Double polymer sheathed carbon nanotube supercapacitors show enhanced cycling stability.

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DOI: 10.1039/C5NR05978J

Double Polymer Sheathed Carbon Nanotube Supercapacitors Show

Wenqi Zhao,1 Shanshan Wang,2 Chunhui Wang,1 Shiting Wu,3 Wenjing Xu,3 Mingchu Zou,3 An Ouyang,3 Anyuan Cao,3* Yibin Li1* 1

Centre for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, P. R.

China 2

School of Marine Science and Technology, Harbin Institute of Technology at Weihai, Weihai 264209, P.

R. China 3

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing

100871, P. R. China *Corresponding authors: Email: [email protected], [email protected]

Abstract Pseudo-materials are effective in boosting the specific capacitance of supercapacitors, but during service their degradation may also be very strong, causing reduced cycling stability. Here, we show that a carbon nanotube sponge grafted by two conventional pseudo-polymer layers in sequence can serve as a porous supercapacitor electrode with significantly enhanced cycling stability compared with single polymer grafting. Creating conformal polymer coatings on nanotube surface and the resulting double-sheath configuration are important structural factors leading to the enhanced performance. Combining different polymers as double sheaths as reported here might be a potential route to circumvent the dilemma of pseudo-materials, and to simultaneously improve the capacitance and stability for various energy storage devices.

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Published on 20 November 2015. Downloaded by New York University on 24/11/2015 05:03:18.

Enhanced Cycling Stability

Nanoscale

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DOI: 10.1039/C5NR05978J

Keywords: pseudo-polymer, double sheath, carbon nanotube sponge, supercapacitor,



cycling stability.

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Introduction Supercapacitors represent an important type of energy storage device which can offer

improve the capacitance of supercapacitors by introducing pseudo-materials which have redox reactions with electrolytes during charging and discharging cycles.[6-9] To this end, numerous hybrid or composite electrodes that contain pseudo-materials grafted on a conductive scaffold have been proposed, including those based on carbon nanotube (CNT) networks[10-12] or graphene oxide aerogels.[13-15] Introduction of pseudo-polymers and oxides in general show considerable enhancement in the specific capacitance, but the presence of those materials also may cause reduced cycling stability due to degradation in various electrolyte environment and undesired structural evolution over many reaction cycles.[16,17] Although both the specific capacitance and cycling stability are important criteria for evaluating new electrode systems, most of research in the field have been focused on the former (achieving maximum specific capacitance) whereas the study on solving the stability issue is relatively less. Recently, Liu et al utilize a deposited thin carbonaceous shell to enhance the cycling stability of conductive polymer electrodes.[18] Our group has investigated the supercapacitor properties of CNT sponges by coating conventional polymers (e.g. Polypyrole, PPy) or oxides (e.g. MnO2) onto the surface of CNT networks.[19,20] A noticeable increase of the specific capacitance was observed in those CNT-PPy and CNT-PPy-MnO2 electrodes, owing to the synergistic effects stemming from the embedded conductive CNT network and the as-coated pseudo-layer interfacing with electrolytes. Furthermore, the porous CNT sponges can be compressed to large strains (up 3



large specific capacitance and high power density.[1-5] It has been a common strategy to

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to 80% volume reduction) with enhanced volume-specific capacitance.[21] Those results prove that our pseudo-material-grafted CNT sponges have a strong potential in developing

Here, we show that the cycling stability, one of the most important consideration of supercapacitors, could be significantly enhanced by creating smooth and uniform double-layer polymer coating throughout the CNT sponge. Although individual polymer layers do not show high stability, a simple combination of two different such polymers leads to much higher capacitance retention after thousands of cycles without sacrificing the specific capacitance. We attribute the enhanced stability to two structural related factors: 1) uniform polymer coating throughout the CNT network and 2) the unique double-sheath configuration obtained by sequential coating of two polymers. Results and discussion A CNT sponge is a bulk, highly porous material containing three-dimensionally overlapped multi-walled nanotubes with diameters of 20-40 nm and lengths of tens of µm, as reported by our group a few years ago.[22] These light-weight and conductive CNT sponges are ideal bulk electrode materials for various energy devices such as supercapacitors and batteries. Here, our basic idea is to utilize the bare CNT surfaces among the as-grown sponge and to deposit desired materials onto CNTs for surface modification. As illustrated in Figure 1a, first one polymer (A) was coated onto the surface of a CNT and then another polymer (B) was coated onto polymer A, forming a double-layer coating with certain thickness in each layer. This sequential deposition ensures that only the polymer A can be in

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high performance and flexible supercapacitors.

