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Realizing High Capacitance and Rate Capability in Polyaniline by Enhancing the Electrochemical Surface Area through Induction of Superhydrophilicity Roby Soni, Varchaswal Kashyap, Divya Nagaraju, and Sreekumar Kurungot ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15534 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Applied Materials & Interfaces

Realizing High Capacitance and Rate Capability in Polyaniline by Enhancing the Electrochemical Surface

Area

through

Induction

of

Superhydrophilicity Roby Sonia,b, Varchaswal Kashyapa,b, DivyaNagarajua,b, and Sreekumar Kurungota,b* a

Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune,

Maharashtra-411008, India. E-mail: [email protected], Fax: +91 20-25902636; Tel: +91 20-25902566. b

Academy of Scientific and Innovative Research (AcSIR), CSIR-National Chemical

Laboratory Campus, Pune, Maharashtra-411008, India. KEYWORDS. polyaniline, superhydrophilic, supercapacitor, electrochemical active surface area, electropolymerization, electrochemical functionalization

ABSTRACT. Polyaniline (PANI) as a pseudocapacitive material has very high theoretical capacitance of 2000 F g-1. However, its practical capacitance has been limited by low electrochemical surface area and unfavorable wettability towards aqueous electrolytes. This work deals with a strategy wherein high electrochemical surface area of PANI has been achieved by the induction of superhydrophilicity together with the alignment of PANI exclusively on the

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surface of carbon fibers as a thin layer to form a hybrid assembly. Superhydrophilicity is induced by electrochemical functionalization of the Toray carbon paper, which further induces superhydrophilicity to the electrodeposited PANI layer on the paper and, thereby, ensuring high electrode-electrolyte interface. The Toray paper is electrochemically functionalized by anodization method which generates a highly active electrochemical surface as well as greater wettability (superhydrophilic) of the carbon fibers. Due to the strong interaction of anilinium chloride with the hydrophilic carbon surface, PANI is polymerized exclusively over the surface of the fibers without any appreciable aggregation or agglomeration of the polymer. The PANIToray paper assembly in the solid-state prototype supercapacitor can provide a high gravimetric capacitance of 1335 F g-1as well as high areal capacitance of 1.3 F cm-2 at a current density of 10 A g-1. The device also exhibits high rate capability delivering 1217 F g-1 at a current density of 50 A g-1and a high energy density of 30 Wh kg-1 at a power density of 2 kW kg-1.

Introduction Polyaniline (PANI), a conducting polymer, has many advantageous properties when used as a capacitive material. High capacitance (2000 F g-1),1 ease of synthesis, low cost, good electronic conductivity2 and environmental benignity make it a suitable material for supercapacitors. However, low practical capacitance, poor rate capability,3,4 and high equivalent series resistance5 are some pertaining issues which hinder its application in commercial supercapacitors. PANI falls under the category of pseudocapacitors where the redox transition between the different reduced and oxidized states of the material results in charge storage.1 Basically, PANI has three distinct oxidation states, the completely reduced state or leucoemaraldine, the half oxidized form


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or emaraldine and the fully oxidized form or pernigraniline.1 The transition among these redox states results in charge storage, and thus, highly active electrochemical surface and facile movement of electrolyte ions are necessary for high rate capability in PANI. Wang et. al.,1 (2009) calculated the theoretical capacitance of PANI by assuming it to be a solid, highly conducting nanofiber and the entire surface of which is used for double layer charge storage. Moreover, the diffusion of counter anions (HSO4-) was assumed to be fast enough so as not to limit the faradaic reactions of PANI. The capacitance for the single electrode was calculated to be 2000 F g-1 with 1700 F g-1contributedfrom pseudocapacitance and 300 F g-1 by the double layer. Despite its very high theoretical capacitance, values reported in the literature vary from 230 to 8006 F g-1in the three-electrode system at a low current density or scan rate, generally exhibiting poor rate capability and high Equivalent Series Resistance (ESR). Recently, it was reported that a Metal Organic Framework (MOF)-based PANI supercapacitor7 showed a very high areal capacitance of 2146 mF cm-2 for a single electrode. However, the device exhibited just 35 mF cm-2 at a low current density of 0.05 mA cm-2. Further, a PANI-based solidstate device fabricated through bar coating method exhibited a good capacitance of 647 F g-1 in the device at a current density of 0.5 A g-1 which could retain 62 % of the capacitance at 20 A g-1 but the corresponding rate performance was poor.8 Nano-structuring of PANI by preparing different morphologies9 such as spheres (136 F g-1), layered flowers (244 F g-1), dendrites (64 F g-1), etc., or creating pores10 in the polymer structure has also been attempted to achieve high performance but none of them has been able to attain near the theoretical capacitance and high rate capability. Most of the PANI-based supercapacitors reported in literature work well under low current density but show drastic deterioration in performance as the discharge current density is increased. Limited material utilization along with restricted diffusion and movement of


