Electrochemical energy storage and adsorptive dye removal of Platanus fruit-derived porous carbon

Abstract

Activated Platanus fruit carbon (a-PFC) was synthesized by pyrolytic carbonization and alkali activation treatment of an easily available biomass, Platanus fruit (PF). Carbonization yielded a Platanus fruit carbon (PFC) sample with a partially graphitized phase, and the following KOH activation created a highly porous texture containing a large fraction of micropores, and therefore a high specific surface area. Both of these factors are beneficial for surface-related applications such as electrode materials for electrochemical capacitors and adsorbents for the removal of organic dyes. A supercapacitor based on a-PFC₃, which was synthesized with a KOH : PFC activation ratio of 3, offers a high gravimetric capacitance of 216 F g⁻¹ at 1 A g⁻¹, a high rate performance and an excellent cycling stability, highlighting the potential of a-PFC₃ in electrochemical energy storage. Its high specific surface area also makes a-PFC₃ an efficient adsorbent for the removal of methylene blue (MB) from aqueous solution.

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Introduction

Energy and the environment are two major topics that have attracted widespread concern in the scientific community. With the ever-growing consumption of fossil energy and the concomitant environmental pressure, it is necessary to develop reliable and sustainable energy sources without polluting emissions. Supercapacitor is an attractive electrochemical power source, having superior power density, cycling stability, charge–discharge duration and operational safety compared to other rechargeable batteries, as well as sharply increased capacitance and energy output.^1–3 Hence it is regarded as a promising energy storage and supply device for portable electric products, electric vehicles, etc. The electrode material is the key component of a supercapacitor. An ideal electrode material requires highly accessible surface for electrolyte, good conductivity, structural stability and chemical inertness against the electrolyte. On the other hand, organic dyes are important aquatic pollutants discharged from the printing, dyeing, textile and paint industries which seriously damage the aquatic environment and may cause some threats to health. Because some dyes are difficult to degrade, adsorptive treatment is a feasible pathway to remove and even recycle them. Adsorptive treatment of organics necessitates a highly porous adsorbent with high surface area and structural stability. Porous carbon can easily meet requirements for the abovementioned capacitive material and adsorbent. As a result, great efforts have been devoted to the preparation of porous carbon materials and their applications in supercapacitors and organic dye removal.

So far, various porous carbon materials, including carbon nanotubes,^4,5 graphene^6,7 and heteroatom-doped carbon,^8–10 have been synthesized as electrode materials for supercapacitors and demonstrated satisfactory performance. Meanwhile, these types of materials also showed high adsorption performance for the removal of various dyes.^11–13 Although excellent performance was achieved from the abovementioned materials, their relatively high costs and somewhat tedious synthesis processes limit their widespread application. More facile and cost-effective synthesis of porous carbon materials in bulk will promote their more extensive application. Direct pyrolysis and activation of carbonaceous substances is a competitive approach for producing porous carbon on a large scale. Biomass, especially vegetation, represents a large class of cheap, easily available and renewable carbonaceous substances widely distributed on our planet that sustains the global water and carbon cycles. Due to the presence of many capillaries for the transport of water and nutrients to sustain its growth, many pores exist in vegetation, which make it an ideal feedstock for the preparation of porous carbons with high surface areas. Pyrolysis is the most direct and facile approach for transforming biomass into porous carbon. Due to being abundant resources with low costs, various forms of natural biomass, including nut shells,^14,15 plant leaves,^16,17 fruit peels,^18,19 fish scales,²⁰ feathers,²¹ hemp fibers²² and wood sawdust^23,24 have been widely employed as precursors for carbonization to generate porous carbon materials, and the high capacitances or adsorption abilities afforded have highlighted the huge potential of biomass-derived porous carbon in energy and environment-related applications.

Platanus is a widely planted street tree for its strong adsorption ability toward dust and poisonous exhausts in the urban atmosphere and excellent resistance against contaminants, as well as ultra-high climate adaptability. The spherical fruit of Platanus, namely PF, contains abundant tiny fluff, which is commonly undesirable and discarded as waste, owing to it causing allergy in certain susceptible populations. Considering the possible high surface area associated with the fine, light fluff, carbonization can achieve resource utilization of PF and afford carbon materials for surface-related applications, so capacitive and adsorptive performances of PFC deserve to be expected.

