Physically and Chemically Activated Carbon from Coffee Waste in High Performance Supercapacitor Electrodes

1. Introduction

Many reasons stand behind considering bio wastes as sources for carbon materials in modern devices. Firstly, recycling bio wastes prevents the widespread environmental contamination from random dumping. Secondly, the produced carbon is useful as a low cost and safe material in electronic devices. Thirdly, the strategy minimizes dependence on the hazardous fossil sources [1-3]. A promising application area is supercapacitor (SC) technology for energy storage. The SC is a device with two high specific surface area (SSA) electrodes, a separator and an electrolyte solution. Compared to conventional capacitors, SCs have much higher capacitance [4-7]. With potentially high power yields, SCs are considered in various electronic devices, and specifically electric cars, competitively with batteries [5, 8].

The Ragone plots, describe how SCs are at intermediary position between batteries and conventional capacitors[9]. The plots combine energy density and power density output values for various energy storage devices, including fuel cells, batteries, SCs together with conventional capacitors. For a given device, lowering the equivalent series resistance increases the energy density, while raising the voltage window increases the power density [10-12].

The need for higher-energy density storage devices continuously increases to meet expanding demands for modern electronic devices. As in batteries, electrochemical capacitors (ECs) attract special attention by virtue of their high storage density, low-temperature performance and multiple charge-discharge cycles. Unlike batteries and fuel cells, SCs involve no real chemical reactions, and function for prolonged life cycles [13], which is one virtue. SCs with both high energy density (like batteries) and high-power density (like capacitors) at the same time, are highly needed. SC research is thus active to achieve this feature.

In SCs, very high specific surface area (SSA) electrodes, with high conductivity, are used. The need for carbon materials in SCs is thus justified. Activated carbons (ACs) are conducting materials, thermally and chemically stable, and can have large SSAs with high micropore volumes. With high adsorption capacity and tunable pore structures [14], ACs are being considered in various energy-storage devices such supercapacitors [15, 16].

AC characteristics (surface, pore structure and adsorption capacity) are affected by preparation and activation methods. Two types of activation are widely known, physical activation and chemical activation. In chemical activation, impregnation with a suitable material is used. Examples of widely used chemical activating agents are: KOH, NaOH, K₂CO₃, ZnCl₂ or H₃PO₄. The activating agents, which are used in combination with mild physical processing, ensure pyrolytic decomposition of the precursor material at lower processing temperature, with less tar formation [17, 18]. In physical activation, the precursor material is partially gasified under inert atmosphere at a high temperature, and then activated with oxidizing gasses such as steam, air, carbon dioxide or mixed gases [18].

Chemical activation is a simple process, and is advantageous in terms of processing time, low temperature and high yield with high porosity. However, physical activation is still preferable at commercial scale, due to higher control on characteristics and simplicity [19].

High temperature pyrolysis can be coupled with poro-genic processes, using NaOH or KOH activation. Hazelnut shell hydrothermal carbonization (HTC) was studied by three various poro-genic methods namely simple-heat treatment, KOH activation and MgO templating. Better electrochemical characteristics (with higher SSA and effective micro-porosity) were reported in anode materials for energy storage [19, 20]. Chestnut shell based ACs, activated by KOH, were reported by Jiang et al. [21]. Effect of pre-carbonized stuff and KOH ratio was studied. SSA values of 1829.7 m² g⁻¹ were observed for samples activated with biochar to KOH at 1 to 3 ratios. The AC showed highest capacitance of 238.2 Fg⁻¹. KOH was also used as activating agent at 600–900 ºC for 2 h, and yielded N-doped AC with a large SSA 3401 m² g⁻¹ and a high specific capacitance 346 F g⁻¹. The energy density was 22.4 W h kg⁻¹ at 0.5 A g⁻¹ with high capacity stability (97.6% after 5000 cycles at 1 A g⁻¹) [22]

Chesnut shell-based AC, activated with ZnCl₂, exhibited SSA of up to 1987 m² g⁻¹ with a specific capacitance105.4 F g⁻¹ [23]. Hong et al. used K₂SO₄ to activate chestnut shell-based AC and reported an SSA 1412 m² g⁻¹ with a specific capacitance 265 F g⁻¹, but with only a low current density of 0.1 A g⁻¹[22].

