Preparation of an Electric Double Layer Capacitor (EDLC) Using

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Preparation of an electric double layer capacitor (EDLC) using Miscanthus-derived biocarbon Xiangyu You, Manjusri Misra, Stefano Gregori, and Amar K. Mohanty ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02563 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Sustainable Chemistry & Engineering

Preparation

of

an

electric

double

layer

capacitor

(EDLC)

using

Miscanthus-derived biocarbon Xiangyu You1, Manjusri Misra1,2* , Stefano Gregori2 and Amar Mohanty1,2* 1

Bioproducts Discovery and Development Centre, Crop Science Building,

Department of Plant Agriculture, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canada 2

School of Engineering, Thornbrough Building, University of Guelph, 50 Stone Road

East, Guelph, Ontario N1G 2W1, Canada Email: *Manjusri Misra, [email protected] *Amar Mohanty, [email protected] Abstract Commercially available biocarbon (BC) derived from Miscanthus was activated by potassium hydroxide (KOH) at different BC/KOH ratios of 1:3, 1:4 and 1:5. The structural and morphological changes and the porosity developed in the resultant products were investigated. In addition, the electrochemical performance of the assembled electric double layer capacitors (EDLCs) incorporating these carbonaceous materials was determined using galvanostatic, voltammetric and impedance spectroscopy techniques. Compared with pristine BC samples, these KOH-activated BC samples exhibited a significant increase in the surface area from 98 m2 g-1 to 3024 m2 g-1, which was mainly attributed to the well-developed micropores. By employing the activated BC as an electrode material for EDLCs, comparable capacitance values up to 110.8 F g−1 at a scan rate of 0.05 V s-1 and 65.4 F g-1 at a current density of 1 A g-1 were obtained during operation in an organic electrolyte. The variations in the electrochemical behavior were considered in relation to the surface area and the porous characteristics.

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Keywords Activation; Biocarbon; Electric double layer capacitor; Pore size distribution; Surface area.

Introduction Energy storage devices play an important role in the consumer electronics, automotive, aerospace and stationary markets due to their even increasing applications.1-2 For example, electric double layer capacitors (EDLCs) have been demonstrated to be useful for applications such as memory backup systems, auxiliary power units, instantaneous electricity compensators, and other energy-storage devices.3-4 From a structural view, EDLCs consist of three parts: two active material-loaded electrodes, an electrolyte and a separator sheet.5 Theoretically, the efficiency and practicality of EDLCs primarily depend upon the active materials on the electrodes (electrode materials) because EDLCs store and release electrical energy by ion adsorption and desorption on the surface of these electrode materials. Their capacitance (C) is proportional to the specific surface area (A) of the electrode material according to the equation C=εA/d, where ε is the dielectric constant of the electrolyte and the electrode material and d is the distance between the electrode material and the electrolyte ions.6 Considering the narrow and limited variation range of ε and d, a large surface area is preferred to obtain a high EDLC performance.

Carbonaceous materials are a prospective candidate for EDLCs.7 Several petro-based carbon materials, such as polyacrylonitrile (PAN), pitch, and phenolic resin-based carbon, have been used in EDLCs, while carbon derived from biomass, which is environmentally sustainable, still needs to be developed.8-11 Woody BC from red cedars, with a high carbon content of 98 wt% and oxygen as the only detectable impurity, has been studied as an electrode material in EDLCs. A surface area of 400 m2 g-1 and a capacitance of 115 F g−1 were achieved during HNO3 activation.12 2


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Distillers dried grains with solubles, a by-product of bioethanol production, have also been used as a biocarbon precursor to be pyrolyzed, potassium hydroxide (KOH) activated and further applied as electrode materials of EDLCs. The final carbonaceous product showed a high surface area of 2959 m2 g-1 with a high capacitance of 150 F g−1.13 Another inexpensive carbon, biocarbon (BC) derived from Miscanthus, has recently become commercially available, and it has been used as a filler in polymer processing to enhance the mechanical properties of polymers.14-16 Compared with the biocarbons mentioned above, Miscanthus is distributed around the world, much more abundant, lower cost, a faster growing crop, and harvestable. Miscanthus yields of 27 to 44 t ha–1 have been reported in Europe and Midwestern US locations. However, it contains a higher ash content (appr.10%) than conventional woody BC (appr.2%), which is a disadvantage in electrochemical applications 16-17 Therefore, it is necessary to develop a corresponding method to use biocarbon derived from such biomass in high-performance EDLC assembly.