Here, PF was employed as a raw feedstock for the synthesis of porous carbon by pyrolysis and subsequent alkali activation. The highly porous a-PFC exhibited a high BET surface area with an overwhelming fraction of micropores, so its application performances in capacitive energy storage and adsorptive dye removal were tested and analyzed.

Experimental

Preparation of a-PFCs

All reagents in this work were of analytical grade and used directly without further purification. Mature PF was collected after it fell from the Platanus trees. The spherical PF was initially rinsed with deionized water to get rid of the adsorbed dust. After being dried and exfoliated, the inner fluff was used as a crude material for carbonization and activation to afford a-PFC. In brief, clean fluff was placed in a tube furnace and heated to 600 °C at a heating rate of 5 °C min⁻¹ under a N₂ atmosphere and then maintained at this temperature for 3 h. In the following activation process, KOH was mixed with the obtained black PFC in a mass ratio of 2, 3 or 4. Then the mixture was heated at 800 °C under a N₂ atmosphere for 1 h for activation treatment. After being cooled naturally, the obtained a-PFC was washed copiously with dilute HCl and deionized water to eliminate the alkali and inorganics. The obtained a-PFC sample was designated a-PFC_n (n = 2, 3 or 4, according to the KOH : PFC mass ratio).

Characterizations

The morphologies and microstructures of the samples were characterized by scanning electron microscopy (SEM, JEOL JSM-6390), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100), X-ray powder diffraction (XRD, Bruker D diffractometer with Cu Kα radiation), Fourier transform infrared spectroscopy (FTIR, Bio-Rad FTS-40) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALab 250 X-ray photoelectron spectroscopy with Al Kα radiation). Nitrogen adsorption–desorption isotherms were measured on a Micromeritics Gemini 2380 surface area analyzer at 77 K. The specific surface area was calculated by the multiple-point Brunauer–Emmett–Teller (BET) method. The micropore size distribution was estimated according to the Horvath–Kawazoe (HK) theory. All the samples were degassed at 200 °C for 3 h prior to measurement.

Electrochemical measurements

Cyclic voltammetry (CV) measurements of the electrodes were performed on a 660D workstation within the potential range 0–1 V in 1 M H₂SO₄. The three-electrode test system included a stainless steel mesh working electrode coated with paste containing 85 mw% active material, 10 mw% acetylene black and 5 mw% polytetrafluoroethylene binder, a platinum wire counter electrode and a AgCl/Ag reference electrode. Galvanostatic charge–discharge curves of the symmetric capacitors (on a Land CT2001A cell test system, Wuhan, China) were recorded in double-electrode mode in a 1 M H₂SO₄ electrolyte over the voltage range −1 to 1 V. The weight of active material on each electrode was in the range 1.5–2 mg, and the precise weights were measured accurately for gravimetric capacitance calculations. Electrochemical impedance spectroscopy (EIS) measurements of the electrodes were also performed on the same CHI 660D electrochemical workstation in the three-electrode test system in a 1 M H₂SO₄ electrolyte. Impedances were recorded over the frequency range 10⁵ to 0.01 Hz with an AC perturbation of 5 mV.

The specific capacitance (C_s, F g⁻¹), energy density (E, W h kg⁻¹) and power density (P, W kg⁻¹) of the symmetric capacitors were calculated from the galvanostatic charge–discharge measurements according to the following equations:

C_s = IΔt/(ΔV × m) (1)

E = C_s(ΔV − IR)²/8 (2)

P = E/Δt (3)

where I (A) represents the discharge current, ΔV (V) is the voltage change within the discharge duration Δt (s), IR is the voltage drop owing to internal resistance in the initial stage of the discharge process, and m (g) is the total mass of active material on each electrode.

Adsorption measurements

The adsorption capability of a-PFC₃ adsorbent was evaluated using MB as adsorbate. In brief, 10 mg a-PFC₃ was sonically dispersed in MB aqueous solution (350 mL, 20 mg L⁻¹) in the dark to form a uniform dispersion. After that, the mixture was stirred at 20 °C. At different intervals, 2 mL of the dispersion was sampled and centrifuged to afford a supernatant, which was diluted to double volume for monitoring the adsorption kinetics and capacity via absorbances. In the adsorption isotherm measurements, 10 mg a-PFC₃ was dispersed in 250, 275, 300, 325, and 350 mL MB solution, respectively, for adsorption treatment at 20 °C, and the relationship between the equilibrium adsorption amounts (Q_eq) and equilibrium concentrations (C_eq) remaining in solution was used for isothermal model fitting after the adsorption systems reached equilibrium.

