Three-dimensional polypyrrole-derived carbon nanotube framework for dye adsorption and electrochemical supercapacitor

Accepted Manuscript Title: Three-dimensional polypyrrole-derived carbon nanotube framework for dye adsorption and electrochemical supercapacitor Authors: Shengchang Xin, Na Yang, Fei Gao, Jing Zhao, Liang Li, Chao Teng PII: DOI: Reference:

S0169-4332(17)31126-1 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.109 APSUSC 35790

To appear in:

APSUSC

Received date: Revised date: Accepted date:

20-1-2017 6-4-2017 15-4-2017

Please cite this article as: Shengchang Xin, Na Yang, Fei Gao, Jing Zhao, Liang Li, Chao Teng, Three-dimensional polypyrrole-derived carbon nanotube framework for dye adsorption and electrochemical supercapacitor, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.109 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Three-dimensional

polypyrrole-derived

carbon

nanotube

framework for dye adsorption and electrochemical supercapacitor

Shengchang Xin1, Na Yang1, Fei Gao1, Jing Zhao1,*, Liang Li2,*, Chao Teng3,*

1

School of Life Sciences, State Key Laboratory of Coordination Chemistry and Collaborative

Innovation Center of Chemistry for Life Sciences, Institute of Chemistry and BioMedical Sciences, Nanjing University, Nanjing 210093, P. R. China. 2

School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073,

P. R. China. 3

Guangdong Provincial Key Laboratory of Nano-Micro Materials Research, School of

Chemical Biology & Biotechnology, Peking University Shenzhen Graduate School, Shenzhen 518055, P. R. China

* Corresponding author. E-mail address: [email protected] (L. Li)

1

Graphical abstract

Highlights 

Three-dimensional polypyrrole-derived carbon nanotube frameworks are prepared.



They display outstanding absorption capacity (609 mg g-1) towards methylene blue.



They possess high specific capacitance (167 F g-1) and good rate capability (64%).



They have excellent cycling performance with no capacitance loss over 1000 cycles.

ABSTRACT

Three-dimensional carbon nanotube frameworks have been prepared via pyrolysis of polypyrrole nanotube aerogels that are synthesized by the simultaneous self-degraded template synthesis and hydrogel assembly followed by freeze-drying. The microstructure and composition of the materials are investigated by thermal gravimetric analysis, Raman spectrum, X-ray photoelectron spectroscopy, transmission electron microscopy, and specific surface analyzer. The results

2

confirm the formation of three-dimensional carbon nanotube frameworks with low density, high mechanical properties, and high specific surface area. Compared with PPy aerogel precursor, the as-prepared three-dimensional carbon nanotube frameworks exhibit outstanding adsorption capacity towards organic dyes. Moreover, electrochemical tests show that the products possess high specific capacitance, good rate capability and excellent cycling performance with no capacitance loss over 1000 cycles. These characteristics collectively indicate the potential of three-dimensional polypyrrole-derived carbon nanotube framework as a promising macroscopic device for the applications in environmental and energy storages.

Keywords: Polypyrrole; Carbon nanotubes; Framework; Dye adsorption; Electrochemical supercapacitor 1. Introduction Assembly of materials with macroscopic sizes from individual nanoscale objects is one of the most effective strategies to construct functional devices in the practical applications. Nowadays, remarkable efforts have been made to integrate nanoscale building blocks into organized hydrogels, aerogels, or mesoporous frameworks with three-dimensional structures [1]. Recently, conducting polymers, such as polythiophene, polypyrrole (PPy), polyaniline (PANi), and its derivatives, have attracted increasing interest in scientific and industrial fields due to easy synthesis, low cost, and adjustable redox activity. Great attention has been paid to develop various methods for the fabrication of conducting polymers hydrogels or aerogels with three-dimensional architectures [2–5]. Lu and co-workers prepared poly(3,4-ethylenedioxythiophene) hydrogels via the interaction between high-valent

