Activated carbon nanospheres derived from bio-waste materials for supercapacitor applications – a review

Abstract

Supercapacitors are perfect energy storage devices; they can be charged almost instantly and release energy over a long time. They can be charged multiple times with minimal degradation in performance. Supercapacitor performance is determined by the composition of the electrode and advanced configurations. In this review, we compare the performance of different electrode materials which are obtained from biowaste based precursors. Our main interest in this review is to study the supercapacitor properties using carbon based spherical natured particles well known as carbon nanospheres. Carbon based electrodes, particularly bio-waste activated carbon nanospheres, have gained interest due to their excellent energy storage ability. In this paper, Activated Carbon Nanospheres derived from several bio-waste materials are reviewed on the basis of their cyclic voltammograms, specific capacitances, surface areas, electrolytes used and fabrication process.

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Divyashree Arvind

Ms Divyashree Arvind is working as Assistant Professor in Sai Vidya Institute of Technology, Bangalore for the last few years. She is having experience in working with supercapacitors and nonotubes. Presently she is working as a PhD Student under the guidance of Prof. Gurumurthy Hegde, BMSCE, Bangalore.

Gurumurthy Hegde

Dr. Gurumurthy Hegde has got PhD in physics, and has been working in the field of liquid crystals since 2002. He has been appointed as the Chief Technology Officer (CTO) for Innovidis company in 2011. Presently he is working as Chair Professor at BMS R and D Centre, BMSCE Bangalore. Main topic of his research work has been focussed on photoinduced effects on organic materials. Under this, variety of important results was reported in reputed journals. During his postdoctoral positions both in Gothenburg University and HKUST, Hong Kong, he worked mainly on photoalignment of liquid crystals, field which is very new and having lot of potential in industrial domain. He also got lot of experience in alignment mechanisms which is crucial for display industry. Recent topic of interest is on flexoelectric polarization, fast liquid crystal light shutters and also nanotechnology. To know more about him, click the, link http://murthyhegde.page.tl/.

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1. Introduction

In today's world, there is not only a need for a continuous supply of highly efficient energy at a low cost but also for the storage of energy.¹ Use of conventional sources of energy is speculated to have a severe impact on the world's economy and ecology.^2,3 This spurs the use of electrochemical energy as an alternative power source.^4,5 The most commonly used devices for the storage and conversion of electrochemical energy are batteries, fuel cell and electrochemical capacitors. The electrochemical properties of all the three devices are the same though their energy storage and conversion mechanisms are different.⁶

Batteries are rechargeable storage devices that convert the stored chemical energy into electrical energy through redox reactions at anode and cathode, the conversion and storage of the energy occurs in the same compartment.^7,8 This principle was first proposed by Becker in 1957.^9,11 The electrochemical reaction involves the transfer of electrons from anode to cathode when the battery is connected to an external load.^12,13

The cross section of a conventional battery, the electrons built in the anode due to the redox reaction causes a potential difference between anode and cathode as shown in Fig. 1. When an external load is connected to anode and cathode, a closed path is established between them and the electrons begin to flow from anode to cathode constituting to the electric current.

Fig. 1 Cross section of a conventional rechargeable battery with anode, electrolyte and cathode connected using an external electrically powered device.¹⁰

When the battery has to be recharged, the direction of the flow of electrons is reversed using another power source. This helps in restoring the original state of anode and cathode for further redox reaction.^14,15

The chemical reactants required for the smooth working of the battery is stored within the battery. Once the chemical reactants are completely used up, the batteries have to be either recharged or disposed. This stands as a major hurdle in the energy storage sector and hence fuel cells came into existence. Fuel cells can generate power as long as there is a fuel supply.

Fuel cell are the electrochemical device that converts chemical energy obtained from the fuel into electricity through a chemical reaction between the positively charged hydrogen ions with an oxidizing agent preferably oxygen.¹⁶ Electricity is produced as long as there is a flow of fuel into the cell.^17,18 Fuel cell comprises of three adjacent layers: anode, cathode and electrolyte. Fuel is consumed due to the chemical reactions that occur at the interface of the three different segments resulting in the precipitation of water which in turn produces an electric arc.¹⁹

Basically fuel cells are the devices that combine fuel with oxygen to produce electricity, heat and water as a by-product see Fig. 2. Apart from anode, cathode and electrolyte fuel cells have an additional layer called the catalyst layer. This layer helps in speeding up the chemical reaction. The reaction between the fuel and air occurs at anode and cathode. The cathode receives oxygen from the ambient air. From Fig. 2, it can be seen that hydrogen is produced in the anode through a reforming process between the hydrocarbon fuel and water. Subsequently the hydrogen is then electrochemically consumed in carbonate electrolyte medium producing water and electrons.

Fig. 2 The cross section of a fuel cell showing the interface between the layers along with the chemical reaction occurring at each layer.²⁰

Further the electrons flow through the external load owing to the electric current. The electrons return to the cathode which is made use of in the electrochemical reaction. Oxygen along with carbon di-oxide that is recycled from the anode reacts with the electrons in the cathode producing the carbonate ions. The anode reaction is supported by these carbonates that reach the anode through the electrolyte.

