(146c) Layer by Layer Assembly of Multi-Functional Graphite/Silica Nanoparticle Coatings for PEM Fuel Cell Applications

Layer by layer (LBL) assembly of chemically heterogeneous nanoparticles is investigated as a method to fabricate proton exchange (PEM) fuel cell bipolar plate composite coatings which are both hydrophilic and electrically conductive. The strong polyelectrolyte, acrylamide/β-methacryl-oxyethyl-trimethyl-ammonium copolymer, is used to adhere ~10 nm thick graphite platelets which are electrically conductive yet hydrophobic and ~20 nm diameter silica nanoparticles which are hydrophilic yet electrically insulating onto gold-coated bipolar plate surfaces. Various LBL assembly schemes are explored to obtain a detailed understanding of multilayer formation and functional performance. The optimum assembly scheme employs cationic binder between the graphite and silica nanoparticles to yield porous silica nanoparticle layers deposited directly onto the graphite platelets. For an ~100 nm thick coating, the electrical contact resistance is ~4 mΩ-cm² (comparable to that of a pure graphite platelet coating) while maintaining an advancing contact angle of 20^o (comparable to that of the pure silica nanoparticle coating). Such behavior represents an order of magnitude reduction in the power loss in state-of-the art proton exchange membrane (PEM) bipolar plate fuel cell applications relative to the use of comparably thick pure silica nanoparticle coatings. In addition to such performance gains, LBL assembly of this heterogeneous graphite/silica nanoparticle system is shown capable of de-convoluting the effects of surface chemistry from those of morphology on dynamic surface wetting.