Polyaniline

Research Article pubs.acs.org/journal/ascecg

Biomass-Based Porous N‑Self-Doped Carbon Framework/Polyaniline Composite with Outstanding Supercapacitance Yijie Hu, Xing Tong, Hao Zhuo, Linxin Zhong,* and Xinwen Peng* State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou 510641, People’s Republic of China

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S Supporting Information *

ABSTRACT: Composites combining electrostatic charge accumulation and faradic reaction mechanisms are especially attractive high-performance supercapacitor electrodes for electrochemical energy storage. Up to now, it is difficult to prepare low-cost carbon composites from renewable resources. In this work, an outstanding and low-cost composite was fabricated by using sustainable N-self-doped carbon framework as a hierarchical porous carbon substrate from renewable resource. The N-self-doped carbon framework was fabricated from chitosan via a facile yet unique self-assembly and ice template method without any physical or chemical activation, and exhibited hierarchical porous structure. This texture not only allowed the efficient infiltration and uniform coating of polyaniline (PANI) in the inner network but also permitted a rapid penetration and desorption of electrolytes. Due to short diffusion pathway, uniformly coating of PANI, and high accessibility of PANI to electrolytes, the composite electrode had a very high supercapacitance of 373 F g−1 (1.0 A g−1) and excellent rate capability (275 F g−1, 10 A g−1) in a three-electrode system. The symmetric supercapacitor also showed a supercapacitance of high up to 285 F g−1 (0.5 A g−1), and a very high energy density of 22.2 Wh kg−1. Furthermore, the composite also presented a good cycling stability. KEYWORDS: Chitosan, Carbon composite, Polyaniline, Supercapacitor

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INTRODUCTION Supercapacitors are promising electrochemical energy storage devices that can be utilized to fulfill some critical requirements in energy storage applications. According to the charge storage mechanism, supercapacitors can be divided into electrical double layer capacitors (EDLCs) and pseudocapacitors. Generally, the capacitance of EDLCs comes from the accumulation of ionic charges at the electrode/electrolyte interface, while the pseudocapacitance is due to the faradic reactions occurring at the electrode/electrolyte interface. Up to now, various carbon materials, such as activated carbon,1 carbon aerogel,2 carbon nanotube (CNT),3 carbon fiber,4 and graphene,5,6 have been conducted to fabricate EDLCs. These carbon materials show various advantages such as high specific surface area, excellent conductivity, physicochemical stability, low density, and low cost, making them ideal alternatives for EDLCs. And especially, nitrogen-doped (N-doped) carbons such as N-doped graphene,7 and N-doped CNT,8 possess more electrocatalytic active sites, higher electrical conductivity, and better surface hydrophilicity, and thus facilitating charge transfer, storage, and electrolyte/electrode interaction.9 However, the crisis of fossil fuel reserve and increasing awareness for pollution control are the main concerns for most of these carbon materials. Renewable resources such as lignin,10,11 cellulose,12 chitin and chitosan,2,13 protein14 are ideal © 2017 American Chemical Society

alternatives for fabricating porous carbons due to their sustainability, biocompatibility, low cost, and abundance. Among these biomass resources, chitin and its derivative chitosan are the most abundant and low-cost N-containing resources on the earth. They are the most promising raw materials for cost-efficiently and sustainably manufacturing Nself-doping porous carbons in large scale for supercapacitors.15−17 In spite of various advantages, the capacitance of carbon materials need to be further improved to meet the requirement of high-performance supercapacitors in the future. Conductive polymers such as polyaniline (PANI),18,19 polythiophene (PTH),20 and polypyrrole (PPy)21,22 are usually used to fabricate electrode materials of pseudocapacitors. In particular, PANI shows great potential for supercapacitor applications owing to its low cost, excellent environmental stability, ease of synthesis, fast redox rate, relatively high level of electrical conductivity, and unusual doping−dedoping chemistry.23 However, the swelling and shrinkage of PANI due to the insertion/deinsertion process of the counterions causes significant volume change and destroys the backbone of polymer. This structure deterioration seriously weakens the Received: May 3, 2017 Revised: August 17, 2017 Published: September 1, 2017 8663

