ARTICLE pubs.acs.org/Langmuir
Facile Preparation and Enhanced Capacitance of the Polyaniline/ Sodium Alginate Nanofiber Network for Supercapacitors Yingzhi Li, Xin Zhao, Qian Xu, Qinghua Zhang,* and Dajun Chen State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, People’s Republic of China
bS Supporting Information ABSTRACT: A porous and mat-like polyaniline/sodium alginate (PANI/SA) composite with excellent electrochemical properties was polymerized in an aqueous solution with sodium sulfate as a template. Ultravioletvisible spectra, X-ray diffraction pattern, and Fourier transform infrared spectra were employed to characterize the PANI/SA composite, indicating that the PANI/ SA composite was successfully prepared. The PANI/SA nanofibers with uniform diameters from 50 to 100 nm can be observed on scanning electron microscopy. Cyclic voltammetry and galvanostatic charge/discharge tests were carried out to investigate the electrochemical properties. The PANI/SA nanostructure electrode exhibits an excellent specific capacitance as high as 2093 F g1, long cycle life, and fast reflect of oxidation/reduction on high current changes. The remarkable electrochemical characteristic is attributed to the nanostructured electrode materials, which generates a high electrode/electrolyte contact area and short path lengths for electronic transport and electrolyte ion. The approach is simple and can be easily extended to fabricate nanostructural composites for supercapacitor electrode materials.
’ INTRODUCTION One of the great challenges in the 21st century is unquestionably energy storage.1,2 Electrochemical capacitors (ECs) attract great interest because of faster and higher power energy storage systems and an exceptional cycle life, filling the gap between batteries and conventional capacitors.3 Supercapacitors have been widely used for power buffer applications or memory back-up in toys, cameras, mobile phones, etc. On the basis of the charge storage mechanism as well as active materials of electrodes, ECs can be divided into two categories. One is the electrochemical doublelayer capacitors (EDLCs), which store energy by forming a double layer of electrolyte ions on the surface of conductive electrode, and the most common devices are carbon-based active materials with high specific surface area at present.4,5 The other is the pseudocapacitors or redox supercapacitors, and their capacitances arise from Faradaic reactions at the surface of active materials.6 Transition-metal oxides7,8 and electrically conducting polymers9,10 are extensively applied to pseudo-capacitive active materials. It is necessary for ECs to further improve energy density and power density to meet the higher requirements, such as portable electronics, hybrid electric vehicles,11 and large industrial equipment. According to the traditional voltammetric theory, the Frumkin effect would only affect the kinetics of the chargetransfer process. If the electrode reaction is reversible (i.e., the electron-transfer rate is very fast compared to the masstransport rate), the concentration of electroactive species at the reaction plane is mainly governed by the slow transport process (migration and diffusion) in the depletion layer (diffusion layer) and the effect of the diffuse double layer is negligible.12 r 2011 American Chemical Society
Montenegro et al.13 described that, when the electrode dimension became very small, the high transport rate exceeded the rate at which reactants can be consumed at the surface. Nanostructured materials are considered as the key to new generations of clean-energy devices. Choi et al.14 prepared nanocrystalline vanadium nitride (VN) with a specific capacitance of 1200 F g1 at a scan rate of 2 mV s1. Sugimoto et al.15 synthesized hydrated RuO2 nanosheets with a capacitance of 1300 F g1, but its specific capacitance sharply increased upon decreasing the thickness of the film. Derrien et al.16 fabricated a nanostructured SnC composite, exhibiting an advanced anode material with a large capacity of about 500 mA h g1 over several hundred cycles. An ordered whiskerlike polyaniline (PANI) grown on the surface of mesoporous carbon reported by Xia et al.17 exhibits an enhanced capacitance of 1221 F g1. Kuila et al.18 prepared arrays of ordered and aligned PANI nanorods of 10 nm in diameter on a transparent indium tin oxide (ITO) substrate using a nanotemplate based on supermolecular assemblies of the block co-polymer as scaffold material, demonstrating a high electrochemical capacitance of 3407 F g1. However, these methods are too complex to fabricate products in bulk quantities. A random nanowire network of PANI synthesized by electrochemical deposition showed a capacitance value of 775 F g1 and high stability of electrode for long cyclic life.19 The electrochemical deposition technique is not an easy method to obtain large quantities of uniform products. Received: January 23, 2011 Revised: April 4, 2011 Published: April 13, 2011 6458
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Scheme 1. Chemical Structure of SA with β-D-Mannuronic (M) and R-L-Guluronic (G)
As one of the intrinsic conducting polymers (ICPs), PANI exhibits excellent electrochemical and environmental stability. Considerable efforts have focused on the synthesis of nanostructured PANI, such as the template-induced and self-assembled process.2023 However, agglomeration of some templates, such as graphene and nanotubes, has an influence on the electrochemical properties. In this report, a facile solution method was used to synthesize the nanostructured polyaniline/sodium alginate (PANI/ SA) composite in bulk quantities. The electrode material of the PANI/SA nanofibers with uniform diameters from 50 to 100 nm exhibits an excellent electrochemical property.
