PANI Nanoporous Electrode and Its

Nov 16, 2012 - Juan Cao , Juchuan Li , Liu Liu , Anjian Xie , Shikuo Li , Lingguang Qiu , Yupeng Yuan , Yuhua Shen. Journal of Materials Chemistry A 2...
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Well-Constructed CNT Mesh/PANI Nanoporous Electrode and Its Thickness Effect on the Supercapacitor Properties Yanli Yin, Changhong Liu,* and Shoushan Fan Tsinghua-Foxconn Nanotechnology Research Center and Department of Physics, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: In this work, a well-constructed carbon nanotube (CNT) mesh/polyaniline (PANI) composite film was fabricated as the electrode of flexible supercapacitor via alternatively cross-stacking superaligned CNT sheets and in situ coating PANI from chemical solution. Compared with other CNT/PANI composites using random-constructed CNT network as backbone, this composite has a remarkable uniform and oriented structure in both the plane and thickness direction with a thickness as low as 0.8 μm. As its electrode thickness can be easy-controlled, the electrode thickness dependence of the assembled supercapacitor performance was precisely measured, and the possible cause was analyzed. The results showed that there exists an optimal electrode thickness of about 5 μm or less for achieving the highest electrode specific capacitance, energy density, and power density. Its highest electrode power density (9.0 kW kg−1) is much higher than that of the ever reported CNT/PANI composites prepared by other methods. Such composite may have great application potentials on new flexible devices for energy storage.

1. INTRODUCTION As new energy storage devices, supercapacitors can fill the gap between batteries and conventional capacitors, providing a higher power density than the batteries and a higher energy density than the conventional capacitors.1,2 Combining the lightweight, high conductive, thin, porous, and freestanding characters of the CNT network and the high pseudocapacitance of PANI, the CNT/PANI composite films have been proved as excellent electrode materials for flexible supercapacitors.3−6 However, the CNT networks in these composites were usually constructed in random forms, whose porosities and conductivities were limited. Compared with these random-constructed CNT networks, a well-constructed CNT mesh7−13 is a more effective template for PANI coating. It shows a super uniform structure in both the plane and thickness direction. With high specific surface of 97 m2 g−1, good conductivity, and high mechanical strength, it is suitable for serving as the template of composite electrode materials. Assisted by precision machinery, this CNT mesh can be produced massively in a high efficiency, united specification and good quality, which may be suitable for real applications. Meanwhile, as refs 14 and 15 reported, the wellordered PANI displayed higher electrochemical performance than the randomly connected PANI. Therefore, the CNT/PANI composite with the well-constructed CNT mesh as backbone may be superior to the corresponding composite based on the random-constructed CNT networks in supercapacitor properties. In this work, we designedly fabricated a well-constructed CNT mesh/PANI composite as the supercapacitor electrodes. This composite has shown a uniform structure in both the plane and thickness direction. With its easy-controlled electrode thickness, © 2012 American Chemical Society

we specially studied the electrode thickness dependence of the electrode specific capacitance, energy density, and power density. The results proved that there are relationships between all above characters and the electrode thickness. Then, we tried to explain these findings from many angles. The results also showed that there exists an optimal electrode thickness of about 5 μm or less for achieving the highest electrode specific capacitance, energy density, and power density. The highest electrode power density (9.0 kW kg−1) is much higher than that of supercapacitors using other CNT/PANI composites as electrodes.

2. EXPERIMENTAL SECTION 2.1. Fabrication of the Well-Constructed CNT Mesh. The preparation of CNT sheet has been described in a previous paper.9 This sheet can be continuously drawn from a superaligned CNT array with a height of 300 μm grown on an 8 in. wafer substrate, which is 20 cm in width and 200 m in length. This CNT sheet is ultrathin with average thickness of tens of nanometers, lightweight with mass per unit area of 1.5 μg cm−2, conductive with sheet resistance of 700−1500 Ω per square, and oriented with the aligned CNTs in the same direction. By alternatively cross-stacking layers of these CNT sheets, a wellconstructed CNT mesh can be fabricated. Here, the wellconstructed CNT meshes containing different layer number (5 to 200) of CNT sheets were prepared as samples. Received: August 22, 2012 Revised: October 22, 2012 Published: November 16, 2012 26185

