Application of Kenaf-derived Carbon as Conductive Electrode Additive in Electric Double-layer Capacitors

Abstract

Biochar produced from bio-waste of cellulose-extracted kenaf residue was utilized as a conductive additive for electrodes in electric double-layer capacitors (EDLCs). The EDLC cells were assembled using electrodes with varying mixing ratios of kenaf-derived carbon (KC), and hydrocarbons-derived carbon black (CB) which had been industrially utilized as the conductive additive. Although the sole use of KC could not provide a sufficient electronic conduction in the EDLC electrodes, the combinational use of KC and CB attained a formation of stable conductive path therein. The energy density of the EDLC cell utilizing electrodes composed of KC and CB at a mixing ratio of 50:50 in mass under the use of 8 M KOH electrolytic solution and the cell voltage range of 0−1.0 V was found to be approximately equivalent to that of electrodes composed solely of CB, particularly at power densities below 100 W kg ⁻¹ . Through the application of KC to the conductive electrode additive, we discovered new potential uses for kenaf residue.

Application of Kenaf-derived Carbon as Conductive Electrode Additive in Electric Double-layer Capacitors

Takuya EGUCHI1, Taiki NAGANUMA1, Reiichi CHIBA1, Kimitaka WATANABE1, Yusuke ABE2 and Seiji KUMAGAI2

¹Department of Electrical and Electronic Engineering, College of Engineering, Nihon University, Koriyama 963-8642, Japan

²Department of Mathematical Science and Electrical-Electronic-Computer Engineering, Akita University, Akita 010-8502, Japan

E-mail:[e-mail], [e-mail]

Biochar produced from bio-waste of cellulose-extracted kenaf residue was utilized as a conductive additive for electrodes in electric double-layer capacitors (EDLCs). The EDLC cells were assembled using electrodes with varying mixing ratios of kenaf-derived carbon (KC), and hydrocarbons-derived carbon black (CB) which had been industrially utilized as the conductive additive. Although the sole use of KC could not provide a sufficient electronic conduction in the EDLC electrodes, the combinational use of KC and CB attained a formation of stable conductive path therein. The energy density of the EDLC cell utilizing electrodes composed of KC and CB at a mixing ratio of 50:50 in mass under the use of 8 M KOH electrolytic solution and the cell voltage range of 0−1.0 V was found to be approximately equivalent to that of electrodes composed solely of CB, particularly at power densities below 100 W kg⁻¹. Through the application of KC to the conductive electrode additive, we discovered new potential uses for kenaf residue.

Keywords : Carbon, Biochar, Conductive additive, Kenaf, Bio-waste, Electric double-layer capacitor

1. INTRODUCTION

Electric double-layer capacitors (EDLCs) are energy storage devices based on the formation and release of an electric double- layer at the interface between a conductor and electrolytic solution. They have been integrated into energy storage systems requiring numerous cycles of and rapid charge-discharge, and long-life use such in vehicles, plant machinery and appliances. They have also found applications in power regulation systems for renewable energy [1,2]. The EDLC electrodes consist of activated carbon (AC) as the active material, a conductive additive, a binder, and a current collector. The conductive additive enhances the electrical contact between the active material particles within the electrode and between the current collector and active material particles. Consequently, it decreases the electrode resistance, leading to improvements in the power and capacity characteristics of EDLCs [3]. Carbon black (CB), derived from hydrocarbons such as petroleum and natural gas, is widely used as a conductive additive in battery electrodes [3,4]. Among the various types of CBs, acetylene black, produced from acetylene gas, exhibits excellent electrical conductivity and is employed as a conductive additive in electrodes [5–9]. The conductivity of electrode is related to contact with CB aggregates by the graphitic conduction and electron tunneling between CB aggregates, which greatly increases conductivity up to the percolation threshold [3,10]. Therefore, the mass ratio of CB to the electrode material needs to reach the percolation threshold for electrode conductivity ranges from 5 to 25 % [10–14].

