Nitrogen-doped porous carbons with nanofiber-like structure derived from poly (aniline-co-p-phenylenediamine) for supercapacitors

Accepted Manuscript Title: Nitrogen-doped porous carbons with nanofiber-like structure derived from poly (aniline-co-p-phenylenediamine) for supercapacitors Author: Dazhang Zhu Ke Cheng Yawei Wang Dongmei Sun Lihua Gan Ting Chen Juxiang Jiang Mingxian Liu PII: DOI: Reference:

S0013-4686(16)32569-5 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.023 EA 28495

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

15-8-2016 22-11-2016 6-12-2016

Please cite this article as: Dazhang Zhu, Ke Cheng, Yawei Wang, Dongmei Sun, Lihua Gan, Ting Chen, Juxiang Jiang, Mingxian Liu, Nitrogendoped porous carbons with nanofiber-like structure derived from poly (aniline-co-p-phenylenediamine) for supercapacitors, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.023 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.

Nitrogen-doped porous carbons with nanofiber-like structure derived from poly (aniline-co-p-phenylenediamine) for supercapacitors Dazhang Zhu a, Ke Cheng a, Yawei Wang a, Dongmei Sun b,* Lihua Gan a, Ting Chen a, Juxiang Jiang a, Mingxian Liu a,* a

Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and

Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, P. R. China. b

School of Life Science & Technology, Tongji University, 1239 Siping Road, Shanghai 200092, P.

R. China. Corresponding Author: Fax: +86 21 65981097 E-mail: [email protected]; [email protected]

Highlights: 1. A facile and efficient synthesis of nitrogen-doped porous carbons (NPCs) was reported. 2. Poly (aniline-co-p-phenylenediamine) was directed carbonized/activated to prepare NPCs. 3. NPCs exhibit unique nanofiber-like structure, high surface area and suitable nitrogen content. 4. NPCs have high specific capacitance, high rate capability and good cycling stability.

ABSTRACT

We demonstrate a simple synthesis of N-doped porous carbons (NPCs) with nanofiber-like structure

via

one-step

direct

carbonization/KOH

activation

of

poly

(aniline-co-p-phenylenediamine) (P(ANI-co-PPDA)) at temperatures from 600 to 900 °C in N2 atmosphere. The increase of heat treatment temperature results in an increased specific surface area (from 776 to 2022 m2 g−1) while decreased nitrogen content (from 6.96 to 1.18 wt.%). At 700 °C, the resultant NPCs (denoted as NPC-700) show a high surface area (1513 m2 g−1), high N content (6.43 wt.%) and nanofiber-like morphology. In 6 M KOH electrolyte, NPC-700 electrode has a capacitance as high as 316 F g−1 at 1.0 A g−1 and remains 167 F g−1 at 10.0 A g−1. Besides, NPC-700 electrode exhibits good cycling stability, with capacitance retention of 81.6% after 5000 cycles at 1.0 A g−1. The simple synthesis route and high electrochemical performance of the NPCs show great potential in supercapacitor application. Keywords: Nitrogen-doped porous carbons; Poly (aniline-co-p-phenylenediamine); Direct carbonization/activation; Supercapacitor electrode.

1. Introduction

To satisfy the explosive growth of power demand, there has been a urgent requirement to develop efficient energy storage and conversion systems (e.g., lithium-ion batteries and supercapacitors) [1−7]. Supercapacitors (electrochemical capacitor) have been attracted more attentions due to high power density, good cycle stability and quick charge/discharge capability [8−12]. Supercapacitors build a bridge between the conventional electrostatic capacitors and batteries and apply to the automotive applications, digital telecommunication systems and renewable energy storage systems. Supercapacitors could be classified as pseudocapacitors and electrical double-layer capacitors (EDLCs) based upon the mechanism of energy storage. The capacitance in EDLCs is mainly caused by the charge separation at electrolyte-electrode interface [13−18]. For the pseudocapacitors, the capacitance comes from the transfer of electrons between electrolyte ions and electrode surface [19]. The common electrode materials for pseudocapacitors are metal oxide [20−23] and conducting polymers [24, 25]. At present, EDLCs take up most of the commercially supercapacitors using various carbons as the active materials because they possess high physical and chemical stability, well processability, good electronic conductivity [26−31]. However, these virtues are not enough to meet the critical needs for supercapacitors. In addition to the benefits mentioned above, suitable pore size and high surface area are the key characteristics for carbon electrode materials to improve the power density and durability of the supercapacitors [32−36]. The goal of giving the carbon materials efficient paths for ion diffusion is accomplished by controlling over their structure and morphology [31, 37, 38]. For example, Xu group demonstrated the fabrication of high-surface-area nanoporous carbons (2872 m2·g−1) for supercapacitor application using furfuryl