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direct contact with inside CNTs, while the polymer B only makes contact to the polymer A, so that creating a strict double-layer configuration without mixture in respective layers. The

CNT. Correspondingly, the outermost surface (interface with electrolyte) changes from the CNT surface (for as-grown sponge) to the polymer A (after the first coating) and the polymer B in the final double-sheathed structure. Both polymer layers (A and B) were coated onto CNTs by electrodeposition method using the sponge as one electrode immersed in solution containing targeted monomers in a three-electrode system (see Experimental for details). This ensures that the entire sponge can be deposited by the polymers uniformly, and also the deposition process does not disturb the initial CNT network. Finally, we obtained a three dimensional (3D) CNT network covered by a uniform double-sheath. By depositing PPy and Polyaniline (PANI) respectively in different sequences, we have obtained two resulting configurations including a CNT/PPy/PANI and a CNT/PANI/PPy composite sponge with reversed sheath structure, as illustrated in Figure 1b. Figure 2a and 2b show scanning electron microscopy (SEM) images of an original CNT sponge and a double-sheathed CNT/PPy/PANI sponge, respectively. To examine the internal structure, samples were broken and inside cross-sections were imaged. Due to the random overlapping of CNTs, every nanotube is suspended among the 3D network, which ensures uniform polymer coating around the nanotube surface. As a result, all CNTs have been coated by polymers and the tube diameters have increased from 20-40 nm to 60-140 nm (Fig. 2b). Corresponding to the diameter increase, the mass loadings of polymers were 5



resulting structure containing two different polymer layers is treated as a double-sheathed

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controlled to be about 40 wt% PPy (1.86 mg) and 60 wt% PANI (2.8 mg), resulting in a total polymer loading of ∼70 wt% (4.6 mg) relative to the initial sponge weight (6.6 mg). The same

CNT/PPy/PANI and CNT/PANI/PPy sponges). Because the sponge has an open-porous structure, monomer solution could infiltrate through the channels between CNTs and access the inner part, leading to uniform deposition throughout the bulk sponge. Enlarged view of the broken sponge reveals distinct characteristics of the double-sheathed structure. Many flexible thin wires are exposed in the cross-section; these are the inner CNTs protruding out from the polymer-wrapped segments (CNT/PPy/PANI) with diameters of about 130 nm (Fig. 2c, 2d) and the schematic for these ternary nanotubes is shown in Figure 2e . It is interesting that on some CNTs, close to their exposed tips, there is a short segment with an intermediate diameter (～60 nm), corresponding to the CNT/PPy structure after the first deposition process. Appearing of such step-wise structure (having different diameters) is due to different broken regions in the PPy or PANI layer, along the axis of CNTs (illustrated in Fig. 2e). This distinct micro morphology also proves the formation of the double-sheath configuration consisting of two different polymer layers coated along CNTs. SEM characterization of bare CNT, CNT/PANI and CNT/PPy/PANI sponges reveals distinct structures of single polymer-sheathed (a uniform smooth coating throughout the CNT network) and double-sheathed sponges (stepwise double-layer coating) in which the second layer is prevented from direct contact to inner CNTs (Supporting Information, Fig. S1). Similarly, we also obtained a reversed double-sheath sample, the CNT/PANI/PPy sponge, with the PANI layer inside and the PPy layer outside (Supporting Information, Fig. S2a, S2b). Transmission electron microscopy (TEM) images of as-grown CNTs show typical multi-walled structure with outer diameters of 33 nm and inner cavities of 15 nm (Fig. S3a). After depositing a single PANI layer, the diameters increase to 78 nm (Fig. S3b). It can be seen that the PANI layer was coated to CNTs smoothly without forming gaps at the interface. For the CNT/PANI/PPy sponge, one cannot distinguish the interface between the two 6



polymer loadings were adopted in samples for subsequent electrochemical tests (both