KHCO₃ was described to activate AC, from chestnut shells, for supercapacitor electrodes [22]. Biochar activation with KHCO₃ yielded porous carbons with high SSA 2298 m² g⁻¹ and specific capacitance 387 F g⁻¹ (at 2 A g⁻¹) with high stability 98.68% after 10,000 charge-discharge cycles at 30 A g⁻¹ [22, 24]. N-doped porous carbon from edible Chinese water chestnut corms, was also reported [23].

Melamine also activated chestnut shell-based carbons [24]. The AC exhibited a current density 0.5 A g⁻¹, specific capacitance 402.8 F g⁻¹ and SSA 691.8 m² g⁻¹.

Depending on their regional agricultural resources and biowastes, researchers tend to focus their study attention. For this reason, in many regions people make their studies on chestnut, hazelnuts and others.

Other sources are globally spread, such as coffee wastes (CWs), since coffee is widely used in many places. It is reported that in early 2020s, more than 5 million tons of coffee was globally consumed [25]. More recently, the amounts of CW were reported to exceed 8 million tons [26]. ACs, with high surface areas and pore volumes from CW, were widely prepared and used for adsorption of various materials [25, 27-29].

CWs thus deserve to be studied. Few reports were published on CWs in supercapacitors. A flexible supercapacitor with a specific capacitance value of 139 F g^-1 at 0.5 A g^-1, a specific energy 12.5 W h kg⁻¹ and a specific power of 202 W kg⁻¹, was prepared from nitrogen 8%-doped AC. The supercapacitor functioned for more than 5,000 cycles with ~90% retained capacitance [30]. Chiu et al prepared AC in one combined process (physical and chemical activations) with various activating agents and found KOH as best choice to reach a specific capacitance 105.3 F g^-1 [31]. M Biegun, et.al, used the hydrothermal acidic hydrolysis, then the KOH activation, at 800 ºC, to activate coffee waste, and obtained a high specific surface area of ~2900 m² g^-1. Using a liquid ionic electrolyte, the observed specific capacitance was 178 F g^-1 at 20 ºC and 50 A g^-1 [32]. Omkar Khadka et al. recently prepared supercapacitor electrodes from coffee wastes, first by chemical activation with ZnCl₂ 1:1 ratio, then by physical activation at 700 ºC, and observed a specific capacitance of 113.8 F g^-1 at 1 A g^-1 [33]. In a recent review, Davidraj et al reported the possibility of food waste materials to produce electrodes for energy storage devices [34]. The report listed a number of papers using CWs in supercapacitor electrodes. In another more recent review, Pagett et al reported on CWs in supercapacitor electrodes [26]. With the exception of one 2008 report that showed high specific capacitance [35], more recent papers showed specific capacitance values of only less than 160 F g^-1 [26].

From literature, it can be noted that the AC prepared from coffee wastes showed lower performance than other ACs prepared from other sources such as chestnuts and hazelnuts. Moreover, the study on the coffee based ACs was not comprehensive, as it did not include important characteristics such as surface area or pore size that critically affects the electrode performance.

As CW is a promising source of AC, more study is needed to increase the CW based supercapacitor performance. The present study will assess the added value of using a chemical activating agent to the physical activation of CWs. Instead of using the highly corrosive KOH activating agent, reported earlier, the environmentally friendly and low cost ZnCl₂ is used here. The choice for ZnCl₂ is based on its acidic nature and efficiency as described in earlier reports where it yielded ACs with high surface areas [36]. ZnCl₂ was described in activating coffee waste for supercapacitor electrodes where specific capacitance was 134 F g^-1 at 1 A g^-1 using organic solvents, with equivalent series resistance more than 20 Ω [37]. Effect of activation method on various AC characteristics, such as specific surface areas, pore sizes and morphology, will be studied. The activating agent should chemically drill pores through the carbon matrix and increase its porosity. Various methods, such as XRD, SEM, BET and Raman spectra, will be used to test this effect. The AC suitability for supercapacitor electrodes, will be assessed, taking into consideration specific capacitance, power density and stability. The goal is to produce supercapacitor, with stable, porous and high surface area electrodes, from coffee-waste based AC, with and higher performance (in terms of specific capacitance and power output) than those described in literature. The basic assumption is that these features are achievable by adding a suitable chemical activating agent during CW pyrolysis. With a targeted specific capacitance value above 250 F g^-1, the results observed here were not precedented to our knowledge.

