Transforming waste polystyrene foam into N-doped porous carbon for capacitive energy storage and deionization applications

Applied Surface Science 511 (2020) 145576

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Full Length Article

Transforming waste polystyrene foam into N-doped porous carbon for capacitive energy storage and deionization applications

T

Namrata Dekaa, Jayshree Barmana, S. Kasthurib, Venkatramaiah Nutalapatib, , Gitish K. Duttaa, ⁎

a b

⁎

Department of Chemistry, National Institute of Technology Meghalaya, Bijni Complex, Laitumkhrah, Shillong 793003, Meghalaya, India Department of Chemistry, SRM Institute of Science and Technology (SRMIST), Kattankulathur, Tamil Nadu 603203, India

ARTICLE INFO

ABSTRACT

Keywords: Waste polystyrene Porous carbon Nitrogen-doped Supercapacitors Capacitive deionization

We have utilized cheap and readily available expanded polystyrene (EPS) foam waste as a raw material for the synthesis of a hypercrosslinked polymer. Using this polymeric scaffold, we have synthesized a series of nitrogendoped porous carbons (PSC-3-X) via KOH activation at different temperatures. The optimized carbon material (PSC-3-700) exhibits a specific surface area of 810 m2 g−1 and optimum porosity suitable for capacitive applications such as supercapacitors and capacitive deionization. Moreover, the high nitrogen (5.23%) and oxygen (8.93%) contents present in the material enhances the electrochemical performance. PSC-3-700 exhibits the highest specific capacitance of 327 F g−1 at 1.0 A g−1 in 1 M H2SO4 aqueous electrolyte and 100% capacitance retention even after 10,000 charge–discharge cycles. It also displays a high electrosorption capacity of 34.8 mg g−1 for 500 mg L−1 NaCl solution at an applied voltage of 1.6 V in capacitive deionization application. The charge storage mechanism and electrosorption kinetics of the material have been investigated in detail.

1. Introduction One of the major crises of the modern world is the growing problem of plastic waste accumulation with no feasible long term solution in sight. Even though the global population is slowly waking up to the adverse effects of discarded plastic in the natural environment, the amount of waste generated per year substantially outweighs the recycled or incinerated waste [1,2]. Current plastic production and waste management trends estimate around 12,000 Mt of plastic and other non-biodegradable waste to be deposited in landfills and surrounding environment by 2050 [1]. Among the various non-biodegradable waste materials, expanded polystyrene (EPS) is a major hydrocarbon-based plastic product often used in the packaging industry. Since the beginning of its industrial production, the demand for EPS products has been on a continuous rise. However, removal of EPS wastes from the environment by standard methods such as incineration leads to the release of hazardous polyaromatic hydrocarbons (PAH) [3]. Common recycling techniques, including mechanical and chemical methods, suffer from certain drawbacks such as lack of space, increasing the cost of transportation, the release of greenhouse gases, etc [4,5]. Therefore, transforming these EPS wastes into high-performing materials for energy and deionization applications may prove beneficial in the long term. A practical approach for reutilizing EPS is their conversion to carbon-based materials for various applications such as adsorption of ⁎

contaminants [6], energy storage [7], gas adsorption [8], etc. However, the challenge lies in controlling the pore generation and active sites in the carbon framework for such applications. The presence of a hierarchical porous system translates to a high surface area material and offers minimal diffusive resistance to electron/ion transport [9,10]. Also, the incorporation of heteroatoms at predefined locations of the carbon framework increases the activity of the material by providing more faradaic sites. The combined symbiotic effect of a hierarchical pore network along with high heteroatom content has been achieved by activating heteroatom-containing porous organic polymers (POPs) [11]. POPs including microporous organic polymers [12], covalent organic frameworks [13], hypercrosslinked polymers [14], metal-organic frameworks [15], etc. have been previously explored as sacrificial templates for capacitive applications due to their high surface area, inherent porosity, heteroatom-containing building blocks, high chemical, and thermal stability [16,17]. Recently, Liao and co-workers have demonstrated the use of waste EPS as a raw material for the synthesis of a series of hypercrosslinked polymers [6]. Similarly, Zhang and his group have applied a similar strategy towards synthesizing Ncontaining porous polymers for CO2 adsorption studies [8]. However, these polymers cannot be used directly for the electrochemical energy storage devices due to their low electrical conductivity and poor electrochemical stability. In this regard, high-temperature carbonization combined with chemical/physical activation converts such POPs into

