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Feasibility of activated carbon derived from anaerobic digester residues for supercapacitors
Journal of Power Sources 412 (2019) 683–688
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Feasibility of activated carbon derived from anaerobic digester residues for supercapacitors
T
Ce Wanga, Jin Wanga, Wentao Wua,∗, Jiang Qiana, Shaomin Songb, Zhengbo Yuea a b
School of Resources and Environmental Engineering, Hefei University of Technology, Hefei, Anhui, 230009, China Hefei Huawei Automation Co., Ltd, Hefei, Anhui, 230009, China
H I GH L IG H T S
derived from anaerobic digester residue was feasible for supercapacitors. • AC achieved specific high surface area and good pore size distribution. • AC • Highest capacitance was 184.91 F g at 1 A g and with 94.80% retention. −1
−1
A R T I C LE I N FO
A B S T R A C T
Keywords: Anaerobic digester residue Activated carbon Cattle manure Electric double layer capacitor Supercapacitor
The question of how to recycle anaerobic digester residue is a critical issue for the anaerobic digestion process of lignocellulosic biomass. In the current study, anaerobic digester residue from a biogas plant for the treatment of cattle manure was utilized to prepare the activated carbon for supercapacitors. The effect of activation temperature on the specific surface area, pore size distribution, and electrochemical performance of activated carbon was investigated. Results show that activated carbon derived from anaerobic digester residue is feasible for supercapacitors. With an increase in activation temperature, the specific surface area of activated carbon first increases and then gradually decreases. Activated carbon obtained at 700 °C shows both high specific capacitance and excellent electrochemical cycle stability. The good capacitive performance confirms that anaerobic digested manure residue can act as an excellent carbonaceous material for high-performance supercapacitors.
1. Introduction With the development of large-scale ecological aquaculture, a series of anaerobic digestion techniques have been developed to treat tons of renewable lignocellulose waste. The anaerobic digestion process has been extensively applied in the bioconversion of agricultural production residues, aquatic plants, farm waste, etc. In addition to biogas, a large amount of digester liquid and residue are generated as well [1]. Anaerobic digester residue (ADR) contains the abundant inorganic nutrients N and P and can be used in aquaculture [2], as crop fertilizers [3] and in soil improvement [4]. However, ADR also contains potential environmental contaminants such as heavy metals, organic pollutants, antibiotics, pesticides, and pathogenic bacteria [5]. It has been reported that random stacking may contaminate the soil environment, surface water bodies, and groundwater systems [6]. Therefore, the question of how to recycle ADR remains critical. However, ADR can be viewed as a novel lignocellulosic biomass
∗
source. At present, lignocellulosic biomass such as cattle manure compost [7], silk cocoon [8], feathers [9]) and synthetic materials [10] are being used to prepare activated carbon. Because of its high specific surface area and porosity, activated carbon is being used to prepare electrode materials for supercapacitors of electric double layer capacitors (EDLCs) [11]. These supercapacitors are widely used in the fields of hybrid electrical vehicles, electronic devices, machinery, and the military [12–14]. Currently the major obstacle for the commercial application of EDLCs is the high cost and low energy density, which severely limit their practical application [15]. Compared to the traditional carbon sources of coal, asphalt, and phenolic resin, the development of supercapacitors derived from lignocellulosic biomass has been attracting much attention [16]. Therefore, ADR might be a potential feedstock of low cost for supercapacitors. In the current study, anaerobic digester residue from a biogas plant for the treatment of cattle manure was used for the preparation of activated carbon which was further applied to supercapacitors. The effect
Corresponding author. Tel.: +86 551 62901707. E-mail address: [email protected] (W. Wu).
https://doi.org/10.1016/j.jpowsour.2018.11.092 Received 3 August 2018; Received in revised form 1 November 2018; Accepted 28 November 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. FESEM images of (a) and (b) (ADR-700) (c) N2 adsorption-desorption isotherms and (d) pore size distribution curves of ADR-650, ADR-700, ADR-750 and ADR-800.