Due to the advantages of the fuel cells, it was used as a source of power for a quite bit of time but fuel cell requires a considerable amount of start-up time making it inevitable to use a short-term energy source till the fuel cell starts to operate in full swing. This drawback is overcome by the supercapacitors which ramps up huge current with negligible amount of voltage drop.

Electrochemical Capacitors (EC) also called as supercapacitors or Electric Double Layer Capacitors (EDLC) stores charge either using ion absorption or redox reactions.^21–23 The performance of the supercapacitor depends on the charge accumulation capability from an electrolytic solution through electrostatic attraction by polarized electrodes.^24,25 EDLCs consist of an isolator in addition to anode, cathode and electrolyte to electrically separate the two electrodes. The metal–electrolyte interface can store charge up to ∼10⁶ Farad.²⁶ The capacitance that is produced from the electrochemical double layer is analogues to a parallel plate capacitor.²¹ The excess or deficiency of the charge, builds up on the surface of the electrode.^27,28 In order to provide the electroneutrality, ions of the opposite charge build in the electrolyte near the electrode–electrolyte interface.^28,29 The structure of an electric double layer capacitor with electrolyte separating the two electrodes is as shown in the Fig. 3.

Fig. 3 Basic structure of electric double layer capacitor.³⁰

2. Supercapacitor

Supercapacitor helps in bridging the disparity in the performance between fuel cells and batteries with high energy storage capacities.³¹ Supercapacitor is an electrochemical cell comprising of anode, cathode, electrolyte and a separator. The composition of electrode is usually oxides of nickel, cobalt, molybdenum, iridium or tungsten which are deposited on a metal foil. The electrolyte separating the electrodes can be basic, acidic or neutral.^32,33

The specific capacitances and the performance of the supercapacitors fabricated with various metal oxide obtained at different scan rates is as shown in the Fig. 4. From the Fig. 4 we observe that the performance of the electrode with Co₃O₄ delivers the best specific capacitance in comparison to all the precursors.

Fig. 4 Specific capacitance obtained at different scan rates for different metal oxide thin film based supercapacitors.³²

Supercapacitor comprises of two non-reactive porous plates which are suspended within an electrolyte with voltage applied across the porous plates.³⁴ The potential applied on the cathode attracts the electrons from the electrolyte while the potential applied on the anode attracts the positive charge as shown in Fig. 5.³⁴ This phenomenon effectively gives room to two layers for the chare storage.³⁵ This helps in making supercapacitors more suitable for electrostatic storage of charges.

Fig. 5 Schematic diagram of charge distribution in a supercapacitor.³⁴

The charging and discharging of the electric double layer supercapacitor is as shown in Fig. 6. It shows the concentration of charge distribution around the electrode during charging and discharging of a supercapacitor. The characteristics of a supercapacitor as listed in Table 1 pronounces the galvanometric characteristics of a typical supercapacitor.

Fig. 6 The structure of double layer capacitor during charging and discharging.⁴⁰

Table 1 Characteristics of supercapacitor⁴¹

Features	Range
Voltage level (V)	50–100 V
Current (I)	100–300 A
Pulse duration (Δt)	1.0 ms to 1 s
Capacitance ©	1–10 F
Power density (kW L^−1)	5–180
Energy density (kJ L^−1)	0.5–0.6
ESR ®	20–30
Temperature cycle	−20 °C to +60 °C

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Comparison of the energy density versus power density characteristics of batteries, supercapacitors and capacitors is as shown in Fig. 6.³⁴

Fig. 7 depicts that energy density of a supercapacitor falls in-between that of a battery and a conventional capacitor. It clearly shows that the power storage capacity with least drop in the voltage is high in batteries followed by supercapacitors and capacitors respectively.

Fig. 7 Energy density versus power density of capacitors, batteries and supercapacitors.³⁴

From the Fig. 8, it is visible that supercapacitors occupy a significant position in specific energy and specific power. The need for energy storage in the present world is met by supercapacitors because of its high power capability and huge energy density.³⁶ Supercapacitors operate in a wide range of temperature; they have long cycle-life and also deliver high power density.³⁷

Fig. 8 Ragone plot of energy storage devices.¹⁷

The comparison between the capacitor, battery and a supercapacitor is as listed in the Table 2. From Table 2, it is clearly visible that the supercapacitors are better than conventional batteries and a simple capacitor.

