Journal of Power Sources 304 (2016) 102e110
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Capacitance properties and structure of electroconducting hydrogels based on copoly(aniline e p-phenylenediamine) and polyacrylamide Michael A. Smirnov a, c, *, Maria P. Sokolova b, Natalya V. Bobrova a, Igor A. Kasatkin b, Erkki Lahderanta c, Galina K. Elyashevich a a b c
Institute of Macromolecular Compounds, Russian Academy of Sciences, Bolshoy pr. 31, Saint Petersburg 199004, Russia St. Petersburg State University, Universitetsky pr. 26, Peterhof, Saint Petersburg 198504, Russia Lappeenranta University of Technology LUT, Skinnarilankatu 34, Lappeenranta 53850, Finland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Electroactive hydrogels based on PAAM and polyaniline copolymers were prepared. Optimal composition for highest properties was found. Hydrogels display a high specific capacitance in wide range of current densities. After 1000 cycles the hydrogels keep more than 100% of their initial capacitance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 June 2015 Received in revised form 6 November 2015 Accepted 8 November 2015 Available online xxx
Electroconducting hydrogels (EH) based on copoly(aniline e p-phenylenediamine) grafted to the polyacrylamide for the application as pseudo-supercapacitor's electrodes have been prepared. The influence of preparation conditions on the structure and capacitance properties of the systems were investigated: we determined the optimal amount of p-phenylenediamine to obtain the network of swollen interconnected nanofibrils inside the hydrogel which provides the formation of continuous conducting phase. Structure and morphology of the prepared samples were investigated with UVeVIS spectroscopy, scanning electron microscopy (SEM) and wide-angle X-ray diffraction (WAXD). The maximal value of capacitance was 364 F g1 at 0.2 A g1. It was shown that the EH samples demonstrate the retention of 50% of their capacity at high current density 16 A g1. Cycle-life measurements show evidence that capacitance of EH electrodes after 1000 cycles is higher than its initial value for all prepared samples. Changes of the copolymer structure during swelling in water have been studied with WAXD. © 2015 Elsevier B.V. All rights reserved.
Keywords: Electroconducting hydrogel Polymer structure Pseudo-supercapacitor Polyaniline Polyacrylamide p-phenylenediamine
1. Introduction * Corresponding author. Institute of Macromolecular Compounds, Russian Academy of Sciences, Bolshoy pr. 31, Saint Petersburg 199004, Russia. E-mail addresses:
[email protected] (M.A. Smirnov),
[email protected] (M.P. Sokolova),
[email protected] (N.V. Bobrova), igor.kasatkin@spbu. ru (I.A. Kasatkin), Erkki.Lahderanta@lut.fi (E. Lahderanta),
[email protected] (G.K. Elyashevich). http://dx.doi.org/10.1016/j.jpowsour.2015.11.035 0378-7753/© 2015 Elsevier B.V. All rights reserved.
During the last decade the supercapacitors attract considerable attention due to their promising properties [1e3]. A large number of organic and inorganic materials was investigated as candidates for supercapacitor electrodes. Great interest of researches is focused on carbon nanotubes [4], graphene with metal oxides [5,6]
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or reduced graphite oxide [7] possessing good electrochemical properties and high stability. However, these materials have significant drawback - namely a high cost [3] in comparison with the organic electroactive polymers such as polypyrrole and polyaniline (PANI) which can be synthesized by relatively simple electrochemical or chemical procedures [8]. Although electroconducting polymers demonstrate cycling stability and specific capacitance lower than the best inorganic materials they have high potential for development [9,10]. Two factors that significantly restrict the properties of polymeric electrodes are: 1) slow diffusion of ions inside the bulk of the electrode [3], and 2) low compatibility of electroconducting polymers with electrolyte phase [11]. The first factor reduces the power density of batteries while the second one restricts its specific capacity. Consequently, thickness of electrode should not be too large for effective charge storage. It was shown in Ref. [12] that an increase in electrode thickness from 300 nm to 6 mm leads to the decreasing of specific capacity by one order of magnitude. The problem of the limited ion diffusion is very important one for supercapacitors [11] because the storage of charge in the whole volume of electrode would improve its effectiveness [13]. For this purpose authors use porous templates for preparation of conducting polymer electrode: for example, graphene-mesoporous silica nanosheet [14] or three-dimensional cross-linked carbon network [15]. In such systems ions rapidly permeate inside the active material due to its through pore structure. Another approach which is less popular but more simple is to use the cross-linked composite of conducting polymer with water soluble component [11]. Such systems, so called electroconductive hydrogels (EH), swell in water and at the same time act as ionic and electronic conductors [16,17]. This permits to