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ions under high current density seriously impede their rate performance. One of the reasons contributing to this below par performance of PANI is the unfavorable wetting of the polymer surface by the aqueous electrolytes, thereby resulting in low ESA and miserable rate performance. These limitations can be mitigated by improving surface wettability and hydrophilicity of PANI and the design and architecture engineering of the electrode and the device. For the supercapacitor utilizing aqueous electrolytes as in the case of PANI, the wettability of the surface plays a highly important role in the device performance as favorable wettability (exhibited by hydrophilic or superhydrophilic surfaces) ensures high electrode-electrolyte interface. The intrinsic capacitance of any material whether EDLC or pseudocapacitive is directly related to the electrochemically active surface area which further depends on the surface wettability. There are a number of strategies to increase ESA of the material. Some of which include creation of defects in the material,11 incorporating hydrophilic functional groups on the surface,12 or morphology manipulation. The wettability of the conducting polymers has been manipulated by the incorporation of polymers like dextran in poly(3-hexylthiophene),13 modifying surface morphology14 or by the anion exchange in the polymer backbone.15 However, all these methods result in the decrement in the conductivity of the active material which further reduces the rate performance of the supercapacitor. Therefore, an appropriate strategy is needed


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Scheme1. Schematic representation of the process involved in the electrochemical functionalization of the Toray carbon paper and PANI electropolymerization. Step 1 depicts the procedure for functionalizing the Toray paper through application of electric potential in aqueous electrolyte. Step 2 is the subsequent electropolymerization of PANI over the surface of the pristine and functionalized Toray papers. which increases the wettability and ESA of the capacitive material without affecting its conductivity. From the above discussion, it is clear that, favorable surface wettability which ultimately leads to high ESA is a prerequisite for achieving high capacitance and rate performance. At the same time, it is utmost important to achieve high electronic conductivity in the system for allowing it to achieve high power density. The literature on the PANI


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supercapacitors lacks the study of the effect of wettability of the polymer surface on supercapacitor performance. The literature mainly focused on the synthesis of different morphologies and compositing with other materials to achieve high performance.

Figure 1.a) IR spectra of the pristine Toray carbon paper (pT) and functionalized paper (fT); b) and c) represent the contact angle measurement of pT carried out with water and PVA-H2SO4 gel, respectively; d) contact angle measurement of fT with PVA-H2SO4 gel electrolyte. In this report, we have attempted to address the prevailing issues of PANI on the wettability and ESA standpoint. For this, a strategy is demonstrated in which the wettability of PANI is modified by polymerizing it on the superhydrophilic conducting surface to accomplish high ESA without affecting its electronic conductivity and other physical properties. At the same time, a few nanometers thin PANI film is achieved which reduces the amount of inactive PANI which is unavoidable in thicker PANI coatings. Firstly, a carbon fiber paper (Toray paper) is


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electrochemically modified by applying a high potential (10 V) for a short time interval; this process incorporates hydrophilic oxygen-containing functional groups on the surface of the carbon fibers. Functionalization makes the surface superhydrophilic (zero contact angle) resulting in a very large electrochemically active surface compared to the unmodified carbon paper. Subsequently, PANI is electrodeposited. The interaction of anilinium chloride (monomer) with the superhydrophilic surface ensured formation of PANI exclusively on the surface of the carbon fibers. Bulk polymerization in the voids of the carbon fiber network is minimized, which indeed results in low density of grain boundaries and high conductivity. The underlying chemical interaction between the superhydrophilic carbon fiber and PANI enhances the surfaces wettability of the latter and, thus, making it superhydrophilc. Thus, the induced superhydrophilicity along with high conductivity of PANI invoke high capacitance and low ESR value compared to the many solid supercapacitor devices reported in the literature.16,17,18 Moreover, a high rate capability is displayed by the PANI deposited using the current strategy which otherwise has remained elusive in the PANI-based solid supercapacitors.

