Preliminary testing – troubleshooting fuel cell operation

The monopolar plates that will be tested throughout the research must be subjected to a steadily operating fuel cell environment. The tests will be conducted for 1000 hours to determine if the performance of the fuel cell degrades and to observe corrosion in the monopolar plates. Since the tests will run for such a long period of time, it is necessary to troubleshoot the fuel cell operation and determine operating conditions that allow for steady 1000 hour tests. Once the cell has been built, the tests will continue until 20% degradation in performance is observed regardless of hours logged.

The goals of these tests are to study the corrosion effects of monopolar plates in the fuel cell operating environment and to verigy any performance degradation over time. The cells are constructed using industry standard graphite plates. For preliminary testing, a square or round-edged flowfield is available to compare the performance with different channel designs. For the following tests, the square-edged flowfield machined in the graphite used. Gore membranes are standard in the cell, and gas diffusion media are exclusive to Spectracorp’s 2050 and 2050-A vesions. The cell are tested at ambient pressure (1 atm). To influence cell performance, the remaining parameters can be controlled:

  • Cell and humidifier temperature
  • Air and hydrogen stoichiometries
  • Resistive load using a potentiometer

The test stand is fully operational and can operate a fuel cell when the leads from the stand are connected to the electrodes of the cell. To ensure proper operation of the fuel cell, the air and hydrogen stoichiometries should be set to approximately 2 and 1.5, respectively. A stoichiometry of 1 for the air is only necessary for one unit of fuel to be completely reacted. Excess air and fuel are supplied to ensure that throughout the entire range of loads the cell is not starved of reactants. At higher loads, more current is drawn from the cell; therefore more gases are consumed. These higher stoichiometries would allow for maximum current densities at all operating conditions. However, water management, flooding, and steady voltage readings at different loads always need to be monitored. Flooding occurs when water accumulates in the plate’s flowfield or gas diffusion layer thus eliminating the chance for the electrochemical reaction to occur throughout the cell. Thesefore, the stoichiometries may be adjusted upward to account for these important factors.

Preliminary tests were run with air stoichiometries between 2 and 4, and hydrogen flow rates were set such that a steady stream of hydrogen bubbles in water were visible from the anode exhaust. As the load on the cell increases, there is a corresponding decrease in unreacted gases exiting the cell. Therefore, steady bubbles in water were deemed necessary fro maximum load on the cell to ensure that all the reactant gas is not being consumed during the electrochemical reaction.

The temperature of the cell and humidifier were varied between 70C and 40C, in order to monitor the bes performance for operation. However, to observe the best corrosive data, the fuel cell should be run at a higher cell temperature. The main limitation is the structural integrity of the MEA, which becomes damaged at temperatures exceeding 85C.

The first fuel cell test was run with Tc and Th both equal to 60C. With the load turned off, open circuit voltages between 0.85 Volts nad 0.88 Volts were reached. Although these readings were indicative of the fuel cell working, better readings near 0.95V were expected. Throughout several hours of testing at medium load, it appeared that the cell was flooding due to an abundance of water being supplied in the cathode air stream as well as water being produced in the fuel cell due to the electrochemical reaction. The flooding condition corresponded to a sudden drop in voltage. As this voltage drop occurred, a large droplet of water filled the cathode exhaust and blocked the pathway for air to exit. This lack of airflow could also be seen on the rotameter as the air flowrate dropped to nearly zero.

 

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