High-Performance Aqueous Electrolyte Symmetrical Supercapacitor using Porous Carbon Derived Cassava Peel Waste

Introduction

The gradual erosion of the atmosphere as a result of the burning of conventional energy resources and the diminishing supply of fossil fuels has diverted the attention of scientists and researchers to explore renewable, sustainable and pollution-free energy [1]. Furthermore, green, effective and efficient technology related to this is highly considered as a critical issue to obtain sustainable energy conversion systems and environmentally friendly energy storage devices. In the last decade, generation III & IV solar cells and fuel cells have been widely pursued by researchers as an effective green energy conversion system that converts solar light energy and chemical energy into stable electricity [2,3]. Meanwhile, researchers are also intensively developing energy storage devices in two main forms of electrochemical devices, namely batteries and supercapacitors, which represent the extraordinary performance of energy storage systems [4,5]. However, cost management mechanisms and harmful by-products are the main focus on the advantages and disadvantages of these devices. From the point of view of electrochemical performance effectiveness, supercapacitors have significant advantages at large power density, increased energy density, countless life cycles, and flexible stable chamber operating temperatures, enabling their application in electrical machinery, electronic components market, and dynamic energy systems [6,7]. However, increasing the energy density of supercapacitors is very important to note to expand their application to a wider range of technologies to support the energy evolution of the modern world.

The main component that needs to be modified to produce high energy density in supercapacitor devices is the basic electrode material. Recent studies reported that the ideal electrode should have high surface area, hierarchical pore distribution, wettability, large electrolyte conductivity, good thermal and chemical stability, and high corrosion resistance which contribute greatly to enhanced specific capacitance, increased energy density, without sacrificing power density of the supercapacitor device [8,9]. Metal oxide materials and conduction polymers as electrode materrials have surprisingly excellent physical properties at high electrical conductivity and confirmed flexibility that can increase the energy density of supercapacitors up to 10 times greater than in previous studies [10,11]. Furthermore, through the template method, they were able to regulate the 3D hierarchical connected pore size distribution enhancing the electrochemical high performance of the supercapacitor. This is certainly a separate consideration in optimizing electrochemical work devices [12,13]. However, complicated techniques followed by raw materials that are considered expensive are their negative considerations. In addition, unfriendly, toxic by-products and high corrosive properties hinder the development of sustainable materials. Recently, carbon-based electrode materials have attractive properties for energy storage devices due to their relatively inexpensive synthesis route, inexpensive original material, high conductivity, tunenable nanostructure, excellent thermal and chemical stability [14,15]. Moreover, the biomass-based porous carbon in the EDLC type supercapacitor system, which presents the breakthrough performance, due to the large surface area, wettability of the self-doping, unique morphological structure which markedly increases the capacitance and energy density is remarkable. In addition, their abundant, sustainable, non-corrosive properties and zero residual discharge allow porous carbon to be a prime candidate as an electrode material for green and sustainable supercapacitors [16,17]. However, they have their own drawbacks such as low energy density compared to metal oxide/conducting polymer materials. Therefore, a reasonable solution is needed to overcome this problem in order to achieve an environmentally friendly supercapacitor product based on porous carbon from biomass.

Recently, it has been known that electrolyte stability is the most critical parameter in improving the electrochemical performance of porous carbon-based supercapacitors by increasing the operating voltage window, ionic conductivity, and ionic pore size [18]. Furthermore, organic electrolytes and ionic liquids have shown their achievements in the expansion of the active window of larger voltages up to 3.7V, and they have shown great cyclical stability in capacitor electrochemical devices [19]. However, its low ionic conductivity increased flammability; instability, toxicity, and high price are the main pitfalls limiting its practical prospects. On the other hand, aqueous electrolytes are considered to be less expensive, and toxic-free, and the ion size conforms to the hierarchical pore structure pattern which is very suitable for biomass-based electrode materials. In addition, these features of electrolytes can result in higher energy densities, relatively low series resistance, and high ion diffusion rates in symmetric supercapacitors. Interestingly, aqueous electrolytes display significantly superior conductivity, low cost, environmental friendliness, high mobility, superior protection, and are non-toxic compared to organic and ionic liquid electrolytes [18]. However, it is necessary to study more deeply their potential in the various pores present in biomass-based bovine carbon, plus their relatively lower potential operating window.