Result and Discussion Although the theoretical capacitance of PANI is very high, its realization is limited by incomplete utilization of PANI. This is due to restricted impregnation by the electrolyte in the interior of the bulk polymer, and formation of thick and uneven polymer films, which reduce its electrochemical surface drastically. Large surface area and high conductivity are essential for achieving high rate capability in conducting polymers.[19] In this work, we have tried to overcome these limitations related to the improper utilization of the electroactive material by explicitly synthesizing a superhydrophilic thin layer of PANI over the surface of carbon fibers in


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an electrochemically modified Toray carbon paper which is used as a template and current collector. Moreover, insulating electrochemically inactive binders which impede the conductivity are not used in this work; the binder-free system helps in the seamless charge transfer and current collection. Also, compared to the pristine counterpart, the polymer surface becomes superhydrophilic, which is an important criterion in facilitating the electrolyte penetration and extending the electrode-electrolyte interface from the favorable wettability attained by the

Figure 2. Scanning electron microscope analysis: a) FESEM image of pT; b) FESEM image of the Toray paper after functionalization (fT); c) low magnification SEM image of fTP where polymerization is carried out at a constant current of 2 mA cm-2; d) high resolution FESEM image of fTP where the PANI polymerization was performed by constant current method (2 mA


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cm-2); e) SEM image of fTP where PANI is polymerized at a constant potential of 800 mV; f) FESEM image of PANI (constant potential method). system during polymerization. The strategy adopted for achieving the uniform wrapping of the carbon fibers with superhydrophilic PANI layer is shown in Scheme 1. The Toray paper which is made up of an entangled network of carbon fibers has been used as a solid substrate for the deposition of the aggregate-free thin layer of PANI on the individual carbon fibers in the paper. The carbon fiber forms the backbone of the system while the PANI forms a thin shell around the fiber. In this arrangement, the amount of idle PANI is significantly reduced as PANI, which is the charge storage component, is confined only to the surface whereas the carbon fiber in the core of the assembly functions as the current collector. Anodization method of functionalization has advantages over constant current electrolysis20 (even at high current of (100 mA, the reaction time is 16-24 h) or acid functionalization.21 Compared to acid functionalization, which requires 4-6 hours, the anodization method is not harsh, corrosive or time-consuming.


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Figure 3.SEM image of pTP and fTP where the polymerization is carried out at constant current mode: a) and b) represent the images of pTP at low and high magnification, respectively; c) and d) depict the morphology of fTP at low and high magnification, respectively. Digital photographs exhibiting the changes induced on the surface of the Toray paper subsequent to the anodization process (hereinafter designated as fT) are shown in Figure S1. Application of the potential results in the incorporation of –COOH functional groups on the carbon fibers which is ascertained through the IR spectrum of fT Figure 1a. The spectrum shows the stretching frequency corresponding to the carboxylic (–C=O) group at 1730 cm-1, which is absent in the pristine Toray paper (designated as pT). The quantification of oxygen has been carried out with the help of XPS (Figure S8). The percentage of oxygen in pT is determined to be 5.36 %, which after functionalization, increases to 18.45 %. These results are further


