Influence of oCVD Polyaniline Film Chemistry in Carbon-Based

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Influence of oCVD Polyaniline Film Chemistry in Carbon-Based Supercapacitors Yuriy Y. Smolin, Masoud Soroush, and Kenneth K. S. Lau Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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Industrial & Engineering Chemistry Research

Influence of oCVD Polyaniline Film Chemistry in Carbon-Based Supercapacitors Yuriy. Y. Smolin, Masoud Soroush, and Kenneth K. S. Lau* Department of Chemical and Biological Engineering, Drexel University, Philadelphia, PA 19104, USA. Oxidative chemical vapor deposition (oCVD), carbide-derived carbon, conducting polymers, polyaniline, supercapacitor

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Polyaniline (PANI) is integrated into Mo2C carbide-derived-carbon (CDC) electrodes using the single-step, solvent-free process of oxidative chemical vapor deposition (oCVD). By optimizing the oCVD processing conditions, CDC electrodes integrated with oCVD PANI exhibits more than double the gravimetric capacitance (115 F/g) vs bare CDC electrodes (52 F/g) and a 79% capacity retention after over 10,000 cycles. The oxidant flowrate, substrate temperature, and reactor pressure were varied, and their influence on film chemistry and supercapacitor performance was explored electrochemically and with FTIR and XPS. The study reveals that a higher substrate temperature, pressure, and oxidant flowrate are critical for depositing emeraldine PANI for optimal electrochemical performance. Interestingly, the optimally performing PANI-CDC devices have a porous PANI morphology as determined by SEM, which may facilitate ion transport, improve scan rate performance, and impart electric double layer capacitance in addition to the intrinsic faradaic pseudocapacitance.


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1. Introduction Carbon-based electrochemical double layer capacitors, also known as supercapacitors, are a promising energy storage device due to their low weight, low cost, and fast charge/discharge kinetics.1 Supercapacitors have been used in many applications such as electric vehicles, uninterruptible power supplies, DC power systems, wearable electronics, and mobile devices.2,3 When compared to batteries, supercapacitors have several advantages such as 10–100 times greater power density, higher rate capability, and excellent cyclability even after millions of cycles.1,4,5 However, most supercapacitors only have 10% the energy density of batteries.5 To improve the energy density of supercapacitors, composite devices have been proposed, which integrate a pseudocapacitive material such as conducting polymers into a carbon electrode, thereby synergistically combining faradaic and capacitive storage mechanisms.4,6 This approach has attracted considerable attention in recent years to increase the energy density of supercapacitors. Integrating conducting polymers onto carbon nanotubes, conductive carbons, and in-between graphene and MXene layers have all shown improved device performance.7-9 However, as electrode designs become more complex and continue to shrink to smaller length scales (i.e., nanoscale), the integration of conducting polymers becomes more challenging due to de-wetting, and non-uniform and non-conformal deposition. This is especially true using traditional solvent-based deposition processes such as chemical bath deposition,10 electrodeposition,11 and solvent casting.12 Control of solvent chemistry and properties become critical and might sometimes not be able to achieve conformal and uniform coatings. Conducting polymers deposited onto carbon electrodes using these methods often lead to composite devices with higher energy density (from the faradaic redox reactions of the polymers) but at the expense