Table 2 Comparison between an electronic capacitor, battery and supercapacitor³⁴

Function	Electronic capacitor	Battery	Supercapacitor
Charge time	μs–ms	Hours	ms–min
Discharge time	μs–ms	1–900 min	ms–days
Energy density	<0.01 W h L^−1	50–300 W h L^−1	0.5–5 W h L^−1
Power density	>10^4 W L^−1	<500 W L^−1	10^3 to 3 × 10^3
Cycle life	10^6 to 10^8	200–1000	10^6 to 10^8

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Supercapacitors are widely preferred over conventional capacitors as they can store more energy in comparison with conventional capacitors.³⁸ Substantially more energy can be stored in the supercapacitors because the interface between the electrolyte and electrode which facilitates the charge separation in the electrical double layer is very bleak and comparatively more amount of charge can be stored due to the high surface area formed as a result of large number of pores. Charging of the supercapacitor is rapid as it just involves in the movement of ions to and from to electrode superficially.³⁵ Supercapacitors have a high degree of reversibility and better cycle life. The distinguishing criteria in supercapacitors are electrode material, structure and electrolyte. On the electrode materials used, supercapacitors can be classified into carbon based and metal oxide based.

The most important criteria in using the metal oxide electrode are the faradaic process.^39,40 These electron-conducting reactions predominantly occur in oxides of ruthenium, iridium, iron, manganese etc. The storage and the discharge of the charge occur with the proton insertion and liberation. The electrodes made from metal oxides are known to have better reversibility and long-time stability. But the cost of production of the metal based supercapacitors is very high. Metal oxide based supercapacitors also suffers from self-discharge poor performance at low temperature and degradation in the collector current.^41,42 In considering all these aspects, supercapacitors fabricated from metal oxides as precursor, the technology is at infancy. Carbon precursor is preferred as the electrode material in supercapacitors due to its low cost, easy availability, high surface area, and easy production methodologies.⁴³ Carbon electrodes also contribute to the high stability and conductivity due to its enhanced pore volume distribution.

2.1. Electrode materials for supercapacitor

The two underlying elements for supercapacitors to meet the pressing requirement of the energy crisis are new materials and advanced configurations.²⁸ Numerous researches is going on in finding the better material to fit in as an idle supercapacitor.^44,45 A proper control over the pore size and specific area of the electrode plays a significant role in choosing the material for the electrode to ensure good performance of the supercapacitors.⁴⁶ The capacitive performance of different carbon based electrodes is shown in the Table 3.⁴⁵ From Fig. 9 it is clear that electrodes made from activated carbon nanospheres are commercially suitable in manufacturing electrodes because of their low cost, greater specific capacitance, large surface area and easy processability.^22,42

Table 3 Characteristics of carbon and carbon based materials as electrodes⁴⁴

Materials	Specific surface area (m^2 g^−1)	Density (g cm^−3)	Aqueous electrolyte		Organic electrolyte
			(F g^−1)	(F cm^−3)	(F g^−1)	(F cm^−3)
Carbon materials
Commercial activated carbons (ACs)	1000–3500	0.4–0.7	<200	<80	<100	<50
Particulate carbon from SiC/TiC	1000–2000	0.5–0.7	170–220	<120	100–120	<70
Functionalised porous carbons	300–2200	0.5–0.9	150–300	<180	100–150	<90
CNT	120–500	0.6	50–100	<60	<60	<30
Templated porous carbons (TC)	500–3000	0.5–1	120–350	<200	60–140	<100
Activated carbon fibers (ACF)	1000–3000	0.3–0.8	120–370	<150	80–200	<120
Carbon cloth	2500	0.4	100–200	40–80	60–100	24–40
Carbon aerogels	400–1000	0.5–0.7	100–125	<80	<80	<80
Carbon-based composite materials
TC–RuO2 composite	600	1	630	630
CNT–MnO2 composite	234	1.5	199	300
AC–polyaniline composite	1000		300

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Fig. 9 600 dpi in TIF format)[QUESTION MARK]?>Capacitive performance of electrodes made from carbon.⁴⁷

Templated process is the most widely used technique in producing porous carbons for supercapacitor application. Templated porous carbons having microporous mesoporous and macroporous sizes with a tailored hierarchical structure as shown in the Fig. 10 acts as superior electrode materials for the supercapacitor.^47,48

Fig. 10 Schematic representation of micropore, mesopore and macropore in a carbon particle.⁴⁹

From Fig. 11 it's clear that the microporous carbon well distributed with the narrow pore size in an ionic electrolyte is well suited for high energy density electrodes in a supercapacitor.

Fig. 11 Normalized capacitance as a function of pore size of the carbon electrode in ionic electrolyte medium.⁵⁰

2.2. Need for carbon based electrodes

In the last few years, carbon and its materials have been studied with great interest due to its excellent electrochemical storage of energy.^52,53 Carbon portrays itself as a suitable material for electrochemical storage of energy due to its ability of existing in various forms (allotropes) and structures, its degree of graphitization owes to a wide variety of micro-textures and its dimensionality extends from 0 to 3D.⁵⁴ The electrodes made of carbon are well polarizable. Carbon possesses amphoteric character which facilitates the use of its electrochemical properties from donor to acceptor state.⁵⁵ Primarily carbon-materials are not hazardous.⁵⁶

Chiefly the electrodes made from carbon because of its low cost, easy accessibility and processability. Carbon-materials is stable both in acidic as well as basic solutions.⁵⁷ It can be used in wide range of temperatures. By using various physical activation methods, it is possible to produce materials with huge surface area and controlled pores distribution that chiefly determines the electrode–electrolyte interface for electrochemical application.⁵⁸

