Nitrogen-Doped Porous Carbon Prepared from Urea Formaldehyde

Article pubs.acs.org/IECR

Nitrogen-Doped Porous Carbon Prepared from Urea Formaldehyde Resins by Template Carbonization Method for Supercapacitors Xiang Ying Chen,*,† Chong Chen,† Zhong Jie Zhang,‡ Dong Hua Xie,† and Xiao Deng† †

School of Chemical Engineering, Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, P. R. China ‡ College of Chemistry & Chemical Engineering, Anhui Province Key Laboratory of Environment-friendly Polymer Materials, Anhui University, Hefei 230039, Anhui, P. R. China S Supporting Information *

ABSTRACT: Through a simple and convenient template carbonization method, nitrogen-doped porous carbon has been successfully achieved by heating urea formaldehyde (UF) resin and magnesium citrate at 800 °C, where the magnesium citrate serves as a template. The mass ratio between the UF resin and magnesium citrate plays a crucial impact on the surface areas, pore structures, and the correlative capacitive behaviors of the final porous carbons, denoted as samples UF-Mg-1:1, -1:3, and -1:5. All present porous carbons exhibited amorphous features with low graphitization degrees. Sample UF-Mg-1:3 displayed the best capacitive performance with a large specific capacitance of 239.7 F g−1 at a current density of 0.5 A g−1 and a high energy density of 33.3 Wh kg−1 at a power density of 0.25 kW kg−1. More importantly, it exhibited a high capacitance retention of 94.4% after 5000 charge/discharge cycles, clearly indicating good cycling durability.

1. INTRODUCTION Today, supercapacitors are key devices for energy-storage applications, as they display energy densities much higher than those of conventional capacitors and much better power delivery capabilities than batteries.1−3 Carbon-based electrode materials for supercapacitors have attracted extensive attention, primarily because of their favorable properties such as high surface area, large pore volume, good chemical inertness, good thermal stability, and low cost of manufacture.4−6 Consequently, electrochemical double-layer capacitors (EDLCs) using carbon as active materials currently represent more than 80% of commercially manufactured supercapacitors.7 The template carbonization method has been shown to be an efficient protocol for producing carbon materials, especially porous carbons, with large surface areas and pore volumes,8,9 where hard and/or soft templates are commonly used, such as MgO,10 Ni(OH)2,11 and CaCO3.12 To significantly improve the electrochemical properties of EDLCs, as an alternative strategy, heteroatoms such as oxygen, nitrogen, and boron are incorporated into the porous carbon.13,14 Of these heteroatoms, nitrogen, having five valence electrons, is superior because it can act as an electron donor in the lattice, thus resulting in a shift of the Fermi level to the valence band in carbon electrodes.15 In addition, the doping of nitrogen atoms into carbon materials also strengthens the wettability of the interface between the electrolyte and electrodes. Doping nitrogen into porous carbon can be realized by reaction with nitrogen-containing reagents (such as NH3, urea, nitric acid, and amines) or carbonization/activation of nitrogen-rich carbon precursors, such as polyacrylonitrile, melamine, urea polymer, and biomass containing nitrogen.16 On the other hand, the nontransparent thermosetting urea formaldehyde (UF) resin derived from urea and formaldehyde contains high nitrogen contents and can exhibit excellent capacitive performance assisted by the addition of calcium acetate.17 © 2013 American Chemical Society

Herein, a template carbonization method has been employed to prepare nitrogen-doped porous carbon by heating a mixture of UF resin and magnesium citrate. UF resin can act as both carbon and nitrogen sources, whereas magnesium citrate is a hard/soft dual template by virtue of the MgO and gases produced in situ at elevated temperature. The effects of the mass ratio of UF resin to magnesium citrate on the surface areas, pore structures, and correlative capacitive behaviors of carbon samples were studied in depth.

