Proton transport medium

Bouwman et al. demonstrated that Pt can be used in the ionic form (Pt" and Pt") by dispersing it in a matrix of hydrous iron phosphate (FePO) via a sol-gel process (Pt-FePO)." The hydrous FePO possesses micropores of approximately 2 nm. It has 3 H2O molecules per Fe atom and is thought to also serve as a proton transport medium. The Pt-FePO catalyst exhibited a higher ORR activity than Pt/C catalysts. This catalyst was also found to be less sensitive to CO poisoning because CO did not adsorb onto the catalyst surface. The ORR catalytic activity was attributed to the adsorption and storage of oxygen on the FePO, presumably as Fe-hydroperoxides. However, these catalysts have poor electrical conductivity. There is no published data on the long-term stability of these catalysts in fuel cell environments. 

As stated above, the membrane acts as the proton transporting medium, is an electrical insulator, and separates the reactant gases from direct chemical reaction. On either side of this membrane are placed two electrodes. The anode at which hydrogen is consumed in the hydrogen oxidation reaction (HOR) and the cathode in which oxygen from air is consumed in the oxygen reduction reaction (ORR). The two half-cell reactions and the overall reaction are shown below. 

The functional and morphological heterogeneity of a lamellar system of chloroplasts indicates that pH values in different compartments (in granal and intergranal thylakoids) differ. This type of structure makes it difficult to measure local pH values at different sites. Therefore, mathematical models taking into account the spatial structure of chloroplasts provide a tool for studying the effect of diffusion restrictions on pH distributions over the thy lakoid on the rates of electron transport, proton transport, and ATP synthesis. The rate of ATP synthesis depends on the osmotic properties of a chloroplast-incubation medium and, therefore, on topological factors. 

The coupling between electron transport from NADH (or FADH2) to O2 and proton transport across the inner mitochondrial membrane, which generates the proton-motive force, also can be demonstrated experimentally with Isolated mitochondria (Figure 8-14). As soon as O2 is added to a suspension of mitochondria, the medium outside the mitochondria becomes acidic. During electron transport from NADH to O2, protons translocate from the matrix to the Intermembrane space since the outer membrane Is freely permeable to protons, the pH of the outside medium Is lowered briefly. The measured change In pH Indicates that about 10 protons are transported out of the matrix for every electron pair transferred from NADH to O2. 

A EXPERIMENTAL FIGURE 8-14 Electron transfer from NADH or FADH2 to O2 is coupled to proton transport across the mitochondrial membrane. If NADH is added to a suspension of mitochondria depleted of O2, no NADH is oxidized. When a small amount of O2 is added to the system (arrow), the pH of the surrounding medium drops sharply—a change that corresponds to an increase in protons outside the mitochondria. (The presence of a large amount of valinomycin and in the... 

M EXPERIMENTAL FIGURE 8-19 Electron transfer from reduced cytochrome c (Cyt c " ) to O2 via the cytochrome c oxidase complex is coupled to proton transport. The oxidase complex is incorporated into liposomes with the binding site for cytochrome c positioned on the outer surface, (a) When O2 and reduced cytochrome c are added, electrons are transferred to O2 to form H2O and protons are transported from the inside to the outside of the vesicles. Valinomycin and are added to the medium to dissipate the voltage gradient generated by the translocation of H, which would otherwise reduce the number of protons moved across the membrane, (b) Monitoring of the medium pH reveals a sharp drop in pH following addition of O2. As the reduced cytochrome c becomes fully oxidized, protons leak back into the vesicles, and the pH of the medium returns to its initial value. Measurements show that two protons are transported per O atom reduced. Two electrons are needed to reduce one O atom, but cytochrome c transfers only one electron thus two molecules of Cyt c are oxidized for each O reduced. [Adapted from B. Reynafarje et al., 1986, J. Biol. Chem. 261 8254.1... 

One of the most powerful features of HPLC stems from the fact that the eluent is not merely a transport medium, it rather contributes significantly to the mechanism of separation. Retention as well as selectivity arise from the combined action of mobile and stationary phase on the solutes. Because of its distinct physicochemical characteristics, a given solvent interacts in a specific manner with the solutes. Its capacity to donate or accept protons or to induce a dipole moment defines the nature of its interaction with the solutes in solution. Slight differences in these interactions complemented by stationary-phase action are often enough to provide the desired selectivity in HPLC. Solvent classification based on their elution strength or polarity represents a classic area of investigation into the fundamentals of retention mechanisms in HPLC (2S-27), and the chapter is not yet closed on its development and refinement (28-30). Here we will present a practical and comprehensive way to exploit the effect that the nature of the solvent has on retention and selectivity in reversed phase HPLC. 