2. Materials and Methods

2.2. Starting Materials

Organic solvents, such as acetone, acids and bases, were purchased Sigma-Aldrich in pure forms. Coffee wastes was taken from freshly used Turkish coffee drinks (Arabica type). The coffee was processed according to Turkish coffee drink protocols, with fine grinding, and boiled. The CW was isolated and stored in a refrigerator for processing. The glass fiber separator (-M 5V5) has been purchased from Alter-Lab.

2.2. Equipment

A hydraulic press (Shimadzu), equipped with pressure gauge, was used to make AC disks. The sample was pressed at 7 tons. The value was based on earlier recommendation [36].

A standard Teflon Swagelok cell, with 10 mm in diameter with stainless steel electrodes, was used for supercapacitor testing. The cell was connected to the potentiostat/Galvanostat (VoltaLabPGZ402). The working electrode was connected to one side, while the counter and the reference electrodes were connected together. The internal reference cell was used as reference. The VoltaMaster 4 software, was used for electrochemical measurements. AC morphology and surface structure were examined by scanning electron microscopy (SEM) (Hitachi- S-4800) on a Field-Emission machine. Transmission Electron Microscopy (TEM) was measured on a JEM-ARM200F system. Brunauer-Emmett-Teller (BET) was measured on a Micromeritics-3Flex 3500 equipment. X-Ray diffraction was measured on A PAN-alytical X’Pert-PRO X-Ray diffraction equipment, using a Cu Kα as a source. X-ray photoelectron spectroscopy was measured on a Thermo Scientific machine model K-alpha+. Raman spectra were measured on an OlympusBX41M equipment. SEM, XRD, TEM, BET, XPS and Raman spectra were all measured in the labs of the KIER, Daejeon, Korea.

2.3. AC Preparation

The CW material was activated by two methods, namely the physical and the chemical activation.

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Physical activation (AC_Phys)

Pre-dried Arabic coffee waste (100 g) was rinsed with hot distilled water (400 mL), then with cold distilled water (400 mL), and finally rinsed with ethanol. This was to remove soluble coffee ingredients. The remaining solid waste was left to dry at room temperature for 24 h, then heated in an oven at 70 ºC for 12 h. The waste was then heated at 800 ºC for 2 h under nitrogen atmosphere. The resulting AC_Phys was stored in a desiccator for further use. The net mass for the AC_Phys was 75.3 g.

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Chemical-physical activation (AC_Phys-Chem)

Pre-dried Arabica coffee waste (100 g) was washed with hot distilled water (400 mL) and then with cold distilled water (400 mL), and finally rinsed with ethanol. The coffee waste was left to dry at room temperature for 24 h, then heated in an oven at 70 ºC for 12 h. The waste was then magnetically stirred with ZnCl₂(s) (in a 1:1 mass ratio) at 60 ºC for 24 h. The solid was dried at room temperature for 24 h and then in an oven at 70 ºC for 7 h. The solid was heated at 800 ºC for 2 h. The resulting AC_Phys-Chem was cooled and stored in a desiccator for further use. The net mass for AC_Phys-Chem was 47.6 g.

2.4. AC Disc Electrode Preparation

A sample of activated carbon was ball milled to fine powder with Agat mortar for 10 min. The AC was mixed with PVDF polymer in a 93:7% ratio, and was magnetically stirred with acetone (15 mL). The mixture container was then sonicated in a bath at 35 ºC for 20 min. The solid was separated and dried in an oven at 70 ºC for 2 h. The AC powder (0.03 g) was then used to make electrode discs, which were pressed in a mold under hydraulic pressure of 7 tons. The AC disc diameter was 10 mm and the disc thickness was ~0.19 mm.

