Evaluating the performances of surface-modified titanium bipolar plates using in situ nitriding by plasma-enhanced chemical vapor deposition

In this study, the surface modification of titanium plates was performed using in situ nitriding via plasma-enhanced chemical vapor deposition to improve the conductivity and corrosion resistance of the plates. A series of titanium nitride (TiN) coatings were synthesized at different nitriding temperatures and durations. The influence of nitriding temperatures and durations on the surface morphology, hydrophobicity, interfacial conductivity, and corrosion resistance of the as-prepared coatings was investigated. The results indicated that faster growth and larger particle size of TiN are observed at higher temperatures. However, lower temperatures are unfavorable for surface reactions; thus, the coating cannot entirely cover the titanium substrate. Moreover, a shorter nitriding time results in irregular nanogrowth nuclei on the surface, leading to an uneven coating and bare titanium substrate. Conversely, longer nitriding time encourages the continuous accumulation of TiN nanoparticles and forms a uniform coating of the titanium substrate but decreases the flatness because of the stacking of the coatings due to the long nitriding time (120 min). The TiN coating prepared by nitriding at 650 °C for 90 min (TiN-650-90) is relatively compact and smooth with the composition of TiN0.26 and has an increased water contact angle of 105.4°. The change from hydrophilicity to hydrophobicity in TiN is beneficial to fuel cell water resistance. At a loading pressure of 1.5 MPa, the contact resistances of the coatings prepared at a nitriding time of 60 min can satisfy the U.S. Department of Energy requirement of less than 10 mΩ·cm2. Despite a contact resistance of 13.2 mΩ·cm2 for the TiN-650-90 coating, the contact resistance decreases with increasing loading pressure and is stable at 6.5 mΩ·cm2 under a loading pressure of 2.75 MPa. The corrosion current density of the TiN-650-90 coating is 0.56 μA·cm−2, and the corrosion potential positively shifts from −0.37 to −0.05 V at room temperature. The corrosion current density tested in the simulated operating environment of fuel cells is higher than that at room temperature but much lower than that of titanium (4.2 μA·cm−2). Furthermore, the current density is stable at 0.67 μA·cm−2 and at a −0.1 V constant potential, indicating superior corrosion resistance and stability than titanium. The titanium bipolar plates modified by this method exhibit the advantages of relatively low deposition temperature, quick deposition speed, and good hydrophobicity, conductivity, and corrosion resistance. This work can pave the way for efficient surface modification of metal bipolar plates.