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Summary

We present a protocol for concentration alternating frequency response analysis of fuel cells, a promising new method of studying fuel cell dynamics.

Abstract

An experimental setup capable of generating a periodic concentration input perturbation of oxygen was used to perform concentration-alternating frequency response analysis (cFRA) on proton-exchange membrane (PEM) fuel cells. During cFRA experiments, the modulated concentration feed was sent to the cathode of the cell at different frequencies. The electric response, which can be cell potential or current depending on the control applied on the cell, was registered in order to formulate a frequency response transfer function. Unlike traditional electrochemical impedance spectroscopy (EIS), the novel cFRA methodology makes it possible to separate the contribution of different mass transport phenomena from the kinetic charge transfer processes in the frequency response spectra of the cell. Moreover, cFRA is able to differentiate between varying humidification states of the cathode. In this protocol, the focus is on the detailed description of the procedure to perform cFRA experiments. The most critical steps of the measurements and future improvements to the technique are discussed.

Introduction

Characterizing the dynamic behavior of a PEM fuel cell is important in order to understand which mechanisms dominate the transient operational states lowering the performance of the cell. Electrochemical impedance spectroscopy (EIS) is the most commonly used methodology for studying PEM fuel cell dynamics, due to its ability to separate different process contributions to the overall dynamic performance^1,2. However, transient processes with similar time constants are often coupled in the EIS spectra, making it difficult to interpret them. For this reason, in the past transient diagnostic tools based on the application of non-electrical inputs with the aim of detecting the impact of a few or individual dynamics have been developed and proposed^3,4,5,6,7.

A novel frequency response technique based on concentration perturbation input and electrical outputs named concentration-alternating frequency response analysis (cFRA) has been developed in our group. The potential of cFRA as a selective diagnostic tool has been investigated theoretically and experimentally^6,7. It was found that cFRA can separate different kinds of mass transport phenomena and discriminate between the different states of operation of the cell. In this protocol, we focus on the step-by-step description of the procedure for performing cFRA experiments. The assembling of the cell, its conditioning and the experimental setup for creating a feed with periodic concentration perturbation, as well as the data analysis will be shown and discussed in detail. Finally, the most critical points of the procedure will be highlighted and several strategies for improving the quality and selectivity of cFRA spectra will be pinpointed.