’ EXPERIMENTAL SECTION Materials. Aniline (chemically pure), ammonium persulfate (APS, analytical reagent), sodium hydroxide (analytical reagent), and SA (chemically pure) with Mw ∼ 2.1 105 were purchased from Shanghai Chemical Co., used without further purification. All of the solutions were prepared using deionized water. Synthesis of Nanostructural PANI/SA. A facile method was employed to prepare PANI nanofibers. In a typical process, 0.03 g of SA was dissolved in 30 mL of 0.1 M NaOH at 60 °C for 12 h. Then, 0.093 g of aniline was added to the above solution with stirring for 1 h, and the resulting solution was cooled to 0 °C. H2SO4 with 1 M was dropped to adjust the pH value to 7, and then APS of 0.228 g was added to synthesize the nanofibers at 0 °C for 24 h. The resulting product was collected by centrifugation and washed with deionized water, and finally, PANI/SA was dried at 50 °C in a vacuum oven. To prepare a parallel sample, nanostructured PANI without SA was synthesized according to the previous report.24 Characterization. The morphology of pure PANI and PANI/SA nanofibers was observed on field emission scanning electron microscopy (FESEM, Hitachi S-4800) at an accelerating voltage of 3.0 kV. Samples were dispersed in an ultrasonic bath for 20 min and carbon-sputtered prior to observation. Transmission electron microscopy (TEM) was performed on a Hitachi H-800 electron microscope at an accelerating voltage of 10 kV. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 8700 FTIR spectrometer. The spectra in the range of 4000400 cm1 were collected by the averaging of 32 scans at a resolution of 4 cm1. Ultravioletvisible (UVvis) absorption spectra were observed on a Lambda 35 (Perkin-Elmer) spectrophotometer at a scanning rate of 480 nm/min. X-ray diffraction (XRD) measurements were conducted on a Rigaku D-max-2500 diffractometer with nickelfiltered Cu KR radiation, with λ = 0.154 nm. Electrochemical experiments were carried out in a three-electrode system. The working electrode was a glassy carbon electrode with a diameter of 3 mm. The counter electrode was a platinum wire. A saturated calomel electrode (SCE) was used as the reference electrode. Typically, 1 mg of PANI/SA was ultrasonically dispersed in 1 mL of deionized water, and 10 μL of the polytetrafluoroethylene (PTFE) emulsion (60%) was added to the dispersion. The above suspension of 5 μL using a pipet gun was dropped onto the glassy carbon electrode and
dried at room temperature. H2SO4 (1 M) was used as the electrolyte solution at room temperature. Cyclic voltammetry (CV) and galvanostatic charge/discharge were measured using a Zahner Zennium electrochemical workstation. CV tests were carried out from 0.2 to 0.8 V at various scan rates of 20, 50, 100, 150, 200, 300, and 500 mV s1. The galvanostatic charge/discharge property was measured at the current densities of 1, 3, 5, 10, and 15 A g1. Measurement of cycle-life stability was performed by CV at a scan rate of 100 mV s1.
