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Supercapacitor Electrodes Based on High-Purity Electrospun Polyaniline and Polyaniline-Carbon Nanotube Nanofibers Silas Kiptui Simotwo, Christopher DelRe, and Vibha Kalra ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03463 • Publication Date (Web): 28 Jul 2016 Downloaded from http://pubs.acs.org on July 28, 2016

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ACS Applied Materials & Interfaces

Supercapacitor Electrodes Based on High-Purity Electrospun Polyaniline and PolyanilineCarbon Nanotube Nanofibers Silas K. Simotwo#, Christopher DelRe# and Vibha Kalra* Department of Chemical and Biological Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA.

Abstract Freestanding, binder-free supercapacitor electrodes based on high purity polyaniline (PANI) nanofibers were fabricated via a single step electrospinning process. The successful electrospinning of nanofibers with an unprecedentedly high composition of PANI (93 wt %) was made possible due to blending ultra-high molecular weight poly (ethylene oxide) (PEO) with PANI in solution to impart adequate chain entanglements, a critical requirement for electrospinning. To further enhance the conductivity and stability of the electrodes, a small concentration of carbon nanotubes (CNTs) was added to the PANI/PEO solution prior to electrospinning to generate PANI/CNT/PEO nanofibers (12 wt% CNTs). Scanning electron microscopy (SEM) and Brunauer-Emmet-Teller (BET) porosimetry were conducted to characterize the external morphology of the nanofibers. The electrospun nanofibers were further probed by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). The electroactivity of the freestanding PANI and PANI/CNT nanofiber electrodes was examined using cyclic voltammetry, galvanostatic chargedischarge and electrochemical impedance spectroscopy. Competitive specific capacitances of 308 and 385 F g-1 were achieved for PANI and PANI-CNT based electrodes, respectively, at a current density of 0.5 A g-1. Moreover, specific capacitance retentions of 70 and 81.4% were observed for PANI and PANI-CNT based electrodes, respectively, after 1000 cycles. The promising electrochemical performance of the fabricated electrodes, we believe, stems from the

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porous 3-D electrode structure characteristic of the nonwoven interconnected nanostructures. The interconnected nanofiber network facilitates efficient electron conduction while the interand intra-fiber porosity enable excellent electrolyte penetration within the polymer matrix, allowing fast ion transport to the active sites. Keywords: electrospinning, pseudocapacitors, conductivity.

high

purity

nanofibers,

freestanding,

microporous,

Introduction With the ever rising need for eco-friendly, sustainable and high power/density energy devices, supercapacitors have gained immense interest for potential applications in electronic circuits, electric vehicles and back-up power generators.1-2 Supercapacitors possess many attractive features, including high power densities, ultrafast charge-discharge rates and long life cycles.3 Supercapacitors are broadly classified into two different groups: electrochemical double layer capacitors (EDLCs), which rely on the accumulation of electrostatic charges at the electrode/electrolyte interface for energy storage and pseudocapacitors, which store energy via fast Faradaic reactions at the surface of the electrode.4 High surface area carbon materials are frequently applied in EDLCs while electrically conducting polymers and transition metal oxides/hydroxide/sulfides are commonly used in pseudocapacitors.5-8 Polyaniline (PANI), an electrically conducting polymer, has shown enormous promise as a pseudocapacitor material owing to its ease of synthesis, good environmental stability, low cost and excellent electroactivity.9 Bulk PANI, however, is not suitable for application in energy storage device electrodes due to its low accessible surface area. Therefore, nanostructured PANI materials with large surface-area-to-volume ratios – and hence shorter ion diffusion paths – have garnered interest as suitable electrode materials for supercapacitors.10-11 Versatile hierarchical