Corresponding authors. E-mail addresses: [email protected] (V. Nutalapati), [email protected] (G.K. Dutta).

https://doi.org/10.1016/j.apsusc.2020.145576 Received 12 November 2019; Received in revised form 18 January 2020; Accepted 28 January 2020 Available online 30 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.

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graphitized porous carbons. The porous carbon materials also possess other advantages such as better electrochemical stability, high surface areas, tunable porosity, easy to functionalize, etc. which makes them suitable as energy storage materials [18]. The POP-derived carbon materials have also shown tremendous potential in capacitive applications such as supercapacitors and capacitive deionization [19–21]. However, the cost of synthesis, availability of starting materials, and the overall green aspect of the method require improvement. Therefore, researchers have turned their focus towards cheap, plentiful, and mostly discarded raw materials [22–26]. Similar research has been ongoing in the field of saline water purification by capacitive deionization (CDI) [27]. CDI is a desalination technique that has attracted much attention because of its easy maintenance, low processing cost, less energy input, and less environmental pollution [28]. CDI techniques share a similar working principle as electrical doublelayer capacitors (EDLC) [29]. Hence, the best performing material for CDI electrodes requires a high surface area with an interconnected network of hierarchical pores [30]. Based on the easy availability of waste EPS foams, a wide variety of practical applications of EPS derived carbon materials can be explored. Moreover, the waste EPS foam-derived synthetic procedure provides scope for large-scale production of nitrogendoped porous carbons for energy storage devices and deionization applications. In general, direct carbonization of polystyrene foam releases toluene, benzene, other polyaromatic hydrocarbons and CO2 gas which can adversely affect the surrounding environment [31]. Converting the waste polystyrene foam into a highly crosslinked polymer increases its chemical and thermal stability and minimizes the evolution of harmful gases during carbonization [32]. Additionally, the incorporation of a nitrogen-rich crosslinker in the polymer introduces nitrogen functionalities in the framework. Therefore, the polystyrene foam-based hypercrosslinked polymer can be transformed into hierarchically porous carbon materials with high nitrogen doping under high-temperature chemical activation [33]. In this regard, we have synthesized a hypercrosslinked polymer from waste EPS and cyanuric chloride as a crosslinker (PSC). The polymer was further activated in the presence of KOH at different temperatures to synthesize a series of waste-derived nitrogendoped porous carbon materials (PSC-3-X). Due to the high surface area, optimum porosity, and high heteroatom contents of the synthesized materials, the electrochemical performance of the materials was investigated for both supercapacitors as well as capacitive deionization. The effect of parameters such as applied voltage and salt concentration on the ion adsorption capacity of PSC-3-700 have also been studied. Also, an investigation into the adsorption mechanism of the material was carried out using the Langmuir and Freundlich isotherms.

denoted as PSC-3-X (X is the carbonization temperature). The char-yields of the synthesized nitrogen-doped porous carbon materials were ~50%. 2.3. Characterization The FT-IR spectrum of the PSC polymer was taken on PerkinElmer Spectrum Two FTIR Spectrometer. JEOL ECX 400 MHz was used to record the 13C solid-state NMR spectrum of PSC. Morphologies of the materials were studied with the help of NOVA NANOSEM 450 (FESEM) and JEOL JEM-2100 (HR-TEM). Surface areas of the materials were calculated from N2 adsorption-desorption isotherms using Autosorb iQ Quantachrome Instruments. Defects in the materials were determined from the micro Raman spectra (HORIBA LABRAM HR Evolution Micro Raman Spectrometer) (λexc = 532 nm with a beam size of 5 μm. XPS spectra (PHI 5000 Versa Probe II, FEI Inc.) was used to analyze the nature and extent of heteroatom doping in the materials. 2.4. Electrochemical measurements Electrochemical experiments such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were performed on a Metrohm Multi Autolab/M204. The threeelectrode setup consisted of an Ag/AgCl reference electrode, a Pt counter electrode, and a working electrode. For the working electrode, the active material was mixed with PVDF binder and Super P in a ratio of 85:5:10 respectively in the presence of NMP as a solvent. The active material slurry was then coated on a 1 cm × 1 cm area of a graphite sheet. The working electrodes mass loading was ~ 2.3 – 2.5 mg cm−2. The specific capacitance (Cs) values of the materials were calculated from the GCD curves using the following equation reported in the literature [11,34].