2. EXPERIMENTAL SECTION All analytical chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and used as received without further treatment. In the present work, porous carbon materials were prepared by the direct carbonization of mixtures of UF resin and magnesium citrate at 800 °C under an Ar flow, accompanied by the release of MgO, gases, and so on. The unit structures of the UF resin and magnesium citrate and a simulated structure of porous carbon with various types of pores are illustrated schematically in Figure 1. 2.1. Typical Synthetic Procedure for Samples UF-Mg1:1, -1:3, and -1:5. UF resin and Mg3(C6H5O7)2·14H2O in mass ratios of 1:1, -1:3, and -1:5 were first ground and then placed in a porcelain boat, flushed with a flow of Ar for 30 min, heated in a horizontal tube furnace to 800 °C at a rate of 5 °C min−1, and maintained at 800 °C for 2 h under an Ar flow. The resultant product was immersed in dilute HCl solution to remove soluble/insoluble substances and then further washed with adequate deionized water until it reached pH 7. Finally, the Received: Revised: Accepted: Published: 10181

March 18, 2013 June 5, 2013 July 9, 2013 July 9, 2013 dx.doi.org/10.1021/ie400862h | Ind. Eng. Chem. Res. 2013, 52, 10181−10188

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic illustration of the production of porous carbon by direct carbonization of UF resin and magnesium citrate at 800 °C under an Ar flow.

sample was dried under a vacuum at 120 °C for 12 h to obtain samples denoted as UF-Mg-1:1, -1:3, and -1:5 in accordance with the initial UF resin/magnesium citrate mass ratio. 2.2. Characterization. X-ray diffraction (XRD) patterns were obtained on a Rigaku D/MAX2500 V instrument with Cu Kα radiation (λ = 1.5418 Å). High-resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) patterns were obtained with a JEM-2100F unit at an accelerating voltage of 200 kV. Fieldemission scanning electron microscopy (FESEM) images were recorded on a Hitachi S-4800 scanning electron microscope. Xray photoelectron spectra (XPS) were obtained on a Thermo Scientific Escalab 250Xi spectrometer with an exciting source of Mg Kα (1253.6 eV). The specific surface area and pore structure of the carbon samples were determined from N2 adsorption− desorption isotherms at 77 K (Micrometrics ASAP 2020 system) after the samples had been vacuum-dried at 200 °C overnight. The specific surface area was calculated by the conventional BET (Brunauer−Emmett−Teller) method. The pore size distribution (PSD) plot was recorded from the adsorption branch of the isotherm based on the Barrett−Joyner−Halenda (BJH) model. 2.3. Electrochemical Measurements. To evaluate the capacitive performances of the as-prepared carbon samples (∼4 mg) in electrochemical capacitors, a mixture of 80 wt % carbon sample, 15 wt % acetylene black, and 5 wt % polytetrafluoroethylene (PTFE) binder was fabricated using ethanol as the solvent. A slurry of this mixture was subsequently pressed onto nickel foam under a pressure of 20 MPa, serving as the current collector. The prepared electrode was placed in a vacuum oven at 120 °C for 24 h. A three-electrode experimental setup with a 6 mol L−1 KOH aqueous solution as the electrolyte was used in cyclic voltammetry and galvanostatic charge−discharge measurements on an electrochemical working station (CHI660D, ChenHua Instruments Co. Ltd., Shanghai, China). Here, the prepared electrode, platinum foil (6 cm2), and a saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. Specific capacitances derived from galvanostatic tests were calculated as

C=

window, and m (mg) is the mass of active material loaded in the working electrode. The specific energy density (E) and specific power density (P) derived from galvanostatic tests were calculated as

E=

1 C ΔV 2 2

P=

E Δt

where E (Wh kg−1) is the average energy density, C (F g−1) is the specific capacitance, ΔV (V) is the potential window, P (W kg−1) is the average power density, and Δt (s) is the discharge time. Specific capacitances derived from cyclic voltammetry tests were calculated as C=

1 mv(Vb

∫ − V) V a

Vb

I dV

a

where C (F g−1) is the specific capacitance; m (mg) is the mass of active material loaded in working electrode; v (V s−1) is the scan rate; I (A) is the discharge current; and Vb and Va (V) are high and low potential limits, respectively, of the cyclic voltammetry (CV) tests.

