CHAPTER 2 EXPERIMENTAL TECHNIQUES

Transcription

1 CHAPTER 2 EXPERIMENTAL TECHNIQUES In the present doctoral work, effort has been made to synthesize different types of biomass based activated charcoal for its application as an electrode material in electrochemical double layer capacitors abbreviated as EDLCs. This chapter briefly describes the general methods of activation for activated charcoal followed by the synthesis techniques used in the present work. It also covers all the experimental techniques used for the characterization of synthesized biomass based activated carbon. General preparation methods for electrode and electrolyte materials were discussed in brief. Finally, the fabrication and characterization of electrochemical double layer capacitor using synthesized electrode and electrolyte materials are discussed in details. The greatest advantage of using carbon as an electrode material is that it can be easily converted into the form that has very high specific surface area. Commonly, the process used in order to increase the surface area or porosity from a carbonized organic precursor or char is called as activation and the resulting broad group of materials is referred to as activated carbons. Generally chars have low porosity and their structure consist of elementary crystallites having large number of interstices between them. These interstices are filled with disorganized carbon residues known as tar that block the pore entrances. These pores are open during the process of activation along with the development of additional porosity. By changing the carbon precursor and activation conditions like temperature, time and gaseous environment its resulting porosity, pore size distribution and the nature of the internal structures can be controlled. The activation processes are generally categorized into two types: physical activation and chemical activation [Pierson (1993), Bansal et al. (1999)] Physical activation also known as thermal activation involves the modification of a carbon char by controlled gasification and is generally carried out at higher temperature ranging between 700 to 1100 C in the presence of suitable oxidizing agents like steam, carbon dioxide, air or 41

2 mixture of these gases. During gasification, the oxidizing atmosphere greatly enhances the pore volume and surface area of the material with the help of controlled carbon burn-off and removal of volatile pyrolysis products. The level of burn-off is possibly the most important factor which governs the quality of activated carbon and is controlled by the temperature and duration of activation. A high degree of activation is obtained when burn-off is increased but additionally there is a decrease in carbon strength, lower density, reduced yield and widening of pores. Chemical activation is generally carried out at comparatively lower temperatures (~ C) and this process involves the dehydrating action of certain agents such as phosphoric acid, zinc chloride and potassium hydroxide. In this method post-activation washing of the carbon is generally required in order to eliminate residual reactants as well as any inorganic residue that originates from the carbon precursor or is introduced during activation. Exceptionally high surface area materials almost >2500 m 2 g -1 have been reported with the help of potassium hydroxide [Lozano-Castell o et al. (2003)]. The details of all the techniques used in the present work are given in the following sections: 2.1 Synthesis Techniques Used in the Present Studies Synthesis of Chemically Treated Activated Charcoal Commercially available charcoal was procured from Loba Chemie and was used as a precursor in the present study. Chemical activation with KOH (potassium hydroxide) was carried out using an impregnation method. In the process, five grams of activated charcoal was mixed by stirring a solution containing 15 ml of double distilled water with activating agent KOH for 2 h at 60 C in 1:1 ratio. The resulting slurry was dried at 110 C for h in an oven and the resulting mixture was used for carbonization. The carbonization was carried out in a muffle furnace and the sample was heated (10 C min -1 ) from room temperature to the final carbonization temperature (T carb = 600 C) in the presence of nitrogen flow (150 ml min -1 ). Sample was kept at the final temperature for 1 h before cooling down up to room temperature under same nitrogen flow. The pyrolyzed sample was washed repeatedly with 5 M HCl solution and later with double distilled water until it gets free from chloride ions. 42

