Membrane Conduction Mechanism

FIGURE 2.11 Schematic illustration of different modes of proton conduction in a solid polymer electrolyte where) Grotthuss, vehicular, and (C) surface mechanisms. (From Peckham, T. J. and Holdcroft, S., Adv. Mater., 22(42), 4667, 2010.) 

Choi et al. proposed a pore transport model to describe proton diffusion within Nafion. The diffusion coefficients are predicted. The surface diffusion coefficient is 1.01 X10 cm /s at room temperature the vehicular diffusion coefficient is 1.71x10 cm /s and the Grotthuss diffusion coefficient is 7x10 cm /s. The Grotthuss diffusion is the fastest proton transport mechanism within Nafion. The surface diffusion coefficient is much lower than the other two diffusion coefficients. The surface diffusion does not contribute significantly to the overall conductivity of protons except at low water levels. 

Choe et al. investigated the nature of proton dynamics in Nafion. The results show the hydrogen bond network of water is completely different in very low water content ( =4.25) and at mediate water content (k= 12.75). It is discontinuous in the former and continuous in the latter. The disconnected hydrogen bonds around the sulfonic groups in X=4.25 hinder the proton transfer. 

Paddison and Paul studied the diffusion of protons in fully hydrated Nafion ( =22.5) with nonequilibrium statistical mechanical transport model. The model calculated a diffusion coefficient for a proton moving along the pore center of 1.92x10 cmVs. Within 4A of the pore center proton transfer via the Grotthuss mechanism, while within 8A of the wall of the pore, the transport of the proton is predominantly vehicular in nature. 

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The proton conductivity plays an important role in determining fuel cell performance. Therefore, it is very necessary to investigate the membrane conduction mechanism. For the pristine SPEK membrane, the proton conduction is dependent on water. The hydrophobic domain (polymer backbone) provides the morphological stability and prevents the membrane from dissolving in water. The sulfonic acid functional groups aggregate to form hydrophilic domains that are hydrated in the presence of water. And the connected hydrophilic domain is responsible for the transport of protons and water. 

During fuel cell operation, ions would conduct through PEMs, which leads to current flow in the cell system. Thus, the conductivity of PEMs plays an important role in fuel cell performances, and membrane conduction mechanism has drawn much attention in fuel cell researches. There are mainly two kinds of conduction mediums in PEMs according to the fuel cell type proton (H+) and hydroxyl (OH ). As aliphatic membranes were usually used for anion-conduction membrane and proton-conduction mechanism has been discussed already in the preceding chapters, we will focus our attention mainly to hydroxyl conduction mechanism in this chapter, although they have a lot in common. 

The physiological role of the ICOR is not clear and may be heterogeneous in the various tissues. In the thick ascending limb of the loop of Henle this channel appears to serve as the exit for CP at the basal cell pole [16,65,66], This conductive mechanism, therefore, is required for the reabsorption of Na and CP by this segment of the nephron [16]. In the rectal gland of Squalus acanthias a very similar channel is utilized for Na" and CP secretion. In these latter cells the CP-channel is present in the luminal membrane and is controlled by cytosolic cAMP [15,56,71]. It has been claimed that this kind of channel is also responsible for the secretion of CP in the colonic crypt cell, in colonic carcinoma cells and in respiratory epithelial cells [17,19,20,22]. Recent data have cast some doubt on this concept ... 

Paddison SJ (2003) Proton conduction mechanism at low degrees of hydration in sulfonic acid-based polymer electrolyte membranes. Ann Rev Mater Res 33 289-319 Rasten E, Hagen G, Tunold R (2003) Electrocatalysts in water electrolysis with solid polymer electrolyte. Electrochimica acta 48 3945-3952... 

To establish the chemical basis of the action potential, A. L. Hodgkin and A. F. Huxley in the 1950s devised the voltage clamp, a sophisticated device by which the transmembrane current can be measured while using a feedback mechanism to fix the membrane potential at a preselected value.413-417 Using the voltage clamp the membrane conductance could be measured as a function of the membrane potential... 

Early research of ionomer membrane degradation was conducted in the context of PEM electrolyzers. The detection of fluoride and other chain fragments in the condensed effluent water indicates the decomposition of PFSA ionomer and has long been noticed. Baldwin15 reported the detection of fluoride in the effluent of PEM electrolyzer and believed that it is the result of membrane mechanical failure. Extensive research has been conducted to elucidate the reaction pathways for membrane decomposition. Many controversial results and mechanisms have been reported in the literature, demonstrating the complex nature and the current inadequate understanding of the membrane degradation mechanisms. 

The above discussion, which mostly considers kinetic and chemical aspects to explain respiration and phosphorylation unconjugation, makes clear the action of uncouplers of different origins. Moreover, no principal difference in the mechanism of uncouplers and open mitochondrial surface is observed, because uncouplers increase membrane conductivity for H+ ions and, therefore, promote its partial break in places of uncoupler injection. 

Both oci and (3 adrenoceptors are postsynaptic excitatory heteroreceptors. Their action is mediated via the activation of Ca2+ current. However, different mechanisms are involved Gq-mediated phospholipase C (PLC) activation and Gs-mediated AC stimulation, respectively (Kandel et al., 2000). When they present on glutamate pyramidal neurons, (3-adrenoceptors receptors decrease the Ca2+-activated after-hyperpolarization K+ influx, making the neurons more responsive for excitatory inputs. The a] adrenoceptors, however, increase membrane conductance and make the pyramidal cells less excitable (Devilbiss and Waterhouse 2000). 

The energy released by electron transfer can be used in the transport of protons through the membrane. One of the proton conduction mechanisms in proteins is through a chain of hydrogen bonds in the protein, i.e. a Grotthus mechanism (Section 2.9), similar to the mechanism of proton movement in ice. Protons are injected and removed by the various oxidation/reduction reactions which occur in the cell there is no excess of protons or electrons in the final balance, and the reaction cycle is self-sustaining. 

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