Proton membranes, conductivity

Kasianowicz et al. [65] described the determination of the transport of niclosamide protons across lipid bilayer membranes by equilibrium dialysis, electrophoretic mobility, membrane potential, membrane conductance, and spectrophotometric... 

In this section, we describe the role of fhe specific membrane environment on proton transport. As we have already seen in previous sections, it is insufficient to consider the membrane as an inert container for water pathways. The membrane conductivity depends on the distribution of water and the coupled dynamics of wafer molecules and protons af multiple scales. In order to rationalize structural effects on proton conductivity, one needs to take into account explicit polymer-water interactions at molecular scale and phenomena at polymer-water interfaces and in wafer-filled pores at mesoscopic scale, as well as the statistical geometry and percolation effects of the phase-segregated random domains of polymer and wafer at the macroscopic scale. 

The effective conductivity of the membrane depends on its random heterogeneous morphology—namely, the size distribution and connectivity of fhe proton-bearing aqueous pafhways. On fhe basis of the cluster network model, a random network model of microporous PEMs was developed in Eikerling ef al. If included effecfs of varying connectivity of the pore network and of swelling of pores upon water uptake. The model was applied to exploring the dependence of membrane conductivity on water content and... 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

Proton exchange membrane fuel cell (PEMFC) working at around 70 °C with a polymer membrane electrolyte, such as Nafion, which is a solid proton conductor (conducting by the H + cation). 

The conductivity data measured in situ in Fig. 1 are within the temperature range from room temperature (27°C) to -30°C. In contrast, Cleghorn et al. reported the proton conductivity for Gore-Select membranes in the temperature range of 40-100°C.23 Extrapolating the correlation of Cleghorn et al. to 27°C and at 100% relative humidity, the membrane conductivity is calculated to be 0.027 S/cm, which is in reasonable agreement with our in-situ measurement of 0.021 S/cm. 

The membrane conductivity was measured in HCl(aq) solutions of different concentrations and in 2 M HC1 + 0.2 M CuCl solution to model the catholyte and anolyte solutions in the electrolyser. All membranes were equilibrated in the same solutions for 20 hours before starting the measurements. Detailed characterisation data for a number of commercial anion exchange membranes are published elsewhere (Gong, 2009). The AHA membrane, which demonstrated the highest conductivity in HC1 (12.61 mS/cm) compared to other membranes with similar IEC and water uptake, was selected to prepare a membrane electrode assembly (MEA) and carry out electrolysis tests with this MEA. The ACM membrane with lower conductivity values was also chosen for the electrolysis tests due to its proton blocking properties and high Cl- selectivity. 

Fig. 17.6. The vectorial pumping of calcium ions and protons across the mitochondrion membranes. A schematic enlargement of the inner (cristae) membrane is shown to indicate the existence of protein-based electron (e ) and proton (H+) conduction pathways (from Ref. 26 with permission).

Specifically speaking, membrane conductivity represents only the membrane s resistance to flow of protons (H+) and is highly dependant on its thickness (cp) and water content. Electrical resistance of electrodes, cell interconnects, and the formation of any insulating layer on the electrode surface are all bundled under the conductivity term. Voltage decreases for a given current as temperature increases and can be controlled to improve stack efficiency. 

There is a class of nonporous materials called proton conductors which are made from mixed oxides and do not involve transport of molecular or ionic species (other than proton) through the membrane. Conduction of protons can enhance the reaction rate and selectivity of the reaction involved. Unlike oxygen conductors, proton conductors used in a fuel cell configuration have the advantage of avoiding dilution of the fuel with the reaction products [Iwahara ct al., 1986]. Furthermore, by eliminating direct contact of fuel with oxygen, safety concern is reduced and selectivity of the chemical products can be improved. The subject, however, will not be covered in this book. 

As to the membrane conductivity, only small losses of protonic conductivity, of the order of 5-10% after 4000 h, have been observed in well-humidified cells during PEFC life tests according to measurements of cell impedance at 5 kHz [42]. The deionized water employed in the humidification scheme [42] had very low levels of metal ions (e.g., Fe " / +, Ca + or Mg +). Such multivalent ions could exchange irreversibly with protons in the PFSA membrane, causing a drop in membrane conductivity. Deionizing the water used for PEFC humidification is therefore required, and appropriate plumbing should also be used in the humidification loop to avoid generation of ionic contaminants by corrosion processes. 

Fontanella, Greenbaum, and co-workers [79] have reported an interesting study of the pressure dependence of membrane conductivity. A significant increase is observed in the activation volume for proton transport as water content drops below 5 H2O/SO3H. This accords well with the expectation of a change in the mechanism for proton conduction when the water in the membrane is essentially only water of ionic solvation. 

Note that convection of protons with the flow of water has been neglected in Eq. (12). (The effect of a convection term in Eq. (12) was considered in Ref. 16. The net effect due to proton convection is a slight worsening of the effective membrane conductivity.)... 

Membrane conductivity losses by ion exchange seem to be easier to prevent only small losses of protonic conductivity, of the order of 5-10% after 4000 hours, have been observed in well-humidified cells... 

Structure of ATP synthase. The F complex situated above the membrane consists of three afi dimers and single y, S, subunits. The membrane segment Fq, also known as a stalk, consists of H" " channels. Protons are conducted via the C-subunit channels. The rotation of C-subunits relative to a subunit of the stalk drives the rotation of the )/-subunit. [Reproduced with permission from Y. Zhou, T. M. Duncan and R. L. Crass Subunit rotation in Escherichia coli FflFi-ATP synthase during oxidative phosphorylation. Proc. Natl. Acad. Sci. 94, 10583 (1997). [ 1997 by the National Academy of Sciences.]... 

A SrTio.4 Mgo.603 x catalyst was used, which had been previously shown to be an effective catalyst for this reaction. The use of the membrane significantly increased the yield of C2 hydrocarbons. This remains an area with significant unexplored potential. Progress can be made here by developing CMR systems with enhanced catalytic activities towards the CH4 coupling reaction, and asymmetric-type proton-hole or proton-electron conducting membranes with significantly increased conductivities. 

An interesting application of proton-electron conducting membranes has recently been reported by Li et al. [2.76]. These authors studied the conversion of CH4 first to C2H4 and its subsequent direct catalytic aromatization to benzene and other valuable aromatic hydrocarbons. Their reactor configuration is shown schematically in Figure 2.3. The two distinct additional features of their work are the use of an active catalyst for the reaction itself, (Mo/H-ZSM5), and the use of asymmetric membranes with a thin (10-30 pm)... 

Charge alone is not sufficient to induce ionic conductivity. Ion exchange capacity (lEC), water uptake, and water retention capabilities help to ensure good electrochemical properties such as membrane conductivity. As water uptake and water retention properties increase in the bulk membrane, the conductivity tends to increase proportionally. lEC provides information regarding the density of ionizable hydrophilic groups in the membrane matrix, which are responsible for the conduction of protons and thus lEC is an indirect and reliable approximation of the proton conductivity [8]. 

Mixed Protonic-Electronic Conducting Membrane for Hydrogen Production from Solid Fuels... 

Mixed Protonic-Electronic Conducting (MPEC) Membrane... 

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