Grotthus-like mechanism

In this chapter, PEC systems using RTILs were introduced. The discussion focused on the relationship between the viscosity of the RTILs and the short-circuit photocurrent. It was shown that a Grotthus-like mechanism observed in a high concentration of iodide/triiodide redox in RTILs compensates for the shortcomings of the relatively high viscosity of RTILs. This is a critical fact for the apphcation of RTILs in actual electrochemical power devices. 

Delocalized H+ counterions are denoted with a subscript f, while H+ species which transfer between tbe film and bulk solution during the redox reaction are identified by the subscripts s. Thus, for each electron injected into the film there is a simultaneous transfer of one proton, i.e. Hs +, from the solution bulk into the hydrous oxide material, while at the same time there is a transfer locally of 1.5 protons into the ligand sphere of the central metal ion for each electron added to the latter. Proton transport is likely to occur via a Grotthus-type mechanism in these films and is much more likely than OH movement as suggested by other authors [144]. 

Going beyond an atomistic description of the aqueous phase and the membrane, Paddison and coworkers [79-88] employed statistical mechanical models, incorporating solvent friction and spatially dependent dielectric properties, to the calculation of the proton diffusion coefficient in Nation and PEEKK membrane pores. They concluded from their studies that, in accordance with NMR based evidence [50], the mechanism of proton transport is more vehicular (classical ion transport) in the vicinity of the pore surface and more Grotthus-like in the center. 

The ease of occurrance of proton transfer reactions is also demonstrated by the apparently high mobilities of the solvent ions. Due to a chain-like mechanism, the essentials of which were first described by Grotthus in 1806, the proton is attached to an associated solvent unit, which in turn may transfer another proton to another bulk of solvent molecules. The high mobilities of the solvent anions... 

At salt concentrations below those shown in Figure 18.5, molar conductivity behavior has been identified with the formation of electrically neutral ion pairs [8]. Between concentrations of 0.01 and 0.1 mol (up to an O M ratio of 50 1) the molar conductivity rises and this can be explained by the formation of mobile charged clusters such as triple ions, a progressive dissociation of ion pairs, or a combination of both. Up to O M = 50 1, however, spectral data indicate very little change in the species concentrations, and this may instead indicate an enhancement in ionic mobility. With a charge separation <5 A and polymer motion restricted by ion coordination, an anion-assisted (Grotthus-like) transport mechanism could be envisaged as Equations 18.5 and 18.4. 

The thermal activation energy of proton conductivity at intermediate and high water content is similar to that in dilute aqueous acidic solutions (see Fig. 6). From this similarity it has been concluded that proton transfer in PEMs proceeds, like in the bulk, according to the Grotthus structural diffusion mechanism (see below). For decreasing water content a very slight trend towards... 

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