nitrogen-doped graphene paper hybrids as binder-free electrode for supercapacitor applications

Accepted Manuscript Free-Standing MnO2/nitrogen-doped graphene paper hybrids as binder-free electrode for supercapacitor applications Xianqiang Feng, Yanhua Li, Guisen Chen, Zheng Liu, Xiaohua Ning, Aiping Hu, Qunli Tang, Xiaohua Chen PII: DOI: Reference:

S0167-577X(18)31209-6 https://doi.org/10.1016/j.matlet.2018.08.026 MLBLUE 24737

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

19 April 2018 19 July 2018 6 August 2018

Please cite this article as: X. Feng, Y. Li, G. Chen, Z. Liu, X. Ning, A. Hu, Q. Tang, X. Chen, Free-Standing MnO2/nitrogen-doped graphene paper hybrids as binder-free electrode for supercapacitor applications, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.08.026

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Free-Standing MnO2/nitrogen-doped graphene paper hybrids as binder-free electrode for supercapacitor applications Xianqiang Feng, Yanhua Li, Guisen Chen, Zheng Liu , Xiaohua Ning, Aiping Hu, Qunli Tang, Xiaohua Chen* College of Materials Science and Engineering, Hunan University, Changsha 410082, China

Abstract: Sandwich-type hybrid structure consisting of etched nitrogen-doped graphene paper (ENGP) and flower-like MnO2 are fabricated through a three-step method. The ENGP were synthesized by vacuum assisted flow-filtration of the dicyandiamide (DCD) and graphene oxide (GO) aqueous solution through filter membrane, followed by a carbonization process, and then by an electrochemical etching method to improve the surface active-sites and wettability. The MnO2 nanoflowers consisting of multiple thin MnO 2 nanosheets were anchored on both side of ENGP by electrodeposition techniques. The optimized hybrid electrodes exhibited a high specific capacitance of 368.3 F/g at 0.2 A/g, and excellent cycling performance of 98.4% capacitance retention over 3000 cycles. Keywords: Sandwich-type hybrid structure; nitrogen-doped graphene paper; MnO2 nanoflowers; supercapacitor; Electrodeposition; Nanocomposites Introduction As the highly efficient power sources, supercapacitors have attracted considerable interest in recent years because of the high power density, long cycling life, and short charging time[1, 2]. It is well known that the electrochemical performance of supercapacitors is mainly determined by the electrode materials[3]. Among the various materials, MnO2 is regarded as one of the most attractive oxide material due to its high theoretical capacitance (~1370 F/g), low cost and environmentally

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benign. However, the poor electrical conductivity of MnO2 significantly hinders its applications[4]. At present, graphene have been combined with MnO2 to enhance the electrochemical performance, because of its extraordinary electrical conductivity, high surface area, and good mechanical properties. Nevertheless, most of the reported MnO2/graphene are in the form of aggregated powders, which will decrease the electrical conductivity of the electrode materials[5]. Nowadays, considerable effort has been made in designing a highly flexible and binder-free electrode substrate for supercapacitors. Moreover, it well agrees on that only the contactable area of MnO2 with electrolyte enables pseudo-capacitive reactions[5]. Therefore, it is ideal to fabricate uniform structures with large and available area. Electrochemical deposition has shown amount of advantages in the preparation of thin films in comparison with other methods, for instance, admirable thickness controllability, high deposition rate and homogeneous coating uniformity [6]. In addition, a number of studies has demonstrated that the nitrogen doped graphene can increase the conductivity and surface active sites of graphene paper and shows superior electrochemical properties[7].