Kenaf has garnered attention as an effective plant for CO₂ reduction, prompting active research into utilizing kenaf-derived biomass resources [15–25]. Kenaf grows to a length of 2.5–3.5 m in about 4 to 5 months and has been cultivated in more than 20 countries, reaching a production volume of 2 million tons in 2015 [15,16,19,21]. Fibers extracted from kenaf find uses in various industries including paper and plastic. In recent years, research on kenaf-produced reinforcing fibers has surged due to their low cost and lightweight nature, particularly for application in automobiles and building parts [17,18,23–25]. Kenaf's primary components are cellulose, hemicellulose, and lignin, with 30–40 % of the fiber extractable from the plant [16,19]. While there are numerous examples showcasing the utilization of kenaf-producing fibers, few instances demonstrate the usage of kenaf residue after fiber extraction. One promising application for kenaf residue is its conversion to biochar. However, its use remains limited to water treatment, fertilizer, and solid fuels [19,20]. While bio-waste-derived materials have been extensively surveyed for their potential for use in batteries and capacitors, there is a scarcity of studies focusing on bio-waste-derived conductive additives. Snowdon et al. reported on carbonized lignin as an alternative to CBs [26], with an electrical conductivity of 9.5 S m⁻¹. In contrast, CBs typically exhibit electrical conductivities ranging from 10 to 10⁴ S m⁻¹, surpassing that of biocarbons [3]. Research on electrode materials has aimed to create superior electrodes using composite materials, such as carbon nanotubes, carbon nanofibers, and CB [27–30]. However,

the combinational use of biocarbon conductive additive with conventional hydrocarbons-derived CB has not been sufficiently explored.

In the present study, kenaf-derived carbon (KC) produced from cellulose-extracted kenaf residues was applied as the conductive additive of EDLC electrodes. To overcome the low electrical conductivity of KC, we endeavored to create an electronically conductive path in the electrode by combining KC and CB. The fundamental properties of KC, encompassing its crystal structure, pore structure, and particle morphology, were investigated by the evaluation of the electrochemical performances of EDLC cells using KC and CB.

2 MATERIALS & METHODS

2.1 Characterization of KC

KC was provided from Herat Plaza Co., Fukushima, Japan, in a form of powder. The KC powder was produced by carbonizing the kenaf residue subsequent to the extraction of cellulose components by digestion. The carbon crystal structure of KC was examined using X-ray diffraction (XRD) equipment (SmartLab, Rigaku Co., Japan) with Cu-Kα radiation within the diffraction angle range of 5–90°. The pore properties of KC were analyzed using the N₂ adsorption-desorption isotherms measured using a gas adsorption analyzer (Autosorb-3B, Quantachrome Instruments Inc., USA), at

−196°C with KC degassed at 200°C for 8 hours under vacuum.

The specific surface area of KC was calculated from the adsorption isotherm based on Brunauer-Emmett-Teller (BET) theory. The pore properties of KC were simulated using quenched solid density functional theory through the software (ASiQwin, version 1.11, Quantachrome Instruments Inc., USA), and the pore size distribution in regard to volume were calculated. The particle morphology of the KC was observed using field-emission scanning electron microscopy (FE-SEM; JSM-6500F, JEOL Ltd., Japan).

2.2 Fabrication of electrodes

EDLC electrodes were prepared using AC (KD-PWSP UedaEnvironmental Solutions Co., Ltd., Japan), with a modal particle

diameter of 6 μm, as the active material, KC and CB (Denka Black, Denka Co., Ltd., Japan) as conductive additives, and polytetrafluoroethylene (PTFE, Daikin Industries Ltd., Japan) as the binder. AC had a specific surface area of 1323 m2 g−1 measured using the BET method. Acetylene black, derived from acetylene gas, with an average particle diameter of 35 nm, a specific surface area of 68 m2 g–1, and an electrical conductivity of 4.8×104 S m−1, was used as CB. The mass ratio of the electrode materials was 80 mass% AC active material, 10 mass% conductive additives, and 10 mass% binder. Ethanol was added to the electrode materials and mixed using a mortar and pestle. The resulting mixture was placed in a mold and machine-pressed to produce an electrode sheet. The sheet was dried at 100°C for 6 hours, then punched into φ12 mm circular disks. Electrodes were obtained by crimping the circular disks onto a Ni mesh collecting electrode using a press machine at 2 MPa. Electrodes with mass mixing ratios of CB: KC = 0:100, 25:75, 50:50, and 100:0 in 10 mass% conductive additives were fabricated. The thickness of the electrode and the total mass of electrode materials were 0.33 to 0.28 mm and 29 to 24 mg, respectively. The EDLC cell comprised two (positive and negative) electrodes having the similar masses, a SUS304 body (HS 2-electrode cell, Hosen Corp., Japan), an 8 M KOH electrolyte solution (Fujifilm Wako Pure Chemical Co., Japan), and paper separators (TF4050, Nippon Kodoshi Co., Japan). Four types of EDLCs were assembled, and the cells were named according to the KC mixing ratio of the corresponding additive.

