Nitrogen-doped porous carbon with a hierarchical structure prepared for a high performance symmetric supercapacitor

Abstract

Here, we present a facile approach to synthesizing nitrogen-doped porous carbon materials (NPCs) through carbonization of poly(o-phenylenediamine) (PoPD) by using molten-salt as a template. The as-prepared NPCs exhibit hierarchically micro-nanometric porous structure and unprecedented nitrogen content (14.86 wt%). The micrometric pores are interconnected which form from the micrometric salt droplets during carbonization, while the nanopores are generated by the exclusion of small molecular gases. This unique structure and high nitrogen content endows the NPCs with excellent specific capacitance (364.93 F g⁻¹ at 2 mV s⁻¹) and good cycling stability (92.3% capacitance retention at 10 A g⁻¹ after 5000 cycles) in 6 M KOH electrolyte. Moreover, the symmetric supercapacitor array fabricated with the NPCs can easily power a light-emitting diode (LED), demonstrating the practical application of the NPCs in energy storage.

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Introduction

In recent years, supercapacitors with high power density and excellent cycling stability have attracted increasing interest for their wide range of applications, including in various portable devices, medical electronics, public transportation, and military defense systems.^1–5 With the advantages of both rechargeable batteries and dielectric capacitors, supercapacitors are crucial alternatives in energy storage devices to meet the continuous energy demands and increasing environmental concerns of the future.^6–9

Carbon materials are regarded as the most competitive materials for supercapacitors due to their outstanding attributes of low weight, good electronic conductivity, and extraordinary chemical and thermal stabilities.^10,11 Recently, numerous strategies have been published on the improvement of the capacitive behaviors of carbon materials, such as improving the effective surface area,¹² changing the pore size distribution,¹³ or introducing heteroatoms into the carbon skeletons.¹⁴ Among these methods, introducing heteroatoms into carbon materials shows superior specific capacity. Doping of heteroatoms, such as oxygen, sulfur or nitrogen, affects the electron distribution of the carbon materials, enhances the surface wettability, and introduces pseudocapacitance to the system, which are all favourable to enhancing the supercapacitor performance.^15–17 On the other hand, improving the surface area of the carbon materials is another efficient method to increase the specific capacity of the carbon materials. Porous carbon materials possess a very large surface area, which can provide a large accessible surface area for ion transport and increase the charge accommodation.¹⁸ In order to improve the surface area of carbon materials, hard and soft template have been widely used. Various hard templates, such as porous clay heterostructures,¹⁹ porous silica,²⁰ magnesium oxide,²¹ or zinc oxide,²² have been explored to synthesize nanoporous carbon materials. Soft-templates include a series of block copolymers, such as polyacrylonitrile (PAN),²³ metal–organic frameworks (MOFs),²⁴ poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO).²⁵ However, these templates always come with some incontrovertible drawbacks, such as applying dangerous chemicals to remove templates, consuming energy and time, and sometimes leading to significant mass loss of the carbon materials.²⁶

In this work, we report a convenient and low-cost method to prepare nitrogen-doped porous carbon materials (NPCs) using inorganic salts (sodium chloride and zinc chloride) as template, and PoPD as precursor. After carbonization, the salt phase is easily removed by simple washing with water, while the carbon is not etched as such.²⁷ The experimental parameters are optimised and the microscopic morphologies and electrochemical performances of the NPCs were investigated. According to the results, it has been found that the NPCs prepared at 800 °C with the salt: carbon precursor mass ratio of 6 : 1 possess hierarchically micro-nanometric porous structure, partial graphitization, large specific surface area, and high heteroatom-doping, which are the keys to strengthen the specific capacity and energy density of supercapacitor.²⁸ As a result, the NPCs manifest high specific capacitance, outstanding specific energy and outstanding cycling stability. In addition, the NPCs were successfully applied to fabricate the symmetric supercapacitor array demonstrating the feasibility for practical application of the NPCs in energy storage.

Experimental

Synthesis of poly(o-phenylenediamine) (PoPD)

The PoPD microspheres were prepared according to the previous reports.²⁹ In brief, 1.20 g o-phenylenediamine (o-PoPD) was dissolved in 80 mL deionized water and 5 mL of ammonium persulfate solution (APS) was added in the solution, subsequently. After maintaining the solution at 0–5 °C for 12 h, the obtained sample was thoroughly washed with deionized water until the filtrate became colorless. Finally, the samples were dried in a vacuum at 60 °C overnight.