Results and discussion

Morphologies and microstructures

Fig. 1 shows the morphologies and microstructures of PFC and a-PFC₃. PFC (Fig. 1a) exhibits a tubular outline with an outer diameter of up to 40 μm and a length on millimeter scale, which resembles the original appearance of raw PF fluff (not shown), implying morphological dependence on the original PF source. KOH is a commonly employed porogen for carbon materials, such as graphene, carbon nanotubes etc., due to the corrosive alkali reacting with the carbon framework to create extensive pores and increase the specific surface area.^7,22,25 The widely accepted mechanism of alkali activation follows the equation 6KOH + 2C = 2K + 3H₂ + 2K₂CO₃. The produced potassium and its carbonate can further intercalate into graphitic carbon layers and etch the carbon skeleton so as to continue producing more open pores.^7,16,26 After being activated and etched by molten KOH at a high temperature, the tubular PFCs are damaged to different extents: for a-PFC₃ activated with an alkali : PFC ratio of 3, the long tubes are broken into segments with lengths shorter than 100 μm, with openings at the tip clearly evidencing the tubular shape. The short tubes are beneficial for easier diffusion of electrolyte ions or organics into their inner surface with a short channel length; hence their improved capacitive performance and adsorption capability are favorable.^27,28 TEM images (Fig. 1b and e) more clearly demonstrate the morphological change due to alkali activation. PFC exhibits a 1-dimensional shape, the tube is dark overall, which is mainly due to its large diameter and the thick tube wall, and electron beams cannot penetrate the tube, which results in a seemingly rod-like shape (Fig. 1b). After alkali activation (Fig. 1e), the enhanced transparency of the tube indicates a largely hollowed interior of the tube wall, which confirms that KOH can etch PFC and create a substantial amount of micro/mesopores that are homogeneously distributed throughout the tube wall. More detailed observation reveals that the tube wall is composed of 1-dimensionally arranged mesopores (narrower than 50 nm) along the longitudinal direction, indicating that an a-PFC₃ macrotube is actually composed of many primary nanotubes embedded in the tube wall. An HRTEM image (Fig. 1c) of a PFC fragment shows alternate dark and light spots, revealing the presence of high-density micropores in the tube wall. Sinuous fringes at the rim can also be easily observed, which is typical of carbon materials, indicating graphitization of PFC. After alkali activation, HRTEM still shows extensive micropores (Fig. 1f). The wave-shaped and discontinuous fringes (inset) give evidence that the high density of micropores destroys the continuity of graphitization regions throughout a-PFC₃. In this unique porous network, with mesopores and micropores embedded into the tube wall of a-PFC₃, macropores and mesopores serve as the main channels for electrolyte diffusion. The shorter length of a-PFC₃ tubes, owing to activation, accelerates this process due to their shorter geometric distance, whereas the micropores provide the major contribution to a high specific surface area. Therefore electrolytes and organic molecules can readily infiltrate into the deep inner voids with their huge interface areas. Hence, a-PFC₃ is expected to be an efficient porous material in surface-dependent applications, e.g. electrode materials of capacitors and adsorbents for water treatment. Control samples with different KOH : PFC ratios were also characterized: a-PFC₂ exhibits longer macrotubes, while the length of a-PFC₄ is similar to that of a-PFC₃. Meanwhile, more severe fragmentation occurs (Fig. S1†), indicating a higher KOH dosage not only etches the carbon framework of PFC, but also causes fracture and fragmentation of the carbon tubes. According to these results, the morphology, surface area, and therefore application performance of a-PFC can be tuned by altering the KOH activation dosage.

Fig. 1 (a and d) SEM and (b, c, e and f) TEM images of (a–c) PFC and (d–f) a-PFC₃ at different magnifications.

