Accepted Manuscript Manganese oxide nanowires wrapped with nitrogen doped carbon layers for high performance supercapacitors Ying Li, Yuan Mei, Lin-Qun Zhang, Jian-Hai Wang, An-Ran Liu, Yuan-Jian Zhang, Song-Qin Liu PII: DOI: Reference:

S0021-9797(15)00488-9 http://dx.doi.org/10.1016/j.jcis.2015.04.070 YJCIS 20483

To appear in:

Journal of Colloid and Interface Science

Received Date: Accepted Date:

19 February 2015 23 April 2015

Please cite this article as: Y. Li, Y. Mei, L-Q. Zhang, J-H. Wang, A-R. Liu, Y-J. Zhang, S-Q. Liu, Manganese oxide nanowires wrapped with nitrogen doped carbon layers for high performance supercapacitors, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.04.070

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Manganese oxide nanowires wrapped with nitrogen doped carbon layers for high performance supercapacitors Ying Li, Yuan Mei, Lin-Qun Zhang, Jian-Hai Wang, An-Ran Liu, Yuan-Jian Zhang, Song-Qin Liu* Jiangsu Province Hi-Tech Key Laboratory for Bio-medical Research, School of Chemistry and Chemical Engineering, Southeast University, Nanjing 210096, China. Fax: +86-25-52090618; Tel: +86-25-52090613; E-mail address: [email protected] Abstract: In this study, manganese oxide nanowires wrapped by nitrogen-doped carbon layers (MnOx@NCs) were prepared by carbonization of poly(o-phenylenediamine) layer coated onto MnO2 nanowires for high performance supercapacitors. The component and structure of the MnOx@NCs were controlled through carbonization procedure under different temperatures. Results demonstrated that this composite combined the high conductivity and high specific surface area of nitrogen-doped carbon layers with the high pseudo-capacitance of manganese oxide nanowires. The as-prepared MnOx@NCs exhibited superior capacitive properties in 1 M Na2SO4 aqueous solution, such as high conductivity (4.167 × 10-3 S cm-1), high specific capacitance (269 F g-1 at 10 mV s-1) and long cycle life (134 F g-1 after 1200 cycles at a scan rate of 50 mV s-1). It is reckoned that the present novel hybrid nanowires can serve as a promising electrode material for supercapacitors and other electrochemical devices. Keywords: Manganese oxide, Poly(o-phenylenediamine), Nitrogen-doped carbon, Supercapacitor Introduction: Supercapacitors have higher power density compared to capacitors and better cycle life than batteries, thus they can meet the demands for various applications, such as consumer electronics, or industrial supporting powers. Nowadays, considerable efforts have been devoted to the researches of electrode materials for supercapacitors, such as transition metal oxides, conducting polymers and carbon materials. Carbon materials, most commonly activated carbons (ACs) and carbon nanotubes (CNTs), exhibit high double-layer capacitance, high electronic conductivity, high specific

surface area, and good electrochemical stability [1-6]. Doping carbon materials with nitrogen holds great potential to enhance the electrical conductivity and surface area of carbon materials, which are benefit to their capacitive properties [7-9]. Unfortunately, the specific capacitance of the nitrogen-doped carbon materials is still limited compared to the pseudo-capacitance of transition metal oxides or conducting polymers. As one of the most promising electrode materials for aqueous asymmetric supercapacitors, manganese oxide is expected to provide a wide operating voltage at high energy, low cost and eco-friendly nature [10,11]. However, few studies have been reported on manganese oxide as pseudo-capacitor materials due to the weak conductivity, poor cycling stability and rate capability of manganese oxide. In the course of handling these puzzles, combining manganese oxide with nitrogen doped carbon materials has been proposed to exploit the relative advantages and alleviate the relative disadvantages of each material [12-14]. Herein, we synthesized manganese oxide nanowires wrapped by nitrogen-doped carbon layers (MnOx@NCs) to construct a balance between the high conductivity and high specific surface area of nitrogen-doped carbon layers (NCs) and the high pseudo-capacitance deriving from manganese oxide nanowires. The NCs were prepared through carbonization of poly(o-phenylenediamine) (PoPD) layers, which were grown onto the surface of MnO2 nanowires by using MnO2 nanowires as the oxidant and template. The morphologies of the PoPD layers and NCs were restricted by the MnO2 nanowire template through the electrostatic and coordinative action at the interface of the MnO2 nanowire and PoPD layers. This strategy provides a new method to prepare nitrogen-doped carbon materials grown onto the surface of metal oxide as a high-performance electrode material for asymmetric supercapacitors. The manganese oxide in our hybrid nanomaterial was efficiently utilized with the assistance of the highly conductive nitrogen-doped carbon shell. Results revealed that the hybrid nanowires exhibited high conductivity (4.167 × 10-3 S cm-1), high specific capacitance (269 F g-1 at 10 mV s-1) and long cycle life (134 F g-1 after 1200 cycles at a scan rate of 50 mV s-1).

