Grotthuss proton ordering

Figure 13.3 Proton ordering in hydrogen bonding, illustrating favorable (Grotthuss-like) ordering in a chain (above) or cyclic hexamer (a), contrasted with the unfavorable isomer (b), one of many similar that cannot survive thermodynamically. All clusters are ice rule-compliant (cf. Sidebar 5.18).

Ab initio methods provide elegant solutions to the problem of simulating proton diffusion and conduction with the vehicular and Grotthuss mechanism. Modeling of water and representative Nation clusters has been readily performed. Notable findings include the formation of a defect structure in the ordered liquid water cluster. The activation energy for the defect formation is similar to that for conduction of proton in Nafion membrane. Classical MD methods can only account for physical diffusion of proton but can create very realistic model... 

For phosphoric acid-doped polybenzimidazole membranes with an intermediate Ap Ta (3.5), hydrogen bonds are likely formed as shown by infrared spectroscopy [61, 62] and molecular dynamic simulation [63]. The overall proton conductivity of the acid-doped membranes is at least an order of magnitude lower than that of pure phosphoric acid due to the presence of the solid polymer phase. The polymer does not seem to interrupt the extensive hydrogen bond network of the phosphoric acid, though it does decrease the percolation within the liqmd-like part of the phosphoric acid domain. As a result, the proton conducting mechanism remains the same, i.e., primarily via the Grotthuss... 

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