Herein, we study the behavior of different aqueous electrolytes (Na₂SO₄, KOH, and H₂SO₄) on porous carbon-based electrode materials for symmetric supercapacitor applications. Porous carbon was prepared from cassava peel waste which was synthesized through chemical impregnation and physical activation approaches. The optimization of the pore morphology structure was evaluated through differences in the concentration of the activating agent solution. Furthermore, the electrode material is designed to resemble a binder-free solid disc. Surprisingly, aqueous electrolytes based on strong acid H₂SO₄ revealed their high potential to enhance the capacitive properties of cassava peel-based carbon electrodes. The specific capacitance obtained from the optimized electrode material in Na₂SO₄, KOH, and H₂SO₄ electrolytes were 112, 150, and 183 F g^-1, respectively. Therefore, the 1M H₂SO₄ electrolyte is highly proportional to the porous carbon-based electrode material of cassava peel for a high-performance symmetrical supercapacitor.

Materials and Methods

Preparation of electrode material derived biomass-based activated carbon

The activated carbon used in this study was derived from cassava peel waste. The route of cassava peel waste conversion into activated carbon is carried out through a simple, hassle-free, and time-saving technique. The based material is cleaned with distilled water until all the adhering soil is completely removed. After that, the precursors were finely chopped with sizes varying from 1 cm² to 3 cm². Precursors were dried using a drying oven at 100°C for 36 hours, it should be noted, that changes in mass were recorded every 12 hours to ensure the moisture content was reduced drastically.

Furthermore, the dry precursor was pre-carbonated in an oven in an airtight container at a graded temperature of 50-250°C. After that, the sample was crushed, grounded, and sieved to obtain a homogeneous powder precursor. Chemical activation was focused on the impregnation of ZnCl₂ at different amounts of 0.3 mol/L and 0.7 mol/L in 50 gr precursor powder. The chemically activated samples were designed in the form of solid discs by pressing them with a hydraulic press, and the metric ton pressure was set to ±8 tons. Furthermore, the disc solid sample was pyrolyzed (carbonization-physical activation) in one step initiated by carbonization from a temperature of 27°C to 601°C in inert gas and directly followed by physical activation in CO₂ gas to a temperature of 850°C. Finally, the obtained activated carbon precursors were neutralized with DI Water and labeled CPAC3 and CPAC7, representing the activated carbon from the chemically impregnated cassava peel in 0.3 mol/L and 0.7 mol/L solutions. Generally, the preparation of electrode material derived biomass-based activated carbon is shown in Figure 1.

Figure 1.Figure 1. Preparation of electrode material derived biomass-based activated carbon

Preparation of symmetrical supercapacitor cell

The supercapacitor cell was prepared in a two-layered coin arrangement consisting of activated carbon electrode material based on cassava peel waste without the addition of a binder such as polyvinyl or others. Activated carbon is set in the form of a 9mm diameter solid disc at 0.21mm thickness. The separator used was an organic semipermeable membrane from egg shells. Further, the current collector is prepared from stainless steel with a square acrylic-based cell body.

Preparation of aqueous electrolyte solution

Aqueous electrolytes are targeted at three types of electrolyte solutions consisting of neutral/salt, base, and acid supplied from Na₂SO₄, KOH, and H₂SO₄. All three chemicals are dissolved at the same concentration at 1 mol/L. Na₂SO₄ and KOH in the form of pellets/solid were dissolved in a concentration of 1 mol/L in DI water by stirring them on a hotplate stirrer at 300 rpm at 80°C for 60 minutes. Furthermore, a 1 mol/L solution of H₂SO₄ was obtained by diluting 98% H₂SO₄ in 1000 mL DI water.

Characterization of material properties

The material-porosity properties of cassava peel-based activated carbons designed to resemble discs were evaluated by their dimensional shrinkage in high-temperature pyrolysis. The dimensions of the solid disc including mass, thickness, and diameter were measured before the pyrolysis process. Furthermore, the same thing is also done after the pyrolysis process is complete. The results of the two measurements were compared comprehensively.

