| Reducing the exhaust emission of ships is the important contents of annex VI in MARPOL73/78convention, aiming to improve the environments of ocean and port. Polymer electrolyte membrane (PEM) fuel cells, which can be assembled into high-power stacks, have gained extensive attention as new power sources and electric stations for ships due to their relatively simple operating mechanisms, high efficiency and low emissions. Bipolar plates are one of the most crucial components, constituting the dominant share of the total weight and the total cost of the high-power fuel cell stack. Stainless steels are potential candidates for bipolar plate materials to replace the traditional graphite bipolar plate. However, bare stainless steels can not be successfully applied into a commercial PEMFC in terms of corrosion resistance and interfacial contact resistance. To address the insufficient corrosion resistance and high surface resistivity of stainless steel, a surface modification technique-plasma surface alloying method was utilized to make the surface modification of the commercial AISI304stainless steel (304SS). Following are the main study and results:Considering the good corrosion resistance of transition metal in acid environment, the tungsten, molybdenum and niobium alloying diffusion layer was respectively prepared on surface of304SS by plasma surface alloying method. The three modification layers are uniform in thickness, dense in microstructures without pinhole, micropore and microcracks, and well in metallurgical adhesion to the304SS substrate with no interfacial defects. The corrosion resistance were investigated and evaluated in simulated PEFMC environment (0.05M H2SO4+2ppm F" solution at70℃, purged with H2to simulate the anodic environment and purged with air to simulate the cathodic environment). The results showed that the modification layers were passivated in PEMFC environment. The corrosion resistance of stainless steel was affected by the composition of the modification layer, and the niobium alloying diffusion layer greately improved the corrosion resistance and stability of304SS.Transition metals nitrides and carbides usually have high electrical conductivity and good corrosion resistance, which can be made up the disadvantages of transition metal. On basis of the transition metal alloying diffusion layer, the niobium nitrid and niobium carbide alloying diffusion layers were respectively prepared on surface of304SS by introducing nitrogen or carbon containing gas to the plasma atmosphere. The niobium nitride alloying diffusion layer was comprised of β-Nb2N、δ-NbN and δ-NbN with a niobium nitride surface layer (8～9μm) and a Nb and N diffusion solid solution subsurface layer (1～2μm); The niobium carbide diffusion layer with a cubic NbC phase was comprised of surface layer (～6μm) and a Nb and C diffusion subsurface layer (-1μm). In simulated PEMFC environments, the compound modification layer improved the corrosion resistance, reduced the passivation current density and interfacial contact resistance (ICR), and increased the hydrophobicity of304SS. The niobium nitride and carbide diffusion layer considerably improves the corrosion resistance of304SS, which reduced the corrosion current density to0.127μA cm-2and0.058μA cm-2in simulated PEMFC environment, respectively. Moreover, the ICR of Nb-C304SS kept at9.04mΩ cm2after10h potentiostatic tests fulfilling the requirement of DOE in2015.The ICP-AES was used to detect the dissolution of metal ions in the corrosion solutions and in combination with the XPS analyses, the influence of corrosion environments on the composition, structure and thickness of passive film formed on304SS before and after surface modification were further discussed. The results reveal that the corrosion environments affected the composition and structure of passive film, which directly decided the corrosion resistance and conductivity of the modification layer. The passive film formed on the niobium carbide diffusion layer was composed of niobium oxide as well as NbC benefiting the improvement of surface conductivity.Besides, the performances of bipolar plate were investigated in aqueous acid methanol solutions with varied methanol concentrations. It was found that the corrosion resistance of the304SS (before and after surface modification) was better when the methanol content is higher. The passive films formed on Nb-C304SS were n-type semiconductor, while those formed on304SS were composed of a duplex electronic structure with an external n-type semiconductor layer and an internal p-type semiconductor layer. |
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