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polymer layers, but sometimes a necklace structure was observed possibly due to the non-uniform deposition of the PPy on the PANI layer (Fig.S3c, S3d). Also, the double polymer sheathed CNT sponges still possess high flexibility (can be bent or rolled up),

Successful coating of polymers has been confirmed by X-ray diffraction (XRD) and Raman characterization. The original CNT sponge only shows a broad band at about 20° coming from multi-walled nanotubes (Fig. S4). For CNT/PANI and CNT/PPy/PANI sponges, we observe pronounced peaks centered at 25.8°, which are attributed to the periodicity parallel and perpendicular to the polymer chains.[23] Raman spectra of the composites show characteristic peaks of PANI and PPy in the CNT/PANI and CNT/PPy/PANI sponges, respectively (Fig. S5). For example, in the CNT/PANI sponge, there are several distinct peaks corresponding to PANI including those centered at 1571 cm-1 (C=C stretching), 1348 cm-1 (C-N+ stretching) and 1169 cm-1 (in plane C-H bending), while in the CNT/PPy/PANI sponge, peaks at 1004 cm-1 (ring deformation), 1082 cm-1 (in-plane C-H bending), 1355 and 1410 cm-1 (C-N stretching) and 1583 cm-1 (C=C stretching) appear due to the presence of PPy. The Raman spectra of the two composite sponges are in well accordance with previous characterization on pure PANI and PPy, respectively.[24,25] The electrochemical properties of two double-sheathed sponges in different sequence (CNT/PANI/PPy and CNT/PPy/PANI) have been characterized by cyclic voltammograms (CVs) and galvanostatic charge/discharge curves, respectively (Fig. 3). First of all, polymer coatings in either sequence can make an excellent supercapacitor electrode with large-area CV curves (scan rates of 2 to 200 mV/s, voltage window of 1 V) and long charge/discharge periods at different current densities. Meanwhile, the CV curves of the two samples show 7



indicating potential applications as flexible or compressible supercapacitors (Fig. S2a).

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similar shapes with redox peaks, indicating that the two polymers give rise to pseudo-capacitance of the electrode (Fig. 3a, 3d). Negligible voltage drop (IR drop) can be

energy electrodes (Fig. 3b, 3e). With decreasing current density, the discharge time increases clearly, indicating a good rate capability. Based on galvanostatic discharge curves, the calculated specific capacitance reaches 395 F/g and 596 F/g (at 1 A/g) for the CNT/PANI/PPy and CNT/PPy/PANI sponge, respectively. Compared with bare CNT sponges typically having capacitances below 30 F/g,[21] coating the two polymers has significantly improved the electrode performance. The above results also suggest that the specific capacitance also depends on which polymer is coated outside and in contact to the electrolyte. So what is the advantage of making double-sheathed structure versus single polymer coating? To investigate this, we have compared the CV characteristics of a single-sheath sponge (CNT/PANI) and the above double-sheath samples (with the same loadings of PPy and PANI). At a scan rate of 5 mV/s, the redox peaks appear more pronounced since electron transportation at a slow scan rate is more efficient (Fig. 4a). All three samples have similar redox peaks, but the CNT/PANI sponge shows the largest areas enclosed by the CV curve and hence the highest specific capacitance (746 F/g at 5 mV/s). Meanwhile, the CNT/PPy/PANI and CNT/PANI/PPy sponges exhibit lower capacitances of 509 F/g and 415 F/g, respectively. If we compare the CNT/PANI and CNT/PPy/PANI sponges, it shows that introducing a PPy intermediate layer has decreased the capacitance to about the average value of the CNT/PANI and CNT/PPy sponge (ca. 300 F/g).[19] Interestingly, the double-layer 8



seen from the galvanostatic curves, indicating a low internal resistance which is important for

