV/s)

-26.40

2.234

8.225 (Cl-)

PPy (15 mV/s)

-26.47

2.240

8.245 (Cl-)

PPy (5 mV/s)

-28.15

2.382

8.770 (Cl-)

CNTPPy41

-39.79

3.367

12.40 (Cl-)

CNTPPy14

-30.03

2.541

9.354 (Cl-)

CBPPy41

-34.72

2.938

10.82 (Cl-)

CBPPy14

-31.54

2.669

9.825 (Cl-)


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Regarding the electrode mass change, all samples containing polymer demonstrate significant mass variation (for some compositions almost double) as compared to pure carbons (Table 2). These results indicate that there is no direct correlation of mass exchange with the BET surface, leading to the conclusion that the charge accumulation/release is more complex or is associated with other processes. Bulk porosity, hydrophobic/hydrophilic porosity, and pore size distribution are another very important parameters. The tendency observed in this study is that for bigger pores the mass change is substantial as compared to the samples with the smallest pores (SWCNTs and its composite with PPy, especially at 4:1 weight ratio, Table 1). Although, all samples showed micro-porosity (pore size < 2 nm) the difference in ion exchange can be primarily correlated with the pore structure. For the pure SWCNTs and carbon black the micropores do not participate in ion exchange (or the contribution is minor). Although they contribute to the BET surface expansion and are hydrophilic53, these electrodes did not show the maximum of mass change (Table 2). The increase in pore size for the pure polymer and its composites is noticeable but not large enough to generate a major difference in mass uptake/release (histograms of the pure carbon black and the polymer shown in supporting information demonstrate very similar pore size distribution). Therefore, larger mass change for the polypyrrole-containing samples is attributed to the open porosity (confirmed by TEM observations, Fig. 1). The open and big pores are more accessible for charged species (as confirmed by an increased total pore volume for pore in similar size, shown in histograms of SWCNTs-PPy 1:4 and 4:1 compositions; supporting information). This is obvious, particularly for the composites with low polymer content (4:1) for both carbons, and is also correlated with TEM imaging. The small polymer concentration facilitates the separation of carbon particle (open porosity) and an even distribution of the polymer on the carbon surface. Both carbons tend to agglomerate for 1:4 formulations (Figure 1 D and F) thus the accessible surface is reduced and the respective mass uptake lower, as compared to 4:1 compositions (Table 2). Considering the hydrophobic/hydrophilic pores, both the SWCNTs and carbon black have oxygen-containing functionalities as confirmed by XPS surface analysis,49 which facilitate good wettability and water (electrolyte) management. More hydrophobic surface is expected for pure polymer. In this study we performed a longterm conditioning of each electrode (by immersing in electrolyte 24 hours prior CV and EQCM measurement and with a proper electrochemical equilibration followed by CV-EQCM test). This allowed to exclude the time decay for an ion exchange due to hydrophilic imbalance.


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The EQCM surface calculated from Eq. 4 is presented in Figure 5 together with BET surface of individual components and their composites. In general, the electrochemical surface area increases with decreasing BET surface. For pure carbons, the difference in EQCM is very small, regardless the enormous variation in BET surface area (Figure 5). This effect can be interpreted in respect to the pore size as larger pores would favor ion accumulation. The second reason are the oxygen-containing functionalities (e.g., carboxylic, phenolic, hydroxyl49) present on the carbon surface - these generate pseudo-capacitance effects, resulting in the scavenging of extra charge and contribute to the improvement of the total charge density. The pure polymer showed large EQCM surface as compared to both carbons. This is rather surprising as its BET surface is the smallest among all tested samples. Considering the pore size, since the polypyrrole morphology is dominated by open porosity, this would explain some improvement in ion accumulation. However, the pore size/structure is not discriminating since the pore width for the PPy and carbon black are very similar. In this regard, higher ion uptake (EQCM surface) observed for polymer-containing should be correlated with different mechanism of charge distribution and storage in these materials, as compared to the carbon-based double-layer capacitors.11 Furthermore, the enhancement in charge density is generated by multiple redox active centers that are localized approximately every third monomer unit in polypyrrole.54 Their number can be increased by an elongation of the polymer chain – this contributes in the charge scavenging. An important observation is that composites with the carbon-to-polymer weight ratio of 4:1 show larger electrochemical surface (EQCM) in comparison to 1:4 formulations, and slightly larger than the polymer alone. This is apparently related to the polymer structure/size and the composite morphology. Firstly, the polymer content, which is adjusted in order to generate a non-continuous polypyrrole coating over the carbon surface (Figure 1 D insert and 1 F with yellow circles highlight the polymer fraction) facilitates an access of ions to the carbon and the polymer redox active centers. This generate the synergy of double-layer (carbon) and pseudocapacitance (polymer). Second, very important features are the carbon-polymer junctions – these are considered as the most effective charge anchoring centers that contribute to the capacitance improvement.11 Another critical parameter is the size of the polymer particles, which is in the range from 40 to 100 nm (Figure 1 E and F). As demonstrated in our previous study,55 an effective diffusion length that facilitates a fast ion exchange can be obtained for very small particles (diameter or thickness less than 100 nm). Correspondingly, the process of ion doping is very fast since they can access the majority or all redox active centers located in bulk polymer, and thus the pseudo-capacitive charge exchange is the most efficient (the 17 ACS Paragon Plus Environment