In this study, we first develop the application of Miscanthus-based BC to assemble EDLCs. Compared with conventional petro-based precursors, this pyrolyzed BC contains pores that are generated from plant cells, which result in a larger surface area and enhance the EDLC capacitance. Furthermore, KOH was also used as an activation reagent to further enlarge the surface area. Finally, EDLCs were assembled using this BC, and their electrochemical properties were characterized.

Experimental Section Synthesis of the KOH-Activated BC BC derived from Miscanthus (ash content, 8.7%; average particle size, 14.89 ± 9.77 µm) was obtained from CGtech, CA and was pyrolyzed at 700°C. Powdery KOH was obtained from its pellets (Fisher Chemical, CA) by pre-grinding with mortar and pestle. The BC and powdery KOH were mixed subsequently (BC: KOH weight ratios 3



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equal to 1:3, 1:4 and 1:5) and activation was carried out in an inert N2 atmosphere (N2 flow rate, 1 L min−1) at 900 °C for 1 h with a heating rate of 10 °C min−1, and then cooled to room temperature within a tube furnace in the same N2 atmosphere. The mechanism of KOH activation is listed as follows: 18 4KOH+(−CH2)→ K2CO3+K2O+H2 (1) 6KOH+2C→2K2CO3+2K+3H2

(2)

K2CO3→CO2+K2O

(3)

4KOH+C→4K+CO2+2H2O

(4)

K2O+C→2K+CO

(5)

K2CO3+2C→2K+3CO

(6)

Subsequently, the activated biocarbon samples were washed with 0.1 mol L−1 hydrochloride acid to neutralize the base residue until the pH value was equal to 7, followed by DI water washing to remove the salt residues. The final activated products were dried in an oven at 105°C overnight and named as BCK1-3, BCK1-4, and BCK1-5, according to the weight ratio of BC to KOH during activation.

Physiochemical property measurements The specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) equation based on N2 isotherm (77 K) adsorption results, which were analyzed using an Autosorbe iQ (Quantachrome, Florida, USA) machine. The total pore volumes were obtained at a relative pressure of 0.995, while the pore size distributions were determined using the NLDFT (slit pore) analysis model for carbon. A desktop SEM (Phenom Pro X, PhenomWorld, Netherlands) operated at an accelerating voltage of 15 kV was also used for morphology observations. The structure of the activated carbon was further characterized using a DXR2 Raman microscope (Thermo Scientific) at room temperature with an excitation wavelength at 532 nm from a diode-pumped solid-state laser.

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EDLC fabrication and electrochemical measurements The BC samples were mixed with conductive carbon black (CB), and the mixtures were suspended in a 2% aqueous sodium carboxymethyl cellulose (CMC) solution at a weight ratio of 80:10:10 (BC:CMC:CB). The mixture was then stirred for 2 h to yield a homogeneous suspension of electrode materials. After this, the suspension was coated on Al foil, and the electrodes were obtained by cutting the coated foil into 15 mm diameter circular sheets. The electrodes and separator (Celgard 2400, Charlotte, North Carolina, USA) were placed in a flat cell with drops of an electrolyte solution (1.0 M triethylmethylammonium tetrafluoroborate (TEAMBF4/PC) solution). Finally, the assembled flat EDLC cell was sealed for the performance measurements. Cyclic voltammetry (CV) with different scan rates from 0.01 to 0.1 V s−1 and galvanostatic charge/discharge (GCD) tests at different current densities from 0.5 to 2 A g−1 were conducting using an electrochemical working station (Autolab PGSTAT302N, Metrohm Autolab, Mississauga, Canada). The gravimetric capacitance values were calculated from the CV and GCD profiles. In the CV-based capacitance calculation, the gravimetric capacitance (C) was obtained from Eq. (7): 4 (7) ∆ where m is the carbon mass of one electrode, ∆ is the voltage window, i is the =

response current, V is the voltage, and v is the scan rate. In the case of the GCD test, the capacitance could be calculated using Eq. (8): =

4 (8) /

where, i is the discharge current and dV/dt is the slope of the discharge curve.19-20 The electrical resistances of the cell system were also measured using electrochemical impedance spectroscopy (EIS); a frequency response analysis was conducted over the frequency range from 0.1 to 1,000,000 Hz with a potential amplitude of 10 mV.

