Protonic conduction hydration dependence

Acidity-basicity effects in proton-conducting materials depend not only on the atomic and bonding properties of the central anionic species, but also on the properties of the aggregate surface and on the state of aggregation as can be observed in a discussion of the true compositions of weak and medium strength acidic materials such as tin dioxide, P"-alumina and antimonic acid hydrates (Table 1.1). 

It has been widely reported [61-64] that temperature can significantly affect the proton conductivity (cr) of a membrane. For a PFSA membrane (e.g. Nafion membrane), the proton conductivity strongly depends on the water content of the membrane. Therefore, when studying the effect of temperature on membrane conductivity, the RH or water content of the membrane must be considered. At a low RH, an increase in the temperature will cause membrane dehydration, resulting in decreased proton conductivity, whereas with a well-hydrated membrane, the proton conductivity will increase with increasing temperatures. For example, the conductivity of Nafion 117 at 100% RH increases from 0.1 to 0.2 S cm when the temperature is raised from 30 to 85 °C [65]. Generally, under weU-hydrated conditions, the temperature dependence of conductivity can be expressed in an Arrhenius form [2] ... 

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. 

The conductivity exponent of about 1.23 indicates 2D character of the percolation transition. Similar values of the conductivity exponent were obtained for the hydration dependence of the conductivity of embryo and endosperm of maize seeds [595, 596], where the percolation threshold is /t = 0.082 and 0.127 g/g, respectively. In hydrated bakers yeast, protonic conductivity evidences 2D percolation transition of water at h = 0.163 g/g, and the value of the conductivity exponent is about 1.08 [597]. In this system, increase in conductivity due to 3D water percolation is observed at essentially higher hydration level h= 1.47 g/g, where conductivity exponent is about 1.94, i.e., close to the 3D value t = 2.0. Conductivity measurements of Anemia cysts at various hydrations show strong increase in conductivity starting from the threshold hydration h = 0.35 g/g [598] (see Fig. 97). The conductivity exponent in this system is 1.635, which is in between the values expected for 2D and 3D systems. DC conductivity of lichens, evaluated from the dielectric studies at frequencies between 100 Hz and 1 MHz [599], shows strong enhancement at some hydration level. Fit of the conductance-hydration dependence to equation (24) gave the following parameters he = 0.0990 g/g, t = 1.46 for Himantormia lugubris and he = 0.0926 g/g, t = 1.18 for Cladonia... 

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. 

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. 

Heteropolyacids are frequently used to modify proton-conducting composites,or they are just dispersed in inert matrixes.However, because the proton conduction mechanism of such hydrated salts is similar to those of hydrated polymeric sys-tems, these composites show qualitatively similar transport properties. The same is true for organically modified inorganic layered compounds such as titanium phosphate sulfophenylenphosphonate, the conductivity of which is dependent on the RH value, in a manner similar to that observed with Nafion. ... 

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. 

Most proton conducting solids until fairly recently were inorganic hydrates and stable conductivities much above 150 °C in such materials are seldom found (i.e. after the water is lost, conductivity disappears) moreover, the conductivity is highly dependent on the water content, including the surface water of the material. Thus there is a need to develop proton conductors for various applications in the higher temperature range, which are not dependent on water content. 

Fig. 13. Hydration dependence of protonic conduction. The dielectric relaxation time, Ts, is shown versus hydration, h, for lysozyme powders. The relaxation time is proportional to the reciprocal of the conductivity. (A) H20-hydrated samples solid curve, lysozyme without substrate , lysozyme with equimolar (GlcNAc)< at pH 7.0 , with 3x molar (G1cNAc)4 at pH 6.5. The relaxation time is nearly constant between pH 5.0 and 7.0. (B) HjO-hydrated samples solid curve, lysozyme without substrate 9, lysozyme with equimolar (GlcNAcb at pH 7.0. From Careri etal. (1985). 

Fig. 15. Critical exponent for protonic percolation on purple membrane. Hydration dependence of the conductivity for HjO (O) and ( ) hydration of lyophilized...

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. 

Figure 12.1. The dependence of proton mobility on water content, (a) Proton selfdiffusion coefficients (D

The next question is which of the above-discussed scenarios is most Kkely to be found at the catalyst/hydrated membrane interface. It is generally accepted that the proton conductivity of the membrane depends on the characteristics of ionic clusters formed surrounding the polymer hydrophilic sites, both within the bulk polymeric structure and at the interface with the catalyst [79]. The ionic clusters located at the membrane/catalyst interface are the ones that close the circuit of this electrochemical system. That is, these ionic clusters act as bridges through which protons and other hydrophilic reactants and products may pass from the membrane to the catalyst surface and vice versa during fuel cell operation. To get some insights into the possible formation of ionic clusters, we have analyzed the conformation of a hydrated model nation membrane over Pt nanoparticles deposited on a carbon substrate via classical MD simulations [80] at various degrees of hydration. 

Fig. 26 Temperature dependence of proton conductivity of anhydrous (a) and hydrated (b) PBI complexes with H3PO4 (1), H2SO4 (2), EtSOjH (3), MeSOjH (4) [7]...

While studying the Ce(IV)/phosphoric acid system, a fibrous material of composition Ce(HP04)2.2H2O was obtained . Similar results were obtained with thorium . The structure of these compounds is unknown. The fibrous materials are of interest for electrochemical devices because they can be used to obtain very thin, autoconsistent membranes. The conductivity of anhydrous and trihydrated cerium phosphate is reported in Table 16.2. The conductivity of the hydrated compound was investigated as a function of temperature at different relative humidities and parametrized on the basis of the Arrhenius equation. The dependence of both activation energy and pre-exponential factor on relative humidity was similar to that of ZrP. This suggests that in cerium phosphate also the proton conduction is due, for the most part, to the hydrated surface of the particles. 

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