Investigation of Different Aqueous Electrolytes for Biomass-Derived Activated Carbon-Based Supercapacitors

Abstract

The present work reports the synthesis of biomass derived activated carbon and its electrochemical behaviour in different electrolytes. Ricinus communis shell (RCS) was used as a raw material in this study for the synthesis of activated carbon (AC) following a high-temperature activation procedure using potassium hydroxide as the activating agent. The physical and structural characterization of the prepared Ricinus communis shell-derived activated carbon (RCS-AC) was carried by Brunauer-Emmett-Teller analysis, X-ray diffraction analysis, Fourier Transform Infrared Spectroscopy, Raman Spectroscopy and Scanning Electron Microscopy. The synthesized AC was electrochemically characterized using various techniques such as Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) tests, and Electrochemical impedance spectroscopy (EIS) measurements in different aqueous electrolytes (KOH, H₂SO_4, and Na₂SO₄). The results show that the double layer properties of the RCS-AC material in different electrolytes are distinct. In specific, the working electrode tested in 3 M KOH showed excellent electrochemical performance. It demonstrated a specific capacitance of 137 F g⁻¹ (at 1 A g⁻¹ in 3 M KOH) and exhibited high energy and power densities of 18.2 W hkg⁻¹ and 663.4 W kg⁻¹, respectively. The observed capacitance in 3 M KOH remains stable with 97.2% even after 5000 continuous charge and discharge cycles, indicating long-term stability. The study confirmed that the synthesized RCS-derived activated carbon (RCS-AC) exhibits good stability and physicochemical characteristics, making them commercially promising and appropriate for energy storage applications.

1. Introduction

A huge amount of effort has been made in the recent years to develop renewable, environmentally sustainable, affordable, and dependable energy systems. The gradual deterioration of the atmosphere and the depletion of fossil resources have drawn the attention of researchers and scientists towards the exploration of renewable energy [1,2,3,4,5]. There are two kinds of electrochemical devices: batteries and capacitors, both of which are promising energy storage systems. Depending on the charge management mechanisms used, all energy storage technologies have certain advantages and disadvantages. Batteries have a low power density but a high energy density, whereas traditional capacitors have a high-power density but a low energy density [6,7,8]. A supercapacitor (SC), also known as an electrochemical capacitor, is an intermediary between the battery and the traditional capacitor that provides significant advantages such as enhanced strength and energy flow, high reliability, flexible operating temperature, and environmental friendliness [9,10]. The energy storage process in EDLCs depends on the efficient electrostatic accumulation of charges at the electrode-electrolyte interface and during this charge-storage procedure, there is no charge transfer. Fast redox processes involving charge transfer at the electrode and electrolyte interfaces controls the amount of energy is stored in pseudocapacitors [11,12]. The development of numerous electrode materials for supercapacitors have been reported in the literature which can be categorised as composite materials [13,14,15,16,17], carbon materials derived from biomass [18], conductive polymers [19], and transition metal oxide [20]. Carbon materials from biomass are frequently used in these electrode materials because of their low cost, nontoxicity, abundance of source, and long cycle life. The most frequently reported electrode material is activated carbon derived from biomass [21,22], which has been used in industrial capacitors.

Due to the wide range of electrochemical and physicochemical properties obtained depending on the precursor and activation method used, activated carbon-based materials are frequently used as electrodes [23]. Activated carbon prepared from various biomass such as human hair [24], bean dregs [25], dead neem leaves [26], tobacco rods [27], cornstalk core [28], cassava peel [29], fungi [30], pine cone petal [31], silk [32], rice husk [33], microalgae [34], sugar cane bagasse [35], eggshell [36] etc. has been previously reported in literature. The high packing density, high surface area, higher porosity, and high chemical and physical stability of these activated carbons are some of their common characteristics [37]. The porous nature of AC materials is considered to be advantageous for supercapcitor applications, with microporous materials having a greater impact on charge transfer processes. The material’s surface area and the electrolyte’s stability are the main factors responsible for the high capacitance values. Electrolyte is found to be one of the most effective elements in supercapacitor performance. There are three categories of aqueous electrolytes commonly used: base, acidic, and neutral liquids. Aqueous electrolytes outperform organic-based and ionic liquid electrolytes in terms of power density while limiting energy density [38]. Electrochemical capacitors tested with aqueous electrolytes provide additional advantages over other electrolyte systems, such as low cost, high-ionic conductivity, safety, and environmental friendliness.