3. RESULTS AND DISCUSSION The crystallinities, purities, and phases of the samples were determined by XRD. When a mixture of UF resin and magnesium citrate with a mass ratio of 1:1 was heated directly at 800 °C for 2 h under an Ar flow, a great deal of black powder was obtained, consisting of carbon and MgO, as indicated in Figure 2a. Apparently, the MgO resulted from the decomposition of magnesium citrate above 200 °C.18 After being washed with aqueous HCl solution and deionized water, a pure carbon sample, denoted as UF-Mg-1:1, was obtained, and its corresponding XRD pattern is shown in Figure 2b. Two broad and low-intensity diffraction peaks centered at 25.3° and 44.2° are present, clearly revealing the amorphous nature and low graphitization degree of sample UF-Mg-1:1. In addition, when the initial mass ratio of UF resin to magnesium citrate was simply adjusted to 1:3 and 1:5, samples UF-Mg-1:3 and UF-Mg-1:5 were obtained; as can be seen in Figure 2c,d, these samples displayed XRD patterns similar to that of sample UF-Mg-1:1. The shapes, sizes, and intrinsic structures of samples UF-Mg1:1, -1:3, and -1:5 were determined by the FESEM and HRTEM

I Δt mΔV

where C (F g−1) is the specific capacitance, I (A) is the discharge current, Δt (s) is the discharge time, ΔV (V) is the potential 10182

dx.doi.org/10.1021/ie400862h | Ind. Eng. Chem. Res. 2013, 52, 10181−10188


Article

present N 1s spectra with peaks located at 398.5, 400.4, and 401.4 eV can be indexed as pyridinic nitrogen (N-6), pyrrolic nitrogen (N-5), and graphitic nitrogen (N-Q), respectively, as shown in Figure 4c. With the help of XPS peak software, we calculated the contents of N-6, N-5, and N-Q species within the porous carbons, and the values calculated are listed in Table S1 (Supporting Information). Interestingly, the total nitrogen contents of samples UF-Mg-1:1, -1:3, and -1:5 decreased gradually, accompanied by a decrease in the N-6 contents and an increase in the N-5/N-Q contents. Regarding the high-resolution O 1s spectra of the three samples (527.5−540.0 eV), we obtained three fitted peaks at 531.3, 532.5, and 533.4 eV, respectively. In detail, the peak at ca. 531.3 eV is due to −OH, CO, and O−CO groups, and that at ca. 532.5 eV arises from contributions of CO and O−CO. The peak at ca. 533.4 eV might be due to C−O−C, C−O−OH, and C−OH moieties.16 In addition, the contents of C, N, and O are listed in Table 1. One can see that the content of carbon increased with increasing initial mass ratio of UF resin to magnesium citrate, accompanied by declines in the contents of nitrogen and oxygen. That is, the final contents of C, N, and O could be readily tuned by altering the initial mass ratio of UF resin to magnesium citrate. The surface areas and pore structures of samples UF-Mg-1:1, -1:3, and -1:5 were analyzed by N2 adsorption/desorption isotherms and pore size distribution curves. Figure 5a−c displays typical N2 adsorption/desorption isotherms for the three samples at relative pressures (P/P0) ranging from 0.0 to 1.0, which have almost the same shapes and are assignable to type-IV species according to the IUPAC classification. The adsorptions at low relative pressure indicate the existence of micropores, and the hysteresis loops at relative pressures between 0.45 and 1.0 suggest the presence of large amount of mesopores within the carbons. In addition, the lack of adsorption plateaus near P/P0 = 1.0 reveals low contents of macropores.23 The specific surface areas of samples UF-Mg-1:1, -1:3, and -1:5 were calculated by the BET method to be 1062, 1059, and 1117 m2 g−1, respectively, as listed in Table 2, which are much higher than that of the carbon obtained by carbonizing UF resin alone without the help of magnesium citrate at 800 °C (∼150 m2 g−1). The Smicro/Stotal ratios of samples UF-Mg-1:1, -1:3, and -1:5 were found to be 0.39, 0.21, and 0.15, respectively, indicating a decrease in micropores with increasing mass ratio of UF resin to magnesium citrate. On the other hand, the total pore volumes of samples UF-Mg-1:1, -1:3, and -1:5 were high, at 1.25, 1.67, and 1.56 cm3 g−1, respectively, and their micropore volumes also decreased with increasing mass ratio of UF resin to magnesium citrate. The average pore widths of samples UF-Mg-1:1, -1:3, and -1:5 were 4.7, 6.3, and 5.6 nm, respectively, and the pore size distribution (PSD) plots recorded from the adsorption branch of the isotherm based on the BJH model are shown in Figure 5d. Cyclic voltammetry (CV) was applied to estimate the capacitive behaviors of the three samples in the potential range from −1.0 to 0.0 V. Their CV curves were approximately rectangular shapes, especially at the low scan rate of 10 mV s−1, as shown in Figure 6a, implying good electrochemical performances as EDLC electrode materials. The deviation from the ideal rectangular shape owes to the resistance of the electrolyte and the contact resistance between the electrode and the current collector.24 Furthermore, no pronounced reversible redox peaks incurred by nitrogen atoms were observed on these CV curves, indicating little contribution from redox reactions.20 Another mechanism for interpreting this phenomenon was also

