Article pubs.acs.org/Langmuir
Easy Synthesis of Hierarchical Carbon Spheres with Superior Capacitive Performance in Supercapacitors Xinhua Huang,† Seok Kim,‡ Min Seon Heo,† Ji Eun Kim,‡ Hongsuk Suh,§ and Il Kim*,† †
The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, South Korea ‡ School of Chemical and Biomolecular Engineering, Pusan National University, Pusan 609-735, South Korea § Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Pusan 609-735, South Korea
Langmuir 2013.29:12266-12274. Downloaded from pubs.acs.org by KAROLINSKA INST on 01/24/19. For personal use only.
S Supporting Information *
ABSTRACT: An easy template-free approach to the fabrication of pure carbon microspheres has been achieved via direct pyrolysis of as-prepared polyaromatic hydrocarbons including polynaphthalene and polypyrene. The polyaromatics were synthesized from aromatic hydrocarbons (AHCs) using anhydrous zinc chloride as the Friedel−Crafts catalyst and chloromethyl methyl ether as a cross-linker. The experimental results show that the methylene bridges between phenyl rings generate a hierarchical porous polyaromatic precursor to form three-dimensionally (3D) interconnected micro-, meso-, and macroporous networks during carbonization. These hierarchical porous carbon aggregates of spherical carbon spheres exhibit faster ion transport/diffusion behavior and increased surface area usage in electric double-layer capacitors. Furthermore, micropores are present in the 3D interconnected network inside the cross-linked AHC-based carbon microspheres, thus imparting an exceptionally large, electrochemically accessible surface area for charge accumulation.
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INTRODUCTION Electrochemical capacitors (ECs), also known as supercapacitors, are attracting increasing interest as promising energy storage devices because of their high power energy density and longer cycle performance than conventional dielectric capacitors.1 Such capacitors are based on the accumulation of electrochemical charge at the electric double layer and the occurrence of Faradaic reactions.2 Since the electrode is the key part of ECs, the electrode materials are the most important factor in determining the properties of ECs.3 Carbon-based materials,4 transition metal oxides,5 and conductive polymers6 are the most commonly used electrode materials. Porous carbon materials are the most promising candidates because of their unique properties including high surface areas, stable physicochemical properties, good conductivity, low cost, and availability.7−9 However, porous-carbon-based ECs suffer from electrode kinetic problems related to inner-pore ion transport, which results in poor rate performance.10−12 The exact mechanism of ion transport within porous materials is very complex: The tortuosity, connectivity, pore-size distribution, and shape of the pores, as well as the nature of the electrolyte and solid−liquid interface must all be considered.13−15 Among these factors, the inner-pore ion transport resistance and diffusion distance are the most important. A common method for reducing the transportation distance of ions in micropore channels is to intersperse mesopores or macropores, which serve as ion© 2013 American Chemical Society
buffering reservoirs and provide convenient ion-transportation pathways into the micropores.16,17 To date, porous carbon materials have been pursued and touted as the most promising candidates for ECs owing to their excellent physicochemical stability and good conductivity.18,19 Many efforts have been made to elucidate the relationship between the pore structure and electrochemical capacitive behavior of carbon materials with the aim of developing advanced supercapacitors.20,21 For example, Oschatz et al. and Korenblit et al. introduced ordered mesopores into carbide-derived carbons, which are still microporous but have enhanced frequency response.22,23 Recently, hierarchical porous carbon (HPC) materials have shown great potential for high performance supercapacitor applications. Hierarchical pores with three-dimensional (3D) ordered/aperiodic porous texture in an interconnected micro-, meso-, and macroporous network exhibit the advantages of each pore size through a synergistic effect during the electrochemical charge−discharge process. The macropores serve as ion-buffering reservoirs to reduce the diffusion distance,17,28,24 the mesopores provide ion-transport pathways with minimized resistance,25−27 and the micropores enhance the electrical double layer.12,13 As a consequence, considerable progress has been made in the design and construction of such Received: July 16, 2013 Revised: August 19, 2013 Published: September 4, 2013 12266