Taking into account all these physical and chemical characteristics which is listed in Table 4, the electrodes made from carbon for the storage of energy in electrochemical capacitors is hugely advantageous. The surface functionalities of the porous activated carbon nanospheres forms an important criterion in the capacitive performance as they affect the wettability of the carbon surface by the electrolyte and this exhibits pseudo capacitance.⁵⁹ In this regard nano-porous carbons which are produced using template methods have well controlled pore size, large specific area and interconnected pore network.^60,61

Table 4 Comparison of specific capacities, surface area, pore volume and average pore size of activated carbons⁵¹

Carbon	Specific capacity (F g^−1)	Specific capacity (F cm^−2)	BET (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore size (Å)
M-10	55.95	0.041	1370	0.500	9.12
M-14	57.20	0.0047	1223	0.561	9.60
M-15A	78.10	0.043	1800	0.629	9.17
M-15B	55.80	0.034	1624	0.563	9.37
M-15C	63.34	0.042	1518	0.600	9.79
M-20	100	0.046	2130	0.709	14.73
M-30	62.9	0.024	2571	1.230	14.95
A-10	35.3	0.031	1150	0.424	—
A-20	41.20	0.020	2012	0.902	14.23
SACF-20	48.8	0.027	1839	0.699	9.74
SACF-25	27.9	0.011	2371	0.977	11.93

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2.2.1 Activated carbon nanospheres (ACNs). ACNs is most popular amongst carbon electrodes due to its large surface area, low cost and good charge holding capacity.⁶² Physical activation or chemical activation of carbonaceous precursor is used to produce activated carbon nanospheres.^63,64 The activated carbon nanosphere's pore size include micropores (<2 nm), mesopores (2–50 nm) and macropores (>50 nm).²⁸ The major part of the internal surface area is contributed by micropore. Bio-waste carbonaceous materials form the source of activated carbon nanospheres. It is either produced by physical reactivation or chemical activation. In physical reactivation, activated carbons are produced using hot gases. The physical reactivation is then followed by carbonization by pyrolysis at a temperature of 600–900 °C. In chemical activation, bio-waste raw materials are impregnated mainly with acids, base or a salt which is then carbonized at a temperature ranging between 450–900 °C. Activated carbon nanospheres have vast applications like treating overdose of drug and poisoning, air and water purification, livestock production, wine making, storing hydrogen and natural gas, filtration of beverages, mercury scrubbing etc.

Activated carbon nanospheres in its crude form before the post processing is as shown in the Fig. 12.

Fig. 12 Activated carbon nanospheres.⁴⁹

2.2.2 Templated carbons. Supercapacitor electrode materials can be produced using templated process. Nanoporous carbons produced using templated process has a controlled pore size and huge surface area.⁶⁵ Template synthesis comprises of infiltration of a carbon material into the pores of the required material, followed by carbonization/polymerization and then the removal of template yielding a carbon porous structure.⁶⁶ This method is proved to be the best technique in controlling the pore structure.⁶⁵

3. Bio-waste materials

The past three decades have seen a rapid economic growth in developing countries.⁶⁷ This growth is being powered by coal and fossil fuels. The downside of a coal powered growth is a suicidal destruction of our habitats due to climate change.^68,69 This transition urgently requires a source of limitless supply of clean energy. Failure to develop such a source could be catastrophic to the existence of humanity. Bio-waste makes up most of the municipal solid waste which is as listed in the Table 5.^70,71

Table 5 Composition of lignocellulose in solid waste⁷⁴

Constituent	Percentage of lignocellulose (%)
Paper and paper products	37.8
Food waste	14.2
Yards waste	14.6
Wood waste	3.0
Total cellulose waste	69.6
Plastic	4.6
Rubber and leather	2.2
Textiles	3.3
Glass and ceramics	9.0
Metals	8.2
Miscellaneous	3.1
Total	100

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The reasonable way to address the global economic issues is to make use the bio-waste.⁷² The structural components of the cells, cellulose and hemicellulose of bio-waste are suitable for the energy storage devices.⁷³

The lignocellulosic biomass is the primary fraction of the solid bio-waste comprising of 35–50% of cellulose.^74,75 The raw materials obtained from bio-waste have vast option in the energy storage industry.⁷⁶ Percentage of lignocellulose composition in various bio-waste materials is as listed in Table 5. The use of bio-waste materials has relatively less harmful environmental impacts; it is cost effective and also helps in the reduction on the dependency on fossil fuels.^77,78

4. Bio-waste materials for supercapacitor

This review highlights the use of several bio-waste materials as a precursor for electrodes used in supercapacitors.⁷⁴ Carbon electrodes prepared from sugarcane bagasse, coffee beans, banana fibres, cherry stones waste, firewood, paper pulp, camellia oleifera shell, corncob residue, lignin, acacia gum, chestnut shells, carrageenan, beer yeast cells and urea, tamarind fruit shell, biomass-derived proteins, chicken eggshell, Eichhornia crassipes, and oil palm were studied. The choice of the bio-waste precursor and the activation methodologies determine the electrochemical performance along with the pore size distribution, surface area and specific capacitance of the carbon electrodes.

