Proton transport surface mechanism

From these data is also possible to calculate the activation energy in pores of given water content from an Arrhenius representation of the calculated diffusion coefficients [63], The results show no indication of a substantial increase in activation energy at water levels as low as A = 4, contrary to the results in [23] and more in line with results from Kreuer (see Fig. 6 and [50]). Rather, the activation energy for proton transfer remains low at all studied water contents. Thus, it appears that in an individual pore the Grotthus mechanism remains rate-determining whereas the surface mechanism does not dominate the transport behavior. Similar results as in slab pores were also found in cylinder pores [63],... 

Going beyond an atomistic description of the aqueous phase and the membrane, Paddison and coworkers [79-88] employed statistical mechanical models, incorporating solvent friction and spatially dependent dielectric properties, to the calculation of the proton diffusion coefficient in Nation and PEEKK membrane pores. They concluded from their studies that, in accordance with NMR based evidence [50], the mechanism of proton transport is more vehicular (classical ion transport) in the vicinity of the pore surface and more Grotthus-like in the center. 

Bacteriorhodopsin is the quintessential transmembrane ion pun ). It consists of a small, seven-helix protein where proton transport across the membrane is driven by photoisomerization of retinal from the all trans to the 13-cis,l5-anti configuration. A number of high-resolution crystal structures of the protein and its photointermediates have been used to propose several competing mechanisms describing proton translocation to fhe extracellular surface. Unresolved issues include understanding how conformational changes couple to proton transfer and the role played by water molecules in the proton transfer process. ... 

The mobility in the pore includes molecular mechanisms of proton transport in bulk water and along the array of charged surface groups. An idealized two-state approach based on this distinction was considered in [82]. This simple model can reproduce a continuous transition from surface-like to... 

The mechanism of protonic transport in ZrP is not yet known. Nevertheless, the fact that the conductivity is dominated by surface transport may be explained considering that, due to steric effects, the diffusion and/or reorientation of protonic species on the surface should be easier than in the bulk in addition the ionogenic groups of the surface can be more hydrated than the inner ones, thus facilitating their dissociation and water protonation. 

Ma et al. reported that the addition of silver (Ag) to Pd improved the proton conductivity of Pd film as the Pd-Ag flhn had higher proton conductivity than the pure Pd flhn (Ma et al. 2003). Using the same sputtering method, a thin Pt/Pd-Ag/Pt alloy fllm was deposited on the surface of the Nation membrane. The platinum (Pt) on the membrane surface acts as a bridge to transfer the protons from the Pd-Ag alloy fllm to the Nation membrane due to the different mechanisms of proton transport between the Pd-Ag alloy flhn and the Naflon membrane. 

When ZrP, ZrTPC, and BP were used, there was an increase in the membrane conductivity compared to the doped pure PBl membranes [42, 50-52]. He et al. [42] reported an optimum percentage of ZrP in the membrane of 15 % and explained the increase in the conductivity in terms of the presence of proton conductor surface groups in the ZrP (Fig. 13.5b). Furthermore, PBl and PBl -1- ZrP membranes presented similar proton conduction activation energies, indicative of an analogous proton transport mechanism. Di et al. [52] reported a drop in the PBl -1- BP activation energy, attributed to a more facile proton transportation in the composite material. 

Acid secretion is a regulated process whose rate is determined by its necessity after a meal. Consequently, the eventual result of the complex mechanisms for regulation of secretion described in previous sections is to activate the H,K ATPase. In contrast to the regulation of many other enzymes, there is no evidence for any chemical factors that directly influence the activity of the H,K ATPase, other than the availability of the necessary substrates MgATP, H, and K. Because within the parietal cell the availability of protons and AAgATP is not likely to be rate limiting, it follows that the major, if not the only, factor that controls proton transport is the availability of K at the extracytosolic surface of the H,K ATPase. Substantial evidence has accumulated to indicate that this is indeed the case, and that activation of the proton pump results from association of the H,K ATPase with a K and Cl" permeability in the membrane of the secretory canaliculus. 

Water-based PEMs exhibit proton transport mechanisms and mobilities similar to those in liquid electrolytes like hydrochloric acid proton conductivities could reach up to 0.1 S cm in the case of PFSA-type ionomers, and up to 0.5 S cm in the case of block copolymer systems. The temperature range of operation of PEMs stretches from —30°C to 90°C, the lower bound being determined by the freezing point of water, which is suppressed because of the high surface energy of water in nanopores. The upper limit is determined by evaporation of water only a few water-based PEMs have been demonstrated that could maintain a sufficient conductivity above lOO C. 

This section provides a systematic account of proton transport mechanisms in water-based PEMs, presenting studies of proton transport phenomena in systems of increasing complexity. The section on proton transport in water will explore the impact of molecular structure and dynamics of aqueous networks on the basic mechanism of proton transport. The section on proton transport at highly acid-functionalized interfaces elucidates the role of chemical structure, packing density, and fluctuational degrees of freedom of hydrated anionic surface groups on concerted mechanisms and dynamics of protons. The section on proton transport in random networks of water-filled nanopores focuses on the impact of pore geometry, the distinct roles of surface and bulk water, as well as percolation effects. 

However, intriguing phenomena arise if the SGs density at polymer-water interfaces is increased. In the regime of high SG density, proton transport in PEMs become similar to proton transport at acid-functionalized surfaces. Surface proton conduction phenomena are of importance to processes in biology. Yet, experimental findings of ultrafast proton transport at densely packed arrays of anionic SG have remained controversial. Theoretically, understanding of the underlying mechanisms is less advanced than for proton transport in bulk water. 

The structural picture of ionomer in catalyst layers, unraveled in this section, suggests that extrapolation of bulk membrane properties in terms of water uptake, water binding, and proton transport skews specific properties of ionomer in CLs and is not generally feasible for the purpose of CL modeling. One needs to adapt mechanisms of water and proton transport to the thin-film ionomer morphology, where (i) proton transport is dominated by surface properties of ionomer and (ii) electrocatalytic properties are determined by the interfacial thin-film structure formed by Pt/C surface, ionomer film, and a thin intermediate water layer. 

Structure and water sorption characteristics of fuel cell media determine their transport properties. The dynamic properties of water determine microscopic transport mechanisms and diffusion rates of protons in PEM and CLs. Protons must be transported at sufficiently high rates, away from or toward the active Pt catalyst in anode and cathode catalyst layers, respectively. Effective rates of proton transport in nanoporous PEM and CLs result from a convolution of microscopic transport rates of protons with random network properties of aqueous pathways. Accounting for the geometry of these materials, namely, their external surface area and thickness, gives their resistances. 

Fig. 14 A conceptual model of Nafion as a micellar structure. The PTFE groups segregate to the core with the perfluoroether groups forming a shell terminated with the sulfonic acid groups forming a hydrophilic channel for water and proton transport. The PTFE chains form cross-links between hydrophilic domains imparting mechanical strength. The surface of Nafion can rearrange when in contact with vapor or liquid water to be either hydrophilic or hydrophobic...

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