Membrane operation in the fuel cell is affected by structinal characteristics and detailed microscopic mechanisms or proton transport, discussed above. However, at the level of macroscopic membrane performance in an operating fuel cell with fluxes of protons and water, only phenomenological approaches are feasible. Essentially, in this context, the membrane is considered as an effective, macrohomogeneous medium. All structures and processes are averaged over micro-to-mesoscopic domains, referred to as representative elementary volume elements (REVs). At the same time, these REVs are small compared to membrane thickness so that non-uniform distributions of water content and proton conductivities across the membrane could be studied. 

I. 0 the protonated species of weak acids (HAc) is expected to diffuse a-cross the PM into the cells along the chemical gradient. Inside the cells HAc dissociates into the anion (Ac ) and H . Since the cytoplasmic pH of acudopklia is close t 7 and that of the medium is 1.0, internal accumulations of Ac and H" are expected. To maintain cytoplasmic pH, the protons are expected to be reexported by ATPases of the PM. This w ll depend on the ATP-ase capacity in vivo and the extent of internal H stress. Monitoring the uptake of weak acids and simultaneously cytoplasmic pH by means of NMR techniques can provide an estimate for the minimal proton transport capacitity, which is a measure for the acid resistance of intact cells. 

On the other hand, the vehicular mechanism involves the movement of the hydrated proton aggregate. Here, in response to the electrochemical difference, hydrated proton (H30 ) diffuses through the aqueous medium [244,245]. A schematic representation of the vehicular mechanism is presented in Fig. 3.17. In the vehicular mechanism, hydrated protons carry one or more molecules of water (H+[H20] ) through the membrane and are transferred with them as a result of electro-osmohc drag. The major condition for proton transport through the vehicular mechanism is the existence of free volumes within the polymer matrix of a PEM, which allow the passage of hydrated protons through the membrane. 

The prevailing class of PEMs exploits the unique properties of water as aproton solvent and shuttle. As a result of the high concentration of protons and the peculiar nature of hydrogen bonding, liquid water is an ideal medium for proton transport. As a blueprint for this fundamental principle, nature relies entirely on liquid water to facilitate proton transfer in intracellular energy transduction (Kuznetsov and Ultrup, 1999). 

Water management is a key issue in view of optimizing PEFC operation. Attributable to the role of the PEM as a medium for proton transport and the sensitivity of its properties to water content, the membrane determines the feasible operational range and water management in all other components of the fuel cell. 

Type I electrodes, the prevailing type, are three-phase composite media that consist of a solid phase of Pt and electronic support material, an electrolyte phase of ionomer and water, and the gas phase in the porous medium. Gas diffusion is the most effective mechanism of reactant supply and water removal. Yet, CLs with sufficient gas porosity, usually in the range Xp 30-60%, have to be made with thickness of Icl — 10 pm. In this thickness range, proton transport cannot be provided outside of the electrolyte environment. Porous gas diffusion electrodes are, therefore, impregnated with proton-conducting ionomer. The concept of a triple-phase boundary, often invoked for such electrodes, is however inadequate. The amount of the electrochemically active interface is usually controlled by two-phase boundary effects at the interface between Pt and water. 

Basically, the nanoporous water-filled medium with chargeable metal walls works like a tunable proton conductor. It could be thought of as a nanoprotonic transistor. In such a device, a nanoporous metal foam is sandwiched between two PEM slabs, acting as proton source (emitter) or sink (collector). The bias potential applied to the metal phase controls proton concentration and proton transmissive properties of the nanoporous medium. The value of cp needed to create a certain proton flux depends on surface charging properties and porous structure of the medium. Moreover, coating pore walls with an electroactive material, for example, Pt, would transform it from a tunable proton conductor into a catalytic layer with proton sinks at the interface. Owing to the intrinsically small reaction rate of the ORR, it would not significantly affect the proton transport properties. 

For simple metals, the metal charging behavior can be described by the potential of zero charge 0 for such a medium, proton transport will be suppressed. For (p — protons with a protonic resistance that decreases upon decreasing (p. This tunability of proton conductivity could be applied as a method for determining (pP of porous metallic materials, for example, by using the linear relation of Equation 3.74. This setup would allow for systematic studies of effects of materials eomposition, surface roughness, and surface heterogeneity on (pP. ... 

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