3

metal ions with macromolecules with negative groups, such as poly(sodium 4-styrenesulfonate) or sulfonated polyaniline [6]. Zhang and co-workers synthesized self-crosslinker PANi hydrogels using aniline hydrochloric salt as the precursor [7]. Yu and co-workers fabricated PANi and PPy hydrogels in interfacial reaction with the aid of phytic acid as both the dopant and the gelator [8,9]. PPy nanotube hydrogels with controlled morphology were also obtained in the presence of methyl orange dye as the template in our previous work [10]. Their unique three-dimensional networks and characteristics, such as adsorption/desorption property, electrical conductivity, energy storage capacity, make it a suitable material as an adsorbent or electrode material for supercapacitors. PPy hydrogels have been used as an adsorbent to remove Cr(VI) from aqueous solutions [11]. PANi hydrogels doped by phytic acid was employed to remove methylene blue [12]. However, the low adsorption capacity of conducting polymer inhibits its efficient application as an adsorbent. In the field of supercapacitors, conducting polymer hydrogels or aerogels unfortunately exhibit poor cycle stability and rate capacity due to the rapid degradation of conducting polymers during sustained operation. Moreover, most of the obtained conducting polymer hydrogels or aerogels show weak mechanical strength. Therefore, it is still a big challenge to construct conducting polymer-based functional materials with desired structures and properties to realize their extensive potential applications. In this situation, transforming conducting polymers into the carbonaceous materials could be a good strategy to achieve an optimized property. Chen et al. synthesized the porous nitrogendoped carbon nanofibers by carbonization of carbonaceous nanofibers coated with PPy [13]. He et al. prepared carbon nanowhiskers wrapped-on graphitized electrospun nanofibers from the starting material of PANi nanowires [14]. Yang et al. demonstrated a template method for the 4

synthesis of ordered mesoporous carbons using PPy as a carbon precursor and FeCl3 as the oxidant [15]. Although the pioneering works have been reported in recent years, it is desirable to simply construct the three-dimensional conducting polymer-based functional materials with improved adsorption and supercapacitor performance. Herein, in this work, three-dimensional carbon nanotube frameworks have been prepared via pyrolysis of PPy nanotube aerogels that are synthesized by the simultaneous self-degraded template synthesis and hydrogel assembly followed by freeze-drying. Compared with PPy aerogel precursor, the as-prepared three-dimensional carbon nanotube frameworks exhibit higher mechanical strength, outstanding adsorption capacity towards organic dyes and better electrochemical capacitor performance.

2. Experimental 2.1 Synthesis of three-dimensional PPy-derived carbon nanotube frameworks In a typical procedure, 1 mmol of APS was dissolved in 20 mL of 5 mmol/L methyl orange aqueous solution. Then 1 mmol of pyrrole monomer was added to the above solution. After stirring for about 2 min, the solution was held without stirring to complete the reaction. After 24 h at room temperature, a black cylinder-like PPy hydrogel was produced. The obtained PPy hydrogel was further freeze-dried and transferred into a furnace for pyrolysis in argon atmosphere. The sample was heated to 800 oC for 5 h, and then cooled to room temperature to yield three-dimensional PPy-derived carbon nanotube frameworks.

5

2.2 Characterization Thermogravimetric analyses (TGA) were performed on a Netzsch STA 409 instrument at a heating rate of 10 oC/min in nitrogen atmosphere. Fourier transform infrared (FTIR) spectra were measured with a Nicolet 510 FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) was conducted with a Kratos XSAM800 spectrometer. The morphology of the sample was analyzed using a JEM-200CX transmission electron microscopy (TEM). The Brunauere-Emmette-Teller (BET) surface areas were measured with a Micromeritics ASAP 2010 instrument. Methylene blue, rhodamine B, congo red and orange G were used to explore the adsorption capacity of the as-prepared three-dimensional PPy-derived carbon nanotube frameworks. 2.0 mg of the framework was added into the dye solution (60 mL, 40 mg L-1). At the time intervals, 1.0 mL of the solution was taken out for measurements. UV-Vis spectrometer was used to measure the maximum absorbance of each dye to calculate the concentration of dye. The absorption capacity was evaluated using the following equation: Q=(C0-Ct)V/m

(1)

where Q (mg g-1) is the absorption capacity, C0 (mg L-1) and Ct (mg L-1) is the initial concentration and the concentration at time t, V (L) is the dye solution volume, and m (g) is the weight of the adsorbent. All of the electrochemical properties were studied in a three-electrode cell using an electrochemical workstation (CHI660D). A Pt wire and an Ag/AgCl electrode were used as the counter and reference electrode, respectively. The working electrode was obtained by mixing the synthesized electrode materials, acetylene black, and PTFE (mass ratio of 85:10:5). A small

6

amount of ethanol was added to get the homogenous slurry, and then pressed on a stainless steel net. It was allowed to dry at 65 oC in vacuum for one day. The active material in the working electrode was about 1.0 mg cm-2. The electrochemical performances were evaluated by cyclic voltammetry and galvanostatic charge-discharge measurements in 0.5 mol L-1 H2SO4 aqueous solution.