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Fourier transform (FT)/Raman spectra were recorded at a resolution of 4 cm−1 using a Bruker IFS 66 interferometer coupled to a Bruker FRA 106 Raman module equipped with a continuous He/Ne laser. The microstructures of the samples were investigated using an S-4800 scanning electron microscope (SEM; Hitachi, Japan), a JEM-2100F HR transmission electron microscope (TEM), and a NOVA 3200e surface area and pore size analyzer. Prior to analysis, the carbon samples were degassed for more than 10 h at 250 °C. The Brunauer− Emmett−Teller (BET) surface area (SBET) was determined using BET theory. The total pore volume (Vt) was estimated from the amount adsorbed at a relative pressure (P/P0) of ∼0.997. The pore size distribution was analyzed via Barrett−Joyner−Halenda (BJH) combined with non-negative regularization and medium smoothing. The thermo gravimetric analysis (TGA; Scinco, TGAN-1000) was performed in the range of 30−800 °C in steps of 10 °C min−1 under high-purity N2 flow (10 cm3 min−1). Electrode Preparation and Electrochemical Measurements. Each carbon material was suspended in a Nafion solution (100 μL of Nafion 117 solution mixed with 900 μL ethanol) and sonicated for at least 1 h to ensure that the particles were well-dispersed and wetted with the solution. The carbon film was prepared on a glassy carbon electrode (MF-2012, Bio Analytical Systems, 3.0 mm diameter) by drop-casting the carbon suspension followed by drying the electrode at room temperature for 12 h. The aqueous electrolyte was 1.0 M H2SO4 and the electrochemical performance of the HPC was characterized by cyclic voltammetry (CV) in a voltage range of 0−0.9 V using a CV50W voltammetric analyzer (Bio Analytical Systems, Inc.) with a platinum-wire counter electrode and a silver/silver chloride (Ag/ AgCl) reference electrode. The electrochemical impedance spectrum (EIS) was obtained by the Iviumstat: Electrochemical & Impedance Analyzer (Ivium Technologies, Netherlands) with a platinum-wire counter electrode, a silver/silver chloride (Ag/AgCl) reference electrode, and 1.0 M H2SO4 aqueous solution as electrolyte.
HPCs and the characterization of their promising electrochemical capacitive properties. To the best of our knowledge, the fabrication of hierarchical structures is usually complicated and time-consuming, owing to the required preparation of templates with special nano- or molecular structures, removal of hard-templates, and/or post activation.28−33 Although these methods are used to successfully prepare various HPCs with precise microstructures, they also inevitably have the limitations mentioned above due to the need for expensive templates. These limitations impart a higher price-to-performance ratio as compared to other materials for any given application, and thus limit their commercial viability. The most simplistic and evident solution to the problem is the introduction of hierarchical porosity without auxiliary templates.34−37 Therefore, new costeffective carbon materials are urgently needed for the development of HPCs. In this work, we report the electrocapacitive performance of template-free HPCs. The primary stage is the thermal decomposition of their hydrocarbon precursors, which form a polyaromatic network from aromatic hydrocarbons (AHCs) in the presence of chloromethyl methyl ether as the linkers. The properties of the amorphous HPC electrode, such as high surface area, short diffusion length, and the presence of hierarchical porosity, afford excellent capacitive performance, such as lower diffusion resistance and higher capacitance retention with a commercial activated carbon in aqueous H2SO4 (1.0 M).
