Preparation of energy storage material derived from a used cigarette filter for a supercapacitor electrode

Supercapacitors are attracting attention as an energy storage system (ESS) due to their high power density, short charging time and long cycle life [1–3]. They are of considerable interest in various applications, such as electrical vehicles, micro sensors and portable electronics [4–7]. For such applications, it is important to develop an advanced generation of supercapacitors that have the capability to supply high levels of power and energy [8]. Therefore, the selection of the proper electrode material is one of the most important parameters in improving the performance of supercapacitors.

Carbon is one of the promising materials considered for use in supercapacitors due to its low cost, high porosity, electronic conductivity and stability [8]. For a high-performance supercapacitor, electrode materials with a high surface area and proper pore size distribution are essential in utilizing a large amount of electrolyte ions with quick transfer mobility at the surface of the electrode material [9]. It has been reported that the mesopores (2–50 nm) can provide desirable pathways for the movement of electrolyte ions [10], while the micropores (<2 nm) further increase the surface area in order to enhance the electrical double-layer capacitance (EDLC) [11]. In addition, chemical doping with heteroatoms, especially nitrogen, can largely enhance the capacitance of a carbon material by supplying pseudocapacitive interactions [12]. Therefore, the development of carbon electrode material with a well-developed meso-/micropore structure and with pseudocapacitive assistance can largely improve supercapacitive performance. Although various attempts have been made to prepare nitrogen-doped carbon materials with a porous structure, most methods involve the use of commercial activated carbon sources and include post-doping routes, which include high energy consumption and multi-step processes [13].

Used cigarette filters are composed largely of cellulose acetate. They are disposable, non-biodegradable, toxic and are a threat to the environment after usage. However, it has been reported that cellulose acetate can be directly utilized in the production of carbon materials containing a meso-/micropore structure by only a carbonization process [14]. That is, used cigarette filters could be used as a proper carbon source for supercapacitors. Importantly, carbonizing used cigarette filters in a nitrogen-containing atmosphere could provide the nitrogen doping on the carbon structure with the formation of such unique pore structures in a one-step process.

In the findings reported herein, the nitrogen-doped porous carbon material derived from a used cigarette filter was employed as an active material for the supercapacitor. Taking advantage of the above-mentioned properties, our concept efficiently supports the combined virtues associated with recycling severe solid waste and developing a cost-competitive supercapacitor material. The present work focuses on (1) recycling used cigarette filters as a carbon source for use in a supercapacitor, (2) characterizing the pore structure and the nitrogen-doping states of the prepared material and (3) studying the effect of physicochemical characteristics on the electrochemical properties of the product. To the best of our knowledge, a study of the feasibility of preparing porous carbon materials derived from used cigarette filters for use in supercapacitors has not been reported.

2.1. Preparation of active materials for the supercapacitor

2.2. Material characterization

2.3. Preparation of electrodes

2.4. Electrochemical measurements

2.5. Calculation

The specific capacitance (F g⁻¹) was calculated from the discharge curves of the galvanostatic charge-discharge results by the following equation [16].

$$\begin{eqnarray}&&C=\frac{I\Delta t}{m\Delta V}\end{eqnarray} \tag{ 1 }$$

where I is the constant discharge current, Δt is the discharging time, ΔV is the discharging voltage and m is the mass of the active material. ΔV is the voltage, excluding the IR drop occurring at the beginning of the discharge.

Scheme 1 illustrates the synthetic procedure used to prepare an NCF by a pyrolysis. Typically, various polymers can be easily transformed into a carbon material in the presence of Ar and NH₃, leading to the formation of a unique pore structure [17, 18]. Figure 1(a) shows the features of the NCF powder and the used cigarette filters. Figure 1(b) reveals the morphology of the cellulose acetate fibers of a used cigarette filter. Figures 1(c) and (d) show SEM and TEM images of the NCF flakes, respectively. In figure 1(d), the pores (white dots) are elucidated within the material after the carbonization process.

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Crystalline properties of the NCF, CF and N-doped TiO₂ were investigated for basal characterization. The broad x-ray structures of the NCF and the carbons obtained by annealing with Ar from non-smoked cigarette filters (PCF, all brands mixed) and from each brand are indicative of amorphous carbon, as shown in figure 2. The two broad peaks positioned at 20 ∼ 25 and 40 ∼ 45 of 2θ for prepared carbon samples indicate the incomplete graphitization by pyrolysis at a relatively low temperature of 900 °C [19]. Interestingly, Peaks for anatase TiO₂ were detected after the carbonization process for all of the as-prepared materials. This result reveals that Ti species do not originate from the cigarette smoke or a specific cigarette filter. TiO₂ is a commonly used additive material during the fabrication of a cigarette filter in order to induce the photo-degradation of the cellulose acetate in ambient surroundings and to whiten the color of the filters [20]. N-doped TiO₂ and TiN were observed in the NCF. TiN begins to be produced from bare TiO₂ while annealing with ammonia at a temperature of over 680 °C with a formation of N-doped TiO₂ [15].