4.1. Supercapacitors from activated carbon nanospheres derived from sugarcane bagasse

Milling of sugarcane and other waste products contributes to the production of sugarcane bagasse.^79,80 The activated carbon nanospheres which is obtained by treating sugarcane bagasse have found application in water treatment, gold extraction and charge storage due to its porosity.^81,82 Activated carbon nanospheres obtained by activation of sugarcane bagasse using ZnCl₂ has a greater surface area and better average pore width which is greater than 1 nm.^83,84

Sugarcane bagasse rinsed in hot water at 100 °C using sonicator for 8 hours is further air dried at the same temperature for 48 hours. The residue obtained after drying is mixed with ZnCl₂ in the ratio 1 : 1. The same mixture is then sonicated four 4–5 hours in room temperature, followed by air drying at 100 °C produces activated carbon nanospheres.⁸⁵

Activated carbon nanospheres after the treatment were found to possess surface area of 2871 m² g⁻¹, specific capacitance of 109 F g⁻¹ and pore volume of 0.81 cm³ g⁻¹ in 1 M H₂SO₄ electrolyte. It showed retention of 77% after 10 000 cycles.⁸⁵

The Ragone plot as shown in the Fig. 13, confirms that the carbons derived by treating sugarcane bagasse is suitable in supercapacitor fabrication.

Fig. 13 Ragone plot for the activated carbon nanospheres obtained from sugarcane bagasse.⁸⁵

4.2. Supercapacitors from activated carbon nanospheres derived from coffee beans

Coffee is one of the most highly consumed beverages in the world. A considerable amount of waste is obtained by coffee beans.⁸⁶ Efforts have been put towards the utilization of the waste obtained from it in energy sector by using coffee beans as the precursor in fabricating supercapacitor electrodes.^87,88

Ground coffee beans mixed with ammonium polyphosphate heated to 1000 °C in a microwave oven for about 30 min and then cooled for about 2 hours yields the carbon that is suitable for supercapacitor application.⁸⁹

Activated carbon nanospheres obtained after synthesising ground coffee beans had the surface area of 999.67 m² g⁻¹, specific capacitance of 286 F g⁻¹ in 1 M H₂SO₄ electrolyte and pore volume of 0.57 cm³ g⁻¹. It is found to be stable even after 2000 cycles in both acidic and alkaline electrolytes.⁸⁹

From Fig. 14, it can be seen that the shape of the cyclic voltammogram is quasi rectangular which indicates that ground coffee beans can be used for energy storage application.

Fig. 14 Cyclic voltammograms of activated carbon nanospheres obtained from ground coffee beans in 1 M H₂SO₄.⁸⁹

4.3. Supercapacitors from activated carbon nanospheres derived from corncob residue

Corncob is one of the most common leftovers in U.S. which is used as a major source of heat.⁹⁰ It has found application in small scale industries in generating power.⁹¹ It is also used as bio-fuel. The electrodes fabricated using corncob as a precursor proved to have an excellent cycle life and well distributes pore.⁹²

Activated carbon nanospheres is obtained by drying ground corncobs for 12 hours at 120 °C followed by heating to 400 °C for hours under the flow of nitrogen. The carbonized powder is then sonicated in KOH solution for two hours and then dried in vacuum for about 12 hours at 120 °C which is further heated to 800–900 °C under the thin flow of nitrogen.⁹³

Activated carbon nanospheres obtained from the process is found to possess surface area of 3530 m² g⁻¹, total pore volume of 1.95 cm³ g⁻¹, specific capacitance of 401.6 F g⁻¹ in 1 M H₂SO₄ electrolyte. The retention is found to be observed after 100 000 cycles.⁹³

It is observed that quasi rectangular structure is obtained from quasi voltammogram as shown in the Fig. 15 proving that corncob is suited for supercapacitor application.

Fig. 15 Cyclic voltammograms of activated carbon nanospheres obtained from corncob.⁹³

4.4. Supercapacitors from activated carbon nanospheres derived from natural flax fiber

Flax is the waste product which is obtained from the textile industry. They are used in making bed spreads. Flexible carbon fibers can be obtained by carbonization followed by pyrolysis of flax textile. Due to their porous nature, they've found their application in supercapacitors.⁹⁴

Carbon fiber cloth immersed in 70 mL KMnO₄ solution for one day at 60 °C followed by rinsing and drying of the same in deionised water at 65 °C yields activated carbon nanospheres which is suitable for the fabrication of electrodes for supercapacitor application.⁹⁵

The MnO₂ film coated with processed natural flax activated using KMnO₄ is found to have specific capacitance of 325.8 F g⁻¹, surface area of 645 m² g⁻¹, 94% retention was found in the capacitance after 1000 cycles.⁹⁵

The cyclic voltammogram curves as shown in Fig. 16 shows the supercapacitance characteristics of carbon fiber.