3. Results and discussion As we reported previously, PPy hydrogels could be prepared via the reactive self-degraded templates [10,16]. After freeze-drying, the obtained PPy aerogel remained the cylindrical shape, but it was fragile and easily broken, indicating that PPy aerogel had weak mechanical strength. Further through pyrolysis at 900 oC, the resultant PPy-derived carbon framework shrank in comparison with PPy aerogel precursor. TGA measurement was carried out to study the weight change during the pyrolysis of PPy aerogel precursor. As shown in Fig. 1, the initial weight loss below 200 oC was ascribed to the release of water from the sample. The second sharp weight loss in the temperature range from 200 to 600 oC was related to the removal of doping anions and the decomposition of PPy chains due to its thermal degradation. The weight of PPy-derived framework was decreased to 48% of the initial weight of PPy aerogel precursor. However, it was unexpected that PPy-derived carbon framework showed a satisfactory stiffness and it can support a balancing weight of about 1600 times of its own weight with little deformation (Fig. 2). The compressive stress-strain curves and rheological behaviors of PPy aerogel and PPy-derived carbon nanotube framework were shown in Fig. S1. The compressive strength of PPy-derived carbon nanotube framework (31.5 KPa) was much higher than that of PPy aerogel (10.8 KPa).

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The storage moduli (G’) of both samples were much larger than the corresponding loss moduli (G’’). Moreover, the storage modulus of PPy-derived carbon nanotube framework was larger than that of PPy aerogel. It might be mainly attributed to the strong interaction among carbon nanotubes in the contracted space and the change of microstructures after the pyrolysis of PPy aerogel. The chemical structure of the as-synthesized PPy-derived framework was analyzed by Raman and XPS. As shown in Fig. 3a, several characteristic absorption peaks at 1610, 1360, and 9301050 cm-1 were ascribed to ring-stretching mode, C=C backbone stretching and C-H in-plane deformation of pyrrole, respectively [17]. It confirmed the formation of PPy. However, these characteristic peaks diminished after pyrolysis. In Raman spectrum of PPy-derived framework, only two peaks at 1322 and 1587 cm-1 could be obviously observed. They corresponded to the Dband (C-C, the disordered graphite structure) and G-band (C=C, sp2-hybridized carbon), respectively [18]. It indicated that the functional groups of PPy disappeared because of the high temperature and the carbon skeleton mainly remained after pyrolysis, which was agreement with the result of TGA and FTIR (Fig. S2). XPS analysis of PPy-derived carbon nanotube framework shown the chemical structure variation compared with PPy aerogel. The element composition in PPy aerogel and PPy-derived carbon nanotube framework was calculated by XPS, given in Table 1. The dopants were the most possibility that acted as oxygen resource in PPy aerogel. After pyrolysis, the content of carbon increased while the content of oxygen greatly decreased. The high-resolution N 1s core-level spectra of PPy aerogel and PPy-derived carbon nanotube framework were also different, as shown in Fig. 3b. A major component at a binding energy (BE) of about 399.7 eV and a high BE tail above 400 eV corresponded to the pyrrolic nitrogen and the positively charged pyrrolylium nitrogen, respectively, in the N 1s core-level spectra of PPy

8

aerogel. However, the N 1s core-level spectra of PPy-derived carbon nanotube framework could be curve-fitted into three different valence states, attributable to the pyridinic-N (398.5 eV), pyrrolic-N (399.7 eV) and quaternary-N (401.5 eV), respectively. After the process of pyrolysis at 900 oC, the aromatization and cyclization reactions led to the formation of more pyridine and quaternary nitrogen groups, implying that the resultant carbon framework had been doped by nitrogen atoms deriving from pyrrole. TEM images of PPy aerogel and PPy-derived carbon naotube framework were further shown in Fig. 4. The network of PPy aerogel consisted of the one-dimensional building blocks, which had the rough surface and the diameter in the range of 200-300 nm. PPy nanostructures of hollow tubes with the diameter of 280-400 nm were fabricated with stirring [19]. Although the diameters of PPy tubes were different in two cases of stirring and no stirring, it had no obvious effect on the formation of PPy hollow nanostructures. After pyrolysis, the diameter of onedimensional building blocks in PPy-derived carbon naotube framework was decreased to 100200 nm and the surfaces of the nanotubes turned smooth to some extent, which were similar with the phenomena of the transformation from the powder of polypyrrole precursor prepared by Fe3+-methyl orange template to carbon nanotubes via carbonization [20]. It was also in agreement with the decrease of the weight during the pyrolysis process from PPy aerogel to PPyderived carbon naotube framework. The effective bundles and junctions in the three-dimensional network composed of nitrogendoped carbon nanotubes were responsible for the mechanical property of PPy-derived carbon nanotube framework. Moreover, compared with the BET surface area (43 m2 g-1) of the original PPy aerogel, the surface area of PPy-derived carbon naotube framework was increased

