Effect of Nitrogen Flow Rate on the Performance of VNbMoTaWNx Film as Modified Layer of Stainless Steel Bipolar Plates for PEMFC

The work aims to reduce the contact resistance on the surface of SS316L and improve its corrosion resistance. VNbMoTaW and VNbMoTaWNx films with different nitrogen flow rates were prepared on SS316L stainless steel by closed field unbalanced magnetron sputter ion plating. The field-emission scanning electron microscopy, XRD diffractometer, XPS photoelectron spectroscopy, electrochemical workstation, and contact resistance testing device were used to study the effects of modified layer composition and structure on contact resistance and corrosion resistance. The scanning electron microscopy results showed that all film surfaces were dense and continuous, with good adhesion to the substrate. With the increase of nitrogen flow rate, the columnar crystal structure decreased, and the film became more dense and compact. The XRD results indicated that the high entropy alloy film without nitrogen flow rate had a high entropy alloy body centered cubic structure and grew along the (110) crystal plane direction. With the increase of nitrogen flow rate, the nitride phase gradually increased, and the film crystal structure began to transition from a body centered cubic structure to a face centered cubic structure. Combined with XPS analysis, the surface of VNbMoTaWNx film was mainly composed of metal nitrides and a small amount of high entropy alloy BCC phase, and with the increase of nitrogen flow rate, the metal nitrides phase gradually increased. Compared to single-layer VNbMoTaW films, VNbMoTaWNx films had better corrosion resistance and conductivity. The high entropy alloy nitride film with a nitrogen flow rate of 12 mL/min had the most excellent comprehensive performance. The contact resistance of the film after surface modification was greatly reduced. Under the pressure of 1.4 MPa, the contact resistance with carbon paper was only 12.2 mΩ·cm2, which was close to the technical goal of the United States Department of Energy (DOE). Under the simulated PEMFC environment, the corrosion current density of VNbMoTaWNx-12 mL/min in the cathode environment was 0.040 μA/cm2 measured by the potentiodynamic polarization curve. Compared to the SS316L substrate, the corrosion resistance of the film was greatly improved. Under a 0.6 V constant potential simulated cathode environment, the current density of VNbMoTaWNx-12 mL/min remained stable at 1.01 μA/cm2, close to the goal of 1 μA/cm2 of the United States Department of Energy. Both VNbMoTaW and VNbMoTaWNx films with different nitrogen flow rates can significantly improve the corrosion resistance and conductivity of SS316L substrate. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.