Cs =

I· t m· V

(1)

where I (A) is the discharging current, Δt (s) is the discharge time, m (g) is the mass of active material, and ΔV (volt) is the potential difference applied. For assessing the performance of the material in a two-electrode system, two graphite sheets with similar mass loading of the active material were assembled using glass fiber as the separator. Copper wires were used to connect the device to the potentiostat/galvanostat. Electrochemical parameters of the device were calculated using equations reported in the literature [35,36].

2. Experimental

2.5. Capacitive deionization (CDI) measurements

2.1. Materials

For preparing the CDI electrodes, a slurry of the active materials was first prepared by homogenously mixing 80% active material, 10% conductive Super P, and 10% PVDF binder in NMP. This slurry was then coated onto a 2.5 cm × 2.5 cm area of the graphite sheet. The total mass loaded onto the electrode was kept between 25 to 30 mg. The electrodes were fixed inside the home-made CDI set-up in a parallel manner and connected to a DC power supply (INDOSAW). NaCl salt solution was continuously pumped into the CDI device and pass through the feed tank with the help of a peristaltic pump. The conductivity of the feed solution was monitored with the help of a Eutech Con 2700 conductivity meter at regular time intervals. The electrosorption capacity (EC) of the materials was calculated as per the following equation [37].

Waste expanded polystyrene (EPS) foam was collected from discarded packaging. All other reagents and chemicals used were commercially available. Solvents were pre-distilled for use in reactions. Polystyrene-cyanuric chloride-based hypercrosslinked polymer (PSC) was synthesized according to a reported procedure available in the literature [6]. 2.2. Synthesis of nitrogen-doped porous carbons (PSC-3-X) PSC polymer was properly ground with potassium hydroxide (KOH) pellets in a 1:3 ratio. After drying in a vacuum oven, the KOH-polymer mixture was transferred in an alumina boat and heated to the desired temperature for 2 h in a tubular furnace (heating rate = 5 °C min−1) under an inert atmosphere. The resulting carbon materials were stirred in a 3 M HCl solution to remove any inorganic residues. The obtained carbon materials were washed with distilled water until the washings were neutral. The materials were dried at 100 °C under vacuum and

EC (mg g 1) =

(C0

Ct )·V m

(2)

where the difference in initial and final salt concentration is denoted as (C0 − Ct); V is the volume of salt solution taken, and m is the total mass of the materials. 2

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Scheme 1. Schematic representation of the formation of PSC and PSC-3-X.

3. Results and discussion

distribution of nitrogen and oxygen doping throughout the materials (Figs. S2 and S3). These results are also confirmed by XPS measurements (vide infra). Further investigation on the porosity of PSC-3-700 was carried out by HRTEM imaging. The HRTEM images confirm the existence of layered particles (Fig. 1e). Also, the porous framework in the materials was noticeable in the HRTEM images in the form of white and dark regions. The darker areas can be attributed to the carbon network, while the lighter areas are due to micropores and mesopores [38]. The SAED pattern of PSC-3-700, shown in Fig. S4 suggests the amorphous nature of the material [39]. The evolution of pore structure with increasing carbonization temperature was analyzed from the N2 adsorption-desorption isotherms (Fig. 2a). The isotherms of PSC-3-600 and PSC-3-700 display type IV isotherms with clearly visible hysteresis loops. The presence of H1 type hysteresis loop in the isotherms is indicative of capillary condensation occurring inside uniform cylindrical pores in the material [40,41]. On the other hand, the polymer carbonized at a higher temperature of 800 °C displays a type 1 isotherm typically observed in microporous solids [42]. The slight presence of H4 hysteresis in the isotherm of PSC3-800 may have arisen due to some mesopores in the material [41]. All three porous carbon materials observe a steep increase in adsorbed

