Conductive passivation via carbon mediated defect control in FeCoNiCrCx high-entropy alloy coatings: breaking conductivity-corrosion trade-off for PEMFC bipolar plates

Stainless steel bipolar plates (BPs) for proton exchange membrane fuel cells (PEMFC) face an inherent corrosion-conductivity conflict. Corrosion-resistant passivation layers, such as Cr 2 O 3 , induce unacceptable interfacial contact resistance (ICR) elevation. We resolve this conflict using high-power impulse magnetron sputtering (HiPIMS) to deposit FeCoNiCrC x coatings with carbon-mediated defect control, where acetylene flow regulation (0–5 sccm) triggers three synergistic effects: carbon-mediated amorphization eliminates grain boundaries to suppress ionic diffusion; in-situ carbide formation, primarily Cr 3 C 2 , consumes reactive metals and inhibits oxide nucleation; and sp 2 -carbon networks establish efficient electron-conduction pathways. The optimized 5 sccm coating delivers exceptional performance in simulated PEMFC cathode conditions: corrosion current plunges to 0.079 μA/cm 2 . Simultaneously, ICR is maintained at 7.9 mΩ cm 2 under 1.4 MPa compaction, outperforming U.S. Department of Energy (DOE) 2025 targets. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) analyses validate a carbon-dominated barrier layer exceeding 80 at.% carbides and amorphous carbon. This layer effectively replaces insulating oxides, ensuring post-polarization ICR stability. This work demonstrates conductive passivation as an effective materials strategy that breaks the persistent corrosion-conductivity trade-off in metallic BPs through carbon-mediated defect control.