alcohol as a carbon source and metal–organic framework as the template [39]. It was reported that graphene-derived carbon with unique 3D network exhibits excellent electrochemical properties [40, 41]. Our group reported the design and synthesis of carbon nanospheres with novel 3D core–shell ultramicroporous@microporous structure as electrode materials for high-performance EDLCs [27]. It was reported that heteroatoms (e.g., N, O, S and P) anchored to carbon matrix could enhance the electrochemical performance of the materials [42−44]. The existence of nitrogen atom could improve the electrical conductivity of carbon materials [45−49]. Generally, nitrogen-containing carbons can be obtained through the carbonization and activation of the N-containing organic precursors, such as polyaniline, polypyrrole, etc. [50−57]. For example, Zhou

et al. prepared nitrogen-riched porous carbon materials by activation/carbonization of

polymerized ethylenediamine and carbon tetrachloride. When used as a supercapacitor electrode, the resultant carbon has 363 F g−1 (0.1 A g−1) in sulphuric acid solution [26]. By carbonization of a N-containing precursor derived from m-phenylenediamine and terephthalaldehyde, our group fabricated high-performance nitrogen-doped microporous carbon nanoparticles for supercapacitor electrodes (397 F g−1 at 0.1 A g−1 using KOH electrolyte) [58]. It is highly attractive to fabricate N-doped porous carbons (NPCs) through simple carbonization of nitrogen-containing polymers. However, this strategy remains rather challenging due to very limited available organic compounds considering the fact that majority of carbon precursors, owning to low thermal stability, are fully evaporated or decomposed into gaseous molecules during high-temperature treatment [59]. Herein, we demonstrate a facile synthesis of NPCs with nanofiber-like structure derived from poly(aniline-co-p-phenylenediamine) (denoted as P(ANI-co-PPDA)). Polyaniline and poly(p-phenylenediamine) are common conducting

polymers which could be used for supercapacitors, but have poor cycle stability [60, 61]. In this work, p-phenylenediamine and aniline was copolymerized to obtain P(ANI-co-PPDA), followed by direct carbonization/KOH activation to fabricate NPCs. P(ANI-co-PPDA) acts as an "all-in-one" precursor for C and N sources, and thus the synthetic process is quite simple. Besides, P(ANI-co-PPDA) shows unique nanofiber-like structure which benefits the introduction of KOH for activation of the polymer to generate abundant micropores. A typical NPC sample prepared at a heat treatment temperature of 700 °C (denoted as NPC-700) show high surface area, high nitrogen content and nanofiber-like morphology. NPC-700 electrode shows high electrochemical capacitance, good cycling life and high rate feature. This reseach provides a simple and efficient method to prepare heteroatom-doped carbons for supercapacitor applications. 2. Experimental 2.1 Synthesis All chemicals used were analytical reagents. 9.1 mL (10.0 mmol) aniline monomer and 0.108 g (1.0 mmol) PPDA were dissolved in 50 mL 0.1 M HCl solution, and 23.028 g APS (oxidizing agent) was dissolved in 50 mL 0.1 M HCL solution. The above solutions were mixed under vigorous stirring for 24 h to obtain P(ANI-co-PPDA). After centrifugation, washed by water and ethanol (three times), P(ANI-co-PPDA) was simple chemically activated by KOH to fabricate NPCs. Firstly, 0.8 g of P(ANI-co-PPDA) was mixed with 0.4 g of KOH solution containing 2 g deionized water. The solution was dried, and then was heated to 400°C (5°C min−1) for 2 h, 600–900°C (2°C min−1) for 2 h protected by N2 flow to decompose and activate the polymer, and to fabricate NPCs (labeled as NPC-x where x denotes the final activation temperature). 2.2 Characterization