Polystyrene-cyanuric chloride-based hypercrosslinked polymer (PSC) was synthesized by a simple and easily scalable Friedel-Craft alkylation reaction (Scheme 1). The use of cyanuric chloride as a crosslinker allows us to incorporate nitrogen atoms in the polymer framework in a controlled manner. The FT-IR and 13C solid-state NMR spectra confirm the formation of cyanuric chloride crosslinking (Fig. S1). The FT-IR spectrum has peaks at 1448 cm−1 (aromatic CeN stretching), 2853 cm−1 (aliphatic CeH stretching), and 3100 cm−1 (aromatic CeH stretching). The peaks for methylene carbon (~42 ppm), aromatic carbons (ca.128 and 138 ppm), and triazine carbon (~170 ppm) were observed in the 13C solid-state NMR spectrum of PSC. The extent of nitrogen doping in PSC was quantified through EDX analysis (Fig. S2a). FESEM image of PSC polymer exhibits a rough and porous surface morphology (Fig. 1a). The polymer undergoes morphological changes after carbonization as visible from the FESEM images. The formation of layered particles can be observed in FESEM images of PSC-3-X (Fig. 1b–d). Moreover, distinct mesopores and larger voids can be seen in all three porous carbon materials. The EDX spectra and elemental mapping of the materials demonstrate the high and uniform

Fig. 1. FESEM images of (a) polystyrene-cyanuric chloride derived polymer, PSC; (b) PSC-3-600; (c) PSC-3-700; (d) PSC-3-800; and (e–f) HRTEM images of PSC-3700. 3

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Fig. 2. (a) N2 adsorption-desorption isotherms and (b) NLDFT pore-size distributions of PSC-3-X. Table 1 Summary of specific surface area and pore-size distributions of PSC-3-X. Sample

SBET (m2 g−1)

VT (cm3 g−1)

Vmicro (cm3 g−1)

Vsuper (cm3 g−1)

Vultra (cm3 g−1)

Vsub (cm3 g−1)

