A fuel cell is a device that generates a type of chemical energy converted into electric energy. It is a highly efficient power supply with environmental pollu-tion free and it can be considered as a generator with fuel and oxidant as inputs. For an efficient and a stable opera-tion of fuel cells, it is necessary to control parameters such as the temperature of the fuel cells under the opera-tion and the humidity, flow rate, and utilization rate of reactive gases. When the fuel cell operation is stopped, it is necessary to stop the reaction completely and extract the excess and absorbed gases to ensure safety. A typical method is the nitrogen purge, which injects nitrogen gas to draw out the excessive fuel and oxidant.
Why is impedance measurement necessary?
Not only fuel cells but also many other types of cells do not allow performance adjustment in the post-manufacturing stages. It is nonethelessnecessary to run and test the cells in order to verify that they provide theexpected levels of performance and meet the required specifications.However, running every manufactured cell for testing purposes is by nomeans easy. One way to examine the characteristics of individual cells in ashort time is to conduct accurate impedance measurement.Knowing impedance characteristics provides clues as to the characteristicsand performance variation of cells as well as their service life.
Impedance measurement method - AC impedance measurement
There are several impedance measurement methods, including:(1) AC impedance measurement, (2) current interrupt method, (3) fast Fourier transform, (4) lithargemethod, (5) impedance bridging, and (6) oscilloscope measurement.Of these, AC impedance measurement is the most popular method. To measure large currents, thecurrent interrupt method has been the technique of choice. This method, however, is often problematic interms of data reproducibility and accuracy.The AC impedance measurement method applies alternate current-induced vibration to the device undertest (fuel cell), calculates the complex impedance from the amplitude of the resulting voltage and current andthe phase difference, and then plots the impedance in a complex coordinate system. By varying the vibrationfrequency of alternate current, the method obtains the equivalent impedance from the plotted trajectory.