2.3 Evaluation of electrochemical properties

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted using electrochemical measurement equipment (PGSTAT-128N, Metrohm Autolab B.V., Netherlands). CVs were executed within the cell voltage range of 0–1.0 V at sweep rates of 1, 10, and 100 mV s⁻¹. The specific capacitance of the cell was derived from equation (1),

$C_{\mathrm{\text{CV}}}\left(\mathrm{\text{F}}{g}^{-1}\right)=\frac{1}{m}\times \frac{d{Q}_{\mathrm{\text{CV}}}}{dV}$ (1)

where C_CV is the specific capacitance obtained from the CV curves, dV is the micro-voltage step, dQ_CV is the amount of charge during the flow of current in dV, and m is the total mass of AC active material both in the positive and negative electrodes. EIS was conducted within the frequency range of 10 mHz to 10 kHz at a maximum voltage amplitude of 4 mV and at the bias voltage of 0

V. The rate test employed galvanostatic charge-discharge (GCD) equipment (EF-7100P, Electrofield Corp. Inc., Japan). Throughout the rate test, the current density remained constant during both the charge and discharge phases. The current density values for the rate test ranged from 0.1 to 50 mA cm⁻². The cell key parameters such as specific capacity, equivalent series resistance, energy density, and power density were obtained from the results of the rate test. The specific capacity corresponds to the amount of charge during charging or discharging, divided by the total mass of AC active material both in the positive and negative electrodes. The equivalent series resistance was calculated from the discharge characteristics using equation (2) and the specific capacitance the EDLC cell was determined using equation (3).

$R\left(\Omega \right)=\frac{V_{IR}}{I}$ (2)

$C_{\mathrm{\text{GCD}}}\left(\mathrm{\text{F}}{g}^{-1}\right)=\frac{1}{m}\times \frac{Q_{\mathrm{\text{GCD}}}}{(V_{\mathrm{\text{cell}}}-V_{\mathrm{\text{IR}}})}$ (3)

In equation (2), R is the equivalent series resistance, V_IR is the voltage drop observed at the start of discharge, and I is the current during charge and discharge. In equation (3), C_GCD is the specific capacitance in the GCD test, Q_GCD is the electrical quantity during discharge, and V_cell is the maximum cell voltage (1.0 V). The energy and power densities, normalized to the total AC active material in the electrodes, were calculated using the discharge electric energy and output power, respectively. The output power was based on the average voltage and constant current values during the discharge process.

1. 3. RESULTS & DISCUSSION
   1.    3.1 Material properties

The XRD pattern of KC in Figure 1 depicts two broad peaks near 25.3° and 43.4° corresponding to the 002 and 10l peaks of crystalline graphite, respectively. The 10l peak indicates the combined appearance of the 100 and 101 peaks as a single peak [31]. The presence of broad peaks at 100 and 002 indicates that KC possesses a turbostratic carbon structure [32]. Kenaf contains some elements besides carbon [20], which leads to the observation of smaller peaks.

Figure 1. XRD pattern of KC.

The N₂ adsorption-desorption isotherms of KC, and the pore size distribution analyzed from the N₂ adsorption isotherms, are depicted in Figure 2. The N₂ adsorption-desorption isotherms of KC exhibited a slight increase in the quantity adsorbed at relative pressures between 0 and 0.4, as well as hysteresis at relative pressures above 0.5 (Figure 2(a)), corresponding to type- IV behavior as defined by IUPAC [33]. Adsorption observed at lower relative pressures suggested the presence of micropores (pore width < 2 nm), while the hysteresis indicated the existence of mesopores (pore width 2–50 nm). The specific surface area, total pore volume, and average pore width of KC, calculated using the BET method, were determined to be 11 m² g⁻¹, 0.018 cm³ g⁻¹, and 6.3 nm, respectively. Figure 2(b) illustrates the pore size distribution of

Figure 2. N₂ adsorption and desorption isotherms of KC, and pore size distribution analyzed from the N₂ adsorption isotherms, (a) N₂ adsorption and desorption isotherms, (b) pore size distribution.

