Conclusions & recommendations of graphite bipolar plates for fuel cell performance.

Throughout the duration of this research, several milestones have been reached to set the stage for further bipolar plate development. A fuel cell test stand have been designed and built to run six single fuel cells for monopolar plate screening and long-term testing. With relatively unsupervised operation, the test stand can control over 1000-hour tests to determine how monopolar plates perform in the fuel cell environment. The stand is used to monitor performance degradation over time as well as subject conductive metals to a fuel cell environment to observe resistance to corrosion. Steady operating conditions were determined to ensure that proper and consistent parameters are set for every test that was conducted. These conditions will also be used for upcoming novel composite monopolar plates undergoing 1000-hour long-term testing.

A baseline model has been established for the industry standard monopolar plate manufactured from CFCCARBON graphite. Six individual cells were tested with the cfc graphite plates and the performance was monitored for over a total of 5,000 hours. Some cells displayed excellent performance over time while others decayed much quicker to unacceptable operating levels. Significant powers were reached by several cells nearing 2.5 Wats. This level of performance is extraordinary for a cell operating at atmospheric pressure and with only an active area of 5 cm2. A performance evaluation was conducted to see trends in power over time at 0.7 V, 0.65V and 0.6V. This power curve can be used to compare new monopolar plates manufactured with novel composite materials.

In order to preserve the lifetime of the fuel cells and membranes, several adaptations could be made to the fuel cell test stand. It was possible that during unsupervised operation that water from the humidifier could have entered the cell due to excessive bubbling in the sight glass. This water would enter the cathode of the fuel cell and possibly damage the thin membrane due to flooding. A possible solution would be to integrate a water trap device into the cathode inlet line of the fuel cell. A water trap would ensure that the humidified reactant air could still enter the cell without condensation occurring but eliminate the possibility of unnecessary abundance of water entering the cell and damaging the internal materials.

A main concern for the longevity of the fuel cells in the failure of the temperature controllers integrated into the design of the test stand. Several times the switch within the temperature controller has failed because of excessively operation and thus causing continuous current to be passed to the fuel cell silicone heaters. To alleviate this problem, an operating parameter for the temperature controller allows for setting the control period. The CP affects how often the switch opens and closes while the PID controller is engaged. The current setting is at 10, so increasing the control period should increase the lifetime of the temperature controller. However, there may be more oscillation in the actual temperature of the fuel cell due to the lack of response time from the controller. The resulting profile at a setting of 70C set point would give a tolerance of +/- 1-2C. This would be deemed acceptable in order to prolong the life of the fuel cell and have less worry that the cell would fail before the 1000-hour goal tests were reached.

A less crucial matter for operating the test stand more effectively is possibly redesigning the water feed system to refill the humidifiers. Currently, one pump is used to feed deionized water through a common spine to the six humidification chambers. The water is fed to each humidifier via needle valves that allow for water to enter when opened. Due o pressure gradients from one chamber to the next, only one needle valve can be opened at a time so air does not rush back into the spine of the feed system. Therefore, the pump can distribute water to only one chamber at a time and cannot be run continuously for the duration of testing. Since the amount of water in the chamber lasts for nearly 12-14 hours of continuous operation, the humidifiers need to be refilled twice daily. In order to alleviate this time consuming refill process, a slight redesign with check valves placed strategically at the inlet of the humidification chamber and the outlet of the sight glass could possibly aid the water feed system. This would allow for only water and air flow in the expected directions and not cause flooding or air pockets in any of the water feed lines.

After all the baseline testing was completed, the MEAs from each fuel cell were collected and sealed for post-mortem analysis. Further studies can be done on the membrane and backing layers to investigate any phenomena that occur during rigorous testing in the fuel cells. Further analysis may unveil evidence as to why some cells outperformed others or why some cells performance dropped faster than expected. Until the membranes have been studied, only discussion conveying what caused the profile of performance for the baseline is available. Hopefully, new discoveries can be made that will lead to improvements in the development of novel monopolar plates and fuel cell operation in the future.

Several observations can be made for the advancements in future research by analyzing the novel composite bipolar plates. Preliminary composite plates have been manufactured usning the injection-molding machine including samples made from K-109 aluminum flakes, carbon and polypropylene resin, as well as all carbon and the resin matrix. After handling the plates for some time, a glittery residue of aluminum flakes became apparent on materials in contact with the plate. This could be troublesome when the plates are inserted and operated in the fuel cells. Particles could leach out of the plate and become embedded in the diffusion media as well as the membrane. Excess reactant flow and water removal could cause an abundance of flakes to build up in the flow channel or outlets, thus blocking crucial pathways for proper fuel cell operation. If aluminum proves to surpass the baseline model in performance, and the aforementioned problems arise, a conductive and protective coating could be used to ensure no flakes are expelled into the fuel cell and negatively alter the performance.

 

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