3 Figure 2.1: Schematic Flow Chart for Preparation of Chemically Treated Activated Charcoal Once the activating agent was removed, the sample was dried at 110 C for h. A schematic flowchart showing the preparation of chemically treated activated charcoal is shown in Fig Synthesis of Biomass Based Chemically Treated Activated Charcoal A schematic flowchart for the preparation of biomass based chemically treated activated charcoal is shown in Fig The samples used in the present studies are eucalyptus leaves, coconut shell and almond shell. In the process, biomass sample was first of all cleaned from other Figure 2.2: Schematic flowchart showing preparation of biomass based chemically treated activated charcoal materials such as wood, soil etc. Sun dried it for 2-3 days. The dried sample was then kept in muffle furnace at 300 C for 5 h. Thereafter it was soaked in chemical solution of CaCl 2 (25 wt 43

4 %) for h to get activated charcoal. Activated charcoal was then washed with double distilled water. Samples were then kept in oven at 110 C (overnight) for drying. Further, chemical activation using impregnation method has been carried out in which KOH was used as an activating agent. For the purpose, five grams of the activated charcoal sample was mixed by stirring a solution containing 15 ml of double distilled water with activating agent KOH for 2 h at 60 C in 1:1 ratio. The resulting slurry was dried at 110 C for h in an oven and the resulting mixture was used for carbonization. The carbonization was carried out in a muffle furnace and the sample was heated (10 C min -1 ) from room temperature to the final carbonization temperature (T carb = 600 C) in the presence of nitrogen flow (150 ml min -1 ). Sample was kept at the final temperature for 1 h before cooling down up to room temperature under same nitrogen flow. The pyrolyzed sample was washed repeatedly with 5 M HCl solution and later with double distilled water until it gets free from chloride ions. Once the activating agent was removed, sample was dried at 110 C for h Preparation of Electrode Materials Fig. 2.3 shows the schematic diagram for the preparation of activated charcoal powder based electrodes. The electrodes were prepared by making slurry of chemically treated activated Figure 2.3: Flow chart for the preparation of activated charcoal powder electrode 44

5 charcoal powder and polymer (PVdF-HFP) in the ratio 90:10 (w/w) in a common solvent acetone by thorough mixing. Fine films of electrodes were coated by spraying the slurry on carbon cloth (Ballard, USA) and kept in oven at 70 C for h. The same procedure was followed for making electrodes of biomass based chemically treated activated charcoal powder Synthesis Technique Used for Electrolyte Materials In the present studies three different types of polymeric materials have been synthesized using standard solution cast techniques namely; nano composite polymer gel electrolyte, blend polymer electrolyte and ionic liquid based gel polymer electrolytes. In the process, different polymers, inorganic salt, ionic liquid and nano filler has been used without further purification. Block diagram for the synthesis of different types of polymer electrolyte is shown in Fig Figure 2.4: General block diagram for the synthesis of polymer gel electrolyte For blend polymer electrolyte, two different polymers in different weight ratios were dissolved in a common solvent to get polymer blend. Secondly, liquid electrolytes were prepared by dissolving suitable amount of dopant salt in a common solvent separately. Thereafter, the mixture of polymer blend and liquid electrolytes in suitable amounts were mixed together and stirred continuously with a magnetic stirrer at a suitable temperature until homogenous solutions 45

6 were obtained. Finally the solutions were poured into glass petri dishes and left to dry at room temperature for some days to form a free standing film. The films were kept in desiccators for further drying and to avoid any atmospheric conditions. For ionic liquid based polymer gel electrolyte, initially the liquid electrolytes were prepared by dissolving different concentration of salt in plasticizer. The different weight percentage of polymer and ionic liquids were dissolved separately in a common solvent. The optimized composition of liquid electrolyte was then mixed with the optimized solution of polymer/ionic liquid blend in different weight ratios and stirred continuously. The viscous mixture was then cast on glass petri dishes for few days until free standing polymer gel electrolyte films were obtained. The films were kept in desiccators for further drying to avoid any atmospheric contaminations. For nano gel polymer electrolyte, initially the liquid electrolytes were prepared by dissolving different concentration of salt in plasticizer. The host polymer was separately dissolved in solvent using magnetic stirrer and thereafter its different amount was mixed in the optimized liquid electrolyte. To prepare composite gel polymer electrolyte, nano filler in different weight ratios were dispersed. In the composite gel polymer electrolyte, the ratio of polymer with respect to the liquid electrolyte was maintained at fixed weight percent throughout the synthesis. Finally the mixtures were poured in glass petri dishes and allowed to evaporate volatile solvent to obtain solid-like free-standing composite gel films. The films were kept in desiccators for further drying to avoid any atmospheric contaminations. 2.2 Physical Characterization Techniques used for Electrode Materials Brunauer, Emmett and Teller (BET) Surface Area Analysis The surface morphology and pore structure studies of the synthesized samples were carried out with the help of a Brunauer-Emmett-Teller (BET), Micrometrics Instruments, Gemini Model 2380 surface area analyzer and Autosorb-1 Quantachrome Instruments. The powder samples were dried at 383 K for almost 6 h prior to the BET analysis using surface adsorption of N 2 at 77 K. Multipoint BET analysis was performed between pressure range of 4 to 730 mm Hg. 46