Fig. 2a presents the XRD patterns of PFC and a-PFCs. Two broad diffraction bands can be observed at 24° and 43° for PFC, which correspond to (002) and (110) facets of graphite-type carbon. The low intensity and width of the diffraction bands indicate the low crystallinity of PFC. After activation, the (002) diffraction band shifts to a lower angle, which is presumably due to an increase in the interlayer distance of adjacent graphitized carbon layers caused by introduced oxygen functional groups as spacers. In contrast, the intensity of the (110) facet increases, which implies higher integrity of the in-plane graphitic carbon skeleton. From this phenomenon, the graphitization degree is actually increased by alkali activation at a higher temperature, although a high density of pores are created in the carbon framework. On the whole, both diffraction peaks are still low, suggesting the largely amorphous nature of the samples. Moreover, the high baseline in the low-angle region for a-PFCs is probably derived from the high density of micropores in the carbon framework.⁷

Fig. 2 (a) XRD patterns, (b) FTIR, (c) XPS survey spectra, (d) N₂ sorption isotherms and HK pore size distribution of PFC and a-PFC₃. Inset in panel c: C 1s spectrum of a-PFC₃.

Fig. 2b shows the FTIR spectra of PFC and a-PFCs. The vibration peaks at 3420 cm⁻¹ (stretching vibration mode) and 1635 cm⁻¹ (bending vibration mode) indicate the presence of –OH groups in PFC and a-PFCs. The enhancement in the intensity of both peaks for a-PFCs gives evidence that the activation treatment produces many hydroxyl groups in the carbon framework. The vibrations at 1726 cm⁻¹ and 1680 cm⁻¹ are attributed to –C=O in carboxyl and carbonyl motifs. All these vibrations indicate the presence of hydroxyl, carbonyl and carboxyl in all the samples, thus offering essential hydrophilicity and polarity and rendering the active surface accessible to electrolyte ions and organics, which is also an indispensable factor for applications as capacitive materials and adsorbents for polar organics.

The surface chemical compositions of PFC and a-PFC₃ were revealed by XPS analysis. From Fig. 2c, the survey spectrum of PFC (black curve) shows three peaks with binding energies at 284.8, 400.2 and 532.4 eV, which are characteristic of C 1s, N 1s and O 1s orbitals, respectively. These peaks indicate the surface composition of PFC comprises carbon, oxygen and nitrogen elements. After alkali activation, the peak belonging to N 1s disappears in a-PFC₃ (red curve), suggesting elimination of nitrogen element by the activation treatment. Compared with PFC, the enhanced intensity of the O 1s peak relative to C 1s in a-PFC₃ indicates the introduction of oxygen functional groups by alkali activation. The deconvoluted C 1s peak of a-PFC₃ shows the presence of C–C bonds in graphite domains (284.8 eV), C–OH (285.7 eV) and C=O (286.5 eV) groups (inset), further verifying the partial graphitization of the carbon framework and the presence of hydroxyl and carbonyl groups on a-PFC₃, which basically coincides with the XRD and FTIR data.