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of reduced rate capability and poor cycle life. These drawbacks are often due to a thick polymer film, which impedes ion transport and degrades mechanically due to large expansions and contractions upon charge and discharge.13-17 Moreover, solvent-based coating techniques often do not preserve the electrode surface area, which reduces the double layer capacitance contribution and leads to sub-optimal performance. To overcome the disadvantages of poor rate capability, loss of double layer capacitance and poor cyclability, this work utilizes oxidative chemical vapor deposition18 (oCVD) as a singlestep, solvent-free polymerization and coating technique to integrate ultrathin polyaniline (PANI) films into porous carbon electrodes. As shown in Figure 1, the oCVD process involves flowing monomer and oxidant vapors, aniline and antimony pentachloride, respectively, into the reaction chamber. These vapors adsorb onto the carbon electrode surface and the oxidant initiates oxidative step-growth polymerization18,19 as well as simultaneously dopes the growing polymer film.20,21 To control the deposition process, the substrate temperature (Ts), reactor pressure (P), and oxidant, monomer and N2 flowrates (Fo, Fm, Fn) are precisely regulated. We have previously demonstrated oCVD’s ability to integrate conformal and uniform ultrathin (down to 4 nm) films of polythiophene (PTh) into porous nanostructures of anodized aluminum oxide, titanium dioxide, and carbon-based electrodes.20 By preserving the surface area and pore spacing of the carbon electrode, the PTh-integrated carbon electrodes exhibited 50% increase in gravimetric capacitance and 90% capacity retention after 5,000 charge-discharge cycles. Work by the Gleason group has also demonstrated the utility of oCVD to conformally integrate poly(3,4-ethylenedioxythiophene) (PEDOT) on aligned carbon nanotubes and graphene flakes for energy storage, demonstrating improved charge storage and stability.7,22-24 oCVD has also emerged as an excellent tool to


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integrate thin and ultrathin (1–10 nm) polymer films within other 3D devices such as batteries, solar cells, and sensors.25,26 Despite previous work, which has shown oCVD as a viable technique to deposit polypyrrole, PTh, and PEDOT,25 there has been only been limited work on oCVD PANI to date. Among conducting polymers, PANI is ideally suited for supercapacitors due to its high theoretical capacitance (750 F/g), which is more than double that of PTh and three times higher than most bare carbon electrodes (250 F/g).14 PANI also has high electrical conductivity and greater stability, and can be made with an inexpensive monomer. Recently, we successfully showed the polymerization of PANI by the oCVD approach,27 and provided an initial demonstration of enhanced energy storage of carbon supercapacitors coated with oCVD PANI.28 However, the question of how oCVD PANI film chemistry influences supercapacitor performance remains to be addressed. This understanding will aid in optimizing the oCVD PANI film chemistry for enhancing carbon-based supercapacitors as well as provide insight on the influence of PANI properties on charge storage. Therefore, this work systematically investigates the effect of oCVD processing conditions on supercapacitor performance of carbide-derived carbon (CDC) as a model system as a result of changes in PANI film chemistry and morphology.


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2. Experimental Methods 2.1 Fabrication of CDC electrodes Porous Mo2C carbide-derived carbon (CDC) electrodes were fabricated from a molybdenum carbide precursor.29,30 By controlling the synthesis temperature (800 °C), these electrodes had a controlled bimodal pore size distribution (pore widths of 0.81, 1.68, 3.38 nm) and a specific surface area of 530 m2/g.29 With a mix of micro- and meso-porosity, the Mo2C-CDC electrode was used as a model system to test and optimize the oCVD processing conditions for integrating PANI.30 To make free-standing electrodes, the carbon powder was mixed with 5 wt% PTFE binder and subsequently rolled to a thickness of ~100 µm using a Durston rolling mill (UK). 2.2 oCVD PANI polymerization To deposit PANI using oCVD, separate source jars containing aniline (Sigma Aldrich, ACS reagents, >99.5%) and antimony pentachloride oxidant (Sigma Aldrich, 99%) were heated to 60 °C to produce vapors, which were delivered into the oCVD reactor using precision metering valves (Swagelok). The monomer and oxidant were separately introduced to minimize polymerization in the gas delivery manifold. To carry the oxidant into the reactor, nitrogen gas was additionally metered through the oxidant line using a mass flow controlled (MKS Instruments). The deposition substrate (silicon, CDC electrode) was placed on a stage contacted on the backside with a thermal fluid recirculating in a heater/cooler (Thermo Electron RTE-7). Pressure in the reactor was measured with a Baratron capacitance pressure gauge (MKS Instruments) and automatically maintained at a prescribed setpoint using a pressure controller (MKS Instruments) connected to a downstream throttle valve (MKS Instruments). Figure 1 shows the key oCVD processing


