Controlling the hydrophilicity and contact resistance of fuel cell bipolar plate surfaces using layered nanoparticle assembly

Hybrid nanostructured coatings exhibiting the combined properties of electrical conductivity and surface hydrophilicity were obtained by using Layer-by-Layer (LBL) assembly of cationic polymer, silica nanospheres, and carbon nanoplatelets. This work demonstrates that by controlling the nanoparticle zeta (zeta) potential through the suspension parameters (pH, organic solvent type and amount, and ionic content) as well as the assembly sequence, the nanostructure and composition of the coatings may be adjusted to optimize the desired properties.;Two types of silica nanospheres were evaluated as the hydrophilic component: X-TecRTM 3408 from Nano-X Corporation, with a diameter of about 20 nm, and polishing silica from Electron Microscopy Supply, with diameter of about 65 nm. Graphite nanoplatelets with a thickness of 5~10nm (Aquadag RTM E from Acheson Industries) were used as electrically conductive filler. A cationic copolymer of acrylamide and a quaternary ammonium salt (SuperflocRTM C442 from Cytec Corporation) was used as the binder for the negatively charged nanoparticles. Coatings were applied to gold-coated stainless steel substrates presently used a bipolar plate material for proton exchange membrane (PEM) fuel cells.;Coating thickness was found to vary nearly linearly with the number of polymer-nanoparticle layers deposited while a monotonic increase in coating contact resistance was observed for all heterogeneous and pure silica coatings. Thickness increased if the difference in the oppositely charged zeta potentials of the adsorbing components was enhanced through alcohol addition. Interestingly, an opposite effect was observed if the zeta potential difference was increased through pH variation. This previously undocumented difference in adsorption behavior is herein related to changes to the surface chemical heterogeneity of the nanoparticles.;Coating contact resistance and surface wettability were found to have a more subtle dependence on the assembly sequence and coating composition. Various LBL assembly sequences were investigated to control heterogeneous coating nanostructure and tune their hydrophilic and electrically conductive properties. Assembly from mixed nanoparticle suspensions yielded competitive nanoparticle adsorption and is denoted as cLBL assembly. The absence of intervening polymer binder during sequential deposition from first carbon then silica nanoparticle suspensions directed the assembly process with each applied layer and is denoted as dLBL assembly. Use of intervening polymer binder as in standard LBL deposition is denoted as sLBL assembly. The cLBL assembly sequence was found to yield nanoparticle competition for available surface sites between the heterogeneous nanoparticles and result in phase separation within each layer, producing coatings with high electrical contact resistance but poor hydrophilicity. Coatings prepared using dLBL assembly exhibited improved contact resistance due to improved alignment of a carbon phase perpendicular to the substrate surface but continued poor hydrophilicity. The sLBL assembly scheme produced optimum coating performance due to the adsorption of highly dispersed silica layers directly onto the graphite platelets, while maintaining through-plane platelet to platelet contact. The wetting behavior of the prepared coatings was satisfactorily described by Johnson-Dettre model while exhibiting little response to changes in surface morphology (in contrast to Wenzel's equation). Hydrophilicity of the cLBL assembled coatings could be enhanced by altering the silica nanoparticle zeta potential in mixed suspensions.;Coating durability was demonstrated through LBL assembly onto industrial-size bipolar plate materials and testing under PEM fuel cell operating conditions.