Research On Plasma Nitriding Of Titanium And Titanium Alloys Used In Bipolar Plates Of Fuel Cells

Proton exchange membrane fuel cell（PEMFC）is the most promising clean energy with its high efficiency,low operating temperature,and environmentally friendly.As one of the most important components of the PEMFC,bipolar plates need to reduce the weight and cost for commercialization.Metal is the most promising material for the bipolar plate due to its lower processing costs,highly thermal and electrical conductivity.However,in PEMFC working environment,most of the anti-corrosion metals can form a passive film on their surfaces resulting in interfacial contact resistance increasing and degradation of cell performance.In this paper,plasma nitriding of TA1 and TC4 for exploring the possibility of application in bipolar plates of PEMFC.The main research and results are as follows:The plasma nitriding with hollow cathode discharge technology is used for this paper.Scanning electron microscope（SEM）,X-ray diffraction（XRD）,and X-ray photoelectron spectroscopy（XPS）were used to characterize the nitrided layer.From the result,a thin and dense nitriding layer was formed on the titanium sample.The nitriding layer is composed of Ti₂N phase and TiN phase.The corrosion test underwent in PEMFC working conditions（70℃,0.5 M H₂SO₄+2 ppm HF,cathode bubbled air,anode bubbled hydrogen）to evaluate the possible application for the bipolar plates.Before and after corrosion test,nitrided samples have a very low corrosion current density and interfacial contact resistance.Plasma nitriding of TA1 titanium was carried at 700℃for 2-4 h in a NH₃ atmosphere to prepare a nitriding layer with a thickness ranging from 0.7 to 2.1μm.The nitriding layer mainly consists of Ti₂N phase.After corrosion test,the current density of 4 h nitrided titanium sample was 0.39μA/cm².The ICR value of 4 h nitrided titanium sample was 4.1 mΩcm².Plasma nitriding of TC4 titanium alloy was carried at 750℃ for 4 h in a NH₃ gas and a NH₃+N₂ mixture gas.A Ti-N compound layer about 2.1μm thick was produced after plasma nitriding in a mixture gas（NH₃:N₂ 1:2）compared with that only about 1μm thick obtained in a pure NH₃.The current density of nitrided in mixture gas samples is 0.71μA/cm²which much lower than that of nitrided in a pure NH₃.After corrosion test,the ICR value of nitrided in a mixture gas sample was only 7.3 mΩcm².