area and pore size distribution (ASAP 2020 V3.03H instrument). To prepare the working electrode, a viscous slurry containing 80 wt % ADR material, 10 wt% carbon black, and 10 wt% polytetrafluoroethylene was mixed and dispersed onto a nickel foam current collector as a 1 cm × 1 cm sheet, followed by drying at 105 °C in a vacuum oven overnight. The electrode was pressed on a tablet press at 12 MPa for 10–15 s. The mass of the loaded active material was approximately 3.0–4.0 mg for one electrode. The electrochemical performance of the synthesized materials was tested using a three-electrode system in a 6 M KOH aqueous electrolyte solution at room temperature. CV and GCD were conducted in a one-compartment cell using a threeelectrode configuration with a CHI 660D electrochemical workstation (Shanghai Chenhua Instruments Co). EIS was conducted in a onecompartment cell using a three-electrode configuration with a BioLogic EC-LAB VMP3 electrochemical workstation. An Hg/HgO electrode and Pt foil acted as the reference and counter electrode, respectively. CV curves were obtained in the potential range of −1 to 0 V by varying the scan rate from 5 to 100 mV s−1. EIS was measured in a frequency range of 10 kHz to 10 mHz at an open circuit voltage with an alternate current amplitude of 5 mV. GCD measurements were galvanostatically completed at 0.5–10 A g−1 over a voltage range of −1.0–0 V. For quantitative considerations, the specific capacitance was calculated from the GCD values using the following equation:
of activating temperatures on the physiochemical characteristic of the activated carbon was investigated. The electrochemical properties of the supercapacitors were evaluated using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS). 2. Materials and methods 2.1. Preparation of activated carbon from anaerobic digester residue ADR was collected from a private farm in the Suzhou, Anhui Province, China. The residue was thoroughly dried at 80 °C and precarbonized at 400 °C for 1 h using a heating rate of 5 °C min−1 under a nitrogen gas atmosphere in a tube resistance furnace. The pre-carbonized materials were mixed with KOH (1:1 based on mass) in a water bath at 80 °C for 10 h and dried at 100 °C. The mixtures were pyrolyzed at 650, 700, 750, and 800 °C for 2 h at a heating rate of 5 °C min−1 under a nitrogen gas atmosphere, respectively. The products were soaked in 100 mL of HCl solution (1.0 M) for 10 min and then washed with hot distilled water at 80 °C until the washed water pH reached 7. The solid products were dried overnight and sieved through 200 mesh. These samples were labeled as ADR-650, ADR-700, ADR-750, and ADR800, respectively.
C = IΔt / mΔV
2.2. Analytical methods
(1)
where I is the constant discharge current, Δt is the discharge time, ΔV is the potential window during discharge, and m is the active mass [17]. Coulomb efficiency as a function of cycle number was calculated using the following equation:
The morphology, structure, and composition of the as-prepared samples were analyzed using field emission scanning electron microscopy (FESEM, FEI Nova Nano SEM 450), Fourier transform infrared spectroscopy (FTIR, Varian 600-IR) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250). The porosity properties and specific surface area of the samples were obtained using the Brunauer–Emmett–Teller (BET) method to obtain the specific surface
η (%) = Td/ Tc × 100
(2)
where Td and Tc are the discharge time and charge time, respectively [18]. 684
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eNeOe (534.1 eV) functional groups [25]. This can participate in Faradaic reactions to provide improved pseudo capacitance and enhance wettability of the carbon materials which increasing the specific capacitance of the supercapacitors [26]. These results confirm the successful synthesis of doped nitrogen and high-oxygen-containing porous carbon materials. FTIR analysis results for the ADR shows peaks at 3357.9, 2924.0, 2852.6, 1579.6, 1087.8, and 798.5 cm−1 (Fig. 3). These peaks may be a result of OeH (polyol or phenol), eCH2e (alkyl), eCH2e (alkyl), NeH (secondary amide), CeOeC (ether), and NeH (secondary amide) bending or stretching vibrational absorption peaks of the functional groups, respectively [27]. With the increase in activation temperature, the strength of the CeOeC peak (1087.8 cm−1) and OeH peak (3357.9 cm−1) gradually increased. Results showed that there were a large number of O-containing groups on the surface of the prepared materials consistent with the results of XPS.
3. Results and discussion 3.1. Porous structure characterization FESEM images of ADR-700 clearly show a rough irregular flake and block structure (Fig. 1a). The sample has a distinct rough surface caused by a large number of micropores (Fig. 1b). It can be seen that micropores and mesopores are dominant in ADR-700 (Fig. 1). According to the classification of the International Union of Pure and Applied Chemistry (IUPAC), the isotherms of all the samples are composed of Langmuir-I and Langmuir-IV (Fig. 1c), because of the coexistence of the micropores and mesopores [19]. The steep uptake volume below the relative pressure of 0.01 suggests the main contribution to the surface area was from the micropores [19]. When the relative pressure is higher than 0.01, the slow rise of the adsorption curve indicates the presence of mesopores (Fig. 1c). The less pronounced H4 hysteresis loops can be seen within the range of relative pressures of 0.5–0.9 which represents the slit pore type [20]. The pore size distribution curves of the ADRs are calculated from the adsorption isotherm curve using the density functional theory calculation method (Fig. 1d). The specific surface area and pore-structure characteristic of all the carbon materials are summarized in Table 1. ADR-700 had the maximum specific surface area of 792 m2 g−1 and the lowest specific surface ratio of micropores to mesopores of 3.77. This meant that ADR-700 contained a large number of micropores and a small amount of mesopores which was consistent with the result predicted by FESEM. The abundant porous structure and high specific surface area of the ADR can provide a channel for the exchange of ions and thus shows promise as an electrode material for supercapacitors [21].