2. Experimental 2.1. Synthesis of MnO2 nanowires Firstly, 1.759 g Mn(Ac)2·4H2O and 2.307 g sodium dodecyl benzene sulfonate (SDBS) were dissolved in 210 mL deionized water at 70 ºC under stirring to get a homogeneous solution. Then 0.948 g KMnO4 was added to the solution with continuous stirring for 30 min. After that, the resulting cloudy solution was transferred into a Teflon-lined stainless steel autoclave, heated at 180 ºC for 5 h, and followed by natural cooling to room temperature. Finally, the precipitates were collected by filtration, washed with deionized water and absolute ethanol to remove the SDBS, and then dried at 60 ºC for 12 hours. 2.2. Synthesis of MnOx@NCs Firstly, a PoPD layer was formed by using MnO2 nanowires as an oxidant and template, using triblock copolymer PEO-PPO-PEO (P123) as a structure-directing agent to generate mesopores in NCs after the heat post-treatment. That is, 0.500 g MnO2 nanowires and 0.500 g P123 were dissolved in 450 mL deionized water under magnetic stirring for 10 min. Subsequently, 0.706 g o-phenylenediamine (oPD) was added to the mixture under stirring for 3 h. Then, the precipitates, i.e. manganese oxide nanowires wrapped by PoPD (MnO2@PoPD), were filtrated, washed several times with water, and dried at 80 ºC overnight. Secondly, the MnO2@PoPD were carbonized to prepare manganese oxide nanowires wrapped by nitrogen-doped carbon layers under flowing nitrogen at a series of temperatures for 3 h. The as-prepared materials were labeled as MnOx@NCs-450, MnOx@NCs-550,

MnOx@NCs-650,

MnOx@NCs-750,

MnOx@NCs-850,

respectively, and the numbers represented the different carbonization temperatures. 2.3. Characterization The crystal structure of the MnOx@NCs was characterized with X-ray powder diffractometer (XRD, Rigaku, D/Max 2500VL/PC, Cu Kα radiation). Surface morphology and microstructure of the materials were investigated with a field emission scanning electron microscope (FE-SEM, Zeiss, Ultra Plus) and a

transmission electron microscope (TEM, Jeol, JEM-2100). Surface area (BET method) was measured by Nova 1000e (Quantachrom, USA). Chemical structure and composition of the materials were characterized with X-ray photoelectron spectroscope (XPS, PHI 5000 VersaProbe, Ulvac-Phi, Japan), respectively. Electronic conductivity of powder materials was measured by four-probe test (SZT-B, Jingge, China). 2.4. Electrochemical measurements The electrochemical measurements were performed on a CHI 660C electrochemical workstation (Chenhua, Shanghai, China) with a traditional three-electrode system in 1 M Na2SO4 solution. The reference electrode was a saturated calomel electrode (SCE) and the counter electrode was a 1 cm2 square Pt sheet. The working electrode was prepared by mixing the active materials (80 wt. %) with acetylene black (15 wt. %), and poly(vinylidene fluoride) (PVDF, 5 wt. %) with a mass loading of 1.25 mg cm-2, which was similar to that reported in literature (a net mass per area of ~1.24 mg cm-2) [10]. N-methyl pyrrolidone (NMP) was used as solvent to disperse those materials to manufacture a homogeneous paste. Then the paste was coated onto the nickel foam (1 cm2), dried at 80 ºC for 4 h, and pressed to a thin film at a press of 2.0 MP. 3. Results and discussion The MnO2 nanowires were synthesized by a typical surfactant-assisted hydrothermal method, by which SDBS acted as soft template and its concentration was beyond critical micelle concentration. As shown in Fig. 1, the as-prepared MnO2 nanowires had a diameter distribution from 30 nm to 50 nm and a length more than 1.5 μm. It was well known that one-dimensional (1D) nanostructures with controlled size, crystallinity and chemical composition could achieve fast redox reactions, high specific surface areas and short diffusion paths for electrons and ions [15]. Nevertheless, the powder conductivity of the MnO2 nanowires was only 6.8 × 10-5 S cm-1, which is detrimental to its application in supercapacitors with high energy and power densities. Using the MnO2 nanowires as an oxidant and template, an adherent PoPD layer was formed on the surface of the MnO2 nanowires and its thickness was estimated to be 6 nm. The powder conductivity of the MnO2@PoPD increases to 2.4

