Effect of different N contents on CrN coatings of aluminium alloy bipolar plates for proton exchange membrane fuel cells

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Aluminium alloy bipolar plates have a unique development potential in proton exchange membrane fuel cells (PEMFCs) because of their light weight and low cost. However, aluminium alloy bipolar plates are highly susceptible to corrosion in the operating environment of PEMFCs, which limits their commercial application. To enhance the corrosion resistance and surface conductivity of aluminium bipolar plates, Cr/CrNx coatings with different N contents were prepared on aluminium alloy 6061 (AA6061) via radio-frequency (RF) magnetron sputtering. The effects of N content on the composition, deposition rate, and microstructure of the CrNx coatings were analysed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Electrochemical testing of the corrosion resistance, electrical conductivity and stability of the coatings. The results showed that with increasing N content, the CrN concentration gradually increased from 15.8 % (10 % N) to 79.5 % (40 % N), while the Cr2N concentration decreased from 81.2 % (40 % N) to 18.1 % (10 % N). The composition of the physical phase was mainly transformed from the Cr phase to a mixed phase of Cr2N + CrN, and increasing the nitrogen content resulted in the gradual transformation of the Cr2N phase to the CrN phase. The density and thickness of the coating surface increased with an increase in N content in the range of 10%–40 %; however, with a further increase in N content, it resulted in the decrease of coating density and the thinning of the coating thickness. At an N content of 40 %, the CrN coatings showed the lowest corrosion current density (1.426 × 10−5 A cm−2), closer to the 2025 DOE targets (10−6 A cm−2). This coatings remained stable during potentiostatic polarisation, which obtained the lowest contact resistance (9.77 mΩ cm2) under a pressure of 150 N cm−2, which was lower than that of the 2025 DOE targets (≤10 mΩ cm2), and the coating exhibited a maximum water contact angle of 92.56°.