Introduction

Inflated energy demand is presently a significant crisis in our human society in the twenty first century. Industrial growth, population or pollution are main sources which are affecting our diminishing resources. Our first step towards saving the fossil fuels should be to conserve as well as preserve the energy [1]. Energy may only be transformed from one form to another, not generated or destroyed, according to the principle of energy conservation. In the same way energy storage is a very demanding research topic among scientists these days which includes batteries, supercapacitors, fuel cells etc. The above said devices used in electric vehicles and portable electronic devices, works on the non-faradic process and energy is converted from electrical form and can be stored as a charge to get energy according to our resource needs. Energy density and power density plays a critical role in differentiating batteries, capacitors and supercapacitors as it is directly related to their charging–discharging ability. Supercapacitor is believed to replace batteries in coming future because of its various advantages like fast charge–discharge cycle and long-life cycle capability but still there is a long way to go. Electric double-layer capacitors (EDLC), pseudo capacitors, and hybrid supercapacitors are the three different categories of supercapacitors. In the present work we are mainly focused on fabricating high capacitive EDLCs using solid polymer electrolytes because it provides high energy density than other supercapacitors which uses aqueous electrolytes [2,3,4,5].

In EDLCs, electrolytes are helpful in providing high ion conduction between the symmetrical carbon-based electrodes, they are basically of two types: aqueous and solid polymer electrolytes. Solid polymer electrolytes are widely used in energy devices because of its various advantages as compared to the aqueous electrolytes such as volatility, flammable issues, leakage and short circuit issues. The only drawback is low ionic conductivity which is resolved by adding different plasticizers, alkaline metal salts and use of low viscosity Ionic liquid (IL) is the most common one. The IL is a class of salts with a low melting point (below 1000 °C), a liquid state at ambient temperature, and a lot of ions (both cations and anions) that serve to boost ionic conductivity. Among natural, synthetic and biodegradable polymers, various efforts have been made to increase the conductivity of bio-derived polymers as it is eco-friendly and it provides high flexibility, stability and fast ion conduction rate [6,7,8]. The other main component of EDLCs are symmetric activated porous carbon (APC) electrode materials which are basically the exotic carbonaceous materials having dominant properties like high surface area, high ion adsorption capability, high conductivity, high dimensional stability, low weight and physical strength. There are several ways to choose and prepare APC materials, out of which pyrolysis of particular carbon precursor using physical activation, chemical activation and hydrothermal activation are widely known. These activation processes are performed at very high temperature (900 °C) under inert environment to avoid oxidation reaction and achieving highly conducting porous carbon material. Bio-mass host-derived APCs material such as waste plastic, human hair, cow dung, sewage sludge, pistachio shell, peanut shell has already been reported and it hence proves the excellent electrochemical performance of supercapacitor [9, 10]. Polymer as a carbon host material has been studied widely and it was seen that the right choice to choose the particular monomer and converting it in porous carbon by tuning the pore size, morphological and various catalytic properties is little bit challenging but this problem are been resolved and it can be seen that the synthesized APCs possess high chemical and thermal stability [11]. Also, the research on biopolymers as a carbon host material for electrode material is in huge demand as is easily available, eco-friendly in nature, cost effective and most importantly it provides high energy storage performance of about 184.8 Fgm−1 as previously reported by our group [12].

In this research work, we have used methyl cellulose as a carbon precursor for electrode material and same precursor for solid polymer electrolytes as it has not been reported elsewhere. The two-step pyrolysis process has been performed using H3PO4 as an activating agent by varying some parameters like temperature and different activation ratios. The solution cast technique has been used for the synthesis of methyl cellulose-derived solid polymer electrolyte with EMImTCM as IL as reported previously by our group [2]. The electrochemical performance of fabricated supercapacitor gives promising results and further compared with previously reported results of other research groups.

Experimental section

Materials used

Methyl cellulose (C8H16O8) has been used as a starting precursor for the synthesis of the biodegradable porous carbon and polymer electrolyte film, and it was purchased from Sigma-Aldrich. Other materials purchased from Sigma-Aldrich, USA, include orthophosphoric acid (H3PO4), hydrochloric acid (HCl), PVDF-HFP (polyvinylidene fluoride co-hexafluoropropylene), acetone.