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sponge shows an average effect, with an overall specific capacitance in between the single PPy (lower bound) and PANI coating (higher bound), due to the physical combination of

relationship between the deposition time of PANI on CNT/PPy sponge (50 wt.%) and the electrochemical performance (Fig. S6). The results show that the specific capacitance depending on the PANI deposition time, in which the capacitance increases to about 509 F/g at 160 min (∼40 wt.% PANI), and then becomes saturated upon excessive PANI loading due to the increase in material weight and less efficient charge collection [19]. However, when the scan rate increases from 5 to 200 mV/s, their specific capacitances become close, and the capacitance retention at 200 mV/s of the double-sheathed samples (48.9% of CNT/PPy/PANI and 48.7% of CNT/PANI/PPy sponge) is higher than the single-sheathed CNT/PANI sponge (36.2%) (Fig. 4b). We have conducted electrochemical impedance spectroscopy （ EIS ） tests to characterize the internal resistances of polymer-sheathed CNT sponges. Figure 4c shows the Nyquist plots of EIS spectra for the CNT/PANI, CNT/PPy/PANI and CNT/PANI/PPy sponge, respectively. All spectra are composed of a typical semicircle in the high frequency region and a straight line at low frequency. The CNT/PANI sponge shows the biggest diameter of the semicircle at high frequency, indicating a relatively large charge transfer resistance (Rct). In comparison, the two double-sheathed samples show decreased semicircle diameter and hence lowered internal resistances. The results suggest that the intermediate PPy layer at the CNT-polymer interface could improve the electrode conductivity by providing superb highways for fast electron transportation and electrolyte 9



these two polymers (with different pseudo-capacitances).[18,26] Moreover, we studied the

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ions diffusion. This also leads to increased capacitance retention in the CNT/PPy/PANI and CNT/PANI/PPy sponges at different scan rates (Fig. 4b).

which is an important criterion for supercapacitor applications. Composite sponges were tested at a scan rate of 200 mV/s over 3500 cycles and their CV curves were recorded (Fig. 4d). Although the CNT/PANI sponge has the highest specific capacitance, it shows a quick decay during initial cycles, with a capacitance retention of 41.5% at the 1000th cycle and 36.5% at the 3500th cycle, respectively. In comparison, samples with two polymer layers exhibit much enhanced cycling stability. Specifically, the CNT/PPy/PANI sponge shows a capacitance retention of 90.1% (1000th cycle) and 70.0% (3500th cycle), much higher than that of the CNT/PANI sponge. For the CNT/PANI/PPy sponge with reversed sheath sequence, the capacitance retention even reaches 103.6% (1000th cycle) and 88.0% (3500th cycle). This is because that the PPy layer, although with lower pseudo-capacitance than PANI, has better stability as reported in literature.[26] Extended cycling tests on a CNT/PANI/PPy sponge showed excellent stability with nearly 60% retention of original capacitance even after 20000 cycles (Fig.S7). To study the mechanism for enhanced cyclic stability, the structural evolution in the polymer-sheathed sponges before and after 1000 CV cycles was characterized by SEM (Fig. 5 and Fig. S8). We can see obvious difference in the final structure of sponges coated by single or double-layer polymers. For the CNT/PANI sponge which showed quick degradation, the diameters of core-shell nanostructures have decreased from about 100 nm to 50 nm, due to the consumption of the PANI sheath (Fig. 5a, 5b). Also, the PANI surface appears 10



Notably, we found that the double-sheath structure could improve the cycling stability,

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more rough than the original smooth core-shell structures. However, for the CNT/PPy/PANI sponge, the polymer surface remains smooth with negligible decrease in the core-shell

morphology should be the underlying factor for the improved cycling stability in the CNT/PPy/PANI sponge. This phenomenon suggests that introducing an intermediate PPy layer is favorable for limiting the consumption of pseudo-polymers and retaining their original morphology (Fig. 5e). Conclusion We have synthesized double polymer-sheathed CNT sponges adopting two conventional pseudo-polymers (PANI and PPy) with controlled layer sequence, and obtained a CNT/PPy/PANI and CNT/PANI/PPy composite sponge correspondingly. Systematic electrochemical tests show that the double-sheath structure significantly improves the cycling stability, with much higher capacitance retention than the single-sheath sample. Mechanism study reveals that the maintenance of microstructure in the double-sheathed sponge is an underlying factor for improved stability. Combining different polymers and designing double or multiple sheath structures in the porous electrodes might be a potential route to simultaneously improve the capacitance and stability for various energy storage devices. Experimental Section 1. Synthesis of CNT sponges CNT sponges were synthesized by chemical vapor deposition (CVD) using ferrocene 11



diameter (Fig. 5c, 5d). Such well maintenance of the micro structure and polymer

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and 1,2-dichlorobenzene as the catalyst and carbon precursors. A solution of ferrocene in dichlorobenzene (0.06 g/mL) was injected into the CVD furnace by a syringe pump at a