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parameters: substrate temperature (Ts), reactor pressure (P), and oxidant and monomer flowrates (Fo and Fm, respectively). Our aim was to investigate how each of the oCVD process conditions influenced deposition behavior, film chemistry and electrochemical performance of oCVD PANI supercapacitors. Table 1 summarizes the reaction conditions explored in this work. The first condition, known as the base case (BC), had a monomer and oxidant flowrate of 1 and 0.8 sccm (standard cm3 min-1), respectively, nitrogen gas flowrate of 1 sccm, reactor pressure of 700 mtorr, and a substrate temperature of 90 °C. From the base case BC condition, three other main cases were explored and are highlighted here: substrate temperature was decreased to 25 °C for the low-T condition; reactor pressure was lowered to 35 mtorr for the low-P condition; and oxidant flowrate was reduced to 0.15 sccm for the low-O condition. Based on previous oCVD work of PEDOT, higher substrate temperatures yielded higher quality films with greater polymer conjugation length and doping that improved film electrical conductivity.31,32 Here with PANI, the situation requires more care. As shown in Figure 2, PANI can exist in three primary oxidation states: fully reduced leucoemeraldine, semi-oxidized emeraldine, and fully oxidized pernigraniline. Out of the three states, the intermediate emeraldine state is the one with the higher electrical conductivity and one that is preferred for supercapacitor applications. We further hypothesize that the reactor pressure and amount of oxidant relative to the monomer would influence polymerization behavior and polymer properties such as the oxidation state. Therefore, a search and optimization of these oCVD conditions were carried out to understand the impact of PANI film chemistry on supercapacitor performance.


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2.3 oCVD PANI integration and electrochemical testing of CDC electrodes CDC electrodes were placed on the temperature-controlled reactor stage and oCVD was performed to deposit PANI following the conditions given in Table 1. The reaction time was varied between processing conditions to have a consistent PANI loading on the CDC electrode (~15%), see Supporting Information Table S1. The reaction time reported in Table 1 together with the mass loading in Table S1 provide an indication of the PANI growth rate, which is on the order of 1–10 mg/min. After PANI deposition, the coated CDC samples were soaked for 3 h in tetrahydrofuran (THF >99.9%, Sigma-Aldrich) and subsequently dried for 14 h in a vacuum oven at 70 °C. This routine washing step was carried out for all samples to remove the oxidant and dopant, antimony pentachloride, as well as soluble short chain oligomers from the as-deposited PANI films. To elucidate morphology, scanning electron microscopy (SEM) was performed on bare and PANIcoated CDCs using a Zeiss Supra 50VP at a 4 mm working distance, with the in-lens detector at 15 kV, and by applying line integration with 7 repeats. Samples were sputtered with Pt for 30 s prior to SEM imaging. The PANI-coated CDC electrodes were punched down to an electrode diameter of 9 mm and assembled into a Swagelok device in a standard three-electrode configuration. The counter electrode was a larger, over-capacitive activated carbon electrode while the reference electrode was Ag/AgCl saturated in KCl. To decrease contact resistance between the electrodes and stainless steel rods, platinum current collectors were used. Sandwiched between the two electrodes were two layers of porous separator (Celgard) filled with 1 M H2SO4 (in deionized water) electrolyte. Cyclic voltammetry (CV) and galvanostatic cycling were conducted using a VMP3 potentiostat/galvanostat (BioLogic) to characterize the resulting supercapacitor devices based on the different washed oCVD PANI films from Table 1. In addition, CDC using oCVD PANI from


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the BC condition with the routine THF washing was compared to that without the post-deposition washing to investigate the effects of washing on electrochemical behavior. Similarly, supercapacitors integrated with oCVD PANI were also compared with bare CDC devices. 2.4 oCVD PANI compositional analysis Further characterization was performed on oCVD PANI films deposited on silicon substrates to facilitate compositional analysis by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Transmission FTIR spectra were acquired with a Thermo Nicolet 6700 spectrometer using an MCT/A detector at a resolution of 4 cm–1 and averaged over 128 scans. XPS was done on a Physical Electronics VersaProbe 5000 using a micro-focused monochromatic scanned X-ray beam from an Al Kα X-ray source (1486 eV) with a 100 µm spot size, and at 25 W and 15 kV. High resolution N1s, Cl2p, and Sb3d spectra were acquired using 23.5 eV pass energy and 0.05 eV energy step for a total of 2048, 256, and 256 scans, respectively.