DOI: 10.1021/acssuschemeng.7b01380 ACS Sustainable Chem. Eng. 2017, 5, 8663−8674

Research Article

ACS Sustainable Chemistry & Engineering

aqueous solution, washed with ultrapure water, and finally freeze-dried. 1 g of the purified chitosan was added into 49 mL of NaOH/urea solution (with a mass ratio of NaOH:urea:H2O of 8:4:88), vigorously stirred for half an hour at room temperature, and then placed in a refrigerator and frozen at −20 °C for 4 h. The frozen solution was moved to an ice bath for further stirring for 1 h to obtain a transparent solution, and then chitosan hydrogel was formed when the chitosan solution was placed at room temperature. The chitosan hydrogel was washed with ultrapure water and freeze-dried. Finally, the chitosan aerogel was placed in a tube furnace under N2 flowing, heated to 200 °C at a heating rate of 5 °C min−1 and held for 2 h, then pyrolyzed to 800 °C at a heating rate of 3 °C min−1 and held for another 2 h to obtain N-self-doped carbon framework (CS). Synthesis of N-Self-Doped Carbon Framework CS/PANI Composites. 0.0003, 0.0006, or 0.0012 mol AN was added into 15 mL of 1.0 M HCl. Then, 0.15 g of CS was immersed in the AN solution and the mixture was shaken for 12 h in the dark. APS (the mol ratio of AN:APS was 1:1.25) in 15 mL of 1.0 M HCl was used as an initiator for the oxidative polymerization of AN at 0 °C for 12 h. The obtained CS/PANI composites were washed thoroughly with water, ethanol, and tert-butyl alcohol, and then freeze-dried. The loadings of PANI on CS, calculating from mass difference before and after polymerization, were 16% (0.0003 mol), 33% (0.0006 mol), and 64% (0.0012 mol). The obtained N-self-doped carbon framework CS/ PANI composites were denoted as CS/PANI16%, CS/PANI33%, and CS/PANI64%, respectively. Pure PANI was also prepared under the same condition. Characterizations. X-ray diffraction (XRD) patterns were monitored by a Bruker D8 diffractometer using Cuα radiation (λ = 0.154 18 nm) as an X-ray source. FT-IR spectra were recorded on a Vector 33 infrared spectrum instrument (Bruker Corporation, Germany). The scan range was 400−4000 cm−1. The morphology was observed from transmission electron microscopy (TEM, JEM2100F) and scanning electron microscopy (SEM, Merlin, Zeiss). Raman spectra were recorded on a Raman spectrometer (LabRAM ARAMIS, H.J.Y.) operating with 532 nm laser. X-ray photoelectron spectra (XPS) were recorded on Thermo Scientific ESCALAB 250Xi spectrometer with an exciting source of Al Kα (1286.6 eV). Electrochemical Measurements. To evaluate the electrochemical performances of the as-prepared CS/PANI, cyclic voltammetry (CV) curves were obtained in the potential range of −0.1−+0.8 V, and charge−discharge measurements were completed at current densities of 1.0−10 A g−1 over a voltage range of −0.1−+0.8 V using a three-electrode system in 1.0 M H2SO4. Ag/AgCl electrode was used as the reference electrode and Pt wire was used as the counter electrode. The monolithic composites were cut into small slices with a dimension of about 1 mm × 10 mm × 10 mm and then pressed onto titanium meshes as working electrodes. All electrochemical experiments were carried out with a CHI 660E Electrochemical Workstation (CH Instruments, China) at the room temperature. The gravimetric specific capacitances of the electrodes at various scan rates were calculated on the basis of CV curves according to eq 1:

charge/discharge cycle life and consequently limits its application in supercapacitors.21 In order to overcome these drawbacks of EDLCs and pseudocapacitors, carbon/PANI composites combining electrostatic charge accumulation and faradic reaction mechanisms are proposed, such as graphene/PANI,24,25 carbon nanotube/ PANI,26 carbon nanofiber/PANI,27 carbon cloth/PANI,28 and graphene/PANI/CNT.29 These composites show both high specific capacitance and good cycle stability, and are ongoing interest. The high cost and complex synthesis of these carbons, however, limit their practical applications. Therefore, alternative carbonaceous materials derived from renewable and abundant biomass are especially attractive for fabricating carbon/PANI composite electrodes. For the synthesis of such kind of composites, the pore size of carbon should be desirable for the infiltration and polymerization of aniline on the substrate. However, the present renewable resource-derived carbons are almost obtained by chemical activation and thus are mainly composed of micropores and mesopores. Although the specific surface area is high, the small-dimension pores disfavor the infiltration and polymerization of aniline in the inner network of activated carbons. Therefore, it is still a big challenge to fabricate highperformance carbon/PANI composites from renewable resources. On the contrary, 3D hierarchical porous carbon with large amounts of macropores, mesopores, and micropores is an ideal support to synthesize high-performance carbon/PANI composite, because such porous structure is very beneficial for the diffusion of aniline and to provide enough space for the polymerization of aniline on carbon surface. Recently, great efforts have been devoted to develop various 3D carbon/PANI composites for supercapacitor application, such as porous carbon/PANI,30 porous graphene/PANI,31 RGO/PANI,32 CNF/CNT/PANI,33 graphene/CNT/PANI.34 Although these composites showed excellent supercapacitance properties, the high-cost carbon materials and unsustainable way limit their future applications. This work made the first attempt to fabricate biomass-based N-self-doped carbon framework/PANI composite with outstanding supercapacitance performance. The biomass-based Nself-doped carbon framework with large amounts of micropores, mesopores, and macropores was fabricated from chitosan by using a facile and unique self-assembly and sequent ice template method. The hierarchical porous structure not only facilitates the infiltration and uniform coat of PANI in the inner network but also allows ions efficiently diffusing through the carbon network. Furthermore, these hierarchical pores are expected to make PANI highly available to electrolyte ions. This structure and the significant synergistic effect of electrostatic charge accumulation and faradic reaction resulted in very high specific capacitances of 373 F g−1 for threeelectrode system and 285 F g−1 for two-electrode system, an excellent energy density of 22.2 Wh kg−1 (713 W kg−1) and rate capability, as well as good cycling stability.

