Proton, mobility

Figure 1.1 The nature of a composite, Pt/C/recast ionomer layer with a structure that enables high electronic and gas mobilities as well as sufficient proton mobility [Gasteiger, 2005].

Eigen pointed out that the mobility of a proton in ice at 0°C is about 50 times larger than in water. In ice, H20 molecules already occupy fixed positions suitable for accepting a proton, so that the proton mobility is directly proportional to the rate of tunnelling. 

Cukierman, S. (2000) Proton mobilities in water and in different stereoisomers of covalently linked gramicidin A channels. Biophysical Journal, 78, 1825-1834. 

When a solution of the ester (155 R = H) in dioxan is warmed at 80 °C, the phosphonate (156) is produced together with about 5% yields of each of dimethyl hydrogen phosphonate and diazoacetoacetic ester (Scheme 8) the enol phosphate (157) could not be detected. The explanation for this sequence of reactions also relies on proton mobility, and the reaction is known to be acid-catalysed.124... 

In the case of PEMs, the situation is more complicated because the sulfonate counter-ions (in the case of a PEM such as Nafion ) are bound to the polymer chain and are thus relatively immobile, in contrast to the free counter-ion in a small molecule acid such as sulfuric or acetic acid. Tethering of the sulfonate group can be considered to be an impediment to the mobility of the proton as it traverses the membrane. Proton mobility is also affected by the effective mean-free path of connectivity of the conduction pathway as shown in Figure 3.2. In situation (a), the increased number of dead ends and tortuosity of the aqueous domains through which proton transport occurs over the situation in (b) leads to lower overall mobility. This has been demonstrated by Kreuer and will be discussed later in this section. 

At low water contents, Dh o decreases more rapidly for SPEEKK than for Nafion. Proton mobility, D, behaves in a similar fashion. When water content is high, is higher than a result of the influence of inter-... 

Proton mobility (D J and water self-diffusion coefficient (D q) as a function of the water volume fraction (X ) in Nafion and SPEEKK, where X, = volume of water in membrane divided by volume of wet membrane. (From Kreuer, K. D. 2001. Journal of Membrane Science 185 29-39.)... 

It is interesting to note, however, that even though the SPEEKK sample shown in Figure 3.22 has a higher lEC content (1.46 meq/g) than sPS02-781 (1.28 meq/g), it exhibits overall lower conductivity for a given water content. Based on an examination of the proton mobility values for these polymers, it was suggested that this may be due to some microstructural differences between the two different systems. Proton conductivity as a function of... 

Peckham, T. J., Schmeisser, J., Rodgers, M. and Holdcroft, S. 2007. Main-chain, statistically sulfonated proton exchange membranes The relationships of acid concentration and proton mobility to water content and their effect upon proton conductivity. Journal of Materials Chemistry 17 3255-3268. 

Herz, H. G., Kreuer, K. D., Maier, J., Scharfenberger, G., Schuster, M. F. H. and Meyer, W. H. 2003. New fully polymeric proton solvents with high proton mobility. Electrochimica Acta 48 2165-2171. 

Proton Mobility near the Polymer-Water Interface.385... 

A question of ufmosf inferesf is whefher high proton mobility in aqueous-based PEMs is possible under conditions of minimal hydration and at elevated temperature. Obviously, the answer could have a tremendous impact on promising design strategies in membrane research. - ° This calls attention to interfacial mechanisms of proton transport (PT). 

Proton conductivities of 0.1 S cm at high excess water contents in current PEMs stem from the concerted effect of a high concentration of free protons, high liquid-like proton mobility, and a well-connected cluster network of hydrated pathways. i i i i Correspondingly, the detrimental effects of membrane dehydration are multifold. It triggers morphological transitions that have been studied recently in experiment and theory.2 .i29.i ,i62 water contents below the percolation threshold, the well-hydrated pathways cease to span the complete sample, and poorly hydrated channels control the overall transports ll Moreover, the structure of water and the molecular mechanisms of proton transport change at low water contents. 

The complications for fhe fheoretical description of proton fransporf in the interfacial region befween polymer and water are caused by the flexibility of fhe side chains, fheir random distributions at polymeric aggregates, and their partial penetration into the bulk of water-filled pores. The importance of an appropriate flexibilify of hydrated side chains has been explored recently in extensive molecular modeling studies. Continuum dielectric approaches and molecular dynamics simulations have been utilized to explore the effects of sfafic inferfacial charge distributions on proton mobility in single-pore environments of Molecular level simulations were employed... 

On the other hand, the merits of such insights are obvious. It would become possible to evaluate the relative importance of surface and bulk mechanisms of PT. The transition from high to low proton mobility upon dehydration could be related to molecular parameters that are variable in chemical synthesis. It could become feasible to determine conditions for which high rates of interfacial PT could be attained with a minimal amount of hghtly bound water. As an outcome of great practical value, this understanding could direct the design of membranes that operate well at minimal hydration and T > 100°C. 

The total electro-osmotic coefficient = Whydr + mo includes a contribution of hydrodynamic coupling (Whydr) and a molecular contribution related to the diffusion of mobile protonated complexes—namely, H3O. The relative importance, n ydr and depends on the prevailing mode of proton transport in pores. If structural diffusion of protons prevails (see Section 6.7.1), is expected to be small and Whydr- If/ ori the other hand, proton mobility is mainly due to the diffusion of protonated water clusters via the so-called "vehicle mechanism," a significant molecular contribution to n can be expected. The value of is thus closely tied to the relative contributions to proton mobility of structural diffusion and vehicle mechanism. ... 

Cation—sulfonate interactions, as well as proton mobility, are also expressed in the electrical conductance behavior of these membranes. Many studies of this property have been reported, and there is no attempt in this review to cite and describe them all. Rather, a few notable examples are chosen. Most testing is done using alternating current of low voltage to avoid complications in the form of chemical... 

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