3.3. Electrochemical properties of ADR as electrodes To explore the advantages of the porous carbons and their potential as electrode materials for supercapacitors, the capacitive performance was investigated in 6 M KOH electrolyte using a three-electrode system. CV curves of ADR samples showed a rectangular shape in the range of −1.0 to 0 V under different scanning speeds (Fig. S2). This indicated that the electrode materials had the characteristics of an ideal doublelayer capacitance [28]. In addition, the presence of some oxygen-containing groups on the activated carbon surface resulted in a weak redox peak and slight deformation of the curve (Fig. S2). This indicated that weak pseudo-capacitance performance was available. The oxygencontaining functional groups of carbonyl and ketone played an important role. The carbonyl group was detected using XPS spectra (Fig. 2d). The mechanism of the O atom in enhancing the capacitance using reversible Faradic redox reactions in the aqueous electrolyte has been widely studied [29–32]. The inductive effects of the σ-bonded structure from the O heteroatom cause a redistribution of the electrons as well as the polarization. In the alkaline aqueous electrolyte, the pseudocapacitance might be caused by the insertion/deinsertion reaction of the hydrated ions in the pore [29,30]. It has also been reported that the reversible redox reactions of the carbonyl (-C=O) groups at the surface of the materials could contribute to the pseudocapacitance [31,32]. ADR-700 is the most promising material for supercapacitor electrodes of the studied materials because it exhibits the highest current density (Fig. S2). To verify the results in CV curves, GCD experiments were performed at various current densities using a three-electrode configuration (Fig. 4b, S3, and S4). The GCD curves with symmetric triangles and straight lines indicated that these electrode materials had good reversibility of charge–discharge and ideal double-layer capacitance characteristics [33]. The GCD curves of the ADRs also showed a good performance rate with nearly no IR drop (Fig. 4b and S4). The capacitance performance of the obtained sample was calculated according to Eq. (1). When the current density was 1 A g−1, the ADR-700 electrode reached a maximum specific capacitance of 184.91 F g−1. Increasing the current density, the specific capacitance gradually decreased (Fig. 4d and S3a). When a high current density was applied, the entire charge–discharge process was completed within a very short period and the pores of the activated carbon material were not sufficient. Immersion of the electrolyte resulted in the reduction of the electric double layer formed in the pores and further reduced the specific capacitance of the porous carbon material electrode. ADR-700 showed the optimal specific capacitance (Fig. 4d). This was consistent with the BET and CV analysis results. A higher specific surface area can provide more adsorption sites for electrolytic ions, while the porous structure can provide short-distance channels [34]. EIS was used to obtain the electrochemical frequency characteristics and equivalent series resistance of the capacitors. Fig. 4c shows a
3.2. XPS and FTIR analysis XPS results of the four carbon materials are shown in Fig. 2 and S1. The presence of heteroatoms O and N prove that doping occurred in situ during the preparation of the activated carbon. The high-resolution C1s spectrum for all of the samples was deconvoluted into four individual component peaks at 284.8, 286.8, 287.6, and 289.8 eV corresponding to the functional groups C=C/CeC, CeO (epoxy and alkoxy), C=O (carbonyl), and O=CeC, respectively (Fig. 2 and S1). This indicated the presence of hydroxyl, epoxy, carbonyl, and carboxyl isooxygenated groups. A narrow trend was observed with the increasing temperature in the XPS C1s spectra of the ADRs meaning an enhanced degree of graphitic order [22]. The N1s spectra were deconvoluted into four different types: a pyridinic-type (398.5 eV), nitrile or imine type (399.5 eV), pyrrolic-type (400.5 eV), and graphitic-type (401.3 eV). Pyridinic and pyrrolic nitrogen species were assumed to be the main configurations responsible for the pseudo capacitance [23]. Meanwhile, graphitic-type nitrogen can enhance electrical conductivity [24]. The O1s peak can be deconvoluted into carbonyl groups (531.2 eV), = C = O/– CeOeCe (532.38 eV), phenol ether bond (533.87 eV), or Table 1 The specific surface area and pore-structure characteristic of the samples. Samples
SBETa [m2 g−1]
Smicb [m2 g−1]
Vtotalc [cm3 g−1]
Vmicrod [cm3 g−1]
Davere [nm]
Smic/Smes
ADR-650 ADR-700 ADR-750 ADR-800
437 792 686 605
389 626 554 536
0.361 0.648 0.591 0.476
0.214 0.340 0.303 0.293
3.30 3.27 3.44 3.15
8.10 3.77 4.20 7.77
Note. a SBET is the specific surface area from multiple BET method. b Smic is the microporous surface area from t-plot method. c Vtotal is the total volume calculated at a relative pressure of 0.99. d Vmic is the microporous volume from t-plot method. e Daver is the average pore width from the equation of 4Vtotal/SBET. 685
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Fig. 2. (a) XPS full spectra of ADR-650, ADR-700, ADR-750 and ADR-800 and (b) C1s, (c) N1s and (d) O1s spectra of ADR-700.
