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High nitrogen-doped carbon/Mn3O4 hybrids synthesized from nitrogen-rich coordination polymer particles as supercapacitor electrodes† Kuaibing Wang,*a Xiaobo Shi,b Aiming Lu,a Xiaoyan Ma,c Zhiyang Zhang,c Yanan Lua and Hongju Wanga High nitrogen-doped carbon/Mn3O4 composites were synthesized by annealing nitrogen-rich Mn-based coordination polymer particles, and investigated by electron microscopy, X-ray diffraction, and electrochemical experiments. To assess the performance of high nitrogen-doped hybrids as electrode materials in supercapacitors, cyclic voltammetry and galvanostatic charging–discharging measurements are performed. High nitrogen-doped carbon/Mn3O4 composites are charged and discharged faster and have higher capacitance than carbon/Mn3O4 nanostructures with low nitrogen amounts and other reported ones. The capacitance of the high nitrogen-doped carbon/Mn3O4 is 94% retained after 1000 cycles at a

Received 12th August 2014, Accepted 1st October 2014

constant current. These improvements can be attributed to the nitrogen-doped carbon matrix, which promotes fast Faradaic charging and discharging of the Mn3O4 motifs. The nitrogen-doped carbon/

DOI: 10.1039/c4dt02456g

Mn3O4 composites could be a promising candidate material for a high-capacity, low-cost, and environ-

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mentally friendly electrode for supercapacitors.

Introduction Carbon-supported Mn3O4 composites have been extensively studied in electrochemical capacitor electrodes, not only because the excellent conductivity of carbon-based materials can promote Faradaic charge-transfer reactions and enhance the accessibility to the electro-sites of active materials, but also due to the low cost, environmental compatibility and intrinsically high capacity of Mn3O4 structures.1–6 Creating carbonbased materials is an effective strategy to solve the problem of the poor electronic conductivity (10−5–10−6 S cm−1) of Mn3O4 and maintains high electrolyte penetration/diffusion rates with the aim of improving its capacitive performance.7,8 However, all of these results are far from meeting the demands of commercial application for their synthetic complexity, relatively

a Department of Chemistry, College of Science, Nanjing Agricultural University, Nanjing 210095, P. R. China. E-mail: [email protected] b College of Agriculture, Nanjing Agricultural University, Nanjing 210095, P. R. China c State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China † Electronic supplementary information (ESI) available: XRD patterns for CPP-2 and CPP-3 with simulated ones separately, crystal structures for bulk-crystals of CPP- and CPP-3, IR spectra for CPP-1, SEM images of C/Mn3O4 composites, and Table S1 for reported N-doped samples. See DOI: 10.1039/c4dt02456g

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worse reversibility or lower stability. Therefore, seeking facile and efficient synthesis approaches for carbon/Mn3O4 (C/Mn3O4) composites with better electrochemical performance remains challenging. Among them, nitrogen-doped (N-doped) carbon materials have drawn much attention due to their unique properties and wide applications in supercapacitors, solar cells and fuel cells.9–11 More importantly, in the process of designing and improving N-doped carbon materials, a great number of chemical and physical approaches have been documented for adjusting the N amount and consequently affecting the resulting properties, such as ammonia treatment, chemical vapor deposition etc.11,12 Although there have been various reports about the synthesis and application of N-doped carbon or N-doped metal oxides,9–13 the literature on the N-doped carbon/metal oxides has been very scarce to date. Herein, we report a simple and straightforward strategy for synthesizing a high N-doped C/Mn3O4 composite from a N-rich coordination polymer particle (CPP) precursor.14,15 The capacitive performance of the investigated N-doped C/Mn3O4 composite is evaluated and confirmed by cyclic voltammetry and constant current charge–discharge measurements and electrochemical impedance spectroscopy for the cycling performance. For comparison, the electrochemical behaviors of C/Mn3O4 composites with different N amounts are also investigated.

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Experimental

Electrode preparation

Synthesis of Mn-based coordination polymer nanoparticles (CPP-1)

The working electrodes were prepared as follows. A mixture containing 80 wt% active material, 15 wt% acetylene black and 5 wt% polytetrafluoroethylene (PTFE) was mixed well and ground, and then was incorporated into a nickel foam (1 cm × 1 cm), and the typical mass loading of electrode materials ranged from 2.5 to 5 mg.

