Proton conductivity electrochemical impedance

Cahan BD, Wainright JS (1993) AC impedance investigations of proton conduction in Nafion . J Electrochem Soc 140 L185-6... 

Sone Y, Ekdunge P, Simonsson D (1996) Proton conductivity of Nafion 117 as measured by a four-electrode AC impedance method. J Electrochem Soc 143(4) 1254-1259... 

Schnumberger [1999] Change of Electrochemical Impedance Spectra (EIS) with Tune during CO-Poisoning of the Pt-Anode in a Membrane Fuel Cell, in Proton Conducting Membrane Fuel Cells II, ed. S. Gottesfeld and T. F. Fuller, Electrochem. Soc. Proc. 98-27,... 

Y. Sone, P. Ekdimge, D. Simonsson, Proton conductivity of Nafionll7 as measiu ed by a foiu electrode AC Impedance method, J. Electrochem. Soc., 143 (1996) 1254-1259... 

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). 

Sugimoto W, Iwata H, Yokoshima K, Murakami Y, Takasu Y (2005) Proton and electron conductivity in hydrous ruthenium oxides evaluated by electrochemical impedance spectroscopy the origin of large capacitance. J Phys Chem B 109 7330-7338... 

At the macroscopic level, proton transport can be studied with electrochemical impedance spectroscopy (EIS). Cappadonia et al. (1994,1995) performed EIS studies to explore variations of proton conductivity with water content and temperature for Nafion 117. The Arrhenius representation of conductivity data revealed activation energies between 0.36 eV at lowest hydration and 0.11 eV at highest hydration, as shown in Figure 2.6. The transition occurs at a critical water content of A-crit 3. At fixed X, the transition between low and high activation energies was observed at 260 K for well-hydrated membranes. This finding was interpreted as a freezing point suppression due to confinement of water in small pores. 

Sone, Y., Ekdunge, R, and Simonsson, D., 1996, Proton conductivity of Nafion 117 as measured by a four-electrode AC impedance method, J. Electrochem. Soc. 143 1254-1259. Fontanella, J. J., McLin, M. G., WintersgUl, M. C., Calame, J. P., 1993, Electrical impedance studies of acid form NAFION membranes. Solid State Ionics 66 1-4. 

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. 

Proton conductivities of fully hydrated membranes (24 h at ambient temperature in double deionized HjO) may be measured using two- or four-probe electrochemical impedance spectroscopy (EIS) at frequency 0.1-10 MHz with AC amplitude of 5 or 10 mV (Rg. 3.2). For good membrane-electrode contact the PEM is placed between two Hg or Pt electrodes in a sealed conductivity cell, thermostated at the desired T for about 5 h before measurements. It is advisable to perform the measurements with dry membranes from 20 up to 100°C in 10°C steps with wet membranes. Each sample should be measured 10 times and the average value of the impedance, R, used for calculating the proton conductivity o = d/RS (S/cm), where d is the membrane thickness, thus the distance between the electrodes. The results are sensitive to the specimen immersion depth, quality of deionized water, and electrode/membrane contact. Usually, the ionic conductivity correlates with the degree of sulf onation, 38. .82,83... 

Figure 4.33. Equivalent circuit of a catalyst layer [8]. (Reproduced by permission of the authors and of ECS—The Electrochemical Society, from Lefebvre MC, Martin RB, Pickup PG. Characterization of ionic conductivity within proton exchange membrane fuel cell gas diffusion electrodes by impedance spectroscopy.)...

The bulk of EAP-based supercapacitor work to date has focused on Type I devices. Polypyrrole (PPy, Figure 9.4C) has been studied [147,151-153] for this application, with specific capacitance values ranging from 40 to 200 F/g. Garcia-Belmonte and Bisquert [151] electrochemically deposited PPy devices that exhibit specific capacitances of 100-200 F/cm with no apparent dependence on film thickness or porosity extensive modeling of impedance characteristics was used. Hashmi et aL [153] prepared PPy-based devices using proton and lithium-ion conducting polymer electrolytes. As is often observed, electrochemical performance suffered somewhat in polymeric electrolytes single electrode specific capacitances of 40-84 F/g were observed with stability of 1000 cycles over a 1 V window. 

Needless to say MEA is the core compartment of DMFC with its electrochemical reaction function. Figure 13.3 shows the typical microstructure of MEA, danon-strating the interface of catalyst and membrane. Its function is to deliver materials, such as catalyst and membrane, and physical functions for fuel delivery and recovery. Mobile application MEAs minor functions, such as fuel delivery and recovery, have become more important. MEA can be defined as three compartments of membrane, a catalyst layer and diffusion electrode with a microporous layer. The catalyst layer consists of catalyst and interface materials with membrane. This layer has to be designed for effective utilization of the catalyst in order to minimize the use of precious metals while maintaining the produced proton path to the membrane. For this reason, this layer has to be electron- and ion-conductive with low fuel flow resistance. The membrane is located at the center of the MEA, with the catalyst layer coated (catalyst-coated membrane, CCM) in some cases. Its ion conduction would be made a lot easier by reducing the impedance at the interface with the catalyst. 

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