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3. Results and Discussion 3.1. oCVD PANI-CDC electrochemical behavior To understand how oCVD processing impacted supercapacitor performance, PANI was integrated at different oCVD conditions (Table 1) into CDC electrodes and electrochemically tested. Typically, oCVD PANI films are washed after deposition but to first investigate the influence of THF washing on resulting device behavior, CV measurements at 20 mV/s were made on supercapacitors integrated with oCVD PANI using the base case condition with (BC) and without washing (BC-unwashed), see Figure 3a. Previous work on other oCVD conducting polymers have shown that post-deposition washing can improve film properties such as conductivity and stability,33-35 as well as make the film more resistant to atmospheric degradation that has been attributed to tighter chain packing.18,36 Nejati et al.19 have shown, using UV-vis, that washing oCVD PTh films removes the oxidant and soluble portions of the film composed of short chain oligomers of five repeat units or shorter. The BC-unwashed device shows redox peaks associated with the Faradaic oxidation and reduction of PANI.2 If we remove the contributions of the redox peaks from the BC-unwashed plot, the shape of the CV is very similar to bare (uncoated) CDC also provided in Figure 3a. This is to be expected since PANI only adds the pseudocapacitive peaks. However, for the CV of BC with the normal THF washing, while the redox peaks of PANI are still present, there is now an increase in the CV envelope throughout the entire voltage window. This is rather unexpected since this suggests that PANI somehow increases the overall electric double layer capacitance (EDLC) in addition to adding the pseudocapacitance. Now focusing on the CV behavior at 20 mV/s of CDC devices integrated with PANI films from various oCVD conditions, see Figure 3b. All the PANI-coated CDC electrodes again show


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the PANI redox peaks.2 Going from low-O, low-P, low-T to BC, the redox peaks increase in intensity. We strived to achieve roughly the same PANI loading of ~15 wt% (of the final PANICDC electrode weight) for all samples (13.6, 14.8, 17.9 and 15.0% for BC, low-T, low-P and lowO, respectively), see also Supporting Information Table S1, so the changes in intensity cannot really be explained by differences in the amount of PANI incorporated. In fact, based on the PANI wt% loading alone, the BC condition should have the lowest charge storage, which is not the case here. Changes in peak intensity could come from changes in film chemistry and morphology with different oCVD PANI synthesis conditions. Likewise, from Figure 3b, there is an increase in the EDLC envelope of the PANI-integrated devices in the order of low-O, low-P, low-T, and BC. Since these films have been washed with THF, the washing process likely contributed in some way to the higher EDLC. In addition, these results indicate that oCVD PANI deposition at higher substrate temperature, pressure, and oxidant flowrate leads to more favorable PANI properties. Further tests were carried out to evaluate the electrochemical performance at different scan rates from 2 to 500 mV/s, as shown in Figures 3c and 3d. For all oCVD processing conditions studied, the CDC devices with oCVD PANI gave higher capacitance values compared to the uncoated, bare CDC counterpart. In many cases, the capacitance is more than doubled. Based on the actual CV data, these improvements can be attributed to the addition of the Faradaic contribution of PANI, although there is also an enhanced EDLC component. These improvements are even present at a high scan rate of 500 mV/s. If we were to compare more closely the different sets of data, washing again enhanced capacitance at all scan rates. With washed films at different oCVD deposition conditions, capacitance increased in the order of low-O, low-P, low-T, and BC for the most part. The BC condition, which yields the highest capacitance, gives a PANI-only gravimetric capacitance of 526 F/g (at 10 mV/s) that is close to the theoretical limit of 750 F/g.14


