A state-of-the-art PEMFC and steady-state current-potential measurements are illustrated in Figure 3.18, which shows a schematic view of the PEMFC geometry, the basic electric circuit of the membrane electrode assembly and the gas diffusion layers at both anode and cathode.
130 X-Z. Yuan, C. Song, H. Wang and J. Zhang
Figure 3.18. Schematic of the PEMFC geometry and basic electric circuit showing the membrane electrode assembly (MEA) and the gas diffusion layers (GDLs) at both anode and cathode [33]. (Reprinted from Electrochimica Acta, 51(13), Tsampas MN, Pikos A, Brosda S, Katsaounis A, Vayenas CG, The effect of membrane thickness on the conductivity of Nafion, 2743–55. ©2006, with permission from Elsevier.)
The impedance can be measured using various instruments and techniques, ranging from a simple oscilloscope display to a fast Fourier transform (FFT) analyzer. The most common instrument used is a frequency response analyzer (FRA), e.g., the Solartron FRA. A potentiostat or a load bank combined with a frequency response analyzer can perform the EIS measurements. The electrical connection between the FRA, the potentiostat (or the load bank), and the fuel cell is illustrated in Figure 3.19.
Figure 3.19. Schematic EIS measurement configuration for a fuel cell
There are two options for controlling the perturbation to the measured system: one is to control the current perturbation then record the voltage response from the system; the other is to control the voltage perturbation then record the current response. For a current control measurement, when the leads are all connected the electrical load is set to a DC constant current. The FRA will generate an AC current perturbation and interrupt the fuel cell through a load bank or a potentiostat. The response to the interruption from the fuel cell will enter into the FRA for analysis to obtain the AC impedance spectra.
The FRA is a four-terminal instrument. Two terminals are used for current or voltage signal generation (usually in the form of voltage). The other two accept the voltage or current response. By dividing the response voltage by the perturbation current, the complex impedance (both magnitude and phase) can be obtained at the measured frequency. This procedure is repeated again and again at any desired frequency within the capabilities of the FRA. Ultimately, a full impedance spectrum can be achieved. Suitable software can be chosen to automate the EIS measurements, collect and process the data, and represent them as figures.
It is worthwhile pointing out that a regular potentiostat cannot provide a heavy load in fuel cell testing. Booster technology can help to improve the load capability. Alternatively the fuel cell load bank can be connected directly to the FRA. Sometimes, a power supply may be needed to adapt to the heavy load. Figure 3.20 shows an example of a connection between a TDI load bank and a Solartron 1260. The TDI is a single-channel load bank (RBL 488 series 100-60-400) that can be controlled with signals via an external program; the values of all signals are referred to as S-. When a controlling DC, AC, DC/AC combination, or FRA- generated waveform is connected into the ports of the REM and S- located on the rear panel of the load bank, the load bank will transfer the signal into the fuel cell load. The accuracy of the measurement is limited by the load bank frequency range.
132 X-Z. Yuan, C. Song, H. Wang and J. Zhang
Figure 3.20. AC impedance measurement by TDI RBL 488 load bank and Solartron 1260 [21]. (Reproduced by permission of ECS—The Electrochemical Society, from Tang Y, Zhang J, Song C, Liu H, Zhang J, Wang H, Mackinnon S, Peckham T, Li J, McDermid S, Kozak P. Temperature dependent performance and in situ AC impedance of high- temperature PEM fuel cells using the Nafion-112 membrane.)
In Tang et al.’s [21] impedance experiments, a differential input with a floating ground was chosen to reduce noise and harmonic signals from externally wired circuits. An oscilloscope was connected to the fuel cell current collectors to monitor the noise level. The results showed that this electrical connection could effectively reduce noise. During the measurements, the FRA port, “GEN OUTPUT”, gave a software-command signal to the load bank through the ports of
“REM” and “S-”. The cell voltage response went to FRA “V1” and “V2” for analysis. The impedance information obtained was sent to a computer for data display, and more sophisticated data analysis was performed by powerful software called Z-plot.
To obtain the fuel cell impedance at different frequencies, the frequency of the stimulus sine wave can be swept across the frequency range of interest. However, at high current density, the cell voltage is unstable because of water flooding. This instability could impair the accuracy of the AC impedance measurement, especially for low-frequency measurements. For example, at 10 mHz in one cycle of measurement, which takes about 100 s, it is difficult to obtain accurate AC impedance information at a high current density. More cycles are needed to average the results by integration in the non-linear regime. As we know, the lower the frequency, the longer each frequency point takes for a full impedance spectrum.
Therefore, at high frequencies, data collection can be done in the blink of an eye with a high average number, but at very slow frequencies it will slow down dramatically. In terms of the software settings (e.g., Zview), although averaging
more points can improve AC impedance results, to speed up a test below a certain frequency it is preferable to collect fewer points.