Proton transport mechanism

The calculated energy difference between the ground state and the barrier state is less than 0.2 eV for most of the perovskites studied, which is much less than the experimentally observed 

FIGURE 5.24 Arrangements for proton transfer between neighboring oxygen ions. The relaxed lattice is represented by solid lines and the ideal lattice is symbolized by the dashed lines. 

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Absorption of a photon by the purple iill-trans retinal chro-mophore (with a single broad absorption band with a maximum at 568 nm) initiates the reaction sequence BR-/zv K L Ml M2 N -o- N - O BR (7, 8), where each state and substate is well defined by spectroscopic and crystallographic means. Although a kinetic scheme that rigorously fits all data into a linear sequence has not yet been produced, the proton transport mechanism can be understood by the molecular properties of the intermediate states and by their interconversions. 

Notwithstanding the non-specific role of water in many non-aqueous solutions, in some cases it may have specific effects due to ion-water interactions which depend on the particular nature of the ions and solvent. Thus the conductance of associated AgNOa and (CHajaTlI in dimethyl-formamide increase with the water content in contrast to the decrease found for dissociated salts In methanol and ethanol the conductance of perchloric acid (and presumably of other Inorganic acids) decreases significantly upon addition of 0.3 % water. This was attributed to a change in the proton transport mechanism. 

In the major part of this article, we have dwelt on different proton transport mechanisms. The two limiting views consider proton transport either as... 

Since the idea of low energy barriers is intuitively appealing, the search for proton conductors has concentrated worldwide on systems with recognizable H-bonding, both in inorganic systems and biological membranes . This has created a bias which may have prevented the recognition of some important proton transport mechanisms that appear to be present in non H-bonded system. 

Quantitative data on bulk proton transport are required especially for the understanding of proton transport mechanisms (see Chapters 29 31) including the implications for the use of solid proton conductors in operational electrochemical cells (see Chapters 32 39). 

Dippel, Th. and Kreuer, K.D. 1991. Proton transport mechanism in concentrated aqueous solutions and solid hydrates of acids. Solid State Ionics 46 3-9. 

The main characteristic to consider for a PEM to be used in potential fuel cell is proton conductivity. To achieve good performance of a PEM fuel cell, high proton conductivity is essential, especially at a high current density. To understand proton transport at a molecular level in hydrated polymeric membranes, there are two principal proton transport mechanisms (1) the Grotthus mechanism or proton hopping mechanism, and (2) the vehicular mechanism or diffusion mechanism [243-245]. 

Hereby, B, A and Tq are material-dependent parameters. The parameter is proportional to the activation energy of ionic transport. In a system with a strict coupling between dynamic viscosity and conductivity, as described by the Stokes-Einstein equation, the parameter B in (8.8) is equal to the parameter B in (8.10). In a system with a higher probability for the motion of ionic charge carriers than for viscous flow events, as it can be found in case of cooperative proton transport mechanisms, the strict coupling between dynamic viscosity and conductivity does not hold [56-58]. In this case the parameter Bg in (8.10) will be smaller than B in (8.8). Combining (8.8) and (8.10) and considering the concentration dependence of cr, by introduction of the molar conductivity one will yield a fractional Walden rule (-product) as shown in (8.11). 

It should also be stressed that the conductivity is only weakly dependent on the Si02 lm filler, for both concerns the as-prepared membranes and the washed ones. This is due to the active nature of this filler, which participates to the proton transport mechanism by means of the imidazole units. The inset of Fig. 11.6 shows the typical behaviour of a nanoscale passive filler (HiSiF ) here there is an initial increase of the conductivity due to acid-base interactions and/or to the formation of space-charge layers, which is followed by a strong decrease due to dilution effects [31]. 

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. 

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. 

FIGURE 2.34 Inteifacial configuration considered in studies of proton transport mechanisms. Donor (D), acceptor (A), and spectator (S) SGs are indicated. The collective variable dev = d 2 — d23 is shown as well. 

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