Chemical Composition and Corrosion Behavior of a-C:H/DLC Film-Coated Titanium Substrate in Simulated PEMFC Environment

: The amorphous hydrogenated (a-C:H) ﬁlm-coated titanium, using different CH 4 /H 2 and deposition times, was prepared by the ion beam deposition (IBD) method, which has the advan-tage of high adhesion because of the graded interface mixes at the atomic level. The chemical characterizations and corrosion behaviors of a-C:H ﬁlm were investigated and evaluated by SEM, AFM, Raman spectroscopy, EPMA, TEM and XPS. An a-C:H ﬁlm-coated titanium was corroded at 0.8 V, 90 ◦ C in a 0.5 mol/L H 2 SO 4 solution for 168 h. The metal ion concentration in the H 2 SO 4 corrosion solution and the potentiodynamic polarization behavior were evaluated. Results indicate that a higher CH 4 /H 2 of 1:0 and a deposition time of 12 h can result in a minimum I D /I G ratio of 0.827, Ra of 5.76 nm, metal ion concentration of 0.34 ppm in the corrosion solution and a corrosion current of 0.23 µ A/cm 2 . The current density in this work meets the DOE’s 2020 target of 1 µ A/cm 2 . Electrical conductivity is inversely proportional to the corrosion resistance. The signiﬁcant improvement in the corrosion resistance of the a-C:H ﬁlm was mainly attributed to the increased sp 3 element and nanocrystalline TiC phase in the penetration layer. As a result, the a-C:H ﬁlm-coated titanium at CH 4 /H 2 = 1:0 with improved anti-corrosion behavior creates a great potential for PEMFC bipolar plates.


Introduction
A fuel cell can be regarded as a kind of equipment, which can directly convert the chemical energy of a fuel into electrical energy with zero emissions [1]. With good efficiency, low operating temperature and clean technology, polymer electrolyte membrane fuel cell (PEMFC) is proposed as one of the most prospective power sources for residential and commercial applications. As the important component of hydrogen fuel cells, bipolar plates must be made of permeability materials, be anti-corrosive, electrically conductive between adjacent cells, have a uniform feed gas diffusion and product removal capacity, be of low cost and be economically viable to manufacture [2]. Traditional bipolar plates are made of graphite material. However, the brittleness and high cost of graphite are considered to be major barriers to its mass production [3]. Instead, metal bipolar plates, such as stainless steel, titanium and aluminum alloys, are used [4][5][6]. The conductivity of metallic bipolar plates is a vital requirement. The low electrochemical impedance of metallic bipolar plates indicates the high performance of PEMFC. Under the PEMFC acid environment, the metal bipolar plate is prone to reaction with electrolyte solution, leading to the dissolution of metal ions. The dissolved metal ions are absorbed by the polymer electrolyte membrane and decrease catalytic activity, which can affect the efficiency of PEMFC [7]. In addition, the lower working potential can easily generate an oxidation film on the surface of metal bipolar plates at the cathode side [8]. The thin passivation layer offers poor conductivity, resulting in a lower output power of fuel cells. Thus, some modification of the material surface has been developed to improve these properties.
The deposition of the DLC film as a protective coating is a promising solution to protect the metallic bipolar plate from a harsh acid operation condition. There are several studies about the deposition method of DLC film, such as sputtering deposition, plasma enhanced chemical vapor deposition (PECVD), ion beam deposition (IBD), cathodic vacuum arc deposition (CVA) and other techniques [9]. In the deposition process, impurities present on the surface of the sample are also coated with the DLC film along with the sample. When the plasma supply is stopped, the impurities and substrate with different thermal expansion coefficients return to the original state, resulting in the presence of pinholes in the DLC film [10]. Due to the processing temperature of IBD, it can be held below 93 • C (200 • F) and no such pinholes are produced on the surface. The IBD is a physical thin film technique that achieves a high degree of precision and uniformity via the simultaneous bombardment of energetic atomic particles. Therefore, the improved film performance, such as high interfacial adhesion, high density, smooth surface morphology and low internal stresses, are obtained by the IBD method. Toro et al. proposed a DLC coating using a PECVD method as the protective film to improve anti-corrosion properties [11]. Wang et al. prepared a dense polypyrrole film on SUS316L to resist corrosion by galvanostatic and cyclic voltammetric methods [12]. Some researchers have developed noble metal film on titanium bipolar plates by pulsed bias arc ion plating, such as Au and Ag, which have a prominent corrosion resistance but easily form an oxide layer and are expensive [13,14]. We prepared DLC film on the SUS316L substrate as the protective coating against corrosion in our previous works, which found that the stainless steel still has intergranular corrosion under a long-term high temperature and acidic environment [15]. Liu et al. applied the magnetron sputtering method to deposit CrCuAgN film onto the metallic bipolar plate [16]. However, there is little publication about the DLC film deposition on metallic bipolar plates using the IBD technique.