Protocol

1. Material preparation Cut and perforate two rectangular pieces of Teflon of the same size as the end plates by using a cutting press; take care and ensure that the holes are in the exact position where the bolts should be placed. Using the same procedure cut Teflon gaskets considering the outer and inner dimensions of the flow field, and the position of the holes where the screws should be placed. Cut the gas diffusion layers using a metal frame fitting the size of the gaskets. Cut the excess Nafion from the catalyst coated membrane (CCM) in order to adjust it to the size of the bipolar plates. Make holes in the membrane at the positions where the screws should go through with the help of the metal frame used previously. Take care to center the frame before making the holes. 2. Fuel cell assembly Place the cathode bipolar plate on a smooth and sturdy surface with the flow field side up. Place the gasket on top. Make sure it aligns with the screw holes. Place the cathode GDL in the middle of the gasket and put the CCM on top. Make sure the CCM is aligned with the screw holes. Place the anode GDL and gasket on top. Make sure the gasket aligns with the screw holes and the GDL is placed in the middle. Place the anode bipolar plate on top (flow field side down) and use screws to clamp the parts together. NOTE: The bipolar plates must not be strongly tightened. The purpose of the screws is just to keep aligned the different parts. Place the cathode stainless steel end plate on a smooth and sturdy surface. Place the rectangular Teflon piece and the copper current collector on top. Make sure they align with the bolt holes. Slot the cathode side of the cell unit assembled in step 2.1 on the cathode current collector taking into account the notches in the flow fields. Slot the anode side of the unit on the anode current collector, position the Teflon gaskets and finish with the anode stainless steel end plate on top. Place the insulating sleeves, the O-ring and the bolts in the holes of the anode end plates; insert the bolts in the holes. Position the insulation sleeves and the O-ring; finish by placing the nuts on the bolts on the cathode side. Tighten the bolts crosswise using a torque-wrench until you reach the recommended torque value of 5 N·m. 5 crosswise cycles are suggested; start by low torque value (1 N·m) and increase by 1 N·m in each subsequent cycle. 3. Integration of a fuel cell with the periphery Place the fuel cell in the heating box and connect the inlets and outlets to the periphery. Use snoop liquid to check for leakages. Insert the thermocouple in the cathode end plate. Interface the fuel cell with the potentiostat; choose 2 electrode-configuration. Connect the cables marked as RE and CE to the anode side and the ones marked as WE and SE to the cathode side. Start the software used to control the cell periphery; a scheme of the experimental setup is visualized (see schematic in Figure 1). Choose the values of the anode and cathode inlet gas flow rates and open the valves. In the experiments shown in this protocol, flow rates of 850, 300 and 300 mL/min were used for hydrogen (anode side), nitrogen and oxygen (cathode side) respectively. Choose the temperature of the inlet gases and turn on the heating tapes. Wait until the set point temperature is reached. In all experiments in this protocol, the set point temperature of the inlet gases at the anode and cathode side was 68 °C. Set the temperatures of the thermostats to define the desired dew point temperature of the inlet gases; turn the thermostats on. Set the chosen temperature of the fuel cell on the control panel of the heating box. Then, turn the heating on. In the experiments described in this protocol a fuel cell temperature of 80 °C was set. Wait until the set point temperature of the fuel cell is reached; check the humidification state of the inlet gases; check the fuel cell open circuit cell potential. The open circuit cell potential value on the display of the potentiostat should be between 1 and 1.2 V. 4. Fuel cell start up procedure NOTE: The procedure described in the following section uses a specific software program and potentiostat (Autolab N104, NOVA 2.0 software). However, it can be also performed using other software and potentiostats without changing the main outcomes. The start-up procedure must be performed if a new CCM is used. Start the Autolab NOVA 2.0 software. Select New Procedure in the Action section of the software; the procedure editing page opens. In Command, click on the Autolab Control icon ; drag the Autolab Control icon to the workspace section. Then, in Properties, select Mode On Potentiostatic. NOTE: The Autolab NOVA 2.0 software does not differentiate between the terms potentiostatic and voltastatic. In Command, select the Cell icon and place it next to the Autolab Control icon. Then, in Properties choose Cell On. Add the Apply icon and in Properties set 0.9 V as Cell Potential with respect to the reference electrode. Add the Wait command and set Duration to 1800 s. Add the LSV Staircase command from Measurement Cyclic and Linear Sweep Voltammetry. Set the Start Potential to 0.9 V, the Stop Potential to 0.6 V, the Scan Rate to 0.4 mV/s and Step to 0.244 mV. Add the Wait command and set Duration to 1800 s. Add the LSV Staircase command from Measurement Cyclic and Linear Sweep Voltammetry. Set the Start Potential to 0.6 V, the Stop Potential to 0.9 V, the Scan Rate to 0.4 mV/s and the Step to 0.244 mV. Add the Repeat command. In the workspace select the commands from step 4.1.4 (the first Wait command) to step 4.1.7 (the last LSV Staircase command); drag and drop the icons into the Repeat box. In Properties asset the number of repetitions to 20. Start the cell start up procedure by clicking the Play button. After 2 h, if the current is stable at 0.6 V stop the program by pressing on the Stop button. If the current is still changing, let the program run until it terminates. 5. Galvanostatic electrochemical impedance spectroscopy experiment Start the Autolab NOVA 2.0 software. Select New Procedure in the Action section of the software; the procedure editing page opens. In Command click on the Autolab Control icon ; drag and drop the Autolab Control icon to the workspace section. Then, in Properties select Mode On Galvanostatic. Add the Cell On command. Add the LSV Staircase command. In Properties set the Start Current to 0 A, the chosen steady state current to Stop Current, the Scan Rate to 0.005 A/s and Step to 0.01 A. Insert the Record Signal command; in Properties set the Duration to 7200 s and the Interval Sampling Time to 0.1 s. Insert the FRA measurement command window. In Properties set the First applied frequency to 1000 Hz, the Last applied frequency to 0.01 Hz and the Number of frequencies per decade to 5. Set the Amplitude to 5% of the steady state current. Add the Cell Off command. Start the cell galvanostatic EIS program by pressing the Play button. Wait until the cell potential value stabilizes by observing the change in the recording window. Then click on the Forward button to start the EIS experiment. Check the stability of the system during the experiment and wait until the program is terminated. 6. Concentration-alternating frequency response experiment NOTE: The following instructions describe the procedure for performing cFRA experiments under galvanostatic conditions. However, the procedure would not differ if performing cFRA experiments under voltastatic conditions, apart from setting the galvanostatic to potentiostatic control in the software and fixing a certain cell potential as a steady state instead of current. Set up the Pyro fiber oxygen sensor for fast dynamic measurements. Push gently down on the plunger in the upper part of the Pyro fiber oxygen sensor in order to remove the sensitive part of the fiber from the protective needle and place it in the center of the tubing at the cell inlet. Open the Pyro software. Click on Options | Advance and choose Enable Fast Sampling. Set the Sampling Interval to 0.15 s. Edit the cFRA procedure by using Autolab NOVA 2.0 software. Open the NOVA software and select New Procedure in the Action section ; the software editing page opens. In Commands select the Control icon and insert it in the workspace. In Properties select Mode On Galvanostatic. Then select the Cell On command and place it next to the Control icon. Add the LSV Staircase command from the Measurement Cyclic and Linear Sweep Voltammetry. In Properties set the Start Current to 0.0 A; set as Stop current the steady state current value at which the cFRA experiment should be performed. Then use 0.005 A/s as the Scan Rate and 0.01 A as the Step. Insert two Record Signal commands; in Properties set Duration to 7200 s and Interval Sampling Time to 0.05 s. Repeat the same step 20 times by adding a Repetition command. The number of repetitions must be equivalent to the number of signal frequencies that need to be measured. NOTE: Two recording signal windows are convenient for the following reasons: one recording window is used to monitor the transient part of the periodic output signal, while the second one is used to register the steady state part of the periodic output signal. The steady state part of the signal is used for transfer function determinations. Press the Play button to start the cFRA program. In the first set of repetitions, check if the cell potential reaches the steady state value by observing the recording window. 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 in order to ensure a linear response (example: set 30 mL/min with 600 mL/min of total flow rate). Then set the switching time of the valve to an initial value of 0.5 s. Press the switching control Start button. Monitor the recording window and wait until the cell potential achieves a periodic steady state; then click on the Next button. Register the periodic steady state signal in the new recording window for 60 s. Then, click again on the Next button. Simultaneously with the previous step 6.7, register the periodic oxygen input. Select the Start button in the sensor software, insert a name which recalls the frequency input (example: 1 Hz), and click on OK. Register the signal for 60 s as in the current output case and press the Stop button. Repeat the previous steps 6.6-6.8 at increasing switching time values in order to measure periodic input/output correlations for a frequency range from 8-1000 mHz by taking 8 frequency points per decade. For experiments at a frequency higher than 100 mHz, register input and output for 60 s. At lower frequencies, sample the signals for a range of time equivalent to 5 periods. 7. Analysis of cFRA data Export measured cell potential responses from the Autolab NOVA 2.0 software. In the recording window click on the diagram with the measured periodic steady state cell potential output. Click on the Show Data | Key | Export buttons. Insert a file name which recalls the frequency of the input (Example: 1 Hz) and click on Save. Repeat steps 7.1.1-7.1.2 for each measured cell potential output at each frequency. Open the Matlab scripts FFT_input.mat and FFT_output.mat. In the Address Folder section insert the specifications of the location of the folder where the measured oxygen pressure and current data files are stored. NOTE: The script was written with the aim of performing the windowing of the collected inputs in order to have an integer number of periodic cycles to analyze, and calculate their Fourier transforms accurately and quickly. Any other procedure which performs the same task does not change the results. Run the FFT_PO2.mat and FFT_Pot.mat scripts; check in the plotted diagrams if the computed algorithm works properly (in the time domain, an integer number of input and output cycles should be extracted from the original input and output samples). CAUTION: A Fourier transform based on a non-integer number of periodic cycles could result in misleading analysis of the inputs and outputs resulting in inaccurate cFRA spectra. Open the Matlab script cFRA_spectra.mat and run it. Magnitude, phase angle and Nyquist spectra of the cFRA transfer function under galvanostatic conditions are plotted. NOTE: The script calculates the cFRA transfer function by using the Fourier transform values at the fundamental frequency of the oxygen pressure (inputs) and cell potential (outputs) signal using the following equation.

