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Jan 8, 2016 - ABSTRACT: CeO2/PPy nanocomposites were synthesized via in situ chemical oxidative polymerization of pyrrole based on the two kinds of ...
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Synthesis and Electrochemical Performance of CeO2/PPy Nanocomposites: Interfacial Effect Xue Wang,† Tingmei Wang,‡ Dong Liu,† Jinshan Guo,† and Peng Liu*,† †

State Key Laboratory of Applied Organic Chemistry and Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China ‡ State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ABSTRACT: CeO2/PPy nanocomposites were synthesized via in situ chemical oxidative polymerization of pyrrole based on the two kinds of amine-functionalized CeO2 nanoparticles with 3-triethoxysilylpropylamine (APTES) or p-aminobenzoic acid (PABA), respectively. The morphology and structure of the CeO2/PPy samples with different interface interactions were comparatively investigated by transmission electron microscopy, Brunauer−Emmett−Teller equation, Fourier transform infrared spectroscopy, UV−vis, X-ray diffraction analysis, X-ray photoelectron spectroscopy, and thermal gravimetric analysis. It was found that the introduction of the amine-functionalized CeO2 nanoparticles into the PPy matrix significantly enhanced the thermal stability and electrical conductivity of the resulting nanocomposites. As an electrode material for electrochemical supercapacitors, the CeO2/PPy nanocomposites with different amine-functionalized CeO2 nanoparticles were characterized by galvanostatic charge/discharge (GCD) and cyclic voltammetry (CV) tests. The CeO2/PPy nanocomposites showed improved electrochemical performance in comparison with the pure PPy. The P-CeO2/PPy nanocomposites synthesized with the PABAfunctionalized CeO2 nanoparticles (P-CeO2) exhibited a higher specific capacitance, while those with the APTES-functionalized CeO2 nanoparticles (A-CeO2) possessed the higher cycling stability.

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INTRODUCTION Polypyrrole (PPy) is one of most promising conducting polymeric electrode materials for a supercapacitor because of its intriguing characteristics such as high electrical conductivity, high specific capacitance, and low cost. Unfortunately, its poor cycling performance restricts severely the practice application due to the degradation at high potential and big volume changes during the long charge/discharge process.1 In recent years, the PPy-based nanocomposites with various inorganic nanomaterials as supports, such as carbon nanomaterials (carbon black,2 nanotubes,3 and graphene4), metal oxides,5−9 clay minerals,10,11 and so on, have attracted more and more attention because of their unique morphology, high electrical conductivity, large specific surface area, and good stability.12 The interface between the inorganic nanomaterials and the conducting polymers might be the most important determining factor for the design of the nanocomposites.13 To improve the interfacial property of the PPy-based nanocomposites, the inorganic nanomaterials usually had been surface modified. For example, the carbon nanomaterials were used after oxidization, or treatment with surfactants.12,14 As for the nanoclays, the desired nanocomposites with well-defined structure could be obtained by using various modifiers such as functional silane,10 dyes,11 or surfactant,15 although polypyrrole could be deposited onto the unmodified nanoclays due to their polar surface.16,17 And the surface modification strategies of the metal oxides for the well-defined core−shell polypyrrole-based nanocomposites are mainly focused on the functional silane,7 and surfactant.5 Especially for the amine group-containing modifiers, the PPy chains could be grafted and coated onto the surface of the © XXXX American Chemical Society

inorganic materials via the specific interactions of the Lewis acid−base type between the surface amine groups and PPy.7,18,19 Although various surface modification techniques have been developed in order to design the polypyrrole-based nanocomposites with better interfacial property, there is no report about the interfacial influence of the different surface modifiers on the morphology and especially electrochemical property of the resultant nanocomposites by now. In the present work, two kinds of surface modifiers, 3-triethoxysilylpropylamine (APTES) and p-aminobenzoic acid (PABA), were used as the electron donors for PPy via the facile in situ chemical oxidative polymerization of pyrrole in the presence of the CeO 2 nanoparticles as effective supporting materials, with ammonium persulfate and sodium p-toluene sulfonate as oxidant and dopant, respectively. After the surface modification, the amino-functional groups had been modified onto the CeO2 nanoparticles, thus two kinds of amino-functionalized CeO2 nanoparticles, the PABAfunctionalized CeO2 nanoparticles (P-CeO2) and the APTESfunctionalized CeO2 nanoparticles (A-CeO2), were obtained and used for the facile chemical oxidative polymerization of pyrrole. The formed polypyrrole could be coated onto the aminofunctionalized CeO2 nanoparticles via the specific interactions of the Lewis acid−base type between the basic amino group (n donor) and the acidic N−H bonds (σ* acceptor) and/or the positively charged PPy backbone (n acceptor).19 The morphology, structure, and properties the two nanocomposites (P-CeO2/ Received: October 15, 2015 Revised: December 10, 2015 Accepted: January 8, 2016

