Cathode exchange current density

The exchange current density is the electrode reaction rate at the equilibrium potential (identical forward and reverse reaction rates) and depends on the electrode properties and operation. The typical expression for determining the exchange current density is the Arrhenius law (3.23), where the constant A depends on the gas concentration. Costamagna et al. [40] provide the following expressions for the anodic and cathodic exchange current density, respectively ... 

At equilibrium (i.e., no current) there exist dynamic currents, measured in amps, at each electrode and are a fundamental characteristic of electrode behavior. The anode and cathode exchange current densities can be defined as the rate of oxidation and reduction respectively. The exchange current density is a measure of the electrode s ability to transfer electrons and occurs equally in both directions resulting in no net change in composition of the electrode.22 A large exchange current density represents an electrode with fast kinetics where there is a lot of simultaneous electron transfer. A small exchange current density has slow kinetics and the electron transfer rate is less. 

The higher the exchange current density the easier it is for reaction to continue when current is supplied to the stack. The cathode exchange current density is thus not the limiting parameter of the activation overpotential term and is often ignored. The current density (/) normalizes the stack current (7) to the active area of the cell. 

The cathode exchange current density is typically four orders of magnitude greater than the anode exchange current density and supported by Choi and Berning in Ref. 24 and 25. The anode side is therefore limiting the reaction and dominates the activation overpotential. 

The total couple cathode exchange current density is the sum of the corresponding exchange current densities for A and B ... 

An estimation of the effect of different parameters on the current density distribution can be made from Fig. 3.2, which shows the dependencies of the current densities at the closer, / > and further, if, part of the cathode from the anode on the cell voltage, (/, for different solution resistivity. As can be seen, the increase of the conductivity of the electrolyte leads to the more uniform current density distribution of deposits at the electrode surface. A similar but less pronounced effect of the increase of the cathodic Tafel slope can be seen, while the change of /q does not affect the current density distribution. It is necessary to note that a soluble anode is considered in this case and, hence, the anodic and cathodic exchange current densities are the same. 

The cathode exchange current density has been calculated according to the following equation ... 

The coefficient on the right-hand side of the equation is 61 mV. Now, assuming no external current is drawn, = 0, and a cathode exchange current density of 10 A/cm, the overpotential caused by current losses versus the internal current density is shown in Figure 5.26. For an internal current density of 1 mA/cm, the cell overpotential is 0.28 V. From the figure, we see a steep increase in overpotential at small internal current density. Thus, even if the cell current density to the load is zero, the open-circuit voltage of the cell is 0.92 V for an internal current density of 1 mA/cm. ... 

The internal short circuit current open-circuit condition with a cathode exchange current density of 10- A/cm. ... 

Cathode exchange transfer coefficient, a = 0.5 Cathode exchange current density, = 0.1 A/cnC Layer thicknesses ... 

Kinetic parameters Cathode exchange transfer coefficient, o = 0.5 cathode exchange current density,/o = 0.00015 A/cm ... 

Example 4.12 Calculating Crossover Losses In ref. [9], the authors noted a hydrogen crossover loss of 3.3 mA/cm for their automotive H2 PEFC applications. Calculate the mass crossover rate of hydrogen through the membrane. Also, calculate and plot the cathode activation overpotential loss at open circuit and 1 A/cm as a function of cathodic exchange current density. Assume the cathodic charge transfer coefficient at the cathode is 1.5 at a temperature of 353 K, and the fuel cell has a 50 cm geometric area. 

Plotting the activation losses as a function of cathodic exchange current density, we find ... 

As discussed in Chapter 4, this is higher than the Nemst voltage increase due to increased cathode exchange current density. 

Cathode exchange current density is 1 x 10" °A/cm of Pt surface (measured with oxygen at 25°C and atmospheric pressure). Calculate the expected current density at 0.9 V (iR corrected) if an MEA is prepared with catalyst specific surface area of 640cm /mg and with Pt loading of 0.4mg/cm and the cell operates with Hi/Air at 60°C and 300 kPa. What potential gain may be expected at same current density if the Pt loading on the cathode is increased to 2mg/cm ... 

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