Membrane proton conductivity and

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 ... 

Table 1 PEM micro-fuel cell prototypes, incorporating mesoporous silicon as the proton exchange membrane in the core system, are reported in the literature. The performance of the membranes (proton conductivity) and the FC, i.e., the open circuit voltage (OCV) and the power density peak during the test, is reported...

As discussed in Chapter 5, PEM fuel cells widely use PFSA membranes, whose proton conductivity strongly depends on their water content. To achieve high membrane proton conductivity and good PEM fuel cell performance, it is necessary to add water to fuel cell systems to maintain a sufficient membrane hydration level. Water is often added externally with the reactant gases at the anode and the cathode. So far, several humidification methods, such as bubble humidification and direct liquid water injection, have been developed for PEM fuel cells. 

Table 5.1 Summary of membrane proton conductivity and methanol permeability performance data.

The microstructure of a catalyst layer is mainly determined by its composition and the fabrication method. Many attempts have been made to optimize pore size, pore distribution, and pore structure for better mass transport. Liu and Wang [141] found that a CL structure with a higher porosity near the GDL was beneficial for O2 transport and water removal. A CL with a stepwise porosity distribution, a higher porosity near the GDL, and a lower porosity near the membrane could perform better than one with a uniform porosity distribution. This pore structure led to better O2 distribution in the GL and extended the reaction zone toward the GDL side. The position of macropores also played an important role in proton conduction and oxygen transport within the CL, due to favorable proton and oxygen concentration conduction profiles. 

In the case of Nafion, a similar situation occurs. There is a sharp increase in proton conductivity and proton concentration as a function of water content followed by a decrease at A > 20. At these higher water contents, Nafion undergoes a similar dilution of proton concentration per BAM membrane in conjunction with a lower mobility value versus ETFE-g-PSSA. However,... 

Kim, Kim, and Jung measured proton conductivity and MeOH permeability for a series of S-SEBS membranes. Both proton conductivity and MeOH... 

Kim, J., Kim, B. and Jung, B. 2002. Proton conductivities and methanol permeabilities of membranes made from partially sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene copolymers. Journal of Membrane Science 207 129-137. 

Kim, Y. J., Choi, W. C., Woo, S. I. and Hong, W. H. 2004. Proton conductivity and methanol permeation in Nafion/ORMOSIL prepared with various organic silanes. Journal of Membrane Science 238 213-222. 

Miyake, N., Wainright, J. S. and Savinell, R. E 2001. Evaluation of a sol-gel derived Nafion/silica hybrid membrane for proton electrolyte membrane fuel cell applications. I. Proton conductivity and water content. Journal of the Electrochemical Society 148 A898-A904. 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

Alberti et al. investigated the influence of relative humidity on proton conductivity and the thermal stability of Nafion 117 and compared their results with data they obtained for sulfonated poly(ether ether ketone) membranes over the broad, high temperature range 80—160 °C and RHs from 35 to 100%. The authors constructed a special cell used in conjunction with an impedance analyzer for this purpose. Data were collected at high temperatures within the context of reducing Pt catalyst CO poison-... 

Hydrated Acidic Polymers. Hydrated acidic polymers are, by far, the most commonly used separator materials for low-temperature fuel cells. Their typical nanoseparation (also see Section 1) leads to the formation of interpenetrating hydrophobic and hydrophilic domains the hydrophobic domain gives the membrane its morphological stability, whereas the hydrated hydrophilic domain facilitates the conduction of protons. Over the past few years, the understanding of the microstructure of these materials has been continuously growing, and this has been crucial for the improved understanding of the mechanism of proton conduction and the observed dependence of the conductivity on solvent (water and methanol) content and temperature. 

TABLE 1. Proton Conductivity and Open-Circuit Testing Results for Electrolytic Membranes Consisting of Selected Polyamidic Acid Sulfamic Acid Derivatives Conducted at 150oC ... 

The cool-down process of the cold-start experiment also provides an opportunity to obtain the membrane proton conductivity as a function of temperature at a known water content. Note that the temperature dependence of proton conductivity with low membrane water content is of particular interest here as PEFC cold start rarely involves fully hydrated membranes after gas purge. In addition, unlike PEFCs operated under normal temperatures, the membrane resistance under low water content and low temperature typical of cold start conditions is much greater than the contact resistance, making in-situ measurements of the membrane proton conductivity in a PEFC a simple but accurate method. 

Polyphosphazene has good chemical and thermal stability. Its polyphosphazene backbone is highly flexible. Various side chains can be introduced to this backbone readily. Cross-linking is needed in order to reduce the dimensional changes in the presence of methanol or water. The membranes have shown reasonable proton conductivity and low methanol crossover. However, an improvement in mechanical strength is needed for practical fuel cell applications. 

Preliminary results with membranes based on sulfonimide-substituted polyphosphazenes (226) show a good proton conductivity and moderate swelling in water, depending on the degree of cross-linking. ... 

On the largest length scale, top picture of Fig. 2, the distribution of water in the membrane is depicted as a porous network. The latter is characterized by a pore size distribution (psd) and a tortuousity factor , which accounts for multiple interconnectivity and bending of pathways in the network. The distribution of pore radii translates into a distribution of pore conductivities. Via this correspondence, the distribution of water in the membrane finally determines its transport properties, namely, proton conductivity and water dif-fusivity. 

This notion is supported by a large number of independent experimental data, related to structure and mobility in these membranes. It implies furthermore a distinction of proton mobility in various water environments, strongly bound surface water and liquidlike bulk water, and the existence of water-filled pores as network forming elements. Appropriate theoretical treatment of such systems involves random network models of proton conductivity and concepts from percolation theory, and includes hydraulic permeation as a prevailing mechanism of water transport under operation conditions. On the basis of these concepts a consistent approach to membrane performance can be presented. 

The effect of temperature on the protonic conductivity in fully hydrated poly(PFSA) membranes can be qualitatively described as a 50% increase in the conductivity between 20 and 80 °C. However, when in contact with saturated water vapor, a rise in poly(PFSA) membrane protonic conductivity with temperature is followed by a fall above 80 °C [25], as result of the trade-off between the thermal activation of the conduction process and... 

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