In neutral electrolytes, the working potential window extends above 1.2 V to 2.2 V while it is restricted to 1.2 V in acidic and basic electrolytes [39]. The protonation or deprotonation of the functional groups is what gives carbonaceous materials derived from biomass their altered surface properties in aqueous electrolytes. Despite significant advancements, some obstacles still exist, including a low energy density and a small potential window for aqueous based electrolytes.

The current work is focussed on the investigation of the effect of aqueous-based electrolytes on the capacitance performance of carbonaceous material derived from biomass. The capacitance of the material is influenced by the relationship between the average pore size of the material and the effective size of the electrolyte ion. Ricinus communis shell is used as a precursor to prepare the highly porous carbon by pre-carbonation followed by KOH activation. The prepared carbon material has a better ionic conductivity and a good specific surface area as the electrode material. It demonstrated good capacitive performance when used as supercapacitor electrode materials in aqueous electrolytes, such as 3 M KOH, 1 M H₂SO₄, and 1 M Na₂SO₄. The established relationship between carbon electrode structures and electrolyte composition will help in understanding the impact of carbonaceous electrodes derived from biomass on the storage properties of symmetric supercapacitors.

2. Results and Discussion

The structural and phase of the prepared activated carbon was investigated by XRD analysis (Figure 1a) and as observed from the figure, RCS-derived activated carbon (RCS-AC) displays diffraction intensities, indicating the predominance of amorphous structures. It is clear that there are sharp shifts of (002) peaks at around 23° and 43° corresponding to various N and O heteroatom contents. The (002) peak represents graphitic stacking originating from randomly oriented aromatic carbon. This might be due to the high temperature activation process.

When such high temperatures are applied, graphitic carbon structures are formed. The turbostratic or random layer lattice structure is an intermediate structure between graphite and amorphous states in the RCS-derived activated carbon [40]. Apart from the amorphous nature of RCS-derived activated carbon evident from the characteristic peaks, the D band located at 1334 cm⁻¹ represents disordered and defective carbon structures, while the G band located at 1587 cm⁻¹ represents the vibration of carbon atoms (Figure 1b). RCS based activated carbons with a low graphitization degree exhibits a high-intensity ratio of the D and G bands (I_D/I_G), which indicates a highly disordered characteristic nature [41]. This improves the electrode ion diffusion and electron transport abilities, as well as its surface wettability towards aqueous electrolytes. The surface functional groups in the synthesized AC were further investigated by FT-IR analysis. The spectrum of RCS derived activated carbon shows peaks at about 3325, 1527, 1127, and 828 cm⁻¹ (Figure 2). The peak at about 3325 cm⁻¹ indicates O-H stretching vibration, while the peak at about 1527 cm⁻¹ is ascribed to C=C stretching vibration. The small peaks at about 1127 cm⁻¹ is attributed to C=O stretching modes, which are due to the high degrees of substitution on the C-H bond in the aromatic ring. RCS sample conductivity is enhanced by aromatization occurring during the activation and carbonization processes [42,43,44].

By storing charge on the surfaces via Faradaic processes, surface functional groups produce a pseudocapacitive effect that enhances capacitive behaviour. In addition to reducing charge-transfer resistance at the electrode/electrolyte interface, it increases wettability for maximum utilization of the accessible area at the electrode/electrolyte interface [45]. N₂ adsorption/desorption measurements were performed on RCS-activated carbon to determine its pore texture. According to the IUPAC classification [46], N₂ adsorption-desorption isotherm curves of all samples at 77 K have micropore and mesopore structures, as shown in Figure 3. Chemical activation of the material with KOH gives microporous and mesoporous textures. The sample exhibits high adsorption capacity at low relative pressure (0.20), indicating the presence of numerous micropores. Upon reaching 900 ℃, potassium is at its boiling point and can diffuse into carbon layers, facilitating pores to form.

