Proton conductivity vehicle mechanism

X 10 cm /s at room temperature) and that the diffusion of protonated water molecules makes some contribution to the total proton conductivity (vehicle mechanism " ). This is --"22% when assuming that the diffusion coefficients of H2O and H3O+ (or H502 ) are identical. However, as suggested by Agmon, " the diffusion of H3O+ may be retarded, because of the strong hydrogen bonding in the first hydration shell. 

A more recent view of proton transport is that of Kreuer, who, compared with the Zundel-based view, describes the process on different structural scales within phase separated morphologies. The smallest scale is molecular, which involves intermolecular proton transfer and the breaking and re-forming of hydrogen bonds. When the water content becomes low, the relative population of hydrogen bonds decreases so that proton conductance diminishes in a way that the elementary mechanism becomes that of the diffusion of hydrated protons, the so-called vehicle mechanism . 

Noda et al. [ 168] reported the details of Bronsted acid-based ionic liquids consisting of a monoprotonic acid and an organic base, in particular solid bis(trifluorometha-nesulfonyl)amide (HTFSI) and solid imidazole (Im) mixed at various molar ratios to form liquid fractions. Studies of the conductivity, H NMR chemical shift, selfdiffusion coefficient, and electrochemical polarization results indicated that, for the Im excess compositions, the proton conductivity increased with an increasing Im molar fraction, with rapid proton-exchange reactions taking place between the protonated Im cation and Im. Proton conduction was found to occur via a combination of Grotthuss- and vehicle-type mechanisms. Recently, Nakamoto [169] reported the... 

If proton conduction is verified, is the conduction mechanism then proton hopping between partially occupied proton sites accompanied by a reorganization of the anion lattice or is the mechanism proton transport by a carrier (e.g. water, hydrazine or ammonia) which possibly counter-difTuses when the proton is discharged at the cathode (vehicle mechanism) ... 

Kreuer et al. have proposed a proton conduction mechanism, where the proton is attached to a vehicle (water, ammonia, hydrazine etc.), and the complex moves as a whole. If, in a d.c. experiment the vehicle is not supplied together with protons at the anode, migration of the proton-vehicle complex will generate a vehicle deficit close to the anode. This will cause the conductivity to decrease, if no comparable alternative conduction path exists. The decline will be observed in both the d.c. and the a.c. conductivities. 

It should be emphasized that a time-independent current cannot be taken as the only proof of a hopping mechanism. If the amount of vehicle is much larger than the amount of protons, the vehicle concentration gradient caused by the current may not be sufficient to influence the conductivity. 

Unfortunately HU As is the only proton-conducting hydrate for which available oxygen diffusion coefficient data allow direct verification of the vehicle mechanism. For other compounds there is just an indication that this may be the mechanism. These are e.g. diffusion bottleneck considerations for ionic conduction in zeolites or the exceptionally high temperature factors for the water oxygen in and some heteropoly-... 

In general, the low thermal stability and high vapour pressure for the anticipated vehicle molecule make plausible a conduction mechanism similar to the one in acid solutions for this family of compounds. If we assume the mechanism presented above, then at least some amplification for proton conduction in the solid hydrate TSA.28H2O is observed A = 1.8) (Fig. 26.3). This is in perfect agreement with a ratio [HjO" ]/ [H2O] = 6 for this compound (Fig. 31.5). A is indeed independent of temperature and appears to be a structural feature as suggested by the model. 

The presence of water in PVPA seems to contribute to the conductivity of PVPA, at a temperature below the boiling point of water, by proton transport in additional proton solvents. Hence, the proton movement in PVPA can be explained by rapid transfer of protons via hydrogen bond-forming and bondbreaking (hopping mechanism) and by self-diffusion (vehicle mechanism). Different conductivity values could be obtained due to a different water content. Also, recent studies have revealed that the behavior of PVPA as a polyelectrolyte is very similar to poly(actylic acid) in aqueous salt solution under identical conditions. ... 

By using hydrophobic fluorinated polymer backbones, the phase separation can be made more distinct in phosphonated membranes, which in turn can enhance the proton conductivity. DesMarteau and co-workers have studied the proton transport characteristics of model perfluoroacid compounds functionalized with phosphonic, phosphinic, sulfonic, and carbo)q lic acids. The results indicated that the proton transfer in phosphonic and phosphinic acids occurs via structural diffusion rather than by a vehicle mechanism. The findings suggested that fluoroallqrlphosphonic and -phosphinic acids are good candidates for further development as anhydrous, high-temperature proton conductors. 

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