(2000 and 300mL/min, respectively) to grow sponges. The reaction temperature was set as 860 °C, and the CVD time was 4 h. A quartz sheet was placed in the reaction zone to deposit and collect sponge samples. As-synthesized sponges were peeled off from the substrate. 2. Fabrication of CNT/PANI core–shell sponges An as-grown bulk CNT sponge block was directly used as the working electrode in the potential range from -0.2 to 0.8 V at sweep rate of 40mVs-1 for 100~500 cycles in 1 M H2SO4 and 0.05 M aniline aqueous solution. The electrodeposition of PANI was performed using a three-electrode electrochemical workstation (CHI660D Instruments, Shanghai, China). A Pt wire and a saturated calomel electrode (SCE) were employed as the counter and reference electrodes, respectively. Subsequent to electrodeposition, the electrode was washed with distilled water several times until pH of the water is larger than 5, and then freeze-dried to maintain the porous structure. 3. Fabrication of CNT/PPy/PANI core-double-shell sponges An as-grown bulk CNT sponge block was immersed in Py/acetone solution with a concentration in the range 0.1–1 M for 0.5 h. Then the CNT sponge with Py monomers adsorbed on the surface of each individual CNT was directly used as the working electrode under a potential of 0.8 V in 0.3 M NaClO4 aqueous solution. The electropolymerization of Py was performed using a three- electrode electrochemical workstation (CHI660D 12



feeding rate in the range of 0.1–0.25 mL/ min, and carried into the reaction zone by Ar/H2

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Instruments, Shanghai, China). A Pt wire and Ag/AgCl were used as the counter and reference electrodes, respectively. A typical electropolymerization time was 5–10 min. After

with distilled water and then attached the PANI shell also through electrodeposition according to step 2. 4. Fabrication of CNT/PANI/PPy core-shell sponges According to step 2 CNT/PANI sponge can be obtained, then according to step 3 to electrodeposited PPy on the CNT/PANI sponge directly. 5. Material characterization Microstructure and morphology of samples including CNT sponges before and after compression, AC-CNT sponges before and after testing, were characterized using SEM (Hit-achi S4800) and TEM (FEI G2 T20, 200 kV). Raman spectra were recorded with a micro-Raman spectrometer (Renishaw in Via plus). The structures of the prepared sponges were analyzed by X-ray diffraction in a 2θ range from 10°to 80°at room temperature. 6. Electrochemical measurements Electrochemical characterization of the sponges including cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) were carried out in 1 M H2SO4 electrolyte with the same equipment in a three-electrode cell as in the electrodeposition. The CV curves were measured with different scan rates from 2-200 mV/s between -0.2 and 0.8 V. EIS measurements were carried out in the frequency rang from 100 kHz to 0.01 Hz at open circuit potential with an ac perturbation of 5 mV. The 13



the electropolymerization process, the crosslinked CNT/PPy core–shell sponge was rinsed

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specific capacitance of the sponge electrode (Cs) was calculated from the CV curves and discharging curves according to eqs 1 and 2, respectively.

CS =

I

∫ m dV / v∆V I∆t m∆U

(1)

(2)

where I is the response current (A), m is the total mass of the sponge electrode (including CNTs, PPy and PANI) (g), ∆V is the potential range in the CV (V), v is the potential scan rate (mV/s), ∆U is the potential window in the discharging process, and ∆t is the discharging time.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (NO. 51325202). Y. Li acknowledges financial support from the Natural Science Foundation in China (NSFC 11272109) and the Ph. D. Programs Foundation of Ministry of Education of China (20122302110065). S. Wang acknowledges funding support from Beijing National Laboratory for Molecular Sciences (BNLMS), NSFC 21301039, HIT.NSRIF.2011096, Promotive Research Fund for Young and Middle-aged Scientists of Shandong Province (BS2013HZ021) and Technology and Development Program of Weihai.