Fig. 16 Capacitive performance of processed natural flax using cyclic voltammogram.⁹⁵

4.5. Supercapacitors from activated carbon nanospheres derived from acacia gum

Acacia gum is one of the natural gums obtained from acacia trees. It is usually found in Africa.⁹⁶ It is used as additive in pharmaceutical industry and also an emulsifying agent. Carbons obtained by hydrothermal carbonization and chemical activation with KOH had a high surface area and a well distributed pore size.⁹⁷

Ground acacia gum dissolved in water and sonicated is vacuum heated to 180 °C for 12 hours. The precipitate obtained after cooling is rinsed with water and ethanol followed by drying at 60 °C for 8 hours. The carbon obtained is mixed with KOH and heated in a tube furnace under the flow of nitrogen for about an hour. The porous carbon is then rinsed with 1 M HCl and dries at 110 °C for 12 hours.⁹⁸

The obtained microporous carbon has a surface area of 1764 m² g⁻¹, total pore volume of 0.99 cm³ g⁻¹ and specific capacitance of 272 F g⁻¹ in 1 M HCl and has a capacitance retention of 59% after 1000 cycles.⁹⁸

The CV curves shows a quasi-rectangular structure which can be seen in Fig. 17 owing to the typical characteristics of a supercapacitor.

Fig. 17 Cyclic voltammograms of activated carbon nanospheres obtained from acacia gum.⁹⁸

4.6. Supercapacitors from activated carbon nanospheres derived from coconut shell

The significant by-product obtained from coconut shell is shell charcoal. It is used as a source of energy from times unknown both in domestic and industrial application.⁹⁹ Activated carbon nanospheres can be obtained by processing coconut shells.¹⁰⁰ It finds application in energy storage domain due to it porous nature.¹⁰¹

Ground coconut shells are rinsing them with distilled water and dried for two days at 110 °C is carbonized by heating it to a temperature of 1000 °C for two hours under nitrogen gas flow yields activated carbon nanospheres which can be used as a precursor in fabricating electrodes for supercapacitor application.^102,103

The activated carbon nanospheres obtained after KOH activation and further post processing of coconut shells were found to have a surface area of 2000 m² g⁻¹, total pore volume of 1.21 cm³ g⁻¹, specific capacitance of 250 F g⁻¹ in 1 M H₂SO₄ electrolyte. 93% capacitance retention was observed after 2000 cycles.^102,104

The cyclic voltammometric curve as shown in the Fig. 18 depicts a quasi-rectangular shape. This proves that the activated carbon nanospheres obtained from coconut shell as a precursor is well suited for supercapacitor application.

Fig. 18 Cyclic voltammograms of activated carbon nanospheres obtained from coconut shell in 1 M H₂SO₄ electrolyte.¹⁰⁴

4.7. Supercapacitors from activated carbon nanospheres derived from sunflower seeds

Sunflower seeds stand amongst the most commonly used oil seeds in the world.¹⁰⁵ An ample amount of bio-waste is produces annually during the processing of sunflower seeds. Sunflower seeds have found its application as a feedstock to the boilers from ages.¹⁰⁶ Sunflower seeds can be used as a precursor in manufacturing the electrodes of supercapacitor due to its mesoporous structure.

Impregnated sunflower seed shells in KOH is dried for 12 hours at 100 °C is carbonized using a tube furnace under nitrogen gas flow at a temperature ranging between 600–800 °C for about an hour. The residue obtained is washed through with 0.1 M HCl followed by distilled water and dried thoroughly yields the carbons which is suitable for supercapacitor application.¹⁰⁷

The carbon sample obtained after the processing of sunflower seed shell is found to have specific capacitance of 244 F g⁻¹ in 0.1 M HCl electrolyte, surface area of 2585 m² g⁻¹ and total pore volume of 1.41 cm³ g⁻¹ and a good cycle stability.¹⁰⁷

From the Ragone plot as shown in the Fig. 19, it proves that the activated carbon nanospheres obtained using sunflower seed shells as a precursor is suitable for supercapacitor applications.

Fig. 19 Ragone plot for the carbons obtained by sunflower seeds as a precursor.¹⁰⁷

4.8. Supercapacitors from activated carbon nanospheres derived from rice husk

Rice is the staple food in most of the countries around the world.¹⁰⁸ Every year rice husk forms 20% of the total harvested agricultural by-product.¹⁰⁹ From time unknown, rice husk is used as a raw material in production of heat. However rice husk contains a higher degree of ash making it more complicated in using it for supercapacitor application.^110,111 But with enhanced activation process, rice husk can be used as a precursor for supercapacitor application due to its excellent absorption characteristics.¹¹²

Cleaned, dried and ground rice husk is impregnated in 0.5 M NaOH solution for 2 hours at 343 K. It is further sonicated and soaked in ZnCl₂ solution for 2 hours and dried for 48 hours at 383 K. The residue obtained is then dried to obtain the porous activated carbon nanospheres.¹¹³

The activated porous carbon obtained from rice husk has the specific capacitance of 171 F g⁻¹ in an organic electrolyte, surface area of 1768 m² g⁻¹ and total pore volume 1.07 cm³ g⁻¹. A negligible degradation in the specific capacitance was observed even after 1000 cycles of charge and discharge.^113,114

The quasi-rectangular shape of the CV curve as shown in the Fig. 20 depicts the ion diffusion and hence proving their absorption quality in supercapacitor application.