Synthesis of activated porous carbon from methyl cellulose biopolymer

The highly activated porous carbon was synthesized from the methylcellulose biopolymer (C8H16O8) which undergoes chemical activation and pyrolysis carbonization. Initially, we had taken the methylcellulose biopolymer as the precursor for the carbon and mixed it well with 40 wt.% of H2SO4 solution to form a homogenous mixture in which the precursor (gm) to the 40 wt.% H2SO4 solution (mL) was in the ratio of 1:10. The resultant precursor solution was covered with aluminum foil and then kept in a magnetic stirrer for 24 h at 120 °C. The slurry was collected, filtered, and washed with deionized water until the pH of the slurry product was neutral after the mixture had been stirred for 24 h. The surplus solvent from the sample was then dried off by placing the thoroughly cleaned product in a hot air oven for around 24 h at 120 °C. Now, this pre-activated carbon is impregnated with 85 wt.% of orthophosphoric acid (H3PO4) in which the mass ratio of the pre-activated carbon (gm) to the H3PO4 (mL) was kept as 1:6. After this process, we now place the resultant material in a tube furnace and pyrolyze it in the presence of a nitrogen (N) atmosphere at 850 °C and hold the temperature for approximately 30 min. The flow of nitrogen was made steady to make sure the sample was burnt in the presence of nitrogen atmosphere. After cooling down the temperature of the tube furnace to room temperature the obtained product was then washed several times so as to get rid of any other impurities and hence the sample was neutralized. The neutralized sample was then placed in a hot air oven for another 14 h and finally taken out sample was named as highly activated porous carbon synthesized from methylcellulose biopolymer.

Fabrication of EDLC

The fabrication of the EDLC requires three main components namely the electrodes, the electrolyte, and the active material. Here for the electrolyte portion, we have used the same material that is methylcellulose incorporated with 60 wt.% of 1-ethyl-3-methylimidazolium tricyanomethanide ionic liquid (IL), showing maximum conductivity of 1.93 × 10−2 Scm−1 which was reported elsewhere by our colleagues [2].

For fabricating symmetric EDLC electrodes or current collectors, graphite sheets were used and were cut precisely with a dimension of 1 cm × 1 cm. The porous carbon that was previously created was employed as the active component in the electrode of the EDLC. The porous carbon was combined with the PVDF-HFP polymer, which acts as a binder, and the resulting paste was then evenly coated in the graphitic electrodes. Here, the ratio of porous carbon to binder which was dissolved in acetone was taken to be 90:10. Each electrode was precisely coated with 1 mg of porous carbon, which was then dried for an hour at 80–90 °C in a vacuum oven. Now the coated electrodes were taken out from the oven and weigh two uniformly coated electrodes so as to confirm that both the electrode are of the same weight after coating for fulfilling the condition for a symmetrical supercapacitor. In the next step, the polymer electrolyte film was also cut in a dimension of 1 cm × 1 cm, and this cut piece of electrolyte was sandwiched in between two symmetrically coated porous carbon electrodes and as a result, symmetrical EDLC was fabricated. The constructed EDLC was further examined in the CH electrochemical workstation for the cyclovoltammetry and low-frequency impedance that determine the performance of the resulting cell, as mentioned in the findings and discussion section.

Dye-sensitized solar cell fabrication (DSSC)

Between the working and counter-electrodes, IL-doped methylcellulose biopolymer electrolyte was utilized to construct EDLC. To compare a data from well-established platinum-based counter-electrode and methylcellulose-based counter-electrode (cost effective) two DSSC cells were fabricated. Conducting FTO glasses measuring 1.5 × 1.5 cm2 were first repeatedly cleaned in an ultrasonic bath with DD water, ethanol, and acetone. After this, on working electrode coating of blocking layer was done using spin coater and was annealed at 500 °C for half an hour. On continuation of this, doctor blade method was used to coat a porous TiO2 paste (~ 10 µm) on working electrode over blocking layer, and annealed at 500 °C for half an hour.

