Smoked cigarette butts are a non-biodegradable pollutant that has damaged the planet. However, carbon materials derived from cigarette butts have proven to be suitable for various applications. We synthesized cigarette butt-derived carbon via hydrothermal carbonization and chemical activation methods and then converted it to an electrode material for capacitive deionization. The fabricated material, SCC-750, exhibited a relatively high salt adsorption capacity of 10.27 mg g−1. The excellent CDI (capacitive deionization) performance is due to the high specific surface area of 3,093.10 m2 g−1 and a pore volume of 1.754 cm3 g−1. This work offers a new method to recycle harmful cigarette butts by converting them into promising electrode materials for capacitive deionization.
Explore the influence of chemical activation temperature and compare the capacitive deionization performance of the carbon material derived from smoked cigarette butts and unsmoked cigarette butts.
Smoked cigarette butt-derived carbon at the activation temperature of 750 °C exhibits high specific surface area of 3,093.10 m2 g−1, high pore volume of 1.75 cm3 g−1, and reaches a high salt adsorption capacity 10.27 mg g−1.
Water shortage has become a global strategic concern to all human survival due to the rapidly growing population and developing industries (Vörösmarty et al. 2010; Naimi Ait-Aoudia & Berezowska-Azzag 2016; Sutcliffe et al. 2021). For example, more than 75% of Chinese cities are suffering from water shortages (Hou et al. 2021). It has been predicted that at least 25% of the population may live under water scarcity by 2050 (Hou et al. 2021), which emphasizes the urgent demand for sufficient clean water. Desalination of saline water is considered one of the feasible strategies to tackle this issue (Biesheuvel et al. 2011; He et al. 2018; Zhang et al. 2019a).
Current desalination techniques include reverse osmosis, thermal distillation, and electrodialysis. These all inevitably have some drawbacks such as high energy cost, the need for subsequent remineralization, and secondary pollution (Strathmann 2010; Porada et al. 2013; Suss et al. 2015). Capacitive deionization (CDI) is advantageous because of the cost-efficiency, high-water recovery ratio, and facile operation compared to the former methods (Mao et al. 2019; Zhang et al. 2019b; Vafakhah et al. 2020; Li et al. 2021). The ions or polar molecules in the saline stream will be adsorbed to the electrical double layers (EDLs) of their counter electrodes with application of an electrical potential difference in CDI. Normally, a 1.23 V or less bias is needed to desalinate water (Porada et al. 2013). The cell can then be short-circuited to regenerate the electrodes (Porada et al. 2013). Increasing the salt adsorption capacity is crucial to improve the performance of CDI (Biesheuvel et al. 2011). Therefore, one of the most effective research angles is to seek better electrode materials (Anderson et al. 2010; Zhao et al. 2020). High specific surface area (SSA), favorable pore size distribution, high stability, and high conductivity are the key factors to consider (Jia & Zhang 2016; Gao et al. 2021; Li et al. 2021). Carbon materials such as activated carbon (Ghaffour et al. 2013; Asquith et al. 2015), carbon nanotubes (Zhou et al. 2018), carbon nanofibers (Wang et al. 2012), graphene (Dong et al. 2014), metal-organic framework-derived carbon materials (Ma et al. 2019), and carbon aerogels (Liu et al. 2020) have been widely studied. Most of these materials are energy-intensive and synthesized from expensive resources, so high-performance electrode materials based on wastes or recycled products with abundant resources should be developed.
Unwanted waste-derived carbons are ideal alternatives because they are cost efficient and environmentally friendly (Idrees et al. 2020; Teong et al. 2020; Wu et al. 2020). Paper waste (Pham et al. 2020), sewage sludge ash (Wang et al. 2019), cotton waste (Sartova et al. 2019), and paper mill sludge (Jaria et al. 2019) have been proven useful in various applications. Nearly six trillion cigarettes are consumed annually, which leads to about 800,000 metric tons of waste cigarette butts. They are non-biodegradable and exert a tremendous impact on environmental and biological health (Blankenship & Mokaya 2017). Extensive efforts have been made to effectively transform waste cigarette butts into suitable porous carbon materials. Li and co-workers prepared cigarette-derived functional carbon materials that had bifunctional applications in supercapacitors and water contaminant removal (Li et al. 2020). Blankenship and Mokaya showed that porous carbons derived from cigarette butts have ultra-high surface areas (4,300 m2 g−1) and exhibit unprecedentedly high hydrogen storage capacity (Blankenship & Mokaya 2017). Li's group fabricated hierarchical porous sorbents combined with the high adsorption capability of the mesopores and the shape-selectivity of the micropores using waste cigarette butts as a starting material (Li et al. 2019). Hou and co-workers recycled waste cigarette butts for sodium-ion batteries (SIB) and demonstrated their high electrochemical Na-storage performance (Hou et al. 2019). It was confirmed that cigarette butt-derived carbon has a large specific surface area, abundant pore structure, and large ion storage capacity (Chang et al. 2015; Meng et al. 2019; Xiong et al. 2019), which are highly desirable for CDI. Therefore, we prepared cigarette butt-derived carbon materials and applied them to CDI. Two current methods for the preliminary carbonization of cigarette butts are high-temperature carbonization and hydrothermal carbonization (Blankenship & Mokaya 2017). High-temperature carbonization requires considerable energy and time to raise and lower the temperature. Hydrothermal carbonization requires the cigarette butts to be placed in a reaction kettle, which uses the high pressure in the closed space during the reaction to perform low-temperature carbonization and obtain a carbon-containing precursor with high porosity (Lee & Park 2021). This makes it more feasible in preparing CDI electrode materials. There are two types of activation methods, physical activation (Bazan-Wozniak et al. 2021) and chemical activation (Olivares-Marín et al. 2006). The chemical activation process is more stable, mature, and suitable for large-scale production compared with the former.