Synthesis of NPCs

Firstly, sodium chloride and zinc chloride were mixed and ground in the ratio of 42 mol% sodium chloride to form eutectic salts.³⁰ Subsequently, the as-prepared PoPD was mixed with the eutectic salts in different proportions. Then the precursor mixtures were thermally treated under nitrogen atmosphere in a tube furnace with a heating rate of 2 °C min⁻¹. And after maintaining the final temperature for 2 h, the obtained products were ground into powder and washed in deionized water repeatedly. Finally the resulting carbon materials were dried overnight at 80 °C.

The as-prepared materials are denoted as NPC-r-T. Where r represents the mass ratio of eutectic salts and PoPD, and T is the synthesis temperature.

Structural characterization

Raman spectra were recorded on DXR-Microscope (Thermo Fisher, USA) at an excitation wavelength of 532 nm. X-ray photoelectron spectra (XPS) were obtained on a PHI 5000 VersaProbe (Ulvac-Phi, Japan) system with monochromic Mg K_α radiation. The crystal structure of the samples was characterized with X-ray powder diffractometer (XRD, Rigaku, D/Max 2500VL/PC, Cu K_α radiation). The microscopic morphologies of the samples were analysed by a field emission scanning electron microscopy (FE-SEM, Zeiss, Germany) using an ULTRA plus microscope and a field emission transmission electron microscope (FE-TEM, JEOL, Japan) with an acceleration voltage of 200 kV. Nitrogen sorption isotherms and Brunauer–Emmett–Teller (BET) surface areas were investigated by NovaWin (Quantachrom, USA).

Electrochemical measurements

All the electrochemical measurements were performed on a CHI700 electrochemical workstation (CH Instruments, China) by using cyclic voltammetry (CV) and galvanostatic charge–discharge cycling with a typical three-electrode cell in 6 M KOH solution. A glassy carbon electrode (GCE) with a diameter of 3 mm was used as the working electrode. A saturated calomel electrode (SCE) and a 1 cm² square Pt sheet were used as the reference and counter electrode, respectively. The modification of electrodes for electrochemical studies is outlined as follows. First, 2 mg of the as-prepared NPCs were added to 0.5 mL deionized water and the mixture was ultrasonically dispersed for 30 min to obtain a homogeneous suspension. Then, the GCE was covered by 20 μL suspension and 5 μL Nafion solution (0.05 wt% in ethanol), successively, to obtain the NPCs modified GCE. The electrode of NPCs-based symmetric supercapacitor was fabricated by pressing a mixture of 10 wt% acetylene black, 10 wt% polyvinylidene fluoride (PVDF) and 80 wt% active material onto a piece of titanium foil (2 cm²). Two electrodes were assembled with a polypropylene membrane sandwiched between them. The mass loading of active material in each electrode was 2.5 mg. The electrolytic solution was 6 M KOH.

Specific capacitances of the working electrodes derived from cyclic voltammetry tests were calculated from the equation:

where C (F g⁻¹) is the specific capacitance; m (g) is the mass of active materials loaded on working electrode; ν (V s⁻¹) is the scan rate; I (A) is the constant current; V_b and V_a (V) are high and low voltage limit of the CV tests.

Results and discussions

To evaluate the effects of the eutectic salt mixture on the structure of the NPCs, the morphology and microstructure of the as-prepared samples were investigated as shown in Fig. 1a–c. As we can see, PoPD and PoPD-800 which was prepared without using the eutectic salt template are comprised of micrometric particles. However, the NPC-6-800 displays interconnected micrometric pores which is probably owing to the low melting point of the eutectic salts and the formation of micrometric salt droplets which act as template during carbonization.³⁰ Meanwhile, the micrometric pore frameworks possess relatively rough surface morphology and abundant nanometric pores (inset of Fig. 1c), which results from the percolation structures of salt droplets and the exclusion of small molecular gases, such as methane, hydrogen, acetylene, ammonia during the thermal treatment,^29,31 indicating the hierarchically micro-nanometric porous structure of NPCs. The image of TEM reveals that the NPCs are made up of graphite-like sheets, as labelled by the parallel yellow bar in Fig. 1d, verifying partial graphitic structures during carbonization. On the other hand, as shown in the XRD spectrum of NPC-6-800 (Fig. S1†), there are two diffraction peaks locating at 26.1° and 43.2°, which can be attributed to the (002) and (100) plane of the standard hexagonal graphite structure, respectively.³² According to Bragg equation, the calculated interlayer distance (d₀₀₂) of 0.342 nm is close to that of graphite,³¹ suggesting the formation of graphitic structures during carbonization, which is well consistent with the TEM spectrum. The elemental mapping (Fig. S2†) demonstrates that there are still homogeneously distributed nitrogen and oxygen atoms in the carbon materials.