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2.2. Synthesis of Composites. Then, the PANI was coated uniformly on the as-prepared CNT mesh samples through the in situ chemical solution method. First, the well-constructed CNT meshes were soaked by ethanol and then immersed in 40 mL aqueous solution containing 0.04 mol HCL and 0.002 mol aniline monomers for a complete infiltration of ten minutes. The aniline monomer used in our experiment has a high purity of ≥99.5%. Second, 40 mL of precooled aqueous solution containing 0.002 mol ammonium persulfate (an oxidant for polymerization) was dropped slowly in the above solution. Then, the mixed solution was put at 0 °C for reacting completely of 24 h. The obtained well-constructed CNT mesh/PANI composite film was picked out from solution and cleaned with deionized water, ethanol, and acetone. Then, this composite was kept at 80 °C in a vacuum for drying of 24 h. The films with different thickness of 0.8, 1.7, 3.5, 5.2, 8.7, 17.4, and 34.6 μm were obtained by the same method. The thickness can be precisely easy-controlled by adjusting the layer number of CNT sheets in 0.17 μm per layer, which makes it convenient for the detailed study on the electrode thickness dependence of the assembled supercapacitor performance. By comparing the mass of the pristine CNT and the corresponding composite, the PANI proportion of these composites were calculated as about 80 wt %. For comparison, a random-constructed CNT network was also fabricated through the method reported in ref 3. The corresponding CNT/PANI composite film was also prepared via the above chemical synthesizing process. 2.3. Characterization and Electrochemical Measurement of Electrode Materials. The microstructure of this composite was characterized by the scanning electron microscope (SEM) (Sirion 200, resolution 1.0 nm). It was assembled as supercapacitor in a sandwich-like system (electrode− separator−electrode) encased in a stainless button mold with the composite as electrodes, the filter paper as the separator, and 1 M H2SO4 aqueous solution as the electrolyte, which is illustrated in Figure 1. The electrochemical properties of the

Figure 2. (a,b) Digital photographs of the well-constructed CNT mesh/ PANI with the thickness of 0.8 and 17.4 μm, respectively. (c,d) SEM images of the well-constructed CNT mesh/PANI in plane view with magnifications of 5 μm and 500 and 100 nm. (e) SEM image of the wellconstructed CNT mesh/PANI with the thickness of 17.4 μm in side view. (f) SEM images of the random-constructed CNT network/PANI in plane view with magnification of 5 μm.

transparent and flexible, while the 17.4 μm thick one is surfacesmooth and elastic. They are both freestanding based on the strong van der Waals interactions among CNTs. The size of sample was limited by square model for CNT sheets supporting, which can be extended to 20 cm, the same as the width of the superaligned CNT sheets drawn from 8 in. arrays in practical production. The microstructures (plane view) of the well-constructed CNT mesh/PANI composite film characterized by SEM with different magnifications are shown in Figure 2c,d. This composite shows an extremely uniform, oriented, and porous structure in the plane direction. Before and after being coated by PANI, the structure of CNT mesh remains stable with superaligned CNTs in the same direction and multilayer sheets in cross-stacking arrangement. The PANI is uniformly coated on the CNTs, between which a lot of pores are formed in a similar size and shape. Figure 2e typically shows the cross-sectional SEM image (side view) of the well-constructed CNT mesh/PANI lamina with the thickness of 17.4 μm. This composite also shows a quite uniform, oriented, and porous structure in the thickness direction. CNTs in each layer of CNT sheets are in good contact with each other and coated uniformly by PANI, leaving a uniform interspace between neighboring CNT sheets. The porosity of the CNT mesh (Pn) and the corresponding composite (Pc) were calculated using ρ Pn = 1 − e ρn (1)

Figure 1. Cross-section schematic and the digital photograph of the supercapacitor encased in stainless button mold.

and Pc = 1 −

assembled supercapacitors were measured through cyclic voltammetry (CV) method (PAR 273A, EG&G, USA) and constant current charging−discharging method (Land Battery Testing System, China).

