Proton conductivity measurements

The dependence of protein and solvent dynamics on hydration fits well into the above three-stage picture for some, but not all, properties. For dynamic properties that do not fit well, analysis on a case-by-case basis within the framework of the time-average picture can be informative. For example, consider protonic conduction, measured by the megahertz frequency dielectric response for partially hydrated powders of lysozyme. The capacitance grows explosively above a hydration level of 0.15 A, in a way characteristic of a phase transition (Section HI, A). The hydration dependence of thermodynamic properties shows, however. 

The formation of spanning H-bonded water networks on the surface of biomolecules has been connected with the widely accepted view that a certain amount of hydration water is necessary for the dynamics and function of proteins. Its percolative nature had been suggested first by Careri et al. (59) on the basis of proton conductivity measurements on lysozyme this hypothesis was later supported by extensive computer simulations on the hydration of proteins like lysozyme and SNase, elastine like peptides, and DNA fragments (53). The extremely interesting... 

Fig. 26.3. H-difTusion coefficient measured on TSA.28H2O by the PFG-NMR technique , (a) Echo attenuation as a function of applied magnetic field gradient showing separation of the contributions from the mother liquor and the solid, (b) The resultant diffusion coefficient as a function of the reciprocal temperature the diffusion coefficient calculated from the proton conductivity measured by a.c.-impedance spectroscopy is given for comparison (see Chapters 29-31).

Aromatic polysulfone (PSf)-based membrane is attractive for fuel cell applications due to its good thermal stability and mechanical properties. However, studies on the performance of these membranes in DMFC applications are very limited. Fu and Manthiram reported that the proton conductivity measurement of sulfonated PSf increased with increasing temperature from 65°C to 80°C, which is similar to... 

Figure 4.8 shows the proton conductivity measurement result at 25°C and 100% RH using the impedance technique. The difference from the previously described arrangement was that the two electrodes were placed on... 

Proton conductivity measurement of PEM by the two-probe impedance technique at 25°C and 100% RH. Courtesy of Dalian Institute of Chemical Physics, Chinese Academy of Sciences. 

Proton conductivity phosphoric acid. Nation N117 and HjPO -PBI membranes as a function of temperature.The membrane prepared by the PPA process has an acid doping level of 32, and the conductivity was measured at 0% relative humidity the DMAc-cast and acid-imbibed membrane has an acid doping level of 5.7 (proton conductivity measurement in an atmosphere with water to air molar ration of 0.7) the membrane cast from trifluoroacetic acid has an acid doping level of 6.0 and the proton conductivity was measured in a relative humidity of 5%. (Source Li ef... 

Chapter 5 Membrane/lonomer Proton Conductivity Measurements... 

To conduct proton conductivity measurements, Buchi et al. [3] designed a current interruption device that used an auxiliary current pulse method and an instrument for generating fast current pulses (i.e. currents > 10 A), and determined the time resolution for the appropriate required voltage acquisition by considering the relaxation processes in the membrane of a PEM fuel cell [3]. They estimated that the dielectric relaxation time, or the time constant for the spontaneous discharge of the double-layer capacitor, t, is about 1.4 x 10 ° s. They found that the potential of a dielectric relaxation process decreased to <1% of the initial value after 4.6r (6.4 x 10 s) and that the ohmic losses almost vanished about half a nanosecond after the current changes. Because there is presently no theory about the fastest electrochemical relaxation processes in PEM fuel cells, the authors assumed a conservative limit of 10 s, based on observations of water electrolysis membranes. They concluded that the time window for accurate current interruption measurements on a membrane is between 0.5 and 10 ns. Another typical application of the current interruption method was demonstrated by Mennola et al. [1], who used a PEM fuel cell stack and identified a poorly performing individual cell in the stack. 

FIGURE 5.11 Schematic diagram of a conductivity cell for proton conductivity measurement with the two-point probe method. TTie membrane size is about 2.4 cm x 1.0 cm the distance between the two Pt strips is 0.4 cm [60]. 

FIGURE 5.13 Schematic of a conductivity cell for proton conductivity measurement with the... 

Composite membranes based on PTA-impregnated SiO and PBI had been prepared and their physicochemical properties were studied. The membranes with a high tensile strength and a thickness of <30 pm were prepared by solution casting. They were chemically stable in boiling water and thermally stable in air up to 400°C [52]. The presence of silica in the composite and 100% relative humidity allowed the membranes to maintain stable proton conductivity at temperatures up to 130°C. For example, the proton conductivity measured at 130°C was 10 S cm" . In these membranes, the PBI formed a network to keep the PTA supported on silica. 

FIGU RE 5.18 The test cell for in-plane proton conductivity measurements. 

Proton conductivity measurements were performed by AC impedance technique. The proton conductivities for all the membranes increased with increase in temperature from 30°C to 100°C as seen in Figure 10.16. It is noteworthy that the proton conductivity increases with increase in SWA content from 5 to 10 wt% and decreases at 15 wt% SWA. Higher content of SWA (>10 wt%) in cross-linked CS-PVA blend disrupts the proton conduction path by blocking the voids of polymer matrix and decreases its proton conductivity, which may be also attributed to the water sorption data. The methanol crossover for the membranes is measured in situ in fuel cells under OCV condition at 70°C. The methanol crossover flux is lower for CS-PVA-SSA-SWA (10 wt%) (about 3.3 x 10" mol s" cm- ) hybrid membranes in comparison with Nation 117 and other membranes. In addition, the electrochemical selectivity of CS-PVA-SSA-SWA (10 wt%) has reached to 2.69 x 10 S cm- s. 

Membrane thickness/ohmic resistance for proton conductivity measurement. NA, Not measured +, Increment —, Reduction. 

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