× 10-4 S cm-1. After thermal treatment at 650 ºC for 3 h, manganese oxide nanowires were transformed into manganese oxide nanoparticles, which were wrapped and linked together by the NCs. Most importantly, the MnOx@NCs-650 maintained the original 1D morphology, except that their lengths decreased to ~800 nm and their diameter slightly increased. In addition, the conductivity of the MnOx@NCs-650 further increased to 4.2 × 10-3 S cm-1.

Furthermore, through electrostatic and

coordinative action between the manganese and nitrogen, NCs can anchor the manganese oxide nanoparticles, avoiding migration and aggregation of manganese oxide nanoparticles, thereby improving the durability of manganese oxide nanoparticles. These features were all anticipated to be very beneficial for capacitive performances. X-ray diffraction (XRD) was conducted to investigate the crystal phase of MnO2 nanowires and the MnOx@NCs-650, as shown in Fig. 2A. It can be clearly seen that sharp peaks representing MnO2 are observed for the as-synthesized MnO2 nanowires, corresponding to the literature [16]. Meanwhile, for the MnOx@NCs-650, the peaks of the MnO2 almost completely disappeared and the presence of the cubic MnO is indicated by the peaks at 35.0º, 40.5º and 58.7º, which is in good agreement with the literature (JCPDS, file No.: 89-2804) [17]. These results revealed that MnO2 nanowires underwent a phase transition and thermal decomposition reaction during heat treatment in the inert atmosphere. From the N2 sorption/desorption isotherms (Fig. 2B), the MnOx@NCs-650 also showed a much higher BET surface area of 87 m2 g-1 compared to the uncarbonizational MnO2@PoPD (25 m2 g-1) and the MnO2 nanowires (38 m2 g-1). The improved BET surface area of MnOx@NCs-650 mainly resulted from the removal of P123 during the carbonization [15]. The high BET surface area of the MnOx@NCs-650 can provide efficient ion transport in the electrode, hence leading to a rapid charge transport mechanism and improved utilization of the composite in pseudo-capacitive reaction. Theoretically, the BET surface area of nitrogen-doped carbon materials should be easily tuned by heat treatment temperature. Therefore, the heat treatment of the MnO2@PoPD was optimized at different temperature.

The chemical states and electronic structure of the Mn and N atoms in the as-synthesized materials obtained at different temperatures were identified by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 3A, the effect of calcination temperature upon the valence of manganese oxides in these samples was studied. For MnOx@NCs-550, the XPS Mn 2p spectrum was deconvoluted into two components. The peaks centered at 654.3 and 642.6 eV with a spin-energy separation of 11.7 eV can be attributed to Mn 2p3/2 and Mn 2p1/2, respectively. These peaks agree with those reported for MnO2. Besides, the binding energies of 652.4 eV and 641.2 eV exhibit a spin orbital splitting between Mn 2p3/2 and Mn 2p1/2, which are well matched with those of the reported Mn3O4 [12]. These XPS results confirmed that most of MnO2 nanowires decomposed to form Mn3O4 at 550 ºC. Hence, we refer the present manganese oxides as MnOx in theses composite materials. At 650 ºC, the two peaks of Mn 2p was located at 652.8 and 641.4 eV with a spin-energy separation of 11.4 eV, confirming that MnO was predominant constitute in agreement with the observation from the XRD measurements. When MnO2@PoPD was calcinated at 850 ºC, the two peaks of Mn 2p spectrum obviously shifted to 652.8 and 641.4 eV, demonstrating that only MnO existed in theses manganese oxide composite materials. The transformation of the nitrogen structure during porolysis has also been evaluated by XPS measurements, as shown in Fig. 3B. It is noteworthy that the N 1s peaks shift to higher binding energies as the calcination temperature increases, meaning that pyrrolic N (400.1 eV) and quaternary-N (401.4 eV) [18] gradually emerge, meanwhile the quinoid imine (=N-) (398.0 eV), benzenoid amine (-NH) (399.7 eV), and pyridinic N (397.5 eV) [19] gradually disappears during carbonization. Pyrrolic N is bonded to two carbon atoms in six-membered rings at the vacancies and edges of graphene layers, resulting in pseudocapacitive effects in aqueous electrolytes. These indicate that the polycyclic-type rings and the good π-conjugated structure arose and the conductivity of the composite materials increased during carbonization [14]. In addition, benefiting from rich N species, the surfaces of MnOx@NCs are favorably hydrophilic, which promotes the deep wetting of the electrolyte into electrode and makes more of the internal surfaces accessible to the electrolyte [20]. Simultaneously,

