Development of Composite Materials for Pefc bipolar Plates
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ABSTRACT The most costly and problematic components of fuel cell stacks may be bipolar plates. Besides meeting cost constraints, bipolar plates must possess a host of other properties including low gas permeability, corrosion resistance, low weight, high strength, and manufacturability. While we have developed a baseline composite material that meets these requirements, further optimization and development have resulted in significant improvements in properties and processability. INTRODUCTION Polymer electrolyte fuel cells (PEFCs) are attractive alternatives to traditional energy sources for both stationary and transportation applications, provided they can be manufactured at a reasonable cost. Thus the search for suitable, low-cost bipolar plate materials is a key element of PEFC stack development. While cost is a major factor in material selection, meeting the host of technical requirements presents quite a challenge as well. Bipolar plates must be electrically conductive, corrosion resistant, relatively impermeable to gases, easily mass produced, and sufficiently strong and/or flexible to withstand stack assembly. For transportation applications, thin cross-sections (for high power density) and low weight are also essential. We have developed composite materials that meet the property, processability, and cost requirements for PEFC bipolar plates. These composite materials, based on graphite powder in a thermosetting vinyl ester matrix, offer several advantages over existing bipolar plate technologies.
Graphite/vinyl ester composites are less brittle than machined graphite and present fewer corrosion concerns than stainless steel. As with other composites, fluid flow fields can be molded directly into plates during fabrication, but composites based on thermosetting resins such as vinyl esters offer substantially shorter process cycle times than thermoplastic-based composites. The light weight, low cost, and commercial availability of the raw materials are also advantageous. With a "baseline" material already identified [I], we describe here the ongoing development and optimization of graphite/vinyl ester composites for PEFC bipolar plates. EXPERIMENT Although most of the specific formulations are proprietary to Premix, Inc., all of the materials studied were based on synthetic graphite powder (Asbury 4012) embedded in a thermosetting vinyl ester resin matrix. The liquid component of the compound is first formulated by incorporating various additives (e.g., catalyst, inhibitor, mold release, thickener) into the resin. Then the liquid is mixed with the graphite powder (and any desired fiber reinforcements) to form a compound with a friable consistency. To form a plate, the compound is compression molded at ca. 1000-2000 psi. Typical cure times of less than 5 minutes can be achieved with an appropriate selection of temperature and resin/catalyst system. 247
Mat. Res. Soc. Symp. Proc. Vol. 575 © 2000 Materials Research Society
The properties of molded plates were evaluated using a variety of methods. Mechanical
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