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C=

I Δt mΔV

(1) −1

where I is the current (A), ν is the scan rate (V s ), ΔV is the applied potential window (V), and m is the mass of the working electrode active material (g). The specific capacitances of the electrodes at different current densities were calculated basing on the galvanostatic charge/discharge curves according to eq 2:

EXPERIMENTAL SECTION

Materials. Chitosan (deacetylation ≥85%, viscosity 200 cps), ammonium persulfate (APS), and aniline (AN) were purchased from Aladdin, China. Other reagents were of analytical grade and used without purification. Preparation of N-Self-Doped Carbon Framework. 2 g of chitosan powder was dissolved in 100 g of aqueous acetic acid (2%, v/ v), and the solution was neutralized by 2 wt % sodium hydroxide

C=

I Δt mΔV

(2)

where I is current loaded (A), V is the potential (V), m is the mass of active material (g), Δt is the discharge time(s), and ΔV is the range of potential (V). Furthermore, the symmetric supercapacitor (SSC) fabricated from CS/PANI composites was also tested in 1 M H2SO4. The SSC cell was 8664


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ACS Sustainable Chemistry & Engineering fabricated from CS/PANI composites with similar weight loadings (the working electrodes were prepared as mentioned above). In order to find the maximum voltage window, the operating potential window was carried out from 0.8 to 1.5 V. The energy density (E) and power density (P) of the supercapacitor were calculated by eqs 3 and 4:

E (Wh kg −1) = P (W kg −1) =

urea/H2O solution at low temperature and then the dissolved chitosan easily became gel due to its self-assembly as a result of the aggregation of chitosan chains (hydrogen bonds) in NaOH/urea/H2O solution at room temperature, resulting in a 3D porous architecture. The formation and growth of ice crystals during freezing step extruded chitosan chains aside into sheets, whereas the removal of these ice crystals led to a lamellar structure. Therefore, ice crystals acted as pore templates. In the presence of initiator APS, aniliniumions began to polymerize on the carbon sheet surface of carbon framework. The morphologies and structures of N-self-doped carbon framework CS and CS/PANI composites are shown in Figure 2 and Figure 3. N-self-doped carbon framework CS has a hierarchical porous structure with well-developed interconnected and open pores (Figure 2a). It is also found that there are some smaller pores and the carbon sheets are smooth (Figure 2b and Figure 3a). The mesopores and micropores will be further investigated by TEM. Such a hierarchical structure is expected to decrease the diffusion distance of ions and thus allows the fast transfer of ions throughout the material. And this open porous structure will also provide abundant interconnected channels and enough space for the infiltration and polymerization of AN. As shown in Figure 2c,e,g, the carbon framework maintains its hierarchical structure when PANI was coated on the carbon sheets by in situ polymerization. At a low AN concentration, the carbon sheet surface of CS/PANI16% (Figure 2d and Figure 3b) shows no significant difference from that of carbon framework CS, indicating that the PANI layer is very thin. As AN concentration increasing, however, the surface of carbon sheet becomes rough and is composed of interconnected nano PANI particles (Figure 2f and Figure 3c). When AN concentration further increasing, the PANI particles grew (with a diameter of around 10 nm) and uniformly distributed on the carbon sheet surface (Figure 2h and Figure 3d). And the height of the particles ranges from 1 to 3 nm (Figure 3d). The as-formed PANI acted as “seeds” for the following polymerization to reduce the interfacial energy. Then PANI gradually grew on the seeds along vertical direction and finally PANI nanorod arrays formed. The π−π stacking force between the carbon sheet and the aromatic rings of PANI was

1 C ΔV 2 2

(3)

3.6

36000 × E t

(4) −1

where C is the total capacitance of the two-electrode cell (F g ), ΔV is the effective potential range during the discharging process (V), and t is the discharging time (s). To prepare simple supercapacitor unit, CS/PANI33% was ground into powder and mixed with small amount of water to obtain slurry. The slurry was then loaded on a nickel foam, dried, and compressed (1 MPa pressure) to obtain an electrode. Two such electrodes were assembled into a supercapacitor unit with a filter paper as a separator between them. Finally, the device was fixed between two sheets of glass, and Na2SO4 was added into the two electrodes until the filter paper was wetted completely.