Here, we synthesized smoked cigarette butt-derived carbon (SCC) by hydrothermal carbonization and chemical activation with three activation temperatures (650, 750 and 850 °C). We also investigated the electrochemical properties of these materials and the performance for desalination in CDI. Unsmoked cigarette butt-derived carbon at an activation temperature of 750 °C (UCC-750) was also synthesized for comparison.
Smoked cigarette butts were collected from public places in Shanghai and unsmoked cigarette butts were purchased from Shandong China Tobacco Industry Co., Ltd. All chemicals were of analytical reagent grade and used without further purification. Carbon black (325 mesh) was obtained from Lion Corporation (Japan). The graphite sheet was purchased from the Tianqi Lithium Corp Company (Shanghai, China). Sodium hydroxide, hydrochloric acid (36–38%), polyvinylidene fluoride, N-methyl-2-pyrrolidone, and sodium chloride were purchased from Sinopharm Chemical Reagent Co., Ltd.
Synthesis of porous carbon from the cigarette butt
Figure 1 shows the synthesis procedure of the cigarette butt-derived carbon material. The outside of the cigarette butt (wrapping paper, ash, impurities) was removed first and cut into several segments. A mixture of 0.5 g of cigarette butts with 50 mL deionized water was hydrothermally carbonized by heating at 250 °C for two hours in a para-polyphenol (PPL) liner and cooled to room temperature. The sediments were washed several times until the upper liquid was clear and dried at 112 °C overnight to obtain the precursor. We prepared a fully ground mixture of KOH/precursor with a 4:1 ratio for chemical activation. This mixture was heated to 650, 750, or 850 °C at a ramping rate of 3 °C min−1 for an hour and cooled under nitrogen. Then 2 M HCl was used to wash the samples at room temperature until the pH of the liquid was neutral. Cigarette butt-derived carbon was recovered after being dried in an oven at 112 °C for six hours. The smoked and unsmoked cigarette butt-derived carbon was named SCC-T and UCC-T, respectively, where T is the activation temperature (650, 750 or 850 °C).
Preparation of the electrodes
The synthesized porous carbon, PVDF dissolved in NMP, and carbon black powder were mixed at a ratio of 8:1:1 to obtain a homogeneous slurry. This slurry was cast onto a 1 × 2 cm graphite sheet for the electrochemical characterization and a 3.5 × 3 cm graphite sheet for the CDI performance test and dried at 60 °C for 10 hours. The final thickness of the electrode was 200 μm.
Scanning electron microscopy (SEM, Sigma 500, Germany) was used to observe the surface morphology of the materials. The defect degree of the samples was recorded using a laser Raman spectrometer (HORIBA LabRAM HR Evolution, France). X-ray diffraction (XRD, Smartlab9, Japan) was used to analyze the crystalline structures of the samples. The surface area was calculated from the N2 isotherms, which were measured by an Autosorb-iQ2-MP analyzer and fitted using the Brunauer–Emmett–Teller (BET) method. The pore size distribution (PSD) was obtained by density functional theory (DFT) fitting of the N2 isotherms. The electrochemical behavior of the materials was characterized by an electrochemical workstation (Autolab PGSTAT302N, Metrohm Instruments Inc, Switzerland). The analytical electrochemical methods employed were cyclic voltammetry (CV), galvanostatic charging/discharging (GCD), and electrochemical impedance spectroscopy (EIS). These measurements were conducted in a conventional three-electrode cell in a 6 M NaOH solution. The Pt and Hg/HgCl electrodes were used as the counter and reference electrodes, respectively.
Here, I is the discharge current in amps, t is the discharge time in seconds, m is the mass of electrode active materials in grams, and V is the voltage change in volts.