Figure 3. FE-SEM images of KC: (a) overall image of KC particles, (b) surface of the KC particles.

KC, indicating the presence of micropores and mesopores in close proximity to the micropores. The calculated micro- and mesopore volumes of KC were found to be 0.004 and 0.012 cm³ g⁻¹, respectively.

Figure 3 displays FE-SEM images of KC particles. These consisted of various sizes, typically less than approximately 10 μm (Figure 3(a)). Some KC particles exhibited pores with several micrometers in size on their surfaces (Figure 3(b)). The differences in the surface condition of KC particles may be attributed to structural differences in the kenaf residue.

3.2 Electrochemical characteristics of EDLC cells

The CV curves of EDLC cells with varying KC ratios used for the electrodes across different sweep rates are illustrated in Figure 4. The curves for KC100 and KC75 displayed a collapsed shape, suggesting a slight electric double-layer capacitance. Meanwhile, the CV curves for KC50 and KC0 at scan rates of 1 mV s⁻¹ and 10 mV s⁻¹ exhibited a symmetrical loop-like, almost rectangular shape, indicating ideal EDLC behavior. However, at a scan rate of 100 mV s⁻¹, distortion became noticeable in the CV curves for KC50 and KC0 due to the impact of the cell's internal resistance, with a more pronounced distortion observed for KC50 compared to KC0.

Figure 5 illustrates the Nyquist plots of the EDLC cells utilizing different KC ratios for the electrodes, and the equivalent circuit model intended for EDLCs. The value for each circuit element was obtained by means of numerical fitting using EIS simulation software (NOVA 2.1.6, Metrohm Autolab B.V., Netherlands). The curves of Fit-KC100, -KC75, -KC50 and -KC0 indicate the fitting simulation results. In the equivalent circuit model, CPE is constant phase element and W is Warburg-open circuit terminus. The charge transfer resistance (R_c), corresponding to the long diameter of the semi-ellipse part, arises from the contact resistance between the electrode and the collector electrode, as well as the electrolyte resistance within the AC pores [34]. The electrolyte and electrode resistance (R_s), observed before the appearance of the semi-ellipse in the high-frequency region, primarily stems from the electrolyte and electrode [34,35]. In the Nyquist plots for KC100 and KC75, a notably large semi-ellipse was observed (Figure 5(a)). The values of R_s and R_c obtained from the EIS simulation are shown in Table 1. R_s of KC100 and KC75 were much lower than their R_c. The minimal R_s values in the Nyquist plots for KC100 and KC75 imply that KC contributes to electronic conduction in the electrode parts. Conversely, the higher R_c value suggests that the low electrical conductivity of KC restricts electron transfer associated with forming an electric double-layer within the AC pores. The resistance component with electrodes can be affected by the degree of electrolyte penetration into the porous electrode [36]. A large difference in R_c between KC100 and KC75 suggests that their electrode internal states varied with the content of KC. The Nyquist plots for KC50 and KC0 depicted a typical EDLC cell behavior (Figure 5 (b)) [34]. The values of R_c for KC50 and KC0 were 3.3 and 2.5 Ω, respectively. The salient decrease in R_c for KC50 indicated the formation of an electronic conduction pathway at its electrodes. In the low-frequency regions, vertical lines were observed in the Nyquist plots for both KC50 and KC0, suggesting the ideal capacitor characteristics, with no leakage currents or side reactions [37].

Figure 6 illustrates the charge and discharge specific capacity of EDLC cells with different KC ratios at charge-discharge current densities ranging from 0.1 to 50 mA cm⁻². The specific capacities of KC50 and KC0 decreased with increasing current density. Specifically, at current densities of 50 and 0.1 mA cm⁻², the discharge specific capacities were 2.8 and 7.6 mAh g⁻¹ for KC50, and 5.4 and 9.0 mAh g⁻¹ for KC0, respectively. KC100 and KC75 exhibited nearly zero specific capacities at all the current densities.

