Proton current

The specimen chamber (or target chamber) may contain a number of samples and standard specimens mounted on the same sample holder. Also here are the X-ray detection system and a Faraday cup which monitors the proton current incident... 

Inasmuch as the protonation of the oxide can be favored by the direction of the field at the O/S interface (cf. Fig. 1) at equilibrium, the proton current in this direction should decrease exponentially with increasing anodic polarization, as the field strength is decreasing and can even change sign. Conversly, the deprotonation should be favored, becoming the main mechanism of formation of the anhydrous oxide. 

Gilbertson, T. A. et al. Proton currents through amiloride-sensitive Na channels in hamster taste cells. Role in acid transduction./. Gen. Physiol. 100 803-824,1992. 

Fairman, W. A., Sonders, M. S., Murdoch, G. H and Amara, S. G. (1998) Arachidonic acid elicits a substrate-gated proton current associated with the glutamate transporter EAAT4. Nat. Neurosci. 1,105-113. 

Under fuel cell operation, a finite proton current density, 0, and the associated electro-osmotic drag effect will further affect the distribution and fluxes of water in the PEM. After relaxation to steady-state operation, mechanical equilibrium prevails locally to fix the water distribution, while chemical equilibrium is rescinded by the finite flux of water across the membrane surfaces. External conditions defined by temperature, vapor pressures, total gas pressures, and proton current density are sufficient to determine the stationary distribution and the flux of water. 

Proton current is determined by Ohm s law and by the continuity of proton flux. 

All acidic proton conductors discussed so far in this review have relied on the presence of large amounts of water (A = 10—30) as a mobile phase for the conduction of protons. Current targets for automotive use of hydrogen/air fuel cells are 120 °C and 50% or lower relative humidity. Under these conditions, the conductivity of the membrane decreases due to low water uptake at 50% relative humidity and thus creates large resistive losses in the cell. To meet the needs of advanced fuel cell systems, membranes will have to function without large amounts of absorbed water. Organic—inorganic composites are one preferred approach. ... 

A key feature in a facility advertising to use such high proton currents and thick targets is the question of handling of components and T/IS changes ... 

A proton ionophore "short-circuits" the proton current, so that both the proton gradient and membrane potential across the inner mitochondrial membrane are collapsed. No phosphorylation of ADP can take place, but electron... 

For the first charge (R362H), the observation was that there is a proton current that is not bell-shaped but increases monotonically as the membrane is hyperpolarized. This proton current has all the characteristics... 

Jurkat-Rott K, Lehmann-Horn F. Do hyperpolarization-induced proton currents contribute to the pathogenesis of hypokalemic periodic paralysis, a voltage sensor channelopathy J. Gen. Physiol. 2007 130 1-5. 

The mitochondrial respiratory chain consists of three proton pumps which act in series with respect to the electron flow and in parallel with respect to the proton circuit (Fig. 2.2a). Two limiting states are frequently referred to for isolated mitochondria - State 4 in which the proton current is limited by the inhibition of proton re-entry through the ATP synthase (due to either actual inhibition of the synthase or to the attainment of equilibrium), and State 3 in which there is ready proton re-entry into the matrix and hence brisk respiration. The State 3 condition can be due to an induced proton leak in the membrane or to the maintenance of AG tp below that required to equilibrate with AfiH+ (by either removing ATP, or following the addition of ADP. 

After discussing the generation and quantitation of the potential term of the proton circuit, we shall now turn to the proton current, and examine the factors which control the flux of protons around the circuit. Although it is not possible to determine the proton current under steady-state conditions directly, the parameter may be calculated indirectly from the respiratory rate and the stoicheiometry of proton extrusion by the respiratory chain. It is outside the scope of this chapter to discuss the contentious issue of the proton stoicheiometries of the complexes, but the important feature is that, unless the complexities of variable stoicheiometry are invoked, respiration and proton current vary in parallel. 

As in an electrical circuit, where the current of electrons flowing through a resistive element is related to the electrical potential difference and the resistance by Ohm s law, the proton current flowing back into the mitochondrial matrix through a leak pathway will be given by the product of the membrane proton conductance and the proton electrochemical potential ... 