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utilization of active centers is the highest). For electrodes with higher polymer content (1:4 weight ratio of the carbon-to-polymer) we observe agglomeration, especially for the samples with carbon black (Figure 1 F). In this case the majority of the electrochemical surface is obstructed, thus both the carbon double-layer and the polymer redox centers become inactive/inaccessible for ions. In order to investigate the effect of the potential scan rate on the electrode ion exchange, the voltammograms were recorded at several different scanning speed for pure polymer (Table 2). The polypyrrole was chosen as the representative sample since the morphology (grain size and film thickness) strongly influences its conductivity (doping process) when compared to the carbon-based samples. As demonstrated in Table 2, the mass change and thus the EQCM area are similar regardless the potential scan rate, indicating that ion dynamics (the rate of adsorption/desorption) has no effect on the measured quantities. Thus the polymer effective diffusion length is optimal for the structures synthetized in this work.55

Figure 5. The specific surface area (A) and electrochemically accessible area (B) for K+ in case of p-SWCNTs and p-CB and by Cl- for PPy and its composites. In the following section the EQCM and BET surfaces are correlated with the specific capacitance estimated by cyclic voltammetry. The specific capacitance of a single capacitor plate, CS, can be calculated from the voltammogram using the following measures56: 18 ACS Paragon Plus Environment

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4 =

56

(6)

∙∙∆∙7

where ACV represents the area of integration of the CV scan, m the mass of active material, ΔV the potential window of the CV measurement and sr is the potential scan rate. Figure 6 represents the specific capacitances of the individual components and their composites standardized to BET (A) and EQCM (B) areas. In general, for BET-corrected capacitance (Fig. 6 A) all carbons and the polymer show similar capacitance. This is very different as compared to the mass-corrected capacitance (F/g) discussed in our previous study, showing the maximum capacitance for the SWCNTs and the lowest for the polymer.49 The BET surface area for SWCNTs is very large relating to the polymer and even to purified carbon black (Table 1). SWCNT’s specific capacitance is affected by the pore structure, and thus the high volume of micropores (not accessible for chlorides), and it also does not correlate with the charge accumulation. Further, the open porosity (through pores) facilitates this process in the polymer. Thus both electrodes demonstrated similar BET-corrected capacitance. For the carbon black, the double-layer effect is accompanied by the pseudo-capacitance from the redox activity of oxygen-containing functionalities present on the surface. Overall, for composites the capacitance is improved as compared to the individual components due to the synergy of the double-layer (carbon) and the pseudo-capacitance (polymer). The SWCNTsPPy sample with high polymer content shows the best capacitance, owing to its morphology (TEM image in Figure 1 D demonstrates very homogenous distribution of the nano-sized polypyrrole beads on the tube surface). Such structure generates many junctions/charge scavengers, leading to the improvement of the total charge storage. This result also agrees with the capacitance standardized to the EQCM effective surface. SWCNTs demonstrate better capacitance than the carbon black, as the redox activity of carbon black occurring in the same potential range will compete/counterbalance the ion storage interphase (some of the redox processes are irreversible, leading to the deactivation of carbon surface). Although the purified SWCNTs showed capacitance comparable to the polypyrrole-containing samples (Fig. 6 B), this difference is still in the same range of magnitude and some traces of solvent co-insertion might be still possible (this could affect the measured mass change and thus a slightly higher capacitance for SWCNTs). In summary, the trend of EQCM-corrected specific capacitance agrees with the mass-specific capacitance (Fig. S8). In order to further investigate the correlation of capacitance with the EQCM surface, the Faraday’s law of electrolysis (Eq. 7) is applied in order to cross-check the molecular weight of the species adsorbed at the electrode surface18: 19 ACS Paragon Plus Environment

