Proton conductivity Grotthuss mechanism

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... 

To conclude this section on proton transfer, we have examined an alternative mechanism to that of von Grotthuss for proton conduction [105-110], We carried out B3LYP/6-31+G and PW91 DFT calculations on model compounds (1,2,3,4-tetrasubstituted benzenes, e.g., 99) showing that these compounds could play the role of proton conductors [104],... 

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

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... 

Figure 4. Leakage vs, conduction models for the permeation of protons. Left transient water wires form very infrequently in the nonpolar environment provided by lipid bilayers, but when they do, they could translocate just one proton very rapidly before breaking up only the hop step ofGrotthuss takes place. Right in polar channels such as GA, water wires are much more long-lived, which is consistent with their rapid relay of proton via a complete Grotthuss mechanism involving both hop and turn steps (l(f s ).

Even if a proton jump along a bond is very rapid, the conductivity is explained by the Grotthuss mechanism (it is not the same proton that jumps). Please notice that this jump is not a rotation it takes place along the bond from one minimum of the potential to another, creating, for short time, an ion H30+ and a correspondent OH-. But, I insist, their concentration is very small. It is out of question to see such species, for example, in scattering experiments. Their impact on the transport properties (self-diffusion, molecular rotations) is totally negligible. 

One of the most interesting systems is the class of imidazoles. They are selfdissociation compounds with high proton conductivity (>100mS cm ) without any acid doping. Further enhancement of the conductivity and also the thermal stability of the system can be realised by the addition of acidic components [45]. This is due to the proton transfer via the Grotthuss mechanism. 

Protons can propagate by two mechanisms. One is by the viscous flow of a complex H+(H20) where n = 1, 2, 3, 4, the second is by the Grotthuss mechanism, hopping of a proton from one hydronium H3O+ to a nearby water molecule. The latter mechanism has some similarity to hopping conduction of ions in solids as discussed in the following text. 

N. Agmon, The Grotthuss Mechanism, Chemical Physics letters, 244,456 (1995). K. D. Kreuer, On the Complexity of Proton Conduction Phenomena, Solid State Ionics, 136, 149 (2000). 

Unlike other acids, in concentrated phosphoric acid solutions (where the water concentration is low) hydrogen ions exist not as hydrated ions H+ H20, but as ions solvated by phosphoric acid molecules H+ - H3P04. For this reason, the solution s conductivity is a complex, nonmonotonous function of concentration. As for the conductivity mechanism, these ions do not move like a spherical particle in a viscous medium when an electric field is applied to the solution (the Stokes mechanism), but rather the protons alone jump in the field direction from one acid molecule to another acid molecule (the Grotthuss mechanism, suggested in 1806 as an explanation for the conductivity behavior in aqueous solutions). 

---