2.5. Electrochemical Methods

Electrochemical characteristics of activated carbon were analyzed by using various methods, namely cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). A Swagelok cell was used in electrochemical studies. One disc was loaded as a coating layer on one stainless steel electrode. A fiberglass disk separator (with diameter of 10 mm) was immersed in KOH electrolytic solution (6 M) for 5 min. The immersed fiberglass separator was then carefully taken by tongs and firmly stacked on the AC electrode, as a coating. The other AC disc was stacked onto the separator. Then the cell was then firmly assembled to prevent gaps and bubbles between electrodes, and to disallow any cell component movements. The assembled Sawgelok cell was connected to the PGZ 402 Potentiostat/Galvanostat, using the 2-terminal mode. The voltaMaster 4 software was utilized in data collections. The CV measurements were performed for each electrode by scanning the potential from 0.0 to 1.0 V at different scan rates

The GCD study was performed at various current densities. The current density was switched between positive and negative values, for charging and discharging, respectively. The results were acquired and analyzed using Origin software.

3. Results

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AC disc characterization AC Morphology

Figure 1 shows SEM micrographs for both AC material in pressed disc forms. The AC_Phys and AC_Phys-Chem materials exhibited different surface morphologies. The AC_Phys-Chem is more porous than the AC_Phys. This is due to the chemical activation which further affects the AC surface, in addition to physical activation, as reported earlier [38, 39]. As stated above, the acidic ZnCl₂ activating agent interacts with the carbon matrix and makes it more porous.

In Figure 2, TEM micrographs for both AC materials are shown. In each case, white dots can be observed indicating pores. The AC_Phys-Chem was more porous, with higher homogeneity, than AC_Phys.

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AC porosity

Porosity for the two AC materials was measured using the BET method. Porosity involves specific surface area (SSA), por size distribution (PSD) and total specific pore volume. Adsorption/desorption isotherms were measured for both AC materials in pressed disc forms. Figure 3 summarizes the results. The Figure shows that at higher relative pressure, the amount of gas that entered the pores increased until all pores were occupied at equilibrium. Naturally, the equilibrium was pushed to higher adsorption with higher relative pressure.

To understand the effect of pore size distribution, in electrodes on the specific capacitance, BET analyses were performed, Figure 3. The isotherms involved small hysteresis loops from high to low pressure ranges. This indicated that the electrodes had meso and macro porous structures. According to IUPAC classification, the sample isotherm can be classified as type I, II and IV [40]. Higher nitrogen adsorption, even at very low relative pressures (P/P^º<0.01), for AC_Phys-Chem, compared to AC_Phys indicated the existence of micropores more in the former than in the latter. The AC_Phys-Chem exhibited higher specific surface area and more micropores than the AC_Phys, as described in Table 1. As the minimal pore sizes for AC_Phys-Chem (0.8 nm) and AC_Phys (1.8 nm) are larger than the ionic sizes for both electrolyte ions (K⁺_(hydrated) 0.4 nm, OH^-_(hydrated) 0.38 nm), both activated carbons readily adsorb the ions and may behave as supercapacitors. Since the AC_Phys-Chem had more total and micropores than AC_Phys, with higher specific surface area, vide infra, the former carbon should have higher specific capacitance and lower equivalent series resistance (ERS) than the latter.

The specific surface area was determined by multiple point Brunauer-Emmett-Teller (BET) method in the regions of the isotherms, which are limited by the range of relative pressure P/P^º = 0.0–0.3 as seen in Figure 4. The adsorption volume showed that BET surface areas for AC_Phys, and AC_Phys-Chem were 458, 830 m²/g respectively. The total volume of pores (V_total, cm³/g) was calculated by the number of adsorbed nitrogen at P/Pº ≈ 0.9932. All results are summarized in Table 1.