the introduction of nitrogen atoms leads to the NCs possessing more concentrated mesopore and effective specific surface area beneficial to the electric double layer capacitors. However, the intensity of the N 1s peaks decreased with the increasing temperature, implying that a high pyrolysis temperature results in a substantial denitrogenation, and consequently yields a carbon product with a low N content. To understand the materials’ capacitive behaviors, electrochemical impedance spectroscopy (EIS) was measured as shown in Fig. 4A. In the high-frequency region, the semicircles in Nyquist plots revealed that the RCT (charge-transfer resistance) of the MnOx@NCs-650 was lower than those of the MnO2 nanowires and MnO2@PoPD, indicating negligible charge transfer impedance and excellent electrical conductivity. Meanwhile, at low-frequency region, unlike the vertical plot for ideal electric double layer capacitors, all of these slopes had deviances deriving from a contribution of pseudo-capacitance. MnOx@NCs-650 had the steepest slope and the smallest Rct among these three materials, showing the best capacitive property among these materials. These results indicated that the samples produced at 650 ºC got higher content and appropriate chemical states of heteroatom doping. The energy storage application of this novel hybrid nanostructure was evaluated by cyclic voltammogram (CV). Fig. 4B showed the CV curves of the MnOx@NCs thermally treated at different temperatures, and the pristine MnO2 nanowires recorded at a scan rate of 10 mV s-1. The pristine MnO2 nanowires showed a reduction peak at low potential (< -0.1 V), and a broad indistinct oxidation peak, which were derived from the pseudo-capacitance. Meanwhile, for the MnOx@NCs, the oxidation peak shifted negatively and reduction peak shifted positively, meaning that the chemical states and structure of MnOx@NCs can enhance the conductivity of the as-synthesized material, facilitate ionic diffusion, and accelerate the electrode reaction to prevent polarization. Thereby, the utilization of the composite in pseudo-capacitive reaction could be improved. From the integral areas surrounded by CV curves, it can be concluded that the specific capacitance of the MnOx@NC-650, as high as 269 F g-1, is much larger than that of the pristine MnO2 nanowires (111 F g-1). Moreover, we can find that as the temperature increases, the specific capacitance increases and gets

maximum of 269 F g-1 at 650 ºC, and then it drops down as temperature gets higher, in agreement with the observation from the EIS measurements. CV of MnOx@NCs-650 electrode at different scan rates (10 mV s-1, 20 mV s-1, 50 mV s-1, 100 mV s-1) was investigated, as shown in Fig. 5A. The steep slopes of the current change at the switching potentials indicated a small mass-transfer resistance. It seems that the nitrogen-doped carbon shells facilitated this ideal capacitive behavior by allowing fast ionic motion, and these capacitive behaviors were maintained at a scan rate of 100 mV s-1. At a low scan rate of 10 mV s-1, the specific capacitance of the MnOx@NC-650 hybrid is 269 F g-1, which is much higher than those of other MnO2-based