A

DOI: 10.1021/acs.iecr.5b03891 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research PPy and A-CeO2/PPy) with different interface interactions were comparatively investigated in detail. Especially, as promising electrode material, the electrochemical performances of the resulting nanocomposites have been focused in the threeelectrode system.

Table 1. Synthetic Conditions for the Samples

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EXPERIMENTAL SECTION Materials and Reagents. Pyrrole (Shanghai Zhongqin Chemical Reagent Co., Ltd., Shanghai, China) was distilled under reduced pressure before use. Analytical reagent grade ammonium peroxodisulfate (APS) (Tianjin Chemical Reagent Co., Ltd., Tianjin, China) and CeO2 nanoparticles (Riel Chemical Technology Co., Ltd., Huizhou, China) were used as received. Carbon black was obtained from Yiping Chemical Factory, Shanghai, China. Polyvinylidenefluoride (PVDF) was received from Funuolin New Chemical Materials Co., Ltd. Zhejiang, China. Nickel foam (thickness, 1.8 mm; pore density, 110 ppi; 30 mm × 5 mm) was purchased from Heze Tianyu Technology Development Co., Ltd. Shandong, China. N,Ndimethylformamide (DMF) was purchased from Tianjin Chemical Reagent Co., Tianjin, China, and was dehydrated with calcium hydride for 24 h. 3-Triethoxysilylpropylamine (APTES, Nanjing Capatue Chemical Co., Ltd., Nanjing, China), p-aminobenzoic acid (PABA, Gracia Chemical Technology Co., Ltd., Chengdu, China), ethanol, and toluene were analytical reagents and used without further purification. Doubly deionized water was used through all the processes. Surface Modification of CeO2 Nanoparticles. APTES. The CeO2 nanoparticles were chemically modified with APTES. Typically, CeO2 nanoparticles (1.5 g) were dispersed into 40 mL of toluene containing 3 mL of APTES. The suspension was refluxed for 10 h under dry nitrogen. The resulting nanoparticles were filtered and washed with toluene and ethanol, and then dried in vacuum at 60 °C overnight to obtain the aminefunctionalized CeO2 nanoparticles (A-CeO2). The content of amino group in the A-CeO2 nanoparticles was calculated to be 1.21 mmol g−1 by elemental analysis. PABA. PABA (0.2489 g) and CeO2 nanoparticles (1.5 g) with a feeding ratio of 1.21 mmol: 1.0 g was dispersed in 40 mL of ethanol. The suspension was refluxed for 24 h under dry nitrogen. The reaction product was filtered off and washed with ethanol. After being washed, the product was dried in vacuum at 60 °C overnight to obtain the PABA-functionalized CeO2 nanoparticles (P-CeO2) with content of amino group of 0.502 mmol g−1 from elemental analysis. Synthesis of the CeO2/PPy Nanocomposites. The CeO2/PPy nanocomposites were prepared by the in situ chemical oxidative polymerization of pyrrole in the presence of the A-CeO2 or P-CeO2 nanoparticles. A certain amount of the ACeO2 (or P-CeO2) nanoparticles were added in 100 mL of deionized water containing sodium p-toluene sulfonate (STS, 4.16 g). The above mixture was ultrasonically dispersed, and freshly distilled pyrrole (1.0 mL) was injected into the mixture with vigorous stirring at 0 °C under nitrogen atmosphere. After 30 min, 20 mL of aqueous solution containing APS (0.90 g), as the oxidant, was added drop by drop to the above mixture to initiate the polymerization of pyrrole. The stirring was kept for 10 h under ice-cold condition. The precipitated powder was washed with deionized water and ethanol several times. Finally, the product was dried in vacuum at 60 °C for 24 h. For comparison, the pure PPy sample without CeO2 nanoparticles was also prepared by the similar procedure. The conditions of the polymerizations were summarized in Table 1.

samples

Py (mL)

CeO2 (g)

APS (g)

feeding ratio of CeO2 (wt %)