Fig. 2.6. Hypothetical localized variants on the proton circuit, a, fully delocalized circuit b, proton current flows along surface of membranes (note that circuit is still in equilibrium with the bulk phases) c, one leg of the proton circuit is conducted through a lateral channel insulated from the bulk phase d, both legs of the proton circuit are conducted through lateral channel insulated from the bulk phases R, respiratory chain A, ATP synthase. Note the necessity for both outward and return legs in all models.

Before considering this information in detail, it is worthwhile to summarize briefly the implications of a localized proton circuit. One possibility is that the major part of the proton current flows not through the bulk aqueous phase (Fig. 2.6a) but along the two surfaces of the membrane (Fig. 2.6b). Note that in this model there is no insulating barrier between the surfaces of the membrane and the bulk phases. Therefore, under steady-state conditions, the electrochemical potential of the protons on the surfaces of the membrane must be the same as in the bulk phases, since otherwise there would be a net flow of protons down the supposed gradient from surface to bulk. This model does not therefore represent true localized chemiosmosis, since the bulk-phase potential measured experimentally will accurately reflect the true potential driving ATP synthesis. 

The number of water molecules carried through the membrane per proton when a protonic current flows through the membrane is a central factor in the determination of the water profiles in the membrane of an operating PEFC. This number has been reported over the years with considerable variation. There is an important difference between the electroosmotic drag coefficient, (A), a characteristic of an ionomeric... 

Fig. 10. Variation of the transmitted proton current with the rotation of a single crystal about the (111) axis...

The channeling effect is illustrated experimentally by letting protons penetrate through a thin aligned gold crystal. The proton current, a measure of the transmission probability, is a strong function of the orientation of the crystal. In Fig. 10 the transmitted ion cunent is plotted as a function of the rotation of the crystal around the (111) axes ... 

Fig. 4 Membrane in operating fuel cell a scheme depicting the transport processes, that is, gas supply, proton current, modes of water flow.

Here, the electroosmotic flow is proportional to the proton current density jp with a drag coefficient n (wx). D Arcy flow as the mechanism of water backflow proceeds in the direction of the negative gradient of liquid pressure, which (for A P% = 0) is equal to the gradient of capillary pressure. The density of water, cw, and the viscosity, /1, are assumed to be independent of w. The transport coefficient of D Arcy flow is the hydraulic permeability K wx). 

Fig. 10 Membrane resistance in H2/O2 fuel cell as a function of proton current density. Experimental data, normalized to the resistance 9ts of the saturated membrane at various temperatures have been extracted from Ref. 94. They are compared to the values calculated in the hydraulic permeation model (main figure) and to the results of the diffusion model, taken from Ref. 7 (inset).

Here, the oxygen partial pressure p is normalized to the absolute O2 -partial pressure Po2 the interface between catalyst layer and GDL (at x = 1), P = Poj/Poj- D is an effective oxygen diffusion constant (in cm2s-1). j-p(x) is the local proton current density (in A cm-2) and jo = jv(x = 0) is its value at the interface with the membrane, where jo is equal to the total current density through the cell. 

The basic equations of catalyst layer operation, Eqs. (42-46), are valid under the assumption of isothermal, stationary conditions. Furthermore, variations of the water vapor partial pressure are neglected. The water content in the PFSI fractions and the corresponding proton conductivity are, therefore, independent of x- Upon proceeding along x, starting at x = 0 with /p(X = 0) = jo, proton current is gradually converted into C>2 flux jo2 = (j-p(x) — y o)/4. At x = 1 the transformation is complete, yp = 0, since no protons are admitted to pass the interface to the GDF. 

Any model of PEFC must cover at least the three basic processes (1) transport of reactants to/from the catalyst sites, (2) charged particles production or consumption at these sites, and (3) transport of charged particles (electron and proton currents). The simplest realistic model of PEFC must take into account these processes. (More sophisticated models, particularly important for high current regimes, ought to take into account on the same footing the production of water at the cathode side and dynamic liquid-vapor phase balance.)... 

Fig. 22 Cell element with conventional feeding through the channels. Shown are maps of electron current densities ja,jc (mA cm-2), reaction rates Qa, Qc (A cm-3), and proton current densityjp (mAcm-2).

The maps of methanol and oxygen concentrations, electrochemical reaction rates, membrane phase potential and proton current density are shown in Fig. 24. The mean current density is 0.3 A cm-2. Several interesting features are seen. 

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