PSC-3-600 PSC-3-700 PSC-3-800

393 810 1250

0.3 0.8 1.1

0.27 0.67 0.86

0.058 0.24 0.37

0.2 0.43 0.08

0.008 0.007 –

volume at low relative pressures indicating their microporous nature [43]. Additionally, the sharp rise of the isotherm at a higher relative pressure (i.e. P/P0 > 0.9) of PSC-3-700 may prove the presence of macropores on the surface [44]. The BET surface areas calculated from the isotherms have been tabulated in Table 1. With the increase in carbonization temperature, the surface areas of the carbon materials increase, i.e., the specific surface area increases as PSC-3-600 < PSC-3700 < PSC-3-800. Also, the distribution of pore-size in the materials has been calculated by the NLDFT method and shown in Fig. 2b. Although all three materials possess both micropores and mesopores, PSC3-600 and PSC-3-700 contain additional supermicropores (pore size between 0.7 nm and 1.2 nm), ultramicropores (pore size < 0.7 nm) and submicropores (pore size < 0.4 nm) [45]. These pore-size systems have been known to cause an anomalous increase in the capacitance of porous carbons [46]. On the other hand, PSC-3-800 contains a narrow pore-size distribution, probably due to the collapse of smaller pores at higher carbonization temperatures [47]. Coupled with the high specific surface area and a hierarchical network of pore sizes may prove favorable for the enhanced capacitive performance of PSC-3-700. The Raman spectra of PSC-3-X shown in Fig. 3a exhibit two bands centered around 1340 cm−1 and 1580 cm−1 corresponding to the D band and G band, respectively. The D band arises from the presence of defects or disordered carbon structures. Whereas the G band relates to the vibrations of sp2 hybridized graphitic carbon atoms [48]. The degree of graphitization of the materials may be quantified from the ratio of intensities of the D band and the G band. As the carbonization temperature increases, the amount of defective structures increases, and consequently, ID/IG value increases. In addition, a slight broad hump at ~2500 cm−1 was observed in the Raman spectra indicative of the layered morphology of the materials [49]. XPS analyses were further carried out to calculate the surface heteroatom contents. The XPS survey spectra of the materials revealed high nitrogen and oxygen content along, which has been summarised in Table S1. Also, the percentages of the different chemical states of doped nitrogen have been measured from the high-resolution XPS spectra. Four different peaks were observed in the deconvoluted N1s spectra (Fig. 3d) of the materials: 398 eV (pyridinic N or N-6), 400 eV (pyrrolic N or N-5), 401 eV (quarternized N or N-Q) and a broad peak at 405 eV (oxidized N or N-X) [50]. Similarly, the deconvoluted C1s spectra (Fig. 3c) also show 3 peaks: 284 eV (C]C), 286 eV (CeO), and 288 eV (C]O) [51]. The heteroatom species like N-6, N-5, and oxygen functionalities have been

previously reported to act as faradaic sites for pseudocapacitance in porous carbon materials [52]. Moreover, heteroatom surface functionalities increase the wettability of the usually hydrophobic carbon materials [53]. The electrochemical response of PSC-3-X was assessed in a threeelectrode cell configuration. Cyclic voltammograms (CV) of the materials display quasi-rectangular shapes, typical of electrochemical double-layer capacitance (EDLC) behavior. However, visible redox peaks in both the cathodic and anodic sweep of the CV curves indicate the presence of pseudocapacitive contribution in the materials [54]. Fig. 4a demonstrates the comparative CV curves of PSC-3-X. The largest CV area for PSC-3-700 can be attributed to the combined effect of pseudocapacitance (arising from its high heteroatom content) and double-layer capacitance (from large surface area and a well-defined porous structure) [55]. The trend observed in the CV curves was further validated by the GCD curves (Fig. 4b). The nearly symmetrical triangular responses of the GCD curves of PSC-3-X suggest the presence of dominant EDLC behavior and high reversibility in the materials [39]. Also, the negligible IR drop in the GCD curves indicates reduced resistance at the electrode-electrolyte interface [56]. In the Nyquist plots (Fig. S8) of EIS studies, low solution resistances were observed for all three materials at the high-frequency region, and steep slopes of the curve noted in the low-frequency region demonstrating low Warburg resistance. PSC-3-700 shows the lowest solution resistance and a more vertical curve, which implies better capacitive behavior of the material [57,58]. The calculated specific capacitance value was highest for PSC3-700 (327 F g−1) followed by PSC-3-600 (257 F g−1) and PSC-3-800 (200 F g−1). Despite PSC-3-700 having a lower surface area than PSC-3800, the superior electrochemical behavior of PSC-3-700 can be attributed to its higher amount of heteroatom functionalities on the surface and a combination of hierarchical micro- and mesopores. The higher amount of nitrogen and oxygen surface functionalities in PSC-3700 (Table S1) are responsible for inducing pseudocapacitance in the material and improving the wettability of the carbon materials [59]. On the other hand, the presence of a hierarchical porous system in PSC-3700 (Table 1) increases the surface areas, provides better accessibility to the active heteroatom sites, and allows faster ion transport [60]. In addition, mesopores present in the porous carbon materials provide more efficient active sites for ion adsorption and faradaic reactions even at high current densities [61]. The combined micro/mesoporosity is also responsible for the good rate performance of the porous carbons 4

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Fig. 3. (a) Raman spectra of PSC-3-X; (b) Full scan XPS spectra of PSC-3-X; (c) High-resolution C1s spectra of PSC-3-700 and (d) High-resolution N1s spectra of PSC-3700.

