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MnO2 on carbon cloth for supercapacitors
Journal of Power Sources 326 (2016) 357e364
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High-performance flexible electrode based on electrodeposition of polypyrrole/MnO2 on carbon cloth for supercapacitors Xingye Fan 1, Xiaolei Wang 1, Ge Li, Aiping Yu, Zhongwei Chen* Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada
h i g h l i g h t s Highly flexible electrodes are designed and fabricated for supercapacitors. High capacitance and excellent rate capability can be achieved. The superior performance is originated from the unique electrode architecture. The facile fabrication technology ensures low-cost practical supercapacitors.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2015 Received in revised form 20 April 2016 Accepted 11 May 2016
A highly flexible electrodes based on electrodeposited MnO2 and polypyrrole composite on carbon cloth is designed and developed by a facile in-situ electrodeposition technique. Such flexible composite electrodes with multiply layered structure possess a high specific capacitance of 325 F g1 at a current density of 0.2 A g1, and an excellent rate capability with a capacitance retention of 70% at a high current density of 5.0 A g1. The superior electrochemical performance is mainly due to the unique electrode with improved ion- and electron-transportation pathways as well as the efficient utilization of active materials and electrode robustness. The excellent electrochemical performance and the low cost property endow this flexible nanocomposite electrode with great promise in applications of flexible supercapacitors. © 2016 Elsevier B.V. All rights reserved.
Keywords: Sueprcapacitors Flexible electrodes High capacitance High rate capability Manganese diodes Polypyrrole
1. Introduction With the rapid development of electronics industry, evergrowing attention has been paid to the electronic devices with small volume, lightweight and flexibility features for portable electronics, medical tools and so on [1]. To power these electronics, suitable energy storage devices such as lithium-ion batteries (LIBs) and supercapacitors (SCs) with corresponding characteristics are highly demanded [2e4]. Compared to LIBs, SCs possess much higher power, longer circle life, and faster charging rate, and have been extensively used as backup powers and supplement to LIBs [5,6]. However, SCs’ broad application is still hindered by the low
* Corresponding author. E-mail address: [email protected] (Z. Chen). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2016.05.047 0378-7753/© 2016 Elsevier B.V. All rights reserved.
energy density (E) which is determined by the equation E ¼ ½ CV2, where C is the total capacitance, and V is the device working voltage [7]. Obviously, higher capacitance and wider voltage window lead to higher energy density. Carbon-based materials with high surface area (e.g., activated carbon) are most commonly used as electrode materials in SCs, which can only deliver a specific electric double layer capacitance (EDLC) of 100e150 F g1 from the charge stored on the surface of the materials, resulting in a low energy density (<10 Wh kg1) [8,9]. Corresponding to EDLC, pseudocapacitance originates from fast reversible faradaic reactions occurring at the surface of the active materials, thus much higher charge (>1000 F g1) can be stored [10]. Therefore, seeking novel pseudocapacitive materials with high capacitance is of great importance for high-energy supercapacitors. So far, various pseudocapacitive materials have been investigated [11], including transition metal oxides (e.g., RuO2) [12], transition metal hydroxides (e.g., Ni(OH)2) [13], and polymers (e.g.,
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polyaniline) [6,14]. Among the many candidates, MnO2 is considered highly promising owing to its abundant natural reverses, nontoxicity and high theoretical capacitance (1370 F g1) [15e18]. Similar to other pseudocapacitive materials, MnO2 usually suffers from agglomeration, low electron conductivity and ion diffusivity, leading to the poor rate capability and inferior pseudo-capacitive behavior (50e250 F g1) resulted from the insufficient utilization of active materials [18e20]. To address these challenges, a common strategy is to decrease the dimensions of MnO2 and incorporate with conductive matrix such as carbon based materials (e.g., porous carbon [21], CNT [22e24], graphene [25e27]) or conductive polymers [28e30]. For example, MnO2/CNT composites exhibit a capacitance of 201 F g1 at a current density of 1.0 A g1 and 140 F g1 at 20 A g1 [31], while the conductive polymer wrapped graphene/MnO2 composites possess a high capacitance of ~380 F g1 at a current density of 0.1 mA cm2 and ~175 F g1 at 5 mA cm2 [32]. Generally, a high-performance electrode should provide both efficient ion and electron transport pathways as well as robustness. Although significant improvement has been achieved on MnO2based composites for supercapacitor applications, it is still quite challenging for the fabrication of MnO2-based flexible electrodes. CNT [6,33], graphene/graphene oxides [27,34,35], and other substrates have been successfully applied in fabrication flexible electrodes with MnO2 based active materials. However, these fabrications either involve the low level of the active materials loading or the elaborate procedures that is not favourable by industries for large production [36,37]. In this context, it is greatly critical to develop novel MnO2-based composite materials and new fabrication technologies for high-performance supercapacitors. Herein, we demonstrate the design and development of highly flexible electrodes based on electrodeposited MnO2 and polypyrrole (PPy) composite on carbon cloth with high capacitance and fast charging rate that enables supercapacitors with both high energy and power (Scheme 1). Such electrodes with multiply layered structure possess several features that are required in highperformance flexible capacitive electrodes: i) the flexible carbon cloth composed of highly conductive woven carbon fibers not only
enables flexible electrode but also provide efficient electron transport pathways; ii) the low dimensions of the MnO2 significantly shorten the ion diffusion length ensuring better utilization of the active material, while the conductive PPy facilitates the electron transfer; iii) the in-situ growth of MnO2 and PPy on CC renders intimate contact between each component, enhancing the robustness, conductivity and stability of the electrodes. Moreover, the facile fabrication process only takes a few minutes, and utilizes low cost precursors and commercially available carbon cloth, which holds great promise in large production and practical application of high-performance supercapacitors [38,39]. 2. Results and discussion Fig. 1A shows the representative scanning electron microscopy (SEM) image of carbon cloth. The smooth surface can be observed (Fig. 1A, inset) on a single carbon fiber with a diameter of ~8 mm. After the electrodeposition of MnO2, a continuously rough surface on the carbon fibers can be obtained (Fig. 1B, inset), indicating the homogeneous and uniform coating of MnO2 (Fig. S1). The highmagnification SEM image in Fig. 1B shows that MnO2 obtained possesses a one-dimensional needle-like morphology and is assembled into bundles vertical to the surface of carbon fibers. The hierarchical structure with high surface area is beneficial for ion transport and better MnO2 utilization. Increasing the deposition time results in the increased thickness of the MnO2 layer, where cracks along the cross section can be found (Fig. S2). Fig. 1C displays the SEM image of the carbon cloth with another layer of PPy coating, where the pyrrole monomers were successfully electropolymerized (Fig. 1C, inset). It is found that the bundled morphology of MnO2 remains and is covered by a vague layer, suggesting the PPy is in-situ and uniformly coated on the surface of MnO2 layer (Fig. 1D). To further prove the feasibility of electropolymerization of pyrrole and in-situ coating of PPy, independent coating of PPy on carbon cloth was also performed. Obviously, PPy was successfully grown on carbon cloth, and the morphology of the vague layer is consistent with that of the PPy layer coated on MnO2 (Fig. S3). After the deposition of MnO2 and PPy, the carbon cloth is
Scheme 1. Schematic view of the fabrication process of the flexible PPy/MnO2 composite electrode.
