1 Introduction

A supercapacitor is a remarkable electrochemical device which is basically the advanced form of the classical capacitor that has been used thoroughly from past centuries. But the capacitor itself cannot provide or give much remarkable results till date. The scientific journey of the capacitor leads to the invention of the supercapacitor which can store much more charge in itself. It yields outstanding specific capacitance, better power density, and great energy density with great capacitance retention having a very less loss of specific capacitance. In the past few years, it has been used in different fields like in electric power distribution system, electrical vehicles, electrical devices, and pulsing techniques [1,2,3,4]. The major criterion in a supercapacitor device is the electrode material. Porous carbon is the best electrode material till date and found to be widely used in order to synthesize a better efficient supercapacitor device. Porous carbon has outstanding properties like higher surface area, larger pore volume, great electrical conductivity, and also efficient chemical stability. Also, it is environmentally friendly and can be synthesized from any matter on earth. As we know, 99% of earth is composed of carbon which made it very cost-effective for use in many applications including the supercapacitor [5,6,7,8,9]. Nowadays, many attempts were made in order to derive highly porous carbon from waste materials which is a greener way of getting good result as an electrode material for energy device. A new way has been established in order to synthesize graphene which is an allotrope of carbon showing high electrical conductivity and also remarkable results by using it as an electrode material in the supercapacitor and other energy devices like DSSC [10]. Also graphene oxide prepared from simple electrochemical exfoliation method shows some satisfying results while using the material as an electrode material for an EDLC device [11]. However, the charging ability or we can say the electrochemical performance of supercapacitor depends on the raw material and the process on how it has been activated during the whole preparation method of porous carbon. Temperature and other factors like time of activation and concentration of the materials used also play a vital role in order to get a good product. The main charge storage mechanism of the porous carbon basically is a two-layer system which undergoes reversible ion adsorption onto the surface of porous carbon; the charges are stored in the interface between the electrodes being coated with the active material and the electrolyte used in the device fabrication, and so this device is called an “electrical double layer capacitor.”

Nowadays for electrode material, high-graded commercial porous carbon (CPC) is being used for supercapacitor application but using CPC is not always worth the money, instead of which common-grade activated carbons and many other naturally carbon-rich materials like coal, shells of palm, shells of coconut, coffee beans, and many other dry matter found on earth are being used for the preparation of porous carbon. These raw materials used are not expensive compared to the CPC and they are easily available anywhere [12,13,14,15,16,17,18]. Here in this paperwork, we have prepared high-graded porous carbon from cornstarch biopolymer. Cornstarch can be called as maize starch or also can be called as corn flour. Basically, the starch is derived from the grains of maize or corn. Though India does not come in the list of most cultivated country of maize/corn in the world, still it produces a reasonable amount of corn which fulfills the requirement of the country. The cornstarch produced are used in medical sector, food products, and also in cattle feed for its great nutritious value. In contrast to other biomass, cornstarch is a very reliable and pure precursor with very negligible amount of impurity. It is a biopolymer with very good yield and hence it can be used to make high-graded porous carbon which will result in a very cleaner and greener way of synthetization. In contrast to physical activation process of porous carbon, chemical activation process is proved to be one of the best ways of getting high-graded porous carbon and it has been used to synthesize many other raw materials to get carbon with high porosity and yield great electrochemical performance. For the chemical activation process, the most widely used chemical activating agents are followed as H3PO4, KOH, NaOH, Na2CO3, and K2CO3 [19,20,21,22,23,24,25]. In many past research works, H3PO4 had been used as an activating agent during the synthesis process of porous carbons which can be treated as one of the most desirable chemical as it is an eco-friendly and economical way of activating the porous carbon. During the process of activation, the precursor material is impregnated in the desired quantity of H3PO4 and then it is dehydrated in a vacuum oven by increasing the temperature [26]. During the course of this process, the micropores and mesopores/macropores having high surface area are evolved and also large pore volume of the porous carbon are formed. Also, the amount of H3PO4 used can be recovered and during the course of activation process the corresponding porous carbon yields high porosity which is a good sign for using the material in the supercapacitor device for better electrochemical behavior [27,28,29,30].

The synthesis of porous carbon spheres from the corn starch was reported elsewhere [31, 32]. The synthesis process of highly porous carbon from corn starch biopolymer using the activating agent H3PO4 which is used as an electrode material for the supercapacitor is very rarely reported. The same two-step process of making high-graded porous carbon is followed by changing some physical parameters like temperature during the activation and carbonization of the material, and also the time of activation results in better yield in contrast to the reported article elsewhere [33]. The product came out showing better results and also better yield in electrochemical performance which is studied in detail in this paperwork. Also, a thorough study of the porous carbon synthesized from the two-step mechanism involving pyrolysis carbonization and chemical activation is done with some remarkable results which we will be discussing further in the paper.

2 Experimental section

2.1 Material used

Cornstarch biopolymer (C6H10O5)n was used as a raw material for synthesis of the porous carbon obtained from the Sigma-Aldrich, USA. Other materials like sulfuric acid (98 wt%), phosphoric acid (85 wt%), PVDF-HFP (polyvinylidene fluoride co-hexafluoropropylene), acetone, and graphite sheets were also procured from Sigma-Aldrich.