Fig. 1 SEM images of (a) PoPD, (b) PoPD-800, (c) NPC-6-800 and inset: the surface morphology of macropore, (d) TEM image of NPC-6-800.

The amount of eutectic salt mixture is a key parameter to control the development of porosity in the NPCs. Therefore, various mass ratios of eutectic salt and PoPD were discussed in details. The SEM images of NPCs were shown in Fig. 2a–c. As we can see, although all the samples possess a porous structure, NPC-1-800 shows the sparse pores with the smallest pore sizes and the NPC-6-800 exhibits the largest pore sizes. The structure of samples was further investigated by nitrogen adsorption–desorption measurements (Table S1†). It is obvious that PoPD-800 and NPC-1-800 presents the low N₂ sorption capacity and small surface area. With increasing the eutectic salt amount, the specific surface areas increase up to the maximum value for NPC-6-800 (266.49 m² g⁻¹). This is mainly due to the fact that during the carbonization process, carbon precursor is condensed and scaffolded in the presence of the molten salt at elevated temperatures and with the amount of the molten salt increasing, the salt mixture is presumably easier to form bigger salt clusters which act as template in the later annealing treatment.³⁰ The N₂ sorption isotherm and the pore size distribution evaluated by the density functional theory (DFT) method of NPCs are given in Fig. 2d and e. The nitrogen isotherm of the NPC-6-800 (Fig. 2d) exhibits a typical characteristic of type I, implying the presence of micropore and relatively small mesopore in the frameworks. The results (Fig. 2e) reveal that the NPC-6-800 with more ultramicropore (<0.8 nm) and supermicropores (0.8–2 nm) has much higher pore volume than the other materials. Some studies have reported that when pore size is close to the ion size (<1 nm), it may minimize the available free space and favor the ion adsorption in the most efficient way. These will lead to the maximum double-layer capacitance and benefit a lot to the energy storage.^33–35 Thus, comparing to the ordinary porous structure, the carbon materials possessing a hierarchical porous network always exhibit a superior capacitive performances.^36–39

Fig. 2 SEM images of the (a) NPC-1-800, (b) NPC-3-800 and (c) NPC-6-800, (d) N₂ sorption isotherm of NPC-6-800, (e) pore size distributions of NPCs and PoPD-800, (f) cyclic voltammograms of NPCs and PoPD-800 at a scan rate of 100 mV s⁻¹.

Considering the unique hierarchical porous features of NPCs, we further evaluated its electrochemical performances. As shown in Fig. 2f, the specific capacitance of NPC-6-800 (245.92 F g⁻¹) is much higher compared to PoPD-800 (9.65 F g⁻¹) and the other NPCs, which is probably due to the hierarchical porous structure and large specific surface area introduced by the molten-salt template. On the one hand, the micropores (especially those ultramicropores) in carbon framework of NPC-6-800 is more efficient for the forming of electrical double layer and result in high charge storage.^39–41 On the other hand, the interconnected micrometric pores which serve as a huge buffering chamber will decrease the diffusion distances between electrolytes and the interior surfaces of the pores. The nanopores on the wall of micrometric pores frameworks will offer a large available surface area for charge storage.⁴² The combination effects of micrometric pores and nanopores facilitate the fast diffusion of electrolyte ions into the pores. Consequently, these experiments above confirm the importance of eutectic salts to alter the microstructure of NPCs and underline the crucial importance of the large specific surface area and hierarchically porous structure of NPCs to improve the supercapacitive performances.