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Fig. 1. SEM images of (A) pristine commercial carbon cloth, (B) MnO2 coating carbon cloth; (C) low and (D) High magnification SEM images of PPy wrapping MnO2/CC; (C inset) 1 4 cm bare carbon cloth and the carbon cloth coated with PPy/MnO2, (D inset) bended PPy/MnO2 electrode; (E) TGA curves of blank carbon cloth and MnO2/PPy/CC electrodes with different MnO2 deposition time, (F) Comparison of TGA curves of MnO2/PPy/CC electrode and MnO2/CC electrode.
still highly flexible, which can be used as flexible electrodes for supercapacitors. It is worth mention that the thickness of both MnO2 and PPy can be adjusted by using different deposition/ polymerization time, and the electrochemical performance may vary when different thickness and ratio is applied (Table S1 and Fig. S4). To precisely determine the content of MnO2 grown on carbon
fibers, thermogravimetric analysis (TGA) was performed to samples with different MnO2 deposition time. As shown in Fig. 1E, the mass of pristine carbon cloth dramatically decreases at around 750e850 C due to the combustion of highly graphitized carbon in the air atmosphere and finally reaches zero after completely consuming. By comparison, the TGA curves of MnO2/PPy/CC electrodes possess similar behavior, however, the combustion
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temperatures are significantly lower due to the catalysis effect by MnO2 with the increase of MnO2 content. After all carbon cloth was consumed completely, the final weight percentage was calculated to be 1.5%, 5.9% and 9.0% for deposition time of 60s, 240s and 360s corresponding to the data shown in Table S1. Fig. 1F further compares the TGA curves between MnO2/PPy/CC electrode and MnO2/ CC electrode. It shows that the PPy coating somehow inhibits the catalysis derived from MnO2, resulting in the delay of combustion of carbon cloth. However, the final mass percentage remained similar. The weight loss of PPy can be observed in Fig. 1F, inset. Compared to MnO2/CC electrode, MnO2/PPy/CC electrode loses more weight due to the consumption of PPy coating. According to our observation, the PPy ratio in the MnO2/PPy/CC composite electrode is estimated to be less than 1%. The structure of PPy/MnO2 nanocomposites on the flexible carbon cloth was investigated by transmission electron microscopy (TEM). Fig. 2A shows the representative TEM image of a shedding piece of PPy/MnO2 nanocomposites from carbon cloth by sonication in anhydrous alcohol for three days. The needle-like MnO2 can be observed, which is consistent with SEM observation. These MnO2 needles are wrapped with some vague, glue-like materials that are mainly from the PPy coating. Fig. 2B shows the highresolution TEM image of a random selected area. The lattice of MnO2 can be clearly observed, indicating that the MnO2 is highly crystalline. However, none of the X-ray diffraction (XRD) patterns of the composite electrodes with different deposition time of MnO2 display any visible diffraction peaks corresponding to MnO2 due to the strong diffraction peaks from carbon cloth substrate, indicating
a long-range disorder of MnO2 [40]. Fig. 2C shows the high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of a small piece of the nanocomposites. The corresponding energy dispersive spectroscopic (EDS) element mapping confirms the existence of element Mn, O and C from MnO2 and carbon cloth, respectively, and element N from the PPy. The homogeneous distribution of these elements indicates that both MnO2 and PPy are deposited uniformly on the surface of carbon cloth (Fig. 2D). The pore structure of composite electrodes was determined by the nitrogen adsorption/desorption technique, and the corresponding information was summarized in Table S2. The pure carbon cloth consisting of graphite fibers doesn’t possess obvious meso- and micro-pores. The extremely low surface area is mainly from the outer surface of these fibers from macroscopic point of view. After the Electrodeposition of MnO2 which exhibits a mesoporous structure, the surface area significantly increases. The coating of PPy leads to a slightly decreased pore size, which is consistent with the SEM observations. The electrochemical behavior of as-synthesized PPy/MnO2 composite flexible electrodes was investigated using a threeelectrode system with a platinum (Pt) wire as the counter electrode and a saturate calomel electrode (SCE) as and the reference electrode, respectively. The cyclic voltammetry was firstly carried out in an aqueous electrolyte containing 1.0 M Na2SO4 with a working voltage window ranging from 1.0 to 0.0 V at scan rates ranging from 2 to 100 mV s1. Fig. 3A and B shows the typical cyclic voltammetric (CV) curves of the flexible electrodes with singlelayer MnO2 coating and PPy/MnO2 composite coating,
Fig. 2. (A) TEM image of the PPy/MnO2 nanocomposites from the electrode; (B) High-resolution TEM image of as-synthesized PPy/MnO2 nanocomposites showing the high crystallinity of the MnO2; (C) HAADF STEM image of PPy/MnO2; and (D) the corresponding elemental mapping of Mn, O, N and C, respectively.