ρe αρn + βρp

(2)

respectively, where ρe is the bulk density of electrode materials, ρn and ρp are the true density of CNT and PANI molecular, respectively, and α and β are the CNT and PANI proportion of the composite respectively. ρe was calculated using m ρe = (3) SL

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure. Figure 2a,b shows the typical appearances of the well-constructed CNT mesh/PANI composite films in a square of 3 × 3 cm with thickness of 0.8 and 17.4 μm, respectively. The 0.8 μm thick composite is partially

, where m, S, and L are the mass, area, and thickness of electrode, which were experimentally measured, respectively. By calculating 26186

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a single CNT model, ρn was obtained as about 1.85 g cm−3. By experimentally measuring the mass and volume of polyaniline obtained from the rest of reacted solution, ρp was calculated as about 1.3 g cm−3. By comparing the mass of electrode before and after polymerization of PANI, we get α and β. The average pore size of these porous materials was obtained by measuring SEM images in different area of samples. By calculation and measurement, the well-constructed CNT mesh has a porosity of 94% and an average pore size of 120 nm, while the corresponding composite has a porosity of 80−90% and an average pore size of about 100 nm. From the above results, this composite belongs to the macroporous materials (pore size > 50 nm). With this composite as supercapacitor electrodes, the uniform and porous structure is helpful for the exchange and diffusion of electrolyte ions and can enhance the ability of PANI molecules to attract ions. For comparison, Figure 2f characterizes the corresponding morphology of the random-constructed CNT network/PANI composite film. The as-comparison composite shows a disordered structure with pores in great difference. Obviously, the porous structure of the former composite is much more uniform than the as-comparison composite. Moreover, the porosity of the as-comparison composite is 70%, which is less than that of the former composite. The differences in structure between the two composites indicate that the former composite may be superior to the as-comparison composite in supercapacitor properties. 3.2. Electrochemical Properties. The CV curves of these supercapacitors with different electrode thickness at a scanning rate of 10 mV s−1 are shown in Figure 3a. The CV curve area

capacitance, and the mass of electrodes can be calculated according to Cs = 4[(i /M )/(dU /dt )]

(4)

where i is the current, M is the mass of the whole two electrodes, U is the discharging voltage, t is the discharging time, and dU/dt is the absolute value of discharging curve slope. The curves show that the absolute value of the discharging curve slope increases with the electrode thickness increasing. By calculation, the results are shown in Figure 4, which indicates that the accurate electrode specific capacitance

Figure 4. Electrode thickness dependence of the specific capacitance and the ratio between the specific capacitance and the PANI proportion at 1 A g−1 charge−discharge current.

decreases with the electrode thickness increasing. Meanwhile, it was accurately calculated and confirmed that the PANI proportions of these composites are not absolutely the same. As ref 3 reported, the pseudocapacitance of PANI is much higher than the electrical double-layer capacitance of CNTs in the CNT/PANI composites. Therefore, the dependence of specific capacitance on the electrode thickness may be affected by the PANI proportion difference between these composites. To eliminate the cause of PANI proportion difference, here the ratio between the electrode specific capacitance and the PANI proportion is also given in Figure 4, which verifies that this ratio also decreases with the electrode thickness increasing, except for the situation when the electrode thickness was less than 5 μm where the data changed not much. This indicates that the dependence of specific capacitance on the electrode thickness is not simply affected by the PANI proportion difference and that other important parameters influencing this dependence may exist. We also found that there have been some literatures previously talking about performance of the battery or capacitor with different electrode thickness,16−18 but the conclusions of those works cannot be applied to the circumstance of this work. For example, Park et al.17 investigated the performance of supercapacitor with electrodeposited ruthenium oxide film electrodes. They also reported that the specific capacitance decreased with the film thickness increase. In that work, the cause was attributed to a decrease in the porosity of the outer layer and formation of a compact inner layer in the RuO2 film with increasing thickness, i.e., the morphological changes. However, the uniform structure used in our work can ensure that the structure of electrodes does not change obviously with the thickness changing. Here, we try to interpret this thickness effect in this material system through the effective electrical potential angle, i.e., the potential distribution in electrode may influence the electrode specific capacitance difference, which is illustrated by the crosssection schematic of supercapacitor in Figure 1. First, from a

Figure 3. Electrochemical measurement results of well-constructed CNT mesh/PANI with different electrode thickness. (a) CV curves at a scanning rate of 10 mv s−1. (b) Constant current charging−discharging curves at 1 A g−1.

decreases with the electrode thickness increasing, which indicates that the electrode specific capacitance decreases with the electrode thickness increasing. Figure 3b shows the constant current charging−discharging curves of these supercapacitors with different electrode thickness at 1 A g−1. The accurate electrode specific capacitance (Cs), the ratio between the 26187

