Proton Transport Phenomena

The proton is formed from the lightest element in the periodic table and has the smallest ionic size. It also has a special relationship to the most common polar solvent, namely, water. These two facts combine to give this ion the highest mobility in water, approximately 10 times that of the Li ion, which is the next larger monoatomic cation. In order to understand these observations one must develop a picture of how the proton is incorporated in the structure of protic solvents. 

The mechanism by which the hydronium ion moves in water is very diflferent than that by which most other ions move. As shown in fig. 6.13, the conductance process involves the transfer of a proton between two adjacent water molecules with the proton in a hydrogen bond between these molecules. Because of the relative orientation of the water molecules with respect to one another, the net motion of the proton is not exactly in the direction of the electrical field but rather in zig-zag motion about this direction. This feature of the net forward motion can also be described in terms of rotation of the water molecule to which the proton is transferred so that the orbitals on this molecule are favorably oriented with respect to the direction of the field. The net effect is that the value of at 25°C is 349.8 cm moU, that is, much higher than any other monovalent ion. 

The hydroxyl ion moves by a similar mechanism, which involves proton transfer in the opposite direction of the electrical field. The molar conductance of the OH ion is smaller (197.6 cm moU ) simply because this ion occupies more space in the water structure. In addition, the field due to OH is much less than that due to because of the much larger size of the former ion. Anomalous proton transfer also occurs in the lower alcohols and their mixtures with water. 

This also reflects the fact that proton transfer in a solvent such as methanol involves the species CH3OH2 However, in the alcohols there is only one site for hydrogen bond formation, whereas the water molecule has two. Thus orientational effects are more important in acidified alcohol systems, so that the enhancement of proton mobility with respect to other monovalent cations is not as great as it is in water. 

An important question regarding proton transfer in water is whether it occurs by a classical mechanism or by quantum-mechanical tunneling. This problem can be elucidated by comparing the rates of proton and deuteron transfer. In this way it was concluded that transfer occurs by both mechanisms [15]. 

---

Proton Transport from the Bottom Up 6.7.1 Proton Transport Phenomena in Membranes... 

Kreuer, K. D. (2000). On the complexity of proton transport phenomena. Solid State Ionics 136 149-160. 

PEM research is a multidisciplinary, hierarchical exercise that spans scales from Angstrom to meters. It needs to address challenges related to (i) to ionomer chemistry, (ii) physics of self-organization in ionomer solution, (iii) water sorption equilibria in nanoporous media, (iv) proton transport phenomena in aqueous media and at charged interfaces, (v) percolation effects in random heterogeneous media, and (vi) engineering optimization of coupled water and proton fluxes under operation. Figme 1.13 illustrates the three main levels of the hierarchical structure and phenomena in PEMs. 

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. 

It is worth mentioning that proton transport phenomena in UTCL pores resemble those in water-filled pores of PEM, discussed in the section Proton Transport in... 

Even so, the occurrence of a charge exchange phenomenon is necessary to produce the conductivity levels observed, even in 50% doped PAn, because of the presence of structural defects other than those caused by inadequate protonation of the nitrogen sites. It is proposed that this involves interchain or intrachain proton exchange as well as electron transport. This explains the observed dependence of conductivity on the ambient humidity, as the presence of water within the polymer lattice would facilitate this proton-exchange phenomenon.16... 

There are different ways to depict membrane operation based on proton transport in it. The oversimplified scenario is to consider the polymer as an inert porous container for the water domains, which form the active phase for proton transport. In this scenario, proton transport is primarily treated as a phenomenon in bulk water [1,8,90], perturbed to some degree by the presence of the charged pore walls, whose influence becomes increasingly important the narrower are the aqueous channels. At the moleciflar scale, transport of excess protons in liquid water is extensively studied. Expanding on this view of molecular mechanisms, straightforward geometric approaches, familiar from the theory of rigid porous media or composites [ 104,105], coifld be applied to relate the water distribution in membranes to its macroscopic transport properties. Relevant correlations between pore size distributions, pore space connectivity, pore space evolution upon water uptake and proton conductivities in PEMs were studied in [22,107]. Random network models and simpler models of the porous structure were employed. 