The nitrogen-rich carboxylate 3-amino-1,2,4-triazole-5carboxylic acid hemihydrate (HATC) was chosen and dissolved in deionized water with the assistance of NaOH (molar ratio: 1/1) to form a 0.5 M NaATC aqueous solution. In a typical synthesis of CPP-1, a 0.5 M NaATC (3 mL) aqueous solution was introduced into a 0.5 M Mn(OAc)2 (3 mL) aqueous solution drop by drop in mixed solvents (water–ethanol: 8/8, v : v) under vigorous stirring for 15 min, and a large amount of pink precipitate was formed immediately. The pink precipitate was collected by centrifugation and washed several times with ethanol and water to remove the unreacted reactants and byproducts. Finally the product was dried in an oven at 50 °C for 6 h, namely CPP-1. Elemental analysis for CPP-1: C, 22.13; H, 2.48; N, 34.25%. IR (KBr pellet, cm−1): 3373s, 3175w, 1603m, 1519s, 1485m, 1425s, 1375s, 1285s, 1120m, 1062s, 890m, 827m, 815m, 723s, 648m, 603w, 518s, 464w, 438s. Synthesis of Mn-based coordination polymer nanowires (CPP-2) The organic building block pyridine-2,6-dicarboxylic acid (H2PDC) was first dissolved in dimethylformamide (DMF) to form a 0.2 M PDC-DMF solution. In a typical synthesis, 0.1 g Mn(OAc)2 was dissolved in 10 mL DMF solvent under vigorous stirring for 5 min, and then a 0.2 M PDC-DMF solution (2 mL) was introduced and stirred for another 10 min to generate a white precipitate. The white precipitate was washed several times with DMF and collected by centrifugation. Finally the product was dried in an oven at 50 °C for 12 h, and denoted as CPP-2. Elemental analysis for CPP-2: C, 33.58; H, 4.51; N, 5.49%. Synthesis of Mn-based coordination polymer spindles (CPP-3) The organic linker 1,4-benzenedicarboxylic acid (H2BDC) was first dissolved in dimethylformamide (DMF) to form a 0.1 M BDC-DMF solution. In a typical synthesis, a 0.5 M Mn(OAc)2 (3 mL) aqueous solution was introduced into a 0.1 M BDC-DMF solution (15 mL) drop by drop under vigorous stirring for 20 min, and a large amount of white precipitate was obtained immediately. The white precipitate was washed several times with water and collected by centrifugation. Finally the product was dried in an oven at 50 °C for 12 h, and denoted as CPP-3. Elemental analysis for CPP-3: C, 45.41; H, 3.92; N, 4.62%. Preparation of N-doped carbon/Mn3O4 hybrids The precursors (CPP-1–CPP-3) were first dispersed in ethanol and dried at 40 °C for 4 h in a porcelain crucible separately in order to make the powder of each precursor pave on the container bottom. Then N-doped carbon/Mn3O4 hybrids were obtained by annealing the precursors under a N2 atmosphere at 450 °C for 30 min. The heating rate of the furnace was kept at 1 °C min−1.

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Methods and measurements Solvents and all other chemicals were obtained from commercial sources and used as received unless otherwise noted; water that was used throughout all experiments was purified with the Millipore system (18.2 MΩ cm). Elemental analyses of C, H, and N were performed on an Elementar Vario MICRO Elemental Analyzer at the Analysis Centre of Nanjing University. X-ray powder diffraction data were collected on a Bruker D8 Advance instrument using Cu-Kα radiation (λ = 1.54056 Å) at room temperature. The morphology of the as-prepared samples and the corresponding energy dispersive X-ray spectra were obtained using a Hitachi S-4800 field-emission scanning electron microscope. Transmission electron microscopy images were captured on a JEOL JEM-1011 instrument microscope at an acceleration voltage of 100 kV. The electrochemical measurements were carried out using an electrochemical analyzer system, CHI660E (Chenhua Instrument, Shanghai, China) in a three-compartment cell with a platinum plate counter electrode, an Ag/AgCl reference electrode and a working electrode. The electrolyte was a 1.0 M Na2SO4 aqueous solution and electrochemical impedance spectroscopy measurements of as-synthesized samples were conducted at an open circuit voltage in the frequency range of 100 kHz to 10 mHz.