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Huang et al.20 reported the maximum specific power of 3.0 kW kg−1 for a prototype supercapacitor with PANI/aligned carbon nanotube composite electrodes. It is also shown that the Ragone plot of electrode with less thickness is nearer to the upper-right edge of the total region, which indicates that the thinner electrode has higher supercapacitor properties and enhanced application potentials. Figure 6 shows the specific capacitance retention curves of the electrodes with thickness of 0.8 and 5.2 μm after 1000 constant

chemical angle, an appropriate potential is needed for the fast faradaic reaction occurring near an electrode surface to produce the pseudocapacitance. Second, from a physical angle, an appropriate potential is needed for the PANI molecules to attract the charged electrolyte ions through the Coulomb force. In the electrode−separator−electrode system, the total potential difference across the total thickness of the two electrodes and separator is a constant decided by the charge−discharge voltage (0.8 V). As Ohm’s law says, i.e., a direct current is proportional to the potential difference and inversely proportional to the resistance of the circuit, the equivalent series resistance of an electrode will share partial potential difference, so that the potential in electrode will decrease from the outer part (close to the collector) to inner part (close to the separator). Because the ability of PANI molecule to attract electrolyte ions is mainly decided by the potential difference between its location in the electrode and the center of the separator, PANI molecules located in the inner part of the electrode will be unable to bind ions when the electrode thickness is more than 5 μm. Therefore, the efficiency to bind ions in electrode pores will decrease from the outer part to inner part. Statistically, the average density of ions bound in the electrode pores will decrease with the electrode thickness increasing, i.e., the effective pseudocapacitance contributed by PANI will decrease with the electrode thickness increasing. As the specific capacitance of the CNT/PANI composite electrodes is mainly provided by the pseudocapacitance of PANI, the total electrode specific capacitance will also decrease with the electrode thickness increasing. Figure 5 shows the electrode thickness dependence of the electrode energy density and electrode power density at 1 A g−1

Figure 6. Specific capacitance retention curves of the electrodes with thickness of 0.8 and 5.2 μm after 1000 constant current charging− discharging cycles at 1A g−1.

current charging−discharging cycles at 1 A g−1. These results reflect the cycling stability of the supercapacitors. The maximum specific capacitance retention is 96.5% obtained by the 0.8 μm thick electrode. All above results show that there exists an optimal electrode thickness of about 5 μm or less for achieving the highest specific capacitance, energy density, and power density of the supercapacitor electrodes. However, the electrochemical properties presented here are simply for the comparative study. We believe that the electrochemical properties of these composites will be improved further by optimizing the measuring parameters and the supercapacitor assembling (including the separator, electrolyte, etc.).

4. CONCLUSIONS In this study, a well-constructed CNT mesh/PANI composite film was fabricated as supercapacitor electrodes. By using its extremely uniform structure, we investigated the electrochemical properties of the assembled supercapacitor with different electrode thickness. The results showed that the electrode specific capacitance decreased obviously with the electrode thickness increasing, which can be explained by the potential distribution across the electrode. Meanwhile, the energy density and power density of electrodes also decreased with the electrode thickness increasing. Moreover, there exists an optimal electrode thickness of about 5 μm or less for achieving the highest specific capacitance, energy density, and power density of the supercapacitor electrodes. These findings are helpful for optimizing the electrochemical performance of the supercapacitor by adjusting the electrode thickness. This composite has shown superior properties such as being tough even at ultrathin thickness (0.8 μm) and high power density for assembled supercapacitor electrodes (9.0 kW kg−1). It may have great application potentials in wearable electronics and flexible energy storage devices.

Figure 5. Electrode thickness dependence of the energy density and power density at 1 A g−1 charge−discharge current and the Ragone plots of the different samples (inset).

charge−discharge current and the Ragone plots (inset). The error bars were mainly aroused by data scattering from different measurements. It shows that the energy density decreases with the electrode thickness increasing with the maximal value of 8.8 Wh kg−1 obtained at the minimum thickness of 0.8 μm. The power density keeps stable at 9.0 kW kg−1 with the electrode thickness less than 5 μm and then decreases from 9.0 to 5.7 kW kg−1 with the thickness increasing from 5 to 34.6 μm, which is much higher than that of other CNT/PANI composites ever reported. For instance, Gupta et al.19 reported a maximum value of specific power at 2250 W kg−1 for 73 wt % PANI/single-wall carbon nanotubes composites. Meng et al.4 reported that the supercapacitors using PANI/random-buckypaper nanocomposite as electrodes had a high power density of 2189 W kg−1. 26188

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 10 62796011. Fax: +86 10 62792457. E-mail: chliu@ tsinghua.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (2012CB932301) and the Natural Science Foundation of China (51173098, 10721404).

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