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PANI nanostructures and nanocomposites with promising electrochemical performance in supercapacitors electrodes have been developed and studied.12-14 PANI nanocomposites have typically been prepared by employing a carbon template. Recently, Luo and co-workers synthesized a self-assembled hierarchical graphene@polyaniline nanocomposite via a simple polymerization route.15 An electrochemical capacitor based on this nanocomposite showed specific capacitance of 488 F g-1, 79% of which was retained after 1000 cycles. Sacrificial templates have also been employed to fabricate high surface area hollow or tubular PANI nanostructures with high electroactivity for use as supercapacitor electrodes.16-17 A hollow PANI nanofiber-based supercapacitor electrode with specific capacitance of 601 F g-1 was recently fabricated by polymerizing aniline monomers on pre-electrospun poly (amic acid) (PAA) nanofibers, followed by selective removal of the PAA nanofiber template. The use of a sacrificial template to fabricate PANI nanostructures is perceived as less efficient technique due to potential structural degradation during the template removal process. Techniques that don’t rely on templates

have

therefore

been

employed

to

directly

synthesize

various

PANI

nanoarchitectures.18-20 Despite the availability of the aforementioned diverse routes for PANI fabrication, there are still many challenges facing these synthesis techniques and the corresponding electrode assembly processes. These challenges include low or negligible capacitance of the substrate for the case of hybrid electrodes, a possibility of over-estimating the capacitance normalized per weight of PANI due to only a small amount of polymer being deposited on the substrates and multi-step fabrication processes leading to additional cost, time, and resources. Moreover, prior to electrochemical testing, the supercapacitor electrodes are almost universally prepared by mixing the PANI nanostructures with binders such as PVDF and conductive additive. 21 The preparation of such paste is not only likely to cause distortion of the


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PANI nanostructures but also introduces electrochemically inert materials in the form of binders into the electrode.

A straightforward, single-step technique for fabricating self-supporting

nanostructured PANI electrodes is therefore highly desirable. Electrospinning offers such an alternative technique. Although electrospinning has widely been adopted in literature to fabricate eclectic polymer nanostructures due to the simplicity of its setup and the ease with which the electrospun nanofiber morphologies can be modulated, PANI is an extremely difficult polymer to process via electrospinning.

The aromaticity in the

polymer’s backbone makes individual PANI chains extremely rigid, preventing PANI solutions from experiencing sufficient chain entanglements to achieve the minimum solution viscosity required for successful electrospinning. 22-23 Additionally, PANI possesses limited solubility and dispersion in most organic solvents, making it difficult to obtain a PANI solution with sufficient viscosity just by increasing the polymer concentration. To circumvent these issues, an electrospinnable carrier polymer can be added to PANI to form an electrospinnable polymer blend. Unfortunately, efforts to electrospin nanofibers with high concentrations of PANI have been hindered by the necessity to include a substantial amount of carrier polymer in order to successfully electrospin continuous and smooth nanofibers.24 The carrier polymer is typically electrically insulating; thus, high concentrations of carrier polymer severely diminish the electrical properties of the electrospun PANI nanofibers. Frontera et al electrospun PANI/PEO with w/w percent ratio as high as 80/20; however, the fiber mats of the resulting nanofibers exhibited a substantial amount of beads and fiber breakage.25 Such beads and discontinuity among the fibers are expected to inhibit optimal electrode performance when applied in a pseudocapacitor device. Chaudhari et al developed supercapacitor electrodes using electrospun PANI/PEO nanofibers with a w/w percent ratio of 50/50. They reported a maximum specific


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capacitance of 267 F g-1 at a current density of 0.35 A g-1 26. To our knowledge, this is the only work in the literature that has reported the capacitance of electrospun PANI nanofibers. In addition to the high fraction of electrochemically inert carrier polymer used, the nanofiber electrodes fabricated in that work were not freestanding – the conventional slurry technique was employed to prepare the supercapacitor electrodes with powdered nanofibers and insulating binders. Our work demonstrates a facile, single-step fabrication of freestanding (self-supported and binder-free), high-purity PANI and PANI/CNT nanofiber mats via electrospinning. We used high molecular weight PEO (8000 kDa) as a carrier polymer, which enabled successful electrospinning of PANI/PEO and PANI/CNT/PEO nanofibers with a low PEO composition of only 7wt%. In addition to the use of high MW PEO, successful electrospinning of high purity PANI nanofibers was enabled by doping PANI, which enhanced its solubility in chloroform.27 The addition of CNTs enhanced the electrical conductivity and in turn the electrochemical performance of the PANI/PEO nanofibers. Specific capacitance of 308 F g-1 and 385 F g-1 was achieved for PANI/PEO and PANI/CNT/PEO, respectively, at a current density of 0.50 A g-1. Both nanofiber mats showed high rate capability and good electrochemical stability after 1000 cycles. To the best of our knowledge, no previous work has reported the preparation of PANI/CNT supercapacitor electrodes via the electrospinning technique. The majority of the PANI/CNT research in the literature reports electrochemical or chemical deposition of polyaniline on a CNT template, often resulting in a high CNT fraction and dead weight in the final electrode. Experimental Methods and Materials