In this paper, a-C:H/DLC film was deposited on titanium substrate by the ion beam deposition method with different gas ratios CH 4 /H 2 and deposition times as bipolar plates for PEMFC. The chemical characteristics, interfacial contact resistance and electrochemical corrosion behavior of the a-C:H film-coated titanium substrate were evaluated in a simulated PEMFC environment.

Materials and Methods
The IBD method combines the advantages of chemical and thermal processes. A schematic diagram of DLC film deposition using the IBD method is presented in Figure 1. In the first step, the hydrocarbon source mixture is ionized by an accelerating voltage [17]. The ionized hydrocarbon ions are accelerated to form ion beam plasma, such as CH 5 + , H + and CH 3 + ions. In the second stage, the coating atoms form the case layer after penetrating the Ti surface. Several reactions take place in surface processes, such as adsorption, direct incorporation of ions, reemission of H, surface etching and sputtering, in the deposition process of a-C:H coatings [18]. The a-C:H coating properties are improved with the graded interfacial layer, leading to the improvement of adhesion and reduction of internal stresses [19]. The substrate material was titanium cut into dimensions of 100 mm (L) × 100 mm (W) × 0.1 mm (H). All the samples were polished with waterproof paper (#2000), ultrasonically cleaned with acetone for 10 min, dried and fixed onto the rotating substrate holder in the ion implantation system chamber as shown in Figure 1. Before deposition, the chamber was evacuated to 4 × 10 −4 Pa for plasma discharge. After that, a mixture of CH4 and H2 gas was introduced into the chamber to make the chamber pressure stabilized at 2 × 10 −3 Pa. The deposition time of the film was set to 6 h and 12 h. The current, accelerating voltage and gas flow were set at 40 mA, 9 kV and 0.5 sccm for all the samples, respectively. Finally, after deposition, all the titanium substrates coated with DLC films were cooled down inside the chamber. Table 1 shows the a-C:H films at different conditions. The micromorphological characterization and surface roughness of the coatings were investigated by a HITACHI-TM3000 scanning electron microscope (SEM) (Hitachi Ltd., Tokyo, Japan) and an SPM-9500J3 atomic force microscopy (AFM) (Shimadzu Ltd., Kyoto, Japan). The samples used for the transmission electron microscopy (TEM) (Hitachi Ltd., Tokyo, Japan) and the SEM were prepared by a focused ion beam (FIB) with a JIB-4500 Multi Beam System (JEOL Ltd., Akishima-shi, Japan). The chemical composition of the cross-section was observed by a JXA-8900R electron probe microanalysis (EPMA) (JEOL Ltd., Akishima-shi, Japan). The DLC film microstructural characterization and element composition were determined by an NRS-4100 Raman measurement (JASCO Inc., Easton, USA) with a laser wavelength of 532.0 nm, a HF-3300 TEM and a Physical Electronics (PHI) Quantum 2000 X-ray photoelectron spectroscopy (XPS) (Pleasanton, CA, USA) equipped with a monochromatic Al Kα X-ray excitation source and an energy of 1486.6 eV. The working conditions of 12 mA and 13 kV were employed. The base pressure of the chamber was about 1.33 × 10 −7 Pa. All the spectra were collected at 20 eV pass energy to ensure an energy resolution of 1.0 eV and high sensitivity. The ICR of the DLC films coated on titanium was measured by a conventional test mentioned in the previous literature [20].
