Proton Grotthuss mechanism

Structure diffusion (i.e., the Grotthuss mechanism) of protons in bulk water requires formation and cleavage of hydrogen bonds of water molecules in the second hydration shell of the hydrated proton (see Section 3.1) therefore, any constraint to the dynamics of the water molecules will decrease the mobility of the protons. Thus, knowledge of the state or nature of the water in the membrane is critical to understanding the mechanisms of proton transfer and transport in PEMs. 

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... 

Figure 16.1 Models of proton transfer process via (a) Grotthuss mechanism and (b) Vehicle mechanism.

Similar thermal effects have also been noted in hydroxide water clusters [54]. While proton transfer is widely accepted to follow the "Grotthuss" mechanism... 

Figure 10 Proton conductivity according to the Grotthuss mechanism. (Reprinted from Ref. 131 2003, with permission from Elsevier)...

Figure 14.8 Proton transfer mechanism ofthe loose complexes R0H---(H20) ---B with a sequential, von Grotthuss-type, hopping of protons through water bridges. For H PTS and monochloroacetate the first transfer to the water bridge forming the hydronium ion HjO+ is ultrafast, and the second transferto the base is slower. (Adapted from Ref [136].)...

The mechanism of proton diffusion, usually referred as the Grotthuss mechanism, differs from that of other ions. Instead of a self-diffusion process, where the mass and the charge are inseparable, the proton diffusion is the movement of the protonic charge independently from the transport of the particle. This mode of propagation leads to the diffusion of the proton 0 + = 9.3 x 10 cm s being faster than the diffusion of all other ions (D <2 x 10 cm s ), and of the selfdiffusion of the water molecules in water as a solvent (for details see Ref [40]). 

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