Thermodynamics charged species transport

Microelectronic devices depend on motion of a charge in and on semiconductor materials. Such phenomena share strong ties with the thermodynamics and transport of charge species in electrolytes. 

Fuel flexibility One of the merits of SOFCs is fuel flexibility. Since the charged species associated with oxidants is transported in SOFCs, any kinds of fuels can be utilized in principles. Since nickel anodes are weak against carbon deposition or sulfur poisoning, fuels are used in SOFCs after fuel processing is made. Thus, fuel chemistry based on the thermodynamics is one of the major components in understanding SOFC systems. 

The equation implies that transport into the cell of either neutral or charged species is thermodynamically favorable if < out or, if we set the activity coefficients to 1, if [A]in < [A]o f An ion also needs to cross a membrane potential difference that arises from differences in Coulomb repulsions on... 

Owing to the size of the subject, it has proved necessary in the following chapters, to be selective in the choice of material presented. Earlier chapters are concerned with ionics and its applications. Here are considered ion interactions in solution, acid-base equilibria, transport phenomena, and the concept of reversible electrode potential. This last named leads to the development of reversible cells and their exploitation. Here one is dealing with electrochemical thermodynamics - with the rapid attainment of equilibrium between species at an electrode surface and charged species in solution. 

As electroneutrality is demanding, it is generally assumed that the field-assisted transport of charged species is more rapid than the transport of neutral species and, consequently, solvation equilibria can only be established slowly. Therefore, the equilibria associated with electronic, ionic and solvation processes may be established on quite different time scales, but at long enough time scales thermodynamics will prevail and processes will attain a state of global equilibrium. However, the relative rates of all the processes involved in the charge compensation are still an open question. 

The thermodynamics of insertion electrodes is discussed in detail in Chapter 7. In the present chapter attention is focused mainly on the general kinetic aspects of electrode reactions and on the techniques by which the transport of species within electrodes may be determined. The electrodes are treated in a general fashion as exhibiting mixed ionic and electronic transport, and attention is concentrated on the description of the coupled transport of these species. In this context it is useful to consider that an electronically conducting lead provides the electrons at the electrodes and compensates the charges of the ions transferred by the electrolyte. 

Cation, anion, and water transport in ion-exchange membranes have been described by several phenomenological solution-diffusion models and electrokinetic pore-flow theories. Phenomenological models based on irreversible thermodynamics have been applied to cation-exchange membranes, including DuPont s Nafion perfluorosulfonic acid membranes [147, 148]. These models view the membrane as a black box and membrane properties such as ionic fluxes, water transport, and electric potential are related to one another without specifying the membrane structure and molecular-level mechanism for ion and solvent permeation. For a four-component system (one mobile cation, one mobile anion, water, and membrane fixed-charge sites), there are three independent flux equations (for cations, anions, and solvent species) of the form... 

It Is assumed that equation 1 Is the complexatlon reaction which facilitates the transport of H S. However, It Is difficult to Justify this reaction taking place within a PFSA membrane since the anionic species HS Is created and anions are excluded from the matrix by electrostatic repulsion. However, if the HS species and the EDA species exist as a Ion pair having a +1 charge, then the complexatlon reaction would not be hampered by unfavorable thermodynamics. Also, the reaction would conform to the A + B - AB reaction... 

When pore radius is in the same order of magnitude than the zeta potential barrier, transport of ionic species is affected by the flxed charge of the membrane. Ions with the same charge as the flxed ions in the membrane are excluded and cannot pass through the membrane. This effect is known as the Donnan exclusion and can be described by equilibrium thermodynamics from which the chemical potential of the ionic species can be calculated in the membrane and in the solution and then the concentration of the different species. If the concentration in the feed is low and the concentration of the fixed charge is high, the Donnan exclusion is very effective. 

However, attention has to be paid not only to the electronic charging of the polymer film (i.e., to the electron exchange at the metal/polymer interface and the electron transport through the surface layer) since, in order to preserve electroneutrality within the film, ions will cross the film/solution interface. The motion of counterions (or less frequently that of the coions) may also be the rate-determining step. The thermodynamic equilibrium between the polymer phase and the contacting solutions requires fi (film) = /zj (solution) for all mobile species. In fact, we may regard our film as a membrane or a swollen polyelectrolyte gel (i.e., the... 

In the electrochemical literature it is useful to refer to a reversible interface or interfacial reaction as one whose potential is determined only by the thermodynamic potentials of the various electroactive species at the electrode surface. In other words, it is only necessary to take into account mass transport to and from the interface, and not the inherent heterogeneous kinetics of the interfacial reaction itself, when discussing the rate of the charge transfer reaction. This nomenclature has two principal disadvantages. First, it neglects the fact that mass transport to the interface, whether migration or diffusion, is inherently an irreversible or dissipative... 

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