Potential activation overpotential

Anode potential Activation overpotential Ohmic overpotential... 

Charge Transport. Side reactions can occur if the current distribution (electrode potential) along an electrode is not uniform. The side reactions can take the form of unwanted by-product formation or localized corrosion of the electrode. The problem of current distribution is addressed by the analysis of charge transport ia cell design. The path of current flow ia a cell is dependent on cell geometry, activation overpotential, concentration overpotential, and conductivity of the electrolyte and electrodes. Three types of current distribution can be described (48) when these factors are analyzed, a nontrivial exercise even for simple geometries (11). 

Seconday Current Distribution. When activation overvoltage alone is superimposed on the primary current distribution, the effect of secondary current distribution occurs. High overpotentials would be required for the primary current distribution to be achieved at the edge of the electrode. Because the electrode is essentially unipotential, this requires a redistribution of electrolyte potential. This, ia turn, redistributes the current. Therefore, the result of the influence of the activation overvoltage is that the primary current distribution tends to be evened out. The activation overpotential is exponential with current density. Thus the overall cell voltages are not ohmic, especially at low currents. 

The anode potential is so positive, due principally to the activation overpotential, that the majority of the impurity metals (Fe, Cu, Co, etc.) in the anode dissolve with the nickel sulfide. In addition, some oxygen is evolved (2 H20 = 02 + 4 H+ + 4 e ). The anodic current efficiency reduced to about 95% on account of this reaction. Small amounts of selenium and the precious metals remain undissolved in the anode slime along with sulfur. The anolyte contains impurities (Cu, Fe, Co) and, due to hydrogen ion (H+) liberation, it has a low pH of 1.9. The electrolyte of this type is highly unfit for nickel electrowinning. It is... 

The other potential losses required to drive an electrode reaction are the activation overpotential, rja, and concentration overpotential, r]conc. The problem of current distribution is then governed Eq. (57) as well as by the following equations ... 

The dimensionless limiting current density N represents the ratio of ohmic potential drop to the concentration overpotential at the electrode. A large value of N implies that the ohmic resistance tends to be the controlling factor for the current distribution. For small values of N, the concentration overpotential is large and the mass transfer tends to be the rate-limiting step of the overall process. The dimensionless exchange current density J represents the ratio of the ohmic potential drop to the activation overpotential. When both N and J approach infinity, one obtains the geometrically dependent primary current distribution. 

Equations (2.144) and (2.145) are extremely interesting as they show that the magnitude of the peak current is independent of the kinetics of the reaction. This is not surprising since from equations (2.139) and (2.141) it can be seen that kc can be increased to any value simply by increasing the potential. However, at a given scan rate, the position at which the peak current occurs is related to the kinetics Ep occurs beyond E° by an activation overpotential related to k°. 

The activation overpotentials for both electrodes are high therefore, the electrochemical kinetics of the both electrodes can be approximated by Tafel kinetics. The concentration dependence of exchange current density was given by Costamagna and Honegger.The open-circuit potential of a SOFC is calculated via the Nernst equation.The conductivity of the electrolyte, i.e., YSZ, is a strong function of temperature and increases with temperature. The temperature dependence of the electrolyte conductivity is expressed by the Arrhenius equation. 

Figure 6.14. Cell Voltage vs. Cell Current profile of a hydrogen - oxygen fuel cell under idealized (dotted-dashed curve) and real conditions. Under real conditions the cell voltage suffers from a severe potential loss (overpotential) mainly due to the activation overpotential associated with the electroreduction process of molecular oxygen at the cathode of the fuel cell. Smaller contributions to the total overpotential losses (resistance loss and mass transport) are indicated.

Acetonitrile, adsorption, 981 Activation overpotential, 1232 Activation potential, in polarography. 1244 Active sites for adsorption, 928 Activity... 

Since in cathodic reactions is always smaller than c°, the concentration polarization has a negative sign, which adds to the activation overpotential in causing the electrode to depart from the equilibrium potential in the negative direction for an electronation reaction. 

It is now necessary to take a more unified view by considering situations in which the rate of the electrodic process at the interface is subject both to activation and to transport limitations. One refers to a combined activation-transport control of the electrodic reaction. Under such conditions, there will be, in addition to the overpotential T)c produced by the concentration change (from c° to c ) at the interface, an activation overpotential because the charge-transfer reaction is not at equilibrium. The total overpotential rj is the difference between the interfacial-potential difference... 

Charge transport is modeled by Ohm s law (Equation (3.10)) and the charge conservation equation (Equation (3.68)), while the current density distribution at the electrode/electrolyte interface is modeled through the Butler-Volmer equation (Equation (3.102)). It should be noted that, contrarily to Section 3.7, Equation (3.102) is here derived from Equation (3.37) rather than Equation (3.39), because the former allows for a better agreement between experimental and simulated results. Equations (3.40)-(3.42) are used to model, the exchange current density, the activation overpotential, and the ideal potential drop at the electrode/electrolyte interface, respectively. Heat transfer is modeled through Equation (3.6), and the appropriate heat terms for each domain. 

At a current density (Fig. 13.6) sufficiently far below the limiting current density values [see Eq. (13.19)] and when the ohmic losses inside the cell are negligible, the activation-overpotential terms dominate the expression for the relation of current to potential, i.e.,... 

Fig. 13.7. Graphical representation of the influence of the internal resistance of an electrochemical energy converter on the cell potential when mass-transfer polarization is negligible. The early nonlinear part of the curve represents the effect of the activation overpotential on the cell potential before ohmic polarization has become important.

It is seen, therefore, that the cell potential V and consequently the efficiency of an electrochemical converter (Fig. 13.9) are determined by the activation overpotential, by the electrolyte conductance, and by mass transfer (i.e., the solubility of the reactants). The factors that dominate the way the efficiency of the conversion of energy changes with an increase in current density are at low current density, the activation... 

Note how, even in the region in which there is linear behavior of V with respect to /, the actual value of the potential that the generator could put out depends on the value of the so-called constant, i.e., on the activation overpotential and thus on the exchange current densities and the catalytic power of the electrodes. 

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