Article pubs.acs.org/EF
Ultramicroporous Carbon Nanoparticles for the High-Performance Electrical Double-Layer Capacitor Electrode Yunhui Zhao, Mingxian Liu,* Lihua Gan,* Xiaomei Ma, Dazhang Zhu, Zijie Xu, and Longwu Chen Department of Chemistry, Tongji University, 1239 Siping Road, Shanghai 200092, People’s Republic of China ABSTRACT: Novel ultramicroporous carbon nanoparticles (UCNs) are synthesized on the basis of solvothermal polymerization of phloroglucinol and terephthaldehyde in dioxane at 220 °C, followed by carbonization at 850 °C. The resultant UCNs show regular 0.54 nm ultramicropores and particle sizes of ∼30 nm. The UCNs as electrode materials for electrical double-layer capacitors (EDLCs) show a specific capacitance of 206 F g−1 at 1.0 A g−1 in a 6 M KOH aqueous solution. The UCN electrode is suitable for charge−discharge operation even at a very high current density of 50 A g−1 coupled with a capacitance of 135 F g−1, indicating excellent high-rate electrochemical performance. The electrochemical capacitance of the UCN electrode has a high retention of 97.6% after 5000 cycles at 1.0 A g−1 tested by a standard three-electrode system (97.3% for the two-electrode cell), which implies a good electrochemical cycling life. This study highlights promising prospects of novel UCNs as advanced electrode materials for EDLCs where a high level of current charge and discharge is required.
1. INTRODUCTION Electrical double-layer capacitors (EDLCs) make up a class of electrical energy storage devices.1 With a high power density, a long cyclic life, and superior reversibility, EDLCs have attracted extensive interest because of their promising applications in mobile electronic devices, pure and hybrid electric vehicles, uninterruptible power supplies, etc.2−5 Electrostatic storage of electrical energy in EDLCs was achieved by accumulation of charges at the electrode−electrolyte interfaces, which takes place on electric double layers.6,7 Therefore, a suitable pore structure and a relatively large surface area are required for the storage of energy in EDLCs. Besides pore structure, it is wellknown that the presence of surface functional and electrical conductivity that mainly depends on the carbonization process is closely related to the electrochemical performance.8,9 As for the pseudocapacitor, a rapid and reversible faradic process takes place due to electro-active species.10,11 Microporous carbons (e.g., activated carbons) offer high specific surface areas and thus have been considered as one of the most promising candidates for EDLC electrodes.12−17 For instance, Jiang et al. synthesized microporous carbon with a surface area as large as 3405 m2 g−1, which showed an electrochemical capacitance of ∼200 F g−1 at 250 mA g−1 in a H2SO4 solution.18 Unfortunately, common microporous carbons suffer the disadvantage of irregular pores, leading to the limited transportation of the electrolyte ion through the island tunnels. Such a shortage has caused difficulty in the lack of a proportional relationship between their specific capacitances and specific surface areas.19 For example, on the basis of the metal−organic framework, Liu et al. synthesized microporous carbon at 530 °C with a specific surface area of 3040 m2 g−1, which showed a specific capacitance of 48 F g−1 at 250 mA g−1; the carbon obtained at 650 °C had a capacitance of 195 F g−1 under the same current condition, although it has a much smaller specific surface area of 1521 m2 g−1.20 Besides, it is difficult to operate common microporous carbon-based EDLCs at a high charge−discharge rate (usually >10 A g−1), which also results from the irregular and island micropores. © 2013 American Chemical Society
