Hierarchical Porous Carbon Spheres from Low-Density Polyethylene

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Hierarchical Porous Carbon Spheres from Low-Density Polyethylene for High-Performance Supercapacitors Hua Zhang, Xiao-Li Zhou, Li-Ming Shao, Fan Lü, and Pin-Jing He ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04539 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Sustainable Chemistry & Engineering

Hierarchical Porous Carbon Spheres from Low-Density Polyethylene for High-Performance Supercapacitors Hua Zhang,†,‡,§ Xiao-Li Zhou, †,‡,§ Li-Ming Shao, ‡,§,┴ Fan Lü†,‡,§ and Pin-Jing He*‡,§,┴ †State

Key Laboratory of Pollution Control and Resource Reuse, Tongji University, 1239

Siping Road, Shanghai 200092, PR China ‡Institute

of Waste Treatment & Reclamation, College of Environmental Science and

Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, PR China. *Email: [email protected] (Pin-Jing He) §Shanghai

Institute of Pollution Control and Ecological Security, 1239 Siping Road,

Shanghai 200092, PR China ┴Centre

for the Technology Research and Training on Household Waste in Small Towns

& Rural Area, Ministry of Housing and Urban-Rural Development, 1239 Siping Road, Shanghai 200092, China

Abstract Plastics are used extensively and provide great convenience in daily life. However, their stable and nonbiodegradable nature incurs challenging threats to the environment and ecosystems. It is essential that a sustainable method for plastic treatment and utilization be developed. We used low-density polyethylene (LDPE) as a precursor to synthesize a hierarchical porous carbon (HPC) through autogenic pressure carbonization followed by potassium hydroxide (KOH) activation. The noncatalytic carbonization in a closed system

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obtained 45% carbon residues from LDPE, which would not yield any carbon residues under normal pressure. The following KOH activation developed hierarchical porous structures in the carbon materials, which can be controlled by KOH dosage. The mechanism of carbonization and activation was proposed considering the nanostructure of carbon materials. The obtained HPC exhibited a micrometer-scale carbon sphere morphology with hierarchical pores, a large specific surface area of 3059 m2 g−1 and abundant surface functional groups. By acting as an electrode material for supercapacitors, the HPC displayed excellent electrochemical performance with a specific capacitance of 355 F g−1 at a current density of 0.2 A g−1 in 6 M KOH electrolyte, a high energy density of 9.81 W h kg−1 at a power density of 450 W kg−1, and an outstanding cycling stability. This research develops a sustainable way for plastic waste utilization and a green approach for HPC synthesis.

Keywords: Plastics utilization, hierarchical porous carbon, supercapacitor, catalyst-free carbonization, formation mechanism

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Introduction Porous carbon has attract a lot of attention and are widely applied as a pollutant adsorbent, catalyst/catalytic carrier, and energy storage material.1-3 This attention is because porous carbon has a large specific surface area (SSA), a controllable pore structure, and an abundant surface functional groups (SFG), in addition to the common properties of carbon materials, e.g., stable physical and chemical properties, high electrical conductivity, high mechanical strength and low cost. Efforts have been made to synthesize porous carbon materials from various precursors, such as biomass4-6, coal/petroleum pitch7-8 and plastics. Among those, plastics are rather promising because the morphology and nanotexture of the obtained carbon material can be easily controlled by pretreatment, optimized reaction conditions and post treatment. Porous carbon with particular characteristics, for instance carbon fibers,9-11 carbon spheres12-13 and carbon sheets14-15 were successful synthesized from plastics. Plastics are used widely in all aspects of daily life because of their cheapness, convenience, and excellent performance. Statistics indicate that global plastics production in 2016 was 335 million British tons,16 roughly half of what is thrown away worldwide each year.17 The mass production and discarding of plastics has produced a huge amount of plastic waste, which has resulted in serious environmental and ecological problems.18-20 Recently, the implementation of China’s import ban on solid waste has challenged conventional plastic waste treatment and the disposal capacity of plastic waste exporters.21 The problems caused by plastic waste have become increasingly prominent. Therefore, it is imperative to develop an economically and environmentally sustainable method for plastic waste treatment and utilization.17 Plastics have a high carbon content and are suitable for carbon material preparation.22 3



