Volumetric exchange current density

The length scale i a 50 — 100 nm determines the effectiveness of catalyst utilization for spherical agglomerates. Analogous relations apply for ultrathin planar catalyst layers with similar thickness, L 100 — 200 nm. We consider layers that consist of Pt, water-filled pores and potentially an electronically conducting substrate. With these assumptions, we can put/(dfptc, XfXptc = 1 and g Sr) = 1. The volumetric exchange current density is, thus. 

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

Here j x) is the local proton current density, is the volumetric exchange current density (the number of charges produced in unit volume per second, A cm ), c is the molar concentration of oxygen, Cref is the reference oxygen concentration, (f> is the conversion function, r] is the local polarization voltage, at is the proton conductivity of the CCL, D is the effective oxygen diffusion coefficient and jo is the cell current. 

Here the axis x is directed from the intercoimect to the electrolyte (Figure 4.18), j x) is the local ionic current density, is the volumetric exchange current density, cm is the molar concentration of hydrogen in the anode, Cref is the reference hydrogen concentration, a is the transfer coefficient, rj is the polarization voltage of the anode side and at is the ionic conductivity of the anode. 

Geometrical constant in Section 5.6.5 Volumetric exchange current density (A m )... 

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... 

In this section, the variables related to oxygen and methanol are equipped with the subscripts ox and mt, respectively. Here, iox, imt are the ORR and MOR volumetric exchange current densities, Cox, cj are the local and reference (inlet) oxygen concentrations, Cmt, c t are the local and reference methanol concentrations, r]mt are the ORR and MOR polarization potentials (overpotentials), box, bmi are the ORR and MOR Tafel slopes, and Dox, Dmt are the oxygen and methanol diffusion coefficients, respectively. 

While the electrochemical reactions occur, fuel and oxidant are consumed over the corresponding electrodes to generate current. Current density distribution is estimated for simulations by Butler-Volmer equation as a function of volumetric exchange current of a given electrode ( o) at reference concentration... 

In Chapter 10 it is stated that corrosion of a surface may be related to the velocity of the corroding fluid across the surface the higher the velocity the higher the rate of corrosion. For this reason the magnitude of the current density required to maintain a particular cathodic potential will increase vith electrolyte velocity. Due to changing cross-sectional area for a given volumetric flow, the necessary current is likely to vary from location to location within a heat exchanger. 

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