’ RESULTS AND DISCUSSION Synthesis of PANI/SA. As shown in Scheme 1, SA consists of a linear block co-polymer of 1,4-linked β-D-mannuronic (M) and R-L-guluronic acid (G), bearing abundant carboxyl and hydroxyl groups. As a polyelectrolyte, SA has the response to specific environmental stimuli, such as pH, temperature, or ionic strength.25,26 To synthesize the PANI/SA sample, the SA was first kept in an alkaline solution at 60 °C to be completely dissolved, and then aniline monomer was added to the above aqueous solution to form biopolymermonomer complexes by the interaction between carboxylic groups of SA and amino groups of aniline monomer.27 Pure SA exhibits a rod-like morphology, as specified in Figure S1 of the Supporting Information. The aniline is adsorbed on the surface of the SA rods by van der Waals forces or hydrogen bonds, producing biopolymermonomer complexes. When the oxidant APS was added, PANI was introduced onto the biopolymer chains; thus, the PANI/SA composite was produced. Figure 1 shows the typical morphology of the products. Apparently, the pure PANI nanorods in Figure 1a appear as random aggregation, and their surface is not smooth because a large number of nanoparticles adsorbed on them because of the secondary growth. However, as shown in Figure 1b, the uniform PANI/SA nanofibers present a wellextended random network with the diameter of 50100 nm. The low-resolution SEM image of the PANI/SA nanofiber exhibits a mat-like nanostructure, and such a composite with a porous network is favorable in supercapacitor applications.28 The TEM image in Figure 1d further illustrates the uniform network of PANI/ SA nanofibers. Because the biomacromolecule is a negatively charged polyelectrolyte in alkalic aqueous solution, the strong electrostatic repulsion among SA carboxylate anions (COO) leads to the expanded network structure of the SA chains.29 The FTIR spectra for SA, pure PANI, and PANI/SA are given in Figure 2. The peaks at 1628, 1419, and 1035 cm1 in the FTIR spectrum of SA are caused by the stretching of COO (asymmetric), COO (symmetric), and COC, respectively.30 For PANI, the peaks at 1565 and 1474 cm1 correspond to the characteristic CC stretching of the quinoid and benzenoid rings, respectively. The peaks at 1305 and 1249 cm1 are related to CN and CdN stretching vibrations. The peak at 1115 cm1 is assigned to the in-plane bending of CH. The peak at 6459
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Figure 1. SEM images of (a) pure PANI, (b and c) PANI/SA, and (d) TEM image of PANI/SA. The content of SA is 0.1%, and the molar ratio of aniline/APS is 1:1.
Figure 3. XRD patterns of SA, pure PANI, and PANI/SA. Figure 2. FTIR spectra of SA, pure PANI, and PANI/SA.
776 cm1 is attributable to the out-of-plane bending of CH.3133 The FTIR spectrum of PANI/SA is similar to that of pure PANI. On the other hand, the PANI/SA composite exhibits the characteristic peak of 1628 cm1 and shoulder peaks at 1419 and 1035 cm1 of SA. The results confirm that the conducting polymer is successfully introduced onto the SA surface. In Figure 3, the XRD pattern of PANI/SA nanofibers is shown relative to pure SA and PANI. The diffraction of SA shows typical peaks at 13.5° and 21.4°.34 The two shape peaks at 31.8° and 45.5° derive from the byproduct of NaCl in SA. The characteristic peaks of PANI at 2θ = 14.8° and 25.3° are attributed to the
periodicity perpendicular and parallel to the polymer chain, respectively, and the peak at 2θ = 20.3° is caused by the layers of polymer chains alternating distance.35 However, the spectrum of PANI/SA clearly exhibits the characteristic peaks at 2θ = 14.8° and 20.3°, becoming broad because of the introduction of SA. The intermolecular interaction between PANI and SA maybe induces the crystallinity of the PANI/SA composite. Figure 4 presents UVvis spectra of SA, pure PANI, and PANI/SA samples. In the UVvis spectrum of SA, an absorption band at about 280 nm can be assigned to double bonds of alginate formed after main-chain scission.36 Characteristic absorption bands at wavelengths of 330360, 420440, and 790820 nm 6460
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Figure 4. UVvis spectra of SA, pure PANI, and PANI/SA.
Figure 5. Cyclic votammograms of SA, pure PANI, and PANI/SA electrodes at a scan rate of 100 mV s1 in 1 M H2SO4.