Thus, higher temperatures would facilitate the formation of mesopores through enlargement or a combination of micropores. A hysteresis loop between 0.4 and 0.9 relative pressure shows mesopores, and a tail at 1.0 shows the presence of macropores. At low pressures, the N₂ uptake increases exponentially, indicating micropores while, the concave curve of the isotherm with hysteresis indicates hierarchical micro/mesoporous structures [47]. These results provide insight into how KOH contributes to microporous structures in RCS at high temperatures. As ions move from the neutral electrolyte bulk to the electrode/electrolyte interface, micropores store charges and adsorb ions, while mesopores create channels for the ions. RCS activated carbon has a specific surface area of 1917 m²g⁻¹. The inset in Figure 3 shows that RCS-AC samples have micropores and mesopores. In Barrett–Joyner–Halenda (BJH) analysis, micropores and mesopores have an average size of 3.8 nm. The porous structure and specific surface area of the AC improves ion access at high charge/discharge rates and reduces ion transport time from electrode to electrolyte, which is an important factor for achieving satisfactory power performance [48]. This study indicates that activation temperature can be used to fine-tune the material structure, which has a significant impact on the supercapacitor performance. Further to understand the morphology of RCS-AC, SEM analysis was carried out (Figure 4). During carbonization, volatile compounds were removed from the Ricinus communis shells, giving rise to a honeycomb structure with different pore sizes and shapes (Figure 4a,b). There is an uneven distribution of microblocks and cavities in the samples regardless of the KOH activation. As a result of the high oxidation properties of KOH and decarboxylation reactions taking place with other functional groups, small pores in the microstructure could be obserevd. It is evident from the results that the porosity of the activated carbon is determined by its activating agent.

As observed from the images, the pores are well connected and uniform and this would facilitate the ease diffusion of electrolyte ions into the mesopores and micropores of the material. In addition, a variety of pores were seen and the majority of which were formed by KOH activation. Fast ion transport required for high-performance supercapacitors could be achieved with this kind of structure, which provides an efficient ion path during electrochemical measurements. The volume of KOH, activation temperature, and its unique reaction, in addition to the carbon source, are believed to play an important role in charge transport [49]. Furthermore, EDAX analysis was carried out to determine their chemical compositions (Figure 4c) and it could be observed that there is a high C and O content in the Ricinus communis shell-derived carbon, which might be due to the activation of KOH.

Further the synthesized materials were fabricated as electrode probe and further analysed for its energy storage properties via various electrochemical tests including cyclic voltammetry, galvanostatic charge discharge and impedance analysis. A three-electrode system was used to evaluate the electrochemical behaviour of the RCS-derived activated carbon with various aqueous electrolytes (acid, base, and neutral). The electrolyte study was carried out to understand the properties of the materials and its behaviour with the applied potential. Cyclic voltammetry was carried out with the electrode in KOH, Na₂SO₄, H₂SO₄ and given in Figure 5. As observed from the figure, KOH and Na₂SO₄ electrolytes have rectangular CV curves, while H₂SO₄ has a highly resistive CV curve due to its lower molar conductivity (Figure 5a). The difference in the electrode material behaviour may be attributed to the differences in the physical characteristics of the ions in different electrolytes. As observed from the figure, a larger current response was obtained in KOH electrolyte (Figure 5a). To obtain more accurate capacitance measurements, galvanostatic charge/discharge measurements were performed. The galvanostatic charge—discharge measurements were carried out with a three-electrode system in the potential window 0.2 V to 0.6 V to provide complementary measurements of the specific capacitances of the electrodes. Figure 5b shows a typical charge/discharge curve for the three electrolytes at current densities ranging from 1 to 10 A g⁻¹with a calculated specific capacitance of 137, 85, and 70 F g⁻¹ in KOH, Na₂SO₄, and H₂SO₄ electrolytes. In 3 M KOH electrolyte, the charge/discharge curves are almost perfect isosceles, giving a high capacitance value. The electrodes exhibit good charge/discharge behaviour and the shape of the curve shows double layer capacitance behaviour and a deviation from linearity is also observed, which is an indicative of pseudocapacitive nature of the carbon material. In general, any energy storage device should have low internal resistance as less energy is wasted as unwanted heat during the charging and discharging processes. As observed from the figure, the curves are reversible thus implying a good capacitive nature. As shown in Figure 5c, specific capacitance varies with current density.

