*3.2. Surface Morphology*

The SEM images of 316L SS, TiNbN/316L SS and TiNb/316L SS before and after potentiostatic polarization corrosion tests are shown in Figure 2.

**Figure 2.** Scanning electron microscopy (SEM) images of the surface of 316L stainless steel (SS), TiNbN/316L SS and TiNb/316L SS.

In Figure 2a, the polished SS surface is uneven due to grinding and polishing, and a number of holes are randomly distributed on it. In Figure 2b, the TiNbN surface is much smoother than that of the SS. However, small holes still exist, and at the bottom of the surface valley, some large particles appear. The micrograph of the TiNb coating shown in Figure 2c is smooth and nearly no obvious holes can be seen. Obviously, the deposition of TiNbN or TiNb has smoothened the SS samples. This may also limit the formation of micro corrosion cells. The morphologies of uncoated and coated 316L SS after potentiostatic polarization tests are shown in Figure 2d–f. After corrosion, 316L SS shows several holes (Figure 2d) which are larger than those observed on 316L SS before corrosion. In Figure 2e, localized corrosion is apparent especially for the valley and defect areas on the TiNbN coating while the surface of TiNb/316L SS in Figure 2f shows few large or deep corrosion pits. This is relevant with the coating defects formed on TiNbN before corrosion. For the hole defect, the electrolyte solution diffusion at the bottom of it is slow. The concentrations of O2 and ions inside the hole differ from that in solution, resulting in the different electrochemical activities of grains. Due to the lack of O2 and the dense passive film, the inner region of the hole becomes the anode of a corrosion battery with a smaller area and a higher corrosion current density, compared with other surface zones. Thus the localized corrosion is induced and more defects are formed. Images in Figure 2g,h under higher magnification indicate that the grain size of sputtering deposited TiNbN varies greatly but with a smaller average value compared with TiNb which might be attributed to the incorporation of N. For the TiNbN coating, the distinction in grain size could result in an inhomogeneous distribution of grain boundaries and finally contributing to the non-uniformity of passive layer [27]. Additionally, the SEM results show the particle agglomeration phenomenon of TiNbN is more serious than of TiNb. And the agglomeration may cause the differences in coating's surface structure and composition by which the localized corrosion of TiNbN can be induced.

In order to characterize the coating composition, EDS was applied and results are shown in Figure 3. Since the coating is thin, the detected images may include intensity peaks from some elements in the substrate, the substrate oxidation layer and adsorbed substances on sample surface. In the film of TiNbN/SS, the atomic ratio of Ti, Nb and N is about 7:22:10 (with the usual reservations regarding quantifications of light elements). For the coating of TiNb/SS, the atomic ratio of Ti and Nb is about 1:4. The EDS results illustrate under the same sputtering power mentioned above, the deposition speed of Nb is faster than that of Ti. Possible reason for this phenomenon is that Nb is a DC power target, and Ti is powered by a RF electric source. For the same material, the sputtering rate of the RF target is lower than that of the DC target at the same set power.

**Figure 3.** Energy dispersive spectrometry (EDS) results of (**a**) TiNbN/316L SS and (**b**) TiNb/316L SS.

#### *3.3. X-ray Photoelectron Spectroscopy Analysis*

The chemical states of Ti and Nb in the surface layers of the TiNb and TiNbN films were examined by XPS. The original Nb 3d and Ti 2p spectra are shown in Figures 4 and 5. After fitting all the spectra with backgrounds subtracted, Nb 3d and Ti 2p peaks were analyzed according to the NIST XPS Database [28], relative literatures and possible reactions in testing environment. In Figure 4a, it's deduced that the highest doublet peaks are related to 3d5/2 and 3d3/2 of Nb in Nb2O5 and the lower peaks represent metal Nb [29–32]. From the Ti 2p fitted spectra in Figure 4b, two peaks are observed corresponding to Ti 2p3/2 and Ti 2p1/2 in TiO2, respectively [33,34]. The testing results reveal that the surface layer of the TiNb coating is primarily composed of Nb2O5 and TiO2, which means the TiNb coating can be oxidized spontaneously in air and form a stable metallic oxide film. Figure 5 shows XPS spectra of Nb 3d and Ti 2p of the TiNbN coating. The fitted Nb 3d spectra depicted in Figure 5a indicate

that the major peaks representing Nb 3d5/2 and Nb 3d3/2 can be ascribed to the formation of Nb2O5, NbN, and NbO [31,32,35]. The Ti 2p fitted spectra in Figure 5b reveal the existence of TiN [36]. But the two largest peaks stem from Ti 2p3/2 and Ti 2p1/2 in TiO2 [37,38]. From the fitted results, it is obvious that under the sputtering conditions mentioned above, TiN and NbN were synthesized. Due to the exposure of the coating to air, stable oxides of Ti and Nb were also formed in the outer layer of TiNbN coating. Since the main surface composition difference between TiNb and TiNbN coatings lies in TiN and NbN, metallic nitrides could play an important role in exhibiting some different properties for TiNb/316L SS and TiNbN/316L SS.