the porous carbon electrode with respect to the Warburg impedance in the intermediate frequency region, and the diffusion control was dominant [37]. The results showed that the charge transfer resistance of the activated material was reduced, the transport resistance of electrolyte ion was small, and the power characteristic has improved. The cycle lifetime of electrode materials is an important indicator of their practical application. The GCD cycling was measured at a current density of 5 A g−1 within a potential window of -1–0 V in the 6 M KOH aqueous electrolyte. Fig. 5 shows the stability of ADR-700 during charge–discharge cycling at a current density of 5 A g−1, representing a current density for high-power application. As shown in Fig. 5, the specific capacitance retained more than 94.80% of its initial value after 5000 cycles, which is significant for practical application. In basic electrolytes, the symmetric charge and discharge curves after 5000 cycles confirm the high degree of reversibility for charge storage in ADR-700 (Fig. 5). Similar to previous reports, the specific capacitance retention rate in some cycles increased the cycle efficiency during the test [31,38]. This might be explained as follows: (i) the surface wettability of the carbon materials had been improved by the hetero atom. With the improvement of the wettability during the cycling process, it was easier for the functional groups to contact ions in the electrolyte, thus allowing the insertion/deinsertion reaction to more readily occur; (ii) the pore utilization ratio of the carbon materials gradually increased during the cycling process [30]; or (iii) the activation process allowed the trapped ions to gradually diffuse outward [39,40]. The Coulomb efficiency of ADR-700 remained approximately 1.0 throughout the entire cycle, even after 5000 cycles (Fig. 5). This fully demonstrates that the ADR-700 can store charge well and discharge. The results prove the good stability of the ADR-700 carbon electrode. The electrode exhibits not only high specific capacitance but also good durability showing that activated carbon from ADR is promising for developing supercapacitors. The superior electrochemical performance of ADR-700 can
Fig. 3. FT-IR spectra of ADR-650, ADR-700, ADR-750 and ADR-800.
Nyquist plot of the electrode materials based on ADR in a frequency range from 10 kHz to 10 mHz with an amplitude of 5 mV. The intercept in the high-frequency region at the real axis represented the equivalent series resistance (ESR). The magnified high-frequency region showed the semicircle of the ADR-700 (Fig. 4C). It showed the charge transfer resistance corresponding to the semicircle was very small (approximately 0. 94 Ω), which means that the electrochemical reaction process was rapid and the material had good conductivity [35]. A nearly vertical line was obtained at low frequencies and indicated that the electrode had good capacitive behavior [36]. In the mid-frequency region, a straight line with an inclination of 45° was the typical characteristic of
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Fig. 4. (a) CV measurements of ADR-700 at different scan rates. (b) GCD curves of ADR-700 at different current densities; (c) EIS of ADR-650, ADR-700, ADR-750 andADR-800 (inset: magnified 0.5–3 Ω region of ADR-700) under the influence of an amplitude of 5 mV; (d) Specific capacitance of ADR-650, ADR-700, ADR-750 and ADR-800 at different current densities.
applied to an electrode of supercapacitors. The activated carbon had a high specific surface area, good pore size distribution, high specific capacitance, and excellent cycle stability in the aqueous electrolytes. ADR-700 showed the maximum specific capacitance of 184.91 F g−1 in 6 M KOH at a current density of 1 A g−1 and a good stability of more than 5000 cycles. This study presents a promising approach for the exciting possibility of preparing a high-performance electrode material for supercapacitors from ADR. Acknowledgements This work was supported by Fundamental Research Funds for the Central Universities of China (JZ2016YYPY0043 and JZ2017YYPY0246) and Hefei Huawei Automation Co., Ltd. Appendix A. Supplementary data Fig. 5. Cycle life of the ADR-700 electrode materials at current density of 5 A g−1 and variation of the specific capacitance retention and the coulomb efficiency over the 5000 cycle times, and the inset is the last 10 cycles of GCD.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2018.11.092. References
be attributed to several aspects. The high specific surface area provides high electronic conductivity and numerous effective electrochemical active sites to accommodate a large amount of charges. Furthermore, the abundant pores are conducive to rapid ion diffusion and provide an unblocked channel during the process of rapid charge and discharge. This leads to long-term electrochemical stability.