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Elements that are known to be present in the cigarette smoke, such as Cd, Cr and As, were not detected by an inductively coupled plasma analysis (ICP). This may be due to their low concentrations (<1 ppm) [21]. The mass ratios of the impurities compared to the total mass of the CF and the NCF were ∼3.6 wt%, which is obtained by TGA analysis, as shown in figure 3; and the amount of nitrogen contents in the NCF was 7 wt%, which was proved by CHNS analysis.

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The porosity properties of the resultant carbon materials by N₂ sorption isotherms are summarized in figure 4(a) and in table 1. It can be seen that the porosity characteristics of the NCF and the CF are considerably different from the AC. The AC showed the isotherm for type-I, which is a general form of microporous structure. However, the isotherms of the NCF and the CF exhibited the type-IV with a slight increase of the slope at higher relative pressures compared to that of the AC. This is typically caused by capillary condensation-evaporation in mesopores. A hysteresis loop constructed from P P₀⁻¹ = 0.45 to 0.8 for the NCF and the CF are caused by the coexistence of both meso- and micropores. Furthermore, it should be noted that the mesopores (3.3 ∼ 4.2 nm) for the NCF and the CF can be beneficial to the easy approach of the electrolyte ions into the pores located deep inside the surface of a particle, as shown in figure 4(b), which are also important for fast electrolyte ion transfer. On the other hand, the majority of the pores for the AC were smaller than ∼2 nm, indicating that the percolation of the electrolyte ions into the pores of the AC electrode could be restricted. The specific surface area of the NCF was larger than that of the CF, as shown in table 1, because the doped nitrogen plays a role in the carbon gasification at over 700 °C, which leads to the formation of additional pores [22]. The mesoporous ratios for CF, NCF and AC in the total surface area were estimated to be 31.06%, 44.68% and 0%, respectively. The binding energy of the chemically bonded nitrogen atoms within the NCF were 396.1 eV, 397.9 eV, 399.8 eV and 400.8 eV, which are assigned to N-Ti-N bonding from Ti species, pyridine, pyrrolic or pyridone, and quaternary species, respectively, as shown in figure 5 [23, 24]. The highest amount of pyridinic nitrogen (39.5 atom%) was observed in the NCF, which can provide the pseudocapacitive interaction between the electrolyte ions in an alkaline electrolyte [12].

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The obtained NCF was applied as electrode materials for the supercapacitor. The electrochemical properties of the prepared NCF were evaluated via galvanostatic charge-discharge measurements (figure 6(a)) and cyclic voltammograms (CVs) (figure 6(b)). These analyses were performed in a potential window of −0.8 to 0 V in 6.0 M KOH aqueous solution with a three electrode system.

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The charge and discharge behaviors of the NCF electrode were examined at current densities in the range from 1 to 10 A g⁻¹. The charge-discharge curves of the NCF show a symmetrical triangle shape with a small internal resistance (IR) drop at a current density of 1 A g⁻¹, indicating a high degree of symmetry in the charge and the discharge [25]. The presence of the IR drop at the initial point of the discharging curve at 1 A g⁻¹ corresponds to the equivalent series resistance (ESR) [26]. The CVs for the NCF electrode were evaluated at scan rates ranging from 5 to 100 mV s⁻¹. The CVs for the NCF electrode have an asymmetrical quasi-rectangular shape, which is due to the charge resistance of the electrodes in the potential range of −0.8 V to 0 V. Despite the asymmetry of the CVs of the NCF, the shapes of the CVs were well retained, in accordance with the increase of the scan rates, which indicate that the NCF retains its substantial capacitive properties, even at a high scan rate of 100 mV s⁻¹. If the ions from the electrolyte are able to easily approach the electrochemically active surface of the electrode, the charge-discharge capabilities become faster with the increase of the scan rate, which indicates the favorable reversibility of a supercapacitor [23].