On the other hand, the temperature of thermal treatment is also a key factor to affect the capacitive properties of the NPCs. Therefore, the electrochemical performance of NPCs prepared at different carbonization temperature were investigated. As shown in Fig. 3a, the rectangular-like shape in the CV curves reveal that the capacitive response mainly comes from the electrical double-layer capacitance. The appearance of humps around −0.2 V at positive potential scans are related to the pseudocapacitance of doped surface heteroatoms.²⁶ The relations between the specific capacitances and scan rates for the NPCs are shown in Fig. 3b. It can be found that NPC-6-800 exhibits a higher specific capacitance than the other samples at different scan rates and possesses the maximum specific capacitance 364.93 F g⁻¹ at the scan rate of 2 mV s⁻¹. On the other hand, when the scan rate is low, both NPC-6-800 and NPC-6-700 possess a high specific capacitance which is ascribed to the pseudocapacitance behavior with low reaction kinetics.^43,44 Nevertheless, with the increasing of scan rate, the specific capacitances of NPC-6-700 decrease more sharply compared to NPC-6-800, probably resulting from the relatively low conductivity, which is caused by the poor graphitization at low carbonization temperature.²⁸ It should be noted that, even at high scan rates of 500 mV s⁻¹, NPC-6-800 maintains a high specific capacitances of 123 F g⁻¹, which is higher than those reported in literatures.^23,43,44 The charge–discharge curves of NPCs and the specific capacitances for NPCs at different current densities are shown in Fig. S3,† which are well consistent with the above CV results. The maximum specific capacitance of NPC-6-800 reaches 218 F g⁻¹ at a current density of 0.1 A g⁻¹. The specific capacitances of NPCs progressively decrease with the increase of the current density, resulting from the diffusion limitation. Nevertheless, the specific capacitances of NPC-6-800 still retain high values of 161.6 F g⁻¹ at 2 A g⁻¹, suggesting the NPC-6-800 has a good rate capability, which is indispensable for the practical applications. The Ragone plots of NPCs presented in Fig. S4† reveal that the highest specific energy of 7.57 W h kg⁻¹ has been found for the sample NPC-6-800 at the specific power of 12.5 W kg⁻¹. At the high current density of 10 A g⁻¹, the specific capacitance of NPC-6-800 is 135.3 F g⁻¹ and the specific energy is still 4.70 W h kg⁻¹ with a high specific power of 1250 W kg⁻¹, indicating the superior energy output performance of NPC-6-800. As shown in Table S2,† the specific capacitance of NPC-6-800 is also higher than those of heteroatom doped carbon materials reported previously, such as nitrogen-containing hydrothermal carbons,²⁶ nitrogen and oxygen co-doped activated carbons,⁴⁵ and sulphur-doped carbon-graphene composites.⁴⁶ The excellent capacitance properties of NPC-6-800 can be ascribed to the combination effect of the high nitrogen doping and hierarchical porous structure, especially these ultramicropores structure which can contribute significantly to the capacitance of supercapacitors.^24,47,48

Fig. 3 (a) Cyclic voltammograms of NPCs at a scan rate of 2 mV s⁻¹, (b) specific capacitances of NPCs at different scan rates.

In order to gain more insight into the high capacitance performances of NPCs, Raman spectra of NPCs prepared at different temperatures are investigated as shown in Fig. S5.† The peak at 1580 cm⁻¹ (G-band) can be attributed to the vibration of sp² hybridized carbon atoms in a 2D hexagonal lattice, while the peak at 1350 cm⁻¹ (D-band) suggests the presence of the defects and disorders of structures in carbon materials.^43,49 The peak intensity ratio of D-band to G-band (I_D/I_G) reflects the disordered crystal structures of the carbon materials and the results show that higher thermal treatment temperature causes a better graphitization, which will enhance the conductivity of materials and well consist with the electrochemical experiments above.^50,51