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RESULTS AND DISCUSSION Physicochemical Structures. The preparation procedure of N-self-doped carbon framework CS/PANI composites is shown in Figure 1. Chitosan could be dissolved in NaOH/

Figure 1. Illustration of fabricating N-self-doped carbon framework CS/PANI composites.

Figure 2. SEM images of N-self-doped carbon framework CS (a and b), CS/PANI16% (c and d), CS/PANI33% (e and f), and CS/PANI64% (g and h). 8665


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Figure 3. AFM images of CS (a), CS/PANI16% (b), CS/PANI33% (c), and CS/PANI64% (d).

Figure 4. TEM images of N-self-doped carbon framework CS (a−c), CS/PANI33% (d−f), and CS/PANI64% (g−i).

microstructure (Figure 4a,d), which well agrees with the SEM images. The transparent sheets indicate that both carbon sheet and PANI layer are thin. High-resolution images (Figure 4b,e) reveal a continuous porous structure with numerous micropores (less than 3 nm) for both CS and CS/PANI33%. Carbon materials usually show well-developed mesopores and micropores.36 But the appearance of mesopores and micropores on the sheet of CS/PANI33% is unexpected because the sheet was

expected to contribute to induce the polymerization of PANI along vertical direction.35 These open networks will provide more effective transport continuity for ions and minimize the diffusion path of ions, as well as make PANI highly available to electrolyte ions. Figure 4 shows the typical TEM images of N-self-doped carbon framework CS, CS/PANI 33% , and CS/PANI 64% composites. CS and CS/PANI33% show a typically layer-like 8666


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Figure 5. FT-IR spectra of N-self-doped carbon framework CS and CS/PANI33% (a), XRD patterns (b), and Raman spectra (c) of N-self-doped carbon framework CS and CS/PANI33%.

Figure 6. XPS surveys of carbon framework and CS/PANI33% (a), high-resolution C 1s spectra of CS (b) and CS/PANI33% (c), high-resolution N 1s spectra of CS (d) and CS/PANI33% (e).

the π-bonded surface of CS is expected and enhances the charge-transfer kinetics between the PANI and CS. Figure 5a shows the FT-IR spectra of CS and CS/PANI33%, recorded in the range of 500−4000 cm−1. The main peaks of N-self-doped carbon framework CS are located at 3457 and 1637 cm−1, attributed to stretching peaks of the −OH and C O, respectively.37 And the broad peak at 1200−1000 cm−1 (observed at 1166 cm−1) in CS is attributed to CO in C OC functional groups and CN stretching vibration.32 For CS/PANI33% composite, however, several new peaks attributed to PANI appear in the spectrum. The CC stretching vibration spears at 1568 and 1495 cm−1 for the quinoid (N) and benzenoid (B) rings, respectively. The aromatic CN (CN+) stretching vibration occurs at 1298 cm−1, accompanying with a shoulder peak at 1230 cm−1, is assigned to the stretching vibration of NBN ring.38 These peaks are typical vibrations of emeraldine salt.39 And the peaks at 796 and 1103 cm−1 are attributed to in-plane and out-of-plane CH bending, respectively.38 There are also many low-intensity

covered with PANI (as revealed by SEM). It is supposed that the PANI layer is so thin that electron beam can traverse across it, and thus the mesopores and micropores on the carbon sheet could be observed. This thin PANI may allow electrode ions penetrating into the inner carbon network. There are also some lattice fringes (aligned graphitic layers) in Figure 4c,f, which correspond to the graphite (002) plane, implying partial graphitic structure of CS. As for CS/PANI64% shown in Figure 4g−i, the sheets are less transparent and fewer mesopores and micropores can be observed under electron beam irradiation, indicating that the PANI layer is thick and it is difficult for the electron beam to pass through. This result is consistent with the result of SEM. Aromatic compounds, such as PANI, will interact strongly with the basal plane of the graphitic carbon through π−π stacking interaction.24,29 The strong π−π interaction is important to improve the cycling stability of composite.34 Therefore, strong interaction between the quinoid ring of conjugated PANI and 8667