In this work, we present a rational design and fabrication of sandwich-type hybrid etched nitrogen-doped graphene paper (ENGP) both side coated with MnO2 nanoflowers, as shown in Fig 1a. The as-prepared sandwich-type hybrid owns several advantages: 1) ENGP serves as flexible, ultrathin, and highly conductive support without binder for electrode, which can provide high pathways for electrons and effectively reduces the connection resistance between the MnO 2 nanoflowers and ENGP; 2) the uniform and tight interfacial contact between MnO 2 and ENGP can not only improve the stability of the electrode but also prevent MnO 2 from conglomeration; 3) MnO2 nanoflowers consisting of multiple thin MnO 2 nanosheets can greatly shorten the diffusion length of

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electrolyte ions and increases the contactable area of MnO 2 with electrolyte, so that increases the electrochemical utilization of MnO2. As a result, the hybrid electrodes exhibited an excellent electrochemical performance.

2. Experiment 2.1. Preparation of nitrogen-doped graphene paper Briefly, 0.6g Dicyandiamide (DCD) powder and 6ml GO aqueous suspension (10 mg/mL) were added in to 20 ml DI water with mild stirring for 2 h. Then, GO/DCD paper was made by filtration of the composite dispersion through filter membrane. The GO/DCD paper was calcined at 750 °C for 5 h under flowing argon to obtain the final nitrogen-doped graphene paper (denoted as NGP).

2.2. Electrochemical etching of NGP The NGP as anode was immersed in the electrolyte and a graphite rod served as cathode. The ammonium bicarbonate was used as electrolytes in concentration of 6%, and the treatment process was performed with a current density of 40mA/cm2 at 50°C for 30sec (denoted as ENGP).

2.3. The deposition of MnO2 on ENGP The electrolyte was prepared by mixing the aqueous solution of 0.05 M Mn(NO 3)2 and 0.1 M NaNO3 in deionized water. MnO2 was electrodeposited by a conventional three-electrode system. The tailored ENGP (1×1 cm2) was used as working electrode while a saturated Ag/AgCl electrode was used as a reference electrode and a platinum foil was counter electrode. The different mass loading of MnO2 on ENGP were obtained by changing the deposition time (1h, 2h and 3h). The resulted materials are denoted as MnO2-ENGP-1, MnO2-ENGP-2 and MnO2-ENGP-3.

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2.4. Characterization The morphologies and microstructures of the prepared samples were characterized by scanning electron microscope (SEM, Hitachi, S-4800, 10 kV). Chemical state analysis was carried out by X-ray photoelectron spectroscopy (XPS) using a K-Alpha 1063 (Thermo Fisher-VG Scientific, UK) with an Al Ka X-ray source operating at 12 kV and 6 mA, with a Thermo Avantage survey and analysis system. The crystal structures were characterized using powder X-ray diffraction (XRD, Cu Ka radiation, 0.15418 nm, 40 kV, 50 mA). Electrochemical testing was performed using a CHI660E electrochemical workstation with a conventional three-electrode system.

3. Results and discussion The surface morphology of the NGP and ENGP was investigated by SEM. It can be seen that the surface of NGP (Fig. 1b) is much smoother than ENGP (Fig. 1c). The ENGP surface has numerous protuberant wrinkles and ripples, which are resulted from the electrolytic etching process. Figure 1d shows the digital photographs of the ENGP material with highly flexible and bendable. The cross-sectional images (Figure. 1e, f) in different magnifications clearly show the continuously cross-linked graphene nanosheets were orderly stacked in a layer-by-layer pattern. This unique layered structure is beneficial to the sufficient permeation of electrolyte and fast diffusion of ions. In Figure 1g-i, the MnO2 nanoflowers with an apparent size of about 1 μm consisting of multiple thin MnO2 nanosheets (insert of Fig. 1g), are homogeneously decorated on the ENGP surface.

Figure. 2 shows the XPS

NGP. The content of nitrogen in the NGP is

calculated to be 7.39%. In Figure 2a, the high resolution N1s spectrum displays four different component peaks, including pyridinic (398.3 eV), pyrrolic (399.6 eV), graphitic (401.1 eV) and

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oxidized nitrogen (402.7 eV), respectively[8]. The O1s spectra as shown in Fig. 2b can be divided into three peaks centered at 531.0, 532.3 and 533.7 eV, which corresponds to O=C-OH, C=O and C-OH configurations. In Fig. 2c,d, the overall oxygen content of ENGP was 14.41 at. %, which is higher than that of NGP (6.61 at. %), implying that the electrolytic etching method successfully introduced oxygen-containing functional groups into ENGP. Fig.2e,f shows the XRD pattern of ENGP and MnO 2-ENGP-2. The sharp peak at 26˚ associating with the (002) reflection of graphite structure presents in both patterns. The broad and weak peaks in Fig. 2f confirm the presence of MnO2 in two types (JCPDS 44-0141 and JCPDS 53-0633).

