VI. EIS STUDIES LEAD NANOPOWDER

Transcription

1 VI. EIS STUDIES LEAD NANOPOWDER 74

2 26. EIS Studies of Pb nanospheres Impedance (valid for both DC and AC), a complex resistance occurs when current flows through a circuit (composed of various resistors, capacitors or inductors). In other words, it is a measurement of the ability of a circuit to resist the flow of electric current. Resistance of an ionic solution is determined by ionic concentration, type of ion, temperature and area of cell. Ohm's law defines, resistance is the ratio between voltage (E) and current (I) i.e. R = E / I. Electrochemical impedance is usually measured using a small excitation signal (by applying an AC potential) to an electrochemical cell and then measuring the current through the cell. Electrochemical Impedance Spectroscopy (EIS) is an analysis tool useful in investigating the mechanisms of electrochemical reactions, passive surfaces, properties of porous electrodes and dielectric / transport properties of materials. It measures dielectric properties of a medium as a function of frequency. EIS data is commonly analyzed by fitting it to an equivalent electrical circuit model. Nyquist plot is based on equivalent circuit. It is a plot between real part (X-axis) Vs imaginary part (Y-axis). It has a drawback, i.e. it does not give frequency detail, at any data point on the plot. Semicircle of such plot is characteristic of a single time constant. Inductors and capacitors have only imaginary impedance component. The impedance of inductors increases as frequency increases. But, capacitors have opposite behavior i.e. impedance decreases as frequency increases. As a result, the current through inductors & capacitors is phase-shifted to -90 degrees & 90 degrees respectively, in respect to the voltage. The Simplified Randles cell is one of the most common cell models. It includes a solution resistance (R s ) and a double layer capacitance (C dl ) parallel with the charge transfer resistance (R ct ) or polarization resistance (R p ). Nyquist Plot for this cell is always a semicircle. The real axis value at the high frequency intercept (near the origin of the plot) gives R s value. The other intercept (low frequency) is the sum of R s and R p. The diameter of the semicircle is therefore equal to the R p. 75

3 26.1. Terms of EIS Analyses: Polarization Resistance (R p ) - At open-circuit, potential of an electrode is forced away from its value, refers as Polarization. It causes current to flow through electrochemical reactions, at the electrode surface. Charge Transfer Resistance (R ct ) or Electron Transfer Resistance (R et ) - Single kinetically controlled reaction causes R ct. Example: metal substrate in an electrolyte. It depends on reaction, temperature, concentration of reaction products, applied potential. Double layer capacitance (C dl ) is an electrical double layer exists on the interface between an electrode and its surrounding electrolyte. This double layer is formed as ions from the solution "stick on" the electrode surface. The charged electrode is separated from the charged ions. The separation is very small, often on the order of angstrom. C dl value depends on electrode potential, temperature, ionic concentrations, type of ions, oxide layers, electrode roughness, adsorption of impurities EIS Experimental Method: Impedance analyses of lead NPs were performed by a Dielectric spectroscopy (EIS), in order to investigate the electrochemical characteristics of the electrode/electrolyte interface. A conventional three electrodes electrolysis cylindrical Pyrex glass cell was used for EIS tests. Prior to the experiments, surface of the working electrode was polished with different emery paper, washed thoroughly with acetone and rinsed with distilled water. The experiments were carried out in room temperature. Pb NPs were coated on the electrode i.e. working electrode was coated with Pb NPs, carbon block, PolyVinylideneDiFluoride (PVDF) and N- Methylpyrolidone. Saturated calomel electrode (SCE), platinum wire (Pt) and 3 Molar dilute sulphuric acid (H 2 SO 4 ) were used as reference, counter electrodes and electrolyte respectively. CV analysis tests were done with two different methods: In first method, CV analysis tests were done in the above EIS setup (100 mv, 5 cycle with Scan Rate (V/s) = 0.1 and 10 mv, 5 cycle with Scan Rate (V/s) = 0.01). In another method, Pb NPs were dispersed in KCl electrolyte solution. In this method, platinum electrode was used as working & counter electrodes and Ag / AgCl as reference electrode. 76

