Preparation And Properties Of Ordered Mesoporous Carbon-based Films On The Stainless Steels

Metal bipolar plates develop a passivating oxide layer on the surface that does protect the bulkmetal from progression of corrosion, but also cause an undesirable effect of a high surface contactresistance, especially as the passivating oxide layer is thickened during operation. This causes thedissipation of some electric energy into heat and a reduction in the overall efficiency of the fuel cellpower stack. The problems outlined above may be overcome or minimized by protecting metalbipolar plates from the corrosive fuel cell operating conditions with coatings which should beconductive and adhere to the base metal without exposing the substrate to corrosive media. In thispaper, ordered mesoporous carbon films are first used as protective coating of stainless steel bipolarplates. Ordered mesoporous carbon films are modified by cooperative assembly with othercomponents in nanometer scale. We discuss the formation mechanism of microstructures of compositefilms, put forward a new idea to select an appropriate component as modifier, and analyse theprotective properties against corrosion with samples exposed to sulfuric acid solution to simulateproton exchange membrane fuel cells environment. The main contents are as follows:1. Ordered mesoporous carbon film is prepared on the304stainless steel by the combination ofsolvent evaporation induced self assembly method and spin-coating method, with block copolymerF127as template agent, phenolic resin as carbon source. Structure tests display, ordered mesoporouscarbon films have a high specific area of500⁶50m2g^-1, pore volume of0.4⁰.5cm³g^-1, pore ratioof greater than70%. Compared with304stainless steel, ordered mesoporous carbon film makescorrosion potential shifted to positive direction by100⁴00mV, corrosion current decreased by1order of magnitude, potentiostatic polarization process in the operation pressure of a fuel cell is stable.The corrosion current density can be as low as0.464μA cm^-2, but the contact angle is71°and theelectrical conductivity is0.0043S m^-1. As the heat treatment temperature further rises, the structure ofordered mesoporous carbon film would shrink and generate some cracks, which could lead to theunstabitily of the polarized process, and reduce the charge transfer impedance, speed the diffusion rateof corrosion product. In order to improve the properties of pure mesoporous carbon film, we try to usefour method to modify mesoporous carbon.2. The rigid silica is introduced in the ordered mesoporous carbon film to strengthen the thermalstability and corrosion resistance. The results show that the mesoporous carbon-silica composite filmhas a highly ordered2D hexangular mesoporous structure. From400to700℃, the bore diameter is contracted by12.2%, whereas the pure mesoporous carbon is contracted by14.7%. Scanning electronmicroscope test shows that there is not crack in the surface of composite film. Electrochemical testshows that, the mesoporous carbon-silica composite film has a corrosion current density (0.0975μAcm^-2), which is1/5of the pure mesoporous carbon film. After heat treatment of700℃, potentiostaticpolarization curve of composite film is also very stable, the results show that composite film still has adensity structure after heat treatment of high temperature.3. Ordered mesoporous hybrid carbon films are prepared by tri-assembly with B, P, Ncompounds as dopant source. During the course of high temperature heat treatment, these elements isin favor of transition of phenolic resin from amorphous state to graphite, and can improve theconductivity and hydrophobicity of mesoporous carbon film. Boracic acid also can improve thethermal stability of the film. For example, when heating temperature is500℃, the conductivity of B,P, N doped mesoporous carbon films can reach to0.15,0.21,0.029S m^-1, the contact angles are86,91,80°, respectively, which are significantly higher than that of pure mesoporous carbon film.Potentiostatic polarization test shows, hybrid mesoporous carbon film in0.5M H₂SO₄has a goodstability. Potentiodynamic polarization tests show that the corrosion current density is0.285,0.0299,0.166μA cm^-2, respectively, which means better anti-corrosion properties.4. Ordered mesoporous carbon-tungsten（molybdenum） composite films are prepared by mixingthe the precursor of W or Mo and phenolic resin. As a result of the catalytic graphitization effect oftungsten and molybdenum compounds, the degree of graphitization and conductivity of mesoporouscarbon film can increase with the content of the two element. When heating temperature is500℃,their conductivity reaches to0.82and0.11S m^-1, respectively. Due to the confinement effect ofmesoporous structure, tungsten compounds, composed of tungsten oxide and tungsten carbide, aregrowing into rodlike. Potentiodynamic polarization shows that the lowest corrosion current densityof the mesoporous carbon-tungsten composite film is0.0559μA cm^-2. Ordered mesoporouscarbon-molybdenum composite films have very dense structure, composed of molybdenum oxide andmolybdenum carbide composite. The molybdenum compounds are embedded in the mesoporouscarbon walls, with a particle size of4nm. In the0.5M H₂SO₄, the corrosion potential and corrosioncurrent density of mesoporous carbon-molybdenum composite films are277mV and0.0273μA cm^-2,respectively.5. In order to enhance hydrophobic property and conductivity of mesoporous carbon film,conductive graphene or carbon nanotubes are added to ordered mesoporous carbon in situ. Theconductivity is0.35and0.41S m^-1, the contact angles are90°and96°, respectively. The corrosion current density is0.140μA cm^-2 and0.008μA cm^-2, in simulated work environment of PEMFC.