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

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... 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  ow 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 di ff erent 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. Further the electrons  ow 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 $ 10 6 Farad. 26 The capacitance that is produced from the electrochemical double layer is analogues to a parallel plate capacitor. 21 The excess or de  ciency 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. 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 speci  c capacitances and the performance of the supercapacitors fabricated with various metal oxide obtained at di ff erent scan rates is as shown in the Fig. 4. From the Fig. 4 we observe that the performance of the electrode with Co 3 O 4 delivers the best speci  c capacitance in comparison to all the precursors. 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 e ff ectively gives room to two layers for the chare storage. 35 This helps in making supercapacitors more suitable for electrostatic storage of charges. 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 galvano- metric characteristics of a typical supercapacitor. 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. From the Fig. 8, it is visible that supercapacitors occupy a signi  cant position in speci  c energy and speci  c 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 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. 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 super  cially. 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 classi  ed 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 su ff ers 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. The two underlying elements for supercapacitors to meet the pressing requirement of the energy crisis are new materials and advanced con  gurations. 28 Numerous researches is going on in  nding the better material to  t in as an idle supercapacitor. 44,45 A proper control over the pore size and speci  c area of the electrode plays a signi  cant role in choosing the material for the electrode to ensure good performance of the supercapacitors. 46 The capacitive performance of di ff erent 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 speci  c capacitance, large surface area and easy processability. 22,42 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 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. In the last few years, carbon and its materials have been studied with great interest due to its excellent electrochemical storage of energy. 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 graphi- tization 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 Chie  y 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. ...