are observed in the curves of pure PANI and PANI/SA, indicating these PANI nanofibers in their emeraldine salt form (ESPANI). The first absorption band arises from ππ* transition. The second and third bonds are related to the polaron band ππ* transition and the π to the localized polaron band of doped PANI (ESPANI), respectively.37 The presence of SA is not likely to modify either the chemical structure or the electronic states of PANI chains because neither an appreciable shift of the absorption bands nor a change in the relative intensity of bands appears. Electrochemical Property. To evaluate the electrochemical characteristics of the samples, CV curves in 1 M H2SO4 electrolyte at a scan rate of 100 mV s1 were performed at the potential window from 0.2 to 0.8 V versus SCE. As shown in Figure 5, the shapes of the CV curves of PANI and PANI/SA are distinct from that of pure SA. The CV curve of SA exhibits a double-layer capacitance characteristic (inset of Figure 5). Moreover, the contribution of pure SA to the capacitance is very small and can be neglected. There are three pairs of redox peaks, i.e., P1/P2 (0.22/0.1 V), P3/P4 (0.49/0.43 V), and P5/P6 (0.76/0.73 V), that can distinctly be found on the CV curves of the pure PANI and the PANI/SA composite, respectively. The first couple of peaks are caused by the redox transition of PANI between the leucoemeraldine form and polaronic emeraldine form. The second couple of peaks are due to the transformation of the
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Figure 6. Cyclic votammograms of a PANI/SA electrode at various scan rates in 1 M H2SO4 and (inset) evolution of the current density at different potential scan rates.
Figure 7. Galvanostatic charge/discharge curves of pure PANI and PANI/ SA nanofiber electrodes at a current density of 1 A g1 in 1 M H2SO4.
p-benzoquinone/hydroquinone couple. The third couple of peaks are ascribed to the formation/reduction of bipolaronic pernigraniline and protonated quinonediimine.19,38,39 Electrochemical capacitance is proportional to their CV curve area. Apparently, the CV curve area of PANI/SA is larger than that of the parallel sample of pure PANI. Scrosati et al.1 reported that nanoelectrodes have many advantages: (i) higher electrode/electrolyte contact area, leading to higher charge/discharge rates, (ii) short path lengths for electronic transport, and (iii) short path lengths for electrolyte ions transport. The BrunauerEmmettTeller (BET) measurement indicates that the specific surface area of PANI/SA (66 m2/g) is larger than pure PANI (28 m2/g), which is favorable for ion transportation and, hence, provides larger capacitance (see the Supporting Information). As indicated in Figure 1, in comparison to pure PANI, the regular nanostructured network of PANI/SA enhances the surface area for redox reactions and shortens the distance for electrolyte ions transport, thereby leading to a high special capacitance. The high-power characteristic is one of the basic requirements for an electrode material in supercapacitors, which can be identified from their voltammetric response at various scan rates.38 This property of PANI/SA is shown in Figure 6. Apparently, each curve exhibits a similar shape, but the total current increases with 6461
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Figure 8. Galvanostatic charge/discharge curves of the (a) pure PANI electrode and (b) PANI/SA electrode at various current densities in 1 M H2SO4.
The galvanostatic charge/discharge curves of pure PANI and PANI/SA are shown in Figure 7. Specific capacitance (Cm) can be calculated by eq 141 Cm ¼ C=m ¼ ðItÞ=ðΔVmÞ
ð1Þ
1
Figure 9. Cycling stability of PANI and PANI/SA electrodes at 100 mV s1 and (inset) CV curves of (a) PANI and (b) PANI/SA at the first charge/ discharge test and at the 1000th cycle.
increasing scan rates. Even at a scan rate of 500 mV s1, the CV curve still appears in three pairs of redox peaks, which illustrates that the PANI/SA network is a benefit to fast redox reactions and a short diffusion path of electronic transport.17 The peak potential shifts less than 30 mV per 10-fold change in the scan rate, indicating that the electrode material possesses electrochemical reversibility.40 A plot of the current density at the oxidation peak at around 0.22 V versus the scan rate is given in the inset of Figure 6. The near linear dependence of the current density upon the scan rate further reveals the good reversible stability and fast response to oxidation/reduction on the current changes. These results demonstrate a high power delivery or uptake of the PANI/SA electrode material. The impedance measurement exhibits that the PANI/SA electrode possesses a lower internal impedance and shorter diffusion path of the ions in the electrolyte (see the Supporting Information). The PANI/SA network is helpful for reducing the internal resistance and increasing the electron transfer and electrolyte ions transport rate in the diffusion layer, which is a benefit to the enhancement of capacitance and energy density of the PANI/SA electrode materials.