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EXPERIMENTAL SECTION
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Materials. In this study, the methylene cross-linking bridges between the AHC compounds were constructed using naphthalene (NT; 98%), pyrene (Py; 98%), anthracene (puriss, >99%), and phenanthrene (98%) as the main materials, chloromethyl methyl ether (CME; 95%) as the cross-linker, zinc dichloride (ZnCl2, 98%) as the Friedel−Crafts catalyst, and Nafion 117 solution (∼5% in a mixture of lower aliphatic alcohols and water). All materials were purchased from Sigma-Aldrich and ZnCl2 was dried at 110 °C before used. Dichloroethane (DCE) was purchased from DAEJUNG Chemicals Co. as the solvent and was distilled before used. General Synthesis of HPC. A representative example of the HPC synthesis is as follows: A slurry of ZnCl2 in DCE (5 mL) and CME was added to a solution of NT (1 g) in DCE (25 mL) under nitrogen atmosphere, and the mixture was stirred for 18 h at 40 °C. The resulting precipitate was washed well with water and methanol until the filtrate was clear. The polymer was then dried under vacuum for 8 h at 80 °C. After extraction in a Soxhlet extractor with methanol for 24 h, the desired polynaphthalene (PNT) polymer was collected (95% yield) and dried overnight in a vacuum oven at 110 °C. The obtained PNT was then pyrolyzed in a quartz tube at a specific temperature for 5 h using a heating rate of 10 °C min−1 under high purity N2 flow (30 cm min−1), and left in the furnace as it naturally cooled to room temperature to produce PNT-C. The polypyrene (PPy) were polymerized by the similar procedure by using pyrene to generate a yellow solid in 94% yield and the desired pyrolysis product PPy-C obtained in a quartz tube at a specific temperature for 5 h using a heating rate of 10 °C min−1 under high purity N2 flow (30 mL min−1). Characterization. The methylene cross-linking bridges were analyzed using a Shimadzu IR Prestige-21 Fourier transform infrared (FT-IR) spectrometer (Kyoto, Japan) and solid 13C NMR spectra were obtained on a Bruker Avance 500 NMR spectrometer. The structures of the HPCs were examined via X-ray diffraction (XRD) using an automatic Philips powder diffractometer with nickel-filtered Cu Kα radiation. The diffraction pattern was collected in the 2θ range of 10−70° in steps of 0.02° and counting times of 2 s step−1. The surface functionality was analyzed using a PHI 5400 X-ray photoelectron spectroscope (XPS, Physical Electronics, Mg Kα source).
RESULTS AND DISCUSSION Generally, the formation of electric double-layer carbon-based supercapacitors involves two processes: (i) ion transport in the porous structure and (ii) electric static adsorption of ions onto the electrode/electrolyte interface.18 Accordingly, the ideal nanoporous carbon structure is expected to feature both fast ion transport and ion-accessible surface area for charge storage. In terms of ion transport, carbon nanostructures with the proper amount of 3D interconnected meso-/macropores promote global ion transport throughout the material by reducing the ion diffusion distance and transport resistance.34 In terms of ion adsorption behavior, micropores play a crucial role in ion accumulation and must be electrochemically accessible to the ions.12 On the basis of previous reports, we propose the novel template-free method shown in Figure 1 for the synthesis of polyaromatics based on intra/inter-sphere cross-linking of PNT and PPy microspheres with perfectly spherical shapes and precise sizes.35,36 In brief, the hyper-cross-linked polymer networks were prepared via the condensation of CME and AHC compounds in the presence of ZnCl2 as a Friedel−Crafts
Figure 1. Schematic of the synthesis of the as-prepared polyaromatic microspheres and carbon materials. 12267
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Figure 2. SEM images of as-prepared polyaromatic microspheres: (A) PNT, (B) PPy, (C) PNT-C, and (D) PPy-C.
Friedel−Crafts cross-linking of the PNT and PPy microspheres with CME is induced by adding anhydrous ZnCl2 to construct the methylene bridges that are essential for carbonization.38 After swelling, the NT/Py phenyl rings inside the microspheres become exposed instead of being tightly entangled; thus, CH2+ carbocations can attack the intrasphere phenyl rings and form CH2 groups, which produce −CH2− bridges between the inner chains as the reaction proceeds. As a result, the intrasphere space becomes subdivided into numerous micropores. Concurrently, the CH2+ carbocations also attack the phenyl rings on the surface of the microspheres to form intersphere −CH2− bridges and generate junctions between the microspheres; this occurs even though the polymerization occurs at a low concentration and leads to close-stacking of the microspheres that eventually forms interconnected meso- and macropores. The DCE solvent also plays an important role in the formation of 3D nanostructure, since DCE have been demonstrated to have a strong solvent effect in the fabrication of microporous materials based on Sonogashira-Hagihara coupling. It leads to the formation of network with higher degree of condensation, thus leading to higher levels of microporosity.39 Figure S1 (in the Supporting Information, SI) shows the SEM image of PNT obtained in the absence of CME by the same polymerization procedure. The network structure of severely aggregated particles is irregular. These results indicate that CME plays an important role in constructuring the hierarchical network. This 3D nanostructure inheritability during carbonization is ascribed to the methylene cross-linking bridges. In the solid 13C NMR spectra of the HPCs (Figure 3), the peak at 34.6 ppm corresponds to methylene, which indicates that the −CH2− groups are fully condensed into the structure of the as-prepared