Based on the molar ionic conductivity of K⁺, Na⁺ and H⁺, KOH electrolyte had the highest specific capacitance, followed by Na₂SO₄ and H₂SO₄. A high molar conductivity electrolyte will allow easy percolation of ions in the electrode material leading to a higher specific capacitance. KOH also exhibits a sustainable capacitance with increasing scan rates when compared to the other two electrolytes. The specific capacitance of a supercapacitor is determined by the pore structure of its electrode material and the degree to which its electrolyte matches its pore size [50]. The Ragone plot of activated carbon derived from RCS is given in Figure 5d, which demonstrates its energy and power density. The specific capacitance of a supercapacitor is directly proportional to its energy density. Consequently, the RCS derived activated carbon exhibits a maximum energy density of 18.2 W hkg⁻¹ at 663.4 W kg⁻¹ in KOH electrolyte and 6.2 W hkg⁻¹ at 2420.9 W kg⁻¹. In Na₂SO₄ electrolyte, it shows a maximum energy density of 10.5 W kg⁻¹ at a power density of 715 W kg⁻¹, while in H₂SO₄ electrolyte, it shows a maximum energy density of 9.06 W kg⁻¹ at a power density of 635.6 W kg⁻¹, outperforming other ACs derived from lotus seedpods [51], chicken feathers [52], corn husks [53], 3D flowerlike carbon [54], jujube pits [55], waste tea leaves [56]. Performance comparison of activated carbon derived from different biomass is given in

Table 1. Performance comparison of activated carbon derived from different biomass.

Biomass	Activating Agent	Specific Capacitance (F g−1)	Energy Density (ED) (W hkg−1)	Power Density (PD) (W kg−1)	Current Density	Electrolyte	Reference
Rotten carrot	ZnCl2	135	29.1	142.5	1 mA cm−2	KOH	Ahmed et al., 2018 [57]
Banana peel waste	-	68	0.75	31	-	H₂SO₄	Taer et al., 2017 [58]
Loofah sponge	KOH	78.2	34.7	26.8	1 A g−1	Na₂SO₄	Luan et al., 2016 [59]
Pinecone	KOH + CO₂	137	19	100	0.1 A g−1	Na₂SO₄	Barzegar et al., 2017 [60]
Cow dung	KOH	124	28	-	1 A g−1	Et₄NBF₄	Bhattacharjya and Yu, 2014 [61]
Pine biomass	KOH	69	24.6	400	0.5 A g−1	KOH/PVA	Bello et al., 2016 [62]
Sugarcane Bagasse	KOH	142	19.74	0.5	0.5 A g−1	KOH	Hao et al., 2014 [63]
Ricinus communis shell	KOH	137	18.2	663.4	1 A g−1	KOH	Present work

.

At higher charge/discharge rates, the impedance and diffusion routes in the pore structure are related to energy and power limitations. Only a portion of the porous structure can absorb electrolyte ions at high diffusion currents, whereas both the medial and lateral porous structures can absorb charge at lower diffusion currents [64]. Therefore, the aqueous-based electrolyte viz. KOH has the potential to increase the energy density of aqueous symmetric RCS-derived activated carbon and is also an ideal carbon material with high capacitance.

It is also important to consider the cycle life of supercapacitors. In Figure 6a, galvanostatic charge/discharge measurements were used to evaluate the long-term cyclic stability of capacitance performance with acidic, basic and neutral electrolytes. Cyclic stability of 97.2%, 93.6% and 84.8% was achieved for the electrolytes KOH, Na₂SO₄ and H₂SO₄ at 5000 cycles. KOH electrolyte demonstrates good cyclic charge and discharge performance for activated carbon derived from RCS. The increased stability of the electrode material may be attributed to the presence of surface carbonyl and oxygen functional groups in the RCS derived activated carbon, which may cause redox reactions with KOH electrolyte. As a result, it is evident that the activated carbon derived from Ricinus communis shell is electrochemically stable with long-term cycle life.

