Gradient Structured Composite Bipolar Plates for Proton Exchange Membrane Fuel Cells

Carbon/polymer binder composite bipolar plates for proton exchange membrane fuel cells have attracted attention for their corrosion resistivity and ease of manufacturing. However, the hot-pressing fabrication process often leads to a resin-rich surface on the plate, which may increase the interfacial contact resistance. A number of studies have been reported to reduce resin accumulation on the surface. However, most reported methods have focused on how to exceed the United States Department of Energy conductivity and mechanical targets. Their fabrication methods are often complicated, which is hard to form flow channels directly in manufacturing. Herein, we propose a simple layer-by-layer method to prepare graphite/resin composite bipolar plates. During compression of the multilayer graphite/resin powder, elevated temperature and pressure make the resin in the inner polymer-rich layer flow toward the outer resin-deficient layers. Scanning electron microscopy with energy-dispersive X-ray analysis shows an unbroken "smiling" curve of carbon content along the whole cross-section, suggesting a continuous distribution of the resin across the whole composite. The resulting composite demonstrates a better interfacial contact resistance (9.2 m omega cm2@100 psi) and a flexural strength (44.3 MPa) than the plates prepared by a single-layer method. Flow channels with a good molding accuracy (+/- 4 mu m) are also observed. Single-cell testing with the gradient-structured composite plate shows excellent cell performance: the cell voltage is 0.67 V at 1 A cm-2, and the maximum power density reaches 0.99 W cm-2. In situ electrochemical impedance analysis demonstrates that compared to an analogue using the single-layer loading method, the cell with gradient structured plates displays lower ohmic and mass transfer resistance than the cell using the plates prepared by a single-layer method.