composite

materials

reported

in

the

literatures,

such

as

MnO2/multi-walled carbon nanotube composites (179 F g-1 at 5 mV s-1) [21], CNT/carbon microfiber/MnO2 composites (180 F g-1 at 10 mV s-1) [22], and highly graphitic carbon-tipped MnO2/mesoporous carbon/MnO2 nanowires (226 F g-1 at 10 mV s-1) [15]. It is noted that even at a high scan rate of 100 mV s-1, the MnOx@NC-650 hybrid retain a capacitance of 135 F g-1, suggesting that such high specific capacitance can be maintained under high power operation. As shown in Fig. 5B, the galvanostatic charge-discharge curves were also measured at current densities of 0.5 A g-1, 1 A g-1 and 2 A g-1, and the calculated specific capacitance values of MnOx@NCs-650 were 201 F g-1, 172 F g-1, and 142 F g-1, respectively. The specific capacitance of MnOx@NCs-650 is also higher than those of graphene oxide-MnO2 nanocomposites (111 F g-1 at 1 A g-1) [23] and MnO2 nanoflower/carbon nanotube array (199 F g-1 at low current density) [24]. Moreover, these galvanostatic charge/discharge curves exhibited lower dynamic voltage (IR) drops, implying an enhanced conductivity of MnOx@NCs-650 and a good contact between the MnOx and the NCs. The long-term cycle stability of electrode materials is a critical requirement for practical applications. Fig. 5C shows the CV curves at a scan rate of 50 mV s-1 for 1200 cycles. Although the specific capacitance decreases with the cycle number, it still remains 134 F g-1 after 1200 cycles, which is much higher than that of graphene oxide-MnO2 nanocomposites (111 F g-1 at 1 A g-1 at the beginning of cycle test) [23].

The results demonstrate that our materials, showing a higher specific capacitance, even after 1200 cycles, are expected to have a significant impact in the community. Through thermal treatment at 650 ºC for 3 h, MnO2 was partially transferred into MnO. Nitrogen atoms were incorporated directly into sp2 carbon lattices, forming stable nitrogen functional groups, such as graphitic nitrogen, pyridinic nitrogen and pyrrolic nitrogen [18], meanwhile the composite retained the original nanowire structure. Compared to the pristine carbon materials, incorporation of nitrogen into carbon structure introduces more active sites for anchoring metal oxide particles through electrostatic and coordinative action, avoiding the migration and aggregation of nanocatalysts, and improving the durability of the active materials. Moreover, the nitrogen doped carbon layers possess high surface area, good conductivity, abundant mesopores, which results in high capacitance and good cycle life. These indicate that the unique structure significantly combines the advantages of different materials, and alleviate the relative disadvantages of each material and achieve better performance. 4. Conclusions MnOx@NC composite materials have been fabricated by doping carbon layer on the MnO2 nanowire surface with carbonization process. The effects of temperature on the crystal phase, chemical states, electronic structure and capacitive behavior of MnOx@NCs were studied. The nitrogen doped carbon layer achieves large specific area, and obtain high specific capacitance and long cycle life. Results revealed that the MnO2 nanowires wrapped in the carbon layer worked as a template and oxidant for the polymerization of PoPD. The composite exhibits a high conductivity of 4.167 × 10-3 S cm-1, specific capacitance of 269 F g-1 at 10 mV s-1 and good cycle capability (134 F g-1 after 1200 cycles at a scan rate of 50 mV s-1). The results obtained here indicate that MnOx@NCs composites are promising for electrochemical applications. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 21035002, 21121091, 21005016) and the Natural Science Foundation of Jiangsu Province (No. BK2011591).

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Fig. 1. (A, B) SEM and TEM images of MnO2 nanowires, respectively. (C, D) SEM and TEM images of MnO2@PoPD, respectively. (E, F) SEM and TEM images of MnOx@NCs-650, respectively. Fig. 2. (A) XRD patterns of MnO2 nanowires and MnOx@NCs-650. (B) Nitrogen adsorption and desorption isotherms of MnO2 nanowires, MnO2@PoPD and MnOx@NCs-650. Fig. 3. XPS spectra of the as-prepared MnOx@NCs obtained at different temperatures: (A) Mn 2p, (B) N 1s. Fig. 4. (A) EIS plots for MnOx@NCs obtained at different temperatures with an amplitude of 5 mV over a frequency range from 0.1 Hz to 100 kHz. (B) CV curves of the MnOx@NCs obtained at different temperatures and the pristine MnO2 nanowires at a scan rate of 10 mV s-1. Fig. 5. (A) CV of MnOx@NCs-650 electrode at different scan rates (10 mV s-1, 20 mV s-1, 50 mV s-1, 100 mV s-1). (B) Charge-discharge curves at 0.05 ~ 2 A g-1 of the MnOx@NCs-650 electrode. (C) Cyclic stability of MnOx@NCs-650 electrode in 1 M Na2SO4 at a scan rate of 50 mV s-1.

Fig. 1 A

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Graphical abstract