Proton Conductivity as a Function of Composition and Temperature

Physiochemical properties and experimental data of phosphoric acid can be found in textbooks and various publications [148-152] and in the literature cited therein. The available works cover, e.g., evaporation and condensation considerations, acidity and proteolytic equilibria, composition specifications and condensation equilibria, vapor pressure of water as a function of composition and temperature, proton conductivity as a function of composition and temperatures, dynamic viscosity, and phase equilibria. The available data builds the foundation for modeling and simulation of a H3PO4-H2O system (and PBI/H3PO4-H2O) which is important to further improve the quality and reliability of HT-PEM fuel cell models. The combination of electrolyte modeling... 

Data on the protonic conductivity of aqueous phosphoric acid can be found in the works of Smith and Menzies [34], Campbell [35], Kakulin and Fedorchenko [36, 37], Greenwood and Thompson [30], McDonald and Boyack [27], Wydeven [38], Tsurko [39] and Chin and Chang [40]. In Fig. 8.5 the conductivity data are plotted as a function of composition and temperature in the range of 0-86 wt% P2O5 and of 0-170 °C. 

From the compiled vapor pressure and conductivity data, the evaporation enthalpy and the activation enthalpy for proton conduction were calculated as a function of composition. The critical temperature according the Vogel-Tammann-Fulcher law was determined from the viscosity data and compared with glass transition temperatures from other studies using NMR spectroscopy. A correlation between dynamic viscosity and molar conductivity was found. As expected, a considerable decoupling between ionic conduction and viscous flow can be determined from a Walden plot, which is based on proton-hopping mechanisms in phosphoric acid. 

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. 

FIGURE 13.11 Proton conductivity of SPEEK/BPO4 composite membranes as a function of BPO4 loading at room temperature. (Reprinted from Othman, M.H.D., Ismail, A.P., and Mustafa, A., J. Memb. ScL, 299, 156-165, 2007. With permission.)... 

By compiling the available literature data on the vapor pressure of aqueous phosphoric acid as a function of temperature and of composition, a fairly comprehensive description of the hquid-gas-phase field can be made for the temperature range and the total pressure range, which is important for HT-PEMFC operation. The same applies to the literature data on the proton conductivity and the... 

Usually, the starting point of model derivation is either a physical description along the channel or across the membrane electrode assembly (MEA). For HT-PEFCs, the interaction of product water and electrolyte deserves special attention. Water is produced on the cathode side of the fuel cell and will either be released to the gas phase or become adsorbed in the electrolyte. As can be derived from electrochemical impedance spectroscopy (EIS) measurements [14], water production and removal are not equally fast Water uptake of the membrane is very fast because the water production takes place inside the electrolyte, whereas the transport of water vapor to the gas channels is difiusion limited. It takes several minutes before a stationary state is reached for a single cell. The electrolyte, which consists of phosphoric add, water, and the membrane polymer, changes composition as a function of temperature and water content [15-18]. As a consequence, the proton conductivity changes as a function of current density [14, 19, 20). 

Variation of cell resistance values for ME As based on these membranes as a function of the temperature is shown in Fig. 2.10. At temperatures lower than 90 °C, the cell resistance of the bare membrane is lower than that of the composite one, whereas at T > 100 °C there is an inversion and the composite membrane became less resistive. The water retention and ionic conductivity of the composite membrane at higher temperature (J > 100 °Q are greater than those of bare membranes. The behavior of the proton conductivity for the studied membranes is typical of thermally activated process. The recorded activatiOTi energy was 17 and 21.6 kJ mol for bare and composite membranes, respectively. Cell resistance values as low as -0.1 Q cm (corresponding to a conductivity better than 7.5 x 10 S cm ) under operating temperatures of 100-120 °C can be achieved (Fig. 2.10). 

Fig. 7.11 Effect of different HPAs on the proton conductivity of composite membranes as a function of temperature. The sulfonation degree for the SPEEK was 70%. Reprinted with permission from Ref. [24] S. M. J. Zaidi, et al., Proton conducting composite membranes from polyether ether ketone and heteropolyacids for fuel cell applications, J. Membn Scl 173,17-34 (2000). Copyright Elsevier...

To improve proton transport of PFSA membranes at high temperatures and in order to operate PFSA membranes at temperatures above 100°C, inorganic compounds such as Si02 and Ti02 were employed as additives to retain water in the Naflon membranes for an acceptable proton conductivity. Proton conductors such as zirconium phosphate, heteropolyacid/ and heterocycle compounds including imidazole,benzimidazole, triazole, and polyfunctional phosphonic acid were also added in the Naflon membrane. In addition, ionic liquids were applied to fabricate composite Naflon membranes due to their anhydrous high conductivity and good thermal stability. Besides, Naflon/Naflon-functionalized multiwalled carbon nanotube composite membrane exhibits a remarkable improvement in proton conductivity compared to the pristine Naflon membrane. ... 

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