The pore properties of PFC and a-PFC₃ were analyzed by N₂ sorption isotherm measurements. As shown in Fig. 2d, both samples demonstrate a type I (Langmuir) isotherm coupled with a faint hysteresis loop characteristic of a type IV isotherm, according to the IUPAC classification. Micropore filling occurs and quickly reaches a saturation plateau at a relative pressure lower than 0.15. The pronounced adsorption in this range gives evidence of the presence of abundant micropores in both samples. These micropores are presumably derived from intrinsic voids by evaporation of less stable substances inside PF during pyrolysis and activation treatment, which agrees well with the TEM and XRD results. At a relative pressure of 0.4–1, the low nitrogen adsorption amount and weak but discernible hysteresis loop verify that only a limited fraction of mesopores exists in both samples. The approximately H1-type hysteresis loop, characterized by a parallel trend between adsorption and desorption branches, indicates a slit-like geometry of the mesopores, which is associated with the embedded mesopores or capillaries in the tube wall of a-PFC₃ (Fig. 1e). The slight increment in the adsorption amount at a relative pressure approximate to 1 suggests the presence of a tiny fraction of macropores, which may be ascribed to the large internal diameter of PFC (or a-PFC) tubes. The BET surface area and overall pore volume of a-PFC₃ are 1215 m² g⁻¹ (1139 m² g⁻¹ for micropores) and 0.65 cm³ g⁻¹ (0.58 cm³ g⁻¹ for micropores) (Table S1†). Both parameters are five times higher than their counterparts for PFC : BET surface area of 243 m² g⁻¹ (216 m² g⁻¹ for micropores) and pore volume of 0.12 cm³ g⁻¹ (0.09 cm³ g⁻¹ for micropores). Relative to PFC, the micropore volume ratio of a-PFC₃ increases from 75% to 88%, indicating the creation of more micropores by KOH activation, which may be due to etching of the carbon framework or opening of dead pores in PFC by alkali activation. Moreover, the HK pore size distribution calculated from the adsorption branch shows enhanced probability below 2 nm, also verifying the increase in micropores in a-PFC₃. No discernible pore size distribution over 2 nm can be observed, which also confirms the much lower fraction of mesopores and macropores in a-PFC₃. The high BET surface area and versatile pore features guarantee a surface highly accessible to electrolyte ions and organic molecules, facilitating the adsorption of more electrolyte ions and organics onto the entire surface of a-PFC₃, which will further result in high electric double-layer (EDL) capacitance and adsorption capacity for organics. The KOH activation dosage-dependent BET surface area and pore volume were also investigated (Fig. S2 and Table S1†). The specific surface areas of a-PFC₂ and a-PFC₄ are 954 and 1513 m² g⁻¹, respectively, indicating a higher alkali dosage is beneficial for a high BET surface area. Whereas the micropore volume ratio slightly decreases from a-PFC₂ to a-PFC₄ and the probability of larger pore sizes increases simultaneously, both these tendencies suggest that, with a low dosage of KOH, the related corrosive species mainly intercalate into carbon layers and etch the carbon skeleton to create a predominance of micropores in a-PFC. If the KOH dosage is increased, the excess corrosive species can further etch the rim of micropores and transform some of them into meso- or macropores. From these results, the specific surface area, pore volume and pore size can be roughly tuned by altering the alkali dosage.

Electrochemical properties

The porous characteristics of a-PFCs, with their high surface area and partial graphitization, make them good candidates as electrode materials for supercapacitors. To evaluate the capacitive performance of a-PFC, CVs of PFC and a-PFC₃ electrodes were measured at first in a three-electrode system within the potential range 0–1 V at a scan rate of 100 mV s⁻¹. From Fig. 3a, both electrodes exhibit a quasi-rectangular loop without obvious redox peaks, characteristic of typical EDL capacitive behavior. The dramatically higher plateau current and loop area of a-PFC₃ relative to PFC indicates a much higher capacitance of a-PFC₃, which is mainly attributed to its dramatic increase in BET surface area and therefore larger electrode/electrolyte interface area for ion accumulation. In addition, the increased oxygen functional groups due to activation increase the wettability of a-PFC₃ in aqueous electrolytes, which also favors ready infiltration and access to the deep inner voids, improving the surface utilization ratio of the active material.

Fig. 3 (a) CVs of PFC and a-PFC₃ electrodes at a scan rate of 100 mV s⁻¹ in 1 M H₂SO₄ electrolyte. (b) CVs of a-PFC₃ electrode at different scan rates. (c) Galvanostatic charge–discharge curves of symmetric capacitor in two-electrode mode based on a-PFC₃ at different current densities; dashed line: PFC capacitor at 1 A g⁻¹. (d) Specific capacitances of PFC- and a-PFC₃-based capacitors at different current densities. (e) Cycling stabilities of PFC- and a-PFC₃-based capacitors at 1 A g⁻¹. (f) Nyquist plots of PFC and a-PFC₃ electrodes; inset: magnified impedance in the high-frequency region.

Fig. 3b presents the CVs of an a-PFC₃ electrode at different scan rates. It is obvious that the plateau current increases accordingly with the scan rate, and the quasi-rectangular loop is largely retained without apparent distortion even at a higher scan rate, providing evidence of low internal resistance and fast electrolyte diffusion kinetics even at high scan rates, which further contributes to a high rate capability.

