Mass transport diffusion layers, composition

The metal ion in electroless solutions may be significantly complexed as discussed earlier. Not all of the metal ion species in solution will be active for electroless deposition, possibly only the uncomplexed, or aquo-ions hexaquo in the case of Ni2+, and perhaps the ML or M2L2 type complexes. Hence, the concentration of active metal ions may be much less than the overall concentration of metal ions. This raises the possibility that diffusion of metal ions active for the reduction reaction could be a significant factor in the electroless reaction in cases where the patterned elements undergoing deposition are smaller than the linear, or planar, diffusion layer thickness of these ions. In such instances, due to nonlinear diffusion, there is more efficient mass transport of metal ion to the smaller features than to large area (relative to the diffusion layer thickness) features. Thus, neglecting for the moment the opposite effects of additives and dissolved 02, the deposit thickness will tend to be greater on the smaller features, and deposit composition may be nonuniform in the case of alloy deposition. 

Mass transport is understood to mean the molecular diffusion in, out and through plastic materials like that shown schematically in Fig. 1-3. This figure represents most applications where there is a layer of plastic material separating an external environmental media from an inner product media. The product can be a sensitive medium with a complex chemical composition, e.g. food, that must be protected from external influences such as oxygen and contaminants. It can also be an aggressive chemical that must not escape into the surrounding environment. Because this plastic material barrier layer usually includes low molecular weight substances incorporated into the polymer matrix, there are many applications in which the transport of these substances into the product and environment must be minimized. 

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 ... 

In membrane extraction of metals, the mass transport of solute from one phase to another occurs by diffusion. It is controlled by phase equilibrium and the resistances of boundary layers in two phases and the membrane material. Both types of materials are used for membrane extraction and stripping, hydrophilic and hydrophobic, and composite hydro-philic/hydrophobic barriers are also developed to avoid the membrane solubilization [122,123]. To enhance separation, the reactive liquids that induce chemical reaction with one of the separated species can be used. In membrane SX of metals, extracting agents, such as tri- -octylphosphine oxide (TOPO), di(2-ethylhexyl)phosphoric acid (D2EHPA), and n-octyl(phenyl)-A,A-diisobutylcarbamoylmethylphosphine oxide (CMPO), and commercial reagents like CYANEX 301, CYANEX 923, LIX622, and LIX622N are applied. 

Because there are no spatial compositional variations complicating the data, it is easy to conclude that the inert gas reduces mass transport across the gas diffusion layer, moving the onset of the mass transport polarization regime to lower current densities. 

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