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Thermosetting plastics, for instance phenolic resins23-24 ， polyurethanes25-26, and polypyrroles27-28 are the most commonly used raw materials because they can maintain their predetermined structure without a template and retain a high yield of residual carbon material after thermal treatment. Thermoplastic plastics, such as polyethylene, polypropylene, and polystyrene are rarely used as precursors for carbon material preparation, although they contribute to approximately 80% of total plastics consumption. Low-density polyethylene (LDPE) is one of the most commonly used plastics in daily life, and it accounted for 17.5% of the total thermoplastic plastics demand in 2016.16 LDPE is mainly applied in shopping bags, food packaging, houseware and agriculture film and is typically discarded as a solid waste after a single use only or after a short lifespan.29-30 Its improper treatment and disposal poses an environmental hazard and wastes resources. The carbon content of LDPE is as high as 85.7%, which makes it a promising precursor for carbon material preparation. However, traditional pyrolysis is unsuitable for LDPE carbonization because LDPE depolymerizes completely to small molecules (short-chain aliphatic hydrocarbons) at approximately 450°C.31 Catalytic chemical vapor deposition can catalyze the carbonization of small molecules in the gas phase to form porous carbon on the template of specific structures.11 However, the catalytic reaction is impeded by the gas–solid mass transfer process, which results in a slow reaction rate and a low carbon yield. Furthermore, catalyst/template reagent costs are high, and the catalyst removal process is complicated by certain environmental risks. Therefore, more efficient and environmentally sound methods are needed to synthesize porous carbon from LDPE.

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Herein, a hierarchical porous carbon (HPC) was synthesized from LDPE through an autogenic pressure carbonization-KOH activation process and used as supercapacitor electrode material. During the carbonization process, a high pressure environment was formed by small molecule substances that were generated from the thermal decomposition of LDPE.32-33 Short chain aliphatic hydrocarbons in the closed reactor underwent further reaction. Through aromatization and polycondensation, hexagonal carbon layers were formed, which serve as the basic structural unit of carbon materials. The obtained carbonization products were activated by KOH, and the HPCs were obtained. The HPCs have a hierarchical porous structure and abundant SFG. As supercapacitor electrodes, the HPCs displayed a high capacitor performance. In addition, the mechanism of activation by KOH and the effect of activator dosage on the microstructure and electrochemical properties of the HPCs were also investigated, which provides a scientific basis to optimize material properties.

Experimental Autogenic pressure carbonization. LDPE powder (0.4 g, 500-μm diameter) from Alfa Aesar was added at room temperature into a 5 mL autoclave reactor made of stainless-steel. The reactor was heated to 600°C at 10°C min−1 in a tube furnace. After 30 minutes, the reactor was cooled and then opened to collect the solid product. The remainder were gaseous products, and no liquid oil was produced. KOH activation. The obtained solid product was mixed and grinded with a certain dosage of KOH (GR, Sinopharm Chemical Reagent Co., China). The mixture was transferred to an iron crucible, heated under nitrogen to 700°C at a 5



