Single Cell Impedance Spectroscopy

Dielectric spectroscopy Single-cell impedance spectroscopy... 

In recent years, several technologies such as white noise stimulation, hydrodynamic focusing and trapping arrays have been implemented within single cell impedance microfluidic cytometry to achieve broadband spectroscopy, improvement in sensitivity and continuous time course measurements. 

Figure 5.20. Calculated Nyquist plot for a single cell with elimination of the transport barrier in the backing [18], (Reproduced with modifications by permission of ECS—The Electrochemical Society, from Springer TE, Zawodzinski TA, Wilson MS, Gottesfeld S. Characterization of polymer electrolyte fuel cells using AC impedance spectroscopy.)...

Figure 6.37. Impedance spectra measured at different times throughout the experiment. The gas flow rates are set to 0.3 L-mkf1 dry hydrogen for the anode and 1 L-min 1 for the cathode [38], (Reprinted from Journal of Power Sources, 154(2), Hakenjos A, Zobel M, Clausnitzer J, Hebling C. Simultaneous electrochemical impedance spectroscopy of single cells in a PEM fuel cell stack, 360-3, 2006, with permission from Elsevier and the authors.)...

Hakenjos A, Zobel M, Clausnitzer J, Hebling C (2006) Simultaneous electrochemical impedance spectroscopy of single cells in a PEM fuel cell stack. J Power Sources 154(2) 360-3... 

S. Gawad, T. Stm, N. G. Green and H. Morgan, Impedance spectroscopy using maximum length sequences Application to single cell analysis. Rev. Sci. Instr., 78, 054301 (2007). 

D. Malleo, J. T. Nevill, L. P. Lee and Morgan H, Continuous differential impedance spectroscopy of single cells. Microfluid Nanofluid (accepted) (2009). 

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). 

In addition to performance tests, diagnosis is also important in single-cell testing. The AC impedance method (called electrochemical impedance spectroscopy (EIS)) is a powerful technique for fliel cell performance diagnosis that can provide detailed information on individual losses such as kinetics, mass transfer, and... 

A. Valero, T. Braschler, P. Renaud, A unified approach to dielectric single cell analysis impedance and dielectrophoretic force spectroscopy, Lab on a Chip 10 (2010) 2216-2225. 

The performance of a fuel cell is closely related to the transport and reaction phenomenon at the electrode/electrolyte interface. For example, porosity and tortuosity affect the effective diffusivity significantly, as well as the triple phase boimdary (TPB) area in a SOFC. This will impact the polarization loss, and changes in the microstructure of the electrode will severely affect the performance of fuel cell. The apparent performance of a fuel cell is a statistical result of every single active site at the catalyst layer. Nevertheless, in the absence of an inner view of the transfer process in the porous electrode, most of the studies, either munerical or experimental, only focus on the overall characteristics of fuel cells such as the J-V curve and the electrical impedance spectroscopy (EIS). A comprehensive understanding of the behavior and mechanisms of a fuel cell is still needed. 

The current interrupt test is particularly easy to perform with single cells and small fuel cell stacks. With larger cells the switching of the higher currents can be problematic. Current interrupts and electrical impedance spectroscopy give us two powerful methods of finding the causes of fuel cell irreversibihties, and both methods are widely used. 

Compared with other methods, such as impedance spectroscopy, the current interrupt method has the advantage of relatively straightforward data analysis. However, one of the weaknesses of this method is that the information obtained for a single cell or stack is limited. Another issue is the difficulty in determining the exact point at which the voltage jumps instantaneously. 

As realised from the above issues in the comparison of test results on the electrodes and on the cells, it is a non-trivial task to break down the total loss measured on a single cell into its components using the results from the electrode studies. Impedance spectroscopy on practical cells is, however, a technique by which a partial break down can be made. Though the impedance spectra obtained in general are difficult to interpret due to the many processes involved, the spectra can at least provide a break down of the total loss into an ohmic resistance (Rj = Rgiyt + Rconnect) and a polarisation resistance reflecting losses due to chemical, electrochemical, and transport processes, as described in more detail in Chapter 9. 

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