catalyst using a one-pot procedure. The design and construction of the cross-linking bridges are the core developments of the present method. The methylene crosslinking bridges, which are derived from CME, ensure that the PNT/PPy microspheres retain stable spherical shapes during swelling and cross-linking, create the hierarchical microporous structure via intra/inter-sphere cross-linking of the phenyl rings, and guarantee good nanostructure inheritability during carbonization. Using our knowledge of dispersion polymerization and the delay addition method,37 we optimized the synthesis and produced well-shaped spherical PNT and PPy microspheres using a low NT/Py nucleation concentration that generates more stable particles during the growth stage. The preparation of the polyaromatic microspheres is illustrated in Figure 1. Micropores are generated from the network inside the cross-linked AHC-based polymer microspheres, while the mesopores and macropores result from compact and loose aggregation of these microspheres, respectively. These micro-, meso-, and macropores are 3D interconnected; this type of hierarchical porous network improves the ion kinetics and facilitates charge accumulation within the pore surface area.17 The details of the hierarchical network are shown in the SEM images of the obtained polyaromatic microspheres in Figure 2A,B. The as-prepared polyaromatics (i.e., PNT and PPy) were polymerized at a low concentration (i.e., 1 g NT or Py in 30 mL DCE), and the samples exhibited spherical particles about 1 μm in diameter. These particles aggregate into grape-like bunches, which interconnect in different directions into a 3D network that results in the formation of interconnected meso- and macropores (Figure 1). 12268
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Figure 3. Solid 13C NMR spectra of as-prepared polyaromatic PNT and PPy microspheres.
polyaromatic microspheres. The spheres were further characterized via FT-IR spectroscopy (Figure 4); the resultant spectra
Figure 5. TEM images of hierarchical porous carbon obtained by carbonization at 600 °C: (A) PNT-C and (B) PPy-C. Parts (C) and (D) show the detailed surface structure of PNT-C and PPy-C, respectively, at high magnification.
carbonization. The high-magnification TEM images shown in Figure 5C,D reveal that there are numerous micropores that are tightly interconnected with the meso-/macropores inside the surface of the microspheres. It is worth noting that this mesoporous structure was obtained regardless of the ramp rate, which ranged from 10 to 20 °C min−1. This demonstrates that the hierarchical porous texture of the as-prepared polyaromatic samples is maintained during carbonization, and the polyaromatic microspheres are stable in their spherical shape during swelling and cross-linking; also, the methylene bridges create the hierarchical pore structure via intra/inter-sphere crosslinking of the phenyl rings in the microspheres and guarantee good nanostructure inheritability during carbonization. The pyrolysis of PPy was monitored using FT-IR spectroscopy. The characteristic peaks remain almost unchanged after heat-treatment at 400 °C (Figure 6a,b), although the color of
Figure 4. FT-IR spectra of as-prepared polyaromatic PNT and PPy microspheres. The marked peaks are the vas and vs of −CH2− for PNT and PPy, respectively.
showed the deformation vibration of the −CH2− groups at 2904 (vas) and 1443 cm−1 (σ). The FT-IR spectra further confirmed the formation of the microporous polyaromatics with methylene cross-linking bridges. To confirm the applicability of the method for widespread polymerization of AHC, we also used anthracene and phenanthrene as raw materials with the same method and conditions. SEM images of the resultant polyanthracene and polyphenanthrene are shown in Figure S2 of the SI and show the generation of some irregular spherical microspheres and 3D structures; therefore, the synthetic method was successful but should be further optimized. To obtain carbon microspheres, PNT and PPy were pyrolyzed at a specific temperature under nitrogen flow. The TGA curves of both PNT and PPy (Figure S3 of the SI) show a sharp weight loss at ∼500 °C; the overall weight changes were about 26% (from 88% to 62%) and 36% (from 94% to 58%), respectively, which presumably correspond to thermal decomposition of the polyaromatic networks. The sharp weight loss at 500 °C plays an important role in producing the HPCs. After heating from room temperature to 600 °C at a ramp rate of 10 °C min−1 and maintaining 600 °C for 5 h, the diameters of the obtained carbon spheres were about 1.0 μm. From the SEM images shown in Figure 2C,D and TEM images shown in Figure 5A,B, we determined that there is no apparent difference in the morphologies or sizes of the samples before and after
Figure 6. FT-IR spectra of (a) as-synthesized PPy microsphere and heat-treated microspheres at (b) 400 °C, (c) 600 °C, and (d) 700 °C for 5 h.
the material changes from yellow to brown and shrinkage occurs. However, when the PPy is heat-treated at 600 °C, the FT-IR pectrum changes completely (Figure 6a,c): The phenyl peak located at 3030 cm−1 for PPy completely disappears and the material becomes black; this indicates that the material is no 12269
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Figure 7. (A) XRD pattern of the HPC and (B) Raman spectrum of the PNT-C (carbonized at 600 °C) microspheres (532 nm, 20 mW).