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rate of 5°C min−1. After an hour of activation, the residue was washed with 3.0 M hydrochloric acid (GR, Sinopharm Chemical Reagent Co., China) and deionized water. The final product was dried at 55°C for 10 hours and designated as HPC-x (x =2, 4, 6, and 8, which refers to a KOH/CMS mass ratio of 2, 4, 6, and 8). For comparison, CMS was also treated by heating and washing without KOH addition and was denoted as CMS-0. Characterization. A scanning electron microscope (SEM, Nova Nano SEM 450, FEI, USA) and a transmission electron microscope (TEM, TALOS F200X, FEI) were used to observe the morphologies of CMS-0 and the HPCs. The mineral composition was investigated by X-ray diffractometry (XRD, D8 Advance, Bruker, Germany) at a wavelength λ = 0.15418 nm, Raman spectrometry (inVia, Renishaw, UK) using a 532 nm laser excitation. The surface chemistry was determined using X-ray photoelectron spectrometry (XPS, AXIS Ultra DLD, Kratos, UK). N2 adsorption–desorption isotherms of samples were measured to study their pore structure by a surface area and porosity analyzer (Autosorb-IQ3, Quantachrome, USA) at 77 K. Before the measurement, the CMS-0 and the HPCs were outgassed at 200°C for 6 h. Based on the isotherms, the SSA was calculated according to the Brunauer–Emmett–Teller (BET) model, and the pore-size distribution (PSD) was evaluated using the density functional theory (DFT) model. Based on the XRD results, the interlayer spacing d002 (Å) of the crystallite was evaluated using Bragg’s law (Eq. 1). The average crystal size L (Å) in the adirection (La, layer diameter) and c-direction (Lc, layer length), were evaluated

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using the Scherrer formula (Eq. 2). The average layer number per crystallite, n, was calculated from Eq. 3, d 002 

L



10

(1)

K 10 B2 cos 

(2)

Lc d

(3)

2sin 

n

where λ is the X-ray wavelength of the XRD measurement, i.e., 0.15418 nm; θ is the diffraction angle (°), B2θ is the peak width at the half maxima (°), and K is a constant of 1.84 and 0.91 for La and Lc calculation, respectively. Electrochemical measurements. The electrochemical performance of CMS-0 and the HPCs was investigated by using a three- and a two-electrode system at room temperature, and the tests were operated using an electrochemical workstation (CHI660D, Shanghai Chenhua, China). The samples (80 wt. %) were mixed with carbon black (10 wt. %) and polytetrafluoroethylene (10 wt. %) in ethanol and ultrasonic treated with a nickel foam. Then, the sample-coated foam was pressed under 20 MPa and dried at 110°C overnight. Approximately 1.0 mg of sample was coated on each of the working electrodes. Three-electrode configurations were tested in 6 M KOH with a Ag/AgCl reference electrode and a platinum-slice counter electrode. Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements were tested in the potential range of – 1.0 – 0.0 V and –1.0 – –0.1 V versus Ag/AgCl, respectively. The cycling ability was measured at 5 A g−1 from –1.0 to 0.0 V. The two-electrode setup was assembled with two working electrodes that were coated with the same sample 7



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by the abovementioned method, a glass-fiber separator and 6 M-KOH electrolyte. The CV and GCD measurements were carried out between –1.0 and 0 V, and the frequency range of the electrochemical impedance spectroscopy (EIS) was 10 kHz–0.01 Hz. The gravimetric specific capacitance (SC) in the three-electrode system (C3e, F g−1) was calculated from the GCD results according to Eq. 4,

C3e 

I t mV

(4)

where I, m, Δt and ΔV are the current (A), mass of coated sample (g), discharge time (s), and the potential range (V), respectively. The SC for each electrode in the two-electrode setup (C2e, F g−1) was evaluated from the GCD results according to Eq. 5, C2 e 

4 I t mV

(5)

where I, m, Δt, and ΔV are the current (A), mass of coated sample (g), discharge time (s), and the potential range (V), respectively. The imaginary part of the capacitance in the two-electrode system (C’’, F g−1) was evaluated based on the EIS data according to Eq. 6, C '' 

Z' 2

2

2 fm( Z '  Z '' )

(6)

where Z’, Z’’ and f are the real part and imaginary part of the electrode resistance (Ω), and operating frequency (Hz), respectively. The energy density E (W h kg−1) was calculated from the result of the twoelectrode system according to Eq. 7. 8


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E

C2 e V 2 1000 8  3600

(7)

The power density P (W kg−1) was evaluated based on the result of two-electrode system, which could be calculated using Eq. 8. P

E  3600 t

(8)

