Storage of energy has a major problem in recent years. Battery, fuel cells and capacitor are some of the examples of energy storage devices in which electrical energy is stored. In the last decade, the supercapacitor has come up as a new energy storage device. In battery and fuel cells, chemical energy is transformed into electrical energy – while in supercapacitor charge storage mainly takes place by electrostatic principle. Every energy storage device has its own advantage and disadvantage – and so is true for the supercapacitor. As compared to capacitor and battery, supercapacitor has moderately high power density and energy density.
Supercapacitors are made up of two electrodes of highly porous material, which are immersed in an electrolytic solution and separated by a separator. Electrolytes provide conducting medium while separator prevents internal short circuit between the electrodes. Voltage rating and internal resistance of supercapacitor are determined by electrolytes. Electrolytes of any energy storage device are basically of two types: i) aqueous electrolyte and ii) non-aqueous electrolyte. Aqueous electrolytes such as potassium sulphate, potassium hydroxide are non-toxic, highly conductive, and they have efficient charge transfer mechanism. So,they are gaining more popularity for the electrolyte of supercapacitor. On the other hand, non-aqueous electrolyte such as aceonitrile is flammable, toxic in nature. Moreover, they are costly and hence they are not preferred. Based on electrode design, supercapacitors are divided into three types (i) electric double layer, (ii) hybrid capacitor and (iii) pseudocapacitor. Electric double layer and pseudocapacitor are symmetric in nature i.e., both electrodes have same material. For hybrid capacitor, one electrode is made up of carbon material and the other of composite material consisting of carbon and other metal oxides.Carbon with high specific surface area, low impurity and high porosity is suitable for electrodes of supercapacitor. Carbon materials like activated carbon, carbon nano-tubes, graphene, carbon aerogels etc., are used as materials for the electrodes of supercapacitor and hybrid capacitor. By mixing graphene or metal oxide with activated carbon, composite materials are formed, which are also suitable for electrodes of supercapacitors. Electrode material determines the capacitance of supercapacitor and hybrid capacitor. Scientists have tried several methods to synthesise high quality carbon, which includes vapour deposition, carbonisation of organic precursors, arc discharge synthesis etc. High capacitance supercapacitors are also developed from multi-walled carbon nano-tubes,synthesised from petroleum based precursor. Activated carbon obtained by petroleum precursor is very expensive and also it will not be available after few decades. Therefore, it is not commercially viable. A recent method of synthesising activated carbon is by burning dead leaves of trees at high temperature, which can be used as an electrode material for the supercapacitor.
Waste management is a major problem faced by big cities. Several environmental communities are taking initiatives for separating out waste material. Carbon content in such waste material is comparatively high. By burning these waste materials, harmful gases are produced along with ash. Dead leaves of trees, which are also a waste material, can be used for obtaining activated carbon. Activated carbon obtained by burning dead leaves at high temperature can be cost-effective and environmental friendly. To synthesise activated carbon, dead leaves are burnt at high temperature in absence of oxygen. After heating, it is washed with sulphuric acid and the solution is then filtered. This article presents a new methodology of synthesising activated carbon from neem tree leaves and coconut shell. The activated carbon derived from neem tree leaves and coconut shell at various temperatures is compared along with commercially available Vulcun XC-72R activated carbon. Area based capacitance is important when volume and shape are design parameters. Specific capacitance is important when weight of the electrode material is significant. Pulse current, energy density and power density of supercapacitor are most significant factors for automobiles applications. Therefore, comparison of various derived activated carbon is made on the parameters of area based capacitance, specific capacitance, pulse current, energy density and power density.
Experimentation with leaves of tree
For the development of supercapacitor, activated carbon plays an important role. It is used as main electrode material in supercapacitors. There are several processes of obtaining activated carbon. A recent method of obtaining activated carbon is from dead leaves of trees. For this purpose, composition of various leaves of trees were studied – and it was found that neem tree leaves and coconut shell have relatively high carbon content as compared to others. So, for obtaining activated carbon, neem tree leaves are collected, and those are thoroughly washed. After washing, it is kept in sunlight for drying purpose and then it is crushed with the help of a grinder. To synthesise activated carbon, neem tree leaves powder is heated in a furnace at 300OC and 500OC respectively in presence of nitrogen. Initially temperature is increased at a rate of 10OC/min from room temperature. After attaining 300OC, it is heated continuously for 5 hours. After heating for 5 hours, colour of neem tree leaves powder changes from light green to black colour carbon. After cooling the sample, it is washed with 3 molar strength sulphuric acid solution – so that ash content gets dissolved in acid and the samples are then filtered with the help of filter paper. After filtering, these samples are kept in oven at 100OC for half an hour for drying purpose. These samples are then crushed into fine powder with the help of mortar pestle. Similar type of procedure is repeated for heating neem tree leaves at 500OC for 2 hours. The obtained activated carbon is ready for use in electrodes of supercapacitor.
Fig. 1: Flow chart for the synthesis of activated carbon from neem tree leaves…
A flow chart shown in Fig. 1 gives the detail process of obtaining activated carbon from neem tree leaves. This process of obtaining activated carbon is generic and can also be applicable to other dead leaves with minor changes.