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Materials. Polyaniline emeraldine base (PANI, Mw = 100,000 g/mole), polyethylene oxide (PEO, Mw = 8,000,000 g/mole), 10-camphorsulfonic acid (HCSA), and chloroform (>99.5%) were purchased from Sigma Aldrich. Carbon nanotubes (CNTs) (multi-walled, 8-15 nm diameter) were purchased from NanoLab Inc. Fabrication of PANI and PANI-CNT nanofibers. Chloroform, PANI, and HCSA were mixed simultaneously and stirred at approximately 300 rpm for more than 6 hours. The PANI concentration in chloroform was 0.67 wt%. A weight ratio of 1:1.29 PANI:HCSA was employed to achieve PANI doping. After stirring for at least 6 hours, the solution was sonicated for 15 minutes. PEO was added immediately after ultrasonication in a 93:7 PANI:PEO weight ratio and the resulting solution was further stirred for 6 hours. For the PANI-CNT solutions, CNTs were sonicated in chloroform for 2 hours prior to mixing. The PANI:CNT:PEO weight ratio was kept fixed at 81:12:7. Electrospinning was carried out in a controlled environment to ensure that the relative humidity never exceeded 20%. The NE-4000 model from New Era Pump Systems, Inc. and a BD 5 mL syringe with a luer-lock tip were utilized as the syringe pump and syringe, respectively. The electrospinning spinneret was a 22 gauge needle from Hamilton. Identical electrospinning parameters were used for the PANI93 and PANI-CNT solutions. The collector plate was placed approximately 25 cm away from the needle tip. The solutions were pumped at a flow rate of 0.6 mL hr-1 and a 5 kV DC voltage was applied to the system. For ease of reference, the electrospun PANI/PEO (93/7) and PANI/CNT/PEO (81/12/7) electrospun nanofiber mats will be referred to as PANI93 and PANI-CNT, respectively. The composition of the nanofiber electrodes was confirmed using thermogravimetric analysis (see supporting info figure S3).


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Electrochemical testing and morphological characterization. Scanning electron microscopy (SEM, ZEIS SUPRA 50VP) was employed to characterize the surface morphology of the electrospun fibers. ImageJ software was utilized to measure the average diameter of the electrospun fibers from the SEM images. The software was used to measure diameters of 100 nanofibers for both PANI93 and PANI-CNT. The percolation of CNTs within PANI nanofibers was observed using transmission electron microscopy (TEM). Fourier transform infrared spectroscopy (FTIR) (Varian Excalibur FTS-3000, range of 4000-800 cm-1), X-ray diffraction (XRD) (Rigaku SmartLab, X-ray diffractometer, Cu Kα, scanning range 5-50° and step size of 0.02°) were used to probe the chemical structure, composition and crystallinity of the PANI based nanofibers. The surface area of the electrospun nanofibers was characterized using nitrogen sorption isotherms at 77 K (Autosorb-1, Quantachrome). Prior to the adsorption– desorption measurement, all samples were degassed at 60°C under vacuum for 24 h to remove impurities. The as-made electrospun PANI93 and PANI-CNT nanofiber mats were punched into freestanding electrodes with a 0.50 inch diameter and tested electrochemically in a three electrode, T-type Swagelok set up, as shown in the supplementary figure S1. To illustrate the electrodes’ freestanding morphology, digital photographs are shown in figure S2. A 0.5 inch diameter graphite rod, Ag/AgCl and platinum mesh were used as the working, reference and counter electrodes, respectively. All electrochemical testing was carried out in a 1M H2SO4 electrolyte. The electrode mass loading used for the reported data was 2.60 mg cm-2. Cyclic voltammetry (CV) experiments were conducted in the voltage range of -0.20 to 0.65 V. Specific capacitance was calculated from CV data using the expression below:

= ∗ ∗


(1)

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Where Cs, I, m and V represent the specific capacitance (F g-1), current (A), mass of the electrode (g), and the voltage window (V). Galvanostatic charge-discharge (GCD) measurements were carried out at varying current densities with a potential range of -0.20 V to 0.60 V vs. Ag/AgCl. Specific capacitance was calculated from the galvanostatic charge-discharge curves using the expression below: ∗

= ∗

(2)

Where Cs is the specific capacitance, I is the constant discharge current, m is the mass of the electrode, ∆t is the discharge time, and ∆V is the potential window. Electrochemical impedance spectroscopy (EIS) in the range of 100 kHz to 10 mHz was used to study the impedance behavior of the PANI93 and PANI-CNT electrodes in a 3-electrode setup. A Reference 3000 instrument from Gamry Instruments was used to measure the CV, GCD and EIS. Results and Discussion Nanofibers characterization. As shown in the SEM image in figure 1 (a), electrospinning the PANI93 solution results in smooth, continuous, nonwoven nanofibers. Well defined inter-fiber porosity percolates throughout the nanofiber mats. The addition of CNTs did not appear to have any negative consequences on the electrospinnability of the solution: smooth PANI-CNT fiber mats were generated as shown in figure 1 (b). TEM images (figure 1c and 1d) show that CNTs are distributed within the nanofibers with no major aggregation. The CNTs are also shown to be well aligned within the nanofibers over a length of more than 1 µm. Additional PANI-CNT TEM image is shown in supporting information (figure S4). The PANI-CNT fiber sample showed a smaller average diameter of 491±86 nm compared to 678±54 nm for PANI93. Introduction of CNTs increases the conductivity of the PANI-CNT solution, which leads to higher charge


8

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density in the polymer jet. The higher charge density increases the extent of jet elongation, resulting in a smaller average diameter. 28 Figure 2 (a) shows the XRD pattern of pristine CNTs, PANI93 and PANI-CNT nanofibers. The XRD pattern of pristine CNTs displayed a prominent diffraction peak at 2θ = 26.3 corresponding to the graphite-like structure.29 For PANI93 samples, prominent peaks at scattering angles of 2θ= ~15° and ~25° were observed. These peaks are attributed to the periodicity of the repeat unit of PANI chain and periodicity parallel to the polymer chain backbone, indicating a degree of crystallinity within the nanofibers that was possibly induced by extensional forces during electrospinning.30-31 Moreover, a weak peak at 2θ=~20.9° attributed to the periodicity perpendicular to the PANI chain was also observed.16 The PANI-CNT nanocomposite XRD pattern exhibited similar peaks to that of PANI93. However, the diffraction peak at 2θ= ~25° was much sharper for PANI-CNT, indicating a more pronounced graphite structure within the nanofibers due to presence of CNTs. Incorporation of CNTs within the PANI fibers does not seem to introduce any additional crystalline order. Figure 2 (b) shows the FTIR spectra of the PANI93 and PANI-CNT nanofibers. The main bands at 1583 and 1492 cm-1 have been ascribed to the C=C stretching vibration of the quinoid ring and benzonoid ring, respectively, in the PANI chain.32 These bands, which suggest the emeraldine salt state of PANI, were observed for both PANI93 and PANI-CNT. The bands at 1307 and 1244 cm-1 are attributed to the C-N stretching of the benzonoid ring while the band at 1138 cm-1 is attributed to N-Q-N (where Q is quinoid). The latter band is a result of electron delocalization in the PANI chains. The bands at 800 and 1735 cm-1 are assigned to the out of plane bending vibration of –CH in the benzonoid ring and C=O stretching, arising possibly due to presence of PEO in the nanofibers. The position of these peaks are not altered in the FTIR