3. Results and discussion Characterization. LDPE was converted to a carbon material with a yield of 45% through autogenic pressure carbonization. As illustrated in Fig. 1a, CMS0 displayed a spherical shape with a 2–8-µm diameter and a smooth surface. After KOH activation, the spherical aspect maintained the same, but some crevices emerged in HPC-2 (Fig. 1b). With further KOH addition, HPC-6 (Fig. 1c) displayed a fragmentized particle shape. The macropores and mesopores were clearly visible in the fractured section of the carbon sphere (Fig. 1d). The HPC nanostructures were further studied using TEM. As illustrated in Fig. 1e and 1f, CMS-0 and HPC-6 possessed randomly distributed carbon layers, indicating their amorphous structure.34 Furthermore, HPC-6 exhibited mesopores and micropores, which indicates the formation of a hierarchical porous structure after the KOH activation. The macropores shorten the electrolyte transfer distance, mesopores accelerate electrolyte transport and the micropores offer abundant formation sites for the electrochemical double layer. The carbon layers endow the samples a low equivalent series resistance (ESR) because of their rapid charge-transport ability.35-36

9



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Fig. 1 SEM images of (a) CMS-0, (b) HPC-2, (c) HPC-6, (d) partial enlarged detail of HPC-6, and TEM images of (e) CMS-0, (f)HPC-6. The crystalline structure of the CMS-0 and the HPCs were studied using XRD patterns (Fig. 2a). Two diffraction peaks were detected, i.e., the (002) and (100) peaks of graphite at 26° and 43° in the XRD patterns.37 The diffraction peaks are broad, which implies the presence of amorphous carbon. The evolution of the crystalline structures can be investigated further by the quantitative results of the interlayer spacing of (002) d002, the average crystal size Lc, and the average crystal diameter La.38 Table 1 shows that with an increase in KOH dosage, the La first increased to 34.46 Å (HPC-6), and then was approximately constant. Lc decreased, whereas d002 increased, and thus n decreased. Carbon layers in crystallites with a larger diameter are more stable, because of the stronger π–π* conjugation bonds.39 During the KOH activation, carbon layers with a small diameter are etched easily, whereas large layers remain, which results in an 10


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increase in La. The decrease in Lc results from crystal segmentation by K intercalation and mineralization of the small carbon layers. The SFGs and defects in the carbon layer that are induced during KOH activation decrease the interlayer π–π* conjugation, which leads to an increase in d002.40 Table 1 Calculated results from XRD, Raman spectra and XPS C Sample

La

(Å)a

Lc

(Å)a

d002

O content na

(Å)a

ID/IG

b

content (% wt.)c (% wt.)c

a

CMS-0

23.41

22.08

3.49

6.33

0.787

94.89

5.11

HPC-2

31.05

15.39

3.53

4.36

0.824

84.80

15.20

HPC-4

32.99

14.72

3.55

4.15

0.801

90.37

9.63

HPC-6

34.46

10.49

3.62

2.90

0.791

90.89

9.11

HPC-8

33.99

4.66

4.09

1.14

0.672

91.47

8.53

XRD result, b Raman spectra result, c XPS result. Raman spectra of CMS-0 and the HPCs were studied for more structural

information. In Fig. 2b, the D band with a Raman shift of 1345 cm−1 is related to the sp3 sites (the crystallite defects), and the G band with a Raman shift of 1410 cm−1 is related to sp2 sites that exists for all carbon samples.41-42 The peak height of the G bands for CMS-0 and the HPCs were higher than that of the D bands, which indicates their good graphitic crystallinity. The graphitization degree of the samples was evaluated according to the ratio of D- to G-band intensity (ID/IG). The larger ID/IG suggests the sample contains less sp2 sites and more sp3 sites. With an increase in KOH dosage, the ID/IG (Table 1) increased and then decreased, 11



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suggesting that the proportion of sp3 sites decrease with the increase of KOH dosage.43 HPC-2 had the most sp3 sites, possibly because an insufficient KOH addition cannot completely mineralize the small carbon layer, therefore, a large amount of erosion debris remains. As the KOH dosage continues to increase, the erosion debris is destroyed gradually, whereas the large carbon layers with better graphite structures remain, as they react more slowly due to their stronger intraand inter-layer π–π* conjugation, which results in a decrease in ID/IG. The additional peak at ~2670 cm−1 (G’-band) is also a typical characteristic peak of the graphitic structure.29 From CMS-0 to HPC-6, the G’-band center frequency decreases, which indicates a decrease in the number of layers,44 which agrees with the XRD results.