**Figure 4.** X-ray photoelectron spectroscopy (XPS) spectra of (**a**) Nb 3d and (**b**) Ti 2p (original-black lines, fitted-red lines) measured on TiNb/316L SS.

**Figure 5.** X-ray photoelectron spectroscopy (XPS) spectra of (**a**) Nb 3d and (**b**) Ti 2p (original-black lines, fitted-red lines) measured on TiNbN/316L SS.

## *3.4. Corrosion Behavior*

The corrosion behaviors of 316L SS, TiNb/316L SS and TiNbN/316L SS were characterized electrochemically by potentiodynamic polarization, EIS, and potentiostatic polarization. Figure 6 displays potentiodynamic polarization curves of bare and coated 316L SS. From the curves, corrosion potentials (φcorr) are obtained. The φcorr is related to coating's thermodynamic stability, and the higher φcorr value normally means better corrosion resistance from the aspect of thermodynamics [3]. In the tests, the φcorr of 316L SS is −0.761 V vs MSE, whereas TiNbN/316L SS has a more positive value of −0.744 V. Compared with the substrate, the TiNb film increases the φcorr by 138 mV. When potentials are swept from negative to positive, all three samples show obvious passivation areas. In the passivation area, as the potential value increases, the current density remains nearly constant. At 0.19 V vs MSE, all the samples are passivated and 316L SS exhibits a current density of 17.3 <sup>μ</sup>A·cm<sup>−</sup>2, while it is 2.28 <sup>μ</sup>A·cm<sup>−</sup><sup>2</sup> and 44.6 <sup>μ</sup>A·cm<sup>−</sup><sup>2</sup> for TiNb/316L SS and TiNbN/316L SS, respectively. The results indicate that the TiNb film has high corrosion resistance at 0.19 V vs MSE. This is owing to the formation of corrosion resistant passive film on TiNb coating obtained from anodic polarization process. Moreover, it is noteworthy that although the TiNbN film is thicker, TiNb alloy establishes better corrosion resistance. This phenomenon is mainly associated with the surface condition and composition of TiNbN film.

**Figure 6.** Potentiodynamic polarization curves of 316L SS, TiNb/316L SS, and TiNbN/316L SS.

EIS was performed and Figure 7 shows Nyquist impedance spectra of coated and uncoated 316L SS measured at OCP and 0.19 V vs MSE. In Figure 7a, influenced by surface defects, TiNbN/316L SS shows two time constants at OCP which reflect the impedance information from coating and substrate [12,39]. In the Nyquist impedance spectra of TiNbN/316L SS at 0.19V, TiNb/316L SS, and 316L SS, incomplete semicircular arcs appear. The depressed semicircular shape of SS is attributed to the roughness of sample's surface. To fit the impedance spectrum of TiNbN/316L SS measured at OCP, an equivalent circuit with two constant phase elements (CPE), shown in Figure 8a [40], is adopted. The CPE is represented by *Q*. Under ideal conditions, a CPE corresponds to a capacitor [41]. *R*s is the resistance of electrolyte solution between working electrode and reference electrode. *R*f and *Q*f are coating's resistance and capacitance, respectively, and *R*ct is the charge transfer resistance, while *Q*1 represents the double layer capacitance [42]. The other spectra in Figure 7a,b are fitted by the equivalent circuit in Figure 8b [43]. Table 1 lists the fitted values of *R*ct. The higher *R*ct suggests the enhanced corrosion resistance [44,45].

**Figure 7.** Nyquist impedance spectra of 316L SS, TiNb/316L SS and TiNbN/316L SS @ (**a**) open circuit potential (OCP), (**b**) 0.19 V vs mercurous sulfate electrode (MSE).

**Figure 8.** Equivalent circuits used for the Nyquist impedance spectra of (**a**) TiNbN/316L SS @ OCP; (**b**) TiNbN/316L SS @ 0.19 V vs MSE, 316L SS, and TiNb/316L SS.