The anti-corrosion property is an important factor for evaluating the performance of bipolar plates. Thus, the DLC film-coated titanium was corroded by potentiostatic polarizations in a 0.5 mol/L H2SO4 solution at 90 °C for one week. The corrosion measurement is illustrated in Figure 2. The precipitated titanium ions in the H2SO4 solution were detected by inductively coupled plasma atomic emission spectroscopy (ICPS-7000) (Shimadzu Ltd., Kyoto, Japan) after the corrosion test. The electrochemical performance of the a-C:H film deposited on the titanium was investigated in 0.5 mol/L H2SO4 solution at 90 The substrate material was titanium cut into dimensions of 100 mm (L) × 100 mm (W) × 0.1 mm (H). All the samples were polished with waterproof paper (#2000), ultrasonically cleaned with acetone for 10 min, dried and fixed onto the rotating substrate holder in the ion implantation system chamber as shown in Figure 1. Before deposition, the chamber was evacuated to 4 × 10 −4 Pa for plasma discharge. After that, a mixture of CH 4 and H 2 gas was introduced into the chamber to make the chamber pressure stabilized at 2 × 10 −3 Pa. The deposition time of the film was set to 6 h and 12 h. The current, accelerating voltage and gas flow were set at 40 mA, 9 kV and 0.5 sccm for all the samples, respectively. Finally, after deposition, all the titanium substrates coated with DLC films were cooled down inside the chamber. Table 1 shows the a-C:H films at different conditions. The micromorphological characterization and surface roughness of the coatings were investigated by a HITACHI-TM3000 scanning electron microscope (SEM) (Hitachi Ltd., Tokyo, Japan) and an SPM-9500J3 atomic force microscopy (AFM) (Shimadzu Ltd., Kyoto, Japan). The samples used for the transmission electron microscopy (TEM) (Hitachi Ltd., Tokyo, Japan) and the SEM were prepared by a focused ion beam (FIB) with a JIB-4500 Multi Beam System (JEOL Ltd., Akishima-shi, Japan). The chemical composition of the cross-section was observed by a JXA-8900R electron probe microanalysis (EPMA) (JEOL Ltd., Akishima-shi, Japan). The DLC film microstructural characterization and element composition were determined by an NRS-4100 Raman measurement (JASCO Inc., Easton, PA, USA) with a laser wavelength of 532.0 nm, a HF-3300 TEM and a Physical Electronics (PHI) Quantum 2000 X-ray photoelectron spectroscopy (XPS) (Pleasanton, CA, USA) equipped with a monochromatic Al Kα X-ray excitation source and an energy of 1486.6 eV. The working conditions of 12 mA and 13 kV were employed. The base pressure of the chamber was about 1.33 × 10 −7 Pa. All the spectra were collected at 20 eV pass energy to ensure an energy resolution of 1.0 eV and high sensitivity. The ICR of the DLC films coated on titanium was measured by a conventional test mentioned in the previous literature [20].
The anti-corrosion property is an important factor for evaluating the performance of bipolar plates. Thus, the DLC film-coated titanium was corroded by potentiostatic polarizations in a 0.5 mol/L H 2 SO 4 solution at 90 • C for one week. The corrosion measurement is illustrated in Figure 2. The precipitated titanium ions in the H 2 SO 4 solution were detected by inductively coupled plasma atomic emission spectroscopy (ICPS-7000) (Shimadzu Ltd., Kyoto, Japan) after the corrosion test. The electrochemical performance of the a-C:H film deposited on the titanium was investigated in 0.5 mol/L H 2 SO 4 solution at 90 • C by a conventional three-electrode method reported in [21]. The DLC film-coated specimen, platinum wire and saturated calomel electrode (SCE) acted as the working electrode, counter electrode and reference electrode, respectively. The test sample was exposed to the corrosive solution on one side with an area of 10 × 10 mm 2 while the other side was sealed. °C by a conventional three-electrode method reported in [21]. The DLC film-coated specimen, platinum wire and saturated calomel electrode (SCE) acted as the working electrode, counter electrode and reference electrode, respectively. The test sample was exposed to the corrosive solution on one side with an area of 10 × 10 mm 2 while the other side was sealed.

