Tuesday, June 21, 2011

Making AC Impedance Measurements on Fuel Cells

Making AC impedance measurements on fuel cells can help identify problems with the fuel cell components and help identify deviations in the fuel cell assembly process. Typically when testing fuel cells, multiple impedance measurements are made at various frequencies, the results are then plotted across the frequency band resulting in an Electrochemical Impedance Spectroscopy (EIS) measurement. In this blog post we will cover building a test system for making AC impedance measurements on fuel cells

To make AC impedance measurements on a fuel cell that is producing dc current we first need stimulate the fuel cell output with a load that is varied sinusoidally at a particular frequency and then digitize the cell's output AC current and AC voltage. The digitized signal data would then by fed to a PC where custom software or a math package, such as Matlab or Excel, would be used to perform post processing on the digitized data that includes:


  1. Perform a Fast Fourier Transform on the digitize current and voltage waveform data.
  2. Using complex math functions, divide the transformed voltage by the transformed current to obtain the complex impedance (both magnitude and phase).
  3. Repeat at multiple frequencies to create a full spectrum of impedance measurements on the fuel cell.
The digitized AC current and voltage measurements needed for the impedance calculations can be done with just two pieces of equipment, a high performance electronic load (eload) and a function generation. Below shows the fuel cell impedance measurement set-up (click to enlarge).



By connecting the function generator to the eload's external analog programming input, we can use it to control the eload's load value or input impedance. By setting the function generator output to a sine wave at various frequencies we can create the necessary AC load variations to create the desired output current and voltage waveforms from the fuel cell under test. Agilent's 33210A function generator is a good cost effective fit for this application. For the eload, the reason I underline "high performance" in the above paragraph is because the eload requires some advance features including parallel voltage and current measurement digitizers built-in to capture the input voltage and current simultaniously. Otherwise we would have to add a two channel external digitizers to capture the AC voltage and current (for current we would also need a current shunt) from the fuel cell. The above figure shows Agilent's N3300 eload series. The N3300 series fills the role of a "high performance" eload because it provides simultaneous voltage and current digitization capability. The max. frequency you can make an AC impedance measurement on is going to be limited by the eload's max bandwidth specification, which is the max.rate you can change its input load or impedance setting. For the N3300 eload series the max. sinusoidal bandwidth is 10 KHz. Because of their design, eloads cannot operate at full specifications at low voltages, typically < 3 V. To over come this characteristic of eloads a "boost" power supply can be added to the circuit in series as shown in the figure. For more info on using an eload with a boost supply click here

Unfortunately going into the details of implementing an FFT in software and performing division of complex equations to make AC impedance measurements and creating a Electrochemical Impedance Spectroscopy plot is beyond the scope of this blog post. The good news is there are plenty of information on the internet for performing these calculations (just google it). Also the first link below is to an Agilent app note that provides more detailed information on preforming AC impedance measurements on fuel cells. The second link below takes you to a complete turn-key solution that provides the hardware and software needed to make AC impedance measurements on fuel cells. 



For more info on a turn-key fuel cell test system click here

8 comments:

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