Investigation of graphite bipolar plates for PEM fuel cell performance

The largest cost in manufacturing PEM fuel cells for automotive applications is due to the bipolar plate. The current graphite material used for the bipolar plate is very brittle and difficult to machine to the rigorous specifications needed for fuel cell stacks. The paper introduces the development of a fuel cell test stand for simultaneous testing of six individual fuel cells. To establish a long-term performance evaluation, the fuel cells incorporate a baseline graphite material that undergoes testing in the fuel cell environment. The graphite is an industry standard material that should not corrode when subjected to continual testing. The baseline model will be used in development of novel composite materials that will be tested under the same conditions for comparison to the graphite. Furthermore, the new materials and applied manufacturing methods could reduce the overall cost of fuel cell stacks in the future.

Over the past decade, rapid progress has been made in the understanding and development of proton exchange membrane (PEM) fuel cells. PEM fuel cells have emerged as a viable energy source for use in the automotive industry and eventually may replace the internal combustion engine. With its ability to attain high power densities and perform with elevated efficiencies, the PEM fuel cell is at the forefront of technology advancement for automotive application. Several key benefits arise from the use of fuel cells with hydrogen as the reactant fuel. Harmful air pollutants and greenhouse gases would be reduced, thus meeting more stringent limits on emissions set forth by the CARB. Also, the increase in hydrogen use as the main fuel source for light duty vehicles could have a corresponding decrease in foreign fuel dependency. However, for fuel cells to the mass-produced and commercially available, the current cost of production needs to be lowered significantly.

Fundamental aspects of the fuel cell have been researched including catalyst loading, membrane and backing construction, and electrodes. However, alternative materials and manufacturing techniques for bipolar plates need to be investigated. To penetrate the automotive industry and ultimately replace the internal combustion engine, fuel cell stacks need to be mass-produced at costs near $25-50/kw. Currently, bipolar plates are made from machined graphite. Although decent performance and corrosion resistance has been proven, graphite bipolar plates are too brittle, heavy, expensive and difficult to machine.

Properties including good electrical conductivity and corrosion resistance make graphite plates a baseline for comparison to investigation of novel materials. However, further requirements for bipolar plates include ease of manufacturability, high electrical conductivity, and thin and lightweight plates, which can yield a higher power density. Possibilities to achieve all of these characteristics lie within using composite materials or other conductive metals. From previous research, a matrix of samples including analysis of aluminum, stainless steels and titanium have been proposed for manufacture and in-cell testing. The composite monopolar plates are injection molded and contain a mixture of polypropylene resin and the conductive filler in question.

The objective of the work being documented in this thesis is to establish a baseline model for fuel cell performance with graphite bipolar plates. The plates being tested are manufactured with industry standard Poco graphite that shown no evidence of corrosion throughout previous operation and testing while being exposed to the fuel cell environment. The baseline is achieved by placing a set of monopolar graphite plates into six single cell test fixtures and monitoring the performance over time for 1000 hours. The six cell are used to record reproducible data as well as for statistical purposes to obtain an average performance over time for the materials under testing. The fuel cell performance average acquired with the graphite plates can be compared to the performance of novel materials that are tested under the same conditions.

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