A scanning electron microscopy (SEM, Hitachi S-4800) was used for the morphology observation of NPCs. X-ray diffraction (XRD) characterization was done using a Focus D8 Advance diffracractometer. Using a Thermo Nicolet NEXUS spectroscopy, the Fourier-transform infrared (FT-IR) spectra were tested. The surface chemical composition and state was measured using a AXIS Ultra DLD X-ray photoelectron spectroscopy (XPS). The pore parameters of NPCs were obtained using N2 adsorption on a Micromeritics TriStar 3000 Analyzer at −196°C. 2.3 Electrochemical measurements

A CHI600D electrochemical workstation was used for electrochemical tests. Hg/Hg2Cl2 electrode was used as a reference electrode and Pt as a counter electrode. NPCs, graphite and polytetrafluoroethylene (PTFE) (8:1:1, w/w) were mixed and dispersed in ethanol. Under 20 MPa, the slurry was pressed between two pieces of Ni foam, and then the obtained circle working electrode (1.0 mm thick, 4 mm diameter and ~2 mg active materials) was dried at 80°C, and then was soaked in 6 M KOH electrolyte overnight. Electrochemical impedance spectroscopy (EIS) was done with frequency between 1 mHz and 103 kHz. The capacitance properties were measured by cycle voltammetry (CV), galvanostatic charge-discharge (GCD) with a potential window of 0–1.0 V. The specific capacitances (C, F g−1) of NPC electrodes are obtained from GCD curves using discharging data according to the equation described in our previous work [51]. 3. Results and discussion Fig.1 presents the SEM images of P(ANI-co-PPDA) copolymer and a typical NPC sample (NPC-700). P(ANI-co-PPDA) shows porous nanofiber-like structure (Fig.1a), which benefits the introduction of KOH solution for activation to generate abundant micropores. After carbonization/KOH activation at 600−800°C, there are no significant changes in morphology in

the NPC sample (Fig.1b). XRD pattern of NPCs (Fig.2) show two broad peaks at 2θ=24° and 43° which reflect amorphous carbon structure of the samples. For P(ANI-co-PPDA), two diffraction peaks at 2θ = 20° and 25° can be ascribed to the (020) and (200) crystal planes of the copolymer [62]. Fig. 3 gives the FT-IR spectrum of P(ANI-co-PPDA). The peaks at 1557 and 1485 cm−1 reflect C=C stretching vibration. 1301 cm−1 peak denotes the N–H bending vibration whereas the N–H stretching vibration appears at 3120, 3414 and 3479 cm−1 [60]. The 1138 cm−1 band corresponds to the aromatic C–H bending in the plane [63]. The absorption peak at 1236 cm−1 is resulted from the C–N stretching frequency of the benzenoid ring. The vibration peak at 818 cm−1 is attributed to the p-substituted aromatic ring [64]. This result and that of XRD shown in Fig.2 indicate that P(ANI-co-PPDA) has been successfully synthesized via polymerization of aniline monomer with p-phenylenediamine. The N2 adsorption/desorption isotherms of P(ANI-co-PPDA) and NPCs are given in Fig.4a. The adsorption and desorption isotherms of P(ANI-co-PPDA) basically show a type I curve (P/P0<0.90), which corresponds to microporous materials [65]. When the relative pressure P/P0>0.90, the isotherms of P(ANI-co-PPDA) show the characteristic similar to type II curve, are the normal form of isotherms obtained with a non-porous or macroporous adsorbent, our case is in accordance with the latter (Fig.1a). The pore parameters of NPCs and P(ANI-co-PPDA) were shown in Table 1. The micropores and macropores existed in P(ANI-co-PPDA) contribute 270 m2 g−1 surface area, and are beneficial for the KOH activation. When the activation temperatures are 600−700°C, the resultant NPCs (NPC-600 and NPC-700) show similar isotherms with a much higher N2 sorption capacity. NPC-600 and NPC-700 show the surface area of 772 and 1513 m2 g−1, respectively. Besides, there are weak hysteresis loops in the isotherms of NPC-600 and