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Result and discussion Morphological and structural properties According to the SEM images shown in Figure 1, the pristine BC particles showed several aligned open channels (Figure 1a), which were probably generated from the plant cell during pyrolysis, but the magnified image (Figure 1b) shows a smooth surface. After KOH activation, these open channels remained, and the surface morphology became rough because of KOH etching. When the KOH/BC ratio increased to 5:1, the BC bulk structure partly collapsed and became interconnected with the channels inside (Figure 1g and 1h).

Figure 1. SEM images of the pristine BC (a, b) and BCK samples at different magnifications: BCK1-3 (c, d), BCK1-4 (e, f) and BCK1-5 (g, h).

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Figure 2. Deconvolution and peak fitting of the Raman spectra (black, original bands; red, fitting curves; green, deconvolution bands) of the BC samples: (a) pristine BC, (b) BCK1-3, (c) BCK1-4and (d) BCK1-5. Table 1. RI and La values of the pristine BC and BCK samples RI=ID/IG

La (nm)

Pristine BC

2.97

6.47

BCK 1-3

4.05

4.75

BCK 1-4

4.47

4.30

BCK 1-5

4.53

4.24

Raman spectroscopy was conducted on the pristine BC and BCK samples to gain insight into the changes in the carbon structure of the material. The Raman spectra (Figure 2) of the pristine BC and BCK samples were deconvoluted. They show a well-known D-band at approximately 1350 cm−1, a G-band at 1580 cm−1 accompanied with a D’’ at approximately 1500 cm-1 and a sp3 phase-rich band at 1180 cm-1. 21 The

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ratio of the integrated intensity of the D peak to G peak, denoted by RI=ID/IG (Table 1), increased from 2.97 to 4.53 when the KOH/BC ratio increased from 0:1 to 5:1, which is not comparable with that of commercial carbon black (2.50). The peaks at 1180, 1350, and 1500 cm−1 were attributed to defects and disordered portions of the carbon materials, and the peak at 1600 cm−1 indicated the presence of ordered graphitic crystallites of carbon. These results indicate that the BC and BCK samples were mainly composed of nongraphitized carbon containing turbostratically disordered graphene sheets. To clearly reveal the KOH activation effects, the in-plane size (La), as an indicator of the graphitic crystallite size, was also calculated using the following equation: = 2.4 × 10 × () ( ) , where λ corresponds to the laser wavelength (nm).22 According to the calculated La values shown in Table 1, the activated BCK samples exhibited smaller La values with higher amounts of KOH, indicating that more defects were generated with a KOH/BC ratio of 5:1, which is in accordance with the previous SEM findings.

1600 1400

dS(d) (m2/nm/g)

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1200 1000

BCK1-5 BCK1-4 BCK1-3 Pristine BC

800 600 400 200 0

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0) Figure 3. N2 adsorption/desorption isotherms for the pristine BC and BCK samples.

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Figure 4 Surface area distribution of the pristine BC and BCK samples in the pore range of (a) 1.0–4.0 nm and (b) 4.0–10.0 nm (calculated using the NLDFT model).

Nitrogen adsorption/desorption measurements were performed at 77 K to track the surface area and pore size distribution of the pristine BC and KOH activated BCK samples. As shown in Figure 3, the adsorption/desorption curves typically increased among the BCK samples in the relative pressure range of 0-0.4, which is similar to previous results for classic microporous activated carbon.23-25 Additionally, the pristine BC showed a low amount of N2 adsorption/desorption and plateaued throughout the entire relative pressure range, indicating a lower surface area and less pores than the BCK samples. Based on these isotherms, the results of the surface area and pore characteristics are summarized in Table 2. The BET surface area of the pristine BC was 98 m2 g-1, which is larger than that of previously reported petro-based carbons and other wood chars (in the range of 1-10 m2 g-1).

26-28

This result can be

explained by the contribution of the existing channels inside the BC. Furthermore, the surface area of the pristine BC significantly increased after KOH activation. A large surface area of 2606 m2 g-1 was obtained at a KOH/BC ratio of 3:1, and it further enlarged to 3024 m2 g-1 at a ratio of 4:1. In addition, the total pore volume largely increased to 1.752 ml g-1 compared with 0.08 ml g-1 for the pristine BC. (Table 2). This result can be explained from the pore size distribution results in which the number of pores ranging from 1 to 4 nm largely increased, especially the number of 9



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micropores (

Preparation of an Electric Double Layer Capacitor (EDLC) Using

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