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Fig. 3. Cyclic voltammogram of (A) MnO2 and (B) PPy/MnO2 at scan rates from 2 to 100 mV s1 under a potential window between 0.0 and 1.0 V. (C) Comparison of specific capacitance of MnO2 and PPy coated MnO2 at different scan rates. (D) Comparison of the rate capability of MnO2 and PPy/MnO2 based on the specific capacitance obtained from CV.
respectively. The CV curves of single-layer MnO2-coated electrode exhibits a nearly rectangular and symmetric shape, indicating the successive redox reaction on the surface of electrode material that corresponds to the capacitive behavior of MnO2 in the aqueous solution [21]. By comparison, the CV curves of the flexible electrode with PPy/MnO2 composite coating (Fig. 3B) remain the nearly rectangular and symmetric shape but possess significantly larger enclosed area and high response current density. Note that the area under a CV curve represents the charge stored on the electrode. The flexible PPy/MnO2 composite electrode exhibits much better capability of charge storage. Such a comparison can be clearly observed in Fig. 3C. At the scan rate of 2 mV s1, MnO2 electrode and PPy/MnO2 composite electrode exhibit a specific capacitance of 257 and 342 F g1 (corresponding to an areal capacitance of 180 and 240 mF cm2), respectively; while at 100 mV s1, the PPy/MnO2 composite electrode still delivers a high specific capacitance of 191 F g1 (134 mF cm2), which is much higher than that of the MnO2 electrode (120 F g1, 84 mF cm2). Note that the specific capacitance of bare carbon cloth, which is less than 1 F g1, is negligible compared to the contribution of MnO2 to the total specific capacitance. The greatly enhanced specific capacitance of PPy/MnO2 composite is mainly attributed to the increased conductivity and the synergistic effects of MnO2 and PPy. On the one hand, the conductive polymer and carbon cloth on each side of MnO2 have established an efficient conductive network, which enables the electrons not only to fast transfer from the MnO2 to the carbon cloth core but also to pass throughout the electrode from the outside PPy. The conductive network can effectively increases the interaction area of MnO2 with the electrolyte and boosts the utilization of MnO2. On the other hand, the presence of MnO2 can interlink the PPy chains and offer short electron pathways between PPy conjugated chains, thus facilitating the conductive behavior of
PPy. As shown in Fig. 3D, a high capacitance retention of 62.3% and 55.8% can be achieved for PPy/MnO2 composite electrode at a scan rate of 50 and 100 mV s1. In contrast, only 56.8% and 46.7% of its initial capacitance can be retained for MnO2 electrode. To further evaluate the excellent capacitive behavior of the flexible composite electrodes, galvanostatic charge-discharge measurement (GCD) was conducted at various current densities ranging from 0.2 to 5.0 A g1 between 1.0 and 0.0 V. Fig. 4A compares the galvanostatic charge-discharge curves of the MnO2 electrode and PPy/MnO2 composite electrode at a current density of 1.0 A g1. Both electrodes exhibit symmetrical and linear profiles, indicating the high columbic efficiency and good capacitive performance, which is consistent with the CV observation. However, the PPy/MnO2 composite electrode possesses longer discharging time than the MnO2 electrode (266 vs. 219 s), suggesting a higher specific capacitance. Such a capacitance difference is even more pronounced at high current densities. As shown in Fig. 4B, the PPy/ MnO2 composite electrode possesses a high specific capacitance of 325 and 226 F g1 at a current density of 0.2 and 5.0 A g1 (corresponding to an areal capacitance of 228 and 158 mF cm2 at 0.14 and 3.5 mA cm2), respectively; while the MnO2 electrode only delivers a capacitance of 275 and 173 F g1. Obviously, the PPy/MnO2 composite electrode exhibits much better rate capability. A capacitance retention of 69.6% can be achieved at a high current of 5.0 A g1 compared to the capacitance at 0.5 A g1 (Fig. 4C). As we mentioned above, such enhancement of the electrochemical behavior is greatly attributed to the improvement of the electrical conductivity and the fast Faradic reaction through better utilization of MnO2. This is further confirmed by the electrochemical impedance spectra (EIS). As shown in Fig. 4D, at high-frequency region, the Nyquist plot of the composite electrode shows lower equivalent series resistance (Rs) of 2.76 U than the Rs (3.41 U) of MnO2 electrode and much smaller semicircle,
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Fig. 4. (A) Comparison of Galvanostatic charge-discharge profiles of PPy/MnO2 and MnO2 at the current density of 1 A g1; and (B) comparison of corresponding specific capacitance of PPy/MnO2 and MnO2 at different current densities. (C) Galvanostatic charge-discharge profiles of PPy/MnO2 at various current densities from 0.2 to 5.0 A g1; (D) Comparison of Nyquist plot of PPy coating MnO2/CC electrode and bare MnO2/CC electrode.
representing the lower charge-transfer resistance of the PPy/MnO2 composites. Moreover, at low frequency region, the plot of composite electrode shows slightly steeper Warburg tail, indicating the lower diffusion-resistance of PPy/MnO2. In addition to the excellent rate capability, the flexible composite electrode also possesses a long-term cycling stability. As shown in Fig. 5A, a stable capacitance retention percentage profile can be obtained over 1000 galvanostatic charge-discharge cycles at the current density of 1 A g1. After over 1000 cycles, the composite electrode still exhibits ~96% of the initial specific capacitance. Such long cycle life is also attributed to the robustness and integrity of the composite materials, which is further confirmed by SEM and electrochemical impedance spectroscopy (EIS) analysis (Fig. 4D). As shown in Fig. 5B, the PPy/MnO2 composite is still integrally and uniformly coated on the carbon cloth after long duration test. The thickness of the active materials does not change significantly, compared to the bare carbon cloth (Fig. 5C), suggesting little loss of active materials during cycling, which is mainly ascribed of the protection of the PPy layer. More importantly, the morphology of PPy/MnO2 composite is preserved very well (Fig. 5D), where the hierarchical arrays of the active materials can still be observed clearly, indicating the robustness of the composite materials synthesized by layer-by-layer electrodeposition technique. 3. Conclusion The low-cost flexible PPy/MnO2 composite electrodes have been successfully developed through in-situ electrodeposition technique. Each component was densely packed and uniformly grown on the flexible carbon cloth substrate to form a robust freestanding electrode with hierarchical structure. Owing to the synergetic effect of PPy and MnO2, the composite electrode has exhibited high utilization of the active materials during charging and discharging and
excellent electron-transfer network, which are both highly demanded in MnO2 based supercapacitor electrodes. Thus, this unique design has provided the electrode with superior capacitive performance, high rate capability and long cycling stability, which makes the PPy/MnO2 composite electrode very promising in the applications of flexible supercapacitors and also highlights the strategy of combining multifunctional materials to achieve demanding high-performance supercapacitor electrodes. 4. Experimental 4.1. Material synthesis The fabrication of flexible PPy/MnO2 electrode is illustrated in Scheme 1. In a typical process, a piece of commercial carbon cloth (CCP-10F, Fuel Cell Earch Inc.) was washed by acetone, methanol and isopropanol under sonication for 30 min in sequence to remove any contaminants on the surface. After drying in the oven, the piece of carbon cloth was cut into small rectangle piece of 2 cm * 1 cm and used as the working electrodes. However, only half of the CC was soaked into the electrolyte to make MnO2 only grow in an area of 1 1 cm. The electrodeposition of MnO2 on CC was conducted in a three-electrode system under a static potential of 0.92 V in an aqueous solution of 0.5 M Mn(CH3COO)2 and 0.1 M Na2SO4. The counter and reference electrode utilized in the electrodeposition is platinum (Pt) wire and SCE electrode, respectively. After electrodeposition, the CC was rinsed by DDI water and dried in a vacuum oven. Generally, the loading of MnO2 determined by weight change of CC before and after the electrodeposition can be controlled by the electrodeposition time (Table S1). The electro-polymerization of PPy, which is similar to electrodeposition of MnO2 is carried out under a static potential of 0.8 V, using an electrolyte of 0.1 M NaClO4 and 0.2% (V: V) pyrrole monomer. To make comparison,
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Fig. 5. (A) Cycling stability of PPy/MnO2 at the current density of 1 A g1. SEM images of (B) PPy/MnO2/CC electrode after duration test over 1000 cycles; (C) Bare carbon cloth; and (D) high magnification SEM images of PPy/MnO2/CC electrode after duration test.