Fig. 3c shows the galvanostatic charge–discharge curves of symmetric capacitors based on a-PFC₃ at various current densities in double-electrode mode. The approximately triangular charge–discharge branches, with high symmetry and a nearly linear discharging trend, are indicative of typical EDL capacitance with a rapid charge–discharge process. No discernible voltage drop can be observed at the beginning of the discharge stage even at a current density of 5 A g⁻¹, confirming the low intrinsic resistance of a-PFC₃. This may be attributed to the high conductivity of partially graphitized a-PFC after high-temperature carbonization and activation treatment, as well as rapid electrolyte diffusion kinetics, owing to its versatile porous network with coexisting micro-, meso- and macropores. The discharge duration of a-PFC₃ at 1 A g⁻¹ (red curve) is apparently longer than that of PFC (black dashed line), which implies a larger C_s of the former. From the discharge branch, the C_s of an a-PFC₃ cell, according to eqn (1), is calculated to be 223, 216, 201 and 194 F g⁻¹, respectively, at current densities of 0.5, 1, 2, and 5 A g⁻¹, all of which are drastically higher than that of a PFC-based cell (Fig. 3d). It should be noted that the decrease in C_s at a higher current density for a-PFC₃ is mainly caused by insufficient diffusion of the electrolyte into deep micropores at a high current density. In spite of this, 87% C_s retention can still be achieved in the range 0.5–5 A g⁻¹ (Fig. 3d), demonstrating the high rate capability of a-PFC₃. We attribute its high EDL capacitance and rate capability to its unique electrolyte diffusion channel, with the embedding of abundant micropores and a small fraction of mesopores into the tube wall of a-PFC, which enables efficient infiltration and transfer of electrolyte ions into deep inner voids of the porous electrode, maximizing charge accumulation. In addition, the partial graphitization of a-PFC benefits rapid charge transfer during the charge–discharge process. Both these factors are essential for high EDL capacitive performance.

The cycling performance of PFC- and a-PFC₃-based capacitors was evaluated by consecutive galvanostatic charge–discharge measurements at 1 A g⁻¹ (Fig. 3e). The C_s of an a-PFC₃ capacitor retains 91.2% of its initial value after 2000 cycles, indicating a high degree of reversibility in the repetitive charge–discharge cycles. The good cycling performance can be attributed to the versatile pores and high structural stability of a-PFC₃. The macropores and mesopores, although in limited amounts, benefit the shuttling of electrolytes, alleviating the over-accumulation of electrolytes in micropores and therefore volume variations during charge–discharge cycles. Besides, the structural stability and chemical inertness of a-PFC₃ also help it undergo successive volume changes free of damage and both of them contribute to the high cycling stability. Although PFC possesses better cycling stability, its much lower C_s relative to a-PFC₃ limits its practical significance in capacitor applications.

To further understand the electrochemical behavior of a-PFC₃, EIS data of PFC and a-PFC₃ electrodes were measured and compared. Nyquist plots of both electrodes show a similar shape, which comprises an inconspicuous semicircle in the high-frequency region and a straight line at low frequency (Fig. 3f). The intercept at the higher-frequency end on the real axis represents the series resistance (R_s), which includes the bulk electrolyte resistance, intrinsic active material resistance and contact resistance between electrode and collector.^29,30 The R_s values of PFC and a-PFC₃ are 2.0 and 1.2 Ω, respectively. The lower R_s of a-PFC₃ is presumably ascribed to its slightly higher graphitization degree relative to PFC (XRD in Fig. 2a) due to activation at higher temperature, although a much hollowed texture is formed. The arc represents charge transfer resistance (R_ct) at the electrode/electrolyte interface.^31,32 The lower R_ct for a-PFC₃ (0.5 Ω) than PFC (2.7 Ω) reflects more rapid ion diffusion and accumulation on a porous electrode surface, i.e. more efficient EDL capacitive behavior, which is due to its increased surface area and hydrophilicity after alkali activation. At lower frequencies, no distinct Warburg impedance can be observed for either sample (a line with slope angle near 45°), which suggests the versatile pore network facilitates rapid electrolyte diffusion kinetics. The more vertical straight line of a-PFC₃ at low frequency is indicative of better EDL capacitive behavior with rapid ion diffusion kinetics. In short, by alkali activation at a higher temperature, lower R_s and R_ct and more efficient electrolyte diffusion kinetics can be achieved simultaneously in a-PFC₃, all of which eventually result in a substantial improvement in capacitive performance.