NPC-700, suggesting the existence of mesopores [66]. The mesopores also could be seen in Fig.4b, which is 3.84 nm for NPC-600 and 3.63 nm for NPC-700. There are already micropores in P(ANI-co-PPDA), further KOH activation enlarges some of the micropores to mesopore scale. Increasing the activation temperature to 800−900 °C makes higher surface area (1894 m2 g−1 for NPC-800 and 2022 m2 g−1 for NPC-900). The hysteresis loops in the isotherms of NPC-800 and NPC-900 are more obvious and the mesopores in the NPCs are more abundant. The heat treatment temperature influences not only the pore structural parameters of the NPCs, but also the chemical composition and the elemental state. The surface chemistry of NPCs is revealed by XPS analysis, and the results were shown in Fig. 5. There are three distinct peaks in the XPS spectra of NPCs, which means that NPCs contain carbon (285 eV), nitrogen (400 eV), and oxygen (530 eV). The elemental composition is given in Table 2. The nitrogen contents of NPCs decrease from 6.96 to 1.18 wt.% with increasing temperature (600 to 900°C). To clarify the effect of nitrogen atoms on the pseudocapacitive of NPCs, high-solution XPS spectra of N 1s were measured to determine the state of nitrogen atoms in the NPCs, as shown in Figs. 5b-e. The N 1s spectra of NPCs can be fitted into five peaks at 405.5, 402.6, 401.0, 400.5, and 398.4 eV, reflecting chemisorbed nitrogen oxides N-Ox, pyridine N-oxide or ammonia, quaternary N-Q, pyrrole or pyridine N-5 and pyridine N-6 [8, 67]. The relative content of nitrogen atoms in different state were shown in Table 2. N-Q improves the conductivity, and N-6 and N-5 exhibit the pseudocapacitance [50, 68, 69]. Fig. 6 exhibits the Nyquist plots of NPC electrodes using KOH electrolyte (6 M). In the low frequency region, the presence of an almost vertical line indicates good conductivity at the interface of electrode−electrolyte. In the intermediate frequency region, a 45° straight line denotes the Warburg impedance, implication of ion diffusion resistance into the carbon electrode

[65]. The existence of the semicircle at the high frequency corresponds to interface resistance of NPCs electrodes [67]. The equivalent series resistance (ESR) of NPC electrodes which comprises the electrolyte resistance, the intrinsic resistance of the active material, and the interfacial contact resistance of the active material/current collector was calculated from the intersection of the Zʹ axis [27, 71]. NPC-600 and NPC-700 have smaller ESR values (0.27 and 0.28 Ω) than those of NPC-800 and NPC-900 (0.48 and 0.47 Ω). N-Q improves the conductivity, which contributes partly to reduced intrinsic resistance. Different pore parameters and nitrogen content in NPCs due to various carbonization temperatures result in different intrinsic resistances and interfacial contact resistances, and consequent ESR values. The electrochemical performance of NPCs was investigated by CV and GCD tests. CV profiles of NPC electrodes at 10 mV s−1 (Fig.7a) show a quasi-rectangular shape, suggesting electric double-layer capacitive behavior of NPC electrodes [72−74]. The electrochemical capacitance

of

NPC

electrodes

decrease

in

the

sequence

of

NPC-700>NPC-800>NPC-600>NPC-900. From the CV curves of NPC electrodes, the obvious redox peaks have not been observed, suggesting that the charge/discharge process of NPC electrodes was conducted at a pseudo-constant rate over the potential window [75]. CV curves of NPC-700 electrode at different scan rates retain a similar shape (Fig. 7b), suggesting a stable capacitance behavior. With increasing scanning rates, there are slight deformations in cathodic and anodic peaks shift. This kind of phenomenon manifests that both pore structure (double layer capacitance) and redox reaction (pseudocapacitance) contribute the electrochemical capacitance of NPC electrodes at slow scanning rate, while the pore structure gives priority to the electrochemical behavior at high scanning rate [25].

GCD profiles of NPC electrodes at 1.0 A g−1 reveal isosceles triangle between 0–1.0 V (Fig. 8a), suggesting good reversibility. The NPC-700 electrode reaches 316 F g−1 (with a high columbic efficiency of 97.9%), considerably larger than those of other NPC electrodes which are 103−128 F g−1 (with columbic efficiencies of only 46.2−68.4%). The potential is not liner with the charge-discharge time and the GCD profile shows an inflexion, which further verifies that the specific capacitance comes from the electric double-layer formation and redox reaction [76, 77]. NPC-700 electrode shows an optimum balance between the specific surface area and nitrogen content (i.e., double layer capacitance and pseudocapacitance), and consequently exhibits the largest capacitance. Fig. 8b shows GCD profiles of NPC-700 electrode at 1.0−10 A g−1. The electrode still remains 167 F g−1 at 10.0 A g−1, indicating good rate performance. Besides, the GCD profiles at large current density (2.0 A g−1 or above) obviously differ from those at low current density (1.0 A g−1), indicating that the double layer capacitance dominates at high current density [77], which is consistent with that of CV result. To investigate the cycle stability, a crucial index for supercapacitors electrode materials, GCD profiles of NPC-700 electrode at 1.0 A g−1 for 5000 cycles were estimated, as shown in Fig.9. The capacitance drops from 316 to 258 F g−1 (with a retention of 81.6%), revealing good cycle stability of the electrode. 4. Conclusions In conclusion, we demonstrate a simple synthetic strategy for NPCs with unique nanofiber-like