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supported by the elemental analysis (C, O analysis). The oxygen content in pT is calculated to be 3.53 %, which increases to 20.23 % in the case of fT. This corresponds to an enhancement in the oxygen concentration by 16.7 % after anodization, which is in close agreement with the XPS data. Thus, electrochemical functionalization drastically changes the surface composition of the carbon fibers and, hence, it is expected to affect the wettability of the polymerization solution towards the carbon fibers. The changes in wettability were analyzed through contact angle measurements. The water contact angle measured for pT is 119°, which indicates an unfavorable surface for wetting. On the other hand, fT becomes highly hydrophilic (superhydrophilic) with improved wettability. However, the water contact angle in the case of fT could not be measured due to instantaneous absorption of water droplet by the paper. Subsequently, PVA-H2SO4 gel (which has been used as the electrolyte for the supercapacitor fabrication) was used in place of water and the contact angle so measured was found to be 48°, the image in the inset of Figure 1d shows 0° contact angle for the fT-water interface. However, the PVA-H2SO4 gel electrolyte is completely absorbed after a few minutes (Figure 1b, c, and d). A video showing the chronology


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Figure 4. a) XRD patterns of pT, pTP and fTP; b) the magnified portion of the XRD profiles presented in a) to clearly revealthe changes in the nature of the XRD patterns in each case; c) and d) depict the XPS survey spectra of pTP and fTP, respectively; e) and f) represent the deconvoluted N 1s XPS pTP and fTP, respectively.


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of the contact angle measurement depicting the electrolyte absorption has been given in ESI. The PVA-H2SO4gel electrolyte is completely absorbed in the case of fT unlike pT where the electrolyte remained unabsorbed even after a long time.

Figure 5. a) Comparative cyclic voltammograms recorded at a voltage scan rate of 50 mV s-1 for the PANI polymerized on fT by constant potential and constant current methods; b) the cyclic voltammograms recorded at a scan rate of 5 mV s-1when PANI was polymerized over fT at constant current with different current densities; c) the effect of different acids employed for the polymerization on capacitance, where the cyclic voltammograms are recorded at a voltage scan rate of 50 mV s-1; d) the wettability of pTP and fTP through PVA-H2SO4 contact angle measurement.


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The expected immediate effect of functionalization is impeding the conductivity of the Toray carbon paper. To analyze its effect on conductivity, the four-probe measurements were conducted on pT and fT to calculate the sheet resistance. Compared to pT for which the sheet resistance was found to be 180 mΩ □-1, the sheet resistance of fT increased to 316 mΩ □-1. This increased sheet resistance in the latter case can be attributed to the disturbance in conjugation on the surface of the carbon fibers during the anodization process. However, the sheet resistance is still sufficiently low to be used as an efficient current collector (for calculations refer ESI, the corresponding I-V curves are shown in Figure S7). The reduction in conductivity is not very significant which again points to the superiority of the anodization process implemented in neutral salt electrolyte for achieving surface functionalization. Changes, if any, in the morphology were investigated with the help of scanning electron microscopy on pT (Figure 2a) and fT (Figure 2b). Long tubes with smooth surfaces forming the entangled network can be clearly seen in Figure 2a for pT. Further, the same characters can be observed for fT in Figure 2b without any structural changes. Hence, electrochemical functionalization keeps the morphology intact, which is necessary for being used as a template for deposition of the PANI layer. One important property expected to be enhanced by electrochemical functionalization is the extent of electrochemically active surface (ESA) of the Toray paper. In order to ascertain this, ESA of pT and fT was measured using the Randles-Savcikequation22 over a geometric area of 1 cm2. The equation is given by, Ip= (2.69X105) n3/2A DOCv1/2, where, Ip is the peak current (A), n is the number of electrons involved in the redox reaction, A is the area of the electrochemical active surface (cm2), DO is the diffusion coefficient (cm2 s-1), C is the concentration of redox species (mol L-1), v is the scan rate (V s-1) (detailed calculations are given in ESI).The ESA measured for pT is 1.77 cm2while that calculated for fT is 15.71 cm2, which is