7 Historically in 1938, Brunauer et al. proposed a theory which was based on a kinetic model of adsorption given by Langmuir in 1916 and represents a solid surface as an array of adsorption sites. Equilibrium takes place when the rate at which molecules arriving from the gas phase and condensing or adsorbing onto unoccupied adsorption sites is equal to the rate at which molecules evaporate or desorb from occupied sites. For the case of monolayer adsorption, the mathematical equation for Langmuir equilibrium adsorption [Langmuir (1916)] can be written as: = (2.1) where, n is the amount of adsorbate (in moles) adsorbed on 1 g of adsorbent, n m is the monolayer capacity (the adsorption of one molecular layer of the adsorbate on the adsorbent), B is an empirical constant, and; P is the partial pressure of the adsorbate Further by assuming multiple adsorptive layers 1, the BET equilibrium adsorption equation can be written as: = { ( ) } (2.2) where, c = exp q q 1 (2.3) R g T P 0 is the saturation vapor pressure of the adsorbate, (q-q 1 ) is the net heat of adsorption, R g is the ideal gas constant and T is the temperature in Kelvin [Gregg and Singh (1982)]. 1 The reader please consult Brunauer, Emmett and Teller (1938) or Gregg and Sing (1982) for assumptions made and the resulting derivation for equation (2.2) 47

8 Since adsorption experiments frequently measures the adsorbed volume rather than moles adsorbed, therefore it is convenient to represent equation (2.3) in terms of equation (2.4). = { ( ) } (2.4) where, v is the volume adsorbed per gram of adsorbent and V m is the monolayer adsorption capacity in terms of volume. By plotting versus over the range of 0.05 < < 0.35, the parameters V m and c can be determined using equations (2.5) and (2.6). V m = (2.5) c = + 1 (2.6) The surface area S of the adsorbent measured in (m 2 g -1 ) can be calculated using equation (2.7). S = (2.7) where, σ is the surface area of an adsorbate molecule, N A is Avogadro s number (6.022 x number/mole), V i is the molar volume of the gas (22.4 L mol -1 at STP) and V m is volume of adsorbent (gas) measured in the units of cm 3 g -1. Different types of adsorbates are generally used to find out the surface area of an adsorbent, amongst which nitrogen at 77 K is the most common one. Some other frequently used adsorbates are benzene at 293 K and carbon dioxide at 195, 273 or 293 K. The above equations can be used for any types of adsorbate, but molecular packing and pore sieving effects should be taken care of while choosing an adsorbate molecule for surface area calculation. Generally an adsorbate whose saturation pressure is comparatively large is preferred for determining surface 48

9 area, so that broad range of relative pressure can be covered at the chosen adsorption temperature. In 1967, McClellan and Harnsberger compiled a list of adsorbate alongwith its molecular descriptions, some of which are enlisted in Table 2.1 (McClellan and Harnsberger (1967)). Table 2.1 Adsorbate Molecule Area of Cross Section (σ) [Å] Molecular Dimension [Å] Water (H 2 O) Nitrogen (N 2 ) Acetone (C 3 H 6 O) Carbon Dioxide Benzene (C 6 H 6 ) (3.7 Å x 7.0 Å) A criticism of BET theory is the assumption that all adsorption sites on the solid surface are energetically homogenous. But actually most adsorption sites are energetically heterogeneous not homogenous. Another anomaly is that the model neglects adsorbate-adsorbate interactions, which actually are not negligible when an adsorption layer is near completion and the average separation is small in relation to their size [Gregg and Sing (1982)]. (a) Adsorption Isotherms and its Types Whenever a solid (adsorbent) is exposed to a gas or vapor (adsorbate), the solid starts to adsorb the gas onto its surface and into its pores, perhaps the solid should be porous. Adsorption takes place due to the force acting between solid and the gas molecules. There are two types of forces that give rise to adsorption: physical (also known as Vander Waal forces) and chemical. These types of forces are called as physical adsorption and chemisorption. In a closed system, the adsorption of a gas onto a solid can be determined by monitoring the decrease in adsorbate pressure within a known volume or by calculating the mass gain of the 49

10 adsorbent due to the adsorbing gas molecule. Both methods are commonly used and these methods give accurate and precise results. Figure 2.5: Five types of adsorption isotherms as classified by Brunauer, Deming, Deming and Teller (BDDT) The amount of gas adsorbed in moles per gram of solid is a function of partial pressure or concentration of adsorbate, temperature of the system, the nature and types of adsorbate and the adsorbent. The amount of compound that is adsorbed on an adsorbent versus concentration or pressure at a constant temperature results in an adsorption isotherm. Adsorption isotherms are very useful in order to characterize adsorbents with respect to different adsorbates. In the literature of adsorption almost thousands of isotherms are reported which are measured for many different adsorbents. The majority of these isotherms are categorized into five different types as classified by Brunauer, Deming, Deming and Teller (BDDT) and are presented in Fig. 2.5 [Gregg and Sing (1982), Brunauer et al. (1940)]. Type I isotherm is observed by the physical adsorption of gases onto microporous solids. Type II isotherm is due to the physical adsorption of gases by non porous solids. Type IV results from the physical adsorption of gases by mesoporous solids. Type III and V may originate from the adsorption of either polar or non polar molecules with a condition that the adsorbate-adsorbent force is comparatively weak. It may also be pointed out that type V isotherm possesses a hysteresis loop. The adsorption of water vapor on activated carbon is an example of type V isotherm. 50

11 (b) Adsorption Forces Adsorption of a gas onto a solid is the result of the attraction forces acting between adsorbate and adsorbent molecules. Presently, adsorption models are idealized and it is not possible to find out an adsorption isotherm which depends upon independently determined parameters of gas and solid [Gregg and Sing (1982)]. Adsorption forces cover dispersion forces which are attractive in nature, short range repulsive forces and electrostatic/columbic forces if either the solid or gas is polar. Dispersion forces or Vander-Waal forces arises due to the rapid fluctuation in electron density within each atom. This induces an electrical dipole moment in neighboring atom, which results to an attraction between the atoms. (c) Pore Size Individual size of the pore can vary in shape and size for different adsorbents as well as within the same adsorbent. Pores are generally classified in terms of their width i.e. the diameter of a cylindrical pore or the distance between two sides of a silt shaped pore. In 1960, Dubinin suggested the classification of pores which are summarized in Table 2.2 which was later adopted by the International Union of Pure and Applied Chemistry (IUPAC) in Table 2.2 Pore Classification Micropores Mesopores Macropores Pore Width less than ~20 Å (2 nm) between ~ Å (2-50 nm) More than ~500 Å (50 nm) The basis for the classification of pores which are enlisted in Table 2.2 is that each size range corresponds to different adsorption effects as can be seen from adsorption isotherm. In case of micropores, the interaction potential is comparatively greater than that in larger pores due to the closeness of the pore walls which in turn results in an enhanced adsorption potential. Moreover, force acting on adsorbate molecule is a function of distance between adsorbate and adsorbent 51

12 atoms (pore size) and its polarity is either permanent or reduced as compared to adsorbate and adsorbent atoms. In mesopores, capillary condensation takes place which in turn results in the formation of hysteresis loop in the adsorption isotherm. The pore size in the macropore range is so wide that it becomes impossible to figure out the isotherm in detail because the relative pressure of the adsorbate (P/P 0 ) would be close to unity. For observing macropore structure, mercury is generally used because of its low vapor pressure X-Ray Diffraction Studies (XRD) X-Ray powder diffraction (XRD) was performed using X Pert PRO, Panalytical, Netherlands equipped with a copper target X-Ray tube with operational conditions; operational voltage of 40 kv and current of 40 ma respectively. The prepared activated carbon samples were placed inside a square shaped sample holder. The samples were scanned between 10 to 70. XRD is one of the most useful and powerful tools for determining structure of the materials [Cullity (1978)]. Along with the determination of the structures, XRD is also used to find out other problems such as chemical analysis, stress measurement, phase equilibrium, particle size measurement etc. The working principle of XRD is based on the scattering of X-rays by the sample. Therefore microscopic structure of the given sample can be determined with the help of XRD. Bragg s law gives the condition for obtaining XRD from a crystalline material, as per the equation: nλ = 2d sinθ (2.8) where, n is the order of diffraction λ is the wavelength of the characteristic line X-rays d is the distance (Å) between the set of parallel lattice planes θ is the angle between the incident collimated X-ray beam and an atomic lattice plane in the crystal 52

13 The angle of reflection for a particular set of lattice planes (hkl) is given as: 2θ = 2 sin -1 (2.9) ( ) where, (hkl) are the miller indices which defines orientation of the plane with respect to the crystallographic axis. When X-rays of a known wavelength falls on the sample whose lattice planes are separated by a distance (d), strong reflection occurs only at those angles where scattering from lattice plane is in phase. This requires the path difference to be an integral multiple n of the wavelength i.e. Bragg s condition has to be satisfied [Cullity (1978)] Scanning Electron Microscopy (SEM) Scanning electron microscope is a type of electron microscope that produces image of a sample by scanning it with a focused beam of electrons. The electrons then interact with atoms in the sample producing various signals that can be detected and contains information about the surface topography and composition. In scanning electron microscope, the electron beam is generally scanned in a raster scan pattern and the beams position is combined with the detected signal to generate an image. By using SEM one can achieve resolution better than 1 nm. In the present work, the materials microstructure was analyzed by scanning electron microscopy (SEM); JEOL Model-JSM 6380LA and JSM-6360, JEOL/EO Thermogravimetric Analysis (TGA) Thermogravimetric analysis (TGA) is the most widely used thermal method for material characterization [Brown (2001), Duval (1962), Wandlandt (1986)]. It is based on the measurement of mass loss of material as a function of temperature. In thermogravimetry studies, a continuous graph of mass change against temperature is obtained when a substance is heated at a uniform rate or kept at constant temperature. A plot of mass change versus temperature (T) is referred to as the thermogravimetric curve also known as TG curve. For the TG curve one generally plot mass (m) which is decreasing downwards on the y-axis (ordinate) and temperature (T) increasing to the right on the x-axis (abscissa). TG curve helps in revealing the extent of 53

14 purity of analytical samples and in determining the mode of their transformations within specified range of temperature. In thermogravimetric studies, the term decomposition temperature is a complete misnomer. In the present work, the TG curve of synthesized samples shows single stage decomposition. In single stage decomposition, there are two characteristic temperatures; the initial temperature T i and final temperature T f. T i is defined as the lowest temperature at which the onset of mass change gets started which can be detected by thermo-balance operating under particular conditions and T f is the final temperature at which the particular decomposition appear to be completed. Though T i has no fundamental importance but still it can be a useful characteristic of a TG curve and then afterwards the term procedural decomposition temperature has been suggested. The difference between T f T i is known as reaction interval [Brown (2001), Duval (1962), Wandlandt (1986)]. The instrument used in thermogravimetry (TG) is known as thermo-balance. It comprises of various basic components in order to provide the flexibility which is necessary for the production of useful analytical data in the form of TGA curve [Gunzzler and Williams (2001), Meyer (2000), Dean John (1995)]. The basic components of a typical thermo-balance are enlisted below: a) Balance b) Furnace (heating device) c) Unit for temperature measurement and control (programmer) d) Recorder (used for automatic recording for the mass and temperature changes) These components can be represented with the help of block diagram, which is shown in Fig

15 Figure 2.6: Block diagram of Thermo-balance (a) Balance The basic requirements of the automatic recording balance are accuracy, sensitivity, reproducibility and capacity. Balances are of two types: null type and deflection type. Null type balance is the most widely used and it consists of a sensing element which detects a deviation of a balance beam from its null position. Whereas in deflection type balance, it involves the conversion of the balance beam deflection about the fulcrum into a suitable mass change trace by (i) photographic recording i.e. change in the path of a deflected beam of light available for photographic recording, (ii) recording electric signals generated by an appropriate displacement measurement transducer, and; (iii) using an electrochemical device (b) Furnace The furnace and control system of TG is designed in such a way that it can produce a linear heating over the whole working temperature range of the furnace with a proper arrangement to maintain a fixed temperature. A wide temperature range varying from -150 C to 2000 C of furnaces are used in the instruments. The range of the temperature basically depends upon the type of heating elements used in the furnace. 55

16 (c) Temperature measurement and control Temperature measurements are usually carried out using thermocouples in which chromal-alumel thermocouple are commonly used for temperature up to 1100 C whereas Pt/(Pt- 10% Rh) is used for temperature up to 1750 C. Temperature can be varied using a program controller with two thermocouple arrangement in which the signal from one actuates the control system while second thermocouple is used to record the temperature. (d) Recorder Graphic recorders are mostly preferred to meter type recorders. X-Y recorders are commonly used, as they can plot weight directly against temperature. The present instrument facilitate microprocessor controlled operation along with digital data acquisition and processing using personal computer which consist of different types of recorder and plotter for better presentation of data. In the present studies, thermal analysis of the prepared sample was conducted by using thermogravimetric analysis (TGA). For these thermal analysis, Diamond TGA/ DTA, Perkin Elmer Instruments, USA was used with nitrogen gas purging for atmospheric control. TGA analysis was conducted in the temperature range of C (heating rate of 10 C min -1 ). 2.3 Electrochemical Apparatus Recent electrochemical workstations comprises of three main components: a single waveform generator (SWG), a potentiostat / galvanostat (PG) and a computer. The user defines all the parameters on the computer, which transfers different instructions to the SWG/PG block. The SWG/PG block transmits the required signal to electrochemical cell and finally the measurements are performed. The block diagram comprising of the complete system is given in Fig Each electrochemical method is defined by its own waveform. There exist several transient techniques like cyclic voltammetry, chronopotentiometry, chronoamperometry etc and stationary techniques like electrochemical impedance spectroscopy, rotating disk electrode etc for electrochemical characterization of cell [Béguin and Frąckowiak (2013)]. 56

17 Figure 2.7: An electrochemical workstation In order to characterize any electrochemical cell, both types of electrode configurations (two electrode or three electrodes) can be performed depending on need and requirement. Fig. 2.8 gives a schematic view of a cell connected to an electrochemical workstation. Figure 2.8: Electrochemical cell configuration (a) two electrode (CE and RE are shorted), and; (b) three electrode Basically, the current flows through the counter electrode (CE) and working electrode (WE), and the corresponding voltage is measured or controlled between the reference electrode (RE) and the WE. For a two electrode cell, the voltage measured is the cell voltage, as the CE and RE is shorted in such a configuration and for a three electrode cell, a third electrode is added and it acts as the RE. The RE used for this purpose should have an ideal non-polarizable characteristics i.e. its voltage is constant over a large range of current densities. Hence, in this way working voltage is measured accurately. 57

18 2.4 Electrochemical Characterization Methods Supercapacitors are characterized by using various electrochemical techniques such as (i) a.c impedance spectroscopy (ii) cyclic voltammetry, and (iii) charge discharge methods a.c Impedance Spectroscopy An electrochemical supercapacitor cell, as discussed in previous section, is fabricated by using two identical electrodes and the polymer electrolyte is sandwiched in between them. a.c impedance spectroscopy techniques have seen a tremendous and remarkable increase in its popularity in recent years. Initially it was used to find out the double layer capacitance but now-a-days, they are also used for the characterization of electrode processes and complex interfaces. These measurements are carried out at different a.c frequencies and hence the name a.c impedance spectroscopy was adopted. By the analysis of impedance data, the information about the interface, its structure and reactions taking place at the interface can be obtained. However, a.c impedance spectroscopy is a sensitive technique and must be used with a great care. It is not always well understood because of the incomplete mathematical developments of equations connecting the impedance data with the physico-chemical parameters. It is a complementary technique and other methods must also be used to elucidate the interfacial processes. a.c impedance spectroscopy gives the direct connection between the real system and idealized equivalent circuit which comprises of discrete electrical components (R, C and L) in their series and parallel combination. In the present case, electrochemical capacitors are the real systems in which either blocking/polarizable electrodes (in case of electrochemical double layer capacitors) or electroactive electrode materials (in case of redox capacitors) are used. The blocking electrodes used in the capacitors have either planar geometry (such as glassy carbon sheets, high density carbon sheets etc) or large surface area porous electrodes (like activated carbon powder, carbon fibers or fabrics etc). The impedance behavior of an ideal and a real capacitor cells are shown in Fig As can be seen from Fig. 2.9(a) the impedance pattern of an ideal capacitor shows a steep rising dispersion line which is overlapping with imaginary axis of the impedance plot. Whereas from 58

19 Fig. 2.9(b), it can be seen that impedance plot of a real capacitor shows an additional semicircle representing a parallel combination of a bulk resistance R b (resistive component due to the ion migration owing to bulk electrolytes) and geometrical capacitance C g (due to dielectric polarization of electrolyte under the influence of electric field). Figure 2.9: Impedance plots of (a) ideal capacitor (b) real capacitor The interfacial polarization is responsible for the formation of a double layer capacitor at the two identical interfaces, which is represented by a steep rising spike in the low frequency region of the impedance spectra. The equivalent circuit of capacitor cells with either rough or porous electrode in contact with the electrolytes can be represented with the help of transmission line model (TLM) and its distributed circuit gives variable time constant. The region of the impedance spectrum associated with a high capacitance at low frequencies is often interpreted in terms of distributed R and C network due to ionic penetration throughout the polymer films or the porous electrodes. In the present studies, impedance studies were carried out using computer controlled CHI 608C, CH Instruments, USA in the frequency range from 1 mhz to 100 khz. The signal level was kept at 10 mv. The overall capacitance of the cell can be calculated by using the relation: C = " (2.10) 59

20 where, ω is the angular frequency and Z is the imaginary part of the total impedance measured in low frequency range. The single electrode specific capacitance values were evaluated by multiplying the overall capacitance by a factor of two and divided by the mass of the single electrode material Cyclic voltammetry Cyclic voltammetry (CV) is a powerful tool in the field of electrochemistry [Bard and Faulkner (2001), Compton and Banks (2007)]. It has been used extensively to characterize the performance of various electrical energy storing devices such as supercapacitors [Conway (1999), Frackowiak and Beguin (2001), Pech et al. (2010)], batteries [Xu (2004), Lee et al. (2010)] and fuel cells [Zhao et al. (2009), Pomfret et al. (2010)]. In CV techniques, electric potential are imposed at the electrodes which varies periodically and linearly with time [[Bard and Faulkner (2001), Compton and Banks (2007)]]. The resulting electric current is recorded. The total charge accumulated at the surface of electrode can be calculated by integrating the electric current with respect to time [Conway (1999), Pell and Conway (2001), Kovalenko et al. (2010), Wang et al. (2010), Yan et al. (2010)]. Finally the capacitance can be estimated as the total charge divided by the potential window [Conway (1999), Kovalenko et al. (2010), Wang et al. (2010), Yan et al. (2010), Byon et al. (2011)]. Capacitance is generally measured at different scan rates in order to characterize the performance of energy storage device like supercapacitors [Conway (1999), Frackowiak and Beguin (2001), Pech et al. (2010), Pell and Conway (2001), Kovalenko et al. (2010), Wang et al. (2010), Yan et al. (2010), Byon et al. (2011)]. At lower scan rates, the capacitance values are higher as compared to higher scan rates and are also closed to ideal shape. Further, the shape of CV curves has been used extensively to deduce the electrochemical processes involved in the charging and discharging of supercapacitors [Salitra et al. (2000), Chmiola et al. (2008), Aurbach et al. (2008) Lin et al. (2009)]. For example, in charging the supercapacitors from zero potential, the current initially increases and then it decreases upon further increase in the electric potential. Therefore a hump is generally observed in the CV curves. In case of supercapacitors, the CV of an ideal capacitor having negligible resistance shows a perfect rectangular shape and it also shows scan rate dependent behavior, as can be seen 60

21 from Fig. 2.10(a). In case of real capacitors, deviations from rectangular shape are observed as shown in Fig. 2.10(b) and it is represented as a series combination of internal resistance R and overall capacitance C. Figure 2.10: Cyclic voltammogram of (a) ideal capacitor (b) real capacitor In the present studies all the cyclic voltammograms were carried out by using computer controlled CHI 608C CH Instruments, USA. The capacitance values can be evaluated by using following relation: C = (2.11) where, i is the current and s is the scan rate. The long cycle-life test was also performed by using cyclic voltammetry technique at a scan rate of 100 mv s Galvanostatic Charge Discharge Techniques The galvanostatic charge discharge is a reliable method to evaluate the electrochemical capacitance of materials under controlled current conditions. This technique is very different from cyclic voltammetry because the current is controlled and the voltage is measured. This is indeed one of the most widely used techniques in the field of supercapacitor because it can be extended 61

22 from a laboratory scale to an industrial one. This method is also called as chronopotentiometry and gives access to various parameters such as: Capacitance Resistance Cyclability In this method, a current pulse is applied to the working electrode and the resulting potential is measured against a reference electrode as a function of time. At the moment current is applied, the measured potential is abruptly changes due to the IR (internal resistance) loss and after that it gradually changes because concentration over-potential is developed across the electrodes, as the concentration of the reactant is exhausted at the electrode surface. The voltage variation is given by the equation: V(t) = ir + i (V) (2.12) where, V (t) is the voltage as a function of time, R is the resistance, C is the capacitance and i is the current. As can be seen from eq. 2.12, the capacitance of the supercapacitor can be calculated from the slope of the galvanostatic charge-discharge curve. For a pseudocapacitor, when the V-t curve profile is not as linear as it should be, the capacitance can be calculated by integrating the current over the discharge time or charge time: C = I (F) (2.13) C = (F) (2.14) where, I is the set current, Δt is the discharge time (or charge time) and ΔV is the potential window. 62

23 The series resistance is deduced from the voltage drop (V drop ) occurring over the current inversion (ΔI) and is given as: R = (Ω) (2.15) When current is inversed or interrupted, the voltage drop is directly linked to the resistance of the cell. By repeating both the parameters of capacitance and resistance measurements over large number of cycles, it is then also possible to determine the cyclic stability of supercapacitor cells (EDLCs or redox capacitors). 63