Electrochemical polarization electrode-electrolyte interface

Figure 48. Kenjo s ID macrohomogeneous model for polarization and ohmic losses in a composite electrode, (a) Sketch of the composite microstructure, (b) Description of ionic conduction in the ionic subphase and reaction at the TPB s in terms of interpenetrating thin films following the approach of ref 302. (c) Predicted overpotential profile in the electrode near the electrode/electrolyte interface, (d) Predicted admittance as a function of the electrode thickness as used to fit the data in Figure 47. (Reprinted with permission from refs 300 and 301. Copyright 1991 and 1992 Electrochemical Society, Inc. and Elsevier, reepectively.)...

Activation polarization effect, which is associated with the kinetics of the electrochemical oxidation-reduction or charge-transfer reactions occurring at the electrode/electrolyte interfaces of the anode and the cathode. 

In a simple case, the electrochemical reaction at the electrode-electrolyte interface of one of the electrodes of the battery can be represented by the so-called Randles circuit (Figure 8.19), which is composed of [129] a double layer capacitor formed by the charge separation at the electrodeelectrolyte interface, in parallel to a polarization resistor and the Warburg impedance connected in series with a resistor, which represents the resistance of the electrolyte. 

Because of the resistance to ion flow at the electrode-electrolyte interface, normal measurement of total ionic conductivity is not possible in polymer electrolytes. In order to overcome this problem the conductivity measurements are carried out by the ac impedance spectroscopy method, which minimizes the effects of cell polarization. The measurements are often made with the electrolyte sandwiched between a pair of electrochemically inert electrodes made of platinum or stainless steel. The detailed methodology of impedance spectroscopy is reviewed thoroughly elsewhere [45-47]. 

Conversely, at the electrode at which it was assumed that > //Na- the Na ions will jump from the SE into the electrode. Once again, the process will proceed until a voltage ii (opposite in polarity to the one developed at interface I) develops that is sufficient to equate the electrochemical potentials Na = TNa across that interface. To summarize the space charge that forms at the electrode/electrolyte interface gives rise to a measurable voltage difference V = (t>ii - 4)1, which is related to the activities of the electroactive species in the electrodes — a fact that is embodied in Eq. (7.93). 

Electrochemical polarization experiments are performed to study the kinetics of charge-transfer reactions at the electrode-electrolyte interface. When cathodic current is applied to the electrode, the electrons accumulate in the metal as a result of the slow charge transfer. This phenomenon causes the cathodic polarization, to be always negative. Conversely, when electrons are removed from the metal as in the case of anode polarization, the polarization is always positive. 

When no current is flowing, the electrochemical changes occurring at an electrode are in steady state, i.e., atoms leave the electrode and become ions and the ions move to the electrode and becomes atoms. The process continuous till equilibrium is reached. A potential difference exists between electrode-electrolyte interfaces, which is known as electrode potential. The electrode potential between the electrode and electrolyte acts as a barrier to a faster rate of reaction, which is the electromotive force (emf) of the cell. External energy must be supplied so that ions are discharged at the required rate to promote current flow. Thus, when the current flows between the electrodes, several phenomena occur at the electrode surface that produce emf, which opposes the current flow. The deparmre of electrode potential from the equilibrium value upon passage of current is termed as polarization. ... 

The activation overpotential is the potential loss to drive the electrochemical reactions from equilibrimn state. Therefore, it is the potential loss when there is a net current production from the electrode, i.e. a net reaction rate. In PEM fuel cell, the activation overpotential at the anode is negligible compared to that of the cathode. Activation polarization depends on factors such as the properties of the electrode material, ion-ion interactions, ion-solvent interactions and characteristics of the electric double l er at the electrode-electrolyte interface. Activation polarization may be reduced by increasing operating temperature and by increasing the active surface area of the catalyst. 

The rate of an electrode reaction depends on the potential drop at the electrodeelectrolyte interface. According to Faraday s law (equation 1.8) the rate of reaction is proportional to the current density that flows through the electrode-electrolyte interface. By measuring the current density as a funetion of potential we can therefore get information about the kinetics of electrochemical reactions. The functional dependence between current density and potential is called polarization curve. To experimentally determine a polarization curve one can control either the potential or the current and measure the other quantity. One thus obtains a potentiostatic polarization curve, i =f[E), or agalvanostatic polarization curve, E = f(i), respectively. 

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