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Feasibility of activated carbon derived from anaerobic digester residues for supercapacitors
T
Ce Wanga, Jin Wanga, Wentao Wua,∗, Jiang Qiana, Shaomin Songb, Zhengbo Yuea a b
School of Resources and Environmental Engineering, Hefei University of Technology, Hefei, Anhui, 230009, China Hefei Huawei Automation Co., Ltd, Hefei, Anhui, 230009, China
H I GH L IG H T S
derived from anaerobic digester residue was feasible for supercapacitors. • AC achieved specific high surface area and good pore size distribution. • AC • Highest capacitance was 184.91 F g at 1 A g and with 94.80% retention. −1
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A R T I C LE I N FO
A B S T R A C T
Keywords: Anaerobic digester residue Activated carbon Cattle manure Electric double layer capacitor Supercapacitor
The question of how to recycle anaerobic digester residue is a critical issue for the anaerobic digestion process of lignocellulosic biomass. In the current study, anaerobic digester residue from a biogas plant for the treatment of cattle manure was utilized to prepare the activated carbon for supercapacitors. The effect of activation temperature on the specific surface area, pore size distribution, and electrochemical performance of activated carbon was investigated. Results show that activated carbon derived from anaerobic digester residue is feasible for supercapacitors. With an increase in activation temperature, the specific surface area of activated carbon first increases and then gradually decreases. Activated carbon obtained at 700 °C shows both high specific capacitance and excellent electrochemical cycle stability. The good capacitive performance confirms that anaerobic digested manure residue can act as an excellent carbonaceous material for high-performance supercapacitors.
1. Introduction With the development of large-scale ecological aquaculture, a series of anaerobic digestion techniques have been developed to treat tons of renewable lignocellulose waste. The anaerobic digestion process has been extensively applied in the bioconversion of agricultural production residues, aquatic plants, farm waste, etc. In addition to biogas, a large amount of digester liquid and residue are generated as well [1]. Anaerobic digester residue (ADR) contains the abundant inorganic nutrients N and P and can be used in aquaculture [2], as crop fertilizers [3] and in soil improvement [4]. However, ADR also contains potential environmental contaminants such as heavy metals, organic pollutants, antibiotics, pesticides, and pathogenic bacteria [5]. It has been reported that random stacking may contaminate the soil environment, surface water bodies, and groundwater systems [6]. Therefore, the question of how to recycle ADR remains critical. However, ADR can be viewed as a novel lignocellulosic biomass
∗
source. At present, lignocellulosic biomass such as cattle manure compost [7], silk cocoon [8], feathers [9]) and synthetic materials [10] are being used to prepare activated carbon. Because of its high specific surface area and porosity, activated carbon is being used to prepare electrode materials for supercapacitors of electric double layer capacitors (EDLCs) [11]. These supercapacitors are widely used in the fields of hybrid electrical vehicles, electronic devices, machinery, and the military [12–14]. Currently the major obstacle for the commercial application of EDLCs is the high cost and low energy density, which severely limit their practical application [15]. Compared to the traditional carbon sources of coal, asphalt, and phenolic resin, the development of supercapacitors derived from lignocellulosic biomass has been attracting much attention [16]. Therefore, ADR might be a potential feedstock of low cost for supercapacitors. In the current study, anaerobic digester residue from a biogas plant for the treatment of cattle manure was used for the preparation of activated carbon which was further applied to supercapacitors. The effect
Corresponding author. Tel.: +86 551 62901707. E-mail address: [email protected] (W. Wu).
https://doi.org/10.1016/j.jpowsour.2018.11.092 Received 3 August 2018; Received in revised form 1 November 2018; Accepted 28 November 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. FESEM images of (a) and (b) (ADR-700) (c) N2 adsorption-desorption isotherms and (d) pore size distribution curves of ADR-650, ADR-700, ADR-750 and ADR-800.
