Literature review of bipolar graphite plates (1) –testing and procedure

Throughout the development of fuel cell technology, many companies and independent laboratories have been improving all aspects of fuel cell components and systems. The specific component analyzed and studied for this thesis is the bipolar plate. While doing research on previous advances of the bipolar plate, a number of aspects were investigated: (1) testing and procedures, (2) materials selection and (3) influence on performance. Within this set of parameters, a number of characteristics were considered:

Every research facility for developing fuel cells has its unique testing apparatus and procedure to perform testing. They all differ so to how many cells and/ or stacks they can operate, the capacity for operating conditions, i.e., temperatures, flowrates, and pressures, and method for evaluating performance. Paganin et al. developed a test station for a unitary fuel cell with an active area of 4.6 cm2. The stand integrates three temperature controllers for both reactant humidification as well as heating of the cell. With proper gaskets, cells may be run at pressurized conditions to enhance cell performance. Galavanostatic control was used in conjunction with a data acquisition system to monitor fuel cell performance with polarization curves.

Davies et al. constructed an endurance test rig that operated eight single cells simultaneously with active areas of 11.8 cm2. The test stand was built using mostly 316 sttainless steel and PTFE to avoid all contamination of reactants entering the fuel cell. Humidification of reactants, as well as cell temperature was controlled. Tests for different monopolar plates were conducted using pressurized fuel cells. To obtain fuel cell performance data, the cell was controlled galvanostatically and the cell potentials and currents were recorded. The fuel cells underwent constant testing with electronic load for 100-hour intervals, then a period of approximately 68 hours under no load conditions and cooling of the cell temperature.

In related work, Davies et al. conducted more testing of monopolar plate materials in similar cell constructions. However, the testing procedure was conducted in a slightly different manner. With the cells being galvanostatically controlled, operation would persist for 5 days at a constant density of 0.7 A/cm2. This time period would be followed by two days of the test stand shut down. Polarization curves were taken every 100 hours during the durability testing using a data acquisition system. Limitations in the test rig required steady flow of gases throughout the duration of acquiring curves.

A single fuel cell with a 7 cm2 active area was tested with different monopolar plate materials by Makkus et al. The set-up allowed for humidified gases to enter the fuel cell at a pressure of 4 bar. With multiple set-ups, several conditions could be run simultaneously to discern the effects of cell and humidifier temperatures on the corrosion resistance of several monopolar plate materials. Cell construction allowed for measurements to be taken for internal resistance as well as cell polarization. To measure the contact resistance between plate and backing layer, a gold wire was inserted between the carbon backing alyer and membrane. As the cell operated, the resistance was calculated by monitoring the voltage drop between the monopolar plate and gold wire. Cell polarization was determined by measuring the current and voltage across the electrodes in contact with the electrically conductive plates.

While looking at stainless steels as a candidate for bipolar plate materials, Makkus et al. suggested several conditions for operation of the fuel cell. Throughout all tests, the cells were subjected to operating conditions closely resembling that of an actual automobile. An alternating load was applied during long term stability tests, as well as shorter tests used for sampling of materials. For the stability tests, the alternating load was applied for 55 minutes at 0.5 V, and then followed by 5 minutes at open circuit voltage. This process produced minimal degradation of the cell over the lifetime of operation. The shorter tests involved 30 minutes of applied load at 0.5 V followed by 30 minutes of no load at open circuit voltage. In both the short and long term tests, the plate material under investigation was inserted in either the anode or cathode of the cell and a standard graphite plate was inserted in the vacant electrode.

Hentall et al. employed a commercially available fuel cell with a 50 cm2 active area while testing a multitude of monopolar plates. Unlike all previous tests stands built, no humidification of the reactant gases was used. Gasketed cells were sealed with silicon in order to operate at elevated pressures. The cells were constructed using Gore Select Primea membranes along with micro-porous diffusion media and a carbon-backing layer. The completed cell was controlled potentiostatically with a separate load unit and additional voltage supplies. The cell polarization was determined via digital multimeters and shunt resistors to produce a current. To determine the effects of contact resistance between plates and MEA, two sheets of carbon diffuser paper separated the structures and the voltage drop between them was measured.

To obtain consistent data for each material investigated, Hentall et al. used the same procedure for every new cell. The cells were operated at 0.5 V while it reached maximum operating temperature and then cooled to ambient temperature. This increase and decrease in temperature was repeated several times until consistent operation was monitored. Hentall et al. began using graphite as baseline and inserted the monopolar plates in both the anode and cathode. All new materials under investigation were inserted in pairs and operated at the same conditions as the baseline.

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