Figure 2. XRD patterns of (a,b) sample UF-Mg-1:1 (a) before and (b) after being washed with aqueous HCl solution and deionized water to remove any unwanted impurities, as well as samples (c) UF-Mg-1:3 and (d) UF-Mg-1:5. Note: $ symbols indicate cubic MgO (JCPDS card 450946).

techniques. Figure 3a shows a representative FESEM image of sample UF-Mg-1:1, which consists of large numbers of irregular particles that are several micrometers in size. The intrinsic structure of sample UF-Mg-1:1 was further studied by HRTEM, as displayed in Figure 3b. It can be clearly seen that the carbon sample contained various types of pores. The corresponding SAED pattern shown in the top inset in Figure 3b reveals its amorphous nature through obscure diffraction rings. Furthermore, the HRTEM image showing disordered lattice fringes in the bottom inset in Figure 3b also evinces a low graphitization degree. With respect to the formation of pores within the carbon sample, MgO particles and gases produced from the decomposition of Mg3(C6H5O7)2·14H2O are believed to serve as hard/ soft dual templates in the carbonization process.10 Additionally, carbon dioxide, isocyanic acid, ammonia, hydrocyanic acid, and carbon monoxide can be produced from UF resin in the carbonization process,19 and these gases also favor the creation of pores within the carbon samples. Samples UF-Mg-1:3 and -1:5 were also found to exhibit amorphous but porous structures, as shown in Figure 3c−f, similar to that of sample UF-Mg-1:1. XPS, as a surface analytical technique, was applied to quantitatively analyze the nature and amount of functional groups on samples UF-Mg-1:1, -1:3, and -1:5. Figure 4a shows the XPS survey spectra of all three samples for binding energies of 0−1400 eV. Three peaks assignable to C 1s, N 1s, and O 1s indicate the high purities of the present carbons without the presence of any kinds of magnesium oxides. In addition, the relative intensities of these peaks, especially those for N 1s, to some extent indicate the different initial mass ratios of UF resin to magnesium citrate. High-resolution C 1s spectra ranging from 282.0 to 295.0 eV of samples UF-Mg-1:1, -1:3, and -1:5 are displayed in Figure 4b. Two strong peaks located at 285.0 and 286.2 eV and one minor peak at 289.5 eV can be obtained with the help of XPS peak software. In detail, the peak at 285.0 eV is due to sp3 C−C bonds,20 and that at 286.2 eV can be attributed to C−O bonds.21 The minor peak at 289.5 eV might arise from −COO or O− COO groups.20 On the other hand, there are 12 types of nitrogen-containing functional groups occurring on the surface of carbons,16 of which pyridinic nitrogen (398.6 ± 0.3 eV), pyrrolic nitrogen (400.5 ± 0.3 eV), graphitic nitrogen (also as quarternary nitrogen, 401.3 ± 0.3 eV), and oxided pyridinic nitrogen (402−405 eV) are the most common.22 As a result, the 10183