For obtaining activated carbon from coconut shell, it is crushed in to small pieces and then phosphoric acid is mixed in it in the ratio of 1:2. The sample is then kept in a rotary kiln at a temperature of 1000OC for 1 hour in the absence of air. After 1 hour, sample is then taken out and is washed with demineralized water. Another commercially available activated carbon i.e., Vulcun XC-72R is used in experimentation. It is generally obtained by pyrolysis process in which carbonaceous material is heated at a very high temperature in absence of air. Vulcun XC-72R is used along with coconut shell derived carbon and neem tree leaves derived carbon for trails. The derived activated carbon by various methods is used for making electrodes of supercapacitor.
For development of prototype, wiremesh SS316 is used as current collector. Loading on the wire mesh is done by mixing 20% Vulcun XC-72R with 80% derived carbon and a paste of two are formed by using iso-propyl alcohol. Then the wire mesh loaded with carbon material is sandwich in between three separators. After developing the prototype of various activated carbons, they are put in an electrolyte solution.
Various prototypes of supecapacitor are made with the activated carbon. Comparison of activated carbon derived from neem tree leaves at various temperatures, coconut shell and commercially available Vulcun XC-72R are made on the basis of specific capacitance, energy density, area based capacitance, pulse current, internal resistance, and power density.
On comparing supercapacitor of various activated carbons, it was found that neem tree leaves derived activated carbon heated at 500OC for two hours gives much better performance. However, specific capacitance and energy density of coconut shell derived carbon heated at 1000OC is equivalent to that of neem tree leaves derived carbon heated at 500OC. From these, it can be concluded that functional properties such as specific surface area and porosity of neem tree leaves derived carbon heated at 500OC for 2 hours sample is similar to that of coconut shell derived carbon.
Fig. 2: Discharge current characteristics for various carbon derived supercapacitor…
A discharge current characteristic of various activated carbons is shown in Fig. 2. Area under the curve gives the charge stored by the supercapacitor, which determines the capacitance. Neem tree leaves derived carbon heated at 500OC has more area under the curve as compared to others. So, it has high capacitance. Also, it has high pulse current and low internal resistance, which results in high power density. Coconut shell derived carbon heated at 1000OC has low pulse current as it has high internal resistance, & hence low power density compared to other activated carbon. But it has almost same capacitance as that of neem tree leaves derived carbon heated at 500OC as its discharge current characteristics after first few seconds almost matches with neem tree leaves derived carbon heated at 500OC. Discharge rate of neem tree leaves derived carbon heated at 300OC is faster as compared to other activated carbon. So, it has less area under curve, which results in lowest capacitance.
Area based capacitance & power density
Fig. 3: Area based capacitance (F/cm2) and power density (kW/gm) for various carbon derived supercapacitor…
Fig. 3 shows area based capacitance and power density of various activated carbons. As per parameters shown in Table 1, neem tree leaves derived carbon heated at 500OC and coconut shell derived carbon heated at 1000OC has almost same area based capacitance. But due to low pulse current,coconut shell derived carbon has high internal resistance, which results in poor power density among all activated carbon. However, discharge characteristic of Vulcun XC-72R is above the neem tree leaves derived carbon at 300OC. Therefore, it has high capacitance and power density than neem tree leaves derived carbon heated at 300OC.
Specific capacitance and energy density
Fig. 4: Specific capacitance (F/gm.) and energy density (Wh/gm) for various carbon derived supercapacitors…
Specific capacitance and energy density of various activated carbons are shown in Fig.4. As per parameter in Table 1,Vulcun XC-72R and neem tree leaves derived carbon heated at 300OC has low specific capacitance & energy density compared to coconut shell derived carbon and neem tree leaves derived carbon heated at 500OC.
However, coconut shell derived carbon heated at 1000OC has almost same specific capacitance and energy density as that of neem tree leaves derived carbon heated at 500OC. But the process of synthesizing activated carbon from neem tree leaves is much easier than coconut shell derived carbon. So, it will be preferred for making electrodes of supercapacitor. Thus, in future neem tree leaves derived carbon can be a better option than coconut shell based carbon.
Pulse current and internal resistance
Fig. 5: Pulse current (mA) & internal resistance (Ω) for various carbon derived supercapacitor…
Pulse current and internal resistance of various activated carbons are shown in Fig.5. Internal resistance depend on the electrode and electrolyte materials. On comparing pulse current and internal resistance of various activated carbons, it was found that coconut shell derived carbon has low pulse current and high internal resistance which is undesirable. Vulcun XC-72R has high pulse current and low internal resistance than coconut shell and neem tree leaves derived carbon heated at 300OC. Neem tree leaves derived carbon heated at 500OC has high pulse current and low internal resistance compared to other activated carbons. The process of obtaining activated carbon from neem tree leaves is much easier compared to Vulcun XC-72R. Moreover, it can be cost effective and enviornmental friendly. So, it will be the preferred more.
Activated carbon synthesised from leaves of neem tree and coconut shell along with commercially available activated carbon i.e. , Vulcun XC-72R is studied for supercapacitor applications. On comparing these carbons materials at various temperatures, it was found that neem tree leaves derived carbon heated at 500OC for two hours gives better specific capacitance, energy density, power density and pulse current as compared to other activated carbons. However, coconut shell derived carbon heated at 1000OC has almost same energy density and capacitance as that of neem tree leaves derived activated carbon heated at 500OC. But the process of synthesising carbon from neem tree leaves is much easier as compared to coconut derived carbon. Activated carbon obtained from neem tree leaves involve heating at low temperature, which is cost effective and environmentally friendly. Specific capacitance and energy density can be further increased by mixing metal oxide in proper proportion along with the derived activated carbon.