Results and discussion The N-doped carbon/Mn3O4 (NC/Mn3O4) hybrids were synthesized by annealing the corresponding Mn-based coordination polymer particles, obtained from Mn ions reacting with N-rich, N-containing and N-absent organic building blocks respectively (for details see the Experimental section). As can be clearly seen in Scheme 1, the chosen linkers are HATC, H2PDC and H2BDC, which possess different numbers of N atoms. After formation into CPPs, the N amount is 34.25, 5.49 and 4.62% for CPP-1, CPP-2 and CPP-3, respectively, according to their elemental analysis (EA) results. The N amount in CPP-3 can be ascribed to the contribution from DMF molecules, which is confirmed by the X-ray powder diffraction (XRD) results as shown in Fig. 1. All diffraction peaks in CPP-3 can be readily indexed to the bulk-crystal structures of {Mn2(BDC)2(DMF)2}n that were reported previously by Xu et al. (Fig. S1, for details see ESI†).16 From the crystal structural unit of {Mn2(BDC)2(DMF)2}n (Fig. S2, ESI†), each Mn(II) ion is sixcoordinated, where two of six coordinate sites are occupied by two oxygen atoms from two DMF molecules. This result is in accordance with the deduced conclusion of EA.

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Scheme 1 The synthetic process for N-doped carbon/Mn3O4 hybrids from different organic building blocks with different nitrogen amounts.

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can be conveniently characterized by IR spectroscopy.20–22 The infrared spectra of CPP-1 shows an intense peak around 3300 cm−1 belonging to the O–H stretching band of the carboxylate group and/or the coordinated water molecules. The characteristic bands of the carboxylate group in CPP-1 are shown at 1485–1603 cm−1 for anti-symmetric stretching and at 1375–1425 cm−1 for symmetric stretching. The absence of the expected bands at 1685–1715 cm−1 for the protonated carboxylate groups illustrates the complete deprotonation after the formation of CPP-1 (for details see the Experimental section and Fig. S4 in ESI†).23 The morphology of the isolated particles for CPPs is characterized by field-emission scanning electron microscopy (FE-SEM) as illustrated in Fig. 2. The image of CPP-1 reveals that the product can be obtained massively and that the structure seems like an arid land, which has lots of fissures and is split into lots of blocks (Fig. 2a). Each block possesses a secondary building unit (SBU), assembling from plentiful nanoparticles with the sizes of about 10–100 nm as shown in the high-magnification SEM image of Fig. 2b. The uniform structure of CPP-2 is displayed in Fig. 2c and 2d. A mass of nanowires randomly arrange together with lengths and the mean width of 10–15 μm and 50 nm, respectively. This morphology is in good agreement with the arrangement of the bulk-crystal structure that facilitates the H2PDC linker and Mn(II) ions in a certain coordination mode to lead to the formation of a onedimensional (1D) motif (for details see Fig. S5, ESI†). In comparison, spindle-like CPP-3 is obtained in great force based on

Fig. 1 XRD patterns for the coordination polymer precursors of CPP-1, CPP-2 and CPP-3, respectively.

Also, the diffraction peaks in CPP-2 can be well indexed to the reported structures of {Mn2(PDC)2(H2O)3}n as depicted in Fig. S3 (ESI†).17 However, the structure of CPP-1 cannot correspond to any known phase of Mn-HATC-based metal–organic frameworks (based on the search results from CCDC18). The deficiency for CPPs is still a challenge for researchers due to their powder nature, and in many cases, the use of singlecrystal X-ray diffraction methods is precluded to characterize them.19 The formation of coordination polymers from metal ions and carboxylate-functionalized organic linkers is well known in transition-metal coordination chemistry, and they

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Fig. 2 (a, b) SEM images of CPP-1 nanoparticles. (c) SEM images of CPP-2 nanowires. (d) The high-magnification SEM image for CPP-2 nanowires. (e) SEM images of CPP-3 micro-spindles. (f ) The high-magnification SEM image for CPP-3 micro-spindles.