Comparing *R*ct values of different materials in Table 1, it is observed that the charge transfer impedance of TiNb coated 316L SS is much higher than for the uncoated substrate. Part of reasons for this phenomenon is that the surface layer of TiNb could be oxidized into TiO2 and Nb2O5 spontaneously in air according to XPS measurements, providing a high corrosion resistance. At 0.19 V vs MSE, the high potential and the continuous supply of oxygen could passivate TiNb and lead to the formation of dense passive film which is corrosion resistant and could act as a barrier to inhibit the bulk solution from approaching the sub-layer directly. In comparison with TiNb, the *R*ct value of the TiNbN film is much smaller, either at OCP or 0.19 V vs MSE. Two major factors may contribute to this phenomenon. From a morphology point of view, the particle clusters and pores observed on the TiNbN coating can cause localized corrosion. The other reason is that the bonds between nitrogen and metallic atoms may impede the formation of more homogeneous and corrosion resistant passive layer. According to the above analysis, from the EIS and potentiodynamic polarization test results draw one can the conclusion that TiNb has good corrosion resistance while TiNbN and bare 316L SS are likely to dissolve in the aggressive cathode environment.

To evaluate the stability of the materials, potentiostatic polarization experiments were conducted at 0.19 V vs MSE. The results are shown in Figure 9. In the beginning, the current density of each coated 316L SS sample declines while that of 316L SS first increases sharply, then decreases gradually. After 2000 s, 316L SS experiences an obvious fluctuation due to the surface states changes affected by electrochemical dissolution, but throughout this period, its average corrosion rate is increasing. As for TiNbN/316L SS, the time needed for total passivation is about 3000 s, and its stabilized current density is approximately 6–7 <sup>μ</sup>A·cm<sup>−</sup>2. Although TiNbN/316L SS and 316L SS show comparable current density values and variation trends at the initial stage of tests, the TiNbN coating is much more stable than bare 316L SS, judging from the smoothness of the curves. That is because the coating deposited by magnetron sputtering is uniform and has less defects than the substrate. The current density of TiNb/316L SS decays rapidly first and then stabilizes at 0.9–1 <sup>μ</sup>A·cm<sup>−</sup>2. This demonstrates that the TiNb coating can be easily passivated and keep a low corrosion current density for a relatively long period, influenced by the high potential and the oxygen saturated acid solution, which shows the good corrosion resistance and stability of the TiNb coating. From the potentiostatic polarization tests above, it is assumed that TiNb/316L SS has the potential to work as a PEMFC cathode flow plate material for a long time.

**Figure 9.** Potentiostatic polarization curves of 316L SS, TiNb/316L SS, and TiNbN/316L SS.

The electrochemical testing shows that both TiNb and TiNbN coatings can form steady passive films at 0.19 V vs MSE, and the long-term corrosion current density of TiNb/316L SS is lower than 1 μA· cm<sup>−</sup>2, illustrating the high corrosion resistance of TiNb coating. Compared with TiNb/316L SS, TiNbN/316L SS is of poorer corrosion resistance.

#### *3.5. Interfacial Contact Resistance*

The conductivity of bipolar plates has grea<sup>t</sup> impact on the PEMFC's working e fficiency [46]. Normally, the bulk resistance of metallic bipolar plates can be ignored since the ICR between the bipolar plate and the carbon paper is much greater. In Figure 10, the ICR of the material with a carbon paper is plotted as a function of the compression force.

**Figure 10.** Interfacial contact resistances (ICR) between 316L SS, TiNb/316L SS, TiNbN/316L SS and carbon paper as a function of compaction force.

In the low-force region, as compaction force raises, the contact area between carbon paper and sample increases leading to the decrease of ICR, and when the force is high enough, the ICR is dominated by the surface composition and nature [46,47]. At the same force, the contact resistance of 316L SS, TiNb/316L SS, and TiNbN/316L SS decreases in turn, and their conductivity increases sequentially. I.e., both the TiNb and the TiNbN coatings provide lower ICR than 316L SS's, but the effect of TiNbN is more remarkable. At 276 <sup>N</sup>·cm<sup>−</sup>2, TiNbN coating's ICR is approximately 48 m <sup>Ω</sup>·cm2, and the ICR value of TiNb/316L SS is nearly 1/5 of that of 316L SS. One of the reasons behind this is that both coatings could reduce the roughness of 316L SS substrate leading to an increased contact area. For the TiNbN coating, the dopant of TiN and NbN in its surface layer improves conductivity. In comparison with TiO2 and Nb2O5, TiN [48] and NbN have higher conductivity. Overall, the TiNbN coating is of relatively high electrical conductivity, and the TiNb film reduces the substrate's ICR. However, considering the moderate corrosion resistance of TiNbN, TiNb/316L SS is more promising for use as bipolar plate. Further studies are relevant assessing a combined film with good conductivity from TiNbN and grea<sup>t</sup> corrosion resistance from TiNb.