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spectra of PANI-CNT composite, indicating negligible or weak chemical interaction between PANI and CNT. It should be noted that chemical interaction between PANI and CNTs has been primarily observed for cases where aniline is polymerized in situ on CNTs. Cochet et al showed that site-selective interaction exists between CNTs and the PANI quinoid ring when the PANICNT composite is synthesized via in situ polymerization.33 Such pronounced interaction has not been reported for ex-situ synthesis. Nevertheless, enhanced electrical conductivity has been observed for PANI-CNT composites fabricated ex situ.34 Electrochemical Impedance Spectroscopy (EIS). Prior to three electrode testing, EIS was employed to establish the through plane electrical conductivity of electrospun PANI93 and PANI-CNT. Graphite current collectors with a diameter of 0.50 inches were used in a two way Swagelok cell. PANI93 and PANI-CNT exhibited electrical conductivity of 0.114 and 0.154 S/cm, respectively, indicating that the addition of CNTs enhanced the electrical conductivity of the nanofibers. EIS was further used to investigate the impedance in a three electrode set up. The corresponding EIS Nyquist plots are shown in figure 3 (a). The Nyquist plots are characterized by the distorted semi-circle located in the high frequency region and a straight line in the low frequency region. The high frequency x-axis intercepts give equivalent series resistance (ESR) values, which were observed to be identical for both electrodes. The intercepts gave an approximate ESR of 0.5 ohms. PANI-CNT electrodes exhibited lower charge transfer resistance compared to PANI93 electrodes, as suggested from the respective diameters of the semi-circles. Additionally, PANI-CNT electrodes possessed faster ion diffusion based on the slope of the inclined straight line in the low frequency regime. The decreased electrical resistance of the PANI-CNT mat compared to the PANI93 mat could be attributed to the combined effect of high electrical conductivity due to the presence of CNTs and the smaller diameter of the PANI-CNT


10

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nanofibers, which facilitates enhanced electron transfer and improved ion penetration and diffusion into the nanofiber electrode. CV study of PANI93 and PANI-CNT nanofiber mats. Pseudocapacitive performance of the freestanding high purity electrospun PANI93 and PANI-CNT composite electrodes was investigated using cyclic voltammetry (CV) measurements at room temperature with a voltage window of 0.85V. Both PANI93 and PANI-CNT exhibited a well-defined reversible pair of redox peaks in the CV curves in the voltage range of 0.10 V to 0.30 V vs. Ag/AgCl. The peaks are represented as A/A’ in figure 3 (b) below. These peaks are attributed to the redox transformation of polyaniline between the polaronic emaraldine state (ES) and the leucoemaraldine state (LS)

35

. Another pair of peaks, denoted as B/B’, was observed for both

systems and is believed to represent the PANI degradation by-products. 36 PANI-CNT electrodes showed higher redox peaks than the corresponding PANI93 electrodes at all CV scan rates. During both the forward and reverse sweeps, the conductivity of the PANI93 electrode varies as the polymer transitions into different oxidation states. The polymer is in its least conductive state when in the leucoemaraldine state. This variation in the conductivity of PANI may inhibit the redox reactions. Conversely, the presence of CNTs in the composite system is expected to stabilize the electrode conductivity and therefore facilitate the redox reactions, which explains the larger redox peaks observed for the PANI-CNT electrode. The PANI-CNT redox peak is marginally shifted to the right for the forward scan and to the left for the reverse scan. We hypothesize that this phenomenon is a result of greater conversion of the ES to the LS state in the presence of CNTs. Specific capacitances of 324 F g-1 and 281 F g-1 were observed for PANICNT and PANI93 at 20 mVs-1, respectively based on the CV measurements. Figure 4 (a,b) shows the CV curves for PANI93 and PANI-CNT at various scan rates from 5 to 100 mVs-1 and


