Tailoring sp²-Cluster Distribution to Suppress Corrosion at Amorphous Carbon/Bipolar Plates Interfaces

With high electrical conductivity and strong corrosion resistance, amorphous carbon (a-C) films are essential for ensuring the durability of metallic bipolar plates (BPs) in proton exchange membrane fuel cells (PEMFCs). However, due to the lack of plasma diagnostics and in situ electrochemical characterization, it is challenging to reveal the real-time corrosion process and to provide theoretical guidance for a-C-coated BPs. Here, we demonstrate a novel approach integrating a rotatable magnetic field with magnetron sputtering to fabricate a-C films under optimized plasma distributions. Results show that varying the magnetic field rotation angle (-60° to 90°) alters the unbalanced coefficient, plasma potential, and electron density. While the sp²/sp³-C ratio remained constant across films, the sp² cluster diameter changed significantly. The a-C film deposited at 30° exhibited the largest sp² cluster (L_a = 1.89 nm). Crucially, BPs coated with this 30° film demonstrated exceptional long-term corrosion resistance during 48 h cathodic/anodic polarization, with interfacial contact resistances of only 2.4 mΩ·cm² before corrosion and 3.5 mΩ·cm² after corrosion. In situ electrochemical impedance spectroscopy and scanning vibrating electrode technique analysis confirmed that performance degradation stems from localized galvanic corrosion initiation and subsequent corrosion product formation at the interface. This study emphasizes controlling sp² clustering and density as a key design principle for enhancing a-C film durability on BPs and advances practical protective film development for PEMFCs.