| PEMFC (Proton Exchange Membrane Fuel Cell, PEMFC) is a clean energy system that can convert the chemical energy of fuel (hydrogen) and oxidant (oxygen or air) directly to electricity. Due to its properties of low temperature operation, high current density, fast start-up and so on, PEMFC can be power of motor vehicle. Bipolar plate is a multi-functional component in a PEMFC stack, it can influence the volume, the weight and the cost. Because of high cost of traditional machined-graphite bipolar plate, it is replaced by the metal bipolar plate with low cost and high stabilization. Stainless steel is the most widespread investigated materials to substitute for traditional machined-graphite as bipolar plate. However, the insufficient corrosion resistance and higher interfacial contact resistance can still inhibit its application as bipolar plate. In this work, the 304 stainless steel with the different surface modification technology was investigated as possible candidate materials for PEMFC bipolar plate.The 304 stainless steel was treated by the electric plating Cr. The Cr-coated 304 stainless steel was studied in the simulated polymer electrolyte membrane fuel cell (PEMFC). The results show that the Cr-coated layer had good binding force with the steel substrate, the Cr-coated 304 stainless steel had lower passive current densities than the substrate in the simulated anodic and cathodic environments, the Cr-coated 304 stainless steel interfacial contact resistance of passive films in the simulated environments had increased.In order to decrease the interfacial contact resistance of passive film, the plasma-assisted nitriding processes with different treating temperatures were adopted on the base of the Cr-coated 304 stainless steel to obtain the surface modification layers of the main content CrxN. With the rise of temperature nitriding, the CrN content of modification layer gradually increased. The electrochemical testing showed that samples by plasma-assisted nitriding processes treated had higher corrosion resistance than the substrate and Cr-coated steel and corrosion resistance was improved. The analysis of XPS revealed that the passive film of the duplex samples by electroplating Cr and plasma-assisted nitriding process treated consisted of a lot of CrN and increased the conductivity of the passive film. After the potentiostatic polarization corresponding cathodic and anodic potential, the interfacial electrical resistance of passive film increased.To enhance the binding force between the Cr-coated layer and the substrate, the method of heating in vacuum was used to machine the Cr-coated 304 stainless steel. The new surface modification technology achieves quietly different surface compositions with high Cr, low Fe and a little Ni. The alloy layer had low passivation current density and lower than 10μA cm-2 in 0.05 mol/L H2SO4+2 mg/L F- solution bubbled with H2 or air (O2) at 70℃. The interfacial contact resistance between the sample and the carbon paper was lower than 55 mΩcm2.The process of low temperature plasma nitriding could formγN phase at steel surface. TheγN phase with an fcc phase supersaturated had good corrosion resistance and low surface conductivity.In addition, the testing technology of XRD, SEM, EDX, AFM and XPS were developed to analyze the surface layer with different modification methods, and obtained its information of composition and pattern. From the results of XPS, it received the base data about the passive film. The data was used to analyze the reason that different surface modification processes resulted in different surface conductivity.
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