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table 1 reveals the specific capacitances calculated from these curves. The cathodic and anodic peaks exhibited marginal shifts to the right and left, respectively, with increasing scan rate. The small shift suggests only a slight increase in internal resistance with increasing scan rate.37 67% of the specific capacitance was retained at 100 mV s-1 for PANI93 electrodes. For PANI-CNT electrodes, the specific capacitance remained relatively steady with increasing scan rate (80% retention at 100 mV s-1), indicating excellent kinetics and rate capability due to the presence of CNTs facilitating fast electron transport. A linear correlation was observed for the wave current as a function of the square root of scan rate (figure S5) for both PANI93 and PANI-CNT electrodes, indicating that the kinetics during the redox reactions in both systems are diffusion controlled. Table 1: Specific capacitance (SC) of PANI93/PANI-CNT electrodes at different scan rates. Scan rate (mV/s) 5 20 50 100

SC (F g-1) PANI93 281 240 213 189

SC (F g-1) PANI-CNTs 324 302 285 259

Galvanostatic charge-discharge (GCD) study of PANI93 and PANI-CNT nanofiber mats. Figure 4 (c,d) illustrates the charge-discharge curves in the potential range of -0.2V to 0.6V. These charge-discharge curves are quasi symmetric due to the redox reactions associated with the transition of PANI to various oxidation states.17 The rate capability of the fabricated systems was investigated by varying the current density from a range of 0.5 A g-1 to 10 A g-1. Figure 4 (e) illustrates the variation of specific capacitance as a function of current density for both PANI93 and PANI-CNT. A maximum


12

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specific capacitance value of 308 F g-1 was obtained for PANI93 at a current density of 0.5 A g-1 and an electrode mass loading of 2.60 mg cm-2. Additionally, when the current density was increased to 5 A g-1, 58% of the maximum specific capacitance value was retained. Area based capacitance of 1.15 F cm-2 with a negligible loss in gravimetric capacitance was obtained when the electrode mass loading was increased to 3.80 mg cm-2. As previously indicated, this is the first time electrospinning has been employed directly to fabricate a freestanding, high purity PANI nanofiber network for supercapacitor electrodes. Our freestanding PANI-based electrode material offers several benefits over PANI based electrodes that employ slurries, such as the aforementioned system reported by Chaudhari et al.

26

Our electrode exhibited higher effective

specific capacitance and enhanced rate capability owing to the binder-free 3D fiber mat morphology and the low concentration of the electrochemically inert PEO. Additionally, the high purity PANI nanofibers developed in our work are expected to be significantly more stable in aqueous electrolyte than those with higher concentrations of PEO, a water-soluble polymer. CNTs were introduced to the PANI nanofibers to address the well documented PANI instability issues

38

as well as to enhance the electrical conductivity of the PANI composite,

particularly when PANI transitions into the less conductive leucoemaraldine state. Specific capacitance of 385 F g-1 at 0.50 A g-1 was obtained, with 72% retention of capacitance at 5 A g-1 and ~66% retention at 10 A g-1. Areal capacitance of 1.37 F cm-2 was observed when electrode loading was increased from 2.60 to 3.80 mg cm-2 with negligible loss in gravimetric capacitance. As mentioned earlier, no prior work in the literature has reported the investigation of the pseudocapacitive behavior of electrospun PANI-CNT nanofibers. The bulk of the PANI/CNT composite electrode literature utilizes CNT as a substrate on which to deposit PANI ex-situ.


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Comparison between the electrochemical performance of our electrospun PANI93/PANICNT electrodes and corresponding literature work is presented in table 2 below. We observe that our electrospun electrodes show competitive electrochemical performance including initial specific capacitance, rate capability and life cycle performance. The promising electrochemical performance of our as-electrospun freestanding electrodes is thought to stem from its porous three-dimensional non-woven nanofiber mat morphology. The inter-fiber porosity enables quick ion transport and diffusion within the electrodes whereas the interconnected nanofiber network facilitate uninterrupted electron conductivity. The standard electrode assembly process – utilization of slurries – in most literature works cited above is likely to result in a lack of such an open, through-connected pore structure. Moreover, the electrospun nanofibers display microporosity on their surface, as observed in the BET data shown in figure 5. The microporosity is possibly the result of evaporative cooling of the polymer jet due to the presence of some humidity during electrospinning. As the surface of the jet cools, water vapor from the ambient air condenses to form tiny water droplets on the fiber surface that eventually evaporate to form pores.39 We believe that this porosity on the surface of the fibers further enhances PANI accessibility and utilization for redox reactions. Table 2. Literature summary of PANI nanostructures/ PANI-CNT nanocomposites as electrodes for supercapacitors —electrochemical performance at low and high current densities (CD), and cycle life.

