Proton Conduction in Aqueous Environments

The dominant intermolecular interaction in vater is hydrogen bonding. The introduction of an excess proton (i.e. the formation of a protonic defect) leads to the contraction of hydrogen bonds in the vicinity of such a defect. This corresponds to the vell-kno vn structure forming properties of excess protons in water (see for example Ref. [26]). Thus the isolated dimer H5O2+ finds its energetic minimum at an O / O separation of only 240 pm [27, 28[ with an almost symmetrical single well potential for the excess proton in the center of the complex. 

The above-described features are reproduced in a high level quantum-molecular-dynamics simulation of an excess proton in water [30, 31]. In accordance with results from several other groups, this finds the excess proton either as part of a dimer (H5O2+, Zundel -ion) or as part of a hydrated hydronium ion (H9O4+, Eigen -ion) (Fig. 23.3). 

Another interesting feature of this mechanism is that the hydrogen bond breaking and forming (hydrogen bond dynamics) and the translocation of protons within the hydrogen bonds take place in different parts of the hydrogen bond network. 

It should also be mentioned that OH hyper-coordination is not found in concentrated solutions of NaOH and KOH [48]. In contrast to acidic solutions where structure diffusion is suppressed with increasing concentration the transference number of OH (for example in aqueous KOH solutions) remains surprisingly high (approximately 0.74) for concentrations up to about 3 M. 

In pure water, excess protons (H3O+, H5O2+) and defect protons (OH ) are present in identical concentration, but owing to their low concentration (10 M under ambient conditions) and the high dielectric constant of bulk water the diffusion of these defects is quasi-independent. 

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Polymer membranes have low proton conductivity until they contain enough proton conductive functional groups, e.g., sulfonic acid, in polymer backbones or side chains. Sulfonic acid in polymer chains shows a high dissociation constant, resulting in high proton conductivity in aqueous environments compared with other acids therefore, most DMFC membranes use the sulfonic moiety as the proton transfer medium. Sulfonation methods of polymer membranes are generally classified as post-sulfonation in the presence of polymers and in situ sulfonation through co-polymerization of sulfonated monomers and nonsulfonated monomers. 

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