Region I Activation Polarization

Activation polarization, which dominates losses at low current density, is the voltage overpotential required to overcome the activation energy of the electrochemical reaction on the catalytic surface. From Eq. (4.1) 

The activation polarization at the anode and cathode are shown as and respectively. Physically, the activation polarization represents the voltage loss reqnired to initiate the reaction. In a somewhat similar fashion, consider a pnrely chemical reaction between gasoline vapor and air in a combustion chamber. There needs to be some ignition energy input to the system to enable the spontaneous reaction to proceed. In an electrochemical system, this manifests as voltage losses, which decrease the original maximum potential energy represented by the theoretical open-circuit potential of the fuel cell. 

Electrical Double Layer In addition to an analytical expression for the activation polarization at an electrode which we will develop in this chapter, an understanding of the microscopic process occurring at the electrode during charge transfer is also important A very natural question often arises when discussion of the activation overvoltage is first introduced What is the physical nature of the activation polarization and how exactly does the charge transfer reaction proceed  

Different activation losses occur at each electrode. In fact, the reactions at each electrode are only linked through conservation of charge. That is, the current passed through the anode must equal the current through the cathode  

Reaction Mechanism In general, the more complex a reaction mechanism, the greater the overpotential required to break the chemical bonds and generate current. For example, the HOR is less complex and involves fewer intermediate steps than the methanol electrooxidation, so that for the same current the overpotential for methanol oxidation is greater than for hydrogen oxidation. There are steric (geometric) and other factors involved as well. 

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Region I Activation Polarization 131 Pendulum analogy beyond equilibrium... 

Region I Activation Polarization 133 In a fuel cell, at each electrode, there is an equilibrium reaction that can be written as... 

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

Let s consider, for example, the cathodic reaction of Fig. 13.12 [12]. Its equilibrium potential is lower than Epp. Since, according to Eq. (13.23), when the corrosion potential Ecorr, i is between Eg, d and a corrosion couple is always created, it happens that the condition Ecorr > Epp is never reached and a corrosive event takes place, though at low power (low icon-, /) The same happens when reaction (2) in which the potential remains in the active region of the polarization diagram, but if reaction (3) is considered it happens that the condition Eporr > Epp is respected and the metal passivizes. In the men time the corrosion... 

The region of micro-polarization, where the j/r) plot is linear, can extend to about il/i) < 0.2 whereas the linear Tafel region starts at about r]/b > 1. As a result, the intermediate region of 0.2 < t]/b < 1 would be left unused, as far as the evaluation of kinetic parameters is concerned. For fast reactions, such as the HER on platinum, this represents a loss of crucial data, since it may be difficult to extend the measurements to overpotentials much above rj/h = 1, because of mass-transport limitations. Fortunately, modern microcomputers allow us to make use of this intermediate region. To do this, we write the full equation for an activation-controlled electrode reaction as follows ... 

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