To understand the chemical state of carbon, oxygen and nitrogen elements, the XPS spectrums of NPCs were recorded. The XPS N 1s spectra of PoPD and NPC-6-800 are shown in Fig. 4a and b. It can be seen that the XPS N 1s spectrum of PoPD was fitted by three component peaks, namely, pyridinic-N (N-I, 398.4 eV), amino-N (400.5 eV) and imino-N (399.2 eV).²⁹ For NPC-6-800, amino-N and imino-N disappeared, whereas pyrrolic-N (N-II, 399.6 eV) and quaternary-N (N-III, 400.7 eV) appeared. Results indicate that good π-conjugated structure and polycyclic-type rings formed and partial graphitization occurred after carbonization at 800 °C, promoting the electrical conductivity of the samples.²⁹ The XPS N 1s spectrum of NPCs obtained at different carbonization temperatures are shown in Fig. S6.† Results demonstrate that with the temperature increasing, the percentage of pyridinic-N and pyrrolic-N decrease due to their conversion to quaternary-N at high temperature. Pyridinic-N and pyrrolic-N can lead to great pseudocapacitance effect,²⁸ and thus the specific capacitance of materials decrease with the carbonization temperature above 800 °C. Fig. 4c present the C 1s spectrum of NPC-6-800. The main peak at 284.7 eV corresponds to C=C, indicating the formation of graphitic carbon. The three small peaks located at 285.9 eV, 287.1 eV and 288.4 eV in NPC-6-800, are attributed to C=N, C–N and O–C=O. In addition, the O 1s spectrum of NPC-6-800 (Fig. 4d) shows the peaks with binding energies at around 530.9 eV, 532.3 eV and 533.3 eV, attributing to C=O, C–OH and –COOH, respectively. The peak at 535.5 eV is contributed to chemisorbed oxygen or water.^52–54 The weight ratio of C, N, and O in the NPCs calculating from XPS spectrums are shown in Table S3.† It has be found that the NPC-6-800 possesses rich N content (14.86 wt%) and O content (11.5 wt%), which is higher than recently reported.^55,56 The N and O species which appeared at the surface of NPCs will enhance the capacitance by facilitating the wettability of materials and introducing the pseudocapacitive effect.^57–59

Fig. 4 XPS survey spectra of samples. High-resolution N 1s spectra of PoPD (a) and NPC-6-800 (b). High-resolution C 1s spectrum of NPC-6-800 (c). High-resolution O 1s spectrum of NPC-6-800 (d).

The CV curves for NPC-6-800 at different scan rates in the potential window from 1.0 to 0.0 V are shown in Fig. S7.† It can be found that, with the scan rate increasing, the rectangular shapes of curves are changed to fusiform shape, which probably can be ascribed to the increasing diffusion limitation between the electrode and electrolyte. The charge–discharge curves in the same potential window are shown in Fig. S8.† The initial part of discharge curves exhibits a small IR drop and the rest is almost linear. With the charge current increasing, the charge curves are similar to the corresponding discharge ones, suggesting that the capacitance of materials mainly originates from the ion adsorption and desorption.^42,60,61

The galvanostatic charge–discharge stability of NPC-6-800 was also analyzed by examination of 5000 cycles. As shown in Fig. 5a, even at the high current density of 10 A g⁻¹, the specific capacitance of NPC-6-800 is still as high as 135.3 F g⁻¹, which is also higher than those of heteroatom-doped carbon materials reported previously, such as nitrogen-doped carbonaceous hybrid,⁴² sulphur-doped micro/mesoporous carbon-graphene composites,²⁹ and nitrogen-doped grapheme.⁴⁸ In addition, NPC-6-800 exhibits an outstanding capacitance retention of 92% compare to its initial capacitance after 5000 cycles, indicating no deterioration takes place despite the large working voltage, and making it directly suited for application.^62,63 A two-electrode symmetric supercapacitor was fabricated to further study the practical application of NPC-6-800 for supercapacitors. In order to meet the required voltage of micro-devices, we connect three supercapacitors in series. According to Fig. 5b, it can be found that the charge or discharge time is similar to a single supercapacitor and the voltage window is broadened from 1 V to 3 V. The LED can be easily powered and sustained for more than 40 min when the supercapacitor array is only charged for 60 s (Fig. 5c). The results demonstrate the feasibility for practical application of NPC-6-800 in energy storage.

Fig. 5 (a) Cycling stability test at 10 A g⁻¹ for NPC-6-800, (b) charge–discharge curves of supercapacitors at the current densities of 1 A g⁻¹. (c) Image of a LED powered by three supercapacitors array in series, the inset: single supercapacitor.

Conclusions

In this work, we have successfully prepared nitrogen-doped porous carbons through salt-templating, a simple and low-cost method, derived from PoPD. The as-prepared NPC-6-800 exhibits hierarchically micro-nanometric porous structure, large specific surface area, high nitrogen-doping and partial graphitization. Due to these advantages, NPC-6-800 shows high specific capacitance, outstanding rate capability, and excellent cycling stability, presenting remarkable capacitive behavior, indicating that NPC-6-800 is a promising electrode material for high capacitance energy storage device.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21505018, 21121091, 21005016) and the Fundamental Research Funds for the Central Universities.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18769b

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