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(=

9 +∙-8

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(7)

∙

Where Q is charge passed thru the electrode in Coulomb, Mw’ is the molecular weight of the species adsorbed on the electrode, n is the valence of the ion, and F is the Faraday constant (96 485 C/mol). Faraday’s law predicts an adsorption of monolayer of the corresponding ions thus the calculated molecular weight should be close to the mass of chloride (samples with polypyrrole), and potassium cation (pure carbons). This allows to estimate the solvation number (nsol) and to correlate the trend of capacitance in Fig. 6 B18: :7 =

9 ;-8 8(=>?@ ABC)

(8)

-8E# F

Where Mw(bare ion) and MwH2O are molecular weight of ions and solvent, respectively. Table 3 summarizes above calculations, indicating that all samples with SWCNTs showed no solvation effect. In contrary, the pure polypyrrole, carbon black and their composites revealed the solvent co-insertion (nsol > 1). In relevance to the EQCM analysis, the mass increase is observed for the PPy-containing samples and for the carbon black where additional species contribute in pseudo-capacitance of the carbon black (e.g., the quinone redox active centers). The solvation is not affected by the scan rate as masses for pure PPy are similar regardless the potential scanning, as shown in Table 2. Regarding the pore size, since all samples demonstrate an average size that is larger than a size of solvated ions, this is not an issue in any case. An open question may be then the shape of the pores and its hydrophilic/hydrophobic nature, which strongly depends on the synthesis method.


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Figure 6. The specific capacitance standardized per BET surface (A) and EQCM surface (B) determined by the cyclic voltammetry at a scan rate of 0.05 V/s.

Table 3. Charge passed through the electrode in Coulomb (calculated from the integration of CV scan in Figure 3 and 4), mass change by EQCM (∆m) and the molecular weight of the species that interact with the electrode during the electrochemical test, M’, (calculated from Faraday’s law according to Eq. 7, solvation number, (nsol) for pure SWCNTs, carbon black, polypyrrole and their respective composites.

Sample

Q [C]

∆m [10-7 g]

M’

nsol

p-SWCNTS

2.77×10-3

1.854

6.54

-1.81

p-CB

1.58×10-4

1.391

84.9

2.54

PPy (50 mV/s)

8.19×10-5

2.538

299

14.6

CNTPPy41

6.63×10-3

3.367

4.90

-1.70


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CNTPPy14

5.44×10-3

2.541

4.51

-1.72

CBPPy41

2.63×10-4

2.938

108

4.02

CBPPy14

9.77×10-5

2.669

264

12.7

4. Conclusion In situ EQCM coupled with voltammetry was applied to verify the electrochemical and specific surface area of the carbon-polymer composites in conjunction with their capacitance. The porosity and surface measurements investigated by the BET method revealed that the average pore size increases for composites as compared to pure carbons. The polypyrrole morphology has significant influence on BET surface, resulting in meso-porous composites in most cases. The electrochemical area accessible for ions was verified for the first time using the electrochemical quartz crystal microbalance technique. The electro-gravimetric tests demonstrated also the significant effect of polymer on the electrode mass change. The mass fluctuations agree with the pore size trend but not with the BET results, and showed that regardless the specific surface, the highest electrochemical area is accomplished for all samples containing the polymer. Furthermore, the total capacitance studied by cyclic voltammetry demonstrated that the best capacitor can be made from the SWCNTspolypyrrole, and the synergy of the double- and pseudo-capacitance can be observed for composites. For pure polymer, the EQCM area is large as the size of polymer particles is optimal for an effective ion exchange (effective diffusion length). However, the capacitances (both EQCM and BET corrected) are lower than for the composites. The combined carbonpolymer system studied in this work demonstrated the capacitance improvement generated by the synergy of double-layer and pseudo-capacitance and with a proper particle design (polymer size and its homogeneous distribution onto the carbon). The EQCM is a valuable screening method allowing to correlate the electrochemical features with the composition and morphology of the carbon-polypyrrole capacitor electrodes.

5. Acknowledgement We thank Carl-Zeiss Foundation and ProChance program at FSU for financial support. The cryo-TEM studies were carried out at the TEM facility in the Jena Center for Soft Matter (JCSM) supported by DFG (German Research Foundation) and the EFRE (European Fund for 22 ACS Paragon Plus Environment

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Regional Development). O.A. thanks the “Evangelisches Studentenwerk Villigst e.V.” for a scholarship.

