Study On Surface Modification Of Titanium Bipolar Plates For Proton Exchange Membrane Fuel Cells

As the core component of proton exchange membrane fuel cell（PEMFC）,the performance of bipolar plate has great influence on the output power and service life of PEMFC.Titanium bipolar plate is widely used because of its low cost,light weight and other advantages.However,the oxide formed on the surface of titanium bipolar plate in the working process will significantly increase the contact resistance,which will lead to a significant deterioration of the battery performance in long-term operation,and causes the battery output power to drop.Therefore,improving the corrosion resistance of titanium bipolar plate is of great significance for the application and development of PEMFC.In this paper,Aluminum-doped Zn O（AZO）and Indium-doped Tin Oxide（ITO）coatings were prepared on the surface of TA2 titanium bipolar plate by sol-gel method.After comparative analysis,graphene was added to improve the properties of titanium bipolar plate.Scanning electron microscope（SEM）,atomic force microscopy（AFM）,X-ray diffraction（XRD）,Raman spectroscopy and X-ray photoelectron spectroscopy（XPS）were used to characterize the surface morphology and phase structure of the coatings.The adhesion between the coatings and the substrate and the hydrophilicity of the coatings were tested by adhesive tape method and water contact angle tester.The polarization curves and electrochemical impedance spectra of the samples before and after coated were measured by electrochemical workstation to evaluate the corrosion resistance of the coatings.The results show that the surface of the prepared AZO coatings are smooth and compact,without obvious defects,and the coatings have good adhesion with the substrate.All the coatings show a hexagonal wurtzite Zn O structure,and Al is successfully doped into the Zn O structure.In contrast,the particle size of the coating with doping concentration n（Al）:n（Zn）=1.0%is the smallest,i.e.28.8 nm,and its surface is the smoothest.The corrosion resistance of this coating sample is the best,and its corrosion current density is decreased by two order of magnitude compared with that of the substrate.Based on the composition of this coating,the coating structure is basically unchanged after the graphite modification,but the average particle size decreases,and the roughness decreases,which apporove that the graphene prevents the grain growth.The diameter of the AZO coating with doping 0.250 g/L graphene is the smallest,i.e.20.4 nm,and its water antenna increases to 89°.Therefore,this coating shows better corrosion resistance,and the corrosion current density is two orders of magnitude lower than that of th e substrate.The prepared ITO coatings show In₂O₃ structure and prefer orientation of（222）crystal plane.Among them,the surface of ITO coating with In³⁺:Sn⁴⁺=1:0.1 is the most smooth and compact,the roughness is the smallest,and the average particle size is the smallest,only 55.4 nm.Therefore,this coating sample shows the best hydrophobicity and corrosion resistance,the water contact angle is as high as 104°,and the corrosion current density is two orders of magnitude lower than that of the substrate.On the basis of the coating composition,the ITO coating modified by graphene are dense and the roughness decreases.There are not only In₂O₃ phase but also In OCl structure in the coating structure,but the incorporation of graphene makes the grain size larger.Graphene modification can effectively improve the hydrophobicity and corrosion resistance of the coating.The water contact angle of ITO coating with 0.375 g/L graphene reaches 126°,and the corrosion current density decreases by 3 orders of magnitude.By comparison and analysis,the corrosion resistance of AZO and ITO coatings after graphene modified can meet the US Department of Energy’s 2020 bipolar plate corrosion resistance index requirements.The sample doped with 0.375 g/L graphene ITO coating shows the best hydrophobic and corrosion resistance.