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Fig. 3 XRD patterns for as-synthesized carbon/Mn3O4 hybrids: (a) NC/ Mn3O4-1, (b) NC/Mn3O4-2 and (c) C/Mn3O4, respectively. EDX patterns for (d) NC/Mn3O4-1, (e) NC/Mn3O4-2 and (f ) C/Mn3O4 hybrids.

the N-absent ligand H2BDC reacting with Mn(II) ions as shown in Fig. 2e. Similar to CPP-2, each spindle has an SBU structure that consists of several sheets overlaying together with lengths, widths and average thickness of 22 μm, 10 μm and 300 nm, respectively. The as-synthesized inorganic hybrids are then prepared by annealing the mentioned CPPs at 450 °C for 30 min under a N2 atmosphere, namely NC/Mn3O4-1, NC/Mn3O4-2 and C/Mn3O4 for the corresponding CPP-1, CPP-2 and CPP-3. The decomposed products are first characterized by XRD spectra as listed in Fig. 3a–3c. The entire observed peaks in all annealing samples match exactly with the standard values reported for Mn3O4, which is a distorted tetragonal spinel structure (JCPDS no. 24-0734; a = b = 5.763 Å, c = 9.456 Å). The chemical composition of the resulting products is further determined using EDX spectroscopy. Fig. 3d and 3e confirm the presence of Mn, C, N and O in samples NC/Mn3O4-1 and NC/Mn3O4-2. The corresponding N amount decreases to 29.87 and 1.96% in comparison with the original CPPs, due to the removal of the organic building blocks during the calcination process. Even so, the obtained N amount of NC/Mn3O4-1 is still much higher than the reported data through different approaches (for details see Table S1, ESI†).24–28 The result suggests that chemical transformation from N-rich CPPs may provide a new methodology for generating high N-doping materials. However, no obvious peak that corresponds to the nitrogen element is observed in the EDX pattern (Fig. 3f ). The reason for this phenomenon is that the N source in CPP-3 comes

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Fig. 4 (a, b) SEM images of rod-like NC/Mn3O4-1 products. (c) The high-magnification SEM image for NC/Mn3O4-1 samples. (d) The highmagnification SEM image of nanoparticles on NC/Mn3O4-1 samples. (e) TEM image of solid NC/Mn3O4-1 rods. (f ) TEM image for a single rod in NC/Mn3O4-1 hybrids.

from the DMF solvent and easily decomposes at high annealing temperatures.29 The morphology of the NC/Mn3O4-1 sample is characterized by FE-SEM as shown in Fig. 4. Fig. 4a and 4b display a rod-like motif with the mean width of 3 μm, which undergoes a significant change from nanoparticles to rods as compared with the original precursor CPP-1. The length of the rods ranged from 4 to 20 μm and each rod has many nanoparticles crowded together on it as vividly depicted in Fig. 4c. These nanoparticles arrange randomly with sizes of 5–100 nm, which may ascribe to N-doped carbon particles (Fig. 4d). The transmission electron microscopy (TEM) image suggests that the rod-like motif is not porous even after annealing from CPPs (Fig. 4e). Fig. 4f confirms the nanoparticles attached to the rod-like motif, which is in good agreement with SEM results. Unlike NC/Mn3O4-1, the morphology of N-absent C/Mn3O4 is spindle-like and is inherited completely from their precursors, shown in Fig. S6a (see ESI†). The side of each spindle is composed of sphere-like particles (50–200 nm) obtained during the annealing process, which distinguishes itself from the original precursor of CPP-3 (Fig. S6b†). Furthermore, the sheets of CPP-3 are transformed to an aggregate of small-spindle-like motifs (mean length: 500 nm and mean width: 250 nm, Fig. S6c and S6d†). Similar to C/Mn3O4, NC/Mn3O4-2 maintained the morphology of CPP-2 as shown in the SEM image of Fig. 5a and 5b. When compared with the CPP-2, it is found

