Research On Surface Modification Of Metal Bipolar Plate Of Fuel Cell Based On MAX Phase Film

As a modified coating on the surface of the fuel cell metal bipolar plate,it is required to meet the requirements of low contact resistance,high corrosion resistance,high film-base(stainless steel)binding force,and certain wear resistance.However,the performance of the coatings currently used cannot meet these requirements simultaneously.Due to its unique crystal structure,the conductive ceramic MAX phase has excellent electrical and thermal conductivity like metal,and has high temperature oxidation resistance and corrosion resistance like ceramic material,so we think it is an ideal thin film material for protecting metal bipolar plates.The research focus of the conventional MAX phase film is on the phase formation process and phase structure.The application research directly on the surface modification coating of the fuel cell metal bipolar plate(stainless steel)is still very little,or the performance cannot meet the application requirements.This article takes the Ti-Al-C system MAX phase material as the research object,and from the perspective of process research,discusses the relationship between the preparation process,structure and performance of the MAX phase thin film material applied to the surface modified coating of the fuel cell metal bipolar plate.We used mid-frequency magnetron and microwave electron cyclotron plasma technology to prepare Ti-Al-C thin films on stainless steel substrates,and studied the crystal structure of different external heat treatment temperatures,substrate types,substrate heating and different overplating layers on the final film,The influence of surface morphology,electrical characteristics,film hardness and corrosion resistance,mainly including the following parts:(1)We used external heat treatment to study the phase formation conditions of the MAX phase Ti-Al-C film,and compared the effects of different stainless steel substrates on the crystal structure and surface morphology of the Ti-Al-C film.Compared with SS304,we found that SS316L is more suitable for preparing the MAX phase,which can better maintain the chemical composition of the magnetron sputtering target,and the film surface structure is better.When the external heat treatment temperature is lower than 700°C,the MAX phase cannot be detected.As the heat treatment temperature increases,the conductivity of the film increases,showing the effect of gradually forming the MAX phase.The hardness of the film is increased and the hydrophobicity is enhanced,but there are more crack defects on the surface of the film,indicating that the preparation of the MAX phase film by external heat treatment has certain deficiencies in controlling the surface quality of the film.(2)We use the substrate heating method to prepare the Ti-Al-C thin film in one step,and directly form the MAX phase Ti-Al-C thin film at 700?-800?.Compared with the secondary heat treatment,the grains deposited by the one-step method are evenly distributed and the surface structure is more dense.When the deposition temperature is higher than 750?,the MAX phase of the film is detected.The contact resistance between the film and carbon paper is 8 m?·cm~2 under a positive pressure of 140 N/cm~2,and good electrical parameters are obtained;the water contact angle of the film is 100.4°,hardness and modulus are 12.5GPa and 133.7GPa respectively;however,the bonding force between the film and the substrate is insufficient,and it is easy to peel off from the substrate during the long-term polarization process,which affects the anti-corrosion performance of the film.(3)In order to improve the corrosion resistance of the coating and the adhesion between the film and the substrate,we explored the effect of the transition layer type on the one-step preparation of Ti-Al-C film at a temperature of 750?.It was found that the Ti C transition layer can effectively reduce the grain size on the surface of the film,increase the surface density,and make the MAX phase easier to form,and the contact resistance between the film and the carbon paper is only 3.2m?·cm~2 under 140 N/cm~2 pressure.It shows good hydrophobicity,the self-corrosion current density is lower than the DOE standard of the US Department of Energy,and the film is well preserved under 10h potentiostatic polarization corrosion.The film hardness and modulus are 15.6 GPa and155.8 GPa,respectively.The film properties we have obtained have met the requirements of modified coatings on the surface of fuel cell stainless steel bipolar plates to a considerable extent,which can greatly promote the application of metal bipolar plates on fuel cells,and then promote the development of fuel cell technology.