Vehicular mechanism

When the proton is attached to a mobile ion, two modes of transport have been proposed. (1) Polyatomic ions like H3O+ or NH4+ may migrate by a simple jump from site to site in the lattice, as has been claimed in the jS-aluminas and in the zeolites.(2) A more involved mechanism based on the simultaneous difiiision of two types of polyatomic units, the so-called vehicular mechanism, was suggested... 

What then is the structural feature most characteristic of a proton conductor system It is generally believed that a proton is transferred through a solid in one of two distinct ways by a vehicular mechanism, whereby the proton rides on a carrier molecule of type NH4 or HjO ion, or by a Grotthuss mechanism, in which the proton jumps from a donor to a suitably placed acceptor molecule (typically, from H30 to H2O, or from H2O to OH ). How then is such a process sensed in a conventional diffraction experiment The nature of the diffraction method is such that we obtain a time- and space-average of the unit-cell content within the characteristic coherence length of the diffraction process (typically, hundreds of Angstroms), and over the duration of the experiment (days to weeks). This follows from the extremely short photon-electron and neutron-nucleus interaction times ( 10 s), which are significantly shorter than the characteristic time of the fastest of the dynamical processes in the structure ( 10 s for the vibration of a covalently bonded atom). It follows then that some type of structural... 

Let us consider an ideal battery of this type. Fig. 37.1. During dischaige, the negative electrode must supply protons to the solid electrolyte in which they are carried to the other electrode by a translocation/vehicular mechanism. Then they must penetrate into the positive electrode material. 

The main characteristic to consider for a PEM to be used in potential fuel cell is proton conductivity. To achieve good performance of a PEM fuel cell, high proton conductivity is essential, especially at a high current density. To understand proton transport at a molecular level in hydrated polymeric membranes, there are two principal proton transport mechanisms (1) the Grotthus mechanism or proton hopping mechanism, and (2) the vehicular mechanism or diffusion mechanism [243-245]. 

On the other hand, the vehicular mechanism involves the movement of the hydrated proton aggregate. Here, in response to the electrochemical difference, hydrated proton (H30 ) diffuses through the aqueous medium [244,245]. A schematic representation of the vehicular mechanism is presented in Fig. 3.17. In the vehicular mechanism, hydrated protons carry one or more molecules of water (H+[H20] ) through the membrane and are transferred with them as a result of electro-osmohc drag. The major condition for proton transport through the vehicular mechanism is the existence of free volumes within the polymer matrix of a PEM, which allow the passage of hydrated protons through the membrane. 

Figure 3.17 Schematic representation of the vehicular mechanism of proton transport. Reproduced with permission from Ref. [246],...

As already explained, IL exhibit numerous advantages as candidates for electrolyte in high temperature PEMFC. The introduction of IL into acid-doped PBI membranes significantly increases the ionic conductivity in the polymeric electrolyte and the mechanism of conduction seems to be the same as that of H3PO4/PBI. Protons are conducted by both hopping and vehicular mechanism, but hopping is the main mechanism of conduction. Vehicular mechanism is dependent of IL content due to the availability of free ions [64]. Ye et al. [66] found that the... 

Under low water content, the ionically conductive hydrated portion of the membranes behave as nearly isolated clusters, and proton transport is dominated by a vehicular mechanism or diffusion, where proton transport is direct, and by purely physical means. 

Figure 5.5 Schematic of connected sulfonated side chains which enable proton conduction through (a) Grotthuss and (b) vehicular mechanisms in wet and dry PEFC electrolyte membranes, respectively. (Adapted from Weber and Newman [5].)...

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