Aqueous layer mass transport limitations

Besides the activation overpotential, mass transport losses is an important contributor to the overall overpotential loss, especially at high current density. By use of such high-surface-area electrocatalysts, activation overpotential is minimized. But since a three-dimensional reaction zone is essential for the consumption of the fuel-cell gaseous reactants, it is necessary to incorporate the supported electrocatalysts in the porous gas diffusion electrodes, with optimized structures, for aqueous electrolyte fuel-cell applications. The supported electrocatalysts and the structure and composition of the active layer play a significant role in minimizing the mass transport and ohmic limitations, particularly in respect to the former when air is the cathodic reactant. In general, mass transport limitations are predominant in the active layer of the electrode, while ohmic limitations are mainly due to resistance to ionic transport in the electrolyte. For the purposes of this chapter, the focus will be on the role of the supported electrocatalysts in inhibiting both mass transport and ohmic limitations within the porous gas diffusion electrodes, in acid electrolyte fuel cells. These may be summarized as follows ... 

Underpotential deposition of metals is a commonly observed phenomenon, which has been found to occur for dozens of metal couples in both aqueous and non-aqueous solvents. The potential difference AFupd is independent of the concentration of the metal ion in solution, (as long as the UPD layer is formed reversibly and mass transport limitation is not involved,) since both Fp and AFrev follow the same Nernst equation, albeit with different values of the standard potential. 

The above brief analysis underlines that the porous structure of the carbon substrate and the presence of an ionomer impose limitations on the application of porous and thin-layer RDEs to studies of the size effect. Unless measurements are carried out at very low currents, corrections for mass transport and ohmic limitations within the CL [Gloaguen et ah, 1998 Antoine et ah, 1998] must be performed, otherwise evaluation of kinetic parameters may be erroneous. This is relevant for the ORR, and even more so for the much faster HOR, especially if the measurements are performed at high overpotentials and with relatively thick CLs. Impurities, which are often present in technical carbons, must also be considered, given the high purity requirements in electrocatalytic measurements in aqueous electrolytes at room temperature and for samples with small surface area. 

One of the physical features that is totally different for prenal and citral is their water solubility. In contrast to prenal (110 gL ), citral is barely soluble in water (1.0 g L ). Since the reaction is supposed to take place in the bulk of the aqueous phase or in the boundary layer, this aspect gives a first hint about whether the reaction could be limited by mass transport or kinetics. 

The concentration polarization is a phenomenon, which influences the pervaporation efficiency. The boundary-layer resistance to the solute transport was reported by Psaume et al. [9] for the recovery of trichloroethylene in aqueous solutions. They showed that under certain operating conditions, transport through the membrane was determined by the hydrodynamic conditions in the feed-side, and thus the resistance of the membrane becomes relatively negligible. Colman et al. [77] showed that resistance to the transfer of the boundary layer is a limiting factor of the dehydration of isopropanol by pervaporation. Various others studies [16,78,79] highlighted the importance of the hydrodynamics on resistance to the solute mass transport in pervaporation. 

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