Magnetron Sputtering Deposition Process And Properties Of Amorphous Carbon Composite Coating On Metallic Bipolar Plates In PEMFCs

Proton exchange membrane fuel cells(PEMFCs)directly converting hydrogen into electricity have been widely regarded as a promising device with many advantages such as quick start-up,high energy efficiency,high power density,low emission,and low operating temperature etc.In recent decades,PEMFCs have been widely used in transportation,portable device,backup power,and space facility.Bipolar plate is one of the key components in fuel cells,supporting the membrane assembly,distributing reactants,collecting electrons,and removing water.Metallic bipolar plates will replace graphite bipolar plates due to several characteristics such as ultrathin material,high mechanical strength,well forming ability and easy for mass production.Nevertheless,to be operated in the harsh acid PEMFCs environment,stainless steel surface will form passive layers and leach metal ions,thus to increase the interfacial contact resistance and poison the catalyst and then decrease the performance and durability of PEMFCs.The application of conductive and corrosion resistant gold coatings can be an effective approach to protect bipolar plates in the harsh environments.However,the high cost of gold coating impedes the commercial application of metallic bipolar plates.In recent years,amorphous carbon coating with good electrical conductivity and corrosion resistance has attracted a lot of attention.There are still two important technical challenges for amorphous carbon coating namely peeling off and decrease of conductivity after durability test in the real stack.This paper mainly focuses on the amorphous carbon coating and carries out the research of anticorrosion mechanism and magnetron sputtering process.According to the vehicle working condition,we established the ex-situ evaluation method for the bipolar plate coating.Based on the performance of a-C film in the real PEMFCs,coating failure modes and mechanisms are proposed.And then efforts are made to adjust the interfacial conductivity and corrosion resistance through ions energy variation.Seed layer element design and amorphous carbon structure doping are also conducted in order to overcome the problem of coating peeling off and conductivity decrease.And finally,we deposit the modified amorphous carbon coating on metallic bipolar plates and durability tests are also investigated.This paper mainly focuses on the following sections:1.Failure modes and evaluation method of metallic bipolar plates coatingsCoating failure modes and ex-situ evaluation method are established for PEMFCs in vehicle working conditions.We first conduct PEMFCs durability tests for 1000 h,and then amorphous carbon coating failure modes and mechanisms are proposed according to the analysis of microstructure before and after durability tests.The seed layer corrosion and oxygen adsorption of amorphous carbon are two important failure mechanisms.Ex-situ evaluation method is established considering potential,pH,and temperature.Failure modes between in-situ and ex-situ are compared in order to verify the accuracy of ex-situ method.2.Effect of magnetron sputtering parameters on the conductivity and corrosion resistance of amorphous carbon filmThe amorphous carbon film is deposited through unbalanced magnetron sputtering system.Process parameters have a significant influence on the plasma energy and thus to change the microstructure of amorphous carbon film.Process parameters range for high interfacial conductivity and well corrosion resistance is obtained through altering depositing atmosphere and substrate bias voltage.And then the multilayered structure through different substrate bias voltage is designed to enhance the conductivity and corrosion resistance.3.Modification process of a-C coating on metallic bipolar plates for high adhesion and stabilityModification process of amorphous carbon coating based on the first principle calculation and E-pH theory to overcome the coating failure modes.Corrosion mechanism of amorphous carbon in high potential environment is investigated and different anti corrosion elements as seed layers are investigated.Doping nitrogen element into the amorphous carbon is effective to increase the compactness,sp~2hybridization percentage and corrosion resistance,and thus to increase the stability in the PEMFCs acid environment in the long run.4.Deposition process and in-situ performance evaluation of metallic bipolar plates deposited with modified amorphous carbon coatingThe modified amorphous carbon film is deposited on the metallic bipolar plates and evaluated using the ex-situ method.Evaluation equipment for metallic bipolar plates is established considering the contact characteristic between bipolar plate and gas diffusion layer.A short fuel cell stack is assembled in order to evaluate the initial and durability performance in the long term.This paper mainly focuses on the amorphous carbon coating on metallic bipolar plates for PEMFCs stack.Four paragraphs including ex-situ evaluation method and failure modes of amorphous carbon coating in the vehicle PEMFCs stack,effect of magnetron sputtering parameters on the conductivity and corrosion resistance of amorphous carbon film,modification process of amorphous carbon coating for high interface bonding and anti-oxidation,performance of modified amorphous carbon coating on the real metallic bipolar plates,are investigated deeply.The research method and conclusion of this paper will shed new light on the metallic bipolar plate coating process and durability of the PEMFCs stack.