Fig. 20 CV curve of electrodes made from porous carbon obtained from rice husk at different scan rate.¹¹³

4.9. Supercapacitors from activated carbon nanospheres derived from carrageenan

Carrageenan is obtained by the extraction of red seaweeds.¹¹⁵ It's composition includes galactose and anhydrogalactose which is linked by glycosidic units. It's well known for characteristics like gelling and stabilizing and hence has been used in food and pharmaceutical industry as a stabilizer.¹¹⁶ The porous carbon obtained from carrageen has a good specific capacitance and tuned porosity.^117,118

Dissolved carrageenan is vacuum heated for 12 hours at 200 °C. The precipitate is collected using centrifugation technique and is dried at 70 °C for 8 hours after it has been washed in ethanol solution. The carbon that is produced is activated chemically using KOH. The mixture obtained is heated using a tube furnace under nitrogen flow for two hours. Activated carbon nanospheres is obtained by cleansing the same using 2 M HCl and water which is further dried at 110 °C for 12 hours.¹¹⁹

Activated carbon nanospheres obtained from carrageenan after KOH activation has a surface area of 2502 m² g⁻¹ in 6 M KOH electrolyte, total pore volume of 1.43 cm³ g⁻¹, specific capacitance of 230 F g⁻¹ in aqueous electrolyte. A small bleak in capacitance retention was observed after 1000 cycles.¹¹⁹

The electrochemical capacitive properties of the activated carbon nanospheres obtained from carrageenan studied using a cyclic voltammometer showed that the activated carbon nanospheres exhibited quasi-rectangular shape as shown in the Fig. 21 proving its suitability in double layer capacitance application.

Fig. 21 CV curves of activated carbon nanospheres obtained from carrageenan.¹¹⁹

4.10. Supercapacitors from activated carbon nanospheres derived from oil palm

Oil palm is one of the major industries in Malaysia, Thailand and Indonesia which is used in producing cooking oil.^120,121 This results in the production of considerable amount of bio-waste which is directly discharged to the environment. The bio-waste produced by oil palm industries include palm kernel cakes, palm kernel shells, empty fruit bunches and mesocarp fibers.¹²² The activated carbon nanospheres obtained from oil palm find application in supercapacitors due to its porous nature and good surface area.^123–125

Self-adhesive carbon fibers prepared by pre-carbonization of oil palm seeds at a temperature of 280 °C which is followed by milling continuously for 8 hours.¹²⁶ The mixture is wetted using deionised water for an hour and is treated with KOH solution. The mixture is carbonised in nitrogen atmosphere at 800 °C to obtain activated carbon nanospheres.¹²⁷

Activated carbon nanospheres obtained from the above process were found to possess a surface area of 1704 m² g⁻¹, total pore volume of 0.89 cm³ g⁻¹, specific capacitance of 343 F g⁻¹ using KOH + CO₂ activation in 1 M H₂SO₄ electrolyte. And it has a cycle life of about 2500 cycles.¹²⁷

Recently Gomma et al.¹²⁵ reported a reasonably promising specific capacitance values using catalyst free carbon nanospheres obtained from bio-waste oil palm leaves.¹²⁷ Template method is considered as a responsible factor for increasing specific capacitance without activating carbon.

The cyclic voltammogram of the activated carbon nanospheres from oil palm leaves is as shown in the Fig. 22. A large window area shows the apt electrode performance of the obtained activated carbon nanospheres.

Fig. 22 CV curves of activated carbon nanospheres obtained from oil palm.¹²⁵

4.11. Supercapacitors from activated carbon nanospheres derived from Scaphium scaphigerum

Scaphium scaphigerum is a perennial tree majorly found in China and Thailand. The drink made from Scaphium scaphigerum is used for medicinal purpose. The seeds of Scaphium scaphigerum are mainly harvested due to its medicinal properties and hence form the major source of bio-waste material.¹²⁸

Double layered hydroxides are synthesized in the presence of graphene by the process of co-precipitation of Co²⁺ and Ni²⁺ ions using NH₃·H₂O as a precipitator. The electrochemical performance of the resultant was found to have a specific capacitance of 2770 F g⁻¹ and shows 93.4% retention after 500 cycles.¹²⁹

4.12. Supercapacitors from activated carbon nanospheres derived from shaddok peel

Shaddoks form the major fruit of Southeast Asia. However the peels of this fruit are not edible and form the major source of bio-waste material. These peels contain 78% hemicellulose which is the major source of non-graphitic carbon when pyrolysed.¹³⁰

Activated carbon nanospheres were synthesized by one step pyrolysis using shaddock peel as the precursor. Shaddock peel is first washed using distilled water which is then dried at a temperature of 80 °C for about 12 hours in vacuum. The resultant is then pyrolysed at 800–1400 °C in inert atmosphere for 2 hours. The product is then washed using HCl and distilled water followed by drying at a temperature of 120 °C for 12 hours.¹³¹

Activated carbon nanospheres obtained from the above process were found to possess a surface area of 1272 m² g⁻¹ with pore volume of 0.49 cm³ g⁻¹ distributed over 0.6–15 nm range. It is found to have a stable cycle life.¹³²

Cyclic voltammograms of the activated carbon nanospheres obtained from shaddock peel is as shown in the Fig. 23. It depicts the CV curves of the carbon obtained at a scan rate of 0.1 mV s⁻¹.

