Oxygen-transport current density

If the oxygen-transport current density hFDci/Icl is much less than jo, the central term jo - crpSrj/lcL in Equation 4.66 is a small difference of two large values. In this case, the potential drop is linear in the cell current ... 

This shows the validity limits of the low-current approximation. A comparison of Equations 4.84 and 4.82 shows that the dimensionless thickness of the conversion domain must be large Zb 1. In other words. Id must greatly exceed the CL thickness. This situation is realized if the cell current is much less than the characteristic oxygen-transport current density in the CCL on the right handside of Equation 4.85. The typical shapes of the local parameters are shown in Figure 4.16a. As can be seen, rjo and Rreac OTO constant, while j decreases linearly with x. 

In the coordinates of Figure 4.27b, the onset of the linear regime occurs regardless of at almost the same value of jo + jcwssfl — 320 mA cm . This value is close to the oxygen-transport current density jo through the CCL. Indeed, the data from... 

Oxygen-transport current density in the CCL, (A cm ). Equation 4.77 Net water flux through the membrane... 

In the corrosion protection of marine structures, it is often found that the corrosion rate decreases strongly with increasing depth of water, and protection at these depths can be ignored. Investigations in the Pacific Ocean are often the source of these assumptions [7], However, they do not apply in the North Sea and other sea areas with oil and gas platforms. Figure 16-1 is an example of measurements in the North Sea. It can be seen that flow velocity and with it, oxygen access, is responsible for the level of protection current density. Increased flow velocity raises the transport of oxygen to the uncoated steel surface and therefore determines the... 

The behavior of metal electrodes with an oxidized surface depends on the properties of the oxide layers. Even a relatively small amount of chemisorbed oxygen will drastically alter the EDL structure and influence the adsorption of other snb-stances. During current flow, porous layers will screen a significant fraction of the surface and interfere with reactant transport to and product transport away from the surface. Moreover, the ohmic voltage drop increases, owing to the higher current density in pores. All these factors interfere with the electrochemical reactions, particularly with further increase in layer thickness. 

The mechanism of facilitated transport involves using the metal ion only in its reduced state in the oxidized state the oxygen-carrying capacity is virtually nil. It is thus natural that electrochemical processes should be attempted to improve both the flux and selectivity obtained with the membranes described above by exploiting this 02 capacity difference. For example, the best of the ultra-thin membranes developed by Johnson et al. [24] delivered oxygen at a rate equivalent to a current density of only 3 mA/cm2, at least an order lower than that achievable electrochemically. Further, the purity was but 85% and the lifetime of the carrier less than a year. 

An experimental method is proposed for estimation of the transport hindrances in air gas-diffusion electrodes. As a measure of the transport hindrances in the air gas-diffusion electrodes is introduced the difference AE between the potentials of one and the same electrode when operating with air and with pure oxygen at one and the same current density. The difference AE can be theoretically described by the equation ... 

Another example of neution imaging is the one presented by Yoshizawa et al. [273], who compared the performance of carbon cloth and carbon fiber paper with a parallel FF design. The CC had a better performance than the CFP at high current densities, but the CFP showed less water content over the whole active area. Thus, it was concluded that the CC diffusion layer was less influenced by the accumulation of water because the transport of oxygen toward the catalyst zones was sufficient while still keeping the membrane humidified. 

The existence of a maximum thickness beyond which the performance deteriorates is due to the concerted impact of oxygen and proton transport limitations. Considered separately, each of these limitations would only serve to define a minimum thickness below which performance worsens due to an insufficient electroactive surface. The thickness of the effective layer, in which current density is predominantly generated, is given by the reaction penetration depth ... 

The last part of the polarization curve is dominated by mass-transfer limitations (i.e., concentration overpotential). These limitations arise from conditions wherein the necessary reactants (products) cannot reach (leave) the electrocatalytic site. Thus, for fuel cells, these limitations arise either from diffusive resistances that do not allow hydrogen and oxygen to reach the sites or from conductive resistances that do not allow protons or electrons to reach or leave the sites. For general models, a limiting current density can be used to describe the mass-transport limitations. For this review, the limiting current density is defined as the current density at which a reactant concentration becomes zero at the diffusion medium/catalyst layer interface. 

Overall, the rib effects are important when examining the water and local current distributions in a fuel cell. They also clearly show that diffusion media are necessary from a transport perspective. The effect of flooding of the gas-diffusion layer and water transport is more dominant than the oxygen and electron transport. These effects all result in non-uniform reaction-rate distributions with higher current densities across from the channels. Such analysis can lead to optimized flow fields as well as... 

---