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almost nine times higher than that for pT. The corresponding cyclic voltammograms are given in Figure S2. Moreover, the enhanced electrochemical surface area was also ascertained by measuring the double layer capacitance (Cdl) (for details refer ESI).23 The Cdl measured for pT is 0.117 mF (Figure S3a and b) whereas the Cdl of fT is found to be 86.51 mF (Figure S3c and d), which corresponds to an enhancement of 740 times in surface coverage by the electrolyte ions. This enhancement in Cdl is due to the availability of increased surface area for ion adsorption corroborated by the achievement of superhydrophilicity. This enhancement in ESA of fT will ultimately lead to high ESA for the PANI, as a large electrochemical surface, which otherwise absent in pT, becomes available for the anilinium chloride monomer to undergo polymerization. To validate the above results and to analyze the beneficial effects of the electrochemical functionalization, PANI deposition was carried out on pT as well as fT. While various methods can be employed for electrochemical polymerization, galvanostatic and potentiostatic methods were tested in this study due to their simplicity and better control of deposition parameters. To determine the deposition potential, a cyclic voltammogram was recorded at a scan rate of 10 mV s-1 in the polymerization solution; the corresponding plot is given in Figure S5. The onset of the electrodeposition determined from Figure S5a is 0.710 V vs. Ag/AgCl. Taking into account the potential losses due to internal resistance, etc., a potential greater than 0.710 is required for the electrodeposition.24 For the above reasons, a potential of 0.80 V was used for polymerization of PANI through the potentiostatic method. For polymerization at constant current, a current density of 2 mA cm-2 was chosen as lesser current density was not able to generate sufficient deposition potential at the electrode. The corresponding chronopotentiograms are given in Figure S5b. Digital images of the PANI polymerized over pT (pT-PANI, designated as pTP) and fT (fT-PANI, designated as fTP) are given in Figure S6a and b, respectively. A thick PANI


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coating is clearly visible in the case of pT. However, the nature of PANI coating is clearly different in the case of fT, where the polymer deposition is extended well deep inside the carbon fibers of the Toray paper by avoiding the thick patches as observed in the case of pT. Polymerization at constant current (current density of 2 mA cm-2) results in uniform coating of PANI over the tubes (Figure 2c) compared to a case when the polymerization was carried out at a constant potential of 800 mV, which resulted into the formation of large aggregates of the polymer on the surface (Figure 2e). Further, in the high-resolution SEM image shown in Figure 2d (constant current), porous polymer structure is clearly visible. Figure 2f shows the carbon fibers with relatively less polymer coverage as most of the PANI forms large aggregates which are concentrated at few places. This change in polymer alignment can be ascribed to the low rate of polymerization in the galvanostatic method (polymerization time of 17 min 24 s) compared to the potentiostatic method (polymerization time of 408 s). Slow polymerization is essential for achieving uniform coating. This is because when the process is slow, the system gets ample time for the monomer to diffuse from the bulk for maintaining constant concentration in the diffusion layer and also over the carbon fibers, leading to a condition favoring for attaining uniform polymer coating. Considering the above, the constant current method was selected as the polymerization technique in this study. Current densities of 2, 5, and 8 mA cm-2 were applied to analyze the effect on the polymerization. It was found that with an increase in the current density, the polymer thickness and non-uniformity in the coating increase. The corresponding images presented in Figure S9 again indicate that slow polymerization is necessary for attaining uniform polymer coating.


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The effect of functionalization on the growth of PANI over the fibers was studied by SEM analysis of pTP and fTP. SEM images of pTP are shown in Figure 3a and b. As is evident from the images, PANI forms a thick coating on the surface of the carbon fibers. Moreover, the open spaces are blocked which will further reduce the electrochemical surface area of the polymer and will hinder the electrolyte infiltration. Also, the polymer coating is non-uniform with several cracks which would cause high resistance due to higher density of grain boundaries. On the other hand, in the case of fTP, the polymerization essentially takes place on the surface of the fibers as shown in Figure 3c and d. To support the SEM results, the thickness measurement of the PANI layer on the surface in fTP and pTP was carried out by TEM analysis; the corresponding TEM images are given in the Figure S10. Thickness measured for fT measured from a single fiber is 315 nm (Figure S10a) whereas thickness measured for fTP is 322 nm (Figure S10b), which suggests formation of a 7 nm thick PANI layer on the surface of the fiber in fTP. Similarly, thickness of pTP obtained from the TEM (Figure S10c) is 622 nm, which gives the approximate thickness of the PANI layer to be 307 nm. Thus, a very thick PANI deposition is obtained for the un-functionalized Toray surface which further limits its electrochemically active surface area whereas a thin PANI layer (just 7 nm) achieved in fTP ensures a low amount of inactive PANI. The alignment of PANI exclusively over the fibers is caused by the superhydrophilicity of the fibers which increases the impregnation by the monomer not only on the surface but also in the inner spaces. This arrangement leaves the pores and vacant spaces intact, thus facilitating an unrestricted movement of the electrolyte ions. Moreover, Electron Energy Loss Spectroscopy (EELS) analysis of pTP and fTP supports the above results. The element maps recorded at the cross-section of fTP shows a uniform distribution of nitrogen over the entire surface, unlike in the case of pTP for which the distribution is limited largely to the surface (Figure S11b and d).