Measurement of electrochemical performance

The performance of electrode materials on various types of aqueous electrolytes is evaluated through comprehensive general methods including cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) in a symmetrical supercapacitor system. In detail, the CV technique was reviewed on a potential scale of 0-1V at scan rates of 1, 2, and 5 mV s^-1. Furthermore, the GCD method was evaluated in current densities of 1, 2, 5, and 10 A g^-1 at a scan rate of 1 mV s^-1. Moreover, the EIS technique is carried out in the low-frequency range of 100 mHz to 100 kHz high-frequency with an amplitude of 10. Electrochemical parameters such as specific capacitance, electrode/cell/electrolyte resistance, coulombic efficiency, relaxation time constant are calculated through general equations widely reported previously.

Results and Discussions

Biomass waste as a raw material for the synthesis of activated carbon which is designed to resemble a solid disc is certainly easy to experience a decrease in its dimensions in the high-temperature pyrolysis process [20]. It can be easily predicted that the mass of precursors decreases drastically with their diameter and thickness. In detail, the tabulation of the decrease in solid disk dimensions of activated carbon is shown in Figure 2.

Figure 2.Figure 2. Reduction of mass, diameter, and thickness in carbonization-physical activation process

The decrease in mass, thickness, and diameter of the solid disk occur due to the carbonization process, physical activation of CO₂, and chemical reactions of ZnCl₂ impregnation [21]. In raw biomass, cassava peels are known to contain large amounts of water and volatiles thus their evaporation at low temperatures tends to decrease disk dimensions in the temperature range of 100-240°C. The removal of the absorbed water content followed by volatile evaporation and the decomposition of hemicellulose that occurs tends to reduce the mass of the disc [22]. The increase in the higher pyrolysis temperature in the inert gas decomposes the cellulose and lignin partially which is characterized at a temperature of >260°C. The concomitant degradation of bio-ligno-cellulose compounds that accompanies the release of H₂O is closely related to a significant decrease in disc mass, diameter, and thickness. At the end of the carbonization process, volatiles, water, hemicellulose, and simple organic components have been removed optimally thus initiating high biochar production [23].

However, their fragile physical condition and suboptimal exploitation of the pore framework are not favorable as electrode materials for energy storage devices. Therefore, it is necessary to treat ZnCl₂ chemical reaction and physical activation of CO₂. The chemical reaction of ZnCl₂ that occurs at a temperature of 600°C and above allows the etching of carbon chains to initiate the formation of various rich pores [24]. Increasing the concentration of ZnCl₂ significantly decreases the mass, thickness, and diameter of the solid disc, as shown in Figure 2 . This proves that chemical impregnation with higher amounts allows the etching of high carbon chains thereby growing new pores, opening narrow pores, and expansion of pores to the meso-macro scale in the precursor. As a consequence, excessive scraping reduces their diameter, thickness, and carbon purity. Furthermore, CO₂ physical activation required to strengthen the resulting pore framework thereby confirming the presence of multiple pores in the activated carbon. As reported by various works of literature, the CO₂ activating agent reacts with the carbon content of the precursor via C + CO₂ → 2CO. As a result, the evaporated carbon monoxide leaves pores on the surface of the precursor. This feature is needed to improve the performance of supercapacitor electrode materials [25]. This analysis was confirmed in depth through electrochemical measurements.

The electrochemical performance of supercapacitors based on CPACs activated carbon electrode materials on different aqueous electrolytes was reviewed comprehensively through a CV approach at a scan rate of 1-5 mV s^-1. The CPAC3 and CPAC7 electrodes were evaluated in the neutral, base and acidic electrolytes of Na₂SO₄, KOH, and H₂SO₄ in a symmetrical supercapacitor system, as shown in Figure 3 a and Figure 3 b. In general, the electrodes of CPACs possess a rectangular perturbed CV profile in all aqueous electrolyte solutions. This confirms the development of an electrically rich double layer as a result of aqueous electrolytes having high electrical conductivity compared to organic and ionic liquid electrolytes [26,27]. Electrolytes play an important role in the performance of electrodes to carry out the phenomenon of charge storage. The electrolytes in supercapacitors play an important role in the development of their electrical double layer [28]. Furthermore, the electrolyte also affects the reversible redox reaction of the pseudocapacitive capacitor, as discussed in the previous study [29,30]. It has been widely reviewed that aqueous electrolytes are believed to optimize the formation of abundant electrical double layers in porous carbon-based electrode materials because higher electrical conductivity and low equivalent series resistance initiate better supercapacitor power delivery [18]. In more detail, the ion concentration, cation-anion conductivity, ion size, type, and interaction with the electrode material are important factors in exhibiting capacitive properties and optimizing the performance of activated carbon-based electrode materials, as well as in this study. The significant difference in the hysteresis loops displayed on the CPAC3 and CPAC7 electrodes was significantly influenced by the specifications of the features present in neutral, basic, and acidic electrolytes. The CPAC3 electrode showed a small CV profile with a rectangular shape resembling a matted leaf in a neutral Na2SO4 electrolyte solution with a specific capacitance of 112 F g^-1.