Pure PANI Morphology

Synthesis Method

Nanotubes 40 Nanofiber 41 Nanorods 7 Hollow nanofibers 16 Nanorods 42

Self-assembly Template, MnO2 Self-assembly Electrospun PAA* Self-assembly

Low CD rate SC in F g-1 (Current Density)

High CD rate SC in F g-1 (Current Density)

% Retention (# cycles)

625 (1 A g-1) 404 (1 A g-1) 455 (1 mV s-1) 601 (1 A g-1) 350 (1 A g-1)

481 (10 A g-1) 385 (10 A g-1) — 268 (10 A g-1) 315 (40 A g-1)

77 (500) 61 (5000) 65 (1300) 62 (500) 99 (500)


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Nanoworms 15

Self-assembly

301 (0.5 A g-1)

—

72 (1000)

This work

Electrospinnig

308 (0.5 A g-1)

177 (5 A g-1)

70 (1000)

PANI-CNT Synthesis

CNT wt%

Biomimetic 32 Polymerization 43 Polymerization 44 Polymerization 45 Polymerization(#) 46

5 25 34 — 35

535 (1 A g-1) 400 (1 A g-1) 560 (1 mV s-1) 568 (10 A g-1) 322 (1 A g-1)

500 (3 A g-1) 230 (5 A g-1) 177 (5 mV s-1) 395 (50 A g-1) 286 (4 A g-1)

90 (1000) 56 (2500) 70 (1000) 91 (1000) 92 (1000)

This work

12

385 (0.5 A g-1)

277 (5 A g-1)

81 (1000)

_____________________________________________________________________________________

PAA* (poly(amide) acid), #All solid state supercapacitor. All polymerization are in-situ Full cell capacitor setup. A full cell capacitor based on symmetric PANI-CNT electrodes was fabricated and tested in a two-way Swagelok set up. The device was characterized using both CV and galvanostatic charge-discharge with a voltage window of 0.8V (0-0.8 V). Figure 6a shows the CV plots for the full cell at various scan rates ranging from 20 to 100 mV s-1. The CV curves display the characteristic pair of peaks observed in the three-electrode test, which are associated with the leucoemeraldine-emaraldine transformation (A/A’) and the formation of by-products (B/B’). Figure 6 (b) shows the rate capability of the symmetric electrodes using the galvanostatic charge-discharge process. Maximum specific capacitance of 320 F g-1 was obtained at 0.5 A g-1 using equation 3 below. 83% of the maximum capacitance was retained after 1000 chargedischarge cycles at a 2 A g-1 current density. ∗

= ∗

(3)

Energy density and power density values for the full device were also calculated according to equation 4 and 5 below, respectively. Ct (Cs/4) is the specific capacitance based on total weight of both electrodes, V is the voltage window and ∆t is the discharge time.


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= ∗ ∗

=

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(4)

(5)