shown in Fig. 4e. The long-term cycle stability of PSC-3-700 was also tested in a three-electrode system. Fig. 4f displays 100% capacitance retention even after 10,000 cycles. The charge storage kinetics of the material provides valuable insight into its capacitive behavior. Generally, the electrode charge storage mechanism is divided into a surface controlled capacitive process and a diffusion-controlled faradaic/intercalation process [60,62]. Dunn’s

equation has been used to quantify the contribution of these charge storage mechanisms in the capacitive behavior of PSC-3-700 [63].

i = k1 v + k2 v1/2

(3)

At lower scan-rates such as 10 mV s−1, a capacitive contribution of 83% was calculated. As the scan-rate increase to 40 mV s−1, the capacitive contribution in the material increases to 94% (Fig. 5). This

Fig. 4. (a) Comparative CV curves and (b) GCD curves of PSC-3-X; (c) CV curves of PSC-3-700 at different scan-rates; (d) GCD curves of PSC-3-700 at different current densities; (e) Plot of specific capacitance of PSC-3-X relative to current densities; (f) cycle stability of PSC-3-700 till 10,000 cycles in a three-electrode system. 5

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Fig. 5. (a) Capacitive contribution (shaded area) to the specific capacitance of PSC-3-700 and (b) the percentage of capacitive and diffusive contribution at different scan-rates.

behavior is to be expected as diffusion-controlled processes require more time than surface controlled processes and are therefore difficult to detect at higher scan-rates. The linear slope of the plot between log of current (i) vs. log of scan-rate (v) also indicates a surface controlled charge storage process (Fig. S9). Nonetheless, PSC-3-700 follows a dominant surface controlled capacitive process even at lower scanrates. In order to further evaluate the practical scope of PSC-3-700, a symmetrical supercapacitor device was assembled with PSC-3-700 as the active material. The CV curves were recorded in an aqueous acid electrolyte (1 M H2SO4) within the voltage range of 0–1 V (Fig. 6a). Similar to its behavior in a three-electrode setup, CV curves of the symmetrical device exhibit nearly rectangular shapes throughout a wide range of scan-rates. The gravimetric specific capacitance of the symmetrical supercapacitor device was calculated from its GCD curves and was found to be as high as 294 F g−1 at a current density of 0.5 A g−1 (Fig. 6b). The rate capability of the device, calculated from the charge-discharge

curves, is depicted in Fig. S10. Moreover, complete retention of gravimetric specific capacitance was observed after 6000 charge-discharge cycles of the device (Fig. 6d). The Ragone plot depicting the energy densities and power densities is shown in Fig. 6c. PSC-3-700 based supercapacitor has a high energy density of 10.2 W h kg−1 with a corresponding power density of 250 W kg−1 at 0.5 A g−1 current density. These results demonstrate that the electrochemical performance of PSC3-700 is promising as an electrode material when compared to similar carbon materials derived from waste materials (Table 2). Due to the impressive supercapacitor behavior of the synthesized materials, the capacitive deionization performance was also investigated under various conditions using a home-made capacitive deionization setup. An initial voltage of 1.6 V was applied between the electrodes of the CDI apparatus. The time-dependent conductance and electrosorption capacity plots of the materials have been depicted in Fig. 7a and 7b, respectively. The electrosorption capacity reaches the highest value of 34.8 mg g−1 for PSC-3-700. A number of factors may

Fig. 6. (a) CV curves and (b) GCD curves of PSC-3-700 based symmetric supercapacitor device; (c) Ragone plot of energy density against power density and (d) cycle stability of the device with first and last few CGD cycles inset. 6

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Table 2 Comparison table of specific capacitance of porous carbon materials derived from waste and other materials. Raw Material

Current Density (A g−1)

Potential Window

Electrolyte

Specific Capacitance (F g−1)