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(17) Chmiola, J.; Yushin, G.; Gogotsi, Y.; Portet, C.; Simon, P.; Taberna, P. L., Anomalous increase in carbon capacitance at pore sizes less than 1 nanometer. Science 2006, 313, 1760 1763. (18) Tsai, W.-Y.; Taberna, P.-L.; Simon, P., Electrochemical Quartz Crystal Microbalance (EQCM) Study of Ion Dynamics in Nanoporous Carbons. J. Am. Chem. Soc. 2014, 136, 87228728. (19) Richey, F. W.; Dyatkin, B.; Gogotsi, Y.; Elabd, Y. A., Ion dynamics in porous carbon electrodes in supercapacitors using in situ infrared spectroelectrochemistry. J. Am. Chem. Soc. 2013, 135, 12818 - 12826. (20) Peng, C.; Zhang, S.; Jewell, D.; Chen, G. Z., Carbon nanotube and conducting polymer composites for supercapacitors. Prog. Nat. Sci. 2008, 18, 777-788. (21) Simon, P.; Gogotsi, Y., Capacitive Energy Storage in Nanostructured Carbon– Electrolyte Systems. Acc. Chem. Res. 2013, 46, 1094-1103. (22) Xie, Q.; Kuwabata, S.; Yoneyama, H., EQCM studies on polypyrrole in aqueous solutions. J. Electroanal. Chem. 1997, 420, 219-225. (23) Weidlich, C.; Mangold, K. M.; Jüttner, K., EQCM study of the ion exchange behaviour of polypyrrole with different counterions in different electrolytes. Electrochim. Acta 2005, 50, 1547-1552. (24) Syritski, V.; Öpik, A.; Forsén, O., Ion transport investigations of polypyrroles doped with different anions by EQCM and CER techniques. Electrochim. Acta 2003, 48, 1409-1417. (25) Mirmohseni, A.; Mohammadreza, M.; Hassanzadeh, V., Ion exchange properties of polypyrrole studied by electrochemical quartz crystal microbalance (EQCM). Polym. Int. 1999, 48, 873 - 878. (26) Baba, A.; Tian, S.; Stefani, F.; Xia, C.; Wang, Z.; Advincula, R. C.; Johannsmann, D.; Knoll, W., Electropolymerization and doping/dedoping properties of polyaniline thin films as studied by electrochemical-surface plasmon spectroscopy and by the quartz crystal microbalance. J. Electroanal. Chem. 2004, 562, 95-103. (27) de Albuquerque Maranhão, S. L.; Torresi, R. M., Quartz crystal microbalance study of charge compensation process in polyaniline films doped with surfactant anions. Electrochim. Acta 1999, 44, 1879-1885. (28) Snook, G. A.; Chen, G. Z.; Fray, D. J.; Hughes, M.; Shaffer, M., Studies of deposition of and charge storage in polypyrrole–chloride and polypyrrole–carbon nanotube composites with an electrochemical quartz crystal microbalance. J. Electroanal. Chem. 2004, 568, 135142. (29) Akieh, M. N.; Latonen, R.-M.; Lindholm, S.; Ralph, S. F.; Bobacka, J.; Ivaska, A., Electrochemically controlled ion transport across polypyrrole/multi-walled carbon nanotube composite membranes. Synth. Met. 2011, 161, 1906-1914. (30) Koehler, S.; Ueda, M.; Efimov, I.; Bund, A., An EQCM study of the deposition and doping/dedoping behaviour of polypyrrole from phosphoric acid solutions. Electrochim. Acta 2007, 52, 3040 - 3046. (31) Bahrami-Samani, M.; Cook, C. D.; Madden, J. D.; Spinks, G. M.; Whitten, P. G., Quartz crystal microbalance study of volume changes and modulus shift in electrochemically switched polypyrrole Thin Solid Films 2008, 516, 2800 - 2807. (32) Eslami, M. R.; Alizadeh, N., A dual usage smart sorbent/recognition element based on nanostructured conducting molecularly imprinted polypyrrole for simultaneous potentialinduced nanoextraction/determination of ibuprofen in biomedical samples by quartz crystal microbalance sensor Sens. Actuators B Chem. 2015, 220, 880 - 887. (33) Ratautaite, V.; Plausinaitis, D.; Baleviciute, I.; Mikoliunaite, L.; Ramanavicience, A.; Ramanavicius, A., Characterization of caffeine-imprinted polypyrrole by a quartz crystal microbalance and electrochemical impedance spectroscopy. Sens. Actuators B Chem. 2015, 212, 63 - 71. 24 ACS Paragon Plus Environment

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Specific Surface versus Electrochemically Active Area of the Carbon

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