increase both the rate of ion diffusion and the compatibility of conducting polymer with electrolyte. One else problem of EH electrode materials is the formation of continuous electroconducting phase inside the hydrogel. This is also related to low compatibility of conducting polymers with water solutions. Methods to increase connectivity of electroconducting phase in nonhydrogelic electrode materials are the preparation of electroactive polymers in the form of nanotubes or nanofibers [18e20] or the introduction of different nanocarbon materials into the polymer matrix [21,22]. Preparation of conducting polymer in anisometric form (for example, nanofibers) decreases the percolation threshold for electroconducting phase. In this work we used p-phenylenediamine (pPhDA) as an effective regulator of PANI morphology [23,24]. In addition, it is reasonable to assume that introducing polar eNH2 groups will increase the compatibility of conducting polymer with water thus reducing the number of contacts between elements of electroconducting phase. The goal of this work is to investigate the influence of different amounts of additive component (pPhDA) on structure and capacitance of electrode materials. It should be noted that usual method of electroconducting hydrogels preparation [25e28] which is based on in situ synthesis of electroconducting polymer inside the cross-linked polymer matrix does not allow to achieve homogeneous distribution of its phase. Another way is the preparation of water compatible dispersion of conducting polymer with subsequent cross-linking of the system [11]. The procedure including synthesis of PANI dispersion in polyacrylamide (PAAM) water solution was elaborated in our previous work [17]. It was pointed out in literature [29] that grafting of conducting polymer on-to back-bone of PAAM takes place during oxidative polymerization of aniline in the solution of this polymer. As a result, the compatibility of PANI with water is significantly increased. In present study this method is modified to achieve higher processability of the system and to improve its properties.
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2. Experimental 2.1. Materials Acryalmide (AAM), ammonium peroxidisulfate (PSA), N,N,N0 N0 tetramethylethylenediamine (TMED), and aniline (ANI) hydrochloride were purchased from SigmaeAldrich and used as received without purification. Para-phenylenediamine (MERK) was recrystallized from water solution prior to use. Methanol and hydrochloric acid were obtained from Vekton (Russia). All solutions were prepared in distilled water. 2.2. Preparation of PAAM PAAM was prepared in 3.75% solution of AAM in deaerated water by radical polymerization with mixed initiator PSA-TMED at 40e50 C. Molar ratio between monomer and components of initiator was AAM:PSA:TMED ¼ 125:1:1. The polymer was precipitated in methanol and dried under vacuum at 40 C. Weightaverage molecular mass (Mw) of PAAM was calculated from characteristic viscosity ([h]) of polymer solution in 1 M aqueous NaNO3 at 30 C according to equation [h] ¼ 3.73 104 M0.66 [30] and was w found to be 330 000. The chemical structure of the polymer was confirmed by Raman spectroscopy. 2.3. Synthesis of EH The chemical oxidative copolymerization of ANI and pPhDA was carried out in water solution saturated with nitrogen and containing 5.7% of PAAM and 1 M of HCl. The mole ratio pPhDA/ANI was equal to 0.025, 0.043, and 0.1 (samples EH-0.025, EH-0.043, and EH-0.1, respectively). For comparison the sample without addition of pPhDA (EH-0) was also synthesized. Reaction was conducted at 0 C in the thermostatic flask connected to refrigeration thermostat and equipped with magnetic stirrer. The solution of oxidant e PSA was cooled till 0 C and then added to the reaction mixture to start the polymerization. The sum of monomer concentrations was 0.26 M and the oxidant/(ANI þ pPhDA) mole ratio was equal to 1. Thus the theoretical mass ratio between conducting polymer and PAAM is equal to 30:70 and the concentration of conducting polymer after polymerization is 2.45% by mass for all prepared samples. Reaction was allowed to proceed for 2 h and then the mixture was precipitated into methanol. Obtained green mass was dried in vacuum without heating and then it was swollen in 1 M HCl water solution. A sample of pure PANI without PAAM and pPhDA was also prepared with the same procedure. 2.4. Characterization methods 2.4.1. Measurement of swelling ability The swelling degree (Q) was measured gravimetrically by immersion of dried EH into the electrolyte (1 M HCl in water) until the constant mass was reached. The equation: Q ¼ (msemd)/md, where ms, and md are the masses of swollen and dried samples respectively, was used for calculation of Q. 2.4.2. Scanning electron microscopy Morphology of synthesized samples in dry state was investigated by scanning electron microscope ZEISS MERLIN (Germany). The specimens were dispersed in water on the ultrasonic bath. Then the dispersion was cast on the table of microscope and investigated at 10 kV voltage. Due to electroconducting nature of the samples the high quality pictures were obtained without covering of them with metal or carbon layer.