Figure 3.Figure 3. a) CV curve of CPAC3 in different electrolyte, b) CV curve of CPAC7 in different electrolyte. c) Curve of specific capacitance at different scan rate of CPAC3, and d) Curve of specific capacitance at different scan rate of CPAC7

Furthermore, the basic KOH solution displays a near-ideal and larger rectangular CV profile with an increased specific capacitance of 150 F g^-1. Surprisingly, the acid electrolyte H₂SO₄ displayed the largest capacitive property of 183 F g^-1 with the largest CV profile. In addition, ion degradation is also found in the CV profile, which initiates a faradaic redox reaction in the CPAC3 electrode material. The CV profile of the CPAC7 electrode also displays a similar behavior, where the neutral electrolyte exhibits a weak capacitive property of 118 F g^-1. Increasing the alkaline nature of the KOH electrolyte can increase the specific capacitance of the electrode up to 160 F g^-1. Furthermore, the highest specific capacitance of the CPAC7 electrode was shown in the H₂SO₄ acid electrolyte of 176 F g^-1. Therefore, the capacitive behavior of the electrochemical capacitors CPAC3 and CPAC7 confirmed the increase in their specific capacitance respectively of electrolyte, neutral, basic, and acidic with the provision of increasing H₂SO₄>KOH>Na₂SO₄. This is due to several factors of the features possessed by each aqueous electrolyte including ionic size, ionic molar conductivity, and the type of interaction that occurs between them. The neutral electrolyte Na₂O₄ has the largest cation and anion radii where the size of the Na⁺ ions is 0.95 Å and SO₄^2- is 2.30 Å with their low ionic molar conductivity 5.01 and 16.00 ms.m².mol^-1, respectively, showing the lowest capacitive properties for CPAC electrodes [19,31]. Furthermore, the basic electrolyte KOH has smaller ion size specifications of 1.38 Å and 0.46 Å for cation K⁺ and anion,OH^-, respectively. In addition, their higher ionic molar conductivity of 19.91 ms.m².mol^-1 and 7.32 ms.m².mol^-1 allows the formation of a dense electrical double layer and accessibility of charge on the electrode surface of the CPACs. Moreover, the H₂SO₄ electrolyte has an H⁺ cation size that is almost 6 times smaller than K⁺, and Na⁺ allows these ions to access all active channels on the electrode material. This significantly produces abundant electrochemical coatings [32]. In addition, the outstanding electrical conductivity of H₂SO₄ 350 ms.m².mol^-1 enables the electrodes of the CPACs to reveal optimum electrode performance with specific capacitances of 183 and 176 F g^-1 for CPAC3 and CPAC7, respectively. Furthermore, the effect of natural, alkaline and acidic electrolytes was further evaluated at different scanning rates from 1 mVs^-1 to 5 mVs^-1, and it significantly affected the high capacitive properties of the CPACs electrodes, as shown in Figure 3 c and Figure 3 d. The CPAC3 electrode displayed a significant decrease in specific capacitance at scanning rates of 5 mV s^-1 of 47, 93, and 130 F g^-1 for aqueous electrolytes of Na₂SO₄, KOH, and H₂SO₄. However, they did show significantly higher capability rates of 41%, 62%, and 70% of Na₂SO₄, KOH, and H₂SO₄ electrolytes. This phenomena shows that the pores that develop in CPAC3 are relatively small and abundant, making it difficult to access the Na₂SO₄ electrolyte which incidentally has a larger bare ion size with a low capability rate of 41% at 5 mV s^-1. On the other hand, they are more accessible to the H₂SO₄ electrolyte which has a relatively much smaller bare ion size resulting in a 70% capability rate at 5 mVs^-1. On the other hand, the CPAC7 electrode revealed decreased rate capability from neutral to alkaline and to acidic electrolytes by 62%, 51%, and 53% for aqueous electrolytes Na₂SO₄, KOH, and H₂SO₄, respectively. This feature confirms that the CPAC7 electrode has a large pore size with a relatively low active site making it suitable only for neutral electrolytes [33]. On the plus side, these electrodes can be applied over a larger potential window up to 1.8 V, as revealed in a previous study.