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Figure 7. CV curves at 20 mV s−1 (a), charge/discharge curves at 1.0 A g−1 (b), and specific capacitances of CS, CS/PANI composites, and pure PANI at 1.0 A g−1 (c).

peaks ranging from 780 to 580 cm−1, which can be assigned to the vibrations of the CH bonds in the benzene rings.40 These results demonstrate the successful polymerization of PANI on the carbon sheet surface of CS. The XRD pattern of N-self-doped carbon framework CS shows a broad and low intensity diffraction peak at around 2θ = 24° (Figure 5b), which is attributed to the graphite (002) plane, suggesting a low degree of graphitization.41 Because the (200) crystal plane of PANI in its emeraldine salt form appears at 2θ = 25.2°,42,43 CS/PANI33% composite shows a similar diffraction feature but a stronger intensity. The Raman spectra of CS and CS/PANI33% display two prominent peaks at 1323 and 1570 cm−1, corresponding to the well-defined D and G bands (Figure 5c), respectively. The D band is associated with a double-resonance effect of disordered carbonaceous structure, whereas the G band corresponds to the ordered graphite inplane vibrations with E2g symmetry. It is clearly that the intensity of the D band is higher than that of G band for both CS and CS/PANI33% composite, indicating a higher amorphous carbon concentration in CS and CS/PANI33%. It is obvious that the intensities of D and G bands in CS/PANI33% are higher than the corresponding ones in CS. Theoretically, the characteristic Raman bands of PANI appearing at 1516−1595 cm−1 represent the CC stretch of the quinoid and benzenoid rings, whereas bands at 1340−1390 cm−1 are assigned to the CN+ stretching of polaronic forms of PANI.44 Therefore, the characteristic Raman bands of PANI overlap with those of CS, resulting in a spectrum with much higher intensity. This result also well agrees with the results from XRD. Figure S1−S1 shows the TGA curves for CS, CS/PANI composites and pure PANI. The weight loss of CS is ignorable with a weight retention of 90.8% when the temperature rises to 695 °C, demonstrating an excellent stability. As the PANI content increases, the stability of composites decreases, with weight retentions of 86%, 80% and 73.5% for CS/PANI16%, CS/PANI33%, and CS/PANI64%, respectively. Pure PANI, however, has a weight loss of high up to 50%, indicating a serious thermal degradation at high temperature. XPS was employed to evaluate the chemical nature of the carbon and nitrogen elements, as shown in Figure 6. Figure 6a shows that C, O, and N present in both CS and CS/PANI33%. The high-resolution C 1s spectra of the two samples (Figure 6b,c) can be fitted to five subpeaks that are related to aromatic CC/CC (284.5 eV), CN (285.4 eV), CN+/CO (286.4 eV), CONO2/CO (287.8 eV), and OCO (289.0 eV).45 N 1s of N-self-doped carbon framework CS can be deconvoluted into four subpeaks, as shown in Figure 6d. These peaks are related to pyridinic N (N-1 at 398.0 eV),

pyrrolic or pyridonic N (N-2 at 399.7 eV), quaternary N (N-3 at 400.8 eV), and oxidized N (N-4 at 402.5 eV),46 confirming the doped N in the carbon network. As for CS/PANI33% composite (Figure 6e), the N 1s region exists in three different electronic states, and is decomposed into 399.0 eV (benzenoid amine, −N= ), 400.1 eV (quinoid amine, −NH−), 401.2 and 403.3 eV (nitrogen cationic radical, N+). Figure S2 shows the N2 adsorption−desorption isotherms and pore size distributions of CS and CS/PANI33%. The two samples display type-IV isotherm pattern with a type-H1 hysteresis loop at a high relative pressure, indicating that mesopores and macropores exist in the materials. As compared with CS/PANI33%, CS shows a sharp increase at low relative pressure, implying a large number of micropores. The same conclusion can be drawn from the pore size distribution in Figure S2b. Both CS and CS/PANI33% have micropores (50 nm), indicating hierarchical porous structures. And especially, CS shows a much higher micropore volume, suggesting that large amounts of micropores were covered by PANI after polymerization, resulting in a lower specific surface area of 45 m2/g (464 m2/g for CS). Electrochemical Performances. Figure 7a illustrates the CV curves of all samples in the potential range from −0.1 to +0.8 V in 1.0 M H2SO4 at a scan rate of 20 mV s−1. The N-selfdoped carbon framework CS shows a quasi-rectangular shape CV, indicating an excellent electrical double layer capacitance (reversible adsorption and desorption of the ions). Pure PANI exhibits a pseudocapacitance characteristic that contains two pairs of redox peaks A/A′ and B/B′. Peaks A and A′ are attributed to the redox transition of PANI between a semiconductive state (leucoemeraldine form) and a conductive sate (polaronic emeraldine form), whereas peaks B and B′ are due to the emeraldine−pernigraniline transformation.47 However, the small area at low potential limits its overall capacitance. By contrast, the CV traces of the CS/PANI composites remain rectangular with clear redox peaks not only confirming high activity and stability of PANI but also indicating that the energy storage is composed of double layer capacitance and faradic pseudocapacitance. The larger areas of CS/PANI33% and CS/PANI64% at the same scan rate further suggesting their higher specific capacitances. The galvanostatic charge/discharge curves of these samples at a current density of 1.0 A g−1 are shown in Figure 7b. The curve of N-self-doped CS exhibits an almost isosceles triangle, suggesting an excellent reversible behavior of the ideal capacitor. It should be noted that the curve of PANI is not triangular, but shows a straight line up and down in a certain 8668


