Fuel cell kinetics

In the previous sections, the electrode kinetics were developed for the halfcell (single electrode). Here, we use Butler-Volmer kinetic equations for the fuel cell anode and cathode and develop the net current density and over-potential relationships (Kordesch and Simander, 2000 Larminie and Dicks, 1999 Mench, 2008 O Hayre et al., 2006 Vielstich et al., 2003). Consider the charge transfer reaction 

From Equation 5.75, the net current density is written in terms of charge transfer coefficients for reduction and oxidation processes and Uox/ respectively, as 

Using Equation 5.68, the exchange current density is now written as 

Since the BV equation is valid for fuel cell reactions in the anode and cathode, the anode and cathode net current densities are given as 

/oa and /oc are the exchange current densities for anode and cathode, respectively. Since reactions are different in anode and cathode, /oa joc- The forward (reduction) reaction charge transfer coefficients for anode and cathode are and aRed,c/ respectively. Similarly, backward (oxidation) reaction charge transfer coefficients for anode and cathode are aox,a and Uox c/ respectively. The oxidation and reduction transfer coefficients are related by the following expressions  

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Unlike the usual types of fuel cell, kinetic fuel cells do not need two gas departments hermetically sealed from each other. Cells of the kinetic type develop power from surface reaction in a mixture of fuel and air (supplied simultaneously to both electrodes, which are made of different materials). Suggest a few possible explanations for the effect. 

However, if we draw current from the fuel cell, the reactions shift from the equilibrium state and the properties of the reaction environment immediately come into play. This is the case of fuel cell kinetics. Before proceeding to the discussion of kinetic relations it is advisable to consider the potentials in a fuel cell. 

There are two major types of charged species electrons and ions. In most fuel cells, ionic charge transport is far more difficult than electronic charge transport as ionic conductivity is generally 4-8 orders of magnitude lower than the electronic conductivity. Therefore, the ionic contribution to ohmic losses tends to be the dominating factor in fuel cell kinetics. From Equation (11.13), we also know that the ohmic loss is proportional to the electrolyte thickness. Hence, fuel cell electrolytes are designed to be as thin as possible in order to reduce the ohmic loss. 

The Effects of Temperature on PEM Fuel Cell Kinetics and Performance... 

Backpressure Effect on Fuel Cell Kinetics (Electrode Kinetics and Mass Transfer Process)... 

The fuel cell kinetics is expressed by the terms on the right-hand side in Eqn (9.14), except for which has been dealt within Eqn (9.24), and IceiiRm, which wiU be discussed later. In acmality, fuel cell kinetics should include both the electrode kinetics and the mass transfer process. If Eqn (9.14) is... 

Electrochemical impedance spectroscopy (EIS) has also been discussed in Chapter 3. EIS is generally used to diagnose the performance limitations of fuel cells. There are three fundamental sources of voltage loss in fuel cells kinetic losses (charge-transfer activation), ohmic losses (ion and electron transport), and mass transfer losses (concentration). EIS can be used to distinguish and... 

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