However, microporous carbons can be used in additional ways if their irregular pores are properly optimized.21−26 For aqueous EDLCs, the 0.5 nm micropores could be electrochemically available for aqueous electrolytes.30,31 Xu et al. recently synthesized ultramicroporous carbons with a regular pore size of 0.55 nm by using poly(vinylidene fluoride) as the carbon source, and the carbons as a supercapacitor electrode have a gravimetric capacitance of 194 F g−1 at 1.0 A g−1 in a KOH electrolyte.21 Besides, the carbon electrode is suitable for charge−discharge operation at a large current density of 10 A g−1 coupled with a specific capacitance of 145 F g−1, indicating good electrochemical performance. Such ultramicroporous carbons are supposed to be powder or flakes on the micrometer scale (not described here). If ultramicroporous carbons could be made into nanoparticles, the homogeneous package among the particles could create mesoporosity, which is proven to shorten the transfer pathways of electrolyte ions to the pore surfaces of carbon and thus lead to improved electrochemical performance.32,33 Thus, ultramicroporous carbon nanoparticles (UCNs) with a regular pore size would represent a novel and promising material for EDLCs. However, UCNs, to the best of our knowledge, have not been reported previously. Herein, we report, for the first time, the synthesis of UCNs as electrode materials for EDLCs. Phloroglucinol and terephthaldehyde, which both have rigid benzene, were used as the carbon source, and dioxane served as the solvent. On the basis of the solvothermal polymerization and carbonization process, UCNs with a regular pore size that peaked at 0.54 nm that consist of nanoparticles with a diameter of ∼30 nm were fabricated. The regular micropores improve the fast transportation and diffusion of aqueous electrolyte ions, and the Received: August 20, 2013 Revised: December 3, 2013 Published: December 27, 2013 1561
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Scheme 1. Illustration of the Preparation of Ultramicroporous Carbons
Figure 1. SEM images of ultramicroporous carbons prepared with phloroglucinol:dioxane mass ratios of 0.15 (a) and 0.10 (b). (Bruker D8) with Cu Kα (λ = 0.154056 Å) radiation, X-ray photoelectron spectroscopy (XPS) (AXIS Ultra DLD), and nitrogen adsorption and desorption isotherms (Micromeritics ASAP 2020 porosimeter). The specific surface area was calculated by the Brunauer−Emmett−Teller (BET) method, and the micropore volume was derived from a Dubinin−Radushkevich plot. Nonlocal density functional theory (NLDFT), the cylindrical model, was utilized to obtain the pore size distribution. 2.4. Electrochemical Measurements. The electrochemical performance was evaluated via a conventional three-electrode system in a 6 M KOH aqueous solution. The Ag/AgCl electrode was used as the reference electrode, and nickel foams were used as the counter electrode. The working electrode was obtained by pressing the mixture of UCNs, carbon black, and PTFE with a mass ratio of 8:1:1 onto nickel foam. Then the 0.50 mm thick circle electrode with a diameter of ∼1.0 cm was dried overnight at 100 °C. Cyclic voltammogram (CV), galvanostatic charge−discharge (GCD) tests were performed on a CHI660D electrochemical workstation with the potential window from −0.8 to 0 V versus the Ag/AgCl electrode. Electrochemical impedance spectroscopy (EIS) was also conducted on the CHI 660D system; the frequency range was set to 0.01−105 Hz, and the open circuit potential of the electrode was −0.19 V. The specific capacitance of the electrode was determined as follows:
stacking of UCNs results in interparticle cavities that could facilitate the movement of electrolyte ions into the interior surfaces of UCNs. As a result, as-prepared UCNs show rapid ion transport and high-rate electrochemical performance, which provides important prospects for EDLC electrodes.
