Consider equation 2.1, if we disturb the equilibrium by pumping current to the system, the equilibrium will proceeds faster in the cathodic reaction direction (equation 2.2) and the potential will become more negative (see Fig. 2.2). In this case, IC>>>IAand the plot of E versus I slopes downward On the other hand, if we take away current from the equilibrium reaction (equation 3), the reaction will proceed faster in the anodic direction and the potential will become more positive (equation 5). In this case, IA>>>IB(see Fig. 2.3)
32 Fig. 2.2: Anodic polarization in a single electrode system Fig. 2.3: Cathodic polarization in a single electrode systemIn Figs. 2.2 and 2.3, E denotes the electrode potential while e stands for the equilibrium potential. The change in electrode potential (E) from the equilibrium potential is called polarization. The difference between E and e is called overvoltage that is, For cathodic polarization, E < e and is negative (equation 2.4) but for anodic polarization, E > e and is positive We have just discussed polarization of a single electrode. However, a typical electrochemical cell consists of two electrodes (anode and cathode), indicating that combined effects of anodic and cathodic polarizations must be examined. Generally, there are two types of polarization in an electrochemical cell, namely concentration polarization (C) and activation polarization (act). Activation polarization is caused by charge transfer across the electrode/electrolyte interface while concentration polarization is caused by changes in the
33 ions concentration near the electrode/electrolyte interface. The sum of C and actgives the total polarization across the electrode, thus, 3.3 Dependence of activation overpotential on current: Tafel law In 1905, Tafel (1862-1981) found that for most electrode reactions, the activation overpotential (is linearly proportional with the log of the current as follows, ()2.6 For anodic and cathodic polarizations, equation 2.6 can be simplified to equations 2.7 and 2.8 respectively, () () and are called Tafel constants. Generally, , where α0.5. Most often, activation polarization is normally studied by using an electrochemical instrument that is able to generate data for the variation of electrode potential with current. Results obtained from such studies are normally treated graphically. A typical plot of this nature (for the reaction, Mn++ ne-= M) is shown in Fig. 2.4. It can be seen from Fig.2.4 that Tafel constants can be obtained from the slopes of the cathodic and anodic arms of the plot. A plot of this nature is called potentiodynamic polarization plot. Specific example of potentiodynamic polarization curve for the system, 2H++ 2e-= H2, is presented in Fig. 2.5. For this reaction at equilibrium, no current flows and the reaction potential is 0 V but at standard condition, io= 1 mA/cm2. If current is allowed to flow, the potential E, changes and the overpotential can be calculated, using Tafel equation (equation 2.9), thus () ()
34 Fig. 2.4: Typical pattern of plot for the variation of E with log(i) Fig. 2.5: Plot of E versus log(i) for the reaction, 2H++ 2e-= H2 4.0 Conclusion Polarization is a concept that has led to the progress of numerous industrial phenomena.