9

significantly to 205 m2 g-1, which should be beneficial for the improvement of adsorption property and capacitor performance. To evaluate the adsorption capacity of the as-prepared three-dimensional PPy-derived carbon nanotube framework, methylene blue, rhodamine B, congo red and orange G were used as the target dyes. 2.0 mg of the three-dimensional PPy-derived carbon nanotube framework was added into the dye solution (4 mL, 40 mg L-1). After standing with agitation for 8 h, the colors of the above four dye solutions were greatly diminished. Even the colors of methylene blue and rhodamine B solutions disappeared completely. The corresponding UV-Vis spectra before and after adsorption were shown in Fig. S3. The decrease of characteristic peak intensity corresponded to the adsorption of dyes. A typical adsorption kinetic experiment was carried out in order to get further insight into the adsorption behavior of PPy-derived carbon nanotube framework towards the four dyes. As shown in Fig. 5, the adsorption capacity increased rapidly in the first stage, and reached to more than 80% of the maximum adsorption within 200 min. Then, a steady value was gradually achieved until adsorption equilibrium. The adsorption capacities of PPy-derived carbon nanotube framework towards methylene blue, rhodamine B, congo red and orange G were calculated to be 609, 572, 151 and 122 mg g-1, respectively. However, the adsorption capacities of PPy aerogel precursors towards the above four dye were only 76, 81, 62 and 58 mg g-1, respectively. The adsorption of PPy-derived carbon nanotube framework towards methylene blue was also much higher that of PANi hydrogels doped by phytic acid (71.2 mg g-1) [12]. The high adsorption ability of PPy-derived carbon nanotube framework towards methylene blue and rhodamine B might be ascribed to two reasons. One was that the larger specific area of PPy-derived carbon nanotube framework could significantly increase the contact opportunity of dye molecules on the framework. The other was that

10

methylene blue and rhodamine B were cationic dyes while congo red and orange G were anionic dyes. The kinetics and isotherms of the adsorption of methylene blue on PPy-derived carbon nanotube framework were shown in Fig. S4 and Fig. S5. The results indicated that the threedimensional PPy-derived carbon nanotube framework with the advantages of simple preparation, mechanical robustness, high adsorption capacity and fast adsorption kinetic could be a potential candidate for water purification, especially for the removal of cationic dyes. Given that the nitrogen doping and large specific surface area of three-dimensional PPyderived carbon nanotube framework, it could be used as active materials for supercapacitor electrodes. Cyclic voltammograms of PPy aerogel and PPy-derived carbon nanotube framework electrodes were recorded at the scan rate of 100 mV s-1 in Fig. 6. Both of CV curves were close to rectangular shape, indicating that the samples had an ideal capacitive characteristic with ion response. It was noted that PPy-derived carbon nanotube framework electrode exhibited the larger current density response in comparison with PPy aerogel electrode. The better capacitive performance resulted from the combination of electric double-layer capacitance and redox reactions, which were related to the heteroatom functionality and the larger specific surface aera of PPy-derived carbon nanotube framework. The galvanostatic charge-discharge curves were further tested to assess the rate capability of the electrodes. As shown in Fig. 7a, the discharging time of PPy-derived carbon nanotube framework electrode was significantly longer compared with that of PPy aerogel electrode, implying that PPy-derived carbon nanotube framework possessed a much larger capacitor, which agreed well with those obtained from CV measurements. Fig. 7b showed the relationship between specific capacitance and discharge current density. It was obvious that the specific capacitance of both of the materials decreased gradually with the increase of discharging current 11

density, which was attributed to the deficient redox reaction under large current density. It was worth noticing that when the discharge current density was increased from 0.5 to 20 A g-1, the capacitance retention of PPy aerogel electrode was about 40% while that of PPy-derived carbon nanotube framework electrode was still up to 64%. It demonstrated that PPy-derived carbon nanotube framework electrode had better rate capability and was very suitable for high current density application. In addition, cycle life is one of the most important criterions for evaluating the supercapacitor performance. Fig. 7c showed the cycling stability of PPy aerogel and PPyderived carbon nanotube framework electrodes at a current density of 1.0 A g-1. The capacitance retention rate of PPy aerogel electrode was only 50.3% after 1000 cycles, indicating that PPyaerogel itself as a supercapacitor electrode material suffered from a poor cycling stability due to the disruption of electrode material with weak mechanical strength during charge-discharge process [21,22]. However, the cycling stability of PPy-derived carbon nanotube framework electrode could be remarkably improved by pyrolysis. It exhibited superior cycling stability with negligible deterioration of specific capacitance after 1000 cycles. The relatively high capacitance, good rate capability and excellent cycling performance of PPy-derived carbon nanotube framework might be ascribed to the synergistic interaction of pseudocapacitive effect, the nanotubular morphology, the large specific surface are and the heteroatom functionality, which was favorable for supercapacitor electrode material.

