1. Selective Laser Sintering of Graphite Composite Bipolar Plates for PEM Fuel Cells Nannan Guo, Ming C. Leu Center for Aerospace Manufacturing Technologies, Department of Mechanical & Aerospace Engineering Missouri University of Science and Technology, Rolla MO, USA. Background Comparison with Conventional Fabrication Methods Research Objectives This research was supported by Air Force Research Laboratory under contract #FA8650-04-C-5704. Future Work Establish the fabrication process based on Selective Laser Sintering (SLS) for graphite composite bipolar plates; Investigate different flow field designs (including bio- mimetic designs) for bipolar plates to improve the performance (energy density) of fuel cell;.Study the effect of different graphite materials on the properties of bipolar plates fabricated by SLS. Fabrication and Post Processing As shown in the table, the electrical conductivity and flexural strength of bipolar plates fabricated by the SLS process are comparable to those of bipolar plates manufactured by injection molding or compression molding. Furthermore, the properties of bipolar plates fabricated by SLS satisfy the targets set by Department of Energy. Synthetic Graphite (SG) As the percentage of SG increases, electrical conductivity decreases and flexural strength slightly decreases. Carbon Fiber (CF) Adding CF increases flexural strength but decreases electrical conductivity. Effect of Different Graphite Materials on Properties Proton Exchange Membrane (PEM) Fuel Cell (I) PEM fuel cell is a promising candidate as a zero- emission power source for transport and stationary applications, having the advantages of:  Low-temperature operation (50-100 o C)  High power efficiency (40%-60%)  Fast start-up  Low green gas emissions (only water) How does PEM fuel cell work?  At the Anode: 2H 2  4H + + 4e -  At the Cathode: O 2 + 4H + + 4e -  2H 2 O Proton Exchange Membrane (PEM) Fuel Cell (II) Applications  Potable electronics  Fuel cell vehicles  Power plants  Off-grid power supply  Emergency power system Components  Bipolar plates, gas diffusion layer (GDL)  Catalyst layer, membrane Graphite and binder were mixed together Green parts were fabricated by SLS Carbonization: convert binder to carbon, increase carbon content and electrical conductivity Infiltration: make bipolar plate gas impermeable Bipolar Plate An important part of fuel cell  60-80% of total weight  40-50% of total cost Functions  Collect current from the cell  Provide support for the membrane  Distribute gas fuels within the cell Requirements (set by Department of Energy) Methods for Making Graphite Composite Bipolar Plates Conventional methods (Compression Molding & Injection Molding)  Mass Production  High cost and time consumed for Research & Development stage (R&D) Selective Laser Sintering (SLS)  Additive Manufacturing technique, suitable for R&D of bipolar plates  Advantages  Build complex geometries ---> gas flow channels  Easily investigate different designs and graphite materials Test the performance of the SLS fabricated bipolar plates in a PEM fuel cell stack; Experimentally study the performance of different flow field designs in order to optimize the design. Methodology Bio-mimetic Design  Inspired from tree leaf veins, which distribute nutrition more uniformly and efficiently within the entire leaf.  Just like leaf veins, we want the flow channels to feed the whole membrane uniformly and efficiently. Material composition: 45%NG+10%CF+10%SG+35%binder Electrical conductivity: 120 S/cm : ~40 MPaFlexural strength: ~40 MPa Carbon Black (CB) Adding CB reduces electrical conductivity due to CB’s lower conductivity compared with nature graphite (NG). Flexural strength decreases slightly with increase in CB. In conclusion, nature graphite provides the highest electrical conductivity and high flexural strength; CF increases flexural strength; both SG and CB lower electrical conductivity and also have slightly negative effect on flexural strength. Flow Field Designs and FEM analysis Electrical conductivity Flexural strength Comparison of Bio-mimetic and Conventional Design  FEM analysis Compared with the conventional serpentine design, bio-mimetic design has lower pressure drop from inlet to outlet and more uniform pressure distribution, which enhance the energy density of fuel cells. SLS Fabricated Bipolar Plates Serpentine design: active area 50×50mm 2, channel width 1.5mm, depth 1.5mm Bio-mimetic design: active area 50×50mm 2, channel width 2, 1.5, 1mm, depth 1.5mm *In all the experiments, SG, CF and CB were mixed with NG in different volume ratios and then with 35 vol% binder.