Electrochemical impedance spectroscopy membrane conductivity measurement

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). 

Electrochemical impedance spectroscopy (EIS) has been used to characterize the ionic conductivity of conducting polymers in the electrode, separating this ionic resistance from the membrane and ohmic contact resistances at the interfaces [24— 31]. Such measurements are helpful in quantifying degradation of the ionomer in... 

In Chapter 3, electrochemical impedance spectroscopy (EIS) was introduced as a powerful technique for PEM fuel cell diagnosis. EIS measurement can be conducted at OCV and under load. The AECD of the ORR (fo ) can be calculated using Eqn (3.8), based on the simulated /f° (charge transfer resistance at the OCV for the ORR) from the Nyquist plot obtained by EIS that is shown in Fig. 3.12. The values of the membrane resistance (/fm), charge transfer resistance (/ t) and mass transfer resistance (/fmt) in a PEM fuel cell at different current densities can also be simulated using measured EIS, based on... 

The conductivity of the polymers was also measured using a galvanostatic four-point-probe electrochemical impedance spectroscopy technique [17]. A four-point-probe cell with two platinum foil outer current-carrying electrodes and two platinum wire inner potential-sensing electrodes was mounted on a Teflon plate. The schematic view of the cell is illustrated in Fig. 6.3. Membrane samples were cut into strips that were approximately 1.0 cm wide, 5 cm long, and 0.01 cm thick prior to mounting in the cell. 

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 5.9. AC impedance spectra of Nafion 115 membrane obtained by the two-probe cell method with different probe distances, at room temperature and under fully hydrated conditions [9], (Reproduced by permission of ECS—The Electrochemical Society, from Xie Z, Song C, Andreaus B, Navessin T, Shi Z, Zhang J, Holdcroft S. Discrepancies in the measurement of ionic conductivity of PEMs using two- and four-probe AC impedance spectroscopy.)...

For conductivity characterization, the membranes were rendered conducting by immersion of PVA/PDDA membranes in 2 M KOH and equilibrated for 24 h to convert it from CF form into OH form. Then the manbranes were taken out and rinsed repeatedly with DI water to remove the absorbed KOH on membrane surfaces for final conductivity measurements. The OH" conductivity of the formed membrane was measured by an AC impedance technique using an electrochemical impedance analyzer. The maximum OH" conductivity value measured at 25°C with AC impedance spectroscopy reaches as high as 0.025 S cm" at a PVA/PDDA polymer composition of 1 0.5 by mass. 

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