The effect of the PFC : KOH activation ratio on the capacitive performance was also investigated (Fig. S3†). For a-PFC₂, due to its longer tubes with a lower BET surface area (Fig. S1 and S2†), as well as a possibly smaller amount of hydrophilic oxygen functional groups owing to low alkali dosage during activation, a lower C_s of 202 F g⁻¹ is obtained at 1 A g⁻¹. The C_s of an a-PFC₄-based capacitor (206 F g⁻¹) is elevated, yet still lower than that of a-PFC₃-based cell, which is presumably due to an overly hollowed texture caused by a higher alkali activation dosage. Too high pore volume limits the conductive channel of a porous electrode, which can be evidenced by the higher R_s but lower R_ct in impedance plots (green plots in Fig. S3d†). In this sense, too high or too low a surface area is not necessarily beneficial for capacitance; an appropriate alkali dosage is essential to balance the contributions from conductivity and BET surface area. In a-PFC₃, balanced factors result in maximized capacitance. The rate performances of a-PFC₂ and a-PFC₄ are both slightly lower than that of a-PFC₃, providing further evidence of the significance of alkali activation dosage for capacitive performance. Ragone plots show that an a-PFC₃-based capacitor offers a high energy density of 30.4 W h kg⁻¹ at a power density of 970 W kg⁻¹ (Fig. S3e†). Both parameters are apparently higher than those of a-PFC₂ and a-PFC₄. The ultra-high energy density here is mainly attributed to the high C_s at a low current density in an aqueous electrolyte and the wide voltage window during the discharge process. Hence, in our case, a-PFC₃ is a preferential electrode material with high capacitive performance.

Adsorption properties

Because of its high specific surface area and porous features with abundant oxygen functional groups on its surface, a-PFC is thought to be an efficient adsorbent for organic dyes in aquatic environments. Here, the dye uptake capability of an a-PFC₃ adsorbent was evaluated using MB, MO (methyl orange), NG (naphthol green), CR (congo red) and RhB (rhodamine B) dyes as model adsorbates. The dye uptake capacity of a-PFC₃ can be easily estimated via monitoring variations in absorbance. When 10 mg a-PFC₃ was dispersed in 180 mL of different dye solutions (each 20 mg L⁻¹), adsorptive decoloration of the dyes occurred to different extents within 24 h (Fig. S4†). As shown in Fig. 4, the uptake ratios of a-PFC₃ toward dyes differ significantly. Almost 100% of MB is removed from solution, which is higher than for other dyes to different extents, revealing high adsorption ability toward MB. The high MB adsorption ability may be attributed to strong π–π stacking interaction between the aromatic rings in MB molecules and the partially graphitized carbon skeleton of a-PFC₃, anion–cation attraction between the positive charge of MB and the oxygen functional groups on a-PFC, hydrogen bonds, the relatively low steric hindrance of MB owing to its linearly aligned aromatic rings, as well as structural merits such as the high BET surface area and versatile pore features of a-PFC.

Fig. 4 Uptake ratios of different dyes by a-PFC₃.

Due to the high uptake ratio with MB, the adsorption capacity for MB was evaluated. Q_eq (mg g⁻¹) for MB can be calculated based on the following mass conservation equation:³³

Q_eq = (C₀ − C_eq)V/m (4)

where C₀ and C_eq are the initial and equilibrium concentrations of MB (mg L⁻¹) in solution, V is the volume of the MB solution (L), and m is the mass of the adsorbent (g). Fig. 5a shows a plot of the adsorption amount as a function of time in MB solution (20 mg L⁻¹, 350 mL); Q_eq for MB is estimated to be 550 mg g⁻¹. The value of Q_eq is higher than or comparable with those of previously reported porous carbon materials,^11,34–37 revealing that a-PFC₃ is a competitive adsorbent for MB uptake. However, our value is still lower than those of other porous carbons or graphene, due to the relatively low surface area or the limited fraction of mesopores.^38–40

Fig. 5 (a) Time-dependent adsorption amount of MB on a-PFC₃. (b) Pseudo-second-order adsorption kinetics. (c) Adsorption isotherm of MB on a-PFC₃ at 20 °C. (d) Plots of C_eq/Q_eq against C_eq based on the Langmuir isotherm model.