PPy A-CeO2/PPy-5 A-CeO2/PPy-10 P-CeO2/PPy-5 P-CeO2/PPy-10

1.0 1.0 1.0 1.0 1.0

0 0.0543 0.1147 0.0543 0.1147

0.9 0.9 0.9 0.9 0.9

0 5 10 5 10

Analysis and Characterization. The Fourier transform infrared (FTIR) spectra (Impact 400, Nicolet, Waltham, MA) were recorded by using the KBr pellet method in the wavelength range of 4000−400 cm−1. The ultraviolet−visible (UV−vis) spectra of the CeO2/PPy composites dispersed in ethanol were recorded on the ultraviolet−visible spectrometer (UV−vis, TU-1901, Beijing Purkinje General Instrument, Co., Ltd.). The X-ray powder diffraction (XRD) patterns were conducted on a Panalytical X′ Pert PRO X-ray diffractometer with Cu Kα (λ = 0.15418 nm) incident radiation. The surface characterization of the CeO2 nanoparticles and CeO2/PPy samples were conducted with a PHI-5702 multifunctional X-ray photoelectron spectrometer (XPS) with pass energy of 29.35 eV and an Mg KR line excitation source. The binding energy of C 1s (284.6 eV) was used as a reference. The element analysis of the samples was conducted with Elementar vario EL instrument (Elementar Analysen systeme GmbH, Munich, Germany). Thermal gravimetric analysis (TGA) was measured by PerkinElmer TGA-7 TG thermogravimetric analyzer (Perkin− Elmer Co., Norwalk, CT, USA) at a heating rate of 10 °C min−1 from room temperature to 800 °C under a nitrogen atmosphere. The morphology of the CeO2, PPy, and the CeO2/PPy samples were characterized by a JEM-1230 transmission electron microscope (TEM, JEOL, Tokyo, Japan). The powders were dispersed in ethanol in an ultrasonic bath for 30 min, and then deposited on a copper grid covered with a perforated carbon film. Nitrogen adsorption experiments (ChemiSorb 2750, Micromeritics Instrument Corp. USA) of the result composites were investigated at room temperature, and the specific surface areas of the samples were calculated by using the Brunauer−Emmett− Teller (BET) equation. The electrical conductivities of the pristine CeO2, A-CeO2, and P-CeO2 nanoparticles and the CeO2/PPy nanocomposites were measured by a RTS-2 four-point probe conductivity tester (Guangzhou four-point probe Technology Co., Ltd., Guangdong, China) at ambient temperature employing the method on a pressed pellet. Each value given is an average of three measurements. Electrochemical performances of the CeO2, PPy, and the nanocomposites were measured using galvanostatic charge/ discharge (GCD) and cyclic voltammetry (CV) techniques with a CHI660B electrochemical workstation (CH Instruments, Shanghai, China). The potential range for the GCD and CV tests was −0.2 to 0.8 V. The working electrodes with mass loadings of about 2 mg were prepared with the mixture containing the active materials, carbon black, and polyvinylidenefluoride (PVDF) with mass ratio of 80:15:5 to make homogeneous mixture in N,N-dimethylformamide (DMF). Then, the slurry was uniformly laid on Ni foam with an area of about 0.25 cm2 which was served as a current collector and dried at 50 °C for 24 h. The Ni foam coated with the CeO2/PPy B

DOI: 10.1021/acs.iecr.5b03891 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. TEM images of the pristine CeO2 nanoparticles, pure PPy and CeO2/PPy nanocomposites.

samples was pressed for 1 min under 1.0 MPa before use. The measurements were carried out in a 1.0 M NaNO3 aqueous electrolyte at ambient temperature. All electrochemical experiments were carried out in a three-electrode system with a working electrode, a platinum counter electrode, and a standard calomel reference electrode (SCE).

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RESULTS AND DISCUSSION Morphological Analysis. The TEM images of the pristine CeO2 nanoparticles, PPy and the CeO2/PPy nanocomposites with different surface modifiers and feeding ratios are shown in Figure 1. It can be seen from Figure 1 that the pristine CeO2 nanoparticles showed a sphere of 40 nm with aggregates to some extent. The pure PPy nanoparticles are amorphous and aggregative due to their high surface energy.20 The CeO2/PPy nanocomposites displayed the aggregative structure with bigger size than the pristine CeO2 nanoparticles, indicating the formation of the PPy shells on the CeO2 nanoparticles. Their increased size was related to the mass of the loaded PPy onto the surface of the amino-functionalized CeO2 nanoparticles, because the CeO2 nanoparticles with surface amino groups might have good compatibility and effective interaction with PPy via specific Lewis acid−base interactions.7,18,19 Compared to the A-CeO2/ PPy nanocomposites, the P-CeO2/PPy nanocomposites exhibited the clearer nanospherical morphology. However, the CeO2/PPy nanocomposites with the high Py/CeO2 feeding ratios displayed the indistinct core/shell morphology, suggesting that too much PPy had been coated onto the surface of the CeO2 nanoparticles.7,18,19 Although the desired core−shell structure could not be seen from TEM analysis because of the segregation of the pristine CeO2 nanoparticles (Figure 1),21 the encapsulation of the PPy shells onto the CeO2 nanoparticles could be revealed with XPS technique (Figure 2). Compared with the pristine CeO2