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The uniform coating of PANI in fTP is expected to yield more electrochemically active surface when compared to pTP. In order to ascertain this, Cdl was measured for the two electrodes by scanning them to the potential range of -0.45 to -0.35 V for pTP and -0.40 to -0.30 V for fTP based on how the double layer charging takes place (for details refer ESI, Figure S4). The Cdl measured for pTP is 123 mF cm-2 whereas that for fTP is 295 mF cm-2, which is authenticating the availability of 2.4 times higher electrochemical surface area for fTP compared to pTP. BET analysis of fT, fTP, and pTP was also carried out to understand surface properties; the adsorption isotherm and pore size distribution profiles of the three samples are given in Figure S12. The adsorption isotherms of all the samples (Figure S12a) show Type VI behavior which is common for non-porous solids with a uniform surface.25 Adsorption isotherm of the fT in the relative pressure range of 0.1-0.3 show very less gas adsorption inferring to its low surface area and non-porous nature. Moreover, it does not show any prominent pore structure which is clear from the pore size distribution profile presented in Figure S12b. The low surface area can be understood from the fact that it is a network of solid carbon fibers which entangled together to form a solid network; the only surface available for the gas adsorption is the outer surface of the carbon fibers. The multipoint BET surface area of fT is very low at 55 m2 g-1 which again indicates its non-porous nature. Adsorption isotherms recorded for pTP and fTP also show features similar to fT. In the case of pTP, a small anomaly i.e. a sudden increase in the gas adsorption or a crossover is observed near the saturation point. This anomaly seems to have been caused by instrument error or disturbance as desorption points have shifted right of the adsorption points and it cannot be due to the material characteristic. However, the surface area obtained for fTP is 50 m2 g-2, which is twice the surface area obtained for pTP. This change in the surface area of pTP and fTP can be explained by uniform PANI growth on the carbon fibers


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on fTP unlike the agglomerated growth of PANI on pTP which reduces available gas adsorption surface as has been seen in the SEM images. However, after PANI deposition, some porosity is developed although to a lesser extent. Both fT and fTP show the pore distribution (Figure S12b) predominantly in the range of 3.45 nm indicating development of mesoporosity while this is absent in pTP. All these results corroborate well with the SEM images. The half pore width in fTP (1.58 nm) is higher than that in pTP (1.38 nm). In addition, fTP shows pore distribution in the range of 7.76 nm which is absent in the case of pTP due to clogging of the pores in this range. The difference in thickness of the PANI coating on the carbon fiber surface has also been reaffirmed by the X-Ray Diffraction (XRD) analysis. XRD patterns were recorded for pTP and fTP with same PANI loading and area as shown in Figure 4a and b. XRD profiles of pTP and fTP show the characteristic crystalline peak of PANI at the 2Θ value of 13.88o. The XRD patterns also reflect a peak corresponding to the amorphous phase of PANI at a 2Θ value of 16.80°. The peak at 25° corresponding to the crystalline phase of PANI is superimposed by the 26° peak of the Toray carbon.26 The intensity of the 16.80° peak is higher in the case of pTP compared to the peak intensity in fTP. This difference in the peak intensity points towards the higher crystalline nature of fTP compared to pTP. One important thing to be noted from XRD is the decrease in the peak intensity of fTP which agrees well with the thin PANI layers deposited on the fibers, as has been observed in the SEM images. X-Ray Photoelectron Spectroscopy (XPS) analysis was carried out to get insight into the electronic properties and the extent of doping in pTP and fTP. The survey spectra of pTP and fTP presented in Figure 4c and d show all the typical peaks of PANI and carbon. The binding energy of N1s in fTP is 399.21 eV while that in pTP is 399.01, showing a shift of 0.20 eV.