4. Conclusions In summary, we report the successful fabrication of three-dimensional carbon nanotube frameworks using PPy aerogels as the precursors and explore its environmental and energy applications as dye adsorbent and supercapacitor. During the pyrolysis process, not only the

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nanotubular microstructure and the self-standing macroscopic appearance remain, but also the specific surface area is increased. These features endow the three-dimensional carbon nanotube frameworks with high mechanical property, excellent adsorption capacity towards organic dyes, and better electrochemical capacitive performance. Therefore, this work may be extended to fabricate the carbonaceous materials with three-dimensional frameworks from conducting polymers and promote their practical applications in versatile fields.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21304003), the Shenzhen Science and Technology Innovation Committee for financial support (JCYJ20150806112401354, JCYJ20160330095448858, JCYJ20140627145346390).

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Captions Fig. 1. TGA of PPy aerogel precusor. Fig. 2. Photograph of three-dimensional PPy-derived carbon nanotube framework. Fig. 3. (a) Raman and (b) N 1s core-level spectra of PPy aerogel and PPy-derived carbon nanotube framework. Fig. 4. TEM images of (a) PPy aerogel and (b) PPy-derived carbon nanotube framework. Fig. 5. Adsorption kinetic curves of methylene blue, rose red B, congo red, and orange G by PPy-derived carbon nanotube framework. Fig. 6. Cyclic voltammograms of PPy aerogel and PPy-derived carbon nanotube framework electrodes at the scan rate of 100 mV s-1. Fig. 7. (a) Galvanostatic charge-discharge curve of PPy aerogel and PPy-derived carbon nanotube framework electrodes at a constant current density of 0.5 A g-1. (b) Capacitance versus discharge current density with PPy aerogel and PPy-derived carbon nanotube framework electrodes. (c) Cycling performance of PPy aerogel and PPy-derived carbon nanotube framework electrodes at a current density of 1.0 A g-1.

17

100

Weight (%)

80

60

40

20

0 0

200

400

600

800

o

Temperature ( C)

Fig. 1.

18

Fig. 2.

19

(a)

Intensity (a.u.)

PPy-derived carbon nanotube framework

PPy aerogel

500

1000

1500

2000

2500

-1

Raman shift (cm )

(b)

PPy aerogel

399.7eV

Intensity (a.u.)

401.1eV

403eV

PPy-derived carbon nanotube framework 399.7eV

398.5eV

401.5eV

396

398

400

402

404

406

Binding Energy (eV)

Fig. 3.

20

Fig.4.

21

800

Methylene Blue Rhodamine B Congo Red Orange G

Adsorption Capacity (mg/g)

700 600 500 400 300 200 100 0 0

200

400

600

Contact Time (min)

Fig. 5.

22

-4

1.2x10

-5

8.0x10

PPy-derived carbon nanotube framework

-5

Current (A)

4.0x10

PPy aerogel 0.0

-5

-4.0x10

-5

-8.0x10

0.0

0.2

0.4

0.6

0.8

Potential (V)

Fig. 6.

23

1.0

(a) PPy aerogel

PPy-derived carbon nanotube framework

Potential (V)

0.8

0.6

0.4

0.2

0.0

0

100

200

300

400

500

Time (s)

(b)

Specific Capacitance (F/g)

160

PPy-derived carbon nanotube framework 120

80

PPy aerogel 40

0 0

5

10

15

20

Current Density (A/g)

(c) 100

Capacity retention(%)

PPy-derived carbon nanotube framework 80

60

PPy aerogel

40

20

0

0

200

400

600

800

1000

Cycle numbers

Fig. 7. 24

Table 1 Parameters of elemental composition and BET surface area Sample

Chemical compositeion

BET

(at%)

(m2 g-1)

C

N

O

PPy aerogel

67.1

10.5

23.4

43

PPy-derived carbon nanotube framework

87.3

6.0

6.7

205

25