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Figure 3. (a,c,e) FESEM and (b,d,f) HRTEM images of (a,b) UF-Mg-1:1, (c,d) UF-Mg-1:3, and (e,f) UF-Mg-1:5. The insets in panels b, d, and f are (top) SAED patterns and (bottom) HRTEM images.

approximate triangles in the potential range from −1.0 to 0.0 V, also suggesting good electrochemical performances. Sample UFMg-1:3 displayed the highest discharge time at a current density of 1 A g−1. In addition, specific capacitances of the three samples at various current densities are shown in Figure 7b. At a current density of 0.5 A g−1, the specific capacitances of samples UF-Mg1:1, -1:3, and -1:5 were found to be ca. 169.1, 239.7, and 195.0 F g−1, respectively. The capacitance retentions at high current densities, namely, 10, 20, and 40 A g−1, are also labeled in Figure 7b, indicating their good rate capabilities. Figure 7c presents Ragone plots showing energy density versus power density for the three samples. When the power density was held at 0.25 kW kg−1, the energy densities of samples UF-Mg-1:1, -1:3, and -1:5 were 23.5, 33.3, and 27.1 Wh kg−1, respectively. When the power density was increased to 20 kW kg−1, the

proposed as faradic reactions by lone electron pairs from the nitrogen groups interacting with the cations in the electrolyte.25 On the other hand, the integral area of the CV curve, as is well-known, is proportional to the capacitance of the supercapacitor; thus, sample UF-Mg-1:3 displayed the largest capacitance at 10 mV s−1. When the scan rate was increased to 50−400 mV s−1, the CV curves gradually deteriorated in shape, especially at the high scan rate of 400 mV s−1, as depicted in Figure 6b−d. The distorted CV patterns, especially at high scan rates, resulted from the increasing equivalent series resistance (ESR).26 Moreover, from the viewpoint of integral area, the capacitance of sample UF-Mg-1:1 fell more quickly. The capacitive behaviors of the three samples were further evaluated by the galvanostatic charge−discharge technique, and the results measured at a current density of 1 A g−1 are shown in Figure 7a. It can be clearly seen that all curves exhibited 10184



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Figure 4. XPS spectra of samples UF-Mg-1:1, -1:3, and -1:5: (a) survey, (b) C 1s, (c) N 1s, and (d) O 1s.

Figure 5. (a−c) N2 adsorption/desorption isotherms and (d) pore size distribution curves of samples (a) UF-Mg-1:1, (b) -1:3, and (c) -1:5 .

corresponding energy densities dropped to 7.0, 12.8, and 11.4 Wh kg−1, respectively. Comparatively, sample UF-Mg-1:3

exhibited the best capacitive behavior among the present carbon samples. As is well-known, multimodal porosity containing 10185



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mass transport of electrolytes to and from the micropores.4 It is therefore concluded that the multimodal porosity within sample UF-Mg-1:3 might be optimum for achieving excellent EDLC

micropores, mesopores, and macropores is currently believed to be ideal, because the actual energy storage occurs predominately in the smaller micropores whereas the larger pores provide fast

Figure 6. CV curves of samples UF-Mg-1:1, -1:3, and -1:5 measured at various scan rates: (a) 10, (b) 50, (c) 100, and (d) 400 mV s−1.