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Fig. 5 (a, b) SEM images of wire-like NC/Mn3O4-2 products. (c) TEM images for NC/Mn3O4-2 samples. (d) The high-magnification TEM image for NC/Mn3O4-2 samples.

that the mean size and the length of a single nanowire decrease to 30 nm and 4–5 μm, respectively, due to the annealing temperature (Fig. 5c and 5d). Moreover, a porous structure can be observed as shown in Fig. 5d, which is different from the solid NC/Mn3O4-1 hybrids. These results demonstrate that CPPs with different N amounts can be used as templates or precursors to generate N-doped carbon/metal oxide composites with a wide range of shapes and N amounts. To verify the applicability of as-synthesized new composites of N-doped C/Mn3O4 as supercapacitor electrodes, their electrochemical properties are investigated in terms of their cycling performance. Cyclic voltammetry (CV) and chronopotentiometry (CP) measurements are performed. For comparison, the properties of C/Mn3O4 are also determined to investigate the influence of N-doped carbon and the effect of the nitrogen amount. CV is first studied using the classical three electrode method in a 1.0 M Na2SO4 electrolyte to measure the capacitance of C/Mn3O4 (shown in Fig. 6a–6c). Clearly, the values that are obtained for the C/Mn3O4 composites directly depended on the scan rate as shown in Fig. 6d. The specific capacitance is determined to be 90, 54 and 28 F g−1 at a scan rate of 10 mV s−1, suggesting that the NC/Mn3O4-1 has the largest specific capacitance. Moreover, the specific capacitance of NC/Mn3O4-1 is also much higher than that of NC/ Mn3O4-2 and C/Mn3O4 at scan rates between 5 and 50 mV s−1. The reason for this increase could be attributed to the effect of morphology at first, especially the distribution of carbon in as-prepared hybrids, which usually affects the Faradaic charging and discharging reaction rate of the Mn3O4 nanostructures, and therefore influences the final electrochemical properties of hybrids. Secondly, the introduction of nitrogen atoms into the carbon-based materials could further cause electron modulation to provide desirable electronic structures and enhance the electron-transfer ability.30,31 In this regard, the different nitrogen amounts in as-synthesized hybrids could be another key reason for explaining the

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Fig. 6 CV curves of as-synthesized (a) NC/Mn3O4-1, (b) NC/Mn3O4-2 and (c) C/Mn3O4 samples at different scan rates. (d) The specific capacitances of NC/Mn3O4-1, NC/Mn3O4-2 and C/Mn3O4 calculated according to the CV curves at different scan rates.

different electrochemical performances. The different shapes of CV curves may support this point, as shown in Fig. 7a. The CV curves of NC/Mn3O4-1 and NC/Mn3O4-2 are more similar to rectangles than that of C/Mn3O4 and similar to the shape of reported graphene/Mn3O4 composites.32 The shape of C/Mn3O4 hybrids is asymmetric, which may be ascribed to functional groups and more defects exist in the carbon framework in carbon/Mn3O4 composites after annealing from CPPs, contributing to the pseudocapacitance, and consequently causing bigger polarization, weaker electrochemical activity and reaction reversibility. The CP curves of NC/Mn3O4-1 and NC/Mn3O4-2 at various current densities are obtained as

Fig. 7 (a) CV curves of as-prepared NC/Mn3O4-1, NC/Mn3O4-2 and C/ Mn3O4 hybrids at 10 mV s−1. Charge–discharge curves for (b) NC/ Mn3O4-1 and (c) NC/Mn3O4-2 hybrids at different current densities. (d) Specific capacitance variations of NC/Mn3O4-1 and NC/Mn3O4-2 were investigated at different current densities.