Reference

Wood Waste Bagasse Cellulose Waste engine oil Packing waste Sorghum vinasse Pomelo Peels Waste sterculia seed Sugercane bagasse Fallen leaves Shiitake mushroom Expanded polystyrene foam

0.5 1.0 1.0 0.5 1.0 1.0 0.2 1.0 1.0 0.5 1.0 1.0

−0.1 V to 0.9 V 0 V to − 1 V 0 V to −1 V 0 V to −1 V 0 V to −0.8 V −0.2 V to0.8 V 0 V to −1 V 0 V to −1 V 0 V to 1 V 0 V to −1 V −0.2 V to −1.1 V −0.2 V to 0.8 V

1 6 6 6 1 1 6 6 1 6 6 1

295 323 162 352 250 311 342 337 280 310 306 327

[64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] This work

M M M M M M M M M M M M

H2SO4 KOH KOH KOH KOH H2SO4 KOH KOH H2SO4 KOH KOH H2SO4

Fig. 7. (a) Plot of conductance against time and (b) electrosorption capacity (EC) for PSC-3-X at 1.6 V and initial salt concentration of 500 mg L−1; (c) Conductance vs time profile of PSC-3-700 at different applied voltages and d) electrosorption capacities of PSC-3-700 with different initial salt concentrations.

have contributed to the high electrosorption capacity of PSC-3-700: (i) high specific surface area of 810 m2 g−1 and total pore volume of 0.8 cm3 g−1; (ii) a hierarchical system of interconnected micro/meso/ macropores which endow the material with appropriate ion adsorption sites and ion-buffering reservoirs; and (iii) high surface nitrogen and oxygen functional groups which increases the wettability of the material [14,75,76]. On the other hand, PSC-3-600, and PSC-3-800 both display a less ion removal capacity than PSC-3-700. The weaker deionization performance of PSC-3-600 may be mainly attributed to its lower surface area and pore volume. Similarly, the limited pore-size distribution of PSC-3-800 hinders its salt adsorption capacity. The existence of a balanced system of micropores, mesopores, and macropores is necessary for carbon materials to achieve high salt adsorption capacity [77]. Moreover, the low heteroatom content of PSC-3-800 may also cause a decrease in the wettability of the material.

Fig. 8. Desalination-regeneration profile of PSC-3-700 with 250 mg L−1 initial concentration and 1.6 V applied voltage.

7

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Fig. 9. (a) Fitting of Langmuir isotherm and (b) Fitting of Freundlich isotherm for PSC-3-700 at an applied voltage of 1.6 V.

Qm

KL

RL

R2

KF

1/n

higher value than that of the Freundlich isotherm. It can be inferred that the electrosorption of NaCl ions onto PSC-3-700 follows monolayer adsorption on uniformly distributed adsorption sites. Another important parameter that can be calculated from the Langmuir isotherm is the constant separation factor (RL) expressed as

50

0.004

0.345

0.987

0.990

0.577

RL =

Table 3 Summary of Langmuir and Freundlich isotherm parameters. Sample

Langmuir Parameters R

PSC-3-700

2

0.998

Freundlich Parameters

Qm KL Ce 1 + KL Ce

(6)

Co describes the initial salt concentration (mg L−1). The value of RL signifies the basics of the adsorption process. For favorable equilibrium adsorption, the value of RL should be between 0 and 1 as is the case for PSC-3-700 (Table 3) [83]. From these observations, we can conclude that waste EPS derived porous carbons are an excellent choice for both supercapacitors and capacitive deionization applications (Tables 2 and S2).