Figure 2. XPS survey of the pristine CeO2 nanoparticles, aminefunctionalized CeO2 nanoparticles, and CeO2/PPy nanocomposites.

nanoparticles, the surface N element contents of the aminefunctionalized CeO2 (A-CeO2 and P-CeO2) nanoparticles were observed, indicating that APTES and PABA had been assembled successfully onto the surface of CeO2 nanoparticles, respectively. For the CeO2/PPy nanocomposites, the surface O and Ce element contents decreased while that of N element increased after the in situ chemical oxidative polymerization of pyrrole, indicating that the PPy shells had been successfully coated onto the CeO2 nanoparticles (Table 2), and the lower surface Ce element contents in the A-CeO2/PPy-5 and P-CeO2/PPy-5 nanocomposites than that in the CeO2/PPy-5 nanocomposite prepared with the unmodified CeO2 nanoparticles also demonstrated that the surface-modification with amino-containing agents was beneficial to the encapsulation with PPy shells, maybe because of the specific Lewis acid−base interactions.7,18,19 Furthermore, the surface S element appeared, revealing the presence of STS as dopant for the PPy shells. C

DOI: 10.1021/acs.iecr.5b03891 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 2. Surface Elemental Content from XPS Analysis surface element content (%) samples

C 1s

O 1s

Ce 3d

N 1s

Si 2p

CeO2 A-CeO2 P-CeO2 CeO2/PPy-5 A-CeO2/PPy-5 P-CeO2/PPy-5

75.84 70.60 58.15 74.88 75.48 77.90

20.73 20.55 37.49 15.72 12.97 13.10

3.43 0.41 2.33 0.10 0.04 0.08

3.07 2.02 7.63 8.54 7.33

5.37

1.20

S 2p

1.67 1.76 1.60

To further confirm it, the specific surface areas (S) of the samples were measured by using nitrogen adsorption experiments, and calculated by using the BET equation. The pristine CeO2 nanoparticles have the large specific surface area of 70.63 m2 g−1. The specific surface area of the pure PPy sample is 12.07 m2 g−1. Increasing the A-CeO2 feeding ratio from 5 wt % to 10 wt %, the PPy content (%) in the CeO2/PPy nanocomposites increased slightly (Table 3), indicating high utilization of the PPy. For the A-CeO2/PPy nanocomposites, as the increase of the A-CeO2 feeding ratio from 5 wt % to 10 wt %, the resulting nanocomposites (A-CeO2/PPy-5 and A-CeO2/PPy-10) were obtained with the specific surface areas of 12.89 and 5.87 m2 g−1, respectively (Table 3). The specific surface area of the A-CeO2/ PPy composites samples decreased with the increase of the feeding ratio of A-CeO2, which is correlated to the aggregative and compact PPy shells formed due to the specific interactions between the NH2-CeO2 nanoparticles and PPy.22 Compared to the A-CeO2/PPy nanocomposites, the P-CeO2/PPy nanocomposites exhibited an increase of specific surface area from 10.16 to 13.47 m2 g−1 as the P-CeO2 feeding ratio increased from 5 wt % to 10 wt % (Table 3), attributed to the smaller diameter and better dispersibility of the P-CeO2/PPy nanocomposites. Additionally, the high specific surface areas of the nanocomposites can also provide more electroactive regions and highways for charge storage and delivery which are beneficial to improve the electrical conductivity and electrochemical performances as an electrode for supercapacitors.9,23,24 Spectral Analysis. Figure 3 shows the FT-IR spectra of the pristine CeO2, A-CeO2, P-CeO2 nanoparticles, PPy, and the CeO2/PPy nanocomposites. The spectrum of the PPy showed the typical absorbance peaks at 1465 and 1554 cm −1 corresponding to the symmetric and antisymmetric stretching modes of the PPy ring, respectively.25 The absorbance bands at 1300 and 3403 cm−1 are associated with the stretching vibration of C−N and N−H, respectively. And the absorbance of SO stretching at 1180 cm−1 of STS was also observed, revealing the presence of PPy in the doped states, as demonstrated by the XPS analysis. The pristine CeO2 nanoparticles showed the characteristic absorbance peaks at 1515, 1265, 1120, 1062, 952, and 862 cm−1.26 For the A-CeO2 nanoparticles modified with APTES, the N−H stretching vibration absorbance at 3430 cm−1 and the