Figure 8. XPS spectra of HPC (carbonized at 600 °C) materials: (A) Broad scan (1100−0 eV) spectra normalized to the carbon peaks of the two HPC materials to show the relative intensities of the oxygen 1s peaks and (B) C 1s (290−280 eV) spectra normalized to the highest peak of each element to show the relative intensities of each functional group.
Figure 9. Nitrogen adsorption (●)−desorption (○) isotherms and the corresponding pore-size distribution curve (inset) of the HPCs: (A) PNT-C and (B) PPy-C carbonized at 600 °C.
The mesoporous walls of the carbon spheres were further confirmed by XRD and Raman analyses. Figure 7A shows a typical XRD pattern of the carbonized samples: There are two diffraction peaks centered at 25.4 and 43.7°, which correspond to (002) and (101) diffractions from the graphitic pore walls.40 The (101) diffraction peak is relatively broad and low in intensity, which is often caused by randomly oriented graphene layers in the turbostratic carbon structure.41,42 A representative Raman spectrum (Figure 7B) of the PNT-C carbonized sample shows two bands centered at 1588 cm−1 (G band) and 1343 cm−1 (D band).43,44 The 2 D bands, 2 G bands, and
longer PPy, from which it can be deduced that the phenyl rings and other organic compounds are completely decomposed. Concomitant with the marked change in the infrared peaks, there is an obvious decrease in the particle size (Figure S4 of the SI): The diameters of the obtained carbon spheres decrease from 1200 to 1100 nm with increased annealing temperature from 400 to 600 °C. This decrease is caused by the decomposition of the polyaromatic network. Up to 700 °C, no further changes to the sphere diameter occurred, which is in agreement with the FT-IR results. 12270
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(2) mesopores (2−50 nm); and (3) macropores (>50 nm). The assembly of micro-, meso-, and macropores forms the hierarchical construction, which was verified by the BJH poresize distribution. It is notable that the micropores exist within the network inside the carbon nanoparticle due to the cross-linking reaction between the methylene groups and the phenyl rings of the AHC compounds, which creates numerous microporous networks. Furthermore, the 3D hierarchical porous carbon was formed from aggregates of spherical carbon spheres composed of turbostratic carbon with a weakly ordered graphitic microstructure; this is strikingly different from conventional micropores that originate from direct carbonization or activation processes. 47 In this design, three electrochemical processes are involved: buffering ions in the macroporous, transporting ions through the mesoporous, and confining ions in the microporous. Moreover, the weakly ordered graphitic microstructure can enhance the electric conductivity. Upon the basis of the above analyses, it is predicted that the hierarchical porous network will impart rapid ion transfer/diffusion throughout the entire surface area leading to both high ion transport ability and large ion-accessible surface area. CV curves were used to characterize the capacitive properties of the HPCs. The capacitances of these carbon materials were calculated from the CV data (Figure 10). The capacitance calculations were conducted as described elsewhere.48 In brief, the gravimetric capacitances (F/g) were calculated by integrating the CV area and then dividing by the scan rate (mV/s), scan range (0−0.9 V), and mass of carbon (g). The measurements were repeated for three different batches of each material to ensure reproducibility. The capacitances of PNT-C and PPy-C (at a scan rate of 5 mV s−1) were found to be 164 and 249 F g−1, respectively. The specific capacity decreases by only 18 and 10% after the scan rates increase from 5 to 50 mV s−1, respectively, suggesting good capacitance sustainability. In order to clearly elucidate the high capacitive performance of these HPC samples, comparisons were performed with some reported representative carbons, including microporous carbons, such as hierarchical porous carbon spheres, doped hollow carbon spheres and carbon microspheres (Table S1 of the SI). From the table, it can be seen that the HPCs have higher capacitance. Taking into account that the selected mesoporous carbon are all phenyl-based carbons with some surface functionalities, their corresponding