Chapter 2 Background and Literature Review
2.10 Electrochemical response
Electrochemical response is a chemical reaction related to electron transfer which occurs on the interface between two different phases. In generally, the most common reaction phase is electrolyte that can provide a conductive surrounding by dissociated ions. Since the electron transfer is indeed for an electrochemical reaction, the applied voltage on an electrode can control the energy of electron to make electroactive species interacting with the surface on electrode, as shown in Figure 2.21 [143]. When a positive voltage is applied, the orbital energy of electrode is reduced and then the electrons of high occupied molecular orbital (HOMO) of reactant will transfer to the electrode, so called oxidation. On contrary, upon a negative voltage is applied, the orbital energy of electrode is enhanced and then electrode electrons will transfer to low unoccupied molecular orbital of reactant, so called reduction.
Furthermore, Hong et al. shows that the reaction rate can be improved more by increasing the voltage than the temperature [144], indicating that electrochemical methods are efficient ways to complete a chemical reaction under room temperature. The major electrochemical methods for this thesis are cyclic voltammerty, i-t chronoamperometry, polarization measurement, and electrochemical impedance spectroscopy, and will be introduced as following sections.
2.10.1 Cyclic voltammerty (CV)
For the cyclic voltammetry (CV), the linearly changing potential will applied versus time between working electrode and reference and the current between working electrode and counter electrode can be detected. When the potential is applied from negative to positive, the oxidation peak will appear during the scan due to the oxidation reaction occurs on the surface
reverse potential from positive to negative is applied and a reduction reaction occurred on the surface of analytes (working electrode), the reduction peak will appear during the reverse scan. Therefore, the cyclic change of current can be detected and be plotted versus the applied potential, so called cyclic voltammetry (CV). Figure 2.22 shows Cyclic voltammerty waveform and standard cyclic voltammetry of oxidation-reduction reaction [145,146], the oxidation peak (anode peak, Epa) and reduction peak (cathode peak, Epc) can be detected and the difference of Epa and Epc can be defined as Delta Ep, shown as follow:
- 0.059
p pa pc
E E E V
n
, (2.11)
where the n is number of transferred electrons. Moreover, the Delta Ep is close to 0.059 V/n, meaning the electrochemical reaction is reversible. The peak current (Ip) can be defined by Randles-Sevick equation:
5 3/2 1/2 1/2
(2.69 10 )
Ip n AD C , (2.12)
where the n is number of mole of electrons transferred from electroactive species, A is the area of electrode (cm2), D is diffusion coefficient (cm2/s), C is the concentration of the analyte (mol/cm3), and υ is the scan rate (V/s). In General, CV is a common way to study the oxidation-reduction behavior of analytes under a given solution.
2.10.2 Amperometry
The amperometry is an electrochemical technique, which is fulfilled by applying a
constant potential to keep analytes reacting under a reaction potential. Then, the reaction current versus time can be plotted and so called i-t curves. The amperometry is major operated for a kinetic system, meaning that the analytes flow through the surface of electrode and then the current can be detected. On contrary, the chronoamperometry is used for a static system to determine the current caused by diffusion under extremely short time. The current measured by amperometry can be defined by Cottrell equation, shown as follow:
nFAC D
I t , (2.13)
where n is number of mole of electrons transferred from electroactive species, F is Faraday’s constant (96500 C/mol), D is diffusion coefficient, C is concentration of the analyte (mol/cm3).
2.10.3 Polarization measurement
Although there are several local cells on surface of metal, the potential of anode and cathode under a corrosion reaction can be regarded as the same due to the polarization, called mixed potential or corrosion potential. The polarization can be divided into three types [147]:
(1) Activation polarization:
Activation polarization is an overpotential for conquering the activation energy of an electrochemical reaction. Because of the analyte and electrolyte are different phases, the forming interface structure will affect the electron transferred ability.
(2) Concentration polarization:
The electrochemical reaction makes the concentration gradient between analyte and electrolyte, due to the depletion of the electroactive species. According to the Nernst equation, the concentration gradient will reduce the equilibrium potential of the electrochemical reaction. Therefore, the overpotential applied to overcome the concentration gradient, which is called concentration polarization.
(3) Ohmic polarization: Ohmic polarization is an electron transfer resistance caused by properties of electrodes and distance between the working electrode and reference electrode. The effect of Ohmic polarization can be minimized via careful experimental setting.
The theory of mixed potential shows the occurring current density of anode and cathode is the same and the overall current density of reaction is zero. The standard polarization curve is shown in Figure 2.23 [148], and the reaction region closed to mixed potential is called kinetic limited region. In kinetic limited region, the concentration polarization can be neglected due to the unobvious mass transport. Therefore, the activation polarization is a key factor only for determining the current density and corrosion potential of analyte in kinetic limited region. The Tafel equation is presented as follow [72]:
log a b i
, (2.14)
where η is overpotential (V), i is current density (A/cm2), i0 is exchange current density (A/cm2) and a and b are the Tafel constants: a= - (2.3 RT/αF)logi and b=(2.3 RT/αF) for anode polarization, or a= (2.3 RT/βF)logi and b= (2.3 RT/βF) for cathode polarization (α and
β are coefficients related to potential drop through the electrochemical double layer).
2.10.4 Electrochemical impedance spectroscopy (EIS)
This method is also called AC impedance, which is mainly operated for measuring the resistance of electron transfer between the surface of analyte and electroactive species of electrolyte. Figure 2.24 shows the structure of the double layer of the electrode interface, the applied negative potential make the positive ion attach on the surface of the electrode [149].
Figure 2.25(a) presents a standard equivalent circuit model of a double layer formed by applying a negative potential on the surface of the electrode [145]. The Rs, Rct, and Cd, are electrolyte resistance, charge transfer resistance, and double layer, respectively. Because the electrochemical reaction and the double layer formation are occurring in the same time, the Cd is in parallel with the Rct. Because the electrolyte affects the electrochemical reaction with the analyte, the Rs is in series with the (Cd-Rp). Generally, the Rs is related to distance between working electrode and reference electrode and is the constant for the same experimental setting.
In an AC circuit, the impedance Z is more appropriate to represent the system's entire resistance. From Euler's relations, we know that Z = Z0 (cosθ + jsinθ). Hence, the impedance can be described by the real and imaginary part (Z = Z’+ jZ’’), and the Nyquist plot is often represented by Z’ versus Z’’. The relation between Rct and Z can be described by:
2
2 2 2 2 2 2
(1 ) (1 )
ct dl ct
s
dl ct dl ct
R C R
Z R j
C R C R
, (2.15)
Figure 2.25(b) shows classical Nyquist plot, the start point of the high frequency region is the Rs and the end point of the low frequency region is the Rs + Rct [145]. Therefore, the Rct can be determined by calculating the diameter of the hemi-circle in the Nyquist plot.