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Similarly, the O1s binding energy in fTP is 531.81 eV while that in pTP is 532.01 eV, corresponding to a 0.20 eV decrease in the case of the former. The above results infer the transfer of electron density from nitrogen to oxygen in fTP, confirming the interaction between the carbon surface and aniline, which is essential for attaining a uniform coating. The N1s core level spectra of the two samples are de-convoluted and fitted into amine (-NH-), protonated amine (-NH2+), imine (=N-) and protonated imine (=NH+-) regions27 arising due to different chemical environments (Figure 4e and f). The corresponding binding energies for each are extracted and presented in Table S1a and b. The oxidation of the polymer chain results in the generation of positive charges which are balanced by the anion incorporation in the polymer backbone. The extent of oxidation or reduction of the polymer is expressed as doping level and is usually measured by the proportion of the dopant ions or molecules incorporated per monomer unit. Increased doping level creates more mobile charges, hence leading to the increased conductivity.28 The level of doping was determined from the area ratio of the protonated components of the N1s core level spectra to the total area of the N1s core level spectra.29 The maximum doping level achievable in emaraldine form of PANI is 0.5.30 Doping level determined from the de-convoluted N1s spectra of fTP is 0.47 while that calculated for pTP is 0.41. Higher doping level achieved in the case of fTP can be attributed to the easy diffusion of the of the electrolyte ions through the PANI thin layers in addition to the higher positive charge density on the nitrogen as discussed above. After thorough physical characterization of pTP and fTP, electrochemical testing was carried out for correlating the physical properties with electrochemical performance; all the liquid state electrochemical measurements were carried out in 0.5 M H2SO4. As discussed in the previous sections, constant current method and constant potential method were used for accomplishing


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electrochemical polymerization. When polymerization was performed at a constant potential of 800 mV, the time taken for the polymerization was 408 s, whereas, when the same amount of charge was passed at a current density of 2 mA cm-2, the polymerization time was 17 min 24 s. As already discussed in a previous section, a longer polymerization time allows for slow and uniform growth of the polymer, providing sufficient time for the monomer to penetrate inside. Cyclic voltammograms recorded for the electrodes prepared by the two methods are shown in Figure 5a. As can be seen from the figure, polymerization at the constant current sweeps an area higher than that attained through the constant potential case, attributable to the increased PANI utilization in the former case. These results are well supported by the FESEM images which have been discussed in the previous sections. The polymerization was also carried out at the current densities of 2, 5 and 8 mA cm-2 in which the amount of charge passed was kept constant in each case to get the same polymer loading. The corresponding CV profiles are


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Figure 6.a) Cyclic voltammograms recorded for the pTP and fTP based solid devices at a scan rate of 20 mV s-1; b) cyclic voltammograms of the fTP device recorded at increasing scan rates; c) Nyquist plots depicting the difference in the ESR of the fTP and pTP based devices; d) and e) show the frequency dependence of capacitance for fTP and pTP, respectively.


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presented in Figure 5b. The one corresponding to the PANI deposition at a current density of 2 mA cm-2 shows the highest capacitance value, exhibiting broad capacitive peaks responsible for the luecomaraldine, emaraldine and pernigraniline transitions. However, the capacitance is found to be reducing when the deposition was carried at higher current densities. This decrease is attributed to the formation of thicker PANI coatings on the surface of the Toray paper, which limits the active polymer surface as can be clearly seen in the SEM images (Figure S9). The supporting electrolyte used during polymerization also plays a critical role in influencing the polymerization kinetics and electrochemical properties. In this study, four different acids, namely, hydrochloric acid (HCl), sulphuric acid (H2SO4), phytic acid (C6H18O24P6) and camphor sulphonic acids (C10H16O4S) containing anions of different sizes were used as the supporting electrolytes and as dopants. The increasing order of the counter ion size for these four acids is as Cl-

Realizing High Capacitance and Rate Capability in ... - ACS Publications

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