The impact of the applied voltage on ion electrosorption was measured and depicted in Fig. 7c. It can be seen that at a higher applied voltage, electrosorption capacity increases, which may be related to the stronger electrostatic attraction between the charged electrode surface and salt ions [78]. The CDI experiments were also repeated with different initial salt concentrations. Fig. 7d compares the highest electrosorption capacities of PSC-3-700 at each of the three NaCl salt concentrations. The decrease in initial salt concentration may lead to an increase in the overlapping effect and a decrease in the mass transfer rate of salt ions [79,80]. Also, a decrease in salt concentration may cause an increase in the resistance of the medium. Hence, we observe that the electrosorption capacity of PSC-3-700 decreases with reduced salt concentration. Moreover, the desalination-regeneration profile of the electrode material is a useful parameter for establishing its practicality. As shown in Fig. 8, PSC-3-700 was subjected to ten desalination regeneration cycles with an initial concentration of 250 mg L−1 at 1.6 V. It is visible that PSC-3-700 can reduce the conductivity of the salt solution and regenerate its electrosorption capacity with negligible changes up to 10 cycles. In order to study the adsorption mechanism of salt ions onto the electrodes, CDI experiments were performed at salt concentrations of 500 mg L−1, 250 mg L−1, and 100 mg L−1. The experimental data were fitted with the help of two isotherms – Langmuir isotherm and Freundlich isotherm (Fig. 9). The Langmuir model is based on the assumption that only monolayer adsorption occurs on uniform adsorption sites and the adsorbed ions do not interact with each other [81]. It can be expressed in terms of the following equation-

Qe =

1 1 + KL Co

4. Conclusion In summary, we have synthesized a series of nitrogen-doped hierarchically porous carbons from expanded polystyrene foam (EPS) waste. The polymeric precursor was obtained from a facile Friedel-Craft reaction between EPS and cyanuric chloride. The carbonization of the obtained polymer with KOH as an activating agent at different temperatures was carried out and was characterized by various analytical techniques. The optimized material, PSC-3-700 has a high specific surface area of 810 m2 g−1 with a combination of appropriate poresizes, and high heteroatom content proves to be a high performing material for supercapacitor and capacitive deionization applications. The specific capacitance calculated for PSC-3-700 has a value of 327 F g−1 in a 1 M H2SO4 electrolyte. The symmetric supercapacitor device based on PSC-3-700 displays a gravimetric specific capacitance of 294 F g−1 and an energy density of 10.21 W h kg−1 (0.5 A g−1 current density). The material also displays nearly 100% capacitance retention up to 10,000 cycles. Meanwhile, PSC-3-700 also displays an electrosorption capacity of 34.8 mg g−1 at an applied voltage of 1.6 V. These results demonstrate that nitrogen-doped porous carbons synthesized by using waste polystyrene foam are suitable for high-performance energy storage and capacitive deionization applications.

(4) −1

Qe is the electrosorption capacity at equilibrium (mg g ), Qm is the maximum electrosorption capacity at monolayer adsorption (mg g−1), KL is the Langmuir constant (L mg−1), and Ce is the equilibrium salt concentration (mg L−1). Similarly, the Freundlich isotherm assumes that adsorption sites are heterogeneous, and adsorption occurs via multilayer coverage [82]. The Freundlich isotherm can be expressed as

Author contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

(5)

Qe = KF Ce1/ n −1

Declaration of Competing Interest

KF is the Freundlich constant (L mg ), and 1/n is the tendency of the adsorbate to adsorb molecules. The regression coefficient (R2) of the linear fitting of Langmuir isotherm is nearly equal to unity and has a

The authors declared that there is no conflict of interest. 8

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Acknowledgment The authors would like to thank Science and Engineering Research Board (SERB), India, and NIT Meghalaya for supporting this work financially (grant No. SB/FT/CS-075/2014). ND is grateful to the Ministry of Human Resource and Development (MHRD), India for a senior research fellowship. Authors would also like to acknowledge NMR Research Centre, IISc Bangalore; ACMS, IIT Kanpur; Materials Science and Engineering Department, IIT Kanpur, and SAIC, Tezpur University for providing analytical support. NVR acknowledges financial support from SERB research grant (SRG/2019/001023) and SRMIST start up research grant. NVR also thanks to Department of Science & Technology, Ministry of Science & Technology (DST-FIST) for the infrastructural development to SRMIST and SRM-SCIF for the Micro-Raman and HR-TEM facility. KS acknowledges to CSIR, New Delhi for Junior Research fellowship.

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Appendix A. Supplementary material

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2020.145576.

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