Representative Results

The preliminary analysis of the fuel cell dynamics based on EIS spectra is shown in Figure 2. EIS magnitude (Figure 2A) and phase Bode plots (Figure 2B) spectra are measured at three different steady state current densities under galvanostatic control. As expected, all main transient processes are observed: the double layer charging/discharging in the high frequency …

Discussion

In contrast to classical EIS, cFRA is a diagnostic tool focused on the characterization of dynamics related to the different mass transport phenomena occurring in the fuel cell. It is not able to detect any transients having a time constant below the oxygen diffusion in the electrode, as for example the charging/discharging of the double layer⁶. Therefore, unlike EIS where several phenomena are coupled, cFRA can help to identify patterns related to specific dynamics more clearly. This would decrea…

Materials

Membrane Electrode Assemby N115 25,8 cm2	QuinTech	EC-NM-115	cathode/anode loding: 1mg Pt/cm2
Potentiostat	Metrhohm	PGSTAT302N
Booster	Metrohm	BOOSTER20A
Retractable fiber oxygen sensor	Pyro Science	OXR430-UHS
Dew Point and Temperature Meter	VAISALA	DMT340
Software process control system	Siemens	Simatic PCS 7
Software MATLAB2012a	Mathworks
Hydrogen	Linde	Hydrogen 6.0
Nitrogen	Linde	Nitrogen 5.0
Oxygen	Linde	Oxygen 5.0