Figure 3. FT-IR spectra of the KBr pellet of the pristine CeO2 nanoparticles, pure PPy, and CeO2/PPy nanocomposites.

aliphatic C−H characteristic absorbance at 2931 and 2870 cm−1 were observed, revealing that the functional organosilane had been successfully assembled onto the surfaces of the CeO2 nanoparticles. It is noted that, after the assembly of PABA onto the surface of the CeO2 nanoparticles, a series of characteristic peaks of benzene ring at 1608, 1584, 1500, 1420 cm−1 appeared in the spectrum of the P-CeO2 nanoparticles. After the chemical oxidative polymerization of pyrrole, the resultant CeO2/PPy nanocomposites showed the similar FT-IR spectra as the pure PPy, due to the high composition of PPy and the weak absorbance of CeO2. Additionally, compared with the pure PPy, the N−H stretching vibration absorbance of the nanocomposites at 3430 cm−1 became weaker after the introduction of the functionalized CeO2 nanoparticles. Especially, the CeO2/PPy showed the weakest absorbance due to the strong interaction between the NH2-CeO2 and PPy.27,28 Figure 4 shows the UV−vis absorption spectra of the CeO2, ACeO2, P-CeO2 nanoparticles, PPy, and the CeO2/PPy nano-

Figure 4. UV−vis spectra of the pristine CeO2 nanoparticles, pure PPy, and CeO2/PPy nanocomposites.

composites dispersed in ethanol. The pristine CeO2 exhibited an absorption peak at around 340 nm, the band gap of the CeO2 nanoparticles.29 The pure PPy showed the characteristic

Table 3. Some Important Parameters of the PPy and CeO2/PPy Nanocomposites PPy (%)a S (m2 g−1) σ (S cm−1) C (F/g) a

PPy

A-CeO2/PPy-5

A-CeO2/PPy-10

P-CeO2/PPy-5

P-CeO2/PPy-10

12.07 12.5 127

47.43 12.89 11.8 126

53.63 5.87 7.89 111

45.53 10.16 23.1 193

52.90 13.47 20.0 143

The PPy content (%) in the CeO2/PPy nanocomposites were determined by element analysis. D

DOI: 10.1021/acs.iecr.5b03891 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research absorption at about 460 nm, and a broad “free carrier tail” above 600 nm in the near-infrared (NIR) region, that can be attributed to the electronic transitions from valence bond to bipolarons and antibipolarons of doped PPy, respectively.30 The UV−vis absorption spectrum of the A-CeO2 was similar to the pristine CeO2 nanoparticles. For comparison, it can be clearly seen from Figure 4 that the P-CeO2 exhibited a distinct peak at around 300 nm, which can be ascribed to π−π* transition of the benzenoid rings. For the UV−vis absorption spectra of the CeO2/PPy nanocomposites, the peak located at 460 nm appeared, which is similar to PPy, revealing formation of the PPy shell based on the surface of the CeO2 core. In addition, the relative peak intensity of the P-CeO2/PPy nanocomposites in the NIR region increased due to a high doping level as compared to the A-CeO2/PPy nanocomposites, which may be favorable for enhancement of the electrical conductivity of the samples.31 X-ray Diffraction Analysis. The XRD patterns of the CeO2, PPy, and CeO2/PPy nanocomposites are shown in Figure 5. For

Figure 6. TGA curves of (a) pristine CeO2, (b) A-CeO2, (c) P-CeO2, (d) A-CeO2/PPy-5, (e) A-CeO2/PPy-10, (f) P-CeO2/PPy-5, (g) PCeO2/PPy-10, and (h) pure PPy.

