Electrochemical transport

1 Mass and Charge Transport under the Chemical Potential 

Let us consider one example of the mass and charge transfer in a simple oxygen- 

At constant temperature, the fluxes of oxygen vacancies and electrons are expressed as 

Solving the latter equation relative to the electrical potential gradient yields 

By introducing Equation (3.97) into Equation (3.93), and taking into account that 

All four of the mechanisms described above probably operate to some degree at every site where mineralisation is buried and it is possible that any one of them could dominate in selected environments. However, the first three are precluded as major contributors to transport in thick, water-saturated Quaternary glacial environments. Since selective leach anomalies are now commonly reported in glacial terrain, electrochemical processes are likely to dominate in at least this environment. 

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Very little work has been done in this area. Even electrolyte transport has not been well characterized for multicomponent electrolyte systems. Multicomponent electrochemical transport theory [36] has not been applied to transport in lithium-ion electrolytes, even though these electrolytes consist of a blend of solvents. It is easy to imagine that ions are preferentially solvated and ion transport causes changes in solvent composition near the electrodes. Still, even the most sophisticated mathematical models [37] model transport as a binary salt. 

This device has not reached commercialization, no doubt in part because bulk electrochemical transport of major gaseous components will rarely be economical compared with more standard separation processes. It is in the transport of minority species from low partial pressure to high (e.g. 02 from seawater, C02 from air) where the benefits of the electrochemical driving force, as detailed at the outset of this chapter can best be exploited. Two final examples of contaminant control of great commercial interest demonstrate this principle. 

With today s computing power and popular use of CFD codes, SOFC modeling is moving toward multiphysics, electrochemical-transport-coupled, and three-... 

Solutions of the combined equations of mass transfer, kinetics and electrochemical transport expressed in terms of the limiting current, i generally are of the form... 

This outline, as brief and superficial as it may be (for a more detailed description of basic electrochemical transport objects, the reader is referred to relevant texts, e.g., [1]—[3]) will permit a formulation of basic equations of electro-diffusion. A hierarchy of electro-diffusional phenomena will be sketched next, beginning with the simplest equilibrium ones. Subsequent chapters will be devoted to the study of some particular topics from different levels of this hierarchy. 

The main point on which the transfer to a clean electrochemical transportation system has stumbled in the past has been the power source, which, with batteries, could provide only a small range, a long charging time, and continued pollution from the... 

Some 109tons of C02 are added to the atmosphere each year, and during the next half century, if present practices continue, the C02 concentration will rise to at least 500 and perhaps even 600 ppm (about a 48% increase over the concentration at the year 2000). One way to deal at least partially with this undesirable development would be to fix the C02 into, e.g., the simplest practical liquid H2 carrier, methanol (compare the use of methanol as a hydrogen source for fuel cells in electrochemical transportation and in other industrial machinery). 

Polyimides have excellent dielectric strength and a low dielectric constant, but in certain electrolyte solutions they can electrochemically transport electronic and ionic charge. Haushalter and Krause (5) first reported that Kapton polyimide films derived from 1,2,4,5-pyromellitic dianhydride (PMDA) and 4,4 -oxydianiline (ODA) undergo reversible reduction/oxidation (redox) reactions in electrolyte solutions. Mazur et al., (6) presented a detailed study of the electrochemical properties of chemically imidized aromatic PMDA- derived polyimides and model compounds in nonaqueous solutions. Thin films of thermally... 

The Nemst-Planck equation is conventionally applied to measure iontophoretic flux and arises from the theoretical development of Eq. 1 to define the flux of an ionic solute /, across a membrane (a) by simple diffusion due to the solute concentration gradient and (b) as a result of the electric potential difference across the membrane (electrochemical transport) [68-70]. 

Similarly, impervious yttria-stabilized zirconia membranes doped with titania have been prepared by the electrochemical vapor deposition method [Hazbun, 1988]. Zirconium, yttrium and titanium chlorides in vapor form react with oxygen on the heated surface of a porous support tube in a reaction chamber at 1,100 to 1,300 C under controlled conditions. Membranes with a thickness of 2 to 60 pm have been made this way. The dopant, titania, is added to increase electron How of the resultant membrane and can be tailored to achieve the desired balance between ionic and electronic conductivity. Brinkman and Burggraaf [1995] also used electrochemical vapor deposition to grow thin, dense layers of zirconia/yttria/terbia membranes on porous ceramic supports. Depending on the deposition temperature, the growth of the membrane layer is limited by the bulk electrochemical transport or pore diffusion. 

Bolviken and Logn (1975) and Smee (1983) include groundwater transport as a possible mechanism for element dispersion from mineralisation. Webber (1975) pointed to the much higher potential migration rate of groundwater as compared with that of diffusion or electrochemical transport and concluded that advective groundwater transport is likely to be the most important dispersal mechanism. 

Hamilton, S.M. and McClenaghan, M.B., 1998. Field data in support of electrochemical transport of elements through thick glacial overburden. Ontario Geol. Survey, Misc. Paper 169 267-274. 

J. Maier, Mass transport in the presence of internal defect reactions-concept of conservative ensembles I, Chemical diffusion in pure compounds. /. Am. Ceram. Soc., 76(5) (1993) 1212-1217 II, Evaluation of electrochemical transport measurements, ibid., 1218-1222 III, Trapping effect of dopants on chemical diffusion, ibid., 1223-1227 IV, Tracer diffusion and intercorrelation with chemical diffusion and ion conductivity, ibid., 1228-1232. 

Electrochemical properties Nearly all electrochemical transport, kinetic, and thermodynamic data in the literature are for aqueous systems at or near room temperature. Exploratory development of other types of systems (nonaqueous solvents, fused salts, polymeric electrolytes) is therefore exceedingly difficult. Creation of a data base that is readily accessed is an essential task but is done poorly at present. 

Systematic studies with the mass transfer process in an electrochemical system date back to the 1940s [137,138]. Later investigators extended the use of the method to both natural and forced convection flows. Extensive bibliographies of natural and forced convection studies using the electrochemical technique are available [139,140]. Convenient sources of information on the general treatment of electrochemical transport phenomena can be found in Refs. 141 and 142. 

Table 3.6-10 indicate that, in general, more charge is carried by the cation. However, the relative fraction of this charge is less than that observed in the electrochemical transport data for the haloaluminate ionic liquids. It is unclear at this time if this difference is due to the different anions present in the non-haloaluminate ionic liquids or to differences in the two types of transport number measurements. The apparent greater importance of the cation to the movement of charge demonstrated by the transport numbers (Table 3.6-10) is consistent with the observations made from the diffusion and conductivity data above. Indeed, these data taken in total may indicate that the cation tends to be the majority charge carrier for all ionic liquids, especially the alkylimidazoliums. However, a greater quantity of transport number measiuements, performed on a wider variety of ionic liquids, will be needed to ascertain if this is indeed the case. 

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