Figure 6b shows the Nyquist plots of RCS-AC from EIS measurements in different electrolytes in the frequency range of 100 kHz to 10 MHz. The observed spectra show different behaviours in three different frequency regions. All the electrolyte capacitive behaviour can be approximated by almost vertical lines in the low-frequency region. In the zoomed region (inset of Figure 6b), a semicircle can be seen indicating the presence of resistance in the electrode. The porous nature of the electrode and the existence of resistance at the electrode-electrolyte interface is the primary cause of the observed semicircle.

At the electrode-electrolyte interface, a wide semicircle indicates resistance dominance, while a smaller semicircle suggests capacitive dominance. As it is a transition range between the low frequency (right angles zone) and high-frequency ranges, this range is also known as the “knee” region (Warburg region). The “knee” is the transition point between the high and low-frequency ranges. The frequency at the knee is a measure of a supercapacitors rate capability since it represents the maximum frequency at which the capacitive characteristic is dominant [65]. Further, the width of the impedance arc at this transition area denotes resistance due to ion diffusion and transportation at the electrode/electrolytes interface. The ESR of the RCS-AC electrode is approximated at ~1.1 Ω for KOH electrolyte and 3.1Ω and 4.5Ω for Na₂SO₄ and H₂SO₄ electrolytes, respectively. Low ESR values indicate low internal resistance within the material, cell components, electrolyte and current collector which further qualifies for electrode application. The as-prepared RCS derived activated carbon sample delivers a specific capacitance of 137 F g⁻¹ in 3 M KOH at 1 A g⁻¹ and excellent cyclic stability with a good energy density of 18.2 W hkg⁻¹ at a power density of 663.4 W kg⁻¹.

Table 2. Electrochemical parameters of RCS-900 in different aqueous electrolytes.

Electrolytes	Specific Capacitance (F g−1)	Energy Density (ED) (W hkg−1)	Power Density (PD) (W kg−1)	ESR (Ωcm2)	Capacitance Retention (%)
KOH	137	18.2	663.4	1.1	97.2
Na2SO4	70	10.5	715.2	3.1	84.8
H2SO4	85	9.06	635.6	4.5	93.6

shows the summary of the electrochemical parameters of RCS-AC in different electrolytes.

Further to understand the practicality of the fabricated electrode with a two-electrode system and a broad operating voltage, the CV tests were performed from 0.0 to 1.8 V vs. Ag/AgCl (Figure 7a). The curves were rectangular shaped and shows that the electrode is beneficial for charge storage. Further the galvanostatic charge discharge curves recorded at 1 A g⁻¹ current density showed that the curves were symmetric with a calculated specific capacitance of 115 F g⁻¹. These results suggest that the symmetric cell has very good electrochemical reversibility and possesses ideal capacitive nature.

Thus, an aqueous-based electrolyte to fabricate a high-energy symmetric supercapacitor with KOH-activated biomass carbon is reported to have a good capacitance and energy density, thereby making it as a promising electrode material for energy storage.

3. Experimental Section

3.1. Preparation of RCS Derived Activated Carbon (RCS-AC)

Ricinus communis shell (RCS), used for the preparation of activated carbon in this study was collected from a local agricultural farm in Coimbatore, Tamil Nadu, India. The transformation of Ricinus communis shell into activated carbon is depicted in Scheme 1. Pre-carbonization and KOH activation were combined to prepare Ricinus communis shell-based carbons. Carbonization of Ricinus communis shell was carried out in N₂ atmosphere in a horizontal tube furnace at 400 ℃ for 3 h. This was followed by KOH chemical activation of the carbonized Ricinus communis shell carbon. The porous carbon materials were obtained by mixing RCS carbon and KOH pellets in a weight ratio of 1:1 and heating at 900 °C under a N₂ atmosphere for one hour at a rate of 5 °C/min. After cooling it to room temperature, it was washed with 5% HCl solution followed by thorough washing with deionized water. The resultant product is designated as RCS-AC.

