Rate of proton conductance

Although the secrets of maximal rates of proton conduction are well illustrated in gA, multifunctional proteins that couple H+ conduction to other events do not exhibit well-formed, proton-conducting hydrogen bond networks. Indeed, in the bacterial reaction center the putative active path is poorly connected by hydrogen bonds detectable in the best current X-ray structures (2.2 A resolution Stowell et al., 1997). Paddock et al. (1999) have shown that chemical blockage or a simple mutational lesion of this active path diminishes proton transfer rates by at least 1000-fold. Thus, the several well-connected (but not quite continuous) files of water that are seen in the X-ray structures, reaching toward the Qg site from the cytoplasmic side, do not conduct protons at significant rates. 

In E. coli, only FO-subunits, and none of the FI-subunits are in contact with the membrane lipids flQ]. Hence a reversal in the direction of catalysis may be accompanied by a reorientation of FO-subunits QlJ. This may result in a change in electrostatic and/or hydrophobic interactions of FO-subunits with the lipids, which in turn would affect the rate of proton conductance through FO. 

The excellent prospects of PEFCs as well as the undesirable dependence of current PEMs on bulk-like water for proton conduction motivate the vast research in materials synthesis and experimental characterization of novel PEMs. A major incentive in this realm is the development of membranes that are suitable for operation at intermediate temperatures (120-200°C). Inevitably, aqueous-based PEMs for operation at higher temperatures (T > 90°C) and low relative humidity have to attain high rates of proton transport with a minimal amount of water that is tightly bound to a stable host polymer.33 37,40,42,43 yj-jg development of new PEMs thus warrants efforts in understanding of proton and water transport phenomena under such conditions. We will address this in Section 6.7.3. 

Rosenberg, whose work on proton conduction in the alcohols led to insights into proton conduction, was also a coauthor of a paper that laid the foundation for the development of the theory of proton conduction in solutions. The theory utilized the idea of proton tunneling as outlined earlier, but added an essential limitation to its rate. Thus, in the Eyring theory, the only prerequisite for a proton to pass from an... 

Conductance. Conway, Bockris, and Linton (423) have made a thorough study of proton conductance in water and alcohols. (Sec Section 2.1.5 for a general description.) Their model was basically that of Lennard-Jones and Pople (Fig. 8-2). However, to account for the rate of transfer, some molecules must rotate. This created some doubly filled H bonds, causing further rotation. These steps [Pg.253]

Fig. 20 Distributions at current density of 1 A cm 2, of electrode potential (top), reactant concentration (middle), and current generation (bottom) in a PEFC anode catalyst layer 5 pm thick, as result of limited transport rate of the hydrogen gas reactant and/or the limited transport rate of protons. Two cases of reactant concentration, 100% hydrogen and 10% hydrogen in the dry gas and two cases of effective protonic conductivity in the catalyst layer, 0.1 and 0.01 S cm-1, are considered in these calculations. A value of 2 x 10-4 cm2 sec-1 was used as estimate for effective Dh2 in the catalyst layer.

Figure 5. The dependence of the rate of proton dissociation from excited 8-hydroxy-pyrene- 1,3,6-trisulfonate on the mole fraction of ethanol in water, and the respective proton conductivity of the mixtures. The rate of proton dissociation was measured by time resolved ( ) or steady-state ( ) fluorescence. The proton conductivity of the solutions (A) is normalized for pure water conductivity. Data taken from Erdey-Grutz and Lengyel (1977).

Not much more is known about the expeditiousness which, in this context, may be defined as the rate at which the protonmotive force rises after the onset of the pump, but some general predictions can be made. The expeditiousness is the higher the fester the pumping rate of protons is relative to the conductance of passive ions, which tend to shunt the electrogenic pump effect, in other words, to maintain electroneutrality. We see that the two components of the protonmotive force, the electrical and the chemical PD, are controlled by different factors and may therefore develop at different rates, as is illustrated by the following two extreme conditions ... 

Which is the mechanism of proton conduction enabling CF to this high turnover rate and extreme selectivity ... 

The addition of reagents containing X-H bonds in which X is more electronegative than H typically lead to addition across the M-C bond in the direction opposite to the addition of silane or borane to the early metal catalysts. Polymerization of etiiylene with lanthanide catalysts in the presence of phosphines generates phosphine-terminated polymers (Scheme 22.12) - by a mechanism in which the alkyl chain is protonated, and a metal-phosphido complex is generated. This phosphido complex then inserts olefin to start the growth of a phosphine-functionalized polyolefin. Marks subsequently showed that a similar process can be conducted witii amines. In this case, the bulky dicyclohexylamine was needed to sufficiently retard the rate of protonation to allow chain growth. The steric bulk also makes the olefin insertion more favorable thermodynamically. 

Here, kr is the CCL thermal conductivity and Rreac is the volumetric rate of electrochemical conversion (A cm ). Equation 4.282 says that the variation of conductive heat flux (the left-hand side) equals the sum of the heating rates from the reaction and the Joule dissipation of electric energy. On the other hand, determines the rate of proton current decay along x ... 

An increased electrolyte concentration within the membrane may enhance accessibility and improve rates of proton sorption and permeation though the membrane. Maintenance of constant electrolyte concentration may be desirable for obtaining stable proton conductivity, and replenishment of vaporized or leached electrolyte may be continuously performed during operation. On a morphological level, dynamic fluctuations between electrolyte domains may provide conductive pathways through the polymeric continuous phase. Thus, the ability of the polymeric phase to mechanically comply with the anodic proton flux may enable proton percolation though the membrane and enhance conductivity. 

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