2.2 Synthesis of porous carbon from cornstarch biopolymer

During the process of synthesis of porous carbon, cornstarch biopolymer is used as a precursor material which involves a two-step mechanism for pyrolysis carbonization followed by chemical activation to obtain the final product. The precursor material, i.e., cornstarch, was carbonized at 120° C for 24 h (overnight) with 40 wt% of sulfuric acid (H2SO4) in which the proportion of cornstarch mass (gm) to 40 wt% of H2SO4(mL) solution volume was 1:10. After 24 h, the slurry product was collected and washed many times with double-deionized water so that the product becomes neutral, which was confirmed by PH paper. The washed product was then dried in a hot air oven at 120° C overnight in order to obtain the initially carbonized product. The obtained dried carbon was then impregnated with 85 wt% of phosphoric acid (H3PO4) where the impregnation mass ratio of carbon (gm) to H3PO4(mL) was kept as 1:6. The obtained carbon product was then finally heated up at tubular furnace for activation at 800° C in the presence of inert atmosphere (nitrogen) for about half an hour. Finally, the sample was taken out from the tubular furnace and washed several times in order to neutralize and then dried in an oven at 120° C for about 12 h which will give us the final porous carbon.

2.3 Fabrication of supercapacitor

In order to fabricate the supercapacitor, we have used pre prepared polymer electrolyte film of PVDF-HFP which was maximized with 300 wt% TCM ionic liquid bearing ionic conductivity of 3.7 × 10−2 S cm−1 and is reported elsewhere by our colleagues [34].

For fabrication of symmetric supercapacitor current collector or electrodes, we have used graphite sheets which were cut in the dimension of 1 cm × 1 cm. The synthesized porous carbon is mixed with the binder where PVDF-HFP polymer is used and the porous carbon-to-binder ratio was kept as 90:10 which is dissolved in acetone. The mixture is then used for coating with approximately 1 mg of coating in each electrode which makes them symmetric. The porous carbon–coated electrodes were then dried in a vacuum oven at 80–90 °C for about 1 h. The dried electrodes were then taken out of the vacuum oven (Fig. 1), and the polymer electrolyte film is cut into 1 cm × 1 cm dimensions which is then sandwiched in between two freshly fabricated porous carbon electrode and hence our supercapacitor device is fabricated. The device is then characterized in CH electrochemical workstation where cyclic voltammetry and low-frequency impedance is measured in order to calculate the specific capacitance of the fabricated device.

Fig. 1
figure 1

Fabricated supercapacitor electrodes

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3 Results and discussion

3.1 Porous material characterization

3.1.1 Field emission scanning electron microscopy

The surface morphology of the synthesized porous carbon is determined by the field emission scanning electron microscopy (FESEM) analysis. Figure 2 shows the FESEM images at different magnifications. Figure 2a shows bit spherical or ball-like structures with some aggregation of the porous carbon stacked with many graphitic layers which are 3 dimensional in shape which can be seen clearly in Fig. 2b. Also, in Fig. 2c and d, it is clearly seen that the average diameter of the porous carbon stakes are found to be 1 to 2 μm. The aggregation is seen in the porous carbon due to the activating agent used during the synthesis process. The activating agent H3PO4 interacts with porous carbon that results in the phosphate and polyphosphate bridges. During the dilation process, the pore structures were created having polyphosphate and phosphate groups[35].

Fig. 2
figure 2

a FESEM scaled up in 10 μm, b FESEM scaled up in 4 μm, c FESEM scaled up in 2 μm, d FESEM scaled up in 1 μm

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3.1.2 X-ray diffraction

The X-ray diffraction (XRD) analysis was performed in the synthesized porous carbon (Fig. 3a). The crystal structure was analyzed from the XRD spectrum. The XRD spectrum shows two typical peaks at 23° and 43° in the 2ɵregion which are under the normal range for porous carbon. The broad nature of the XRD pattern in the 2ɵregion confirms the amorphous nature of the synthesized porous carbon which results in better layer alignment.

Fig. 3
figure 3

a XRD profile, b Raman spectroscopic profile, c EDX profile, d TGA profile

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3.1.3 Raman spectroscopy

Raman spectroscopic analysis has been carried out in the synthesized porous carbon which confirms the uniformity and depicts the purity of the porous carbon before using it as an electrode material in the supercapacitor device. The porous carbon shows two peaks at 1327/cm and 1584/cm in the x-axis region which confirm the D and G band of the synthesized carbon material (Fig. 3b). The D stretching depicts the sp3 hybridized carbon atoms and the G stretching corresponds to the sp2 carbon atoms. During the process of synthesis of the porous carbon, some sp2 hybridized carbon atoms gets converted into sp3 carbon atoms due to the presence of various functional groups as reported earlier. So, the D band specifies the distorted geometry of some of the sp2 carbon atoms while on the other hand G stretching represents the degree of graphitization in the sp2 hybridized carbon atoms.