All above results indicated that AC_Phys-Chem high porosity and suitability for super capacitor applications. The increased porosity is due to using the activating agent.

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Pore size distribution by the BJH method

The Brunauer-Joyner-Halenda (BJH) method is very useful to analyze mesopores and macropores in a diameter wide range (17-3000 Ả). The pore size distributions in AC_Phys and AC_Phys-Chem are summarized in Figure 5. The pore size distributions for the present electrodes were in the micro- , meso- and macropore scales. The Figure shows that for the present electrodes, the domain of pore sizes were centered in the ranges of 1.8-3.3 nm for AC_Phys, and 0.8-2.7 nm for AC_Phys-Chem. Correlation between BET surface area and pore sizes, with specific capacitance can be understood using Equation (1):

C = εA/d

(1)

where C denotes specific capacitance, ε electrolyte-dielectric constant, A specific surface area that is accessible to the ions, and d is spacing between the ions and the electrode pore surface (nm). Based on this Equation, two methods can be followed to improve charge storage for supercapacitors:

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Increasing specific-surface area

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Reducing spacing between ions and electrode surfaces.

The large surface area and high amounts of meso and micro pores together yield high transport of charges leading to high specific capacitance.

The results showed that, for both present ACs, higher adsorbed gas occurred in the smaller pore size. Small pore sizes were more dominant in the AC_Phys-Chem, as observed from pore-size distribution (differential pore volume), Figure 5. The observed AC_Phys-Chem material peaks were distributed within the pore size range 1.8 – 3.3 nm, with the maximum adsorption at 3.3 nm. Thus, mesopores were dominant in the AC_Phys-Chem, with higher adsorption at smaller diameter mesopores. The results justified the high specific capacitance for AC_Phys-Chem, as summarized in Table 1.

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Pore size distribution by the (HJ) t-plot

The volume of micropores and the values of micropore surface areas of (S_micro, m²/g) were investigated by the t-Plot Harkins and Jura. The Harkins-Jura t-plot method can analyze micropores with surface areas. The amount of adsorbed N₂, for a given P/P₀ range, is plotted against pore thickness (t). The micropore volumes and surface areas (S_micro, m²/g) were investigated by the t-Plot Harkins and Jura method. The results are summarized in Figure 6 and Table 1. From Figure 6, the present electrodes involved high micro-surface areas of 557 and 385 m²/g for AC_Phys-Chem and AC_Phys electrodes, respectively. This justified the high specific capacitance for AC_Phys-Chem as described below.

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X-Ray patterns

The XRD pattern is presented in Figure 7. Both AC_Phys and AC_Phys-Chem exhibited a reflection at 2ɵ=~24º. The refection is consistent with earlier literature [41]. However, as both materials are amorphous, only broad and low reflections for the (002) signals were observed. The reflection for AC_Phys-Chem was sharper than for AC_Phys, which indicates that the former is more crystalline. This means that the former has more carbon content than the latter. Another broad reflection in the range 2ɵ=40-50º can be observed more obviously for AC_Phys-Chem than for AC_Phys. The AC_Phys-Chem thus showed more resemblance to earlier chemically activated carbons [42]. The XRD patterns were consistent with the AC_Phys-Chem having higher porosity.

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Elemental analysis

For both AC_Phys and AC_Phys-Chem, elemental analyses were performed by the EDS, as described in supplementary Figures S1(a) and (b). The results are summarized in Table 2.

For AC_Phys-Chem, the atom percentage for carbon was 93%, while for AC_phys it was only 63.20%. So, more carbon appears in AC_Phys-Phem, which is a virtue for this material. The EDS results were consistent with the XRD results, in the sense that AC_Phys-Chem material had higher graphitization.