Equation (7.37) through equation (7.43) form the set of governing equations for the transport phenomenon taking place in the catalyst layer. Before proceeding on to discuss the parameters needed to close this set of equations, it should be pointed out that the transport of electrons and protons respond almost instantaneously to the change in electrical potential compared to the slow process in the transport of gas species. Thus, in equation (7.39) and equation (7.40), the transient terms are neglected. This behavior has been studied by Wu et al. [80]. 

The ionic conductivity of PEM is significantly dependent on the membrane hydration. Inadequate membrane hydration results in high electrical resistance as well as the formation of dry and hot spots leading to membrane failure. The electroosmotic transport occurs due to the proton transport. Proton migrations drag water along with it from the anode side to the cathode side that can eventually reduce the membrane hydration and block the active reaction site in the CCL. Water transport process in a PEM fuel cell is a complex phenomenon, hence it is essential to make a delicate water balance for better and optimum fuel cell performance, and prevent material degradation (Das et al., 2010). 

That is, the H-bonded network provides a natural route for rapid transport. This phenomenon of proton jumping thus occurs with little actual movement of the water molecules themselves. Ice has an electrical conductivity close to that of water because such proton jumps also readily occur even when the water molecules are fixed in a crystal lattice. Such conduction of protons via H-bonded networks has been offered as an explanation for a number of rapid proton transfers of biological significance. 

The intrinsic catalytic properties of enzymes are modified either during immobilization or after they were immobilized [25-27], In heterogeneous catalysis such as is carried out by immobilized enzymes, the rate of reaction is determined not simply by pH, temperature and substrate solution, but by the rates of proton, heat and substrate transport, through the support matrix to the immobilized enzyme. In order to estimate this last phenomenon, we have studied the internal mass transfer limitation both in hexane and in SC C02, with different enzymatic support sizes. 

For oxides to become dispersions with relaxed double layers, charge transfer through the interface should take place. Experience has shown that such transport is usually realized via uptake or release of protons, which leads to equilibria such as I3.6.38a and/or b]. For that, some hydration of the surface, leading to surface hydroxyl groups, is needed. Most oxides exhibit this phenomenon. As a consequence, H and OH" ions may be considered charge-determining. This premise is supported by the observation that several oxides, if made into electrodes demonstrate Nemst or pseudo-Nemst behaviour as a function of pH. Such behaviour has never been observed as a function of the metal ions apparently these are too deeply embedded in the solid to be liberated without any... 

The 13-c/j retinal-chromophore in dark-adapted bacteriorhodopsin exhibits a very different photocycle, whose predominant intermediate has an absorption maximum at 610 nm [199], and which contains no intermediate [202,238] analogous to M. The 610 nm intermediate will decay to either the 13-c/s chromophore or the dW-trans form, the latter pathway being responsible for the phenomenon of light-adaptation [199]. This pathway does not explain, however, why monomeric bacteriorhodopsin shows poor light-adaptation [168,239]. The chromophore in the 13-c/s configuration is not associated with proton translocation [240]. Indeed, reconstitution of bacterio-opsin with 13-demethyl retinal, which traps the retinal moiety in the 13-c/s configuration, results [241] in a non-transporting photocycle. 

The carrier-mediated transport of sodium in exchange for protons across membranes is a virtually universal phenomenon in biology, from bacteria to man. It is carried out by a family of Na /H exchangers which is often referred to as antiporters. They are classified as secondary active transporters, since the driving force is the... 

Figure 10.4 illustrates the transfer of electrons through complex I. Electron transport is accompanied by the movement of protons from the matrix across the inner membrane and into the intermembrane space. The significance of this phenomenon for ATP synthesis will be discussed. 

---