3.1.4 Energy-dispersive X-ray

The porous carbon has been characterized for energy-dispersive X-ray analysis, commonly known as energy-dispersive X-ray (EDX) analysis which confirms the chemical composition present in the synthesized porous carbon. It has been seen that there is doping of phosphorus which is showing a peak in the EDX plot next to the carbon peak as shown in Fig. 3c. The phosphorus peak is seen in the plot due to the activating agent H3PO4 which is used in the process of activation of the porous carbon. The primary peak of carbon is present in almost all the samples of carbon. The amount of carbon present in the sample is 94.61% and the rest 5.39% consist of the phosphorus which confirms the purity of the porous carbon more than 90%.

3.1.5 Thermogravimetric analysis

Figure 3d shows the thermal decomposition plot of porous carbon derived from cornstarch biopolymer. The thermogravimetric analysis shows the weight loss of a prepared porous carbon which tells the material is stable up to a 600 °C. Additionally, in the initial stage, the weight loss is because of the loss in water molecules. After 600 °C, the material started degrading at a higher pace which reaches up to 58 wt% at 800 °C.

3.1.6 Brunauer–Emmett–Teller

The N2 adsorption/desorption Brunauer–Emmett–Teller (BET) isothermal plot of porous carbon from H3PO4 is shown in Fig. 4. The synthesized porous carbon is having a total surface area of 741.380 m2/g. The pore volume of micropores filled at relative pressures up to 0.1 was 0.3099 cm3/g. Using the total BET area as a reference, the average size of the micropores would be 0.707 nm. This compares with the overall average pore radius of 1.138 nm calculated from the total pore volume of 0.422 cm3/g. Therefore, from the N2 isotherm analysis in Fig. 4, it was confirmed that on a volume basis, the sample is predominantly microporous (~ 73%) and has some mesopores (~ 22%) along with a few macropores (~ 5%).

Fig. 4
figure 4

N2 adsorption/desorption isotherm profile

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3.2 Electrochemical characterization

The electrochemical performance of the fabricated supercapacitor cell is measured using the CH electrochemical workstation in which cyclic voltammetry and low-frequency impedance were done from which the specific capacitance of the device is measured along with the stability of the device.

3.2.1 Cyclic voltammetry

The cyclic voltammetry of the fabricated supercapacitor device is measured by the formula C = i/s, where c stands for the specific capacitance, i for current and s for scan rate in which the device was measured. The fabricated device was measured at 5 mV/s which was showing a very remarkable performance having a specific capacitance of 184.8 F/gm and also the CV plot can be seen mostly rectangular in shape without obvious distortion as shown in Fig. 5a which specifies that the supercapacitor device is showing ideal capacitive behavior. The device performance was also characterized at different scan rates from 5 to 100 mV/s in which there was not much difference in the values of specific capacitance as shown in Fig. 5b and Table 1. The little decrement in the specific capacitance values were seen in the plot with the increase of scan rates which is basically seen due to the diffusion of ions in the electrolyte into the whole pore surface at higher scan rates [36].

Fig. 5
figure 5

a Cyclic voltammetry at 5 mV/s, b cyclic voltammetry at different scan rates, c low-frequency impedance plot, d stability plot

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Table 1 Difference in the values of specific capacitance in device performance
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3.2.2 Low-frequency impedance

The fabricated device has been also characterized in the CH electrochemical workstation in order to find the specific capacitance from the low-frequency impedance plot. The frequency was set from 0.01 to 105 Hz. The graph shown in Fig. 5c is seen to be much steeper towards the low-frequency region (Z″/ohm). The specific capacitance was calculated using the formula C =  − 1/2πfZ″. The specific capacitance of the device is found to be 188.4F/gm which is showing a very effective device performance. If we compare both the result of cyclic voltammetry and low-frequency impedance result, then we can say that there is not much difference in the calculated values of specific capacitance in both cases.

3.2.3 Stability plot

A series of studies were made for about 15 days at 50-mV/s scan rate in order to check the stability performance of the supercapacitor device which is shown in Fig. 5d and Table 2. The device shows good performance but also it can be seen that the specific capacitance of the device decreases as the device run for more days. There was decrement in the specific capacitance of the supercapacitor device which is basically due to the diffusion of ions in the polymer electrolyte film as specified before. Also, the device performance depends on the temperature and humidity as the separator used in the device is polymer electrolyte film.

Table 2 Stability performance of the supercapacitor device
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4 Conclusion

In this paperwork, we have established the studies of a highly efficient porous carbon material that has been synthesized from cornstarch biopolymer by pyrolysis carbonization method. All the studies of the porous carbon have been made scientifically which showed better results compared to other CPC carbon available in the market [37]. We have also seen the electrochemical performance of the supercapacitor device that has been fabricated using the same porous carbon and showed remarkable results like 184.8 F/gm which is far better than those of the previously reported paper [33]. Some changes had been made with the concentration and time of reaction during the synthesis process which leads to better specific capacitance values of the device. Since cornstarch is easily available and it is a biodegradable solid material, it can be used in order to synthesize porous carbon which can yield great performance while using as an electrode material.