A Guide to Concentration Alternating Frequency Response Analysis of Fuel Cells

Fuel cells are going to play a major role in the future. Our protocol describes a new method for diagnosing of major fail states of these devices such as degradation, flooding, or poisoning. Unlike other techniques as for example electrochemical impedance spectroscopy, this methodology can be used to decouple the impact of specific dynamics on the polymer fuel cell performances facilitating a less ambiguous fault identification.

The procedure will be demonstrated by Tobias Franz, a master student from my laboratory. For fuel cell assembly, place the cathode bipolar plate on a smooth and sturdy surface with the flow field side up and place the gasket on top of the plate aligned with the screw holes. Place the cathode gas diffusion layer in the middle of the gasket and add the catalyst-coated membrane top aligned with the screw holes.

Place the anode gas diffusion layer and gasket on top of the catalyst-coated membrane making sure the gasket aligns with the screw holes. Place the anode bipolar plate flow field side down on top of the gasket and fix the parts together with screws. Next, place the cathode stainless steel end plate on a smooth and sturdy surface and place a rectangular piece of Teflon and a copper current collector on top of the Teflon aligning both pieces with the bolt holes.

Slot the cathode side of the assembled cell unit onto the cathode current collector taking into account the notches in the flow fields. Slot the anode side of the unit onto the anode current collector with the Teflon gaskets positioned with the anode stainless steel end plate on top. Place the insulating sleeves, O-ring, and bolts in the holes of the anode end plate, turn the cell vertical, and place the insulating sleeves, O-ring, and nuts on the bolts on the cathode side of the unit.

Then use a torque wrench to tighten the bolts crosswise until the recommended torque value of five Newton meters is reached, increasing the torque by one Newton meter per crosswise cycle. For integration of the fuel cell with the periphery, place the fuel cell unit in a heating box and connect the inlets and outlets to the periphery. Insert the thermocouple into the cathode end plate and interface the fuel cell with the potentiostat to electrode configuration.

Start the software used to control the cell periphery and select the values of the anode and cathode inlet gas flow rates. Select the temperature of the inlet gases. Turn on the heating tapes and wait until the setpoint temperature is reached.

Set the temperatures of the thermostats to define the desired dew point temperature of the inlet gases and turn on the thermostats. Set the chosen temperature of the fuel cell on the control panel of the heating box and turn the heating on. When the setpoint temperature of the fuel cell is reached, check the humidification state of the inlet gases and check the fuel cell open circuit cell potential.

To perform a concentration-alternating frequency response experiment, push down gently on the plunger of the upper part of a fiber oxygen sensor to expose the sensitive part of the fiber. Then place the fiber into the center of the tubing at the cell inlet. Open the sensor software and set the sampling interval to 0.15 seconds in order to enable the detection of a periodic signal up to a period of one hertz.

Open the electrochemistry software to edit the concentration-alternating frequency response analysis procedure and in the action section select new procedure. In commands, select the control icon and insert the icon into the workspace. In properties, select mode on galvanostatic and the cell on command placing the command next to the control icon.

Add the linear sweep voltammetry staircase command from the measurement cyclic and linear sweep voltammetry. In properties, set the start current to 0.0 amps and the stop current value to the steady state. Set the scan rate to 0.005 amps per second and the step to 0.1 amps.

Insert two record signal commands. In properties, set the duration to 7, 200 seconds and the interval sampling time to 0.5 seconds for both commands. Note that the first recording window is used to monitor how the output signal approaches the periodic steady state conditions while the second one is to register the steady state periodic output signal which is analyzed.

Add a repetition command to set the same step to be repeated 20 times. Press play to start the concentration-alternating frequency response program. In the first set of repetitions, observe the recording window to check if the cell potential reaches the steady state value.

To ensure a linear response, open the additional oxygen valve and set the mass flow controller to 5%of the value of the total flow rate of the main feed. Set the switching time of the valve to an initial value of 0.5 seconds and click start. Then wait until the cell potential achieves a periodic steady state in the monitoring window before clicking next.

Sampling of the potential under quasi steady state condition is necessary to obtain artifact-free spectra as the presence of drifting signal could lead to misleading conclusions. Register the periodic steady state signal in the new recording window for 60 seconds and click next again. At the same time, register the periodic oxygen input and click start in the sensor software.

Enter a name that recalls the frequency input and click OK.Then register the signal for 60 seconds and click stop. Repeating the previous steps, measure the periodic input/output correlations for signals with a period in a frequency range from eight to 1, 000 microhertz while acquiring eight frequency points per decade. At frequencies lower than 100 microhertz, sample the signals for a range of time equivalent to five periods.

To analyze the concentration-alternating frequency response data, open the MATLAB scripts FFT_input. mat and FFT_output.mat. In the address folder, insert the specifications of the location of the folder in which the measured oxygen pressure and current data files are stored.

Run the FFT_po2. mat and FFT_pot. mat scripts and check the plotted diagrams to determine whether the computed algorithm is working properly.

Then open and run the MATLAB script cfra_spectra.mat. The magnitude, phase angle, and Nyquist spectra of the concentration-alternating frequency response analysis transfer function under galvanostatic conditions will be plotted. In this representative analysis, the electrochemical impedance spectroscopy magnitude and phase bode plot spectra were first measured at three different steady state current densities under galvanostatic control.

Here, exemplary periodic oxygen pressure inputs at two different frequencies and their Fourier transforms can be observed. The magnitudes of the harmonics were normalized with respect to the fundamental harmonic and the pressure input at a 49 microhertz frequency was characterized by a sinusoidal shape. The pressure input at a lower frequency resembled a periodic square wave shape and the related normalized Fourier transform perfectly reflected that of a square wave signal presenting descending harmonic components at multiple odd integer frequencies with respect to the fundamental one.

The cell potential responses presented identical features. Note that a spectral analysis of the input and output performed on a non-integer number of periodic cycles could lead to misleading results due to the effect of the spectral leakage. In this case, the signal is characterized by a more expressed noise bandwidth at fundamental frequency.

Additionally, the magnitude is approximately 90%of the properly processed signal. To avoid the spectral leakage, a procedure of windowing should be applied on any signal analyzed. Here, concentration-alternating frequency response analysis spectra measured under voltastatic and galvanostatic conditions under the same steady state conditions as in the electrochemical impedance spectroscopy spectra are shown.

As observed in the high frequency region, both voltastatic and galvanostatic concentration-alternating frequency response analysis spectra demonstrate no sensitivity to double layer charging/discharging dynamics. The cFRA spectra are sensitive only to transients related to mass transport phenomena. To avoid undesired contributions to the evaluated spectra, measure the cell potential under quasi steady state conditions and sample a sufficient number of period in order to increase the signal-to-noise ratio.

So in addition to diagnostics, the operation of electrochemical fuel cells and reactors under periodic conditions introduces additional possibility to impact energy conversion efficiency as well as product selectivity of electrochemical processes.