Characteristic current density oxygen diffusion

Volumetric exchange current density of agglomerate or ultrathin CCL Characteristic current density of oxygen diffusion... 

In the nonideal case, one has to evaluate the interplay of three effective electrode parameters, viz. volumetric exchange current density P, proton conductivity ap, and oxygen diffusivity D. Instead of considering these parameters, it is more insightful to evaluate and compare three corresponding characteristic current densities. These include the current conversion capability f = PIcl, defined above, the characteristic current density due to proton transport... 

Equations 47-49 describe variations of parameters along the y coordinate of the catalyst layer (y = z/lc 1), where z is the catalyst layer thickness coordinate, y = 0 specifies the catalyst layer/gas interface, and y = 1 specifies the catalyst layer/ionomeric membrane interface (see Fig. 44), in which Rc (= /Ci/cr) is the protonic resistance through a unit cross-sectional area of the catalyst layer and 7d (=nFDC /lc ) is a characteristic diffusion-controlled current density in the catalyst layer. The thickness of the catalyst layer disappears from the equations by introducing Rc, ax, and I ). The experimental variables considered include the overpotential 1], the current density /, and the oxygen concentration C, when pox = 1 atm at the catalyst layer/gas interface. The O2 partial pressure, pox, at the catalyst layer/backing layer interface is determined, in turn, by the cathode inlet gas stream composition and stoichiometric flow rate and by the backing layer (GDL) transport characteristics. 

A schematic representation of this polarisation curve is given in Rg. 15.2. The first part of the curve corresponds to activation loss, mainly the kinetics of oxygen reduction (with jo = 10 -10 A cm ) involved the second part is linear and due to ohmic loss, mainly the electrolyte resistance the third part is due to mass transfer or diffusion loss when the value of becomes close to /umcat or E(j) tends to zero. The optimal operating point is located in the linear part of the curve. These current-density-potential curves are very important for any type of fuel cell, because they summarise the influence of all the important parameters on the performance of a cell. Even though the equation is more complex in the case of high-temperature fuel cells, the general features of the current density vs potential characteristics are similar. 

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