weight loss of the A-CeO2 was higher than that of the P-CeO2 nanoparticles because of their higher organic content. PPy showed two typical mass loss stages. The first stage, from room temperature to 250 °C, was due to evaporation of water in the sample. Second stage, from 250 to 680 °C, results from the degradation of PPy chains. Finally, the mass residue of 5.73% upon 680 °C is the carbonized products.33 The TGA curves of the CeO2/PPy nanocomposites exhibited similar two stages with pure PPy, while the thermal stability of the P-CeO2/PPy nanocomposites increased with an increase of the P-CeO2 feeding content. However, the thermal stability of the ACeO2/PPy nanocomposites decreased with increasing A-CeO2 feeding content. This should be due to the inhomogeneous distribution of aggregative PPy nanoparticles on the surface of the A-CeO2 core, as observed from TEM images. In a word, the improved thermal stability of the CeO2/PPy nanocomposites, attributed to the synergistic effect between CeO2 and PPy, led to a more compact PPy structure in the CeO2/PPy nanocomposites. Furthermore, the A-CeO2/PPy nanocomposites showed slightly higher thermal stability than the P-CeO2/PPy nanocomposites, indicating the much compacter PPy shells had been formed on the CeO2 nanoparticles with higher amino group content. It is in good agreement with the BET results, in which the A-CeO2/PPy nanocomposites possessed the lower specific surface area. The compacter PPy shells are expected to provide the better cycling stability of the nanocomposite electrode materials, and the A-CeO2/PPy nanocomposites showed higher residual quantities at 800 °C than those of the P-CeO2/PPy nanocomposites prepared with the same feeding ratio CeO2 nanoparticles, although the A-CeO2/PPy nanocomposites had a slightly higher PPy content. This illustrates the silane coupling agent might be favorable to the carbonization of PPy. Electrical Conductivity. The electrical conductivity of the CeO2, PPy, and CeO2/PPy nanocomposites at room temperature are presented in Table 3. The pristine CeO2 nanoparticles showed a very low electrical conductivity of 0.00085 S cm−1. The electrical conductivity of the A-CeO2 and P-CeO2 nanoparticles increased to 0.042 and 0.044 S cm−1 after the surface was modified with APTES and PABA. The electrical conductivity of the pure PPy sample was 12.5 S cm−1. Compared with the NH2CeO2 nanoparticles and PPy, the CeO2/PPy nanocomposites exhibited improved electrical conductivity. It resulted from the strong interaction between the NH2-CeO2 and PPy, which may provide a good conductive pathway and bring a synergistic effect

Figure 5. XRD patterns of the pristine CeO2 nanoparticles, pure PPy, and CeO2/PPy nanocomposites.

the pristine CeO2 nanoparticles, the Bragg diffraction peaks (2θ) at 28.6, 33.1, 47.6, 56.5, 59.2, 69.5, 76.8, 79.0 and 88.5° are attributed to its (111), (200), (220), (311), (222), (400), (331), (420) and (422) planes, respectively. It is consistent with the JCPD card 34-394, demonstrating it has a cubic lattice.32 The semicrystalline PPy showed the broad diffraction peak at 15−30° belonged to the amorphous structure of PPy. The A-CeO2 and P-CeO2 nanoparticles displayed similar diffraction peaks with the pristine CeO2, indicating that crystal structure of the NH2-CeO2 well remained after different chemical modifications. The CeO2/PPy nanocomposites simultaneously showed the weak diffraction peak of the semicrystalline PPy and a series of characteristic diffraction peaks of the pristine CeO2. Compared with the A-CeO2 and P-CeO2, the diffraction peak intensity of CeO2 in the CeO2/PPy nanocomposites decreased, which was attributed to the CeO2 core being encapsulated by the PPy shell, as ascribed in the TEM and XPS analysis. Thermogravimetric Analysis. The TGA curves of the pristine CeO2, A-CeO2, P-CeO2, PPy, and CeO2/PPy nanocomposites are compared in Figure 6. As can be seen, the thermal stability of the unmodified CeO2 still remains at a higher level at 800 °C, with only slight mass loss which might be due to the organic residues of the surfactant. In contrast, the weight loss of the A-CeO2 and P-CeO2 nanoparticles increased due to the introduction of the organic groups onto the surface of the pristine CeO2 nanoparticles by chemical modification, and the E