electrodes show similar CVs to that of our HPCs. Hence, the differences in the capacitance confirm that micropores can accumulate more charge than masopores, which is in good agreement with conclusions of previous reports.49−51 Long cycle life is an important requirement for supercapacitor electrodes. Cyclic tests show that 97 and 98% of the initial capacity of PNT-C and PPy-C, respectively, was maintained even after 250 cycles (Figure S7 of the SI). The superior performance is attributed to the excellent kinetic properties of the large mesoporous structure, which facilitates electrolyte transport. EIS analysis was carried out to understand the properties of conductivity and ion diffusion in the electrode/electrolyte interface. The Nyquist plots of HPCs are depicted in Figure 11. It is well-known that the diameter of the semicircle on the Z′ axis in the high-frequency region is referred to as the polarization resistance (Rp). Generally, the lower the Rp, the higher the mass transfer/diffusion rate of ions into the pores of
combination bands (D + G) are also evident in the Raman spectrum; these are consistent with the Raman spectrum of graphene with defects.45 The G band is closely related to a graphitic carbon phase with an sp2 electronic configuration, e.g., graphene layers, while the D band is a common feature of all disordered graphitic carbon. The relative intensity of these two lines depends on the type of graphitic materials present and reflects the degree of graphitization.42 As can be seen in the spectrum, the sample exhibits a strong G-band signal and a lower intensity D band with a G/D intensity ratio of 1.51. These results clearly demonstrate that the carbon microspheres comprise turbostratic carbon with a weakly ordered graphitic microstructure. In contrast, the spectrum of PPy-C (Figure S5 of the SI) only features a strong D band near 1375 cm−1, and the spectrum is blue-shifted compared to that of PNT-C; only the D peak indicates that PPy-C is disordered graphitic carbon. The surface composition of each HPC sample was further studied by XPS (Figure 8). The C 1s XPS spectra of PNT-C and PPy-C display sharp peaks at 284.8 and 284.6 eV, respectively, which are attributed to sp2 graphitic carbon. The oxygen peaks (532.5 eV) present in the XPS spectra of the HPCs is almost certainly due to adsorbed H2O in the air. The full-width at half-maximum values (Figure 8B) were estimated to be ∼1.9 and ∼1.3 eV, respectively; these values are higher than that of graphitized carbon black (0.82 eV);46 the observations of the present work also suggest that these HPC samples are disordered graphite carbon. The textured morphology of the HPC samples was further investigated by nitrogen adsorption−desorption isotherms. In all cases, the nitrogen absorption−desorption isotherms showed a type I curve (Figure 9). Obviously, after carbonized at 600 °C, the surface area have changed 48.6% and 32.0% for PNT-C and PPy-C, respectively, but the change is small when carbonized at 600 to 700 °C (Figure S6 of the SI). Table 1 Table 1. Summary of the Nitrogen Adsorption−Desorption Data, Specific Capacitance (Cesp), and Capacitance Per Surface Area in 1 M Aqueous H2SO4 Obtained for PNT-C and PPy-C (Carbonized at 600 °C) 2
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SBET (m g ) total pore volume (cm3 g−1) %Smic mean pore diameter (nm) Cesp (F/g)c Cesp/SBET/F (m−2)
PNT-C
PPy-C
321.06 0.28a 24 2.42 164 0.51
455.55 0.41b 40 10.26 249 0.54
a According to single point adsorption at P/P0 = 0.991. bAccording to single point adsorption at P/P0 = 0.991. cObtained at scan rate of 5 mV s−1 in 1.0 M aqueous H2SO4 solution between 0 and 0.9 V.
summarizes the specific BET surface area (SBET) and surface area of the micropores of each sample carbonized at 600 °C. The BET specific surface areas of PNT-C and PPy-C are 321.06 and 455.55m2 g−1, respectively, indicating that the HPCs have good porosity. The total pore volume of PNT-C was measured to be 0.28 cm3 g−1 according to the single-point adsorption at P/P0 = 0.991, while that of PPy-C was 0.41 cm3 g−1 at P/P0 = 0.990. In addition, the surface areas of the micropores in PNT-C and PPy-C are 24 and 40%, respectively. As shown in the BJH pore size distribution in Figure 9 (inset), the nanopores of the HPCs can be divided into three major regions: (i) micropores (
