Phosphorus- and nitrogen-co-doped particleboard based activated carbon in supercapacitor application

Abstract

Nitrogen-containing activated carbon (AC7525) was prepared by waste particleboards and modified by ammonium biphosphate (PAC7525) for supercapacitor application. The surface areas of AC7525 and PAC7525 are 1204 m² g⁻¹ and 1407 m² g⁻¹, respectively. Galvanostatic charge–discharge curves, cyclic voltammetry and alternating current impedance were employed to investigate the electrochemical properties of the samples. After modification, the element phosphorus was introduced to the surface of the activated carbon cloth. The specific capacitance was improved from 176 to 227 F g⁻¹ under a current density of 50 mA g⁻¹ in a 7 mol L⁻¹ KOH electrolytic solution. The promising electrochemical performances can be attributed to the synergetic effect of (1) pseudocapacitance that originated from rich and tunable surface groups by co-doping of phosphorus and nitrogen; and (2) the electric double layer capacitance that came from the uniform porosities developed by in situ activation.

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1. Introduction

Supercapacitors are one of the most promising energy storage devices useful for automobiles, electronics and energy systems. The advantage of supercapacitors is that they can be charged and discharged at high rates and can be coupled with fuel cells to deliver high energy in vehicles.¹ Electrochemical double layer capacitors (EDLCs) are a class of electrochemical energy storage devices with ideal characteristics for the rapid storage and release of energy.² This capability is mainly achieved by a non-faradaic mechanism through the separation of ions across a very small distance in the double-layer at the electrode/electrolyte interface, and it is dependent on the surface area of the electrode and the accessibility of the electrolyte to its porosity. Activated carbons in various modifications is the electrode material most frequently used in electrochemical capacitors. Activated carbon (AC) based supercapacitor is more useful due to its low cost, abundant availability, and high surface area.³ Capacitance of the carbon material can be improved further by the introduction of various functionalities with several heteroatoms like phosphorus, nitrogen, oxygen, sulfur and boron.^4–6

The methodologies most frequently used for the introduction of several heteroatoms to the structure of ACs include: by carbonization and further activated of polymer containing heteroatom in their structure.⁷ As far as I know, waste particleboards bonded with formaldehyde-based adhesives, especially urea-formaldehyde (UF) adhesive, which is rich in nitrogen. Besides, particleboard wastes are zero-cost materials which have already been tested as attractive precursors for producing activated carbons from a two steps thermo-chemical process.⁸ Currently China is a big country in wood-based panel production and consumption, and there is a great quantity of wood-based panel (especially particleboards) produced every year. In our work, we develop a straightforward methodology directed to prepare nitrogen-containing activated carbon by waste particleboards for recycling waste particleboards and taking advantage of UF resin. Then, the nitrogen-containing activated carbon modified by ammonium biphosphate was studied to improve the carbon materials capacitive performance.

2. Materials and methods

The waste particleboards was kindly provided by Beijing Jiahekailai Furniture and Design Company, which was obtained in the furniture manufacturing process containing 10% urea-formaldehyde resin adhesive of the mass. Other chemicals were analytical grades and were purchased from Beijing Lanyi Chemical reagent.

The carbonization process was carried out in a high-purity nitrogen atmosphere at the temperature increase rate of 10 °C min⁻¹ to the final temperature of 500 °C, separately, maintained for 60 min. In activation step, 6 grams of the oven-dried sample was soaked in a 50% KOH solution for 24 h at the mass ratio impregnation 2.5 : 1. The sample was then activated at the temperature 750 °C for 60 min in nitrogen atmosphere. The obtained activated carbon (AC7525) was boiled first with 1 M HCl solution and then with distilled water until the pH of solution reach to about 6–7. Finally, these activated carbon was dried at 105 °C in an oven for 8 h. During modification step, 3 grams of the oven-dried AC7525 was soaked in 25% NH₄H₂PO₄ solution for 24 h at the mass ratio impregnation 4 : 1. The sample was then modified at the temperature 300 °C for 60 min. After washed and dried, the obtained modified activated carbon was marked PAC7525.

Chemical surface composition and state of the samples were determined by X-ray photoelectron spectroscopy (XPS). XPS was performed on an ESCALAB250 (VG Scientific, UK) using a monochromatic Al K_α radiation. The N₂ adsorption–desorption isotherms were measured with an accelerated surface area and porosimetry system (ASAP 2010, Micromeritics) for determining the surface areas. After analyzed to obtain information about: (i) the surface area by the BET method (S_BET), (ii) the total pore volume (V_tot) as calculated from N₂ adsorption at a relative pressure at 0.995 (P/P₀), (iii) the micropore volume (V_mi) by applying the t-plot method and (iv) pore volume (V_dft) calculated by means of the Density Functional Theory (DFT). The Brunauer–Emmett–Teller (BET) surface area was calculated using the BET equation from the selected N₂ adsorption data, within a range of relative pressure, P/P₀, from 0.1 to 0.3.