DOI: 10.1021/acs.iecr.5b03891 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research to the electrochemical properties for the composites.23 However, decreasing the PPy content in the CeO2/PPy nanocomposites by increasing the NH2-CeO2 feeding content in the in situ chemical oxidative polymerization caused the electrical conductivity of the resulting CeO2/PPy nanocomposites to decrease (Table 3). This may result from the looser accumulation masses of the CeO2/ PPy nanocomposites prepared with higher NH2-CeO2 feeding content, which showed larger specific surface area. Furthermore, the electrical conductivities of the P-CeO2/PPy nanocomposites were higher than those of the A-CeO2/PPy nanocomposites with the same feeding ratio of the NH2-CeO2, although the electrical conductivities of the two nanocomposites decreased with increasing NH2-CeO2 feeding content due to the introducing of excessive A-CeO2 with lower electric conductivity and reduced specific area of the nanocomposites. This may be because the conjugating PABA can facilitate the electron transfer between the CeO2 and PPy than APTES. Electrochemical Performance. To evaluate the electrochemical behavior of the CeO2/PPy nanocomposite electrodes, their galvanostatic charge/discharge (GCD) and cyclic voltammetry (CV) performance was investigated in a three-electrode system in 1.0 M NaNO3 electrolyte. The electrochemical property of the CeO2 and PPy electrodes was also measured similarly for comparison. The specific capacitance was calculated according to the following equation:

∫

Cm = ( I dV )/(mVυ)

(1)

where Cm is the specific capacitance, I is the response current density, V is the potential window, υ is the potential scan rate, and m is the mass of the composites in the working electrode. Galvanostatic Charge/Discharge (GCD) Test. Galvanostatic charge/discharge curves of the PPy, CeO2, and CeO2/PPy nanocomposites are presented in Figure 7. Figure 7A displays the galvanostatic charging−discharging curves of the pristine CeO2, A-CeO2, and P-CeO2 nanoparticles at a current density of 1 A g−1. The specific capacitance of the pristine CeO2 nanoparticles only was 0.6 F g−1. Notably, after chemical modification, the specific capacitance of the P-CeO2 increased, which may due to the conjugate structure of PABA. As shown in Figure 7B, a specific capacitance (127 F g−1) was obtained for the pure PPy electrode. Figure 7C shows the galvanostatic charge/discharge curves of the CeO2/PPy nanocomposites. The specific capacitance values of the samples were calculated from eq 1 and illustrated in Table 2. In comparison of the A-CeO2/PPy nanocomposite electrodes, the GCD curves of the P-CeO2/PPy electrodes showed the triangle symmetric shape and negligible IR drop, indicating their good capacitive behavior. For the A-CeO2/PPy composites, the specific capacitance decreased with increasing the A-CeO2 feeding ratio, as compared with the pure PPy. After introducing A-CeO2 into the PPy matrix, the interaction between the A-CeO2 and PPy enabled the specific capacitances of the A-CeO2/PPy nanocomposites to retain higher values (>100 F g−1). The specific capacitance of the A-CeO2 nanoparticles was very low (0.2 F g−1), therefore introducing A-CeO2 was bad with respect to the specific capacitance of the A-CeO2/PPy nanocomposites. However, the specific capacitance of the P-CeO2/PPy nanocomposites increased first and then decreased with the increase of the P-CeO2 feeding content. This result can be attributed to the more regular structure and higher specific surface area, as well as higher electrical conductivity. The decreasing in specific capacitance of the P-CeO2/PPy electrode

Figure 7. Galvanostatic charge/discharge curves of (A) pristine CeO2, (B) pure PPy, and (C) the CeO2/PPy nanocomposites ((a) A-CeO2/ PPy-5, (b) A-CeO2/PPy-10, (c) P-CeO2/PPy-5, and (d) P-CeO2/PPy10) electrodes at a current density of 1.0 A g−1 within the potential window −0.2 to 0.8 V (vs SCE).

with increasing the P-CeO2 feeding content was also due to the lower specific capacitance of the P-CeO2 nanoparticles. By comparing the composition, specific surface area, electrical conductivity, and specific capacitance of the A-CeO2/PPy-5 and P-CeO2/PPy-5 nanocomposites, the P-CeO2/PPy-5 nanocomposite with the slightly low PPy content and specific surface area, which might be due to the low amino group content of the P-CeO2 nanoparticles, interestingly possessed the slightly higher electrical conductivity and specific capacitance than those of the A-CeO2/PPy-5 nanocomposites. This might be due to the conjugated structure of the surface modifier PABA. Cyclic Voltammetry (CV) Test. Figure 8 panels A and B show the CV curves of the CeO2/PPy electrodes at different scan rates from 1 to 50 mV s−1 with a potential range between −0.2 and 0.8 F

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Industrial & Engineering Chemistry Research