4 26.3. EIS Analyses of Pb Nanospheres: Fig.25 shows Faradaic impedance spectrum, presented as Nyquist plot (Z Vs. Z ) correspond to the impedance of the electrode with Pb NPs. The Plot is the output from the electrical circuit of the inset figure. The resulted plot consists of a semi-circle and a straight line. Tsai et al. in their report, semi-circle is related to R ct and C dl. Straight line ascribes to Warburg impedance due to diffusion process [109]. The complex impedance is presented as the real Z (ω), and imaginary Z (ω), components that originate mainly from the resistance and capacitance of the cell. The electron-transfer kinetics and diffusion characteristics are extracted from the shape of this impedance spectrum. The small depressed semicircle arc parameters correspond to the R et and C dl of the modified electrode. The R et value of the electrode with Pb NPs is 1.1(Z /Ω). This resistance value corroborates the semi-conducting nature of Pb NPs. The straight line (diagonal line with a slope of 45 ) represents the characteristics of diffusion limited electron-transfer process on electrode surface. This diffusion creates Warburg impedance and it depends on the frequency of the potential perturbation. At high frequencies, the Warburg impedance is small since diffusing reactants don't have to move very far. At low frequencies, the reactants diffuse farther which increases the impedance. In general, a super-capacitor behaves as a pure resistor at high frequencies and as capacitor at low frequencies. In the mid frequency range, it behaves as a combination of resistor and capacitor. It can be seen from the plot that the cell shows a very small kinetic arc semicircle in the high frequency region (implying R ct charge transfer controlled regime) and straight line in the low frequency region (implying C dl - capacitive regime). It means that the sample has blocking behaviour (resisting) at high frequencies and capacitive behaviour at low frequencies. This result implies material is suitable for fabrication of low leakage capacitors. The Bode Plot related to the electrode with Pb NPs is shown in Fig.26 & 27. It differs from Nyquist Plot and it gives the frequency information. In this Plot, impedance is plotted with log frequency on X axis and both the absolute values of impedance and phase-shift on Y-axis. The Warburg impedance exhibits phase shift, in this plot. 77

5 CV analysis confirms the electrochemical performance of Pb NPs. Fig.28 & 29 illustrate the CV curves of Pb NPs in 3 Molar dilute sulphuric acid (H 2 SO 4 ) electrolyte. The electrochemical parameters related to these figures are enumerated in Table.21. Ipa and Ipc represent anodic and cathodic current respectively. Epa and Epc represent anodic (oxidation onset) and cathodic (reduction onset) potential respectively. Karami et al. in their report, that in thin layer of electro-active materials, smaller particle size, increase active surface to take part in electrode reactions. The decreased size can facilitate oxidation and reduction currents / reactions more easily than bigger particles. Also, the smaller NPs are more sensitive and reactive than bigger particles [110]. The report of the sample correlated to Karami et al. report is discussed in coming section. Table.21. Electrochemical Parameters from CV analysis of Pb NSs Scan Rate Ipa (ma) Ipc (ma) Epa (V) Epc (V) E g (ev) Particle Size (nm) 10 mv/s mv/s Subtracting Epc from Epa yields band gap (E g ) value of Pb NPs. This E g value confirms the resistance behavior of the sample (analyzed from impedance study) as well as the semi-conducting nature. The E g value agrees with the E g value of Pb NPs (0.73 ev) calculated from the CV analysis with KCl electrolyte. The calculated particle size from this study confirms the nano nature of the sample i.e. 8 nm and agrees with particle size assessed from TEM analysis. Fig.30 and 31 illustrate the capacitance values C=500 µf and C=3200 µf (oxidation and reduction peak respectively) from CV curves of Pb NPs dispersed in KCl electrolyte solution. The electrochemical parameters and other related studies of this CV analysis are discussed in coming section. These values confirm the capacitance behavior of the sample (analyzed from impedance study). 78

6 Fig.25. Niquist plot of Pb NSs Fig.26. Bode plot of Pb NSs - Modulus against Frequency 79

7 Fig.27. Bode plot of Pb NSs - Phase against Frequency Fig.28. CV of Pb NSs 100 mv with Scan Rate (V/s) =

8 Fig.29. CV of Pb NSs 10 mv with Scan Rate (V/s) = 0.01 Fig.30. Capacitance from Oxidation peak - C = 500 µf 81

9 Fig.31. Capacitance from Reduction peak - C = 3200 µf 82