single layer MnO2/CC was also fabricated. Moreover, to optimise the loading of MnO2, MnO2 with different electrodeposition time was synthesized and denoted as MnO2-Xmin.
The specific capacitance based on discharge curve and CVs was determined by the Eq. (i) and (ii), respectively
C ¼ I=ðDV=DtÞ
(1)
4.2. Material characterization To characterize the structure and morphology of the assynthesized materials, scanning electron microscopy (SEM) was adopted on LEO FESEM 1530. Besides, transmission electron microscopy (TEM) investigation was carried out at Canadian Center for Electron Microscopy (CCEM, McMaster University) on a JEOL 2010F TEM/STEM field emission microscope. X-ray diffraction (XRD) measurement was obtained from XRG 3000 X-ray diffractometer (monochromatic Cu Ka X-rays). The pore structure was analyzed using a Micromeritics ASAP 2020 with nitrogen gas. The thermal gravity analysis was performed on a TA instrument Q500 with a ramping rate of 5 C min1 in air. 4.3. Electrochemical measurement Electrochemical measurements were conducted in a threeelectrode system using our flexible materials as working electrode with a mass loading of ~0.7 mg cm2 (120 s for MnO2 and 30 s for PPy coating), Pt wire as the counter electrode and SCE as the reference electrode. Cyclic voltammetry (CV), galvanostatic chargedischarge, and electrochemical impedance spectroscopy (EIS) measurements were conducted in 1 M Na2SO4 aqueous electrolyte under a cutoff voltage of 0.0 and 1.0 V with a VMP3 potentiostat/ galvanostat (Bio-Logic LLC, Knoxville, TN).
Z0 C¼
idV=vmDV
(2)
i
where DV is the potential window, I is applied constant current density, DV/Dt is the slope of the discharge curve, i is the voltammetric current, m is the loading of active material, and v is the scan rate.
Acknowledgement This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), University of Waterloo, and the Waterloo Institute for Nanotechnology. The authors acknowledge Dr. Carmen Andrei and Canadian Center for Electron Microscopy at McMaster University for TEM characterization.
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.05.047.
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93 (2015) 258e265. [22] S.-L. Chou, J.-Z. Wang, S.-Y. Chew, H.-K. Liu, S.-X. Dou, Electrodeposition of MnO2 nanowires on carbon nanotube paper as free-standing, flexible electrode for supercapacitors, Electrochem. Commun. 10 (11) (2008) 1724e1727. [23] Y. Cheng, S. Lu, H. Zhang, C.V. Varanasi, J. Liu, Synergistic effects from graphene and carbon nanotubes enable flexible and robust electrodes for highperformance supercapacitors, Nano Lett. 12 (8) (2012) 4206e4211. [24] Y.J. Kang, B. Kim, H. Chung, W. Kim, Fabrication and characterization of flexible and high capacitance supercapacitors based on MnO2/CNT/papers, Synth. Met. 160 (23) (2010) 2510e2514. [25] Y. He, W. Chen, X. Li, Z. Zhang, J. Fu, C. Zhao, et al., Freestanding threedimensional graphene/MnO2 composite networks as ultralight and flexible supercapacitor electrodes, ACS Nano 7 (1) (2012) 174e182. [26] A. Davies, P. Audette, B. Farrow, F. Hassan, Z. Chen, J.-Y. Choi, et al., Graphenebased flexible supercapacitors: pulse-electropolymerization of polypyrrole on free-standing graphene films, J. Phys. Chem. C 115 (35) (2011) 17612e17620. [27] G. Yu, L. Hu, M. Vosgueritchian, H. Wang, X. Xie, J.R. McDonough, et al., Solution-processed graphene/MnO2 nanostructured textiles for highperformance electrochemical capacitors, Nano Lett. 11 (7) (2011) 2905e2911. [28] Y. Hou, Y. Cheng, T. Hobson, J. Liu, Design and synthesis of hierarchical MnO2 nanospheres/carbon nanotubes/conducting polymer ternary composite for high performance electrochemical electrodes, Nano Lett. 10 (7) (2010) 2727e2733. [29] J. Tao, N. Liu, W. Ma, L. Ding, L. Li, J. Su, et al., Solid-state high performance flexible supercapacitors based on polypyrrole-MnO2-carbon fiber hybrid structure, Sci. Rep. 3 (2013). [30] J. Duay, E. Gillette, R. Liu, S.B. Lee, Highly flexible pseudocapacitor based on freestanding heterogeneous MnO2/conductive polymer nanowire arrays, Phys. Chem. Chem. Phys. 14 (10) (2012) 3329e3337. [31] L. Li, Z.A. Hu, N. An, Y.Y. Yang, Z.M. Li, H.Y. Wu, Facile synthesis of MnO2/CNTs composite for supercapacitor electrodes with long cycle stability, J. Phys. Chem. C 118 (40) (2014) 22865e22872. [32] G. Yu, L. Hu, N. Liu, H. Wang, M. Vosgueritchian, Y. Yang, et