In Fig. 3a, the CV curves of MnO2-ENGP-1 and MnO2-ENGP-2 display nearly rectangular shape. However, the curve for MnO2-ENGP-3 has significant deformation, indicating serious polarization. Due to slow diffusion rate of electrolyte ions, concentration of electrolyte ions occurred and brought about the difference in the concentration in the interface and bulk of the electrode, which may be the main cause of the polarization. The polarization is a reflection of the increase in contact resistance, resulting from the thicker and dense deposition of MnO2 in MnO2-ENGP-3. The galvanostatic charge–discharge (GCD) curves for all samples are nonlinear during the charge and discharge process, indicating the pseudocapacitive behavior due to the Faradaic redox reactions of Mn4+/Mn3+ (Fig.3b). Fig.3b also exhibits that the MnO2-ENGP-2 has a higher specific capacitance (418.1F/g) than MnO2-ENGP-1 (130.6F/g) and MnO2-ENGP-3 (347.6F/g) at the current density of 0.1A/g. The typical CV performance of MnO2-ENGP-2 at a scan rate of 2 mV/s to 50 mV/s as shown in Fig.3c. There is only a small distortion in each CV curve, even at a scanning rate as high as 50 mV/s, indicating a good rate capability. Fig. 3d shows the GCD curves of MnO2-ENGP-2 retain

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symmetry, suggesting the great reversibility of the electrode. Fig. 3e shows the Nyquist plots of ENGP, MnO2-ENGP-2 and MnO2-ENGP-3. Although the charge transfer resistance (Rct) of MnO2-ENGP-2 electrode is slightly larger than that of ENGP, it is still smaller than that of many MnO2/conductive material composites in previous reports [9, 10]. It is obvious that he Rct and Warburg resistance of MnO2-ENGP-3 are larger than that of MnO2-ENGP-2, which arises out of the thicker and dense deposition of MnO 2 in MnO2-ENGP-3, may be the cause of serious polarization in CV curves as showed in Fig. 3a.The cyclic performance of MnO2-ENGP-2 is shown in Fig. 3f. It can be seen that the capacitance retention of MnO2-ENGP-2 is 98.4% after 3000 cycles, which is prior to some other MnO2/graphene supercapacitors reported [11, 12]. In addition, there is an increase during 1200 cycles, which may be due to an increase of cation intercalation in the interior of MnO 2 during cycles and the Faradaic reactions increase, where the redox reactions not only take place on the surface but also happen in the inside. This great cycle stability may be contributed to three features: First, ENGP serves as flexible support can prevent structural breakdown and electrode pulverization during long-term cycling. Second, the strong connection between the MnO2 and ENGP can improve the stability of the electrode. Third, the unique sandwich-type hybrid structure can effectively accommodate the strain and stress of volume change, which enhances cycling stability. The improved capacitive behavior resulted from the unique sandwich-type hybrid architecture. Fig.4 shows a schematic illustration of MnO2-ENGP electrodes architecture for supercapacitor application.

