Mass transport limited region

If kc is very large such that we are in the mass transport-limited region of the current and the second term in equation (2.162) — 0, then the observed current is its limiting value ... 

The mass transport limited region is the region in which essentially all the reactant that reaches the substrate decomposes to be incorporated into the film. Hence, the term mass transport limited — increase of the mass transport increases the growth rate. If pre-reactions or contamination are not a problem, this is the most ideal growth region since uniformity and composition are controlled primarily by the mass transport to the surface and the distribution of this transported material. For the arsenides and phosphides, this spans the temperature range from =575°C to =700°C. 

Fig. IL4.6 Current-time curves for different step magnitudes. E is the initial potential where no current flows, while E2 is the potential in the mass transport limited region.

Results with and without phase change for the cell operates at nominal current density of 1.4 A/cm are discussed in this section. The selection of relatively high current density is due to illustrate the phase change effects, where it becomes clearly apparent between single and multi-phase model in the mass transport limited region. 

The results show that the multi-phase model is capable of identifying important parameters for the wetting behavior of the gas diffusion layers and can be used to identify conditions that might lead to the onset of pore plugging, which has a detrimental effect of the fuel cell performance, especially in the mass transport limited region. 

Fig. 11.4.6. Current-time curves for different step magnitudes. Ei is the initial potential where no current flows, while E2 is the potential in the mass transport limited region. E2 > 21 > 22 > 1, and E21 and 22 correspond to potentials in the rising portion of the voltammogram where mixed charge transfer-mass transport control prevails...

In general, the effects of mass-transport limitations are not as easy to characterize. The direction of fluid flow, the flow regime, and the local fluid velocity all influence the current distribution. Fluid flow to the rotating disk is unusual in that fluid velocity normal to the disk is dependent only on the normal distance from the disk surface, and not on radial distance. Because the disk surface is uniformly accessible to incoming reactants, mass-transport limitations tend to reduce the current density in regions of high... 

Activation less process — is an electrochemical reaction occurring with zero - activation energy. This behavior is predicted for the region of high - overvoltage which is rarely available in experiments because of - mass transport limitations. The prediction of such type of processes follows from the theory of - Levich and his school [i, ii]. For the diabatic - electron transfer processes the total current density j can be estimated by integrating over the energy levels of a metal electrode e ... 

Additional Complete or Incomplete Semicircle. In EIS measurements, an additional complete or incomplete semicircle can sometimes be observed at the high-frequency end at different potentials. This high-frequency component does not vary with potential, as shown in Figure 5.28. Fisher et al. [32] attributed the high-frequency loop to the internal ohmic resistance and the contact capacitance in the granular electrode structure. Springer et al. [18] considered this region associated neither with any Faradaic process nor with mass transport limitations on the cathode electrode. 

Figure 6.5 shows the AC impedance spectra of the same fuel cells measured at different cathodic potentials. It is evident that as the overpotential increases, the diameter of the kinetic arc decreases due to the increasing kinetic rate. At low overpotential, the kinetics dominates and only the kinetic arc appears. At high overpotentials, the low-frequency region shows additional arcs, which are associated with mass-transport limitations across the gas diffusion layer and within the catalyst layer. 

In this transport-limited region, the conversion increases at a slower rate than the increase of amoimt of catalyst, and thereby, the overall rate decreases instead of remaining constant as in the kinetic regime. In other words, in this region conversion is influenced by different factors (a) the additional catalyst surface area provided does not come in full contact with the pollutant due to external mass transfer resistance, and/or... 

The other common reactor type is a cold-wall reactor. Here only the substrate is heated, and the gas in the forced convection region as well as the reactor walls are considerably colder than the substrate. This design has limited capacity the substrate is usually coated on one face only and although uniform gas ffow to the substrate is easier to control, heating the substrate is relatively difficult. These reactors are usually operated under mass transport limited conditions. Heating is accomplished in one of four ways ... 

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