Electrodes used for fabrication of supercapacitors were prepared by mixing AC, acetylene black, and polytetrafluoroethylene (PTFE) in a mass ratio of 87 : 10 : 3. The capacitive performance of all carbon samples was investigated in 7 M KOH using two-electrode (the electrodes preparation were based on our previous study⁹) cells. Cyclic voltammetry, galvanostatic charge–discharge and alternating current impedance were used in evaluation of the capacitive performance of the assembled supercapacitor. Constant current density charge–discharge and rate performance were tested using the BT2000 battery testing system (Arbin Instrumecnts, USA) at room temperature. Cyclic voltammetry (CV) and alternating current impedance were employed for the electrochemical measurements of each sample using the 1260 electrochemical workstation (Solartron Metrology, UK) at room temperature.

3. Results and discussion

3.1 Physical characterization

BET analysis was performed using nitrogen adsorption–desorption isotherms to determine the porous nature of the material. The surface area (S_BET), total pore volume (V_tot), micropore volume (V_mi) and pore volume calculated by means of the DFT (V_dft) of all the samples are listed in Table 1. PAC7525 has a well-developed porosity with higher S_BET (1407 m² g⁻¹) and larger V_tot (0.834 cm³ g⁻¹). Besides, the effect of loading phosphorus on AC porosity can be clearly appreciated by analyzing pore size distributions. As illustrated in Fig. 1, both the micropore (increased from 0.519 to 0.622 cm³ g⁻¹) and mesopore show slight increase in the accessible volume after modified by ammonium biphosphate, the increase being especially relevant when pore with radius around 0.6 nm are considered. X. L. Wang et al.¹⁰ modified activated carbon fibers by nitric acid, phosphoric acid, ammonium biphosphate and copper nitrate solutions, the result showed that the microvolume of the AC fibers modified by ammonium biphosphate increased most obviously (from 0.603 to 0.700 cm³ g⁻¹). It maybe attribute to the reaction of ammonium biphosphate with carbon materials at high temperatures. When the carbon materials are treated with ammonium biphosphate at high temperatures, ammonium biphosphate will decompose to radicals such as NH₄PO₃, H and N. These radicals maybe can etch carbon fragments, leading to increased porosity. In addition, the N₂ adsorption–desorption isotherms, as shown in Fig. 2, are also used to determine the surface area and pore-size distribution of the ACs. And the isotherms resemble a combination of type-I and type-II isotherms, indicating that this adsorption behavior attributes to a combination of microporous–mesoporous structure, which is correspond to the results of Table 1 and Fig. 1.

Table 1 The porosity parameters of ACs

Sample	AC7525	PAC7525
SBET (m^2 g^−1)	1204	1407
Vtot (cm^3 g^−1)	0.681	0.834
Vmi (cm^3 g^−1)	0.519	0.622
Vdft (cm^3 g^−1)	0.600	0.703

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Fig. 1 Pore size distribution for AC7525 and PAC7525.

Fig. 2 Nitrogen adsorption–desorption isotherms of AC samples.

Fig. 3 shows the FT-IR spectra of samples. Bands at ∼3435 cm⁻¹ and 1635 cm⁻¹ were identified as O–H stretching vibration and C–X (X = C, N, or O) stretching vibration, whilst shoulder peaks at 3220 cm⁻¹ are attributed to N–H stretching vibration. The bands at 1635 cm⁻¹ are attributed to C–X (X = C, N, or O) stretching vibration, whilst shoulder peaks at ∼1400 cm⁻¹ and 1160 cm⁻¹ are identified as P–O stretching vibration and C–O stretching vibration and –OH bending modes in P–O–C et., stretching of P=O groups, and/or stretching of P=OOH.

Fig. 3 FI-IR spectra of all samples.