area and pore size distribution (ASAP 2020 V3.03H instrument). To prepare the working electrode, a viscous slurry containing 80 wt % ADR material, 10 wt% carbon black, and 10 wt% polytetrafluoroethylene was mixed and dispersed onto a nickel foam current collector as a 1 cm × 1 cm sheet, followed by drying at 105 °C in a vacuum oven overnight. The electrode was pressed on a tablet press at 12 MPa for 10–15 s. The mass of the loaded active material was approximately 3.0–4.0 mg for one electrode. The electrochemical performance of the synthesized materials was tested using a three-electrode system in a 6 M KOH aqueous electrolyte solution at room temperature. CV and GCD were conducted in a one-compartment cell using a threeelectrode configuration with a CHI 660D electrochemical workstation (Shanghai Chenhua Instruments Co). EIS was conducted in a onecompartment cell using a three-electrode configuration with a BioLogic EC-LAB VMP3 electrochemical workstation. An Hg/HgO electrode and Pt foil acted as the reference and counter electrode, respectively. CV curves were obtained in the potential range of −1 to 0 V by varying the scan rate from 5 to 100 mV s−1. EIS was measured in a frequency range of 10 kHz to 10 mHz at an open circuit voltage with an alternate current amplitude of 5 mV. GCD measurements were galvanostatically completed at 0.5–10 A g−1 over a voltage range of −1.0–0 V. For quantitative considerations, the specific capacitance was calculated from the GCD values using the following equation:
of activating temperatures on the physiochemical characteristic of the activated carbon was investigated. The electrochemical properties of the supercapacitors were evaluated using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS). 2. Materials and methods 2.1. Preparation of activated carbon from anaerobic digester residue ADR was collected from a private farm in the Suzhou, Anhui Province, China. The residue was thoroughly dried at 80 °C and precarbonized at 400 °C for 1 h using a heating rate of 5 °C min−1 under a nitrogen gas atmosphere in a tube resistance furnace. The pre-carbonized materials were mixed with KOH (1:1 based on mass) in a water bath at 80 °C for 10 h and dried at 100 °C. The mixtures were pyrolyzed at 650, 700, 750, and 800 °C for 2 h at a heating rate of 5 °C min−1 under a nitrogen gas atmosphere, respectively. The products were soaked in 100 mL of HCl solution (1.0 M) for 10 min and then washed with hot distilled water at 80 °C until the washed water pH reached 7. The solid products were dried overnight and sieved through 200 mesh. These samples were labeled as ADR-650, ADR-700, ADR-750, and ADR800, respectively.
C = IΔt / mΔV
2.2. Analytical methods
(1)
where I is the constant discharge current, Δt is the discharge time, ΔV is the potential window during discharge, and m is the active mass [17]. Coulomb efficiency as a function of cycle number was calculated using the following equation:
The morphology, structure, and composition of the as-prepared samples were analyzed using field emission scanning electron microscopy (FESEM, FEI Nova Nano SEM 450), Fourier transform infrared spectroscopy (FTIR, Varian 600-IR) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250). The porosity properties and specific surface area of the samples were obtained using the Brunauer–Emmett–Teller (BET) method to obtain the specific surface
η (%) = Td/ Tc × 100
(2)
where Td and Tc are the discharge time and charge time, respectively [18]. 684
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eNeOe (534.1 eV) functional groups [25]. This can participate in Faradaic reactions to provide improved pseudo capacitance and enhance wettability of the carbon materials which increasing the specific capacitance of the supercapacitors [26]. These results confirm the successful synthesis of doped nitrogen and high-oxygen-containing porous carbon materials. FTIR analysis results for the ADR shows peaks at 3357.9, 2924.0, 2852.6, 1579.6, 1087.8, and 798.5 cm−1 (Fig. 3). These peaks may be a result of OeH (polyol or phenol), eCH2e (alkyl), eCH2e (alkyl), NeH (secondary amide), CeOeC (ether), and NeH (secondary amide) bending or stretching vibrational absorption peaks of the functional groups, respectively [27]. With the increase in activation temperature, the strength of the CeOeC peak (1087.8 cm−1) and OeH peak (3357.9 cm−1) gradually increased. Results showed that there were a large number of O-containing groups on the surface of the prepared materials consistent with the results of XPS.
3. Results and discussion 3.1. Porous structure characterization FESEM images of ADR-700 clearly show a rough irregular flake and block structure (Fig. 1a). The sample has a distinct rough surface caused by a large number of micropores (Fig. 1b). It can be seen that micropores and mesopores are dominant in ADR-700 (Fig. 1). According to the classification of the International Union of Pure and Applied Chemistry (IUPAC), the isotherms of all the samples are composed of Langmuir-I and Langmuir-IV (Fig. 1c), because of the coexistence of the micropores and mesopores [19]. The steep uptake volume below the relative pressure of 0.01 suggests the main contribution to the surface area was from the micropores [19]. When the relative pressure is higher than 0.01, the slow rise of the adsorption curve indicates the presence of mesopores (Fig. 1c). The less pronounced H4 hysteresis loops can be seen within the range of relative pressures of 0.5–0.9 which represents the slit pore type [20]. The pore size distribution curves of the ADRs are calculated from the adsorption isotherm curve using the density functional theory calculation method (Fig. 1d). The specific surface area and pore-structure characteristic of all the carbon materials are summarized in Table 1. ADR-700 had the maximum specific surface area of 792 m2 g−1 and the lowest specific surface ratio of micropores to mesopores of 3.77. This meant that ADR-700 contained a large number of micropores and a small amount of mesopores which was consistent with the result predicted by FESEM. The abundant porous structure and high specific surface area of the ADR can provide a channel for the exchange of ions and thus shows promise as an electrode material for supercapacitors [21].