structure

using

one-step

direct

carbonization/activation

of

a

novel

nitrogen-containing carbon precursor of P(ANI-co-PPDA). Carbonization/activation at a suitable temperature of 700 °C shows an optimum balance between the specific surface and nitrogen content. The resultant NPC-700 has a high surface area (1513 m2 g−1) with a high N content (6.43

wt.%). In 6 M KOH solution, NPC-700 electrode exhibits 316 F g−1 at 1.0 A g−1. NPC-700 also exhibits high rate capability and still remains 167 F g−1 at 10.0 A g−1. Besides, the electrode shows 81.6% capacitance retention after 5000 cycles at 1.0 A g−1, exhibiting good cycle life. We believe that the very straightforward synthesis approach combined with well-structured and high-performance NPCs highlight the great potential for advanced energy storage applications.

Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Nos. 21207099, 21273162, and 21473122), the Science and Technology Commission of Shanghai Municipality, China (No. 14DZ2261100), and the Fundamental Research Funds for the Central Universities, and the Large Equipment Test Foundation of Tongji University.

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Captions of Figures and Tables

Fig.1. SEM images of P(ANI-co-PPDA) (a) and NPC-700 (b). Fig.2. XRD patterns of P(ANI-co-PPDA) and NPCs. Fig.3. FT-IR spectra of P(ANI-co-PPDA). Fig.4. N2 adsorption and desorption isotherms (a) and the pore size distribution curves (b) of P(ANI-co-PPDA) and NPCs. Fig.5. Wide-scan XPS spectra of NPCs (a) and fitted high-solution XPS spectra of N 1s of NPC-600 (b), NPC-700 (c), NPC-800 (d), and NPC-900 (e). Fig. 6. Nyquist plots of NPC electrodes in 6 M KOH electrolyte solution. Fig.7. CV curves of NPC electrodes at a scan rate of 10 mV s−1 (a) and NPC-700 electrode at different scan rates (b) in 6 M KOH electrolyte solution. Fig.8. GCD curves of NPC electrodes at 1.0 A g−1 (a) and NPC-700 electrode at different current densities (b) in 6 M KOH electrolyte solution. Fig.9. Cycle stability of NPC-700 electrode at 1.0 A g−1 in 6 M KOH electrolyte solution. Table 1. Pore structural parameters of P(ANI-co-PPDA) and NPCs. Table 2. The elemental composition of NPCs and relative content of nitrogen species to N 1s in the NPCs.

Table1. Pore structural parameters of P(ANI-co-PPDA) and NPCs. SBET

Smicro

Sex

Vt

Vmicro

(m2 g−1)

(m2 g−1)

(m2 g−1)

(cm3 g−1)

(cm3 g−1)

P(ANI-co-PPDA)

272

238

34

0.24

0.10

NPC-600

776

745

31

0.47

0.34

NPC-700

1513

1445

68

0.84

0.67

NPC-800

1894

1821

73

1.09

0.86

NPC-900

2022

1943

79

1.09

0.91

Sample

SBET, the specific surface area; Smicro, the micropore specific surface area; Sex, the external specific surface area; Vt,the total volume; Vmicro, the microporous volume.

Table 2. The elemental composition of NPCs and relative content of nitrogen species to N 1s in the NPCs. Samples

C (wt.%)

N (wt.%)

O (wt.%)

N-6

N-5

N-Q

N-X

N-OX

NPC-600

86.33

6.96

6.71

31.24%

36.94%

22.36%

-

9.46%

NPC-700

83.38

6.91

9.72

16.18%

34.49%

39.46%

-

9.87%

NPC-800

93.15

1.87

4.98

16.31%

21.60%

37.51%

10.94%

13.64%

NPC-900

92.49

1.18

6.33

8.88%

6.69%

52.76%

16.61%

15.06%