Proton-hopping mechanism

Theory suggests that the electrical conductance A exhibits an anomalous contribution A oc f . With regard to the critical exponent 0, one may think of several scenarios. Scaling behavior with 0 = 1 — a is expected for short-range fluctuations [127] and also for a proton-hopping mechanism [128]. A... 

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]. 

From the compiled vapor pressure and conductivity data, the evaporation enthalpy and the activation enthalpy for proton conduction were calculated as a function of composition. The critical temperature according the Vogel-Tammann-Fulcher law was determined from the viscosity data and compared with glass transition temperatures from other studies using NMR spectroscopy. A correlation between dynamic viscosity and molar conductivity was found. As expected, a considerable decoupling between ionic conduction and viscous flow can be determined from a Walden plot, which is based on proton-hopping mechanisms in phosphoric acid. 

FIGURE 1.24 Proton hopping mechanism in the H3PO4 doped PBI membranes, in which H3PO4 molecules act as proton carrier. (Modified with permission from Ma, Y.L. et al., J. Electrochem. Soc., 151, A8, 2004.)... 

Apart from the problems of low electrocatalytic activity of the methanol electrode and poisoning of the electrocatalyst by adsorbed intermediates, an overwhelming problem is the migration of the methanol from the anode to the cathode via the proton-conducting membrane. The perfluoro-sulfonic acid membrane contains about 30% of water by weight, which is essential for achieving the desired conductivity. The proton conduction occurs by a mechanism (proton hopping process) similar to what occurs... 

The mechanism of conduction is believed to be that of proton hopping. Under the influence of an electric field a proton can be transferred from one OH- group to an adjacent O2- ion. The electric field ensures that the H+ ion rotates in the field direction, making the next jump easier. 

The 0-0 and H-H RDFs (not shown) indicate that no 0-0 or H-H covalent bonds are formed during the simulations at all densities. The g(Roti) shows a lattice-like structure at 115 GPa, which is consistent with proton diffusion via a hopping mechanism between lattice sites.65 At 34 GPa, the coordination number for the first peak in g(RQH) is 2, which indicates molecular H20. Between 95 GPa and 115 GPa, however, the coordination number for the first peak in g(RQH) becomes four, which indicates that water has formed symmetric hydrogen bonds where each oxygen has four nearest-neighbor hydrogens. 

The mechanism of long-range proton transfer processes in solutions is complex becanse seqnential proton hops from initial proton donors to proton acceptors are mediated by water (or solvent) molecules or other groups capable of ionization. 

Proton conduction can then occur via a hopping mechanism between hydroxyl ions. 

Advances were still being made in the 1990s concerning the mechanism of these reactions. In the first reaction (Stage A), the mechanism has been called the proton-electron mechanism and is portrayed in Fig. 13.42. The oxide is a mixture of Mn4+, Mn3+, O2", and OH" ", and the proton hops along between O2- and OH", whereas the electron leaps onto Mn4+ from Mn3+. This picture is a simplification and more details have been elucidated in terms of the Mn02 structure, but our scope will be limited to the above. 

Berg et al. 711 proposed that the adenine and cytosine residues in native DNA are reduced by a so-called electron hopping mechanism, the only condition for this being adsorption of protonated DNA at the electrode surface at the reduction potential of these bases. It was also assumed that the DNA is adsorbed in its A-form, exhibiting semi-conducting properties. There is consequently no surface denaturation of the DNA. 

Structural diffusion is provided by various complexes bare hydronium ion, Eigen complexes, and - Zundel complexes. Structural diffusion of bare hydronium ion and Eigen complexes occurs by proton hops between two water molecules. Two or more protons and several water molecules are involved in the structural diffusion of Zundel complexes. The contribution of mechanisms to the overall mobility depends on the temperature. Eigen and Zundel complexes prevail at room temperature, whereas bare hydronium ions dominate at high temperatures. Excess proton mobility of water has Arrhenius-like (-> Arrhenius equation) temperature dependence with the - activation energy about 0.11 eV. 

Proton transfer proton transfer is a translocation of the protons in condensed media and molecular systems by means of any microscopic mechanism of proton transfer. The latter may involve (1) vehicle motion of the hydrogen ion, (2) proton hopping between two molecular species, (3) a shift of the proton within a molecular structure. The motion of the hydrogen ion is a diffusion process. Two other mechanisms represent elementary reaction steps involving breaking and formation of chemical bonds and may be characterized by the reaction rate constant... 

The Hj molecular ion plays the pivotal role in the ion-neutral reactions scheme now generally believed to be the major mechanism for the chemical evolution of dense molecular clouds. Hj is produced through cosmic ray ionization of H followed by the ion-neutral reaction (1). Since the latter reaction is extremely fast, the rate-determining process for the production of HJ is the cosmic ray ionization, whose rate is generally taken to be f a 10"g-i 63.68 main destruction mechanism of Hj is the proton-hop reaction (2). 

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