Meanwhile, the comparison of the performance of CPAC3 and CPAC7 electrodes in the best electrolyte H₂SO₄ at a scan rate of 1 mV s^-1 is confirmed in Figure 4 a. Both electrodes exhibit almost normal rectangular hysteresis loops revealing the dominant double-layer electrochemical behavior. Interestingly, the CPAC3 electrode displayed a larger hysteresis loop with a specific capacitance of 183 F g^-1, whereas CPAC7 showed a specific capacitance of 176 F g^-1. This event is due to the different porosity properties of the two electrodes. Impregnation of ZnCl₂ in 0.3 mol/L solutions allows activated carbon to produce high pore diversity properties thus providing more active channels and shorter charge migration paths [34] .

Figure 4.Figure 4. a) CV curve of CPAC3 and CPAC7 in H₂SO₄ electrolyte at 1 mV s^-1, and b) Curve of specific capacitance at different scan rate of CPAC3 and CPAC7 in H₂SO₄ electrolyte

Increasing the concentration of the chemical activating agent solution of 0.7 mol/L allows excessive carbon chain etching which damages the pore structure of the base material and hinders the accessibility of ions. At higher scan rates, their capacitance clearly affects the pore structure that develops on the electrodes of CPACs. In H₂SO₄ electrolyte, CPAC3 can maintain its high specific capacitance with a high capability rate of 70% compared to CPAC7 electrode which only maintains of 53% (see Figure 4 b). This confirms the high performance of the CPAC3 electrode for symmetric supercapacitor applications in the 1 M H₂SO₄ electrolyte [35].

The electrochemical performance of the CPAC3 and CPAC7 electrodes in the best aqueous electrolyte of H₂SO₄ was confirmed more thoroughly through GCD measurements, as represented in Figure 5. Figure 5 a displays the GCD curves of the CPAC3 and CPAC7 electrodes in a supercapacitor symmetrical system, showing the shape of an isosceles triangle that is perturbed revealing the type of EDLCs normal supercapacitor.

Furthermore, the degradation of electrolyte ions was clearly identified which was identified with a coulomb efficiency of <90%. This is because the presence of functional groups N, O, and S as doping heteroatoms in the cassava peel-based activated carbon base material provides a pseudo-capacitance effect as well as confirmed EDLCs [36,37] . In addition, the weak iR drop in the GCD profile reveals maximum ion accessibility in the pore framework of the electrodes with their relatively low series resistances of 0.12 and 0.083Ω, respectively. This proves that the H₂SO₄ electrolyte can access all available active channels on the activated carbon electrodes of CPACs. Moreover, the long process of charging and discharging the GCD profile reflects the specific capacitance of the CPACs electrodes. Based on the standard equation, the specific capacitance obtained from the CPAC3 and CPAC7 electrodes were 184 and 147 F g^-1, respectively. Chemically activated and high-temperature carbonization of CPAC3 electrodes allow the precursor cassava peel to produce rich active channels, diverse pore structures, and enhanced wettability thereby initiating high capacitive behavior [38,39].