2. EXPERIMENTAL SECTION 2.1. Materials. Phloroglucinol, terephthaldehyde, dioxane, and KOH were analytical grade and were obtained from Sinopharm Chemical Reagent Co., Ltd. Graphite was supplied by the Shanghai Colloid Chemical Plant. Polytetrafluoroethylene (PTFE, catalog no. FR301B) was produced by Shanghai 3F New Materials Co., Ltd. Nickel foam was bought from the Shanghai Hongxiang Plant. Highly pure N2 was provided by Shanghai BOC Special Gases Sales Service Co., Ltd. 2.2. Synthesis. In a typical procedure, 1.51 g of phloroglucinol and 1.21 g of terephthaldehyde were dissolved in 10 mL of dioxane (ρ = 1.04 g cm−3), and then the mixture was heated to 70 °C while being stirred for 1 h. The obtained phloroglucinol/terephthaldehyde prepolymer (12 mL) was cooled to room temperature and transferred to a 30 mL Teflon-lined autoclave. After being purged with N2 to remove the oxygen, the Teflon-lined autoclave was heated to 220 °C for 4 days to obtain the phloroglucinol/terephthaldehyde polymer nanoparticles (or microspheres). The polymer was carbonized at 850 °C in a purified N2 flow at a heating rate of 5 °C min−1 to fabricate UCNs. Ultramicroporous carbon microspheres (UCMs) were obtained when 15 mL of dioxane was used (without changing other experimental conditions). The schematic illustration for the preparation of ulatramicroporous carbons is shown in Scheme 1. The char yield is 53 wt % for UCNs and 51 wt % for UCMs. 2.3. Characterization. The morphology of ultramicroporous carbons was characterized by scanning electron microscopy (SEM) (Hitachi S-4800) and transmission electron microscopy (TEM) (JEM2100) with an accelerating voltage of 200 kV. The structural information could be obtained by powder X-ray diffraction (XRD)
Cm =
C I × Δt = m m × ΔV
(1)
where Cm is the specific capacitance (farads per gram), C is the capacitance (farads), m is the mass of the electrode (grams), I is the discharge current (amperes), Δt is the discharge time (seconds), and ΔV is the potential window of the discharge (volts). A symmetric two-electrode cell configuration was used to obtain an appropriate parameter for the retention of the capacitance and stability of the electrodes. For the preparation of one electrode, UCNs, PTFE, and carbon black in a mass ratio of 8:1:1 were dispersed in a small amount of ethanol to form a slurry. It was spread onto a nickel foam (1 cm2), held under a 20 MPa atmosphere, and dried at 100 °C overnight 1562
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to prepare the electrode. After that, two electrodes with same weight of active materials were put together, and a polypropylene membrane was sandwiched between them as a separator. Two identical electrodes acted as the cathode and anode. Before each measurement, the electrodes were soaked in 6 M KOH for 24 h to force the electrolyte ions to thoroughly transfer the electrode porosity. The specific capacitance of the electrode was determined as follows: C=
2 × I × Δt ΔV × m
increase in the dioxane level, the rate of polymerization of phloroglucinol/terephthaldehyde becomes slow. Under the solvothermal condition, the obtained phloroglucinol/terephthaldehyde colloids may serve as seeds for further polymerization, leading to the formation of microspheres.34 The XPS spectrum of UCNs (Figure 3a) indicates that UCNs contain C and O, at compositions are 95.06 and 4.94 atom %, respectively. XRD patterns of UCNs and UCMs shown in Figure 3b show broad diffraction peaks displayed at ∼23° and ∼44° that belonged to (002) and (100) planes of amorphous carbons, respectively, indicative of a nongraphitized carbon structure of UCNs and UCMs. N2 physisorption isotherms and corresponding pore size distribution curves of the UCNs and UCMs are shown in Figure 4. The isotherms of UCMs, as shown in Figure 4a, belong to a type I curve according to the classification of the International Union of Pure and Applied Chemistry (IUPAC).35 The steep increase in the adsorbed volume at a very low relative pressure (P/P0 = 0.05) is related to the existence of abundant micropores. UCNs show similar adsorption and desorption isotherms in that region. The pore size distribution curves shown in Figure 4b indicate a sharp peak centered at 0.54 nm for UCNs and 0.50 nm for UCMs, in addition to 0.6 and 1.5 nm micropores in both UCNs and UCMs. Compared with UCMs, UCNs composed of nanoscale particles could more effectively resist the shrinkage during carbonization. Thus, UCNs have larger micropores than UCMs. Besides, a N2 condensation step with a hysteresis loop is observed for UCNs at a relative pressure (P/P0) of 0.90, suggesting the presence of mesopores that could be ascribed to the packing of UCNs, as shown in Figure 2. Table 1 lists the pore structural parameters of UCNs and UCMs. The micropore specific surface area is 549 m2 g−1 for UCNs and 505 m2 g−1 for UCMs. There is also a similar micropore volume for UCNs (0.25 cm3 g−1) and UCMs (0.23 cm3 g−1). However, the BET area and total pore volume of UCNs are 842 m2 g−1 and 0.74 cm3 g−1, respectively, higher than those of UCMs (579 m2 g−1 and 0.29 cm3 g−1, respectively). This should also be ascribed to the mesoporosity that resulted from the stacking of UCNs that have a particle size much smaller than that of UCMs. Figure 5a shows CV curves of UCN and UCM electrodes at a scan rate of 10 mV s−1. The UCN electrode exhibits a quasirectangular voltammogram shape in a potential window between 0 and −0.8 V, typical of electric double-layer capacitive energy storage. As a comparison, the CV curve for UCMs
(2)
where C is the capacitance of each electrode (farads per gram), m is the total mass of the material on two electrodes (grams), I is the discharge current (amperes), Δt is the discharge time (seconds), and ΔV is the potential window of the charge and discharge (volts).