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Figure 8. Electrochemical behaviors of CS and CS/PANI33%: CV curves at different scan rates (a and b), specific capacitances as a function of scan rate (c), charge−discharge curves at different current densities (d and e), specific capacitances as a function of current density (f), and Nyquist plots of CS and CS/PANI33% (g). Cycling stability of CS, CS/PANI33%, and pure PANI (h).

potential, and the effective voltage window ranges from 0.16 to 0.8 V, which limits its application in a wide potential window. The charge/discharge curves of CS/PANI composites are approximate isosceles triangles with somewhat distortion,

indicating a good reversibility during the charge/discharge processes and again implying the double layer capacitance and faradic pseudocapacitance behaviors due to a faradic reaction of active functional groups in PANI. These results are consistent 8669


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resistance of CS/PANI33% can be attributed to the excellent conductivity of CS and PANI, as well as the hierarchical porous structure that can shorten the path of external electrolyte to the interior network. Furthermore, comparing with an only 40% capacitance retention of pure PANI after 5000 cycles, CS and CS/PANI33% show much better cycling stabilities, with capacitance retentions of high up to 110% and 80%, respectively, as shown in Figure 8h. The slightly increasing capacitance after 5000 cycles may be ascribed to that more micropores can be utilized to storage ions during the long-cycle duration. Therefore, CS/PANI33% show an excellent cycling stability. Table 1 shows the comparison of various carbon/PANI composites in electrochemical capacitive performance. Acti-

with the CV results. The specific capacitances of CS/PANI16%, CS/PANI33%, and CS/PANI64% composites are 237, 373, and 348 F g−1 at a current density of 1.0 A g−1 (Figure 7c), which are much higher than that of CS (200 F g−1), with increases by 20%, 89%, and 77%, respectively. Although a higher PANI loading is significantly beneficial to increase the specific capacitance, too high PANI loading, for example, 64% PANI cannot to further boost the specific capacitance. It is supposed that excessive PANI may slow down the charge transport to the underlayer of carbon sheets, leading to a lower capacitance. It should be noted that the specific supercapacitance of CS/ PANI64% is very close to that of pure PANI. To further demonstrate the superior electrochemical properties of CS/PANI33%, its cyclic voltammetry performance at different scan rates and charge/discharge property at different current densities were investigated, as shown in Figure 8. The CV curve of N-self-doped CS exhibits an approximately rectangular shape at a low scan rate and maintains a quasirectangular shape even at a high scan rate of 200 mV s−1 (Figure 8a), indicating that a high conductivity ensures an excellent charge propagation. As for CS/PANI33% (Figure 8b), the cathodic/oxidation peaks A and B shift positively whereas the anodic/reduction peaks A′ and B′ shift negatively when the potential scan rate increases from 5 to 200 mV s−1, which is mainly due to the resistance of the electrode.24,42 The variations of specific capacitances of CS and CS/PANI33% with the current density are plotted in Figure 8c. The specific capacitances of CS and CS/PANI33% slightly decrease with the increase of scan rate. When the scan rate increases from 5 to 200 mV s−1, the specific capacitances of CS and CS/PANI33% composite decrease from 432 to 279 F g−1 and from 740 to 513 F g−1, respectively, indicating an excellent rate capability. The charge/discharge curves of CS and CS/PANI33% at different current densities (Figure 8d,e) almost maintain the same shape in the potential range from −0.1 to +0.8 V, indicating their sustainable behaviors in a broad current range. Poor accessibility of electrode to electrolyte ions usually leads to a decline in capacitance at higher scan rates for electrochemical energy storage devices. Nevertheless, the CS/PANI33% composite possessing a synergistic impact of both CS and PANI shows high capacitance. As shown in Figure 8f, the specific capacitance of CS/PANI33% is 373 F g−1 at 1.0 A g−1, and remains 275 F g−1 even at a current density of 10 A g−1, indicating that the hierarchical porous structure guarantees a rapid transfer of ions from outer part into the inner network. The symmetric charge/discharge curves suggest a high electrochemical reversibility.48 The voltage drop of CS/ PANI33% at the initiation of the discharge is extremely small even at a high current density of 10 A g−1, indicating a very low equivalent series resistance (ESR). Figure 8g shows the electrochemical impedance spectroscopic (EIS) of CS and CS/PANI33%. In the low-frequency region, both CS and CS/PANI33% have a steep capacitive spike with an almost 90° angle, indicating a good capacitive behavior (vertical line for an ideal capacitor). The real axis intercept corresponds to the equivalent series resistance (ESR), whereas the radius of the semicircle impedance of electrode material commonly represents charge-transfer resistance between electrode material and the electrolyte. The ESRs for CS and CS/PANI33% are 1.12 and 0.82 Ω, respectively, suggesting a lower resistance for CS/PANI33%. Both CS and CS/PANI33% display a small semicircle impedance at a high frequency region, revealing their lower charge-transfer resistances. The lower

