Layer by layer assembly of hybrid nanoparticle coatings for proton exchange membrane fuel cell bipolar plates

Optimum proton exchange membrane (PEM) fuel cell operation at low reactant gas stoichiometry requires that bipolar plate surfaces combine negligible electrical contact resistance with a high degree of hydrophilicity. Unfortunately, no single material can simultaneously satisfy both requirements. In the present work, we demonstrate that electrostatic layer by layer (LBL) assembly may be employed to design hybrid coating architectures composed of 5–10 nm thick graphite platelets and 19 nm diameter silica nanospheres. The strong cationic polyelectrolyte, acrylamide/β-methacryl-oxyethyl-trimethyl-ammonium copolymer, is used to deposit the two anionic nanoparticles from both discrete and mixed aqueous suspensions onto gold substrates. Low contact resistance is achieved by maintaining connectivity of the graphite nanoparticles throughout the coating thickness while good hydrophilicity is achieved by controlling graphite surface domain size. For ∼100 nm thick coatings, contact resistances as low as ∼4 mΩ cm2 may be obtained (comparable to that of a pure graphite platelet coating) while maintaining an advancing contact angle of ∼20° (comparable to that of a pure silica nanoparticle coating). This result represents an order of magnitude reduction in ohmic power loss for state-of-the-art PEM fuel cells relative to the use of pure silica nanoparticle coatings. While LBL assembly is a well-established technique for producing thin, layered structures of nanoparticles and polyelectrolytes, this work provides a unique architectural methodology by which domain distribution in heterogeneous nanoparticle coatings may be controlled.