The supercapacitive performance of the NCF was further compared with the CF and the AC, as shown in figure 6(c). The results of the electrochemical performance for the CF and the AC are shown in figure S1. The discharging time of the NCF was obviously longer than those of the other samples, as shown in figure 6(c), which reveals the highest specific capacitance. The specific capacitances of NCF, CF and AC were compared, as shown in figure 6(d). The decrease in specific capacitances with increasing current density for all of the electrode materials is due to the increase in the charge resistance of the material [27]. The lower specific capacitance of the CF compared to that of the AC is due to the fact that the lower surface area. The rate capabilities of the NCF and the CF were better than that of the AC by means of the changes in specific capacitances. This is due to the presence of higher mesoporous ratios compared to that of the AC, which serve as a favorable pathway for electrolyte ion movement. The NCF electrode has specific capacitances of 153.8, 145.0, 138.2 and 132.4 F g⁻¹ at current densities of 1, 2, 5 and 10 A g⁻¹, which are higher than those of CF and AC electrodes. Importantly, the specific capacitance of the NCF at 5 A g⁻¹ (138.2 F g⁻¹) exceeds the values for the CF and the AC at a lower current density (1 A g⁻¹), 86.9 F g⁻¹ and 125.0 F g⁻¹, respectively. This higher current density indicates that a higher energy for quick action is available, which is essential for ideal capacitive behavior. This may be ascribed to the synergistic contribution of the enlarged high surface area, the proper pore structure and the superior surface functionalities of the NCF, as confirmed by the results from BET and XPS measurements [28]. Furthermore, the specific capacitance of the NCF (153.8 F g⁻¹) is higher than previously reported values for N-doped CNT (44.3 F g⁻¹) and N-doped graphene (110 F g⁻¹) [29, 30]. The specific capacitances of 3.5 wt% for both the bare TiO₂ in the CF and the N-doped TiO₂/TiN in the NCF were measured to be 0.04 F g⁻¹ and 0.53 F g⁻¹ (data not shown), respectively, indicating that the effects of such impurities can be negligible. This result demonstrates a high potential for the application of the NCF to be commercialized. The NCF also showed reproducible and highly stable cycle life during 6000 charge-discharge measurements at a current density of 10 A g⁻¹ (figure 7). The slight increment in the specific capacitance with cycling could be due to the release of pore clogging, which results in the increase in the accessible areas for electrolyte ions for charge storage within relatively small pores. It is generally accepted that pore clogging, which can be caused by trapped or blocked electrolyte ions in small pores, may reduce the specific capacitance of the porous electrode material [8, 31, 32].

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The diameter of the semi-circle indicates the interfacial charge transfer resistance between the surface of the electrode material and the electrolyte. The value for the NCF (1.4 Ω) was bigger than those for the CF (0.82 Ω) and the AC (0.78 Ω). This is expected, since the charge transfer of the NCF involves surface redox reactions between the electrolyte ion and the large amount of pyridinic nitrogen functional groups, in addition to the pure EDLC [33, 34]. In an alkaline electrolyte, it has been reported that pyridine-type nitrogen plays a significant role for redox reaction [33]. We can also confirm that the CF and NCF electrodes are not involved in Warburg impedance, which is related to mass transfer resistance, such as ion diffusion and transport from the electrolyte to the electrode's surface, based on the fact that there is no line that intersects the real axis near a −45° angle [35, 36]. A slope of a vertical line at a low frequency region for the NCF and the CF was higher than the one for the AC, suggesting that ion diffusion is more rapid. It is attributed to their unique pore structures with the presence of mesopores, which leads electrolyte ions to be accessed easier than the micropores (∼1 nm) of the AC [37]. This is in good agreement with the higher rate capability of the NCF compared to that of the AC, which was examined by galvanostatic measurement, as shown in figure 6(d). The magnitudes of the ESR obtained from the intercept (x-axis) for the NCF electrode were determined to be 0.27, which is attributed to the IR drop at the beginning of the galvanostatic discharge curve in figure 6(a) [25, 38].

In summary, a nitrogen-doped mesopore/micropore hybrid carbon material was synthesized from used cigarette filters by a one-step process. The results of the electrochemical measurement suggest that the porosity characteristics of the NCF and the CF can provide efficient ion transfer. Moreover, it was suggested that the pseudocapacitive interactions between the electrolyte ions and the nitrogen dopants, as the result of nitrogen doping for the NCF, leads to an increase in specific capacitance. We conclude that further exploration of cigarette filters for a supercapacitor electrode is warranted in the field of commercial power-consuming devices with the reduction of the environmental burden.

This research was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (NRF-2011-0031571).