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2.4.3. Wide-angle X-ray diffraction The systems in dry and swollen state were investigated with wide-angle X-ray scattering (WAXS) on the D2 PHASER diffractometer (Bruker, Germany) equipped with CoKa radiation source (l ¼ 1.79 Å). To compare XRD patterns with ones measured with CuKa sources all results were recalculated. Patterns were obtained in the range of scattering angles 2q ¼ 5 e60 with step of 0.05 , and 2 s time per step. 2.4.4. NMR investigation The samples for NMR were prepared by treatment of dry and grinded hydrogels with 1 M water solution of NH3 on the ultrasonic bath. Obtained dispersion was precipitated in methanol, dried and grinded again. The 13C NMR experiments were conducted at room temperature on a Bruker Avance III 400WB solid state NMR spectrometer (magnetic field of 9.4 T) using a 4-mm CP/MAS probe. The resonance frequencies for 13C and 1H were 100.64 MHz and 400.23 MHz respectively. Sample spinning rate was 12 kHz. The 13C NMR spectra were acquired with 3072 scans, an excitation time of 3.8 ms, and recycle delay of 10 s. The 13C CP/MAS spectra were acquired with 4096 scans, a contact time of 2.0 ms, and recycle delay of 10 s. 2.4.5. Ultravioletevisible spectroscopy Ultravioletevisible (UVeVIS) spectra were obtained with SF2000 spectrophotometer (OKB “Spectr”, Russia) in the range 250e1000 nm for the samples dispersed in water on the ultrasonic bath. 2.5. Electrochemical measurements Samples for electrochemical measurements were prepared by casting of PAAM/PANI water dispersions onto the graphite electrode with subsequent immersion in methanol for precipitation of the polymers and drying at room temperature. The amount of polymerization mixture on one electrode varied from 177 to 417 mg. A working electrode prepared by this procedure was mounted into the cell with silver-chlorine reference and Pt-wire counter electrodes. Then the cell was filled with electrolyte (1 M HCl) and the polymer was allowed to swell until the constant open circuit potential was reached. Redox properties of the electrodes were studied by cyclic voltammetry at the scan rate 4 mV s1. Specific electrical capacitance of EH samples was determined in the galvanostatic mode with current densities in the range 0.2e32 A g1 relatively to the mass of conducting polymer in the EH sample. Cycling stability was measured by voltammetry with the scan rate of 100 mV s1 during 1000 cycles of chargeedischarge. The cyclic voltammetry (CVA), galvanostatic chargeedischarge and cycle life measurements were conducted in three-electrode cell. All electrochemical experiments were performed with P-30J potentiostat-galvanostat (Elins, Russia).
polymerization the viscosity is still higher than its value for initial PAAM solution. It can be concluded that some amount of PANI side chains remain in swollen state. It was observed that increasing of the pPhDA content leads to the growth of system viscosity. It can be explained by introduction of the additional polar eNH2 group which increases compatibility of the polymer and the solvent (water). Coloration of the solution proceeds faster when the amount of pPhDA increases. This is in accordance with results published by authors [31] who measured the changes of UVevisible spectra during polymerization. In literature it is pointed out that pPhDA accelerates the reaction due to lower oxidation potential [32] in comparison with ANI. Polymer phase can be separated from the reaction mixture by precipitation in methanol. After drying, it forms dark-green mass which is capable to swell in water solution of hydrochloric acid and does not dissolve completely without additional stimuli such as ultrasonic treatment. It can be supposed that during drying the physical cross-links are formed by intermolecular contacts of hydrophobic PANI chains which are grafted onto PAAM backbone. The cross-links are reversibly aroused even after the complete destruction of hydrogel. Swelling ability (Q) of hydrogels in electrolyte was equal to 19, 16, and 23 g g1 for the samples EH-0.025, EH-0.043, and EH-0.1, respectively. The data for EH-0 could not be obtained reliably because this sample has extremely poor mechanical properties in swollen state. Swelling ability of hydrogels depends on the crosslink density, difference of osmotic pressure inside and outside of the sample, and the thermodynamical compatibility of the polymer with solvent. In our case increasing of amount of hydrophilic eNH2 groups which are protonated in acidic electrolyte leads to increasing of the compatibility of the conducting polymer with water. At the same time the consequence of this is the growth of the probability for PANI molecules to form intermolecular hydrophobic contacts due to interaction of benzene rings thus increasing the degree of cross-linking of the sample. These two factors act in the opposite directions leading to the non monotonous dependence of swelling degree on the pPhDA content. The minimal value of Q was achieved for EH-0.043 which means that copolymer with this composition is characterized by the maximal degree of crosslinking. 3.2. Spectroscopic characterization Chemical structure of prepared samples was characterized with UVeVIS spectroscopy. The spectra for dispersion of EH in 1 M HCl are presented in Fig. 1. It is seen that position of bands for EH prepared without pPhDA (Fig. 1a) is typical for conventional PANI.
3. Results and discussion 3.1. EH preparation and swelling ability At the initial periods of oxidative copolymerization of ANI and pPhDA in the presence of PAAM the viscosity of solution increases (it was not observed in the case of PANI synthesis without PAAM). This is in accordance with work [29] where it was pointed that ANI is able to graft onto the PAAM backbone in the process of oxidative polymerization that increase the solubility of PANI oligomers and suppress their nucleation. In the course of reaction the viscosity gradually decreases. This can be explained by the contraction of growing PANI chains. Nevertheless at the final stage of
Fig. 1. UVeVIS spectra of EH prepared with different pPhDA/ANI ratio: 0 (a), 0.043 (b), and 0.1 (c).
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Absorption bands located at 350 and 435 nm correspond to the pep* transition of the benzenoid rings [33] and to the presence of cation-radicals [34], respectively. Broad absorption band centered near 800 nm is the sum of transitions in localized polaron structures and in quinoid rings [35]. These results confirm that PANI in electroconducting form (emeraldine salt) in the presence of PAAM is successfully formed. The polymers prepared with pPhDA (Fig. 1b and c) have similar spectra as it was also observed by authors of [36]. At the same time some small distinctions can be seen: the new band near 300 nm appears with the increase in the pPhDA/ANI ratio from 0.043 to 0.1. This confirms that chemical structure of copolymer is slightly changed when pPhDA is introduced to the reaction mixture. The composition and chemical structure of prepared samples were characterized also with NMR study. Typical 13C NMR CP/MAS and 13C NMR spectra of prepared EH are shown in Fig. 2a and b, respectively. Peaks in the Fig. 2a centered at the 42 and 181 ppm correspond to the carbon atoms in the backbone (eCH2e and eCHe groups) and to the one in the amide group (eCONH2), respectively. Carbon atoms which belong to the conducting polymer (aromatic) give three peaks with maximums at 125, 141 and 158 ppm. Peak with maximum at 61 ppm which is visible in the Fig. 2a can be related with the carbon in the PAAM backbone which is connected to the PANI chain. Appearance of the covalent bond in this position during oxidation polymerization of aniline in the presence of PAAM can be explained by the scheme presented in Fig. 2c. The formation of radical on the carbon which is adjacent to the eCONH2 group is possible due to stabilization effect of amide group. Then the recombination process of poly- or oligoaniline radical with one on the PAAM backbone can take place. As a result grafted PANI chain is formed. The ratio between the areas of the peaks of amide group and aromatic carbons obtained from 13C NMR spectra (Fig. 2b) allows to
Fig. 2. Typical solid state
13
C NMR CP/MAS (a) and
13
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calculate the actual amount of conducting polymer in the samples. The content of PANI was found to be in the range of 27e29% from the mass of dry hydrogel. This value is close to the theoretical amount (30%) calculated from polymerization conditions. 3.3. Morphological study As it can be seen in Fig. 3a the sample prepared without pPhDA contains agglomerated rod-shape clusters with the length of 700e1000 nm and diameter of 110e170 nm which consist of compactly packed nanospheres with the sizes about 30 nm, that are clearly seen in the pictures obtained with higher magnification (Fig. 3b). Addition of even a small amount of pPhDA leads to the considerable changes in the sample morphology (Fig. 3c). A high number of smooth fibers with length in the range of 1600e2600 nm and diameter of about 110e160 nm appeared. Few smaller rods with length of 300e1000 are observed also (Fig. 3d). Distribution of polymer anisotropic objects becomes more homogeneous. The influence of the pPhDA on the structure may be explained as follows. During ANI homopolymerization the insoluble nucleates which are formed at the early stages of polymerization [37] are precipitated to form a large amount of small particles. In the later stages they are merged together due to “secondary growth” [38] to form rod-like agglomerates with rough surface. The addition of pPhDA, the more active reagent than ANI, increases the rate of oligomers formation and makes the conditions for homogeneous nucleation of PANI preferable [24,39]. At the same time the additional eNH2 group may lead to higher solubility of oligomers. Acceleration of polymerization and slowing down of precipitation gives enough time for polymer chains to pack parallel to each other forming long smooth fibers. At the final stages of polymerization a coalescence of the fibers takes place, and the interconnected electroconducting phase with good electrical
C NMR (b) spectrum of dry dedoped EH samples and possible mechanism of grafting of PANI onto PAAM backbone (c).
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Fig. 3. . SEM images of EH prepared with different pPhDA/ANI ratio: 0 (a,b), 0.043 (c,d), and 0.1 (e,f).
contacts between structural elements is formed. In Fig. 3f it is seen that fiber structure becomes diffuse and inhomogeneous when increasing the pPhDA/ANI ratio up to 0.1. The sample EH-0.1 consists of smooth rods with their lengths in the range of 180e800 nm and small amount of larger objects (Fig. 3e). As it is seen in Fig. 3e the structural elements are less connected to each other in comparison with EH-0.043. These observations may be explained by the distortion of chains packing due to spatial difficulties which appears because of increasing amount of side groups on the PANI chains. The reason also may be the electrostatic repulsion between eNHþ 3 groups which arises due to protonation of pPhDA fragments in the acidic electrolyte (1 M HCl). 3.4. XRD study The structure details of the samples were investigated with wide angle X-ray diffraction (Fig. 4a,b). Values of interplanar distances (d) were calculated from the Bragg angles (2q) of the reflection according to Bragg equation 2dsinq ¼ nl, where l is a Xray radiation wavelength (Table 1). In the WAXD pattern of PANI prepared without PAAM (Fig. 4a, curve 1) six strong peaks centered at 2q ¼ 9.1, 14.6 , 20.5 , 25.1, 26.8 and 30.0 are observed. According to [40,41] these peaks are attributed to the diffraction from orthorhombic unit cell of emeraldine salt and correspond to the (001), (010), (100), (110), (111) and (020) planes. The first peak (9.1 ) corresponding to the d-spacing 9.8 Å is attributed to the periodicity along the polymer chain. Peak centered at 14.6 (d ¼ 6.1 Å) is attributed to the distance between dopant ions included into the crystalline lattice. The next two
diffraction maximums at 20.5 and 25.1 (d ¼ 4.3 and 3.5 Å, respectively) correspond to the characteristic distance between two adjacent chains of PANI and to the distance between 110 plane respectively. WAXD pattern obtained for PAAM is shown in Fig. 4a curve 2. This polymer is characterized by essentially amorphous structure with two broad peaks, centered at 2q ¼ 16.1 and 21.2 , corresponding to the interlayer spacing d ¼ 5.5 Å and 4.2 Å. In contrast, the WAXD pattern presented in Fig. 4a curve 3 shows that EH-0 is characterized with semicrystalline structure: four peaks at 2q ¼ 9.3 , 16.1, 20.5 and 25.1 are seen in addition to the amorphous diffraction of PAAM. These peaks are located at the same position as the reflections for pure PANI, and can be attributed to the crystalline PANI areas inside the composite. The polymerization of PANI in the presence of PAAM leads to the broadening of X-ray reflections. This indicates a decrease in crystallite sizes in comparison with conventional PANI and may be related to the strong interaction between PANI and PAAM. It can be supposed that fixation of PANI chains at the one end with PAAM decreases the ability of conducting polymer to pack into crystallites. Addition of pPhDA into the polymerization mixture leads to a decrease of intensity of crystalline peaks. This means the decreasing of the degree of crystallinity. It was pointed in Ref. [42] that the ratio of intensity of two peaks at 2q ¼ 25.1 and 20.5 (I25.1 / I20.5 ) is proportional to the doping degree of the PANI. This ratio decreases with increasing amount of pPhDA from 0 to 0.043 which suggests the lower doping degree of copolymers in comparison with homopolymer. Further increasing pPhDA content (Fig. 4a, curves 4 and 5) leads to the increasing I25.1 /I20.5 ratio which may
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Fig. 4. X-ray diffraction patterns for the samples in dried state (a): PANI (1), PAAM (2), EH-0 (3), EH-0.043 (4) and EH-0.1 (5); and for the samples swollen in water (b): PANI (1), EH-0 (2), EH-0.043 (3) and EH-0.1 (4). Table 1 Interplanar distances for the samples in dried state. Sample
d, Å
PANI PAAM EH-0 EH-0.043 EH-0.1
9.8 5.5 9.5 5.7 5.7
6.1 4.2 5.7 4.3 4.3
4.3 3.5 3.3 2.9 4.3 3.5 3.5 3.5
Fig. 5. Cyclic voltammograms of electrodes covered with EH prepared with different amount of pPhDA.