al., Enhancing the supercapacitor performance of graphene/MnO2 nanostructured electrodes by conductive wrapping, Nano Lett. 11 (10) (2011) 4438e4442. [33] Y. Jin, H. Chen, M. Chen, N. Liu, Q. Li, Graphene-patched CNT/MnO2 nanocomposite papers for the electrode of high-performance flexible asymmetric supercapacitors, ACS Appl. Mater. Interfaces 5 (8) (2013) 3408e3416. [34] L. Peng, X. Peng, B. Liu, C. Wu, Y. Xie, G. Yu, Ultrathin two-dimensional MnO2/ graphene hybrid nanostructures for high-performance, flexible planar supercapacitors, Nano Lett. 13 (5) (2013) 2151e2157. [35] Y. He, W. Chen, X. Li, Z. Zhang, J. Fu, C. Zhao, et al., Freestanding threedimensional graphene/MnO2 composite networks As ultralight and flexible supercapacitor electrodes, ACS Nano 7 (1) (2013) 174e182. [36] J.-G. Wang, Y. Yang, Z.-H. Huang, F. Kang, Rational synthesis of MnO2/conducting polypyrrole@carbon nanofiber triaxial nano-cables for highperformance supercapacitors, J. Mater. Chem. 22 (33) (2012) 16943e16949. [37] L. Hu, W. Chen, X. Xie, N. Liu, Y. Yang, H. Wu, et al., Symmetrical MnO2ecarbon nanotubeetextile nanostructures for wearable pseudocapacitors with high mass loading, ACS Nano 5 (11) (2011) 8904e8913. [38] Y.-K. Hsu, Y.-C. Chen, Y.-G. Lin, L.-C. Chen, K.-H. Chen, High-cell-voltage supercapacitor of carbon nanotube/carbon cloth operating in neutral aqueous solution, J. Mater. Chem. 22 (8) (2012) 3383e3387. [39] G. Wang, H. Wang, X. Lu, Y. Ling, M. Yu, T. Zhai, et al., Solid-State supercapacitor based on activated carbon cloths exhibits excellent rate capability, Adv. Mater. 26 (17) (2014) 2676e2682. [40] Y.-C. Chen, Y.-K. Hsu, Y.-G. Lin, Y.-K. Lin, Y.-Y. Horng, L.-C. Chen, et al., Highly flexible supercapacitors with manganese oxide nanosheet/carbon cloth electrode, Electrochimica Acta 56 (20) (2011) 7124e7130.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
High-performance flexible electrode based on electrodeposition of polypyrrole/MnO2 on carbon cloth for supercapacitors Xingye Fan 1, Xiaolei Wang 1, Ge Li, Aiping Yu, Zhongwei Chen* Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada
h i g h l i g h t s Highly flexible electrodes are designed and fabricated for supercapacitors. High capacitance and excellent rate capability can be achieved. The superior performance is originated from the unique electrode architecture. The facile fabrication technology ensures low-cost practical supercapacitors.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2015 Received in revised form 20 April 2016 Accepted 11 May 2016
A highly flexible electrodes based on electrodeposited MnO2 and polypyrrole composite on carbon cloth is designed and developed by a facile in-situ electrodeposition technique. Such flexible composite electrodes with multiply layered structure possess a high specific capacitance of 325 F g1 at a current density of 0.2 A g1, and an excellent rate capability with a capacitance retention of 70% at a high current density of 5.0 A g1. The superior electrochemical performance is mainly due to the unique electrode with improved ion- and electron-transportation pathways as well as the efficient utilization of active materials and electrode robustness. The excellent electrochemical performance and the low cost property endow this flexible nanocomposite electrode with great promise in applications of flexible supercapacitors. © 2016 Elsevier B.V. All rights reserved.
Keywords: Sueprcapacitors Flexible electrodes High capacitance High rate capability Manganese diodes Polypyrrole
1. Introduction With the rapid development of electronics industry, evergrowing attention has been paid to the electronic devices with small volume, lightweight and flexibility features for portable electronics, medical tools and so on [1]. To power these electronics, suitable energy storage devices such as lithium-ion batteries (LIBs) and supercapacitors (SCs) with corresponding characteristics are highly demanded [2e4]. Compared to LIBs, SCs possess much higher power, longer circle life, and faster charging rate, and have been extensively used as backup powers and supplement to LIBs [5,6]. However, SCs’ broad application is still hindered by the low
* Corresponding author. E-mail address: [email protected] (Z. Chen). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2016.05.047 0378-7753/© 2016 Elsevier B.V. All rights reserved.