A maximum energy density of 7.11 Wh kg-1 (corresponding to 28.4 Wh kg-1 per weight of a single electrode) was obtained for the full cell at the power density of 201 W kg-1. The low energy density per total weight of both electrodes is due to the limited electrochemical window (0.8V) of the symmetric cell. A Ragone plot has been included in the supporting information section (figure S8). Life cycle performance. The cyclic stability of PANI93 and PANI-CNT electrodes was investigated through continuous charge—discharge cycling at 2 A g-1. From post-mortem SEM characterization of the electrodes shown in figure 7 (a,b), we observe that the electrospun nanofiber mats largely retain their porous interconnected network after 1000 cycles, indicating that any PANI degradation might be occurring at molecular level. The electropsun PANI93 supercapacitor electrode showed 70% retention of its initial specific capacitance after 1000 cycles while PANI-CNT electrodes showed a retention of 81% of its initial value (figure 7c). Furthermore, a decrease in the intensity of peak A/A’ was observed in the CV plots of the PANICNT electrode after 1000 cycles at 20 mV s-1 (figure S7), attributed to the increase in internal resistance with cycling as shown in the EIS curves (figure 7d). PANI is known to undergo volumetric changes during the doping/de-doping process as a result of repeated insertion and deinsertion of ions, which results in the deterioration of its performance over the course of operation.47 The presence of CNTs enhanced the mechanical stability of these electrodes, a result that is consistent with previously reported findings in literature.30 Addition of CNTs boosted the electrochemical stability of our nanofibers by approximately 11%. The relatively lower


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capacitance retention of PANI93 is attributed to the larger PANI domain size corresponding to the larger average nanofiber diameter. Efforts to reduce the nanofiber diameter led to the need for higher composition of the electrochemically-inert carrier polymer. Future work on generating additional porous within high purity PANI nanofibers to enhance accessibility, stability and rate capability of PANI is underway. Conclusion We have demonstrated a facile methodology to fabricate binder-free PANI and PANI-CNT nanofiber electrodes. The electrodes exhibit a specific capacitance of up to 385 F g-1. The strong electrochemical performance of the fabricated electrodes is enabled by the combination of low electrochemical impedance and good inter and intra-fiber porosity, which facilitates ion diffusion to the PANI active sites. The freestanding format of the electrodes eliminates the need for electrochemically inert binders, thereby allowing us to not only minimize cell assembly procedures, but also to preserve the 3D porous structure of the nanofiber mats throughout the lifespan of the capacitor. Supporting information. Digital photographs, characterization methods and supplementary figures included. This material is available free of charge via internet http://pubs.acs.org. Authors Information Corresponding author. [email protected], Author Contributions. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally; Silas K. Simotwo and Christopher DelRe.


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Acknowledgement We would like to thank National Science Foundation (award numbers: 1463170 and 1150528) for funding of this work. We are also grateful to Drexel University Centralized Research Facility for the use of their characterization equipment. We will also like to thank M. Boota and Prof. Y. Gogotsi at Drexel University for their help with BET surface area measurement.

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Figures


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Figure 1 Figure 1: SEM images of PANI93 (a) and PANI-CNT (b) electrospun fibers with an average nanofiber diameter of 678±54nm and 491±86nm, respectively; TEM images of PANI-CNT nanofiber showing distribution of CNTs (c,d).


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Figure 2: XRD patterns (a) and FT-IR spectra (b) of electrospun nanofibers and pristine CNTs.


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Figure3: Nyquist plots of electrospun PANI93 and PANI-CNT nanofiber mats (a) and CV curves (b) of PANI93 (red) and PANI-CNT (blue) nanofiber mats at 50 mVs-1.


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Figure 4: Cyclic voltammetry of PANI93 (a) and PANI-CNT (b) at different scan rates; cyclic charge-discharge curves of PANI93 (c) and PANI-CNT (d); CV curves of symmetric PANI-CNT capacitor at different scan rates (e).


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Figure 5: Pore size distribution (a) and cumulative surface area as a function of pore size for PANI93 nanofibers (b).


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Figure 6: Plots showing electrochemical performance of symmetric PANI-CNT capacitor: CV curves at different scan rates (a) and specific capacitance as a function of charge-discharge rates (b). The specific capacitance is based on the total mass of both electrodes.


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Figure 7: SEM micrographs showing post-mortem electrode morphologies of PANI93 (a) and PANI-CNT (b); life cycle performance of PANI93, PANI-CNT and symmetric PANI-CNT capacitors (c) and EIS plot of PANI-CNT before and after cycling for 1000 cycles (d). Scale bar is 2 µm and inset scale bar is 200 nm for (a,b).


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