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Fig. 2 (a) XRD patterns, (b) Raman spectra, XPS spectra of (c) C1s and (d) O1s, (e) N2 adsorption–desorption isotherms, and (f) PSDs of CMS-0 and the HPCs. The surface chemistry of the CMS-0 and the HPCs was investigated by XPS measurements. Only elemental carbon (C) and oxygen (O) were detected near the surface of the samples (XPS cannot detect H), and their relative contents are shown in Table 1. The O contents of the HPCs was higher than those of the CMS-0, which indicates an increase in O-containing SFGs after activation. From HPC-2 to HPC-8, the relative content of O decreased with an increase in KOH 13



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dosage, which may occur because of the gasification of O-containing SFGs into CO/CO2 by excess KOH. To characterize the chemical state of the C and O, the XPS peaks were deconvoluted according to XPS PEAK Software. The XPS spectra of C1s (Fig. 2c) possessed three peaks at 284.8, 286, and 288.5 eV, which represent C–C, C–O, and C=O, respectively. The XPS spectra of the O1s (Fig. 2d) possessed two peaks, C–O (531.1 eV) and C=O (532.7 eV).45 The C–O peak weakened, whereas the C=O became stronger from CMS-0 to HPC-8, because of the gradual oxidation of C–O into C=O with an increase in KOH dosage. Therefore,

these

O-containing

SFGs

can

generate

an

excellent

pseudocapacitance and improve the hydrophilicity of the HPCs to enhance the aqueous electrolytes penetration.46-48 The pore structures of CMS-0 and the HPCs were studied using N2 adsorption–desorption measurements. The CMS-0 sample exhibited almost no N2 adsorption, which indicated a poor pore structure (Fig. 2e). After KOH activation, the quantity of N2 that was adsorbed by the HPCs increased significantly. As the KOH dosage increased, the isotherm transferred from a type-I to a type-IV indicating that the pore structure changed from micropores to hierarchical pores. The H4 type hysteresis loop suggests the pores in the HPCs are slit-shaped.49-50 A large amount of N2 was adsorbed at low pressure, which is related to the abundant existence of the micropores. Fig. 2f shows the PSD of the HPCs. The mesopore proportion of the HPCs increased gradually with an increase in KOH dosage. The SSA and pore volume of the HPCs were calculated from the isotherms (Table 2). CMS-0 exhibited a very low SSA, which reached an adsorption saturation rapidly and therefore there were few data points for the 14


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accurate calculation of the other indices. The SSA and mesopore proportion increased with an increase in KOH dosage, and HPC-8 exhibited the largest SSA of 3059 m2 g−1. The micropore proportion decreased after an increase from CMS0 to HPC-6, which may occur because of the micropore collapse and the enlargement into mesopores. The hierarchical porous structures of the HPCs should benefit their supercapacitor performance. Table 2 Calculated results from N2 adsorption–desorption measurements Sample

a

SBET

a

(m2 Smicro

b

(m2 Smeso

c

(m2 Vtotal

d

(m3 Vmicro e (m3

g−1)

g−1)

g−1)

g−1)

g−1)

CMS-0

7.001

--

--

--

--

HPC-2

1801

1652

149.3

0.729

0.599

HPC-4

2236

1792

443.8

1.07

0.566

HPC-6

2994

2499

494.9

1.56

0.825

HPC-8

3059

2290

768.9

1.73

0.788

SSA calculated by BET method,

b

micropore SSA calculated using V–t plot

method, c mesopore SSA calculated using V–t plot method, d total pore volume calculated using DFT method, e micropore volume calculated using DFT method. Mechanism of HPC synthesis. Based on the foregoing discussion, a mechanism of the autogenic pressure carbonization and KOH activation is proposed (Scheme 1). LDPE, used as the precursor, decomposes into short-chain aliphatic hydrocarbons under high temperatures. These small-molecule products are confined in the closed reactor and undergo a series of further reactions.28 Hexagonal carbon layers, i.e., planar polycyclic aromatic hydrocarbon 15