Figure 7. continued 10186



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Figure 7. (a) Galvanostatic charge−discharge curves of samples UF-Mg-1:1, -1:3, and -1:5 measured at a current density of 1 A g−1. (b) Specific capacitances of samples UF-Mg-1:1, -1:3, and -1:5 at various current densities. (c) Ragone plots showing energy density versus power density for samples UF-Mg-1:1, -1:3, and -1:5. (d) Cycling durability of sample UF-Mg-1:3. (e) CV curves of the 1st and 5000th cycles of sample UF-Mg-1:3 at a scan rate of 100 mV s−1. (f) Galvanostatic charge−discharge curves of the 1st and 5000th cycles of sample UF-Mg-1:3 at a current density of 10 A g−1.

4. CONCLUSIONS We demonstrate a simple and efficient template carbonization method to prepare porous carbon containing nitrogen species derived from UF resin. Several scientific advantages are believed to occur as follows: (1) The UF resin used can provide both the carbon source and nitrogen source. (2) Magnesium citrate acts as a dual template in the carbonization process, in which the MgO and gases produced serve as hard and soft templates, respectively. (3) Sample UF-Mg-1:3 displays excellent cycling durability as 94.4% after 5000 charge/discharge cycles. The present template carbonization method, especially using magnesium citrate as a dual template, is expected to be useful for preparing other kinds of porous carbons as excellent EDLC electrode materials.

Table 1. XPS Peak Analysis of the Carbon Samples sample

C (at. %)

N (at. %)

O (at. %)

UF-Mg-1:1 UF-Mg-1:3 UF-Mg-1:5

85.58 88.97 91.08

8.28 5.20 3.69

6.14 5.83 5.23

Table 2. Surface Areas and Pore Structures of the Carbon Samples BET surface area (m2 g−1)

sample

total

Smicroa

Sextb

total pore volume (cm3 g−1)

UF-Mg-1:1 UF-Mg-1:3 UF-Mg-1:5

1062 1059 1117

417 219 173

645 840 944

1.25 1.67 1.56

micropore volume (cm3 g−1)

average pore widthc (nm)

0.19 0.11 0.08

4.7 6.3 5.6

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ASSOCIATED CONTENT

S Supporting Information *

Nitrogen species and contents of carbon samples. This material is available free of charge via the Internet at http://pubs.acs.org.

a

Micropore area calculated from the t-plot. bExternal surface area calculated from the t-plot. cAverage pore width calculated based on 4 V/A by the BET method.

■

AUTHOR INFORMATION

Corresponding Author

performance. Of course, both the large total pore volume of 1.67 cm3 g−1 and the large average pore width of 6.3 nm are favorable for electrolyte transportation. On the other hand, nitrogen-doped carbon has also been indentified to be efficient for altering/enhancing EDLC behavior. In the present work, the nitrogen species within the carbons mainly included N-6, N-5, and N-Q with different contents, as shown in Figure 4 and Table S1 (Supporting Information). Even though the contribution derived from nitrogen species in the basic electrolyte is evident, its mechanism remains unknown.27 Taking into account the excellent electrochemical performance of sample UF-Mg-1:3 reported in this work, its cycling durability was subsequently investigated, as shown in Figure 7d. Interestingly, this sample exhibited a high capacitance retention of 94.4% even after 5000 charge/discharge cycles. This is a good premise for the present carbon to be employed in the practical application of supercapacitors. The long-term cycling durability of sample UF-Mg-1:3 can be confirmed by the integral areas surrounded by the CV curves of the first and 5000th cycles at a scan rate of 100 mV s−1, as depicted in Figure 7e. Furthermore, the nearly equal IR drops of the first and 5000th cycles revealed in Figure 7f also indicate the good cycling stability of sample UF-Mg-1:3.

*Tel./Fax: +86-551-2901450. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21101052), China Postdoctoral Science Foundation (20100480045), and National Training Programs of Innovation and Entrepreneurship for Undergraduates (201210359033). Z.J.Z. acknowledges financial support from Anhui Province Key Laboratory of EnvironmentFriendly Polymer Materials, Anhui University, Hefei 230039, China (KF2012009).

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REFERENCES

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