Effect of Surface Modifications on the Corrosion Resistance and Interfacial Contact Resistance of Ferritic Stainless Steel as a Bipolar Plate for PEMFC

Abstract

Stainless steels are considered the best candidate material for the bipolar plate of polymer electrolyte membrane fuel cells (PEMFC) because they have high mechanical strength, high chemical stability, low gas permeability, a wide range of alloy compositions, applicability to mass production, and low cost. However, the major concerns for stainless steel bipolar plates in PEMFC have been the corrosion and interfacial contact resistance (ICR). To explore a cost-effective method of surface modification, various chemical and heat treatments are performed with 446M and 30%Cr-Fe ferritic stainless steel to understand the effect of the surface modifications on the ICR and the corrosion resistance. In addition, the effect of the surface roughness on the ICR is evaluated both by direct measurement and by finite element method (FEM) simulation. From the results of FEM simulation, when the surface of stainless steel is in contact with the gas diffusion layer (GDL) under load, the surface roughness of stainless steel can affect the degree of deformation of the carbon fibers in the GDL. Moreover, the higher deformation of carbon fibers on the rough stainless steel under load leads to larger real contact area between the carbon fiber and stainless steel. Consequently, the rough stainless steel has a lower ICR value than the fine one. The results indicate that the roughness of stainless steel as a bipolar plate significantly affects the ICR value. The roughness of bipolar plate can be one of the key factors to decrease ICR. The chemical treatments were performed in the acidic and alkaline solutions to modify the surface of stainless steel bipolar plate. In acidic treatment, the ICR value of the specimen immersed in the HCl solution was the lowest among the chemically treated specimens. This result is caused by two factors which are the surface roughening and the precipitation of metallic Cu on the surface. After consequent heat treatment, the ICR of specimen is increased and the corrosion resistance is improved by a thickening of the passive film and the transformation of the metallic Cu to Cu oxide. However, the heat treatment only improved the corrosion resistance, but resulted in no beneficial effect on the ICR at all. The ICR and the corrosion resistance of 446M can be effectively controlled by a proper surface modification with a combined treatment of immersion in the acidic solution, followed by a heat treatment. The combined acidic and heat treatment not only improves the corrosion resistance but also reduces the ICR. Immersion in NaOH solution under optimum condition can lead to decrease the ICR value of 446M with no decrease in the corrosion resistance. Immersion of 446M in NaOH solution increases the ratio of Cr oxy-hydroxide/oxide, which contributes to a decrease of the ICR value. This means that the bound water present in the form of the OH- groups in the passive film acts as a donor-type impurity and provides the active sites for electrical conduction in the oxide. This has a positive effect on the electrical conduction and leads to decrease of the ICR.RuO2 electrodeposition was performed on ferritic stainless steel for the bipolar plate of PEMFC to evaluate the effect of RuO2 on the corrosion resistance and the ICR value. The surface morphology of deposited RuO2 is greatly stabilized by the addition of HNO3 in a 10 mM RuCl3•xH2O solution. The RuO2-deposition on stainless steel shows a high contact angle indicating the high surface energy and hydrophobic characteristics. The ICR measurement indicates that the deposition of conductive RuO2 on stainless steel is very effective in decreasing the ICR value. Moreover, after potentiostatic polarization, the ICR value shows only 2.4 and 2.2 mΩ•cm2 at 150 N/cm2 under air and H2-purged environments, respectively. In electrochemical tests, even though the current density of RuO2-deposited stainless steel was slightly higher than that of bare stainless steel, this is an acceptable value in terms of the DOE 2015 target for metallic bipolar plates (less than 1 μA/cm2). Because the RuO2 deposition on stainless steel results in a low ICR value, a good corrosion resistance and a high contact angle, the RuO2 deposition is a sufficiently feasible method for the bipolar plate material of PEMFC.