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macromolecules, which are formed through aromatization and polycondensation work as the basic structural unit of the graphite crystallite.51 The high-pressure pyrolysis system produces a “chemical cocktail” that consists of hundreds or thousands of distinct components. To keep surface energy at a minimum, a spherical shape is formed spontaneously during quenching.

52-53

The obtained

CMS has an amorphous nanotexture, which contains randomly distributed graphite crystallites. During activation, the porous structure is formed through KOH etching, metallic K intercalation, and CO/CO2 release; this has been reported via the following reactions.54 6KOH + 2C  2K + 3H 2 +2K 2 CO3

(9)

K 2 CO3 +C  K 2 O+2CO

(10)

K 2 CO3  K 2 O+CO 2

(11)

2K+CO 2  K 2 O+CO

(12)

The destruction rate of carbon layers with different sizes and crystallite defects is different, because the carbon structures with stronger intra- and inter-layer π– π* conjugation react more slowly. As a result, the La increases and ID/IG decreases with an increase of the KOH dosage. In addition, KOH oxidation can induce SFGs on the HPC surface. Autogenic pressure carbonization is essential for CMS formation and for the special nanostructure to be established. With regards to activation, the KOH dosage is a key factor that can control the formation of a porous structure, the internal graphite crystal structure and surface chemistry.

16


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Scheme 1 Mechanism diagram of autogenic pressure carbonization and KOH activation of LDPE. Electrochemical performance The CV curves of CMS-0 and the HPCs in the three-electrode system are illustrated in Fig. 3a. A scan rate of 50 mV s−1 were used in all of the CV measurements. The CV curve of the CMS-0 exhibits an elliptical shape, whereas those of the HPCs exhibit quasi-rectangular shapes, which demonstrate the combined double-layer capacitance (contributed by ion adsorption/desorption) and pseudocapacitance (contributed by redox reactions of SFGs).55 Moreover, the CV curves of the HPCs encircle a much larger curve area than the CMS-0, which suggests their higher SCs are obtained by activation. The CV curve of the HPC-6 encircles the largest curve area, followed by HPC-4, HPC-8, HPC-2, and CMS-0. An oxygen evolution reaction can be observed above –0.1 V from the CV curves, therefore, the potential range of the GCD measurements were changed to –1.0 – –0.1 V. The GCD results of CMS-0 and the HPCs at 1 A g−1 were used to calculate their SCs. The approximately symmetrical triangular shapes of the GCD curves (Fig. 3b) are contributed by 17



double-layer

capacitance,

and

the

curve

deviations

Page 18 of 36

result

from

the

pseudocapacitance.55 The SCs of the CMS-0, HPC-2, -4, -6, and -8 electrodes were calculated to be 3, 186, 232, 307, and 219 F g−1, respectively. In the GCD measurements of HPC-6, the sample with the best performance was subjected to various current densities (Fig. 3c). The curves display nearly symmetric triangular shapes with a deviation from the pseudocapacitance.55 The SC decreased gradually from 355 to 274 F g−1 with an increase of current density from 0.2 to 10 A g−1, which is ascribed to the transport limitation of the electrolyte and conduction resistance.56 Compared with the carbon materials that are derived from other plastics (summarized in Table 3), HPC-6 exhibits remarkable capacitive performance. The rate performance of the HPCs is shown in Fig. 3d. The SC retentions of HPC-2, -4, -6, and -8 at 10 A g−1 were 81.2%, 88.8%, 89.3%, and 90.0% respectively, compared with those at 1 A g−1. The SC retentions of the HPCs increase with an increase of KOH dosage, attributing to the formation of hierarchical pores and the nanotexture of the carbon layers, which determines the ion-electrolyte diffusion rate and conduction resistance, respectively.35-36 The electrochemical capacitance of the carbon material relates to its SSA, PSD and SFG. Among which, SSA is the most important parameter, because pore surfaces provide the formation sites for the electrochemical double layer. However, not all the pore surfaces are accessible. The accessibility of pore surfaces is related to the PSD and SFG because a hierarchical PSD could facilitate ion transport by shortening diffusion pathways and SFGs could enhance the hydrophilicity of the carbon material. In addition, the SFGs could undergo 18