For the A-CeO2/PPy and P-CeO2/PPy electrodes, the specific capacitance decreased with increasing scan rate because the effective interaction between the electrode and electrolyte gradually reduced. The specific capacitance values of the PCeO2/PPy-5 nanocomposite electrode were 240.7, 169.4, 151.5, 127.5, 109.4, and 85.2 F g−1 at the scan rate of 1, 5, 10, 20, 30, and 50 mV s−1, respectively. It can be observed that the specific capacitance of the P-CeO2/PPy-5 electrode reached a maximum value (240.7 F g−1) at the scan rate of 1 mV s-1. As shown in Figure 8C, the P-CeO2/PPy-5 composite electrode retained its 35% capacitance as scan rate was increased from 1 to 50 mV s−1. However, the specific capacitances of the PCeO2/PPy-5 electrode were still much higher than that of the ACeO2/PPy-5 electrode at the same scan rate. This result is mainly attributed to the fast charge-transfer of the P-CeO2/PPy-5 nanocomposite dominated by its higher conductivity and larger specific surface area than the A-CeO2/PPy-5 nanocomposite, due to the conjugative linking between the CeO2 and PPy by PABA (Scheme 1).35,36 Scheme 1. Model of the Conjugative Linking between the CeO2 and PPy by PABA

Cycling Stability. The cycling stability of the PPy and CeO2/ PPy electrodes were compared using CV curves between −0.2 and 0.8 V at 50 mV s−1. It can be seen from Figure 9 that the

Figure 8. Cyclic voltammogram of (A) A-CeO2/PPy-5 and (B) PCeO2/PPy-5 electrodes at different scan rates, and (C) specific capacitance versus scan rate for A-CeO2/PPy-5 and P-CeO2/PPy-5 electrodes.

Figure 9. Cycling stability test of the pure PPy, A-CeO2/PPy-5 and PCeO2/PPy-5 electrodes as a function of cycle numbers measured at 50 mV s−1.

V (vs SCE). It can be seen that the curves at different scan rates did not show obvious redox peaks in the entire voltage range during the positive and negative sweeps, indicating that the electrode was charged and discharged at a pseudoconstant rate over the whole CV process.34 All the CV curves showed rectangle-like shape when the scan rates were lower than 10 mV s−1, revealing that the CeO2/PPy nanocomposite electrodes possessed pure electrical double-layer (EDL) capacitance. The near rectangle-like shape of the CV curves indicated that the PCeO2/PPy electrodes should have better electrochemical behavior than the A-CeO2/PPy electrodes. The specific capacitance of the CeO2/PPy electrodes was calculated at different scan rates, as shown in Figure 8C.

CeO2/PPy electrodes showed improved cycling stability than the pure PPy. In particular, the specific capacitance of the A-CeO2/ PPy-5 sample still remained at a high level above 90% because the strong interaction between the CeO2 and PPy prevents the volume swelling and shrinking of PPy during the charge/ discharge process. In addition, the cycling stability of the ACeO2/PPy-5 electrode was better than that of the P-CeO2/PPy-5 electrode, because of the covalent interaction in the surface modification of the CeO2 nanoparticles and/or high amino group content. G

DOI: 10.1021/acs.iecr.5b03891 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

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CONCLUSIONS In summary, the CeO2/PPy nanocomposites were successfully fabricated via the in situ chemical oxidative polymerization of pyrrole in the presence of the amino-modified CeO2 nanoparticles (A-CeO2 and P-CeO2) using APTES and PABA, respectively. The asymmetric supercapacitor with the CeO2/PPy nanocomposite as electrode material has high specific capacitance of 193 F g−1, compared with 0.6 F g−1 for the CeO2 nanoparticles and 127 F g−1 for the pure PPy. The interface interaction between the NH2-CeO2 nanoparticles and PPy led to the higher electrical conductivity and enhanced thermal stability than pure PPy. The P-CeO2/PPy nanocomposite was highlighted because of its highest electrical conductivity and specific capacitance due to the conjugative linking between the CeO2 and PPy by PABA. However, the ACeO2/PPy nanocomposites with the covalently modified surface of the CeO2 nanoparticles exhibited prominent cyclic stability. This understanding is a benefit to the design of promising conductive polymer-based electrodes for supercapacitors.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: 86 0931 8912582. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Gansu Province (Grant No. 1107RJZA213), and the Fundamental Research Funds for the Central Universities (No. lzujbky2011-21 and lzujbky-2013-237).

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DOI: 10.1021/acs.iecr.5b03891 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX