Design, fabrication and testing of graphite bipolar plates for direct methanol fuel cells by indirect laser sintering
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Direct Methanol Fuel Cells (DMFCs) are electrochemical energy conversion devices that convert chemical energy into electrical energy. The bipolar plate component of the DMFC is required to be fluid impermeable to prevent fuel leakage and electrically conductive to collect the electrons produced within the cell. Graphite possesses the properties of high electrical conductivity, low weight and resistance to corrosion that make it an attractive material for bipolar plates. However, the poor mechanical properties of graphite lead to prohibitive machining costs and increased production times. The objective of this research is to develop an indirect laser sintering (LS) process, involving the laser sintering of graphite powders mixed with a phenolic resin binder which offers the advantage of complex part production and testing of prototype bipolar plates in short times. Due to the nature of the indirect LS process, the as-produced (green part) plates are porous and possess low electrical conductivities (< 0.1 S.cm-1). This research describes a viable method to rapidly fabricate and test multiple graphite bipolar plate designs using indirect LS. This process involved identifying and selecting suitable graphite powder and binder systems based on their thermal and electrical properties and developing a post process heat treatment method for achieving electrical conductivity of 250 S/cm for LS graphite parts which exceeds the DOE target of 100 S/cm for bipolar plate materials. The post processing also covered a method of infiltration using cyanoacrylate which was capable of rendering porous brown parts fluid impermeable and suitable for use in DMFCs. The cyanoacrylate infiltrated LS graphite parts were characterized for flexural strength and electrical and thermal conductivities and bipolar plates were made and evaluated in a DMFC test stand. Various flow field designs including plates with varying channel and rib widths and triangular, elliptical and rectangular flow field cross sections were fabricated using indirect LS and their respective polarization curves were compared to commercially machined graphite plates. The fuel cell tests show the improvement in mass transport performance could be due to improved methanol distribution and water removal characteristics of triangular and elliptical cross sectional channels over rectangular channels of equivalent dimensions.