XPS analysis was performed to obtain a clear understanding of the nitrogen and phosphorus functionalities in the AC7525 and PAC7525. The photoemission spectrums are presented in Fig. 4, and the summary elemental analysis for all samples are summarized in Table 2. Survey scans and narrow scan spectra for the main elements of interest-nitrogen phosphorous, carbon and oxygen-can also be found in the table for AC7525 and PAC7525. After modified, the photoemission spectrum of PAC7525 shows an increased intensity of P 2p and O 1s, and a decreased intensity of C 1s, as well as a slight increased intensity of N 1s. These results demonstrate that modification promotes the presence of P 2p and O 1s. Moreover, the population of oxygen functionalities is strongly dependent on not only the heat treatment temperature but also the introduction of phosphorus. C. L. Wang et al.¹¹ reported that a significant increase of surface oxygen content (over 8.4%) was found in phosphorus- and nitrogen-co-doped carbons due to the introduction of phosphorus. Consequently, the existence of P=O groups maybe the important influencing factor of oxygen content. X. L. Wang et al.¹⁰ found that the surface acidic oxygenous functional groups of the AC fibers modified by ammonium biphosphate increased obviously (over 7 times, especially phenolic hydroxyl group). The surface concentrations of nitrogen and phosphorus species for both samples are summarized in Table 2. Besides, the transformation of functionalities groups during carbonization, activation and modification process is showed in Fig. 5. The photoemission peaks from N 1s narrow scans reveal the existence of four types of nitrogen functionalities at binding energies of 398.7 ± 0.3 eV (N-6, pyridinic nitrogen), 400.3 ± 0.3 eV (N-5/N-P, pyrrolic nitrogen and pyridine), 401.4 ± 0.4 eV (N-Q, quaternary nitrogen), and 402–405 eV (N-X, oxidized nitrogen). In addition, the photoemission peaks from P 2p narrow scans reveal the existence of two types of phosphorus functionalities at binding energies of 134.1 eV (P1, P=O), and 136.5 eV (P2). Compared with AC7525, PAC7525 takes more N-5/N-P and N-Q, less N-6 and N-X.

Fig. 4 XPS curves of whole spectra (a) AC7525; (b) PAC7525.

Table 2 Detailed XPS analysis of AC samples

Sample	AC7525	PAC7525
N 1s, P 2p, C 1s and O 1s core level XPS spectra
N 1s (wt%)	1.14	1.16
P 2p (wt%)	—	2.05
C 1s (wt%)	56.43	53.45
O 1s (wt%)	32.03	34.18
N surface concentration (%)
N-6, pyridinic nitrogen (%) (398.7 ± 0.3 eV)	8.86	4.37
N-5/N-P, pyrrolic nitrogen, pyridone (%) (400.3 ± 0.3 eV)	52.76	56.67
N-Q, quaternary nitrogen (%) (401.4 ± 0.4 eV)	25.34	30.58
N-X, oxidized nitrogen (%) (402–405 eV)	13.04	8.39
P surface concentration (%)
P1, P=O (%) (134.1 eV)	—	72.52
P2 (%) (136.5 eV)	—	27.48

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Fig. 5 The transformation of functionalities groups.

3.2 Electrochemical characterization

For the purpose of studying the effect of loading phosphorus on AC electrode, the galvanostatic charge–discharge, CV and alternating current impedance are used, as shown in Fig. 6–8.

The galvanostatic charge–discharge curves of the AC electrodes at a current density of 50 mA g⁻¹ are presented in Fig. 6(a). And Fig. 6(b) shows the variation of specific capacitance versus current densities. Calculation indicate that 27.6% decrease of AC7525 in specific capacitance and only 14.6% decrease of PAC7525 in specific capacitance. Meanwhile, PAC7525 shows higher capacitance (227 F g⁻¹, at current density of 50 mA g⁻¹), which emphasized the better electrical performance of PAC7525. L. L. Cheng et al.¹² prepared chestnut shell-based carbons (CSCs) by a chemical activation method, and the CSCs-based electrodes had the specific capacitances of 105.4 F g⁻¹ at a current density of 0.1 A g⁻¹. Moreover, it is revealed that the phosphorus groups in the PAC7525 can improve the performance of the capacitance. C. L. Wang et al.¹¹ drew the same conclusion and confirmed that the occurrence of pseudocapacitance which can affect the electrochemical performances, associated with rich surface groups. As the research showed¹³ that nitrogen functional groups providing additional pseudocapacitance mainly due to the redox reactions, which is aroused by the electrons come from nitrogen atoms. Meanwhile, phosphor in main group with nitrogen maybe provide additional pseudocapacitance similarly.

Fig. 6 (a) Charge–discharge curves of AC electrodes in 7 M KOH at a constant current density of 50 mA g⁻¹; and (b) calculated specific capacitance as a function of current density of AC electrodes.

Fig. 7 presents the CV curves for AC7525 and PAC7525 at various scan rates. When the scan rate increase to 200 mV s⁻¹, a rectangular shape can still remained with slightly distorted, indicating good capacitive behavior even at high current density. Besides, PAC7525 shows a desired more rectangular voltammogram shape at high scan rate, suggesting PAC7525 has better electrical performance. Pandolfo et al.¹⁴ have mentioned that functional groups can enhance the wettability of carbon electrodes and, consequently, increase the specific capacitance of the carbon through improve pore access and greater surface utilization. Meanwhile, our conclusion corresponds with this analysis.

