Preparation,Structure And Properties Of NbN-based Protective Coating For Stainless Steel Bipolar Plate Of Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cell(PEMFC)has attracted extensive attention from scholars and enterprises around the world due to such merits as small volume,high energy density,zero-emission,quick launch,low operating temperature.Bipolar plate is one of the key components of PEMFC,it's crucial to develop a bipolar plate material with high quality,low cost and stable performance.Stainless steel is regarded as one of the ideal material for bipolar plate due to its pretty good machinability,thermal conductivity,corrosion resistance and electric conductivity.But the unsatisfactory stability and contact resistance of stainless steel in an acid environment restrict its application,so the surface modification process is necessary to improve the performance and durability of the stainless steel.In this study,a series of Nb N-based protective coatings,including Nb N,Nb-Zr-N,Nb/Nb N multilayer coating and Zr N coating,were deposited by multi-arc ion plating on 316L stainless steel,at different deposition parameters.The chemical composition,microstructure and surface morphology of the coatings were investigated and the corrosion resistance and electric conductivity of the coatings were evaluated by electrochemical and interfacial contact resistance measurements in simulated PEMFC environment.The research results obtained in this paper are as follows:(1)A series of Nb N coatings were deposited at different nitrogen partial pressure(P_N2)using multi-arc ion plating.The results show that the improvement and difference of coating properties are the results of the coordination of phase composition,microstructure and surface morphology.The variation of P_N2 has no significant influence on the chemical composition of the coatings.With the increasing P_N2,the phase structure of the coatings was transfer to?-Nb N from?-Nb₂N/?-Nb N phases with grain refinement,the surface roughness of the coating was decreased simultaneously.The performance and durability of the Nb N coating was improved significantly compared with that of the bared substrate in simulated PEMFC environment.The coating deposited under relatively high nitrogen partial pressure showed better properties which exhibited great potential as the protective coating for stainless steel bipolar plate.(2)To obtain good performance coating materials with more economy,a series of Zr N coatings were deposited at different nitrogen partial pressure using multi-arc ion plating.The results show that the different phase composition and surface morphology among coatings influence on their properties.The variation of P_N2 has no significant influence on the chemical composition of the coatings.With the increasing P_N2,the phase structure of the coatings was transfer to Zr N from Zr N/Zr₃N₄ phases,and the surface roughness of the coating was decreased simultaneously.The performance and durability of the Zr N coating was improved compared with that of the bared substrate in simulated PEMFC environment.The coating deposited under relatively high P_N2 showed better properties.The corrosion resistance of Zr N has already met the requirement of bipolar plate but the ICR still not.Zr N coatings are not suitable as the protective coating for stainless steel bipolar plates directly.(3)To investigate the effect of solid solution element on the properties of the coating,a series of Nb-Zr-N coatings were deposited using multi-arc ion plating by different arc currents of Zr targets.The coatings was(Nb,Zr)N solid solution phase.The content of Zr increased firstly and then decreased,and the content of Nb changed with a contrary tendency.The surface roughness of the coating was increased with increasing arc currents.The performance and durability of the Nb-Zr-N coated 316L stainless steel were improved compared with that of the bared substrate in simulated PEMFC environment.Nb-Zr-N coating deposited using relatively low arc currents exhibited better performance.However,the insufficient ICR before and after corrosion of the coatings makes it hard to be a satisfactory protective coating for stainless steel bipolar plate.(4)To investigate the effect of multilayer structure on the properties of the coating,a series of Nb/Nb N multilayer coatings were deposited using multi-arc ion plating based on the previous deposition parameters.The results showed that the multilayer structure enhance the corrosion resistance but is harmful to ICR of the coating.The coatings mainly composed of?-Nb N and?-Nb phases,and Nb₂N phase existed in the Nb/Nb N interface.The columnar crystal growth was inhibited due to the multilayer structure.The coating with Nb N surface had lower roughness than that of the coating with Nb surface.The performance and durability of the Nb/Nb N multilayer coating were improved compared with that of the bared substrate in simulated PEMFC environment.Coating with less interface and Nb surface revealed better ICR,while more interface and Nb N surface exhibited better corrosion resistance and ICR stability.The adopted multilayer structure showed negative effects on the ICR of the sample.Nb/Nb N multilayer coating exhibited potential as the modified coating for stainless steel bipolar plate,but still need to further decrease its ICR.(5)By the comprehensive comparison,Nb N coating deposited at high P_N2exhibited better performance among all coatings in the research which showed most potential as a protective coating material for stainless steel bipolar plate.