3.3. Electrochemical properties of ADR as electrodes To explore the advantages of the porous carbons and their potential as electrode materials for supercapacitors, the capacitive performance was investigated in 6 M KOH electrolyte using a three-electrode system. CV curves of ADR samples showed a rectangular shape in the range of −1.0 to 0 V under different scanning speeds (Fig. S2). This indicated that the electrode materials had the characteristics of an ideal doublelayer capacitance [28]. In addition, the presence of some oxygen-containing groups on the activated carbon surface resulted in a weak redox peak and slight deformation of the curve (Fig. S2). This indicated that weak pseudo-capacitance performance was available. The oxygencontaining functional groups of carbonyl and ketone played an important role. The carbonyl group was detected using XPS spectra (Fig. 2d). The mechanism of the O atom in enhancing the capacitance using reversible Faradic redox reactions in the aqueous electrolyte has been widely studied [29–32]. The inductive effects of the σ-bonded structure from the O heteroatom cause a redistribution of the electrons as well as the polarization. In the alkaline aqueous electrolyte, the pseudocapacitance might be caused by the insertion/deinsertion reaction of the hydrated ions in the pore [29,30]. It has also been reported that the reversible redox reactions of the carbonyl (-C=O) groups at the surface of the materials could contribute to the pseudocapacitance [31,32]. ADR-700 is the most promising material for supercapacitor electrodes of the studied materials because it exhibits the highest current density (Fig. S2). To verify the results in CV curves, GCD experiments were performed at various current densities using a three-electrode configuration (Fig. 4b, S3, and S4). The GCD curves with symmetric triangles and straight lines indicated that these electrode materials had good reversibility of charge–discharge and ideal double-layer capacitance characteristics [33]. The GCD curves of the ADRs also showed a good performance rate with nearly no IR drop (Fig. 4b and S4). The capacitance performance of the obtained sample was calculated according to Eq. (1). When the current density was 1 A g−1, the ADR-700 electrode reached a maximum specific capacitance of 184.91 F g−1. Increasing the current density, the specific capacitance gradually decreased (Fig. 4d and S3a). When a high current density was applied, the entire charge–discharge process was completed within a very short period and the pores of the activated carbon material were not sufficient. Immersion of the electrolyte resulted in the reduction of the electric double layer formed in the pores and further reduced the specific capacitance of the porous carbon material electrode. ADR-700 showed the optimal specific capacitance (Fig. 4d). This was consistent with the BET and CV analysis results. A higher specific surface area can provide more adsorption sites for electrolytic ions, while the porous structure can provide short-distance channels [34]. EIS was used to obtain the electrochemical frequency characteristics and equivalent series resistance of the capacitors. Fig. 4c shows a
3.2. XPS and FTIR analysis XPS results of the four carbon materials are shown in Fig. 2 and S1. The presence of heteroatoms O and N prove that doping occurred in situ during the preparation of the activated carbon. The high-resolution C1s spectrum for all of the samples was deconvoluted into four individual component peaks at 284.8, 286.8, 287.6, and 289.8 eV corresponding to the functional groups C=C/CeC, CeO (epoxy and alkoxy), C=O (carbonyl), and O=CeC, respectively (Fig. 2 and S1). This indicated the presence of hydroxyl, epoxy, carbonyl, and carboxyl isooxygenated groups. A narrow trend was observed with the increasing temperature in the XPS C1s spectra of the ADRs meaning an enhanced degree of graphitic order [22]. The N1s spectra were deconvoluted into four different types: a pyridinic-type (398.5 eV), nitrile or imine type (399.5 eV), pyrrolic-type (400.5 eV), and graphitic-type (401.3 eV). Pyridinic and pyrrolic nitrogen species were assumed to be the main configurations responsible for the pseudo capacitance [23]. Meanwhile, graphitic-type nitrogen can enhance electrical conductivity [24]. The O1s peak can be deconvoluted into carbonyl groups (531.2 eV), = C = O/– CeOeCe (532.38 eV), phenol ether bond (533.87 eV), or Table 1 The specific surface area and pore-structure characteristic of the samples. Samples
SBETa [m2 g−1]
Smicb [m2 g−1]
Vtotalc [cm3 g−1]
Vmicrod [cm3 g−1]
Davere [nm]
Smic/Smes
ADR-650 ADR-700 ADR-750 ADR-800
437 792 686 605
389 626 554 536
0.361 0.648 0.591 0.476
0.214 0.340 0.303 0.293
3.30 3.27 3.44 3.15
8.10 3.77 4.20 7.77
Note. a SBET is the specific surface area from multiple BET method. b Smic is the microporous surface area from t-plot method. c Vtotal is the total volume calculated at a relative pressure of 0.99. d Vmic is the microporous volume from t-plot method. e Daver is the average pore width from the equation of 4Vtotal/SBET. 685
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Fig. 2. (a) XPS full spectra of ADR-650, ADR-700, ADR-750 and ADR-800 and (b) C1s, (c) N1s and (d) O1s spectra of ADR-700.