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Raman spectra

Figure 8 below summarizes the Raman spectra for AC_phys and AC_phys-chem. The AC_Phys-Chem exhibited higher homogeneity (Figure 8b) than AC_Phys (Figure 8a). Both ACs showed two sharp peaks at 1360 cm^-1 (D peak) and 1590 (G peak). Comparison between the two spectra indicated that D peak intensity (I_D) to the G peak intensity (I_G) is 0.7 for AC_Phys-Chem compared to 0.8 for AC_Phys. A lower I_D/I_G ratio means higher graphitization in the AC, as reported in literature [43,44]. Therefore, the AC_Phys-Chem here had more graphitization than the AC_Phys. This result corroborates the XRD and EDS results discussed above, in the sense that the former has more C content than the latter.

All in all, the AC_Phys-Chem showed superior characteristics compared to AC_Phys. Based on that, the present electrochemical study here was restricted to only AC_Phys-Chem electrode only, in comparison with earlier literature.

3.2. Supercapacitor Testing

As described above, the AC_Phys-Chem electrode was singled out for further electrochemical study, unless otherwise stated. Cyclic voltammetry (CV), Galvanostatic charge/discharge behaviors (GCD) and electrochemical impedance (EIS) behaviors were investigated for the AC_Phys-Chem, using KOH (6.0 M).

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Cyclic voltammetry (CV)

CV is the most fitted method to recognize the capacitive characterization. Cyclic voltammograms were measured in the range of 0.0 to 1.0 V at different scan rates (5, 10, 20, 50 and 100 mV/s) for the AC_Phys-Chem electrode, as shown in Figure 9. Figure 9(a) shows the CVs measured at various scan rates and Figure 9(b) shows CV at low scan rate (5 mV/s) only. At all scan rates, the electrode exhibited symmetrical CV curves. At all scan rate, also, the CV curves exhibited semi-rectangular shapes without any peaks and with high reversible charge-discharge process. The electrode thus behaved as an electronic double layer capacitor (EDLC) [45-49]. The specific capacitance was calculated from the CVs using Equation (2).

$$C_s={\displaystyle \frac{{2*(q}_a+\left|q_c\right|)}{m∆V}}$$

(2)

where C_s is the specific capacitance in F g^-1, m is the mass of the active material in g, ∆V is the voltage window in V, q_a and q_c are the anodic and cathodic charges in C, respectively.

The high reversibility for AC_Phys-Chem electrode at various scan rates, and the response (in charging/discharging) were due to high conductivity, as described earlier [50-52]. Based on Figure 9(a) and (b), the specific capacitance values for AC_Phys-Chem were plotted vs. scan rates. Figure 9(c) indicates that specific capacitance decreased at higher scan rate. Table 3 summarizes the results. The Table shows that the present AC_Phys-Chem electrode was superior to earlier coffee waste-based electrodes in terms of specific capacitance value.

Typically, high specific capacitance is associated with low scan rate. In AC_Phys-Chem the same behaviour was observed. This is due to ions having enough time to penetrate the micropores (with less than 2 nm). At higher scan rates, only larger pores mesopores (2-50 nm) may contribute to capacitance. This is a result of the electrolyte diffusion rate difference in the pores with different sizes. It may also be due to network connections between larger and smaller pores [52-54].

The high specific capacitance of AC_Phys-Chem, was partly due to its higher surface area and microporous structure. This increases accessible areas for electrolyte ions storage within the relatively smaller pores. Other factors, such as conductivity, may have influence, as described below.

The increased current density peak with higher scan rate was due to higher ion mobility. This is associated with high concentrations that diffuse in close proximity to electrode surfaces [55, 56].

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Galvanostatic charge/discharge (GCD)

The supercapacitor performance can be further tested and verified by GCD method, Figure 10. The AC_Phys-Chem electrode was charged and discharged, galvostatically here, in the potential range (0.0-1.0 V) at various constant discharge current densities of (0.33, 0.67, 1.67, 3.3, 5.0 A g^-1). Figure 10(a) summarizes the potential (V) vs. time (s) plots, showing the charge and discharge processes. A potential drop (V_IR) was observed in the discharge after the maximum potential value was reached. From Figure 10(a), the potential drops occurred in the discharge processes. The potential drop values were: 0.064, 0.134, 0.297, 0.66 and 0.76 V at the above current density values, respectively. The specific capacitance (C_s) values from the charge discharge curves were estimated using Equation 3 [58], as summarized in Table 4.

$$C_s={\displaystyle \raisebox{1ex}{$\left(2\times I\right)$}\!\left/ \!\raisebox{-1ex}{$\left[\left(dV/dt\right)\times M\right]$}\right.}$$

(3)

where C_s is the specific capacitance, dv/dt is the slope of the linear discharge curve, I is the discharge current in A, and M is the total electrode mass (in g).