Fig. 7 Cyclic voltammograms of (a) AC7525 and (b) PAC7525 electrode in 7 M KOH at different scan rate.

Nyquist plot, also known as electrochemical impedance spectroscopy (EIS), shows the frequency response of the electrode/electrolyte system and is a plot of the imaginary component of the impedance against the real component. The Nyquist plot toward the prepared carbons were measured and further fitted by Chi604d software, as indicated in Fig. 8. As depicted in this figure, the semicircular arc of PAC7525 has slightly decreased in size compared with that of AC7525, primarily revealing the decrease of the charge-transfer resistance.¹⁵ In addition, the vertical shape at lower frequencies indicates a pure capacitive behavior, representative of the ion diffusion in the electrode structure. The more vertical the curve, the more closely the supercapacitor behaves as an ideal capacitor. Obviously, in the low frequency region, the straight line part of PAC7525 is more close to vertical line along the imaginary axis, suggesting PAC7525 has better capacitive behavior than AC7525.

Fig. 8 Nyquist polt AC7525 and PAC7525 electrodes (inset: enlarge high-frequency region of Nyquist plot).

The life cycle of the supercapacitor based on PAC7525 electrode was tested at a constant current density of 5 A g⁻¹ as shown in Fig. 9. After 3000 cycles, this device showed ∼93.8% retention of capacitance compared to the initial value, reflecting that the PAC7525 electrode has good electrochemical stability and a high degree of reversibility in the repetitive charge–discharge cycling test.

Fig. 9 Cycle test for PAC7525 electrode at 5 A g⁻¹.

4. Conclusion

Modified nitrogen-containing activated carbon electrode is a better choice for supercapacitor due to its effective cost and ecofriendly nature. The existence of phosphorus functional group provides pseudocapacitance and enhances the wettability of carbon electrodes which improves pore access and greater surface utilization leading to high charge–discharge rate and electrochemical performance. For electrochemical supercapacitor, specific capacitance is the most important parameter. It is found that the specific capacitance of PAC7525 electrode (227 F g⁻¹) than that of AC7525 electrode (175 F g⁻¹) under the current density of 50 mA g⁻¹.

Acknowledgements

This study was funded by State Forestry Administration, project 201204807: the study on the technology and mechanism of the activated carbon electrode from waste hard board.

Notes and references

1. H. Zhao and A. F. Burke, Fuel Cell Powered Vehicles Using Supercapacitors: Device Characteristics, Control Strategies, and Simulation Results, Institute for Transportation Studies, University of California, Davis, 2010.
2. B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum, New York, 1999.
3. P. L. Taberna, P. Simon and J. F. Fauvarque, J. Electrochem. Soc., 2003, 150, 292–300.
4. D. W. Wang, F. Li, M. Liu and H. M. Cheng, New Res. Carbon Mater., 2007, 22, 307–314.
5. H. F. Li, H. A. Xi, S. M. Zhu, Z. Y. Wen and R. D. Wang, Microporous Mesoporous Mater., 2006, 96, 357–362.
6. Y. J. Kim, Y. Abe, T. Yanagiura, K. C. Park, M. Shimizu, T. Iwazaki, S. Nakagawa, M. Endo and M. S. Dresselhaus, Carbon, 2007, 45, 2116–2125.
7. J. Lahaye, G. Nanse, A. Bagreev and A. Strelko, Carbon, 1999, 37, 585–590.
8. P. Girods, A. Dufour, V. Fierro, Y. Rogaume, C. Rogaume, A. Zoulalian and A. Celzard, J. Hazard. Mater., 2009, 166, 491–501.
9. T. X. Shang, M. Y. Zhang and X. J. Jin, RSC Adv., 2014, 4, 39037–39044.
10. X. L. Wang and Y. P. Sheng, Environ. Sci. Manage., 2012, 37, 94–96.
11. C. L. Wang, Y. Zhou, L. Sun, P. Wan, X. Zhang and J. S. Qiu, J. Power Sources, 2013, 239, 81–89.
12. L. L. Cheng, P. Z. Guo, R. Y. Wang, L. F. Ming, F. F. Leng, H. L. Li and X. S. Zhao, Colloids Surf., A, 2014, 446, 127–133.
13. Y. H. Lee, K. H. Chang and C. C. Hu, J. Power Sources, 2013, 227, 300–308.
14. A. G. Pandolfo and A. F. Hollenkamp, J. Power Sources, 2006, 157, 11–27.
15. B. Z. Fang, J. H. Kim, M. S. Kim, A. Bonakdarpour, A. Lam, D. P. Wilkinson and J. S. Yu, J. Mater. Chem., 2012, 22, 19031–19038.

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