be explained by incorporation of additional dopant acid molecules due to increasing content of eNH2 groups in the polymer. To characterize the structural changes which take place at sorption of water by EH, the XRD patterns were obtained for swollen samples (Fig. 4b). For measurement of pure PANI its concentrated dispersion in water was prepared. A diffuse scattering from solvent molecules at 2q ¼ 28 is observed in every experiment. Additionally, the pattern of the PANI (Fig. 4b, curve 1) contains diffraction peaks from polymer crystallites. These peaks are at the same positions as for a dry polymer. This means that crystalline structure of PANI is preserved, and water cannot penetrate inside the PANI nanostructures because of low compatibility of the conducting polymer with the solvent. The EH-0 sample is characterized with considerably lower intensity of crystalline peaks (Fig. 4b, curve 2) which
almost disappear when pPhDA is added (Fig. 4b, curves 3 and 4). This observation proves that polymerization of ANI in the presence of PAAM apparently increases the compatibility of conducting polymer with water. It is possible to conclude that remaining weak crystalline peaks in EH-0 show that volume of the polymer prepared without pPhDA is partly unavailable for the solvent. 3.5. Electrochemical studies 3.5.1. Cyclic voltammetry To investigate the electrochemical characteristics of prepared EH the CVA curves were recorded in the range 100e800 mV (against standard hydrogen electrode) at the scan rate of 4 mV s1. Measured currents were normalized on the mass of conducting polymer in the EH sample. It can be seen in Fig. 5 that CVA curve for EH-0 covers much smaller area than the ones measured for the samples prepared by copolymerization of ANI with pPhDA. In the CVA curves of EH-0.025, EH-0.043, and EH-0.1 the peaks which correspond to the red-ox processes are clearly seen. The C1/A1 pair can be attributed to the transitions between leucoemeraldine and emeraldine forms of PANI [43] e the first oxidation step of this conducting polymer. Another pair (C2/A2) appears at a lower potential compared to the one at which the second oxidation step of PANI (transition between emeraldine and pernigraniline forms) usually takes place and cannot be attributed to it. It can be supposed that these peaks correspond to the electrochemical transitions in phenazine rings [44] which are believed to be formed at early stages of ANI polymerization [37]. On the curve for EH-0 the peaks corresponding to the red-ox processes are less pronounced. Such differences between homopolymer and copolymer samples can be explained by the higher compatibility of electrolyte and conducting polymer formed in the presence of pPhDA that leads to increasing specific area of the surface between ionic and electroconducting phases. Thus CVA measurements demonstrate a substantial increase of double layer capacity and also pseudocapacity when adding pPhDA. It can be seen (Fig. 5) that the curve for EH-0.1 is closer to the parallelogram shape than CVA for EH-0.043 and EH-0.025. It means that double layer capacitance is increased and pseudocapacitance is decreased with the growth of pPhDA/ANI ratio from 0.043 to 0.1. This fact can be explained by SEM results which were discussed earlier. At the maximal content of pPhDA the fiber structure is
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diffusing, and specific surface area is increasing. 3.5.2. Galvanostatic chargeedischarge The electrochemical capacitance of the prepared electrodes was tested by galvanostatic chargeedischarge cycling method under the same conditions as for CVA measurements. The current density was applied in the range of 0.2e32 A g1, and the specific capacities (C) were calculated from the equation:
C¼
It ; ðU2 U1 Þm
where I is loaded current; t is time of attainment of the final potential during discharge; U2eU1 is the difference between the final and initial potentials; and m is the mass of electroactive polymer or copolymer in the hydrogel deposited on the electrode. Chargeedischarge curves at the current density of 0.2 A g1 (Fig. 6a) have triangular shape showing an approximately linear dependence of potential against time thus confirming capacitance behavior of the prepared samples [45]. For the samples synthesized with pPhDA two distinct waves in the charge and discharge branches are clearly seen. They originate from pseudocapacitance nature of investigated electrodes and reflect Faradic processes in electroconducting polymer. The curve for the sample EH-0 is characterized by smallest chargeedischarge times. This is supported by the results obtained with CVA measurements and is attributed to lower specific area of the boundary between ionic and electronic conducting phases inside the EH-0 in comparison with the samples prepared by copolymerization. As it is seen from Table 2, the increase in pPhDA content in the polymerization mixture leads to the growth of the specific capacitance, and the strongest change of capacity is observed at small amounts of additive. At high current density, equal to 16 A g1 (Fig. 6b), chargeedischarge curves also have a triangle shape for all samples. Waves in chargeedischarge curves have practically disappeared showing decreasing contribution of pseudocapacitance in charge storage processes at this condition. At high current densities potential drop (DU) is seen in the beginning of the discharge curves (shown with arrows in Fig. 6b). This DU reflects equivalent series resistance (ESR) and is equal to I ESR, where I is the discharge current. ESR was calculated from linear fitting of DU measured at different currents. Specific capacitance and values of ESR are presented in Table 2. The EH-0.025 and EH-0.043 have the lowest values of ESR due to their well-defined nanofiber structure. The higher values for other samples can be explained as follows. The EH-0 consists of relatively thick and short fibers which are hardly available for electrolyte. At the same time the EH-0.1 is characterized by low connectivity of the electroconducting phase because it contains shorter fibers in comparison with EH-0.043.