The values for equivalent series resistance (ESR), from GCD method, described in Table 4, can be evaluated by Equation (4). The ESR values were within a narrow domain, which confirms the electrode stability at different densities. This is another superior feature for the electrode.

ESR = V_IR/2I

(4)

It can be seen that each charge and discharge processes is nearly symmetric, which illustrates excellent electrochemical reversibility of the electrode. The small V_IR drop in the discharging curves of AC_Phys-Chem implies a small equivalent series resistance, which is essential to power characteristics of supercapacitors. The low V_IR drop of AC_Phys-Chem is due to its high conductivity, as achieved by ZnCl₂ activation. The specific capacitance of AC_Phys-Chem decreases from 150 F g^-1 (at current density 0.33 Ag^-1) to 39 Fg^-1 (at 5 Ag^-1). The specific capacitance for AC_Phys-Chem decreased with increased current density as shown in Figure 10(b), which common in supercapacitors. This mainly caused by electrolyte ion diffusion limitations.

As the current increases the time needed for charging and discharging processes decreases due to higher ion mobility. This leads to lower C_s, as expected, in congruence with literature [45, 46, 59]. Figure 10(a) shows that as the current density increases, shorter time for charging and discharging is needed. Thus, the voltage drop increases at higher current density, corresponding to the resistance exhibited by the supercapacitor [58, 60].

The specific power and specific energy have been calculated using Equations (5) and (6) [61, 62].

$$E={\displaystyle \raisebox{1ex}{$\left[C_s\times {\left({∆V}_{op}\right)}^2\right]$}\!\left/ \!\raisebox{-1ex}{$\left(2\times 3.6\right)$}\right.}$$

(5)

$$P={\displaystyle \raisebox{1ex}{$E\times 3600$}\!\left/ \!\raisebox{-1ex}{$∆t$}\right.}$$

(6)

where C_s is specific capacitance (Fg⁻¹) extracted from the discharge curve, ΔV_op is operating voltage range (V) defined asΔV_op = ΔV_appl−V_IR, ΔV_appl (= 1 V) is applied voltage used during charge/discharge process. Δt is discharge time (s) andV_IRis voltage drop observed in the discharge plot caused by immediate impact of charge-to-discharge process transition.

Figure 10(c) shows the Ragone plot for electrode. The maximum specific energy is 18.3 Wh/kg at the specific power 360 W/kg at current density 0.33 A/g, while the maximum specific power is 1444 W/kg at specific energy 6.4 Wh/kg at current density 1.66 A/g. These values are higher than most recent literature [33] results for coffee waste electrodes, which showed specific energy 4.78 Wh/kg and specific power 137 W/kg at 1-5 A/g current density. The results confirm the potential value for the present electrode in future application, as it combines high specific energy and specific power together.

The electrode stability on recycling is depicted in Figure 10(d). The plot has been constructed at current density 0.33 A/g for 5,000 cycles, and shows a slight decrease in specific capacitance from 149 to 148 F g^-1. The results depict the high electrode stability upon charge-discharge cycling, which is an important feature in supercapacitor technology.

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Electrochemical impedance spectra (EIS)

To further confirm the above results, EIS was studied, Figure 11. Nyquist plots were constructed for the electrode within the frequency range 0.01 Hz - 10 kHz, Figure 11(a). The ESR values indicate looseness inside the supercapacitor. The x-axis intercept, at the higher frequency, represented the combined resistance (Rs= 0.48 Ω) involving electrode material inherent resistance, ionic resistance in electrolyte solution and the contact resistance between the current collector and the electrode. The semi-circle loop described the electrode conductivity together with its charge-transfer resistance (R_ct) [63]. The measured R_ct here was 0.2 Ω). Higher electrode conductivity normally indicates smaller semicircle loop. The calculated ESR (R_s + R_CT) here was 0.6 Ω. At low frequencies, the vertical line describes the electrode capacitive behaviors. In the present electrode, the Nyquist plot showed almost a straight line parallel with the imaginary axis, indicating exactly polarized systems. Deviation from the vertical line, at lower frequencies, to smaller slopes corresponds to higher ionic-diffusion resistance contribution.