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From Figures 3c and 3d, capacity generally fades with higher scan rates in all cases that includes the bare CDC. This is to be expected since the shorter timescale at a higher rate limits the full penetration of electrolyte ions. However, capacity fade is slower, particularly for the BC condition, when compared with bare CDC or with the unwashed BC case. This suggests that electrolyte ions are less impeded with the BC PANI film. In terms of cycle life, as shown in Figures 3e and 3f, all the washed films have excellent capacitance retention at least up to the 10,000 cycles that were performed. The capacity retention for BC, Low-P, Low-O was 79, 89, and 93%, respectively, while the capacity of Low-T actually increased slightly with cycling. With all these cases, such high cycling stability may be due in part to the ultrathin oCVD coatings, which do not swell and contract appreciably during cycling. In contrast, typical conducting polymer coatings are thicker, with significant swelling and contraction that leads to mechanical failure and device degradation by 1,000 cycles.14 For example, electrodeposited PANI films on carbonized polyacrylonitrile aerogel electrodes lost 15.3% of the initial capacitance after only 200 cycles.37 Our films show an initial degradation within the first few cycles, but then show excellent capacity retention. However, with BC-unwashed PANI, capacity decayed with cycling down to 70% of its initial value. 3.2. oCVD PANI film chemistry and morphology We hypothesize that the differences in electrochemical behavior of the CDC electrodes are a result of differences in oCVD PANI film chemistry and morphology. Figure 4 shows the electrode morphology of bare CDC and coated with oCVD PANI at different processing conditions. The oCVD PANI coating has been shown to deposit throughout the CDC electrode.28 Focusing first on the effect of THF washing, the electrochemical data show washing improved capacitance as well


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as capacitance retention with scan rate and cycling. CV data in particular show that washing increased the CV envelope. SEM of the oCVD PANI coatings before (BC-unwashed) and after washing (BC), see Figure 4, reveal that washing produced a porous morphology in the BC film with an average pore diameter of 12.2 ±15.7 nm (as determined by ImageJ), with pores between 5–40 nm in diameter. This porous morphology was present throughout the BC film. In contrast, the BC-unwashed film is smoother with none of these openings, and this film appears to connect across CDC grains when compared with the bare CDC electrode. The formation of the pores after washing most likely increased electrode surface area, and may explain the increase in the CV envelope in upon washing, since EDLC relies on physical electrostatic interactions that scale with the electrode-electrolyte interfacial area. This argument is further corroborated by looking at the electrode morphologies with various oCVD PANI deposition conditions in Figure 4. Film surface roughness increases in the order of Low-O, Low-P, Low-T, and BC, with BC clearly showing the most prominent porous morphology. This aligns with the observed increase in the EDLC contribution to the CV envelope of these electrodes in the same order. Besides explaining the source of the EDLC contribution in oCVD PANI, the increase in surface roughness and the appearance of the porous morphology seen in Figure 4 can also help with understanding the scan rate improvement and capacity fade slowdown. The pores can facilitate ion movement during charge and discharge that otherwise would be impeded with smoother PANI films. Both the BC, Low-T, and Low-P films have rougher morphologies and pores that slow down capacity fade. In contrast, the Low-O and BC-unwashed films have much smoother surfaces that see a faster capacity fade as scan rate increased. Thus, the capacity fade trends seem to align well with the observed morphology trends.


