Optimization Of Stainless-steel Composition For Proton Exchange Membrane Fuel Cells Bipolar Plates

Emissions of greenhouse gases and volatile organic compounds from the use of fossil fuels are causing serious environmental problems,which will cause significant economic damage every year.It is well known that vehicle emissions are one of the potential causes of the global warming phenomenon.Therefore,fuel cell vehicles are able to convert chemical energy directly into electrical energy,which is an alternative solution that can reduce vehicle emissions,which is clean and environmentally friendly without pollution.Bipolar plate is one of the important parts of fuel cell.Commonly used metal bipolar plates mainly include titanium alloys,aluminum alloys,stainless steel,nickel-based alloys,etc.,all of which require composition optimization or surface modification,and therefore cannot be used directly as bipolar plates.One of the most economical and applicable choices is stainless steel,with low cost and high yield.The current preferred material for bipolar plates is 316L stainless steel,which will naturally form a passivation film under service conditions,suffering nevertheless from passivation-induced corrosion and conductivity deficiencies.Thus the contradictory properties of corrosion resistance and electrical conductivity break the balance between its service life and cell output power.This paper refines Fe-(Cr,Mo)-Ni alloy compositions,using the cluster-plus-glue-atom model,to obtain stainless steels with balanced corrosion and electrical performance.This model identifies a molecule-like structural unit in a solid solution,covering a nearest-neighbor cluster plus a few next-neighbor glue atoms.For austenite 316L stainless steel,the unit is described by a 16-atom cluster formula[Ni-Fe₁₁Ni₁]Cr_2.8Mo_0.2,where the Ni-centered cluster is nearest-neighbored by Fe₁₁Ni₁ and glued by three next-neighbor glue atoms Cr_2.8Mo_0.2.By fixing the three glue atoms,which is required to reach sufficient corrosion resistance,new compositions with varying Ni contents are designed following[Ni-Fe_13-xNi_x-1]Cr₃=Fe_13-xNi_xCr₃(x=1?5)and[Ni-Fe_13-xNi_x-1]Cr_2.8Mo_0.2=Fe_13-xNi_xCr_2.8Mo_0.2(x=2.5,3).The designed alloys were arc-melted for at least five times and were copper-mold suction-cast into 10mm cylindrical rods under argon atmosphere and were homogenized at 1150°C for 2 h and water-quenched.Under the simulated bipolar plate service environment(0.5mol/L H₂SO₄+2ppm HF),as the Ni content increases in Mo-free alloy series,the self-corrosion current density decreases to 1.034?A/cm²after acid passivation and to 0.286?A/cm² after electrochemical nitridation,well below the values of commercial 316L stainless steel(respectively 7.51 and 0.466?A/cm²),reaching the current industry target of 0.55?A/cm² for bipolar plates.At the same time,the contact electrical resistance(under 0.064 MPa)maintains the same level as 316L stainless steel,?1?·cm².The Mo addition further improves the performance:the optimal self-corrosion current density and contact resistance are respectively 0.131?A/cm²and 0.91?·cm²after electrochemical nitridation.The optimal alloy composition is[Ni-Fe₁₀Ni₂]Cr_2.8Mo_0.2=Fe-19.6Ni-16.2Cr-2.1Mo,wt.%.