where Cm is the specific capacitance (F g ), I is the charge/ discharge current (A), ΔV is 0.8 V, and m is the mass of active material within the electrode. The discharge specific capacitance of the PANI/SA composite is as high as 1612 F g1 at a current density of 1 A g1, while that of pure PANI is 755 F g1. The specific capacitance of SA is very small and can be neglected, as discussed in Figure 5; therefore, the specific capacitance of the PANI/SA composite derives from PANI contribution. Because the weight percentage of SA within this composite is 23 wt %, the discharge specific capacitance of the conducting polymer PANI within this composite is 2093 F g1. The galvanostatic charge/discharge curves of the PANI/SA and pure PANI electrodes at various current densities of 1, 3, 5, 10, and 15 A g1 are illustrated in Figure 8, and discharge capacitances with different current densities are shown in the inset of the figure. As a result, the capacitance values of PANI/SA are higher than that of pure PANI at various current densities. On the other hand, the specific capacitance decreases with the increase of the charge/discharge current density. From 1 to 10 A g1, the discharge capacitance of the PANI/SA electrode reduces from 1612 to 1375 F g1, i.e., a decrease by 15% of the initial capacitance, whereas the value for the pure PANI electrode decreases by 40%. When the current density rises to 15 A g1, the discharge capacitance of PANI/SA and PANI decreases by nearly 50 and 55%, respectively. This result is attributed to the fact that the redox reaction rates and the charge diffusion cannot follow the current density increase.42,43 The PANI/SA electrode is more favorable to the fast redox reaction rates and the rapid charge diffusion than the pure PANI, and therefore, the capacitance of PANI/SA is more stabile than pure PANI. The cycle-life test of the PANI electrode was performed at a scan rate of 100 mV s1 for 1000 cycles. As shown in Figure 9, there is a slight increase in the discharge specific capacitance values in the first cycles and, thereafter, the specific capacitance decreases with a further increase of the cycle times. The decrease slope of the PANI/SA composite is less than that of the pure PANI. After 1000 cycles, the discharge capacitance retention of the PANI/SA composite is 74% while that of the pure PANI only 6462
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Langmuir is 50%. The CV curves at the first charge/discharge test and at the 1000th cycle at a scan rate of 100 mV s1 are shown in the inset of Figure 9. Apparently, a large and wide area of the CV curves with evident three pairs of redox peaks at the beginning test implies the EDLCs and pseudo-capacitance performance of the electrodes. However, after 1000 cycles, the redox peaks become weak. At the cycle-life test, the dope or dedope of Hþ into or from the PANI chains results in the swelling and shrinkage of the nanostructured conducting polymer.44 In comparison to the pure PANI electrode, the better cycle life of PANI/SA may be mainly caused by the intermolecular interaction between the SA template and the PANI chains, which restricts the change of the nanostructure at the cycle-life test.
’ CONCLUSION In summary, we have successfully fabricated uniform PANI/ SA nanofibers with the diameter of 50100 nm by the templateinduced method, forming mat-like networks. The nanostructured PANI/SA electrode demonstrates a good reversible stability and fast response to oxidation/reduction on high current changes and an excellent electrochemical discharge capacitance as high as 2093 F g1. In comparison to the pure PANI electrode, PANI/SA exhibits an enhanced retention life, caused by the intermolecular interaction between the SA template and the PANI chains. The outstanding electrochemical characteristic is attributed to the nanostructured electrode materials, which generate a high electrode/electrolyte contact area and short path lengths for electronic transport and electrolyte ions. This facile approach provides an extensive and facile route to prepare supercapacitive electrode materials using conducting composites. ’ ASSOCIATED CONTENT
bS
Supporting Information. Formation mechanism of PANI/SA nanocomposites (S1), elemental analysis (F2), nitrogen adsorption/desorption test (S3), Cycle-life stability (S4), and impedance spectra of PANI and PANI/SA electrodes (S5). This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Telephone/Fax: 0086-21-67792854. E-mail:
[email protected].
’ ACKNOWLEDGMENT Financial support of this work is provided by the National Natural Science Foundation of China (NSFC) (50873021), the Shuguang Plan (09SG30), the Shanghai Leading Academic Discipline Project (B603), and the 111 Project (111-2-04). We thank Prof. Bin Ding and Huifang Chen for the BET measurement and elemental analysis, respectively. ’ REFERENCES (1) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; van Schalkwijk, W. Nat. Mater. 2005, 4, 366. (2) Guo, Y. G.; Hu, J. S.; Wan, L. J. Adv. Mater. 2008, 20, 2878–2887. (3) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845. (4) Frackowiak, E. Carbon 2001, 39, 937. (5) Chmiola, J.; Yushin, G.; Gogotsi, Y.; Portet, C.; Simon, P.; Taberna, P. L. Science 2006, 313, 1760.
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