Conclusions In summary, we have rationally synthesized the sandwich-type hybrid etched nitrogen-doped graphene paper both side coated with MnO2 nanoflowers via a simple and feasible method. This

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ideal electrode architecture helped to reduce the ion diffusion length, increase the contactable area of active materials with electrolyte and improve the utilization of MnO 2. The assembled MnO2-ENGP-2 electrode exhibited a high capacitance of 368.3 F/g at current density of 0.2 A/g and high cycle-life stability with capacitance retention of 98.4% at 5A /g after 3000 cycles. We believed that this hybrid film has the potential application in high performance supercapacitors.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51572078, 51772086, 51272073 and 51541203) and the Scientific Research Fund of Hunan Province (2015JJ2033). References [1] Z. Liu, Y. Zeng, Q. Tang, A. Hu, K. Xiao, S. Zhang, W. Deng, B. Fan, Y. Zhu, X. Chen, Journal of Power Sources 361 (2017) 70-79. [2] X.a. Chen, X. Chen, F. Zhang, Z. Yang, S. Huang, Journal of Power Sources 243 (2013) 555-561. [3] H. Zhou, X. Yang, J. Lv, Q. Dang, L. Kang, Z. Lei, Z. Yang, Z. Hao, Z.-H. Liu, Electrochimica Acta 154(Supplement C) (2015) 300-307. [4] M. Pang, G. Long, S. Jiang, Y. Ji, W. Han, B. Wang, X. Liu, Y. Xi, Materials Science and Engineering: B 194(Supplement C) (2015) 41-47. [5] M. Kim, Y. Hwang, J. Kim, Journal of Power Sources 239(Supplement C) (2013) 225-233. [6] Z.G. Ye, T. Li, G. Ma, X.Y. Peng, J. Zhao, Journal of Power Sources 351 (2017) 51-57. [7] Y. Qin, J. Yuan, J. Li, D. Chen, Y. Kong, F. Chu, Y. Tao, M. Liu, Advanced Materials 27(35) (2015) 5171-5175. [8] Z. Liu, K. Xiao, H. Guo, X. Ning, A. Hu, Q. Tang, B. Fan, Y. Zhu, X. Chen, Carbon 117(Supplement C) (2017) 163-173. [9] X.H. Zhang, X.Y. Meng, S.L. Gong, P. Li, L.E. Jin, Q. Cao, Materials Letters 179 (2016) 73-77. [10] M.H. Tahmasebi, K. Raeissi, M.A. Golozar, A. Vicenzo, M. Hashempour, M. Bestetti, Electrochimica Acta 190 (2016) 636-647. [11] S. Sun, P. Wang, S. Wang, Q. Wu, S. Fang, Materials Letters 145(Supplement C) (2015) 141-144. [12] Y. Liu, D. Yan, R. Zhuo, S. Li, Z. Wu, J. Wang, P. Ren, P. Yan, Z. Geng, Design, Journal of Power Sources 242(Supplement C) (2013) 78-85.

Figure captions

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Fig. 1. Schematic illustration for preparation of MnO2-ENGP electrode (a); Surface SEM images of NGP (b) and ENGP (c); The digital image of the curled ENGP (d); (e,f) Cross sectional views of ENGP; (g-i) SEM images of MnO2-ENGP-1(g), MnO2-ENGP-2(h) and MnO2-ENGP-3(i). Inset in (g) shows high-magnification morphology of an individual MnO2 flower-like particle.

Fig. 2. High-resolution spectra of N1s (a) and O1s (b) of the ENGP; XPS spectra of ENGP (c) and NGP (d); XRD patterns of ENGP (e) and MnO2-ENGP-2 (f).

Fig. 3 CV (a) and GCD (b) curves of MnO2-ENGP electrodes; CV (c) and GCD (d) curves of MnO2-ENGP-2; Nyquist plots of MnO2-ENGP-2 and ENGP (e); Cyclic performance of MnO2-ENGP-2 (f).

Fig. 4. Schematic illustration the ideal architecture of MnO2-ENGP composites for supercapacitors application.

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Highlights 

Sandwich-type hybrid consisting of ENGP and flower-like MnO2 are obtained

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ENGP serves as flexible and highly conductive support without binder for electrode

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Flower-like morphology of MnO2 can increase the contactable area with electrolyte

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The tightly contact between MnO2 and ENGP can improve the stability of electrode

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Composite exhibits a high capacity retention of 98.4% after 3000 cycles at 5 A/g

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