Batch mode CDI tests
Here, C0 and C are the initial and final NaCl concentrations in mg L−1, respectively, V is the NaCl solution volume in L, and m refers to the total mass of the carbon materials in g. Additionally, i represents the absolute value of the electrical current during either the adsorption or the desorption step in amps over a time, t, in min. F is the Faraday constant (96,485 C mol−1) and M is the molar weight of NaCl (58.44 g mol−1).
RESULTS AND DISCUSSION
Morphology and structural characterization
The SEM images of the samples are shown in Figure 3(a)–3(d). They all demonstrate a three-dimensional structure with abundant pores of different sizes. SCC exhibits a closer honeycomb-like structure compared to unsmoked cigarette butt-derived carbon as shown in Figure 3(b)–3(d). The pore area becomes slightly larger with the increase of the activation temperature, which is shown in Table 1.
|.||SBET .||Smicro .||Vtotal .||Vmicro .||Vmic/Vt .||Smicro/SBET .||Dave .|
|Samples .||(m2 g−1) .||(m2 g−1) .||(cm3 g−1) .||(cm3 g−1) .||(%) .||(%) .||(nm) .|
|.||SBET .||Smicro .||Vtotal .||Vmicro .||Vmic/Vt .||Smicro/SBET .||Dave .|
|Samples .||(m2 g−1) .||(m2 g−1) .||(cm3 g−1) .||(cm3 g−1) .||(%) .||(%) .||(nm) .|
Samples were degassed at 200 °C for 15 hours for the pretreatment before the nitrogen isothermal adsorption and desorption experiments. As shown in Figure 4(a), all samples are type I isotherms except SCC-750. The adsorption capacity rises rapidly in the low-pressure area (P0, P−1 < 0.1) revealing the presence of a large number of micropores, which help to increase the specific capacitance (Chmiola et al. 2006; Huang et al. 2011). The nitrogen adsorption isotherm of SCC-750 is type IV. There is an obvious adsorption hysteresis loop in the medium pressure area (P0, P−1 = 0.1–0.9), implying that the sample is rich in mesopores. The PSDs of the samples are in Figure 4(b), where it is shown that all samples display a bimodal pore structure with micropores ranging from 0.5 to 2 nm and mesopores between 2 and 4.5 nm. Such a pore structure increases the specific surface area and the adsorption/transmission of ions (Fernández et al. 2009). UCC-750, SCC-650, and SCC-850 possess more micropores than mesopores, as shown in Figure 4(b), while SCC-750 shows a rise in mesopore distribution with pore sizes ranging from 2 to 3 nm. Table 1 lists the detailed pore structure parameters of all samples. All SCC materials have larger SSA and higher pore volumes compared with UCC materials, whereas more pores are observed from the SCC materials. This is confirmed in the SEM images. This is likely to be due to the impurities from the smoked cigarette butts that could create a porous structure during the degradation process (Li et al. 2019). SCC-750 has the highest BET area (3,093.10 m2 g−1) and highest total pore volume (1.75 cm3 g−1) compared with other SCC materials fabricated at 650 °C and 850 °C, which implies that it is difficult to generate the micropores at the low temperature. There is an increase in the micropore area at 850 °C and the higher temperature also leads to the loss of the mesopores, as shown in Figure 4(b). The mesopores are considered to be important for the ion transportation of the deionization processes (Dykstra et al. 2016). Therefore, SCC-750, with the highest mesopore area, also exhibits the best electrochemical performance, as discussed in section 3.2.
XRD and Raman spectroscopy were carried out to investigate the inner structures of the samples. The dashed line is to guide the eye. All samples have a (002) peak centered at 2θ = 26o that originates from the interlayer space between carbon layers as shown in Figure 5(a). This peak indicates the random orientation of aromatic carbon sheets in amorphous carbon, which can stimulate structural disorder and increase the specific surface area (Zhao et al. 2016). There is a weak peak at 2θ = 44o representing the (100) diffraction mode of the graphitic structure. The Raman spectra are shown in Figure 5(b). Two peaks at 1,320 and 1,565 cm−1 are observed, which correspond to the D band (defect-induced band) and G band (crystalline graphite band), respectively. The intensity ratio of the D peak to G peak (ID/IG) is widely accepted as a parameter to measure the defect degree of carbon materials (Zoromba et al. 2017). These ratios of the as-prepared materials ranged from 0.92 to 0.99, suggesting relatively high defect density, and SCC-750 had the highest ratio among the samples. The high defect degree is important to increase the surface area and the pore volumes.
Figure 6(a) shows the CV plots of the UCC-750 and SCC electrodes at different activation temperatures in an electrical potential window between −0.9 and 0 V. All CV curves exhibit quasi-rectangular features and there are no evident redox peaks. This suggests a representative EDL capacitance behavior without pseudo-capacitance. The integral area under the CV curves represents the specific capacitance (Meng et al. 2019). All SCC electrodes have a larger integral area than the UCC-750 electrode, which is due to the SCC electrode's larger BET area. The SCC-750 electrode has the largest integral area of all CV measurements, suggesting the highest specific capacitance. All the CV curves of the SCC-750 electrode keep their shape even at a 100 mV s−1 scan rate, indicating a good performance rate, as shown in Figure 6(b).