the porous carbon electrode with respect to the Warburg impedance in the intermediate frequency region, and the diffusion control was dominant [37]. The results showed that the charge transfer resistance of the activated material was reduced, the transport resistance of electrolyte ion was small, and the power characteristic has improved. The cycle lifetime of electrode materials is an important indicator of their practical application. The GCD cycling was measured at a current density of 5 A g−1 within a potential window of -1–0 V in the 6 M KOH aqueous electrolyte. Fig. 5 shows the stability of ADR-700 during charge–discharge cycling at a current density of 5 A g−1, representing a current density for high-power application. As shown in Fig. 5, the specific capacitance retained more than 94.80% of its initial value after 5000 cycles, which is significant for practical application. In basic electrolytes, the symmetric charge and discharge curves after 5000 cycles confirm the high degree of reversibility for charge storage in ADR-700 (Fig. 5). Similar to previous reports, the specific capacitance retention rate in some cycles increased the cycle efficiency during the test [31,38]. This might be explained as follows: (i) the surface wettability of the carbon materials had been improved by the hetero atom. With the improvement of the wettability during the cycling process, it was easier for the functional groups to contact ions in the electrolyte, thus allowing the insertion/deinsertion reaction to more readily occur; (ii) the pore utilization ratio of the carbon materials gradually increased during the cycling process [30]; or (iii) the activation process allowed the trapped ions to gradually diffuse outward [39,40]. The Coulomb efficiency of ADR-700 remained approximately 1.0 throughout the entire cycle, even after 5000 cycles (Fig. 5). This fully demonstrates that the ADR-700 can store charge well and discharge. The results prove the good stability of the ADR-700 carbon electrode. The electrode exhibits not only high specific capacitance but also good durability showing that activated carbon from ADR is promising for developing supercapacitors. The superior electrochemical performance of ADR-700 can
Fig. 3. FT-IR spectra of ADR-650, ADR-700, ADR-750 and ADR-800.
Nyquist plot of the electrode materials based on ADR in a frequency range from 10 kHz to 10 mHz with an amplitude of 5 mV. The intercept in the high-frequency region at the real axis represented the equivalent series resistance (ESR). The magnified high-frequency region showed the semicircle of the ADR-700 (Fig. 4C). It showed the charge transfer resistance corresponding to the semicircle was very small (approximately 0. 94 Ω), which means that the electrochemical reaction process was rapid and the material had good conductivity [35]. A nearly vertical line was obtained at low frequencies and indicated that the electrode had good capacitive behavior [36]. In the mid-frequency region, a straight line with an inclination of 45° was the typical characteristic of
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Fig. 4. (a) CV measurements of ADR-700 at different scan rates. (b) GCD curves of ADR-700 at different current densities; (c) EIS of ADR-650, ADR-700, ADR-750 andADR-800 (inset: magnified 0.5–3 Ω region of ADR-700) under the influence of an amplitude of 5 mV; (d) Specific capacitance of ADR-650, ADR-700, ADR-750 and ADR-800 at different current densities.
applied to an electrode of supercapacitors. The activated carbon had a high specific surface area, good pore size distribution, high specific capacitance, and excellent cycle stability in the aqueous electrolytes. ADR-700 showed the maximum specific capacitance of 184.91 F g−1 in 6 M KOH at a current density of 1 A g−1 and a good stability of more than 5000 cycles. This study presents a promising approach for the exciting possibility of preparing a high-performance electrode material for supercapacitors from ADR. Acknowledgements This work was supported by Fundamental Research Funds for the Central Universities of China (JZ2016YYPY0043 and JZ2017YYPY0246) and Hefei Huawei Automation Co., Ltd. Appendix A. Supplementary data Fig. 5. Cycle life of the ADR-700 electrode materials at current density of 5 A g−1 and variation of the specific capacitance retention and the coulomb efficiency over the 5000 cycle times, and the inset is the last 10 cycles of GCD.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2018.11.092. References
be attributed to several aspects. The high specific surface area provides high electronic conductivity and numerous effective electrochemical active sites to accommodate a large amount of charges. Furthermore, the abundant pores are conducive to rapid ion diffusion and provide an unblocked channel during the process of rapid charge and discharge. This leads to long-term electrochemical stability.
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