The adsorption kinetics for MB was investigated by monitoring the decoloration rate relative to its initial absorbance. From Fig. 5a, it is clear that the adsorption rate is rather high in the initial 2 h and then gradually slows down and reaches a saturation plateau within 24 h. The long adsorption duration here is mainly due to the much higher volume of MB solution (350 mL) relative to the a-PFC₃ dosage (10 mg), which restricts diffusion to, and sufficient contact with, a-PFC₃. The adsorption kinetics data was analyzed using a pseudo-second-order kinetics model, based on the assumption that chemisorption is the rate-determining step. The pseudo-second-order kinetics model can be described as follows:

t/Q_t = 1/k₂Q_eq² + t/Q_eq (5)

where Q_t (mg g⁻¹) is the MB adsorption amount at an arbitrary time t and k₂ is the pseudo-second-order rate constant (g mg⁻¹ min⁻¹). A plot of t/Q_t versus t is shown in Fig. 5b. The plot strictly obeys a fitted straight line, with a correlation coefficient of 0.9986, which is apparently higher than that when fitted by a pseudo-first-order kinetics equation (correlation coefficient: 0.9150, Fig. S5†), which confirms that the adsorption behavior follows pseudo-second-order kinetics more, and k₂ is calculated to be 5.79 × 10⁻⁵ g mg⁻¹ min⁻¹. According to this pseudo-second-order kinetics, the adsorption rate is dependent on both concentrations of adsorbent and adsorbate, so a higher a-PFC₃ concentration in the dispersion should accelerate this chemisorption process and therefore shorten the adsorption duration.

An adsorption isotherm can be employed to elucidate interactions between adsorbates and adsorbent. Fig. 5c shows the adsorption isotherm of MB on a-PFC₃, where Q_eq increased accordingly with C_eq. The experimental data was fitted by a Langmuir isotherm, which assumes homogeneous monolayer chemisorption of adsorbates on identical sites and uniform adsorption energy due to high affinity between adsorbate and adsorbent; interactions between adsorbate molecules can be neglected. The Langmuir isotherm can be expressed as follows:

C_eq/Q_eq = C_eq/Q_max + 1/Q_maxK_L (6)

where C_eq (mg L⁻¹) and Q_eq (mg g⁻¹) are the equilibrium concentration and adsorption capacity of MB, respectively, Q_max (mg g⁻¹) is the maximized MB adsorption amount, and K_L (L mg⁻¹) is the Langmuir adsorption constant. Fig. 5c displays a plot of Q_eq as a function of C_eq for different initial concentrations, where Q_eq increases monotonically with C_eq. A plot of C_eq/Q_eq vs. C_eq exhibits a good linear relationship with a correlation coefficient of 0.9831 (Fig. 5d). This value is higher than that fitted by the Freundlich isotherm (correlation coefficient 0.8699, Fig. S6†), indicating the adsorption mode in our case follows the Langmuir isotherm more, in which homogeneous monolayer chemical adsorption of MB occurs onto possible homogeneous sites of a-PFC₃. Desorption of MB was attempted using acid, alkali or salts as eluents, but failed, which also suggests irreversible chemisorption by strong interactions between MB molecules and a-PFC₃.

Conclusions

In summary, a-PFC₃ was facilely synthesized by carbonization and alkali activation of PF. The as-prepared product exhibited a highly porous network with a large fraction of micropores, high BET surface area and partial graphitization, and all of these led to the desired capacitive performances, including high specific capacitance, rate capability and cycling stability, when employed as an electrode material in a supercapacitor. Moreover, a-PFC₃ also showed high adsorption capacity for the uptake of MB in aqueous solution. The good capacitive and adsorptive performances confirm the huge potential of a-PFC in energy and environmental materials.

Acknowledgements

This work was supported by NSFC (nos 61204078, U1304505, 61176004), Program for Innovative Research Team (in Science and Technology) in University of Henan Province (no. 13IRTSTHN026), Key Project of Science and Technology of Henan Province (no. 122102210561), Program for Changjiang Scholars and Innovative Research Team in University and the Key Project of Science and Technology of Xinxiang City.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14357d

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