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To further rationalize the observed electrochemical behavior, oCVD PANI film chemistry was elucidated through FTIR. The films were deposited on silicon to facilitate FTIR analysis and FTIR spectra based on the different processing conditions are given in Figure 5. Again, first focusing on the effect of washing, the spectra of the as-deposited (BC-unwashed) and washed (BC) films are strikingly different. The as-deposited BC-unwashed spectrum has peaks at 3304, 3064, 1577, 1490, and 1168, cm-1 that suggests PANI is primarily in the salt (doped) form of the fully oxidized pernigraniline state.27 For instance, the 3304 and 3064 cm-1 peaks are attributed to –NH+= stretching and C–H stretching of the aromatic ring, respectively, for PANI salt.38. Furthermore, the 1168 cm-1 peak is attributed to –NH+= stretching and in-plane C–H vibrations that suggests the formation of PANI in the salt (doped) form.38,39 The 1577 and 1490 cm-1 peaks are due to quinoid and benzenoid groups, respectively, and their positions correspond to doped PANI.39 Furthermore, by taking the ratio of the 1577 to 1490 cm-1 peak intensities, it is possible to estimate the oxidation state of the polymer.40 For the unwashed BC film, this value is 1.77 which indicates that the film is primarily composed of quinoid groups, indicating that the film is mostly in the pernigraniline state. Therefore, antimony pentachloride, besides acting as the oxidant in the oxidative polymerization of PANI during oCVD, most likely dopes the growing PANI film as well. Our previous work on oCVD PTh has also shown that the oxidant can dope the film.19,20 Upon washing, the BC spectrum indicates that PANI has reduced to the base (undoped) form of the semi-oxidized emeraldine state.27 The washed BC film quinoid and benzenoid peaks have blue-shifted blue to 1588 and 1510 cm-1, respectively, indicating a transition from the salt (doped) to the base (undoped) form.39 Furthermore, taking the intensity ratio of the 1588 to 1510 cm-1 peaks leads to a value of 0.87, suggesting mostly the emeraldine state. Previous reports on other oCVD conducting polymers have also demonstrated this dedoping phenomenon with the removal of oxidant from the


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polymer.31,41,42 Compared to the pernigraniline state, the emeraldine state provides greater electrical conductivity and stability, which may explain why the BC case is superior in capacitance, retention, and cyclability compared to the BC-unwashed case. Now looking at the influence of oCVD synthesis conditions on film chemistry, the FTIR spectra of Low-P and Low-O show that the 1588 cm-1 peak, assigned to the oxidized quinoid, is smaller compared to that of BC. This suggests that a lower reactor pressure or oxidant flowrate during oCVD produced a less oxidized film,27 which is reasonable given the lower oxidant concentration in either case for polymerization and doping. This further provides support to the electrochemical observations with the smaller redox peak intensities and lower capacitance values of Low-P and Low-O compared to BC. What is missing from the FTIR data is the spectrum for the Low-T condition. During THF washing of the Low-T film, which was deposited on silicon for FTIR, portions of the film would dissolve while the remainder would delaminate from the silicon substrate, rendering FTIR analysis near impossible. The partial dissolution indicates that the film contained appreciable amounts of soluble short chain oligomers, which our studies of oCVD PANI at low substrate temperature confirmed to be only a few repeat units long.27 However, the Low-T PANI deposited on the CDC electrode seems to be physically more stable. The electrochemical data still show redox contributions even though the Low-T film has been washed. It is possible that the Low-T PANI was more stable or not as accessible when interfaced within the porous CDC carbon, which might have prevented or reduced dissolution and delamination. Alternatively, it is possible that the short chain fragments dissolved but somehow were re-incorporated into the film. In either case, the presence of low molecular weight chains may explain the observed electrochemical behavior of the Low-T case. The slight increase in capacitance with cycling may be due to coupling of these shorter chains into longer ones that hold more charge. As a result, the