Table 1. Comparison of Various Carbon/PANI Composites in Electrochemical Capacitive Performance carbon/PANI compositesa activated carbon/PANI activated carbon/PANI graphene/PANI graphene/PANI graphene/PANI β-graphene/PANI RGO/PANI RGO/PANI RGO/PANI NH2-RGO/PANI CNT/PANI CNT/CNF/PANI carbon cloth/PANI MnFe2O4/CB/PANI MnFe2O4/grephene/PANI CS/PANI

conditionb 1.0 1.0 2.0 2.0 1.0 1.0

M M M M M M

H2SO4 H2SO4 H2SO4 H2SO4 H2SO4 H2SO4

2.0 1.0 1.0 1.0 1.0 1.0 0.5 1.0 1.0

M M M M M M M M M

H2SO4 H2SO4 H2SO4 HCl H2SO4 H2SO4 H2SO4 KOH H2SO4

capacity (F g−1) 210, 316, 210, 321, 215, 307, 323, 360, 194, 388, 230, 315, 210, 205, 455, 373, 670,

20 mV s−1 0.5 mV s−1 1.0 A g−1 1.0 A g−1 1.0 A g−1 20 mV s−1 1.0 A g−1 1.0 A g−1 20 mV s−1 1.0 A g−1 1.0 A g−1 1.0 A g−1 1.0 A g−1 1.0 A g−1 0.2 A g−1 1.0 A g−1, 20 mV s−1

ref. 49 50 51 52 53 54 55 32 56 57 34 33 58 59 60 this work

a

GO, RGO, CNT, CNF, and CB refer to graphene oxide, reduced graphene oxide, carbon nanotube, carbon nanofiber, and carbon black, respectively. bThree-electrode test system.

vated carbon/PANI composites show the lowest capacitance values,49,50 which is mainly due to the small pores (micropores and/or mesopores) and the narrow distribution of pore size in activated carbons. The univocal micropores and/or mesopores disfavor the infiltration and polymerization of AN inside the activated carbons, or make the active sites poorly accessible to electrolyte ions. Graphene/PANI,51−53 reduced graphene oxide (RGO)/PANI,32,55−57 and CNT/PANI,33,34 are commonly fabricated via in situ oxidative polymerization of AN, and PANI could be evenly coated on the surfaces of the carbon blocks. The high accessibility of PANI to electrolyte ions imparts higher supercapacitance to these composites. 3D RGO/PANI with hierarchical structure via postcoating method shows a very high capacitance due to various interconnected macropores, mesopores, and micropores.34 These results confirm the important role of hierarchical porous structure in fabricating high-performance carbon/PANI composite electrode. In this work, CS/PANI33% composite has a high specific capacitance (373−275 F g−1 at current densities of 1.0−10 A g−1) that is not only superior to those of activated carbon/PANI composite49,50 but also superior or comparable to those of graphene/PANI,51−54 RGO/PANI,32,55−57 CNT/PANI,33,34 8670


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Figure 9. Two-electrode capacitive performances of the CS/PANI33%//CS/PANI33% capacitors in 1.0 M H2SO4 electrolyte. (a) CV curves measured at different potential windows at a scan rate of 20 mV s−1. (b) CV curves at different scan rates with a potential window of 0−1.5 V. (c) Galvanostatic charge−discharge curves at a current density of 1.0 A g−1. (d) Galvanostatic charge−discharge curves at different current densities with a potential window of 0−1.5 V.