energy density (E) which is determined by the equation E ¼ ½ CV2, where C is the total capacitance, and V is the device working voltage [7]. Obviously, higher capacitance and wider voltage window lead to higher energy density. Carbon-based materials with high surface area (e.g., activated carbon) are most commonly used as electrode materials in SCs, which can only deliver a specific electric double layer capacitance (EDLC) of 100e150 F g1 from the charge stored on the surface of the materials, resulting in a low energy density (<10 Wh kg1) [8,9]. Corresponding to EDLC, pseudocapacitance originates from fast reversible faradaic reactions occurring at the surface of the active materials, thus much higher charge (>1000 F g1) can be stored [10]. Therefore, seeking novel pseudocapacitive materials with high capacitance is of great importance for high-energy supercapacitors. So far, various pseudocapacitive materials have been investigated [11], including transition metal oxides (e.g., RuO2) [12], transition metal hydroxides (e.g., Ni(OH)2) [13], and polymers (e.g.,
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polyaniline) [6,14]. Among the many candidates, MnO2 is considered highly promising owing to its abundant natural reverses, nontoxicity and high theoretical capacitance (1370 F g1) [15e18]. Similar to other pseudocapacitive materials, MnO2 usually suffers from agglomeration, low electron conductivity and ion diffusivity, leading to the poor rate capability and inferior pseudo-capacitive behavior (50e250 F g1) resulted from the insufficient utilization of active materials [18e20]. To address these challenges, a common strategy is to decrease the dimensions of MnO2 and incorporate with conductive matrix such as carbon based materials (e.g., porous carbon [21], CNT [22e24], graphene [25e27]) or conductive polymers [28e30]. For example, MnO2/CNT composites exhibit a capacitance of 201 F g1 at a current density of 1.0 A g1 and 140 F g1 at 20 A g1 [31], while the conductive polymer wrapped graphene/MnO2 composites possess a high capacitance of ~380 F g1 at a current density of 0.1 mA cm2 and ~175 F g1 at 5 mA cm2 [32]. Generally, a high-performance electrode should provide both efficient ion and electron transport pathways as well as robustness. Although significant improvement has been achieved on MnO2based composites for supercapacitor applications, it is still quite challenging for the fabrication of MnO2-based flexible electrodes. CNT [6,33], graphene/graphene oxides [27,34,35], and other substrates have been successfully applied in fabrication flexible electrodes with MnO2 based active materials. However, these fabrications either involve the low level of the active materials loading or the elaborate procedures that is not favourable by industries for large production [36,37]. In this context, it is greatly critical to develop novel MnO2-based composite materials and new fabrication technologies for high-performance supercapacitors. Herein, we demonstrate the design and development of highly flexible electrodes based on electrodeposited MnO2 and polypyrrole (PPy) composite on carbon cloth with high capacitance and fast charging rate that enables supercapacitors with both high energy and power (Scheme 1). Such electrodes with multiply layered structure possess several features that are required in highperformance flexible capacitive electrodes: i) the flexible carbon cloth composed of highly conductive woven carbon fibers not only
enables flexible electrode but also provide efficient electron transport pathways; ii) the low dimensions of the MnO2 significantly shorten the ion diffusion length ensuring better utilization of the active material, while the conductive PPy facilitates the electron transfer; iii) the in-situ growth of MnO2 and PPy on CC renders intimate contact between each component, enhancing the robustness, conductivity and stability of the electrodes. Moreover, the facile fabrication process only takes a few minutes, and utilizes low cost precursors and commercially available carbon cloth, which holds great promise in large production and practical application of high-performance supercapacitors [38,39]. 2. Results and discussion Fig. 1A shows the representative scanning electron microscopy (SEM) image of carbon cloth. The smooth surface can be observed (Fig. 1A, inset) on a single carbon fiber with a diameter of ~8 mm. After the electrodeposition of MnO2, a continuously rough surface on the carbon fibers can be obtained (Fig. 1B, inset), indicating the homogeneous and uniform coating of MnO2 (Fig. S1). The highmagnification SEM image in Fig. 1B shows that MnO2 obtained possesses a one-dimensional needle-like morphology and is assembled into bundles vertical to the surface of carbon fibers. The hierarchical structure with high surface area is beneficial for ion transport and better MnO2 utilization. Increasing the deposition time results in the increased thickness of the MnO2 layer, where cracks along the cross section can be found (Fig. S2). Fig. 1C displays the SEM image of the carbon cloth with another layer of PPy coating, where the pyrrole monomers were successfully electropolymerized (Fig. 1C, inset). It is found that the bundled morphology of MnO2 remains and is covered by a vague layer, suggesting the PPy is in-situ and uniformly coated on the surface of MnO2 layer (Fig. 1D). To further prove the feasibility of electropolymerization of pyrrole and in-situ coating of PPy, independent coating of PPy on carbon cloth was also performed. Obviously, PPy was successfully grown on carbon cloth, and the morphology of the vague layer is consistent with that of the PPy layer coated on MnO2 (Fig. S3). After the deposition of MnO2 and PPy, the carbon cloth is
Scheme 1. Schematic view of the fabrication process of the flexible PPy/MnO2 composite electrode.
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Fig. 1. SEM images of (A) pristine commercial carbon cloth, (B) MnO2 coating carbon cloth; (C) low and (D) High magnification SEM images of PPy wrapping MnO2/CC; (C inset) 1 4 cm bare carbon cloth and the carbon cloth coated with PPy/MnO2, (D inset) bended PPy/MnO2 electrode; (E) TGA curves of blank carbon cloth and MnO2/PPy/CC electrodes with different MnO2 deposition time, (F) Comparison of TGA curves of MnO2/PPy/CC electrode and MnO2/CC electrode.
still highly flexible, which can be used as flexible electrodes for supercapacitors. It is worth mention that the thickness of both MnO2 and PPy can be adjusted by using different deposition/ polymerization time, and the electrochemical performance may vary when different thickness and ratio is applied (Table S1 and Fig. S4). To precisely determine the content of MnO2 grown on carbon
fibers, thermogravimetric analysis (TGA) was performed to samples with different MnO2 deposition time. As shown in Fig. 1E, the mass of pristine carbon cloth dramatically decreases at around 750e850 C due to the combustion of highly graphitized carbon in the air atmosphere and finally reaches zero after completely consuming. By comparison, the TGA curves of MnO2/PPy/CC electrodes possess similar behavior, however, the combustion