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redox reactions in aqueous electrolytes to provide pseudocapacitance28,48 For the SSA, the basal and edge planes of the crystallites both contribute to the SSA, but their conductive and capacitive properties are different from each other. The electrical conductivity of the graphite crystallite along the c-direction (edge plane) is as high as a conductor, while the conductivity along the a-direction (basal plane) is much lower. Furthermore, the defects and SFGs are mostly distributed on the edge plane. Therefore, the edge plane can provide an order of magnitude larger capacitance than the basal planes with a same area.57 The ratio of Lc/La which is calculated from the XRD results could be used to estimate the ratio of the edge plane area and the basal plane area. HPC-6 possessed the largest SC, which is attributed to its appropriate properties, including SSA, PSD, graphite structure, and SFGs. For the HPC-8, its SSA and SFG were similar to those of HPC-6, while its Lc/La (0.14) was less than half of that of HPC-6 (0.30). As a result, the SC of HPC-8 was poorer.

19



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Fig. 3 (a) CV curves of CMS-0 and the HPCs at 50 mV s−1, (b) GCD curves of CMS-0 and the HPCs at 1 A g−1, (c) GCD of HPC-6 at different current densities, (d) rate performance of CMS-0 and the HPCs. All the measurements are operated in a three-electrode system. Table 3 Comparison of SC and ESR of HPC-6 with other results SC

ESR

Precursor (F

LDPE

Scan Electrolyte

g−1)

355

(ohm)

0.88

Reference rate 0.2 A

Present

g−1

work

6 M KOH

Phenolic

0.2 A 175

1.2

23

6 M KOH g−1

resin

20


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246

1.4

6 M KOH

1 A g−1

24

188

5.6

6 M KOH

1 A g−1

25

19

--

2 M KCl

Polyurethane

20 mV 26

s−1 1 mV 256

3.3

27

6 M KOH s−1

Polypyrrole 0.5 A 318

1.1

28

6 M KOH g−1

CV and GCD measurements were conducted in a two-electrode system to study the practical electrochemical performance of CMS-0 and the HPCs. The CV measurements were operated at 50 mV s−1. All of the CV curves exhibit an almost rectangular shape (Fig. 4a), which is the typical characteristic of ideal double-layer capacitance.55 GCD curves of CMS-0 and the HPCs are obtained at 1 A g−1 (Fig. 4b). The symmetrical triangular shape of the GCD curves implies their electrochemical performance is similar to an ideal double-layer capacitance.55 HPC-6 possessed the largest SC, which agrees with the threeelectrode-system result. Compared with the CV and GCD curves in the threeelectrode system, the shape deviation decreases significantly, which implies a lower pseudocapacitance performance. In the two-electrode setup, the actual electrode voltage is lower than the set value because of the voltage drop that is caused by an electrode polarization with no reference electrode for calibration. In that case, the reaction potential of the redox reactions cannot be reached, so 21



Page 22 of 36

partial pseudocapacitance does not work. The SCs of the CMS-0, HPC-2, -4, -6, and -8 electrodes were calculated to be 2.6, 214, 238, 283, and 210 F g−1, respectively. Compared with the three-electrode-system results, the lower SCs can be attributed to a decrease in pseudocapacitance. The GCD measurements of the HPC-6 were operated at various current densities (Fig. 4c). The GCD curves exhibit symmetric triangular shapes even at a low current density, which implies the ideal double-layer capacitance characteristic. Fig. 4d presents the rate performances of the HPCs. Compared with SCs at 1 A g−1, the retentions of HPC2, -4, -6, and -8 at 10 A g−1 were 86.9%, 85.7%, 84.1%, and 86.3%, respectively, which implies their high rate performances.

