Electrical Conductivity And Corrosion Resistance Of TaN/(Ta,Ti)N/TiN/Ti Multilayer Coatings On Metal Bipolar Plate

Proton exchange membrane fuel cell stacks（PEMFCs）are an efficient and clean energy source that can directly convert hydrogen energy into electrical energy.They have the advantages of low emissions,high energy conversion efficiency and high-power ratio.It is widely used in many fields such as type power stations.The bipolar plate is one of the core components in the PEMFC,which plays the role of supporting the membrane electrode,separating the reaction gas,dissipating the reaction heat,and conducting electrons in the stack.However,the lack of corrosion resistance and interfacial conductivity of stainless-steel bipolar plate material restricts its practical application in PEMFC.To solve this problem,the physical vapor deposition technology（PVD）was used in this paper to systematically study the process parameters of TiN coatings and their influence on the microstructure and corrosion resistance.On this basis,the idea of TaN/（Ta,Ti）N/TiN/Ti multilayer coating was designed and proposed,the influence of deposition process parameters on composition and structure was explored,and the intrinsic relationship between structure and performance was established.The main findings are as follows:The deposition pressure has a significant effect on the crystal growth orientation of the TiN coating:as the deposition pressure increases,the concentration of nitrogen ions increases gradually,resulting in a decrease in the diffusion distance of Ti atoms on the（111）surface,increasing the surface of the substrate.The island-like nucleation increases the deposition rate.At the same time,the chemical potential of the（200）surface is lower than that of the（111）surface,so that the growth of TiN in the[111]direction is inhibited,and the growth is in the[200]preferred orientation.The deposition time mainly affects the crystallinity and coating thickness of the coating.Through a series of potentiodynamic polarization tests,it can be concluded that a certain thickness of TiN coating can significantly improve the corrosion resistance of the substrate,but if the coating thickness is too large,the protective effect will decrease,mainly because the coating It is caused by the excessive growth of the"V"-shaped columnar crystal structure.The optimal deposition process of TiN is deposition pressure 0.5 Pa,deposition time 30 min,deposition power 50 W,and its corrosion current density is 2.5μA/cm².The existence of the transition layer in the TaN/（Ta,Ti）N/TiN/Ti multilayer coating significantly improves the interfacial conductivity of the coating,mainly because the existence of the transition layer reduces the grain boundaries between the layers,reducing the electron scattering phenomenon,thereby improving the interface conductivity of the coating.Its ICR value is only 6 mΩcm² at a pressure of 138 N/cm².The effects of modulation period,deposition time and deposition pressure on coating properties and structure were investigated.As the modulation period increases,the number of coating interfaces increases,which improves the corrosion resistance of the coating,but the ICR value increases.The deposition time mainly affects the thickness of the coating,and the ICR value increases with the thickness of the coating.TaN/（Ta,Ti）N/TiN/Ti multilayer coatings exhibit an ultra-low corrosion current density of 0.369μ·cm^-2(+0.6 V_vs.SCE)and contact resistance under optimal deposition parameters 6 mΩcm²（after 5 hours of potentiostatic polarization at 138 N/cm²pressure）,which has reached the US Department of Energy standard.Electrochemical impedance spectroscopy（EIS）and equivalent circuit models further elucidate that the TaN/（Ta,Ti）N/TiN/Ti coating significantly hinders the oxidation reaction and dissolution of metals and provides good protection for the substrate.