Table 2 Specific capacitance and equivalent serial resistance (ESR) of prepared EH. Sample
EH-0
EH-0.025
EH-0.043
EH-0.1
Specific capacitance F g1 at 0.2 A g1 Specific capacitance F g1 at 16 A g1 ESR, Ohm
138 19 1.95
304 165 0.83
333 176 0.93
364 133 1.79
As it can be seen in Fig. 7 the specific capacitance decreases with increasing current density for all samples. The systems prepared with medium amounts of pPhDA (EH-0.043 and EH-0.025) are characterized by the lowest slope of dependence of specific capacity on current density. As it is known [18,45], the well-defined nanofiber structure of conducting polymer ensures reducing the ion's diffusion pathways and enhances the connectivity of electroactive regions. That is the case for samples prepared with addition of pPhDA. Improving compatibility between conducting polymer and water due to the appearance of additional polar groups increases the distance between PANI chains and makes easier the penetration of electrolyte inside the PANI nanostructures. The dependence of specific capacitance on current density has a maximal slope for the sample EH-0.1, i.e. when adding high amount of pPhDA. Increased compatibility with water provides high surface area for this sample, and it demonstrates highest capacitance at low currents (0.2 A g1). On the other hand, a higher compatibility of polymer with water leads to decreasing number of hydrophobic cross-links which acts as electrical contacts between PANI structural elements. As a result, ESR value increases (Table 2), and at high current densities the charge storage efficiency of EH-0.1 decreases more rapidly than this value of EH-0.043 and EH-0.025. As mentioned above, it is hard to prepare electrodes with suitable properties and thickness higher than 1 mm. The decreasing
Fig. 7. Specific capacitance of EH at different current densities.
Fig. 6. Chargeedischarge curves of EH at current densities 0.2 (a) and 16 (b) A g1. Arrows show the values of DU.
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Fig. 8. Stability of EH electrodes during the cycle chargeedischarge process.
active layer thickness leads to the growth of specific capacitance of electrode but diminishes its absolute capacitance and scaling potential. In our case, due to grafting of conducting polymer onto PAAM backbone and hydrogel nature of electrode the conducting polymer swells at molecular level and high permeability of the material for electrolyte is achieved. At the same time the formation of physical cross-links by means of contacts between hydrophobic PANI chains leads to a good connectivity of electroconducting phase. The effect of thickness of electrode on its specific capacity was investigated. Four different amounts of polymerization mixture (177, 229, 328, and 417 mg) were used for the preparation of electrode EH-0.025. The measured specific capacitances at current density 1 A g1 were 248, 266, 288, and 284 F g1, respectively. This confirms that the total volume of EH is involved in the charge storage processes. 3.5.3. Cycling performance The cycle-life stability was measured by applying 1000 of CVA cycles to the electrode with scanning rate of 100 mV s1. The capacity retention at each 100th cycle (Fig. 8) was calculated as the ratio between areas of each 100th and the first cycles. It was observed that for EH-0 the capacitance sharply increases during first hundred cycles and then starts a gradual decrease. For the sample prepared with pPhDA the capacity grows up to 600th cycle and finally reaches 105% of the initial value for EH-0.043. It is pointed out in Ref. [45] that cycle-life stability of conducting polymers is restricted by degradation processes which arise from conformational changes of PANI macromolecules during oxidationereduction cycles. This process causes the mechanical destruction of the electrode material and diminishes the quality of electrical contacts between its structural elements. In our samples the “soft” structure of hydrogel electrodes and liability of the physical cross-links give the possibility to change the conformations of PANI and not to disturb the continuity of conducting phase. Moreover it can be assumed that the number of hydrophobic contacts between PANI chains increases with cycling because they are thermodynamically preferable in water environment. Consequently the connectivity of the elements of electroconducting phase increases thus larger amount of conducting polymer is included into the charge/discharge process. As a result the capacitance of prepared EH samples increases up to the formation of the maximal number of feasible contacts. 4. Conclusions In this study the EH on the base of copolymers of aniline with different amount of pPhDA were obtained by chemical oxidative