Likewise, on the same way other FTO glass was used in a counter-electrode for DSSC fabrication. For that, two materials were used in counter-electrode, i.e., chloroplast acid coated on FTO glass and methylcellulose-derived porous carbon-coated on FTO glass and were annealed at 500 °C for half an hour. The performance of both cells was assessed under one solar condition with the working electrode immersed in (N3) dye for 6–8 h and the IL-incorporated methylcellulose biopolymer electrolyte sandwiched between the working and counter-electrodes.

Results and discussion

Porous carbon characterization

Field emission scanning electron microscopy (FESEM)

FESEM characterization method was established to determine the surface morphology and microstructure of the synthesized porous carbon. In Fig. 1, the surface morphology of the porous carbon can be seen at different magnifications. Figure 1a depicts that the porous material was highly dispersed and bears a spherical structure with many graphitic layers in between. Figure 1b confirms the active pores over the region with three-dimensional spherical structures of the porous material and from Fig. 1c, d, it can be concluded that the active porous materials were in the range of 1–2 μm in size. Also, we can see there is a lot of aggregation in the material which is due to the effect of activating agent we used. The activating agent, orthophosphoric acid (H3PO4) interacts with the porous carbon and forms phosphate and polyphosphate bridges. During the dilation process, the pore structures were discovered to include phosphate and polyphosphate groups [13].

Fig. 1
figure 1

a–d Low- and high-magnification FESEM images of synthesized porous carbon

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X-ray diffraction (XRD)

Figure 2 displays the synthesized porous carbon's XRD pattern, which was obtained from methylcellulose. The pattern shows the numerous dominating high-intensity peaks at 2θ of 15.76°, 18.25°, 20.49°, 24.40°, 25.84°, 29.13°, 30.60° and 33.25° narrow broadening natures on comparing with amorphous carbon XRD pattern which indicates the larger defects and more graphitization degree [14]. The addition of H3PO4 on carbon makes a crystalline nature which perks up the conductivity nature of prepared porous carbon.

Fig. 2
figure 2

X-ray diffraction pattern

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Raman spectroscopy

The synthesized porous carbon was then characterized with Raman spectroscopy techniques to confirm the purity and uniformity of the material before being used as an active material for EDLC fabrication. As shown in Fig. 3 the porous carbon shows its 1st peak which resembles the D band at 1328 cm−1 and the 2nd peak which resembles the G band at 1596 cm−1. Here the D band confirms the presence of the sp3 hybridized carbon atoms and on the other hand the G band confirms the presence of sp2 hybridized carbon atoms. During the chemical activation and pyrolysis of the carbon material some of the sp2 hybridized carbon atoms gets transformed into sp3 hybridized carbon atoms due to the presence of various functional groups for which the D band of the Raman spectroscopy depicts the defects or distorted geometry of some of the sp2 hybridized carbon atoms whereas the G band resembles the graphitic character or the degree of graphitization in the sp2 hybridized carbon atoms [12].

Fig. 3
figure 3

Raman spectroscopy of the porous carbon

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Brunauer–Emmett–Teller (BET)

Figure 4 displays the porous carbon's N2 adsorption/desorption Brunauer–Emmett–Teller (BET) isothermal curve from H3PO4. The overall surface area of the synthetic porous carbon is 1394.544 m2/gm. Micropores filled at relative pressures up to 0.1 had mesoporous 0.754843 cm3/gm pore volumes. The average size of the micropores, taking the entire BET area as a reference, would be 0.837067 nm. Though, on reaching higher pressure up to 1.0, indicating macropores in less number present in matrix carbon. The capillary condensation can be seen in the isotherm plot which confirms that there is formation of mesopores in the carbon matrix [15]. It is also confirmed from the isothermal graph of carbon derived from methyl cellulose activated with H3PO4 created pores in the carbon matrix of methylcellulose which is the most dependable for the high value of specific capacitance of developed EDLC.