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temperatures are significantly lower due to the catalysis effect by MnO2 with the increase of MnO2 content. After all carbon cloth was consumed completely, the final weight percentage was calculated to be 1.5%, 5.9% and 9.0% for deposition time of 60s, 240s and 360s corresponding to the data shown in Table S1. Fig. 1F further compares the TGA curves between MnO2/PPy/CC electrode and MnO2/ CC electrode. It shows that the PPy coating somehow inhibits the catalysis derived from MnO2, resulting in the delay of combustion of carbon cloth. However, the final mass percentage remained similar. The weight loss of PPy can be observed in Fig. 1F, inset. Compared to MnO2/CC electrode, MnO2/PPy/CC electrode loses more weight due to the consumption of PPy coating. According to our observation, the PPy ratio in the MnO2/PPy/CC composite electrode is estimated to be less than 1%. The structure of PPy/MnO2 nanocomposites on the flexible carbon cloth was investigated by transmission electron microscopy (TEM). Fig. 2A shows the representative TEM image of a shedding piece of PPy/MnO2 nanocomposites from carbon cloth by sonication in anhydrous alcohol for three days. The needle-like MnO2 can be observed, which is consistent with SEM observation. These MnO2 needles are wrapped with some vague, glue-like materials that are mainly from the PPy coating. Fig. 2B shows the highresolution TEM image of a random selected area. The lattice of MnO2 can be clearly observed, indicating that the MnO2 is highly crystalline. However, none of the X-ray diffraction (XRD) patterns of the composite electrodes with different deposition time of MnO2 display any visible diffraction peaks corresponding to MnO2 due to the strong diffraction peaks from carbon cloth substrate, indicating
a long-range disorder of MnO2 [40]. Fig. 2C shows the high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of a small piece of the nanocomposites. The corresponding energy dispersive spectroscopic (EDS) element mapping confirms the existence of element Mn, O and C from MnO2 and carbon cloth, respectively, and element N from the PPy. The homogeneous distribution of these elements indicates that both MnO2 and PPy are deposited uniformly on the surface of carbon cloth (Fig. 2D). The pore structure of composite electrodes was determined by the nitrogen adsorption/desorption technique, and the corresponding information was summarized in Table S2. The pure carbon cloth consisting of graphite fibers doesn’t possess obvious meso- and micro-pores. The extremely low surface area is mainly from the outer surface of these fibers from macroscopic point of view. After the Electrodeposition of MnO2 which exhibits a mesoporous structure, the surface area significantly increases. The coating of PPy leads to a slightly decreased pore size, which is consistent with the SEM observations. The electrochemical behavior of as-synthesized PPy/MnO2 composite flexible electrodes was investigated using a threeelectrode system with a platinum (Pt) wire as the counter electrode and a saturate calomel electrode (SCE) as and the reference electrode, respectively. The cyclic voltammetry was firstly carried out in an aqueous electrolyte containing 1.0 M Na2SO4 with a working voltage window ranging from 1.0 to 0.0 V at scan rates ranging from 2 to 100 mV s1. Fig. 3A and B shows the typical cyclic voltammetric (CV) curves of the flexible electrodes with singlelayer MnO2 coating and PPy/MnO2 composite coating,
Fig. 2. (A) TEM image of the PPy/MnO2 nanocomposites from the electrode; (B) High-resolution TEM image of as-synthesized PPy/MnO2 nanocomposites showing the high crystallinity of the MnO2; (C) HAADF STEM image of PPy/MnO2; and (D) the corresponding elemental mapping of Mn, O, N and C, respectively.
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Fig. 3. Cyclic voltammogram of (A) MnO2 and (B) PPy/MnO2 at scan rates from 2 to 100 mV s1 under a potential window between 0.0 and 1.0 V. (C) Comparison of specific capacitance of MnO2 and PPy coated MnO2 at different scan rates. (D) Comparison of the rate capability of MnO2 and PPy/MnO2 based on the specific capacitance obtained from CV.
respectively. The CV curves of single-layer MnO2-coated electrode exhibits a nearly rectangular and symmetric shape, indicating the successive redox reaction on the surface of electrode material that corresponds to the capacitive behavior of MnO2 in the aqueous solution [21]. By comparison, the CV curves of the flexible electrode with PPy/MnO2 composite coating (Fig. 3B) remain the nearly rectangular and symmetric shape but possess significantly larger enclosed area and high response current density. Note that the area under a CV curve represents the charge stored on the electrode. The flexible PPy/MnO2 composite electrode exhibits much better capability of charge storage. Such a comparison can be clearly observed in Fig. 3C. At the scan rate of 2 mV s1, MnO2 electrode and PPy/MnO2 composite electrode exhibit a specific capacitance of 257 and 342 F g1 (corresponding to an areal capacitance of 180 and 240 mF cm2), respectively; while at 100 mV s1, the PPy/MnO2 composite electrode still delivers a high specific capacitance of 191 F g1 (134 mF cm2), which is much higher than that of the MnO2 electrode (120 F g1, 84 mF cm2). Note that the specific capacitance of bare carbon cloth, which is less than 1 F g1, is negligible compared to the contribution of MnO2 to the total specific capacitance. The greatly enhanced specific capacitance of PPy/MnO2 composite is mainly attributed to the increased conductivity and the synergistic effects of MnO2 and PPy. On the one hand, the conductive polymer and carbon cloth on each side of MnO2 have established an efficient conductive network, which enables the electrons not only to fast transfer from the MnO2 to the carbon cloth core but also to pass throughout the electrode from the outside PPy. The conductive network can effectively increases the interaction area of MnO2 with the electrolyte and boosts the utilization of MnO2. On the other hand, the presence of MnO2 can interlink the PPy chains and offer short electron pathways between PPy conjugated chains, thus facilitating the conductive behavior of
PPy. As shown in Fig. 3D, a high capacitance retention of 62.3% and 55.8% can be achieved for PPy/MnO2 composite electrode at a scan rate of 50 and 100 mV s1. In contrast, only 56.8% and 46.7% of its initial capacitance can be retained for MnO2 electrode. To further evaluate the excellent capacitive behavior of the flexible composite electrodes, galvanostatic charge-discharge measurement (GCD) was conducted at various current densities ranging from 0.2 to 5.0 A g1 between 1.0 and 0.0 V. Fig. 4A compares the galvanostatic charge-discharge curves of the MnO2 electrode and PPy/MnO2 composite electrode at a current density of 1.0 A g1. Both electrodes exhibit symmetrical and linear profiles, indicating the high columbic efficiency and good capacitive performance, which is consistent with the CV observation. However, the PPy/MnO2 composite electrode possesses longer discharging time than the MnO2 electrode (266 vs. 219 s), suggesting a higher specific capacitance. Such a capacitance difference is even more pronounced at high current densities. As shown in Fig. 4B, the PPy/ MnO2 composite electrode possesses a high specific capacitance of 325 and 226 F g1 at a current density of 0.2 and 5.0 A g1 (corresponding to an areal capacitance of 228 and 158 mF cm2 at 0.14 and 3.5 mA cm2), respectively; while the MnO2 electrode only delivers a capacitance of 275 and 173 F g1. Obviously, the PPy/MnO2 composite electrode exhibits much better rate capability. A capacitance retention of 69.6% can be achieved at a high current of 5.0 A g1 compared to the capacitance at 0.5 A g1 (Fig. 4C). As we mentioned above, such enhancement of the electrochemical behavior is greatly attributed to the improvement of the electrical conductivity and the fast Faradic reaction through better utilization of MnO2. This is further confirmed by the electrochemical impedance spectra (EIS). As shown in Fig. 4D, at high-frequency region, the Nyquist plot of the composite electrode shows lower equivalent series resistance (Rs) of 2.76 U than the Rs (3.41 U) of MnO2 electrode and much smaller semicircle,
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Fig. 4. (A) Comparison of Galvanostatic charge-discharge profiles of PPy/MnO2 and MnO2 at the current density of 1 A g1; and (B) comparison of corresponding specific capacitance of PPy/MnO2 and MnO2 at different current densities. (C) Galvanostatic charge-discharge profiles of PPy/MnO2 at various current densities from 0.2 to 5.0 A g1; (D) Comparison of Nyquist plot of PPy coating MnO2/CC electrode and bare MnO2/CC electrode.