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capacitance of Low-T PANI even approached that of the electrochemically more favorable BC PANI at the end of the 10,000th charge-discharge cycle. Finally, the oCVD PANI film chemistry of BC and BC-unwashed films deposited on silicon was probed by XPS in order to understand more quantitatively the changes with THF washing that was revealed by FTIR. The high-resolution N1s XPS spectra of BC-unwashed and BC PANI can be resolved into up to four separate nitrogen bonding environments, as previous work by Kumar et al. has shown,43 with N1, N2, N3, and N4 assigned to the chemical structures indicated in Figure 6. The corresponding resolved position and relative fraction of each peak are summarized in Table 2. From the resolved peak intensities, the oxidation level of PANI films can be approximated by the ratio (N1+N3+N4)/Ntotal, since N2 is the fully reduced species,43 so for example, the semi-oxidized emeraldine has a theoretical ratio of 0.50. Here, for the BC-unwashed case, the ratio is 0.785, which corresponds to roughly 80% oxidation, and agrees well with the qualitative FTIR analysis that the film is predominantly in the pernigraniline state. For the BC case after washing, the N4 peak, which corresponds to doped PANI disappeared, while the ratio of (N1+N3)/Ntotal is 0.496, which indicates that the BC film is in the undoped emeraldine state, again corroborating the qualitative FTIR evidence. Further, the antimony and chlorine XPS contributions (not shown) of the oxidant are 11.6 and 6.9%, respectively, for the BC-unwashed film while these values decreased to 0.34 and 1.24%, respectively, for the BC film with washing. The loss of the oxidant dopant and the formation of the more favorable emeraldine state with washing creates the more optimal PANI in electrochemical supercapacitors.


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4. Conclusions From this work, the most optimal PANI for enhancing the capacitance, capacitance retention, and cycle life of CDC supercapacitors is made by oCVD at higher substrate temperature, pressure, and oxidant flowrate. The PANI from these conditions was found to have the ideal semi-oxidized emeraldine state for more favorable electrochemical response after THF washing of the polymer that removed the antimony pentachloride oxidant and dopant. In addition, the washing produced a rough porous morphology that improved surface area and enhanced the EDLC contribution above the EDLC offered by the CDC alone. While this work reveals the sensitivity of oCVD PANI film chemistry and morphology on resulting electrochemical behavior of coated CDC electrodes, it highlights the viability of oCVD in tuning PANI properties for optimizing carbon supercapacitor performance. oCVD overcomes many of the challenges faced by current PANI synthesis and processing methods, with its ability to make ultrathin films, control oxidation level, film chemistry and physical morphology.


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AUTHOR INFORMATION Corresponding Author *Kenneth K. S. Lau, [email protected], Department of Chemical and Biological Engineering, Drexel University. 3141 Chestnut Street, Philadelphia PA 19104. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources U.S. National Science Foundation

ACKNOWLEDGMENT The authors gratefully acknowledge support from the U.S. National Science Foundation (CBET1236180, 1264487, 1463170). We would also like to acknowledge the use of Drexel University’s Core Facilities and Prof. Giuseppe Palmese's lab facilities. We thank Katherine L. Van Aken and Prof. Yury Gogotsi for the CDC electrode fabrication and testing.


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34. Howden, R. M.; McVay, E. D.; Gleason, K. K., oCVD poly(3,4-ethylenedioxythiophene) conductivity and lifetime enhancement via acid rinse dopant exchange. Journal of Materials Chemistry A 2013, 1, (4), 1334-1340. 35. Lee, S.; Paine, D. C.; Gleason, K. K., Heavily doped poly(3,4-ethylenedioxythiophene) thin films with high carrier mobility deposited using oxidative cvd: conductivity stability and carrier transport. Advanced Functional Materials 2014, 24, (45), 7187-7196. 36. Nardes, A. M.; Kemerink, M.; de Kok, M. M.; Vinken, E.; Maturova, K.; Janssen, R. A. J., Conductivity, work function, and environmental stability of PEDOT:PSS thin films treated with sorbitol. Organic Electronics 2008, 9, (5), 727-734. 37. Talbi, H.; Just, P.-E.; Dao, L., Electropolymerization of aniline on carbonized polyacrylonitrile aerogel electrodes: applications for supercapacitors. Journal of Applied Electrochemistry 2003, 33, (6), 465-473. 38. Šeděnková, I.; Trchová, M.; Blinova, N. V.; Stejskal, J., In-situ polymerized polyaniline films. Preparation in solutions of hydrochloric, sulfuric, or phosphoric acid. Thin Solid Films 2006, 515, (4), 1640-1646. 39. Trchová, M.; Šeděnková, I.; Tobolková, E.; Stejskal, J., FTIR spectroscopic and conductivity study of the thermal degradation of polyaniline films. Polymer Degradation and Stability 2004, 86, (1), 179-185. 40. Abdiryim, T.; Xiao-Gang, Z.; Jamal, R., Comparative studies of solid-state synthesized polyaniline doped with inorganic acids. Materials Chemistry and Physics 2005, 90, (2–3), 367372. 41. Howden, R. M.; McVay, E. D.; Gleason, K. K., oCVD poly (3, 4ethylenedioxythiophene) conductivity and lifetime enhancement via acid rinse dopant exchange. Journal of Materials Chemistry A 2013, 1, (4), 1334-1340. 42. Borrelli, D. C.; Barr, M. C.; Bulović, V.; Gleason, K. K., Bilayer heterojunction polymer solar cells using unsubstituted polythiophene via oxidative chemical vapor deposition. Solar Energy Materials and Solar Cells 2012, 99, 190-196. 43. Kumar, S. N.; Bouyssoux, G.; Gaillard, F., Electronic and structural characterization of electrochemically synthesized conducting polyaniline from XPS studies. Surface and Interface Analysis 1990, 15, (9), 531-536.


