Cell frequency-conductivity spectra

It has been demonstrated that EIS can serve as a standard analytical diagnostic tool in the evaluation and characterization of fuel cells. Scientists and engineers have now realized that the entire frequency response spectrum can provide useful data on non-Faradaic mechanisms, water management, ohmic losses, and the ionic conductivity of proton exchange membranes. EIS can help to identify contributors to PEMFC performance. It also provides useful information for fuel cell optimization and for down-selection of the most appropriate operating conditions. In addition, EIS can assist in identifying problems or predicting the likelihood of failure within fuel cell components. 

EIS is the experimental technique based on the measurement, under equilibrium or steady-state conditions, of the complex impedance of the cell at different frequencies of an imposed sinusoidal potential of small amplitude. As a result, a record of the variation of impedance with frequency (impedance spectrum) is obtained. Typically, EIS experiments are conducted from millihertz to kilohertz, so that available information covers a wide range of timescales (Retter and Lohse, 2005). 

Fig. 9. A rotation spectrum is produced by observing the motion of a cell in a rotating electric field of constant amplitude and plotting the rotation speed of the cell against frequency of the field. In solutions of low conductivity, the cell rotates in the opposite direction to the field (anti-field rotation) at low frequencies. This rotation reaches a peak when the field frequency corresponds to the charge relaxation time of the membrane. The position of this peak therefore contains information about membrane permittivity and conductivity. As the frequency increases further, the rate of cell spinning falls, becoming zero at about 1 MHz. Above this frequency, the cell starts to spin with the field (co-field rotation) and a second peak is reached. The frequency at which this peak occurs depends in practice mainly on the conductivity of the interior of the cell. It may be used for non-destructive determination of cytosolic electrolyte concentration.

As the readers may see, quartz crystal resonator (QCR) sensors are out of the content of this chapter because their fundamentals are far from spectrometric aspects. These acoustic devices, especially applied in direct contact to an aqueous liquid, are commonly known as quartz crystal microbalance (QCM) [104] and used to convert a mass ora mass accumulation on the surface of the quartz crystal or, almost equivalent, the thickness or a thickness increase of a foreign layer on the crystal surface, into a frequency shift — a decrease in the ultrasonic frequency — then converted into an electrical signal. This unspecific response can be made selective, even specific, in the case of QCM immunosensors [105]. Despite non-gravimetric contributions have been attributed to the QCR response, such as the effect of single-film viscoelasticity [106], these contributions are also showed by a shift of the fixed US frequency applied to the resonator so, the spectrum of the system under study is never obtained and the methods developed with the help of these devices cannot be considered spectrometric. Recent studies on acoustic properties of living cells on the sub-second timescale have involved both a QCM and an impedance analyser thus susceptance and conductance spectra are obtained by the latter [107]. 

Using the circuit schematic in Fig. 2, simulations can be performed using PSpice (Oread Capture, Cadence Inc. USA). The advantage of circuit simulation compared to mixture theory is that the impedance spectrum of the system not only includes the properties of the cell and the medium but also the passive (resistor and capacitor) and active (op-amps) components in the detection circuit. It is important to determine the effect of these parameters on the spectrum. The simulations can also indicate optimal parameters (i.e. frequency, medium conductivity, etc.) and also provide guidance in optimizing the circuit. 

DEP collection is a complicated function of conductivity and frequency. Figure 6A shows the DEP spectrum of a cell. As can be seen, the response as a function of frequency varies greatly. There a regions of minimal response and a number of peaks of strong response. As can be seen in Figure 6A these peaks decrease with increasing conductivity. The frequency at which these peaks occur is also seen to shift. 

The experimental sequence used for these analyses is as follows ions are formed by laser-induced ionization in the source cell (residual pressure 10 Pa). During the ionization event, the conductance limit plate between the two cells (source and analyze) is kept at the trap potential (typically +0.75 V) to confine positive ions to the source side. A variable delay period follows, during which ion/molecule reactions can occur. A frequency excitation chirp allows the ion excitation the resulting image current is finally detected, amplified, digitized, apodized (Blackman-Harris, three terms), and Fourier-transformed to produce a mass spectrum. 

In relative terms, the largest single source of ohmic losses is from the membrane. A simple way to determine the ohmic resistance is employ the impedance spectroscopic method. In a fuel cell impedance spectrum, the ohmic resistance is the real value of the impedance of the point for which the imaginary impedance is zero at the maximum frequency. The effects of these losses are most pronounced at intermediate current densities. Minimizing the ohmic losses requires effective water management in the membrane, excellent electron conductive materials, and minimal contact resistance. 

Electrolyte a substance, such as sodium chloride, that dissolves in water to give an electrically conducting solution. (4.1) Electrolytic cell an electrochemical cell in which an electric current drives an otherwise nonspontaneous reaction, (p. 808) Electromagnetic spectrum the range of frequencies or wavelengths of electromagnetic radiation. (7.1)... 

Proton conductivity is one of the key properties for predicting the PEM suitability, since a high conductivity is necessary for their effective utilization in fuel cell devices [21]. Conductivity measurements are performed on the acid form of the membranes using the special cells. This cell geometry is chosen to ensure that the membrane resistance dominates the response of the system. An impedance spectrum is recorded from 10 MHz to 10 Hz using an impedance/gain-phase analyzer. The resistance of the film is taken at a frequency that produces the minimum imaginary response. All the impedance measurements are performed at various temperatures and various RHs. 

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