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Figure 1. Schematic of the double-sheath structure. (a) Cross-sectional illustration of a multi-walled CNT and the resulting CNT/polymer structures after two electro-deposition steps. Dashed circles indicate the interface with electrolyte in each step. (b) Illustrations of a 3D porous CNT sponge and the resulting two double-sheathed composite sponges with different polymer sequences, including the CNT/PPy/PANI and the CNT/PANI/PPy sponge, respectively.

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Figure 2. SEM characterization of original and polymer-sheathed sponges. (a) SEM image of the inside of an as-grown CNT sponge. (b) SEM image of the internal area in a CNT/PPy/PANI sponge (30 wt% PPy, 40 wt% PANI). (c, d) Higher magnification SEM images of the CNT/PPy/PANI sponge, clearly showing that the CNTs have been wrapped by two polymer layers. (e) Schematic for the double-sheathed structure.

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Figure 3. Capacitive properties of double polymer sheathed CNT sponges. CV curves of (a) CNT/PANI/PPy sponges and (d) CNT/PPy/PANI sponges at different scan rates from 5 mV/s to 200 mV/s. Galvanostatic charge/discharge curves of (b) CNT/PANI/PPy sponges and (e) CNT/PPy/PANI sponges at different current densities. Schematic for (c) CNT/PANI/PPy and (f) CNT/PPy/PANI microstructures.

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DOI: 10.1039/C5NR05978J

Nanoscale


Figure 4. Comparison of capacitive properties from single or double-sheathed sponges. (a) CV curves of three samples including the single-sheathed CNT/PANI, double-sheathed CNT/PPy/PANI and CNT/PANI/PPy sponges, respectively, at a scan rate of 5 mV/s. (b) Calculated specific capacitances of the three samples at different scan rates. (c) Nyquist plots of the EIS for the three samples. (d) Cycling tests of the three samples over 3500 CV cycles at 200mV/s.

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DOI: 10.1039/C5NR05978J

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Figure 5. Characterization of the structural evolution after charge/discharge cycles. SEM images of a CNT/PANI sponge (a) before electrochemical tests and (b) after 1000 CV cycles, and SEM images of a CNT/PPy/PANI sponge (c) before and (d) after 1000 CV cycles. (e) Schematic for the microstructure change in the CNT/PANI and CNT/PPy/PANI sponges, respectively.

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DOI: 10.1039/C5NR05978J




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DOI: 10.1039/C5NR05978J

graphene oxide nanoflakes and hierarchical porous carbon with enhanced cycling stability.

polymer fiber solid-state supercapacitors.

Carbon Redox-Polymer-Gel Hybrid Supercapacitors.

Ultracompressible, high-rate supercapacitors from graphene-coated carbon nanotube aerogels.

Double-walled carbon nanotube processing.

Carbon nanotube balls and their application in supercapacitors.

Core-spun carbon nanotube yarn supercapacitors for wearable electronic textiles.

polymer composites.

Porous Carbon as Electrode for Enhanced Electrochemical Performance Supercapacitors.

Carbon Nanotube Electrode: Freestanding Buckypaper with Polymer-Enhanced H2-Evolution Performance.

Carbon nanotube network film directly grown on carbon cloth for high-performance solid-state flexible supercapacitors.

carbon nanotube hybrid transparent electrodes by plasmonic welding.

MnO(2) double-walled nanotube arrays with fast ion and electron transmission for high-performance supercapacitors.

Polymer Coating of Carbon Nanotube Fibers for Electric Microcables.

polymer composites.

Carbon Nanotube Nanocomposites and Their Electrorheological Response.

carbon nanotube nano composite fibers--a review.

Thermal rectification in a polymer-functionalized single-wall carbon nanotube.

Optical fiber ultrasound transmitter with electrospun carbon nanotube-polymer composite.

Bio-inspired hierarchical polymer fiber-carbon nanotube adhesives.

Stretchable Wire-Shaped Asymmetric Supercapacitors Based on Pristine and MnO2 Coated Carbon Nanotube Fibers.

Hydrous ruthenium oxide nanoparticles anchored to graphene and carbon nanotube hybrid foam for supercapacitors.

High-performance transparent and stretchable all-solid supercapacitors based on highly aligned carbon nanotube sheets.

Impact of the atomic layer deposition precursors diffusion on solid-state carbon nanotube based supercapacitors performances.