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Table 1. Processing studies of oCVD PANI. Run

P

Ts

Fo

Fm

Fn

τrxn

(mtorr)

(°C)

(sccm)

(sccm)

(sccm)

(min)

BC

700

90

0.8

1

1

4

Low-T

700

25

0.8

1

1

1

Low-P

35

90

0.8

1

1

0.67

Low-O

700

90

0.15

1

1

37

P = reactor pressure; Ts = substrate temperature; Fo, Fm, Fn = flowrates of the oxidant (antimony pentachloride), monomer (aniline), and nitrogen gas, respectively; τrxn = reaction time.


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Table 2. Resolved N1 XPS spectral data of oCVD PANI. N1

N2

N3

N4

as-deposited binding energy (eV)

398.5

399.4

400.4

402.0

at %

65.2

21.5

11.1

2.2

washed binding energy (eV)

398.7

399.3

400.4

–

at %

34.2

50.4

15.4

–


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Figure 1. oCVD process for depositing PANI into the pores of a carbon electrode, showing key process parameters, including substrate temperature (Ts), gas species flowrates (Fm, Fo, Fn), and reactor pressure (P).


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Figure 2. Chemical structure of primary oxidation states of PANI in the base, undoped form. (a) Fully reduced leucoemeraldine PANI composed of benzenoid groups, (b) fully oxidized pernigraniline PANI composed of quinoid groups, and (c) emeraldine PANI composed of a 1:1 ratio of benzenoid and quinoid groups.


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Figure 3. Electrochemical measurements of oCVD PANI CDC supercapacitors, comparing the effect of THF washing (top) and the effect of oCVD processing conditions (bottom). (a,b) Cyclic voltammograms at 20 mV/s, (c,d) scan rate dependence, and (e,f) cycle life (at 20 mV/s) of oCVD PANI, respectively, before and after washing at the BC condition, and deposited at the Low-O, Low-P, Low-T, and BC conditions, see Table 1. Bare, uncoated CDC served as a control.


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Figure 4. Top-down SEM images of (a) bare CDC, and CDC coated with oCVD PANI at the conditions: (b) BC-unwashed, (c) BC, (d) Low-O, (e) Low-P, and (f) Low-T, see Table 1. Scale bar = 2 µm, inset = 400 nm.


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Figure 5. FTIR of oCVD PANI films based on the conditions shown in Table 1. Washed films at various oCVD conditions (top panel), and unwashed BC (bottom panel).


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Figure 6. High-resolution N1s XPS spectra of oCVD PANI deposited at the BC condition before (top, BC-unwashed) and after (bottom, BC) THF washing.


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TOC Graphic


30

Influence of oCVD Polyaniline Film Chemistry in Carbon-Based

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