| Proton exchange membrane fuel cells（PEMFCs）could directly convert chemical energy into electrical energy using hydrogen and oxygen as fuel.One of parts in PEMFC is bipolar plates.The primary function of bipolar plates in PEMFCs includes a dense structure to isolate the reaction gas,a good conductivity to connect individual cell in series.Metallic materials are potential candidates for bipolar plates since they present good mechanical stability,electrical as well as thermal conductivity.However,metal corrodes easily when used in a humid acidic and high-temperature environments in PEMFC systems,which causes the release of metal ions and poisons the membrane-electrode assembly（MEA）via proton exchanging reactions.Moreover,the oxide layer featuring lower conductivity could lead to a higher interfacial contact resistance（ICR）between BP and carbon paper.Consequently,surface modification by depositing corrosion resistant and conductive coatings on stainless-steel is a research hotspot for PEMFC BPs.Conductive ceramic MAX phases are a new class of materials that bridge the gap between metals and ceramics.Owing to their remarkable properties,including both their good electrical and thermal conductivities,outstanding oxidation resistance,and good machineability.Therefore,it is considered to be an ideal anticorrosive as well as conductive film material for metal bipolar plate.Therefore,this work focuses on the study of the synthesis of MAX phase composite coatings as well as the investigation of the films’behavior for applications in BPs.First,a series of multi Ti-Si-C or Ti-Al-C films were deposited on SS304 substrates using physical vapor deposition followed by annealing with different temperatures to form Ti3SiC2 or Ti3AlC2,respectively.The composition,morphology,phase composition and hardness of the film were analyzed by EDS,SEM,XRD and nano-composite mechanical tester,respectively.Exploring the effect of annealing temperature and carbon content on the formation of TiC and the corresponding MAX phase in films.The formation of the Ti3SiC2 or Ti3AlC2 phases strongly depended on temperature and temperature higher than 900℃is required.The hydrophilicity of the samples surface can be directly and simply determined by measuring the contact angle with water using a contact angle goniometer,and the experimental results show that the coating samples can greatly improve the hydrophobicity of SS304.The Ti-Si-C films can even more than double the water contact angle of bare SS304 to 126.2°.The ICRs of both uncoated and coated SS304 were analyzed.The experimental results show that the coating samples can greatly reduce ICR of bare SS304.The ICR value of the Ti3AlC2-coated samples is 3.725mΩ·cm2 at 140 N/cm2 pressure,which is very low as compared to the bare SS304.Electrochemical corrosion experiments of BPs were performed using a CHI660E instrument,including open circuit potential（OCP）,AC impedance spectrum（EIS）,potentiodynamic,and potentiostatic method tests.All coated samples were at least two orders of magnitude lower Icorrorr valued than the bare SS304 substrates,and therefore,it was concluded that the coatings were conducive to improving the corrosion resistance of the substrate.Among them,Ti-Si-C films show the best corrosion resistance with the lowest Icorrorr value of 3.3×10-2A/cm2.The current densities of the coated SS304 in the simulated PEMFC cathode environment met the requirement of the U.S department of energy. |