The relationship between the cell voltage and the specific capacity of the EDLC cells at different current densities in the rate test is depicted in Figure 7. Almost linear variations were observed for electrode. This indicates that the low electrical conductivity both within the positive and negative electrodes constrained the cell's capacity and capacitance.

The R_c values of KC100 and KC75 were very large, suggesting that charge transfer associated with double-layer formation in the electrode was very difficult. For the electrodes with KC50, the electrical conductivity of the electrode reached the percolation threshold. This suggests that the specific capacity and thereby energy density of the EDLC cell increased due to the reduced internal resistance across the electrodes. Combining KC with CB can reduce R_c, showcasing potential of cellulose-extracted kenaf- based biochar as an environmentally friendly conductive additive.

KC functioned effectively as the conductive additive for EDLC electrodes when mixed with CB at a 50:50 mass ratio. From a material conductivity perspective, CB outperforms KC; however, the energy density difference between EDLC cells of KC50 and KC0 could be minimized within 1 Wh kg⁻¹ up to 100 W kg⁻¹. The mesopores in KC, and small spaces created by the addition of KC was likely to facilitate ion migration within the electrodes, suggesting that they contributed to the seamless formation of electric double-layers produced along the surface of AC particles.

Figure 4. CV curves of EDLC cells with the different KC content used for electrodes, recorded at the different scan rates: (a) KC100, (b) KC75, (c) KC50, and (d) KC0.

Figure 5. Nyquist plots of the EDLC cells utilizing different KC ratios for the electrodes, and fitting simulation results using an equivalent circuit: Nyquist plots and fitting curves of (a) KC100 and KC75, and (b) KC50 and KC0, and (c) employed equivalent circuit model.

Table 1. Electrolyte and electrode resistance (R_s), and charge transfer resistance (R_c) of EDLC cells with the different KC content used for electrodes, calculated by the EIS fitting simulation.

	Rs (Ω)	Rc (Ω)
KC100	0.34	7.6×104
KC75	0.34	4.1×105
KC50	0.66	3.3
KC0	0.52	2.5

Figure 6. Charge and discharge specific capacities for the EDLC cells with different KC ratios at current densities ranging from 0.1 to 50 mA cm⁻². Square and triangle marks indicate the charge and discharge specific capacities, respectively.

Figure 7. Relationship between cell voltage and specific capacity of EDLC cells at different current densities in the rate test: (a) KC100, (b) KC75, (c) KC50, and (d) KC0.

4. CONCLUSION

Biochar prepared from bio-waste of cellulose-extracted kenaf residue was utilized as a conductive additive for the EDLC electrode. The EDLC electrode was composed of a blend of 50 mass% KC and 50 mass% CB, forming a stable electronically conductive path. The energy density of the EDLC cell using the blended conductive additive was almost similar to that of solely using CB up to the power density of 100 W kg⁻¹. The utilization of KC as a key component for EDLC electrodes, combined with hydrocarbons-derived CB, has proven to be instrumental in maintaining the EDLC performance even introducing bio- waste-derived conductive additives. This study highlights the viability of bio-waste-derived conductive additive in bolstering the performance of EDLC electrodes, paving the way for more eco- friendly and sustainable energy storage systems in the future.

-Figure 8. IR-drop-corrected equivalent series resistance of EDLC cells of KC50 and KC0 during discharge at various current densities in the rate test.

Figure 9. Specific capacitance of EDLC cells of KC50 and KC0 at different current densities in the rate tests.

Figure 10. Ragone plots of EDLC cells of KC50 and KC0, calculated based on the rate tests.

Acknowledgements

This study was supported in part by the Casio Science Foundation (Grant Number 30), JSPS KAKENHI (Grant Number 22H01460), and Nihon University SDGs Project (Grant Number 24SDG02).

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Acknowledgments

This study was supported in part by the Casio Science Foundation (Grant Number 30), JSPS KAKENHI (Grant Number 22H01460), and Nihon University SDGs Project (Grant Number 24SDG02).