Fig. 4
figure 4

BET N2 adsorption/desorption isotherm profile

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Energy-dispersive X-ray (EDX)

To confirm the chemical composition that is functionalized in the synthesized carbon material, energy-dispersive X-ray (EDX) characterization has been performed on the material. As shown in Fig. 5, the EDX analysis confirms the presence of phosphorus (P) peak which is due to the chemical activation process carried out on the porous material with orthophosphoric acid (H3PO4). Due to the activating chemical employed in the process of activating the porous carbon, a phosphorus peak can be noticed in the plot. The phosphorus content in the sample is about 0.30%. We can also see two chlorine (Cl) peak which is basically due to some impurity present in the sample during pyrolysis process of the porous material. The content of Cl is about 0.42%. The carbon (C) peak of the synthesized material resembles about 99.28% of carbon content which reflects with the conclusion that the synthesized porous carbon bears about 99% purity.

Fig. 5
figure 5

Energy-dispersive X-ray (EDX) profile

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Thermogravimetric analysis (TGA)

To determine the thermal decomposition plot of the synthesized porous carbon material we established the thermogravimetric analysis of the sample in which the weight percentage of the porous material that degrades with respect to external temperature up to 900 °C can be seen in Fig. 6. The loss of water molecules from the sample causes the curve in the graph to suddenly drop from 100 to 90% at the beginning. In the region of 200 °C to 700 °C there were very negligible loss of the material. After 700 °C, we can see that the material started to degrade at higher rate till 900 °C in compared to the 200–700 °C region. But at 900 °C we are still left with about 85% of the material which we can conclude that the synthesized porous carbon is pretty much stable at even higher temperature with a loss of 15% of the material synthesized. In contrast to previously reported article [12] the synthesized porous carbon is much more stable at higher temperature and only a few wt% of materials degrade at higher temperature, so we can conclude that the material could be of great importance even for devices bearing higher temperature.

Fig. 6
figure 6

Thermogravimetric analysis (TGA) profile

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Electrochemical characterization

To determine the electrochemical performance of the cell, the constructed EDLC device was evaluated in the CH electrochemical workstation (CH instrument, model 604D, USA). This section goes into depth about the cyclovoltammetry and low-frequency impedance tests that were done.

Cyclovoltammetry

Cyclovoltammetry of the EDLC device was measured in the CH electrochemical workstation. At 5 mV/s scan rate the device showed a specific capacitance of 60 F/gm. The specific capacitance was calculated from the cyclovoltammetry curve using the formula C = i/s, in which i is the current provided in the device and s is the scan rate at which the device was measured. The CV curve shown in Fig. 7 was not similar to the ideal supercapacitor curve but it shows a quite similar nature of super capacity to the ideal supercapacitor plot which is usually bit rectangular in shape [16, 17]. After that, the EDLC device was described at various scan rates, including 10 mV/s, 20 mV/s, 30 mV/s, 40 mV/s, 50 mV/s, and 100 mV/s. The specific capacitance at various scan rates, as shown in Table 1, did not change significantly from that at a scan rate of 5 mV/s. Because ions in the electrolyte diffuse into the whole pore surface at high scan rates, there are variances in specific capacitance at various scan rate levels.

Fig. 7
figure 7

a Cyclovoltammetry profile at 5 mV/s; b cyclovoltammetry at different scan rates

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Table 1 Device performance at different scan rates
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Low-frequency impedance spectroscopy

In the CH electrochemical workstation, a low-frequency impedance spectroscopy approach has been developed to determine the produced EDLC device's specific capacitance. The main parameter necessary for this characterization is to set the frequency low at initial stage which is about 0.01 Hz and the maximum frequency was set as 105 Hz. The plot of the low-frequency impedance can be seen in Fig. 8. It can be seen that the graph shows a linear step towards the low-frequency region, i.e., in the Z´´/ohm region (y-axis) and is in the form of a straight line. Using the formula C = − 1/2fZ´´, we can compute the peak value at the Z´´/ohm area from this figure. The EDLC device's specific capacitance was estimated to be around 126.31 F/gm, which is over double the value of the specific capacitance determined from cyclovoltammetry analysis. This could be because of the dissociation of the ions at different scan rates during the cyclovoltammetry analysis.