3. RESULTS AND DISCUSSION Figure 1 shows SEM images of the ultramicroporous carbons prepared with different phloroglucinol:dioxane mass ratios. When the phloroglucinol:dioxane mass ratio is 0.15, the obtained sample consists of UCNs that are composed of carbon nanoparticles (Figure 1a). The particles observed by TEM show a mean diameter of ∼30 nm, as shown in Figure 2.
Figure 2. TEM image of UCNs.
With the increase in the solvent level, the resultant products are ultramicroporous carbon microspheres (UCMs) with diameters of 1.5−2.0 μm (Figure 1b). Phloroglucinol, which has three hydroxyl groups, shows high reactivity in the electrophilic aromatic substitution. The polymerization of phloroglucinol and terephthaldehyde with a proper amount of solvent could generate monodisperse and nanoscale colloids.33 With the
Figure 3. (a) XPS spectrum of UCNs and (b) XRD patterns of UCNs and UCMs. 1563
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Figure 4. (a) N2 physisorption isotherms and (b) pore size distribution curves of UCNs and UCMs.
Table 1. Pore Structural Parametersa of UCNs and UCMs sample
SBET (m2 g−1)
Smicro (m2 g−1)
Sex (m2 g−1)
Vt (cm3 g−1)
Vmicro (cm3 g−1)
UCNs UCMs
842 579
549 505
293 74
0.74 0.29
0.25 0.23
rate increases to 200 mV s−1, CV curves of UCNs still maintain a rectangular-like shape, which implies that ideal capacitance behavior occurred at the electrode with a very quick charge− discharge feature. Figure 6a exhibits the GCD curves of UCN and UCM electrodes at a loading current density of 1.0 A g−1. The GCD curve of UCNs shows regular triangular shapes between 0 and −0.8 V versus the Ag/AgCl electrode. No obvious IR drop was found in the charge−discharge curve of UCNs, which suggests a tiny ohmic resistance and their good capacitive performance. The calculated capacitance of UCNs in a 6 M KOH solution is 206 F g−1 at 1.0 A g−1, much larger than that of UCMs (45 F g−1). Moreover, UCNs display regularly triangular shapes from 1.0 to 50 A g−1, as shown in panels b and c of Figure 6, and there is also only a tiny IR drop in the charge−discharge curve at a high current density of 10 A g−1, indicating the good Coulombic efficiency and ideal capacitor behavior of UCNs. The effect of carbonization temperature on electrochemical capacitance properties of UCNs was studied. Generally, a high carbonization temperature is required to ensure the complete carbonization of the polymer, which contributes to the enhancement of the conductivity of electrode materials. However, a high carbonization temperature also causes significant skeleton shrinkage and a loss of surface area. Different temperatures such as 700, 800, 850, and 900 °C were chosen as the carbonization temperature. When the carbonization temperature increases from 700 to 850 °C, the specific capacitance of UCNs increased from to 158 to 206 F g−1 at 1 A g−1. However, the capacitance decreased to 145 F g−1 as the carbonization temperature increased to 900 °C. Thus, 850 °C is an optimized condition for UCNs as the EDLC electrode.
a
Abbreviations: SBET, specific surface area; Smicro, micropore surface area; Sex, external surface area; Vt, total volume; Vmicro, micropore volume.
derives much from a rectangular shape. Besides, the specific capacitance of an electrode is closely related to the integrated area of its CV curve; that is to say, the larger the integrated area of an electrode, the higher its capacitance. Therefore, UCNs have an electrochemical capacitance much higher than that of UCMs at the same scanning rate. It has been reported that the
