Study On The Surface Modification Of Stainless Steel Bipolar Plate For PEMFC

Proton exchange membrane fuel cell(PEMFC) is a clean energy conversion device that can transform the chemical energy of fuel and oxidizer directly into electricity.It has high efficiency of energy conversion and it is pollution-free.Its operation is quiet and it's reliability it is high.It is a very promising new energy device.Bipolar plate is one of the key components of PEMFC.It occupy 60%-70%weight of the stack and 40%of the cost of production.The traditional graphite bipolar plate cost high and is breakable.So it is development trend to apply thin metal plates to replace the traditional graphite bipolar plate.Our group has studied the nitrided Cr-plating 304 stainless steel as bipolar plate material and has achieved good results.However,the cracks of the coating sometimes can not be completely closed after nitriding.We analyze the reason for why the cracks of the composite layer can not be completely closed,and electroplate three sets of chrome-plated 304SS under different plating conditions,then we nitride them at 550℃for 10-hour.Finally test their electrochemical capability and contact resistance.The results showed that white chrome coating in the surface of 304ss after nitriding do not exist any cracks through optical metallographic microscope.The white chrome gained by electroplating in the solution of 250g / L CrO₃ +2.5 g / L H₂8O₄ at 70℃and the current density is 25A/dm².While the 304ss with the coating of hard chromium after nitride has many but small cracks on the surface;The coating of multiple cracks chromium after nitride exist many large cracks.Electrochemical testing results show that white chromium nitride coating has superior corrosion resistance than other ones in the simulated PEMFC environment(0.05mol / L H₂SO₄ +2 mg/LF^-) accessing H₂/O₂.The blunt-current density is 5～6μA/cm² and 3～5μA/cm².The corrosion current density maintain stability at a very low value at both 0.6V cathode polarization purging in air and the environmental of constant voltage -0.1V anode polarization purging in hydrogen.The contact resistances of the three sets of samples are all around 25mΩcm² at the pressure of 150N/cm².It shows that different technics of chrome plating have little effect on the contact resistance of the samples after combined treatment.