Fig. 23 CV curve of activated carbon nanospheres derived from shaddock peel.¹³¹

4.13. Supercapacitors from activated carbon nanospheres derived from asparagus lettuce cells

Asparagus lettuce cells is the cultivar of the white bark which is majorly grown in China for its thick stem. It is considered to have lot of medical value and hence forms one of the major sources of bio-waste materials.

Change in texture under different pressure of processing showed that the asparagus lettuce cells at low pressure were found to be spherical and small. At moderate pressure, it was found that there was an initial texture loss in the asparagus lettuce due to loose skeleton wall. At high pressures, there was apparently less texture loss of asparagus lettuce which was the resultant of the unchanged pectin distribution. This shows that the asparagus lettuce cells synthesized at high pressure are spherical in shape and are applicable in supercapacitors.¹³³

Several carbon nanospheres obtained from bio-waste materials showed excellent performance in the area of supercapacitors. Advantages of reducing pollution in the environment followed by the cost reduction are the key factors for the future generation supercapacitors. Different bio-waste materials showed different characteristics which is listed in Tables 6 and 7 based on their lignocellulosic content. So it is worth-while to study more in this aspect, so that one is able to get high energy supercapacitors using carbon nanospheres which are obtained from cost effective bio-waste materials.

Table 6 Showing different activation methods for obtaining high surface areas and specific capacitance. It is to be noted that, electrolyte selection is also crucial to obtain high specific capacitance

Bio-waste carbon source	Activation method	Surface area (m^2 g^−1)	Specific capacitance (F g^−1)	Electrolyte	Reference
Sugarcane bagasse	ZnCl2	2871	109	1 M H2SO4	85
Coffee beans	ZnCl2	999.67	286	1 M H2SO4	89
Corncob	KOH	3530	401.6	1 M H2SO4	93
Natural flax	KMnO4	645	325.8	1 M H2SO4	95
Acacia gum	KOH	1764	272	1 M HCl	98
Coconut shell	KOH	2000	250	1 M H2SO4	102
Sunflower seeds	KOH	2585	244	0.1 M HCl	107
Rice husk	ZnCl2	1768	171	0.5 M NaOH	113
Carrageenan	KOH	2502	230	6 M KOH	119
Oil palm	No activation	—	342	5 M KOH	125
Scaphium scaphigerum	No activation	—	2770	—	129
Shaddok peel	HCl	1272	—	0.1 M HCl	132
Asparagus lettuce cells	—	—	—	—	133

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Table 7 Showing stability cycle along with pore volume

Bio-waste carbon source	Pore volume (cm^3 g^−1)	Stability (cycle life)	Reference
Sugarcane bagasse	0.81	10 000	85
Coffee beans	0.57	2000	89
Corncob	1.95	100 000	93
Natural flax	—	1000	95
Acacia gum	0.99	1000	98
Coconut shell	1.21	2000	102
Sunflower seeds	1.41	—	107
Rice husk	1.07	1000	113
Carrageenan	1.43	1000	119
Oil palm	0.02	2500	125
Scaphium scaphigerum	—	500	129
Shaddok peel	0.49	—	132
Asparagus lettuce cells	—	—	133

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5. Conclusion

Electrodes for supercapacitors that are made using carbon nanospheres derived from several bio-waste substrates were reviewed. We used specific capacitance, cycle time and electrode life as metrics for comparison of various electrode materials. Amongst the papers reviewed, supercapacitors made using activated carbon nanospheres obtained from corncob residue was found to have the highest cycle life of 100 000 which is an order of magnitude higher than the next best material oil palm leaves. The highest specific capacitance reported amongst the papers is 401.6 F g⁻¹ for capacitors made using corncob followed by oil palm leaves having the specific capacitance of 343 F g⁻¹. The largest surface area per unit weight is reported to be 3530 m² g⁻¹ for electrode fabricated from bio-waste material obtained from corncob. The experimental procedures for fabricating electrodes using carbon nanospheres were reviewed. The performance of the electrode depends on the electrolyte used. Since various end users have used different electrolyte materials, it is difficult to quantify and compare performance of the devices. Bio-waste derived carbon nanospheres hold a lot of potential in fabricating electrode materials for supercapacitors. They have the added advantage of being bio-degradable. Further studies are required to explore new sources of carbon nanospheres synthesised from wide variety of bio-waste materials.

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