The GCD curves of UCC-750, SCC-650, SCC-750, and SCC-850 electrodes are all nearly triangular, which suggests a good reversibility and ideal electrical double-layer capacitance, as shown in Figure 6(c). Longer charging/discharging times reflect higher specific capacitance  and this can be calculated quantitatively according to Equation (1). The results of UCC-750, SCC-650, SCC-750, and SCC-850 electrodes are 130.80, 152.57, 201.09, and 160.31 F g−1, respectively. The SCC-750 electrode has the largest specific capacitance, which agrees with the CV results. This is because the SCC-750 electrode has high porosity and specific surface area (Table 1) providing well developed pore channels for ion transmission, which is highly desired for CDI applications. The GCD tests were also conducted in 1 M and 2 M KOH solutions to compare the specific capacitance with the currently published cigarette butt-derived carbon electrode and commercial activated carbon electrode. The results are summarized in Figure 7 with the specific capacitance from this work indicated by a dashed line, which demonstrates that the SCC-750 electrode's performance is improved over these materials.
Figure 6(d) shows the EIS plot of all the electrodes. they all shape a slope line at low frequency and a small quasi-semicircle at high frequency. The first intersection of the curve and Z′ corresponds to the equivalent series resistances (ESRs). The diameter of the semicircle represents the charge transfer resistance. The SCC-750 electrode has the smallest ESRs and the diameter of the semicircle is smaller than the other electrodes, which implies its faster charge transfer.
A series of CDI tests was carried out to verify the desalination performance of the SCC-750 electrode and electrodes from unsmoked cigarette butts. The Na+ and Cl− are separately adsorbed into their opposite electrodes and the salt concentrations and conductivity decrease when applying a voltage over the electrodes. The electrosorption process reaches equilibrium after 30 min. Then, during the discharging step, the CDI cell is short-circuited to release ions. The conductivity changes of the UCC-750 and SCC-750 electrodes are shown in Figure 8(a). The SCC-750 electrode exhibits a larger conductivity change compared to the UCC-750 electrode during charging step and discharging step, which is due to its porous inner structure and excellent electrochemical performance. An increased conductivity change was observed with an increased charging voltage, indicating that more ions were absorbed and released as shown in Figure 8(b). This is because a higher voltage leads to larger electrostatic interactions and creates a thicker EDL (Sriramulu et al. 2019) where the Na+ and Cl− can transfer faster from the NaCl solution to the cigarette-derived carbon electrode surface. Capacitive performance can be boosted as a result.
The calculations of the electrosorption capacity and charge efficiency of the SCC-750 electrode with different initial NaCl concentrations are shown in Figure 8(c). The electrosorption capacities increased from 8.95 mg g−1 to 10.27 mg g−1 and the charge efficiencies increased from 0.34 to 0.43 when the initial NaCl concentrations increased from 600 to 1,000 mg L−1, which suggests that higher initial NaCl concentrations result in better electrosorption capacity. The compression of the EDL is more intense and the effect of the double-layer overlap decreases (Shen et al. 2021). This enables faster ion transmission and leads to higher deionization capacity at higher initial concentrations. The repetitive CDI tests were carried out to further investigate the regeneration performance, as shown in Figure 8(d). The SCC-750 electrode shows no obvious decline in desalination performance after 20 cycles of 1.2 V charge/−1.2 V discharge in a 600 mg L−1 NaCl solution, indicating a good regeneration performance.
We recycled non-biodegradable cigarette butt waste into a three-dimensional porous activated carbon material after carbonization and chemical activation for CDI applications. The carbon derived from smoked cigarette butts showed better morphologic and electrochemical performance compared with unsmoked cigarette butt-derived carbon. The desalination capacity was improved by smoking the cigarette, which is ideal because smoked cigarette butts are more toxic and accessible. The activation temperature was proven to have an influence on the performance of the as-prepared electrode. The SCC-750 electrode exhibited the highest specific surface area (3,093.10 m2 g−1), the highest pore volume (1.75 cm3 g−1), and a suitable micro/mesopore distribution among all samples, which led to high specific capacitance (201.09 F g−1). Therefore, the SCC-750 electrodes achieved a high SAC (10.27 mg g−1) in a 1,000 mg L−1 NaCl solution at 1.2 V and was reversible even after 20 cycles of CDI experiments. This work provides a new and effective method of tackling environmental pollution concerning cigarette butts and promotes the development of electrode materials for CDI application.
This work was supported by the National Natural Science Foundation of China (NSFC: 21606085).
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.