3.2. Physicochemical Characterization

The structure of RCS-AC was analysed using powder X-ray Diffractometer (XRD BRUKER, D8 ADVANCE, Billerica, MA, USA) with Cu Kα radiation (λ = 1.5418 Å). The degree of graphitization of the biomass-derived activated carbon was determined using Raman spectroscopy (HORIBA Jobin Yvon LabRAM ARAMIS, Kyoto, Japan) excited at 633 nm laser. To characterize the surface chemistry, FT-IR spectra were recorded on a Fourier Transform Infrared spectrometer (IR Affinity-1, Shimadzu, Kyoto, Japan). Carbon and oxygen contents of the samples were also determined by EDX using the ZEISS Neon 40 Energy-Dispersive X-ray analyser. The morphology of the activated carbon was studied using Scanning Electron Microscopy (SEM) (ZEISS Neon 40, Jena, Germany. A Quantachrome Autosorb iQ, Odelzhausen, Germany) was used to determine porous textural properties. Brunauer–Emmett–Teller (BET) surface area and pore structure was calculated by using ASiQwin’s multi-point BET-plot calculation.

3.3. Electrochemical Measurements and Electrode Preparation

To prepare the electrode the active material (RCS-AC), carbon black and PVDF binder were used in the weight ratio of 85:10:5 and then mixed, ground, and dispersed in N-methyl pyrrolidone (NMP) solution. The obtained mixture was coated on a graphite foil substrate (1 cm²) and subjected to drying in an oven at 80 °C for 12 h. The prepared activated carbon was tested for its performance as supercapacitor electrode material using a three-electrode system. A CHI660C electrochemical workstation (Austin, TX, USA) was used for the electrochemical analysis including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) testing and electrochemical impedance spectroscopy (EIS). In the above technique, activated carbon was used as the working electrode, Ag/AgCl as the reference electrode, and platinum wire as the counter electrode in different electrolytes viz. 3 M KOH, 1 M Na₂SO₄ and 1 M H₂SO₄. Various electrochemical tests were carried out viz. cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques in the frequency range from 100 kHz to 0.1 Hz and an amplitude of 5 mV. The specific capacitance of an electrode material was calculated using the equation given below [66]:

$$\mathrm{Cs}=\frac{\mathrm{I}\times \Delta \mathrm{t}}{\mathrm{m}\times \Delta \mathrm{v}}$$

(1)

where Cs represents the specific capacitance at constant current in F g⁻¹, mass (m), discharge time (t) and potential window (V). With the three-electrode system, the galvanostatic charge/discharge measurement was used to calculate the energy density (E) and power density (P) [67]:

$$\mathrm{E}=\frac{1}{2}\mathrm{Cs}{\left(\Delta \mathrm{V}\right)}^2$$

(2)

$$\mathrm{P}=\frac{\mathrm{E}}{\Delta \mathrm{t}}$$

(3)

where C_s, $\Delta$V and $\Delta$t are the specific capacitance, discharge cell voltage window and discharging time (s), respectively. For the two-electrode system, two symmetrical electrodes, each measuring 15 × 15 mm² were prepared following the same procedure. A glass fibre separator was used between the two electrodes and the electrode was soaked in 3 M KOH electrolyte. A two-electrode measurement was carried out to understand the practicality of the fabricated electrode and the electrochemical investigation was carried out in 3 M KOH in a cell type symmetric device. Initially CV tests were carried out to verify the operating voltage range of the fabricated device.

4. Conclusions

The work reports the development of activated carbon material for supercapacitors from Ricinus communis shell biomass. Porous RCS-derived activated carbon was synthesized through a combination of pre-carbonization and KOH activation. The surface morphology, crystalline structure, and vibrational response of the synthesized RCS-AC were investigated using standard material characterization techniques. The synthesized RCS derived activated carbon was evaluated for supercapacitors in different aqueous electrolytes. The electrochemical behaviour was investigated using cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy in various electrolytes viz. 3 M KOH, 1 M Na₂SO₄ and 1 M H₂SO₄. With a power density of 663.4 W hkg⁻¹ and a maximum energy density of 18.2 W hkg⁻¹, the RCS derived activated electrode material showed good electrochemical storage capabilities in 3 M KOH. At various current densities, the electrode was more efficient at charging and discharging in 3 M KOH electrolyte compared to 1 M Na₂SO₄ and 1 M H₂SO₄. The electrode delivered an excellent specific capacitance and long cyclic life in KOH. The observations from this study reveal that the as-prepared Ricinus communis shell derived activated carbon could make a significant influence in the production of low-cost efficient electrode material for supercapacitors.