Interfacial processes exchange

Figure 9.6 Visual representation of the platinum oxide growth mechanism, (a) Interaction of H2O molecules with the Pt electrode occurring in the 0.27 V < < 0.85 V range, (b) Discharge of 5 ML of H2O molecules and formation of 5 ML of chemisorbed oxygen (Ochem)- (c) Discharge of the second ML of H2O molecules the process is accompanied by the development of repulsive interactions between (Pt-Pt) -Ofi m surface species that stimulate an interfacial place exchange of Ochem and Pt surface atoms, (d) Quasi-3D surface PtO lattice, comprising Pt and moieties, that forms through the place-exchange process. (Reproduced with permission...

It is worthwhile mentioning that the interfacial potential created at the liquid-liquid interface is governed by single ionic or redox equilibrium only in the simple cases. The presence of various, often two, interfacial processes is a source of the steady-state potential, named also the mixed or the rest potential. Its value is situated between the two equilibrium potentials, near that one which corresponds to the higher exchange current... 

In relation to describing the fundamental interfacial processes we are interested in determining the exchange current Iq. If the current which flows immediately after the application of an overpotential r is I l we have the relationship for < 10 mV ... 

The other important interfacial process is ion exchange, during which the ions in the interlayer space or the external surfaces are changed to other ions. It happens in connection with the change of the composition of the liquid phase interacting on the solid. For example, the cation exchange of the interlayer space of a clay mineral is... 

On the basis of those discussed here, we can say that the thermodynamic parameters of ion exchange determined by linear isotherm equations are not correct. We determine the mechanism of sorption (ion exchange and/or adsorption) and then choose the isotherm equation. When simultaneous interfacial processes take place, the dominant process, if any, has to be selected or experimentally separated, or Equation 1.119 should be used. 

The negative layer charge is mostly neutralized by the hydrated cations in the interlayer space. These cations are bonded to the internal surfaces by electrostatic forces, and they are exchangeable with other cations. The interaction strength between the hydrated cation and the layers (the internal surface) increases when the charge of the cation increases, and the hydrated ionic radius decreases. Cations with hydrate shell can be considered as outer-sphere complexes. Cation exchange is the determining interfacial process of the internal surfaces of montmorillonite. 

As seen earlier, zinc ions can be a part of different interfacial processes on cal-cium-montmorillonite. The ion exchange in the interlayer space and the other sorption on the external surfaces can be separated by measuring ions entering and leaving the montmorillonite phase. The quantitative separation of sorption isotherms will be discussed in Section 2.6. 

The other interfacial process involving hydrogen ion is the cation-exchange process in the interlayer space. When montmorillonite is suspended in water or in an electrolyte solution, a part of exchangeable cations can be dissolved. In Table 2.7, the relative quantity of calcium ions dissolved in water or in acidic solutions is shown. 

The same process of interfacial ion exchange determines the potential difference for other ion conductors, like membranes or SEs. The existence of this potential difference again leads to the formation of the EDL, but this time it is formed by excessive ionic charges in the surface layers of both solutions. Since this EDL is created by the ion transfer across the interface, without charge transport across the bulk media, the Donnan potential drop corresponding to the composition of the phases is established very rapidly. 

The correlation between the behaviour of U, U and v, t-curves found in Ref. [52] may be the basis for developing quantitative methods controlHng the hydrodynamic flux structure near the electrode surfaces, as well as methods for influencing the velocity of the heterogeneous processes by means of controlled changing of the hydrodynamic conditions of interfacial mass exchange. 

The first part of this chapter returns to the topic of transport processes in a PEM. Proton transport and electro-osmotic water drag in PEMs have been discussed extensively in Chapter 2. To understand the operation of the PEM in a fuel cell, other transport phenomena, including liquid water diffusion, hydraulic permeation, and interfacial vaporization exchange of water, must be addressed as well. Eor this purpose, the corresponding transport parameters must be found and their impact on performance rationalized. 

In the past, it has been a common albeit dubious practice to adopt an equihb-rium sorption isotherm for the relation between Xm and This approach demands an infinite rate constant of vaporization exchange. It is problematic for two reasons. First, the relative importance of interfacial water exchange grows with decreasing membrane thickness. Below a critical thickness, interfacial kinetics, rather than bulk transport, will limit the net water flux, implying an out-of-equilibrium condition. Second, if gases adjacent to the membrane are moving, water may be convected away from its surfaces. It is inherently contradictory to assume equilibrium in the presence of any kinetic or convective process. 

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