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polymerization in the solution of linear PAAM. The electrodes with thickness of active layer of about 1 mm were prepared on graphite substrate and their capacitance properties were determined. The maximum capacitance 364 F g1 was observed with current density 0.2 A g1 for EH-0.1. This is in 2.8 times higher than this value for hydrogel prepared without pPhDA. Such increasing of electrical capacitance is achieved due to the better compatibility of conducting polymer containing eNH2 groups with electrolyte and higher connectivity of conducting phase. At the same time at high current densities (32 A g1) the best properties are demonstrated by the samples containing mediate quantities of pPhDA: EH-0.043, and EH-0.025. As it was shown by SEM and measurements of swelling these samples consists of the longest electroconducting fibers and they have maximal number of cross-links. The stability at cycling of the samples is higher than 100% after 1000 cycles. This is related to a “soft” structure of electroconducting hydrogel which protects the electrical contacts between PANI chains from disruption during oxidationereduction cycles. Acknowledgment Maria Sokolova acknowledges St. Petersburg State University for a research grant (12.50.1195.2014). Authors are also grateful to Russian Foundation for Basic Research for financial support (grants 14-03-31411 mol_a, and 13-03-00219). The experimental work was facilitated by the equipment of the Resource Centre of X-ray Diffraction Studies, of Nanotechnology Interdisciplinary Resource Centre, of Centre for Innovative Technologies of Composite Nanomaterials, and of Magnetic Resonance Research Centre at St. Petersburg State University. Authors acknowledge the research fellow of Institute of Macromolecular Compounds Z.Zoolshoev for measurements of characteristic viscosity of PAAM. References [1] P. Simon, Yu. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845e854. [2] J.R. Miller, P. Simon, Electrochemical Capacitors for Energy Management, Mater. Sci. 321 (2008) 651e652. [3] G.A. Snook, P. Kao, A.S. Best, Conducting-polymer-based supercapacitors devices and electrodes, J. Power Sources 196 (2011) 1e12. [4] E.O. Fedorovskaya, L.G. Bulusheva, A.G. Kurenya, I.P. Asanov, N.A. Rudina, K.O. Funtov, I.S. Lyubutin, A.V. Okotrub, Supercapacitor performance of vertically aligned multiwall carbon nanotubes produced by aerosol-assisted CCVD method, Electrochim. Acta 139 (2014) 165e172. [5] S. Sun, P. Wang, S. Wang, Q. Wu, S. Fang, Fabrication of MnO2/nanoporous 3D grapheme for supercapacitor electrodes, Mater. Lett. 145 (2015) 141e144. [6] N.B. Trung, T.V. Tam, D.K. Dang, K.F. Babu, E.J. Kim, J. Kim, W.M. Choi, Facile synthesis of three-dimensional grapheme/nikel oxide nanoparticles composites for high performance supercapacitor electrodes, Chem. Eng. J. 264 (2015) 603e609. [7] B. Lobarto, V. Vretenar, P. Kotrusz, M. Hulman, T.A. Centeno, Reduced graphite oxide in supercapacitor electrodes, J. Colloid Interface Sci. 446 (2015) 203e207. [8] G.G. Wallance, G.M. Spinks, L.A.P. Kane-Mahuire, P.R. Teasdale, Conductive Electroactive Polymer, CRC Press Taylor & Frances Group, London, New-York, 2009. [9] D.P. Dubal, S.V. Patil, G.S. Gund, C.D. Lokhande, Polyaniline-polypyrrole nanograined composite via electrostatic adsorption for high performance electrochemical supercapacitors, J. Alloys Compd. 552 (2013) 240e247. [10] T.G. Girija, M.V. Sangaranarayana, Analysis of polyaniline-based nickel electrodes for electrochemical supercapacitors, J. Power Sources 156 (2006) 705e711. [11] S. Ghosh, O. Inganas, Conducting polymer hydrogels as 3D electrodes: applications for supercapacitors, Adv. Mater. 11 (14) (1999) 1214e1218. [12] H. Zhang, J. Wang, Y. Chen, Z. Wang, S. Wang, Long-term cycling stability of polyaniline on graphite electrodes used for supercapacitors, Electrochim. Acta 105 (2013) 69e74. [13] M. Mastragostino, C. Arbizzani, F. Soavi, Conducting polymers as electrode materials in supercapacitors, Solid Sate Ion. 148 (2002) 493e498. [14] Q. Wang, J. Yan, Z. Fan, T. Wei, M. Zhang, X. Jing, Mesoporous polyaniline film on ultra-thin graphene sheets for high performance supercapacitors, J. Power Sources 247 (2014) 197e203. [15] H. Hu, S. Liu, M. Hanif, S. Chen, H. Hou, Three-dimensional cross-linked carbon
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