representing the lower charge-transfer resistance of the PPy/MnO2 composites. Moreover, at low frequency region, the plot of composite electrode shows slightly steeper Warburg tail, indicating the lower diffusion-resistance of PPy/MnO2. In addition to the excellent rate capability, the flexible composite electrode also possesses a long-term cycling stability. As shown in Fig. 5A, a stable capacitance retention percentage profile can be obtained over 1000 galvanostatic charge-discharge cycles at the current density of 1 A g1. After over 1000 cycles, the composite electrode still exhibits ~96% of the initial specific capacitance. Such long cycle life is also attributed to the robustness and integrity of the composite materials, which is further confirmed by SEM and electrochemical impedance spectroscopy (EIS) analysis (Fig. 4D). As shown in Fig. 5B, the PPy/MnO2 composite is still integrally and uniformly coated on the carbon cloth after long duration test. The thickness of the active materials does not change significantly, compared to the bare carbon cloth (Fig. 5C), suggesting little loss of active materials during cycling, which is mainly ascribed of the protection of the PPy layer. More importantly, the morphology of PPy/MnO2 composite is preserved very well (Fig. 5D), where the hierarchical arrays of the active materials can still be observed clearly, indicating the robustness of the composite materials synthesized by layer-by-layer electrodeposition technique. 3. Conclusion The low-cost flexible PPy/MnO2 composite electrodes have been successfully developed through in-situ electrodeposition technique. Each component was densely packed and uniformly grown on the flexible carbon cloth substrate to form a robust freestanding electrode with hierarchical structure. Owing to the synergetic effect of PPy and MnO2, the composite electrode has exhibited high utilization of the active materials during charging and discharging and
excellent electron-transfer network, which are both highly demanded in MnO2 based supercapacitor electrodes. Thus, this unique design has provided the electrode with superior capacitive performance, high rate capability and long cycling stability, which makes the PPy/MnO2 composite electrode very promising in the applications of flexible supercapacitors and also highlights the strategy of combining multifunctional materials to achieve demanding high-performance supercapacitor electrodes. 4. Experimental 4.1. Material synthesis The fabrication of flexible PPy/MnO2 electrode is illustrated in Scheme 1. In a typical process, a piece of commercial carbon cloth (CCP-10F, Fuel Cell Earch Inc.) was washed by acetone, methanol and isopropanol under sonication for 30 min in sequence to remove any contaminants on the surface. After drying in the oven, the piece of carbon cloth was cut into small rectangle piece of 2 cm * 1 cm and used as the working electrodes. However, only half of the CC was soaked into the electrolyte to make MnO2 only grow in an area of 1 1 cm. The electrodeposition of MnO2 on CC was conducted in a three-electrode system under a static potential of 0.92 V in an aqueous solution of 0.5 M Mn(CH3COO)2 and 0.1 M Na2SO4. The counter and reference electrode utilized in the electrodeposition is platinum (Pt) wire and SCE electrode, respectively. After electrodeposition, the CC was rinsed by DDI water and dried in a vacuum oven. Generally, the loading of MnO2 determined by weight change of CC before and after the electrodeposition can be controlled by the electrodeposition time (Table S1). The electro-polymerization of PPy, which is similar to electrodeposition of MnO2 is carried out under a static potential of 0.8 V, using an electrolyte of 0.1 M NaClO4 and 0.2% (V: V) pyrrole monomer. To make comparison,
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Fig. 5. (A) Cycling stability of PPy/MnO2 at the current density of 1 A g1. SEM images of (B) PPy/MnO2/CC electrode after duration test over 1000 cycles; (C) Bare carbon cloth; and (D) high magnification SEM images of PPy/MnO2/CC electrode after duration test.
single layer MnO2/CC was also fabricated. Moreover, to optimise the loading of MnO2, MnO2 with different electrodeposition time was synthesized and denoted as MnO2-Xmin.
The specific capacitance based on discharge curve and CVs was determined by the Eq. (i) and (ii), respectively
C ¼ I=ðDV=DtÞ
(1)
4.2. Material characterization To characterize the structure and morphology of the assynthesized materials, scanning electron microscopy (SEM) was adopted on LEO FESEM 1530. Besides, transmission electron microscopy (TEM) investigation was carried out at Canadian Center for Electron Microscopy (CCEM, McMaster University) on a JEOL 2010F TEM/STEM field emission microscope. X-ray diffraction (XRD) measurement was obtained from XRG 3000 X-ray diffractometer (monochromatic Cu Ka X-rays). The pore structure was analyzed using a Micromeritics ASAP 2020 with nitrogen gas. The thermal gravity analysis was performed on a TA instrument Q500 with a ramping rate of 5 C min1 in air. 4.3. Electrochemical measurement Electrochemical measurements were conducted in a threeelectrode system using our flexible materials as working electrode with a mass loading of ~0.7 mg cm2 (120 s for MnO2 and 30 s for PPy coating), Pt wire as the counter electrode and SCE as the reference electrode. Cyclic voltammetry (CV), galvanostatic chargedischarge, and electrochemical impedance spectroscopy (EIS) measurements were conducted in 1 M Na2SO4 aqueous electrolyte under a cutoff voltage of 0.0 and 1.0 V with a VMP3 potentiostat/ galvanostat (Bio-Logic LLC, Knoxville, TN).
Z0 C¼
idV=vmDV
(2)
i
where DV is the potential window, I is applied constant current density, DV/Dt is the slope of the discharge curve, i is the voltammetric current, m is the loading of active material, and v is the scan rate.
Acknowledgement This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), University of Waterloo, and the Waterloo Institute for Nanotechnology. The authors acknowledge Dr. Carmen Andrei and Canadian Center for Electron Microscopy at McMaster University for TEM characterization.
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.05.047.
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X. Fan et al. / Journal of Power Sources 326 (2016) 357e364
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