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shown in Fig. 7b and 7c. The increase in charging time represents the higher capacitance of NC/Mn3O4-1, which is in good agreement with the CV results. The specific capacitances of the NC/Mn3O4-1 and NC/Mn3O4-2 at different current densities are calculated and the corresponding relationships are shown in Fig. 7d. The specific capacitance for NC/Mn3O4-1 is determined to be 136, 123, 73, and 59 F g−1 at current densities of 0.1, 0.25, 0.5, and 1.0 A g−1, respectively. The value is comparable to that investigated in other recent reports on Mn3O4 architectures or CNT-supported Mn3O4 composites.33–35 The Nyquist plots of all as-synthesized composites are presented to further confirm the above-observed results as listed in Fig. 8a. The electrochemical impedance spectroscopy (EIS) data can be fitted by an equivalent circuit, which consists of a bulk solution resistance (Rs), a charge-transfer resistance (Rct), a pseudocapacitive element (Cp) from the redox process of Mn3O4, and a constant phase element (CPE) to account for the double layer capacitance, as shown in Fig. 8a, inset. The Rs values of three as-obtained samples is calculated to be 1.25, 1.39 and 1.54 Ω, respectively (for details see the inset of Fig. 8a), while the corresponding Rct values were calculated to be 1.1, 1.4 and 4.3 Ω, respectively. The result clearly demon-

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strates the reduced charge-transfer resistance of the NC/Mn3O4-1 electrode. Moreover, the charge-transfer resistance Rct, also called the Faraday resistance, is a limiting factor for the specific power of the supercapacitor. It is the low Faraday resistance that results in the high specific power of the NC/Mn3O4-1 electrode.36 Because it is important for a supercapacitor material to have good cycling performance, an endurance test is conducted using galvanostatic charging–discharging cycles at 0.25 A g−1 (Fig. 8b). During the first 200 cycles, the specific capacitance is decreased slightly for NC/Mn3O4-1 and maintained at an approximate value over the next 800 cycles. For NC/Mn3O42, the specific capacitance is decreased to the valley during the first 10 cycles and gradually increased up to a certain value and remained stable afterward, which may be ascribed to the shrink and activation of the electrode as electrolytes in general require a certain period of time to penetrate the entire inner space of an active electrode material.37 Even after 1000 continuous charge–discharge cycles, the capacitance retention is more than 94% and 86% for NC/Mn3O4-1 and NC/Mn3O4-2, respectively. This result is in contrast to experiments on other kinds of Mn3O4 electrodes, which showed significant degradation of the capacitance within 100–600 charging–discharging endurance tests.33–35 Both the N-doped carbon and the Mn3O4 nanostructures can make electrochemical contributions to the specific capacitance. First, the N-doped carbon serves as a conductive matrix to promote fast Faradaic charging and discharging of the Mn3O4 structures. Second, Na+ ions in the electrolyte (Na2SO4) can freely intercalate and deintercalate themselves onto/from the surface of the N-doped carbon and Mn3O4 motifs. Third, N-incorporation can enhance the chemical reactivity of carbon and thus serve as a kind of barrier to protect the Mn3O4 motifs and maintain its high capacity even in the endurance test.

Conclusions

Fig. 8 (a) Electrochemical impedance spectra of electrodes of as-prepared carbon/Mn3O4 hybrids at room temperature; the inset shows an equivalent circuit and a magnification image of electrochemical impedance spectra. (b) Cycling test of NC/Mn3O4-1 and NC/Mn3O4-2 at a constant current of 0.25 A g−1.

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In summary, high N-doped C/Mn3O4 hybrids were synthesized from N-rich CPPs, which give clear evidence of the ability of N-doped C/Mn3O4 to improve the electrochemical performance of these materials as electrodes for supercapacitors. The N-doped C/Mn3O4 hybrid could be further explored for goodcycling, low-cost and nontoxic electrode materials for electrochemical applications. Our high N-doped approach should also offer an effective and convenient technique to improve the rate capabilities of carbon-decorated composite electrode materials in the supercapacitor area. Our research on the chemical transformation approach for high N-doped supercapacitor electrode materials is still in progress.

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Acknowledgements This work was supported by the Foundation of College of Science (050802001) and the Scientific Research Foundation of Nanijng Agricultural University (050804087).

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Mn3O4 hybrids synthesized from nitrogen-rich coordination polymer particles as supercapacitor electrodes.

High nitrogen-doped carbon/Mn3O4 composites were synthesized by annealing nitrogen-rich Mn-based coordination polymer particles, and investigated by e...
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