Figure 10. (a) Specific capacitances for the SSC. (b) Ragone plot (power density vs energy density) for the SSC. The values reported for other carbon/PANI composite materials are also given here for a comparison. (c) Photograph of a series of luminous diodes lit by two simple supercapacitors in series. (d) Photograph of driving electric fan by one simple supercapacitor.

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ACS Sustainable Chemistry & Engineering carbon cloth/PANI,58 and MnFe2O4/CB/PANI,59,60 demonstrating an excellent supercapacitance. Figure 9a shows the CV curves of the as-fabricated SSC device with different operating potential windows varying from 0.8 to 1.5 V. The SSC exhibits a good capacitive behavior with significant oxidation and reduction peaks. It should be noted that, unlike the three-electrode system, there is only one cathodic/oxidation peak C (at around 0.4 V) and one anodic/ reduction peak C′ (at around 0.2 V), suggesting the feeble faradaic reactions and the predominance of an electrical doublelayer capacitance.29 As shown in Figure 9b, the CV curves at 0− 1.5 V from different scan rates retain a similar shape, indicating a good charge propagation. The discharge profiles show no significant IR drop even at 0−1.5 V (Figure 9c,d), suggesting a good reversibility and high electrical conductivity,33 which confirms the advantage of the hierarchical porous carbon CS supported PANI. The specific capacitances of the supercapacitor from SSC at different current densities are presented in Figure 10a. The super capacitance of CS/PANI33% at 0.5 A g−1 is as high as 285 F g−1, which ranges the highest value among carbon/PANI composite materials.54,56,59−63 The energy density (E) and power density (P) calculated based on the total mass of active materials on both the cathode and the anode are shown in Figure 10b. The SSC exhibits a high energy density of 22.2 Wh kg−1 at 713 W kg−1, and remains 12.2 Wh kg−1 at a high power density of 7314 W kg−1, which also rang the highest value among carbon/PANI composite materials in aqueous electrolyte.30,31,33,54,56,58,60,61,64 To further illustrate the practical application of the CS/PANI electrode material, two tandem devices were constructed, as shown in Figure 10c,d. First, the tandem device can light up a series of luminous diodes for more than 5 min. And more impressively, one supercapacitor unit could drive a little electric fan for 30 s. These results indicate the potential application of CS/PANI material for high performance energy storage systems. The excellent super capacitance of CS/PANI33% composite can be attributed to (1) the hierarchical porous structure with various macropores, mesopores, and micropores that not only permits the complete infiltration of AN and uniformly coating of PANI throughout of the carbon network; (2) the good conductivity of N-self-doped carbon framework that provides an excellent conductive substrate to PANI to guarantee highly efficient charge transport and avoid significant degradation of PANI; (3) the hierarchical porous architecture offers high accessibility of active sites inside the composite to electrolyte ions and short pathways for the fast transports of ion and charge; (4) the combination effect of the double layer capacitance of N-self-doped carbon framework and the pseudocapacitive nature of PANI.

performance of N-self-doped carbon framework/PANI composite can be ascribed to the uniformly coating of PANI on the carbon sheet surface of N-self-doped carbon framework, improved electronic conductivity, low diffusion pathway for charge percolation, high availability of PANI to electrolytes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01380. TGA thermograms for CS, CS/PANI composites and pure PANI, N2 adsorption−desorption isotherms and pore size distributions calculated from the adsorption isotherms of CS and CS/PANI33% (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Linxin Zhong). Tel./Fax: +86 87111861. *E-mail: [email protected] (Xinwen Peng). Tel./Fax: +86 87111861. ORCID

Linxin Zhong: 0000-0002-0639-4333 Xinwen Peng: 0000-0002-4575-256X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by State Key Laboratory of Pulp and Paper Engineering (2016C15), National Natural Science Foundation of China (21506068), Pearl River S & T Nova Program of Guangzhou (201506010003), Guangdong natural Science Foundation (2014A030310319 and 2016A030313487), Science and Technology Planning Project of Guangzhou City (201504010033), and Special Funds for the Cultivation of Guangdong College Students’ Scientific and Technological I n n o v a t i o n ( “ C l i m b in g P r o g r a m ” S p e c i a l F u n d , pdjh2017b0044).

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REFERENCES

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CONCLUSIONS N-self-doped carbon framework from biomass chitosan is an excellent hierarchical porous architecture for fabricating carbon/PANI composite material. The unique hierarchical porous structure provides an ideal substrate for the infiltration and polymerization of aniline. The obtained N-self-doped carbon framework/PANI composite electrode exhibits very high specific capacitances of 373 F g−1 for three-electrode system and 285 F g−1 for two-electrode system, an excellent energy density of 22.2 Wh kg−1 (713 W kg−1) and rate capability, as well as good cycling stability, outperforming other many carbon/PANI composites. The excellent electrochemical 8672


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