22


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Fig. 4 (a) CV curves of CMS-0 and the HPCs at 50 mV s−1, (b) GCD curves of CMS-0 and the HPCs at 1 A g−1, (c) GCD curves of HPC-6 at various current densities, (d) rate performance of CMS-0 and the HPCs, (e) Nyquist plots, and (f) C’’/C’’max calculated from impedance data versus frequency. All the measurements are operated in a three-electrode system. The Nyquist plots are shown in Fig. 4e. In the low-frequency range of all the samples, the nearly vertical line is the typical impedance characteristic of double-layer capacitors.58-59 The ESR can be evaluated from the impedance 23



Page 24 of 36

curve in the high-frequency region. The ESR is comprised of a bulk resistance including the electrode resistance and electrolyte resistance, and an interfacial charge-transfer resistance of the electrode–current collector interface and electrolyte–electrode interface. The intercept of the semicircle with the real axis corresponds to the bulk resistance and the diameter of the semicircle is related to the interfacial resistance.

46, 60-61

The bulk/interfacial charge-transfer resistances

of HPC-2, -4, -6, and -8 were 0.62/0.04, 0.52/0.06, 0.68/0.2, and 0.66/0.15 Ω, respectively. A comparison of the ESR of the HPC-6 with the carbon materials derived from other plastics is summarized in Table 3. The low ESR is attributed to the good conductivity of the carbon layers, high ion-diffusion efficiency of the hierarchical porous structure, and low contact resistance of the HPCs’ smooth surfaces with each other and the current collector. Figure 4f shows calculated C’’/C’’max values from the EIS data versus the frequency dependencies. The values of C’’/C’’max showed maxima at 21.1, 21.1, 54.9, and 30.9 Hz for HPC-2, -4, -6, and -8, respectively, which suggests that HPC-6 is suitable for supercapacitors working at high-frequency conditions.37 The energy density, power density, and cyclic stability of the electrode materials are important indices for practical application of the supercapacitor. Ragone plots of the HPCs are shown in Fig. 5a. The HPCs deliver a significantly high performance in the supercapacitor zone.62 Among them, HPC-6 possessed the largest energy and power density of 9.81 W h kg−1 and 450 W kg−1 at 1 A g−1. To investigate the cyclic stability of HPC-6, GCD measurements were cycled 5000 times at 5 A g−1 (Fig. 5b). As the cycle time increased, the SC decreased

24


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slightly. After 5000 cycles, the retained capacitance was 82.4% of the initial SC, which indicates the high reversibility and energy-storage stability of HPC-6.

Fig. 5 (a) Ragone plot of all electrode materials and (b) cyclic stability of HPC-6 at 5 A g−1 for 5000 cycles. Inset plot compares the GCD curves of the 1st and 5000th cycles.

Conclusion An effective and green method for HPC preparation from LDPE was developed. The HPCs that were obtained through autogenic pressure carbonization and KOH activation possessed abundant SFGs and a KOH-dosage controlled hierarchical porous structure. As supercapacitor electrodes, the HPCs display a remarkable capacitive performance with a high SC (355 F g−1 at 0.2 A g−1), low ESR and good cycling stability. The energy density and power density reached as high as 9.81 W h kg−1 and 450 W kg−1, respectively. This research advances the highvalue utilization of LDPE plastics, provides a solution to control ecological hazards induced by waste plastics, and paves a sustainable way to synthesize porous carbon material for energy-storage devices.

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Acknowledgements This work was supported by the National Natural Science Foundation of China (21577102) and the Fundamental Research Funds for the Central Universities (22120170050).

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Page 36 of 36

Abstract Graphic

Synopsis.

Hierarchical porous carbon spheres were synthesized from low-density polyethylene through a catalyst-free method and were used for high performance capacitors.

36


Hierarchical Porous Carbon Spheres from Low-Density Polyethylene

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