The time constant divides between the supercapacitor capacitive and resistive regions. This can be determined by taking the reciprocal of the maximum frequency f₀ as (t₀ = 1/f₀). Generally, a low time constant value is a higher-performance indicator, meaning a supercapacitor delivering higher powers in short times. The knee frequency is 0.08 Hz and the time constant is 12.5 s for electrode. Time constant is affected by various factors, including the material conductivity, the electrolyte and the electrode thickness [64]. When the electrode thickness and the electrolyte are kept the same in the supercapacitors, then time constant is influenced only the electrode materials conductivity.

Specific capacitance values, for the supercapacitor, were calculated from impedance analysis using the impedance imaginary component by Equation (7) [60, 65].

$$C_s=4\times \left({\displaystyle \raisebox{1ex}{$-1$}\!\left/ \!\raisebox{-1ex}{$2\pi f{z}^{\text{'}\text{'}}M$}\right.}\right)$$

(7)

where f is frequency (in Hz), z’’ is impedance imaginary component (in Ω), and M is the mass of electrode mass (in g)

Figure 11(b) shows how specific capacitance varies with frequency. The obtained specific capacitance for electrode 179 F g^-1 at frequency of 0.01 Hz.

As described above, the specific capacitance for the AC_Phys-Chem was calculated by three methods. The CV study yielded a specific capacitance value 261 F g^-1, at scan rate 5 mV/s, that higher than literature values for coffee waste electrodes. The GCD study yielded specific capacitance 150 F g^-1 at discharge current 0.01 A, and ESR values in the range 2.53-3.2 Ω at discharge currents ranging 0.15 – 0.01 A. The EIS study showed specific capacitance of 179 F g^-1 at frequency 0.01 Hz with low resistance. All values are superior to earlier reported values, as described above.

All in all, the results indicate that the AC_Phys-Chem electrode exhibited favorable physical characteristics leading to super capacitor performance. As described above, the chemical activating agent improved all characteristics of the activated carbon in many ways. Graphitization and C content were improved. The solid porosity was improved and yielded higher ion uptake. The relative crystallinity was improved, which increased the electrical conductivity within the activated carbon. All these characteristics, enabled the present AC_Phys-Chem electrode to exhibit high specific capacity and output power at the same time. The present electrode here exceeds earlier coffee waste-based electrodes, physically and chemically activated with various materials, both in specific capacitance and in power output density. By combining high supercapacitance and power output together, the present electrode is superior to many electrodes prepared from other biowastes and commercial carbon. This feature makes the electrode a potential candidate for future commercial supercapacitors. Therefore, it is necessary to examine new chemical activating agents, such as ZnCl₂, in producing activated carbons from coffee and other biowastes. Research is active in these laboratories to further improve coffee waste-based ACs by other additional methods.

4. Conclusions

Two activated carbons were prepared from coffee wastes, one by physical method (AC_Phys) with no added activators, and the other with ZnCl₂ addition (AC_Phys-Chem). The latter exhibited super characteristics in terms of specific surface area, porosity, carbon content, graphitization and surface morphology. For these reasons, the AC_Phys-Chem electrode was singled in assessment as a supercapacitor electrode. The new electrode exhibited high specific capacitance values that exceeded literature coffee waste-electrodes. The electrode also exhibited stability with power output values higher than earlier electrodes. With these combined features, the new electrode competes with other reported electrodes produced from other biowastes that showed high performance in literature. Using new types of chemical activators for coffee waste-based electrodes, and using other preparation methods, is recommended in future research.