Fig. 8
figure 8

Low-frequency impedance profile

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Galvanostatic charge–discharge (GCD)

Now, finally GCD was performed for first six cycles up to 1 V at current of 4 mA for a developed EDLC shown in Fig. 9. For all six segments, the performance of developed EDLC demonstrates an excellent electrical property having symmetrical triangle shape with consistency. With a consistent rate for all six segments of the discharging period, a high capacitance of 36 F/gm at a current density of 4 A/gm is achieved, which is also closer to the specific capacitance that was predicted using cyclic voltammetry and low-frequency impedance spectroscopy. The fabricated EDLC using methyl cellulose-derived porous carbon delivers a coulombic efficiency of 95%, power density 4020 W/kg and energy density 5.2 Wh/kg. The EDLC was also tested for 500 cycles to check the stability of a cell using GCD technique. The coulombic efficiency, power density and energy density were again tested for 500 cycles which shows the stable nature of fabricated EDLC.

Fig. 9
figure 9

Galvanostatic charge–discharge profile

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J–V characteristics

A solar simulator was used at the Centre of Excellence on Solar Cells & Renewable (Enlitech, model no-SS-F5-3A, Taiwan). Energy to measure the photovoltaic performances (J–V) of both the manufactured DSSC (shown in Fig. 10) utilizing platinum and MC generated porous carbon on counter-electrode under a 1 sun situation. Following equations was used to calculate how well the constructed DSSC performed.

$${\text{FF}} = \frac{{V_{\max } \times J_{\max } }}{{V_{{{\text{oc}}}} \times J_{{{\text{sc}}}} }} ,$$
(1)
$$\eta = \frac{{V_{{{\text{oc}}}} \times J_{{{\text{sc}}}} }}{{P_{{{\text{in}}}} \times {\text{FF}}}} .$$
(2)
Fig. 10
figure 10

JV characteristics of DSSC with a platinum as counter-electrode and b methyl cellulose-derived porous carbon as counter-electrode

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In Eqs. (1) and (2), fill factor is represented by “FF”, voltage is Vmax, current is Jmax, photon to electricity conversion efficiency is \(\eta\), open circuit voltage is Voc, short circuit current density is Jsc (µA/cm2) and the incident light power is represented as Pin. The photocurrent density–voltage behavior of fabricated both DSSC and the several parameters calculated using J–V characteristics are shown in Fig. 10. The overall performance at 1 sun condition of a cells, i.e., efficiency, Jsc, FF and Voc are listed in Table 2.

Table 2 Solar cell performance at 1 sun condition
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Conclusion

This article mainly focuses on the studies of highly activated porous carbon synthesized from methylcellulose biopolymer bearing a specific surface area of approx. 1400 m2/gm and its potential application to use it as a biodegradable porous carbon material for EDLC application. SEM micrograph reveals the porous nature of carbon driven from biopolymer, XRD pattern which indicates the larger defects and more graphitization degree, D band of the Raman spectroscopy depicts the defects or distorted geometry of some of the sp2-hybridized carbon atoms whereas the G band resembles the graphitic character or the degree of graphitization in the sp2-hybridized carbon atoms, EDX analysis of carbon (C) peak of the synthesized material resembles about 99.28% purity of carbon content, TGA analysis of synthesized porous carbon is pretty much stable at even higher temperature with a loss of 15% of the material synthesized. Using the same raw material followed by TCM ionic liquid as an electrolyte we are able to get about 61 F/gm specific capacitance by cyclovoltammetry analysis and about 126.31 F/gm by low-frequency impedance measurement. Though it is not showing much specific capacitance in contrast with the other porous material used for supercapacitor application [18], we are working on it to make it use for future application in supercapacitor. The right electrolyte with this porous material can yield very high specific capacitance which could be very beneficial for its potential application in coming days. The DSSC performance of fabricated cells using standard platinum-coated counter-electrode showed an efficiency of 0.81% and MC-derived porous carbon-coated counter-electrode showed an efficiency of 0.86% at 1 sun condition, which depicts that replacement of high costly platinum in counter-electrode is possible with conducting methylcellulose-derived porous carbon.