Figure 5.Figure 5. a) GCD profile of CPACs, b) GCD profile of CPAC3 in different current density, c) GCD profile of CPAC7 in different current density, and d) Curve of specific capacitance at different current density in H₂SO₄ electrolyte

However, higher activating agent concentration the CPAC7 electrode allows the destruction of the carbon structure as a result of excessive carbon chain etching. This event reduces the porosity of the carbon material so that it gets low capacitive properties. The high capacitive behavior of CPACs is also reviewed at different current densities as shown in Figure 5 b and Figure 5 c. Both electrodes display a consistent isosceles triangular shape confirming the nature of their normal EDLCs. Furthermore, the increase in higher current density surprisingly improved their coulombic efficiencies by 87% and 89% for CPAC3 and CPAC7, respectively. On the other hand, their specific capacitance is also evaluated at each applied current density, as shown in Figure 5 d. The CPAC3 electrode can maintain a specific capacitance of 116 F g^-1 at 10 A g^-1 and the CPAC7 electrode maintains a capacitive property of 71 F g^-1 at 10 A g^-1.

Figure 6.Figure 6. a) Nyquist plot, and b) Bode phase plot of CPAC3 dan CPAC7 in H₂SO₄ electrolyte

Moreover, the symmetrical cell performance of PCAC3 and PCAC7 electrodes on 1M H₂SO₄ electrolyte was studied in more detail by means of electrochemical impedance spectroscopy in the frequency range of 0.001Hz to 100,000Hz with a constant amplitude of 10. Figure 6 a shows the Nyquist plot of the CPAC3 and CPAC7 symmetric supercapacitor devices obtained in an open circuit potential shows an ideal pattern as an energy storage device for EDLCs [40]. The semicircular profile in the high-frequency region reflects the excellent ionic conductivity of the H2SO4 electrolyte with an electrolyte ion resistance of about 0.04-0.09Ω. Furthermore, the x-axis intercept reveals the electrolytic ion charge transfer and migration behavior which characterizes abundant porosity proportions and high electrode wettability properties [41] . Significantly, they showed equivalent series resistance of 0.21–0.42 for CPAC3 and CPAC7 cells, respectively. Furthermore, the characteristic Warburg impedance shown at low frequencies reveals a mechanism of electrochemical behavior dominated by anion–cation insertion at the electrolyte/electrode interface. Moreover, the near-ideal vertical curves displayed at lower frequencies indicate that the CPACs cells have near-ideal supercapacitor behavior of EDLs embellished with pseudocapacitance effects. In addition, Figure 6 b displays a Bode phase plot of the CPAC3 and CPAC7 electrodes in the 1 M H₂SO₄ electrolyte. At low frequencies, the CPAC3 electrode displays a phase angle of about -60° and the CPAC7 electrode ranges at -55° indicating the capacitive behavior of the normal electrical double layer embellished with the porous carbon-based pseudo-capacitance effect of the biomass [35,42]. These features clearly prove that the symmetric cell sets of CPAC3 and CPAC7 supercapacitors exhibit high performance for developing applications of high-performance supercapacitors with high stability in aqueous electrolytes of 1 M H₂SO₄.

Conclusions

In summary, the porous carbon-based electrode material from cassava peel waste studied in depth on aqueous electrolytes as a symmetric energy storage supercapacitor device. The criteria for selecting aqueous electrolytes are complex, including neutral (Na₂SO₄), basic (KOH), and acid (H₂SO₄) in a solution of 1 mol/L. The porous carbon was obtained through a safe, inexpensive, pollution-free controlled synthesis route in 0.3 and 0.7 mol/L ZnCl₂ chemical impregnation solutions. Through the CV approach, the electrochemical properties of the supercapacitor showed its best performance in the H₂SO₄ electrolyte, followed by KOH and Na₂SO₄. Furthermore, the CPACs electrode material designed in the form of a compact disc revealed a high coulombic efficiency performance of up to 89% at 10 A g^-1. Moreover, the supercapacitor cell resistance is relatively low with high conductivity found in the aqueous electrolyte H₂SO₄. Therefore, the 1M H₂SO₄ electrolyte is highly proportional to the porous carbon-based electrode material of cassava peel for a high-performance symmetrical supercapacitor.

Acknowledgements

The research was founded by in Direktorat Jenderal Pendidikan Tinggi, Riset dan Teknologi, Republic of Indonesia in master thesis research-postgraduate program scheme, contract No.: 1657/UN19.5.1.3/PT.01.03/2022.

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