Surface Topography
The surface topography of the uncoated titanium and the DLC film-coated titanium substrates were observed through AFM and SEM, and the photos are presented in Figure  3, which shows that the a-C:H coatings are dense and composed of spherical particles on the entire specimen surface, with diameters of 0.2 µm~1.6 µ m. The surface morphology of a-C:H coatings deposited at CH4/H2 = 1:0 presents a smoother surface and a smaller granularity than that of a-C:H coatings at CH4/H2 = 1:1, which means that the high CH4 fraction is positive for the smooth surface morphology with nearly no pinholes. According to Ortiz-Medina [22], the increasing of CH4 would change the surface topography. Titanium substrate can be protected from the sulfuric acid solution by depositing dense a-C:H coatings. From the surface microstructure, the high CH4 fraction and long depositing time are more favorable to the denser a-C:H coatings. Figure 4 shows the AFM pictures and average roughness values of the DLC film coated on titanium with random areas of 2 µ m × 2 µ m. As shown in Figure 4a-e, the undulating hills appear on the surface of the bare specimen and DLC films with a height of about 158.21 nm, 126.26 nm, 68.48 nm, 91.75 nm and 66.03 nm, respectively. The average roughness Ra values of the a-C:H coatings at CH4/H2 = 1:0 (7.38 nm and 5.76 nm) are lower than that of DLC films at CH4/H2 = 1:1 (8.98 nm and 6.95 nm). All of them are lower than the Ra of the uncoated titanium surface (14.01 nm). This is because during the deposition process, upon implantation of the hydrocarbon ions, they conveyed substantial energy to the film for substrate heating. The process of substrate heating provides a denser and more uniform coating. The undulating hills are more flattened with the increasing methane fraction, which has a higher sp 3 ratio, and an increase in the hydrogen fraction leads to an increase in the surface roughness; these may be caused by the etching effect [23]. Liang et al. [24] believed that hydrogen etching was a variable that affects the average roughness. The Ti specimen would be destroyed under the process of deposition. Moreover, the incident CHx group ions can form sp 2 and sp 3 hybridizations following the chemical reaction on the Ti substrate surface. Many sp 2 C and sp 3 C structures result in the growth of CHx group ions on the substrate surface. However, the H ions/atoms/molecules that promoted

Surface Topography
The surface topography of the uncoated titanium and the DLC film-coated titanium substrates were observed through AFM and SEM, and the photos are presented in Figure 3, which shows that the a-C:H coatings are dense and composed of spherical particles on the entire specimen surface, with diameters of 0.2 µm~1.6 µm. The surface morphology of a-C:H coatings deposited at CH 4 /H 2 = 1:0 presents a smoother surface and a smaller granularity than that of a-C:H coatings at CH 4 /H 2 = 1:1, which means that the high CH 4 fraction is positive for the smooth surface morphology with nearly no pinholes. According to Ortiz-Medina [22], the increasing of CH 4 would change the surface topography. Titanium substrate can be protected from the sulfuric acid solution by depositing dense a-C:H coatings. From the surface microstructure, the high CH 4 fraction and long depositing time are more favorable to the denser a-C:H coatings. Figure 4 shows the AFM pictures and average roughness values of the DLC film coated on titanium with random areas of 2 µm × 2 µm. As shown in Figure 4a-e, the undulating hills appear on the surface of the bare specimen and DLC films with a height of about 158.21 nm, 126.26 nm, 68.48 nm, 91.75 nm and 66.03 nm, respectively. The average roughness Ra values of the a-C:H coatings at CH 4 /H 2 = 1:0 (7.38 nm and 5.76 nm) are lower than that of DLC films at CH 4 /H 2 = 1:1 (8.98 nm and 6.95 nm). All of them are lower than the Ra of the uncoated titanium surface (14.01 nm). This is because during the deposition process, upon implantation of the hydrocarbon ions, they conveyed substantial energy to the film for substrate heating. The process of substrate heating provides a denser and more uniform coating. The undulating hills are more flattened with the increasing methane fraction, which has a higher sp 3 ratio, and an increase in the hydrogen fraction leads to an increase in the surface roughness; these may be caused by the etching effect [23]. Liang et al. [24] believed that hydrogen etching was a variable that affects the average roughness. The Ti specimen would be destroyed under the process of deposition. Moreover, the incident CH x group ions can form sp 2 and sp 3 hybridizations following the chemical reaction on the Ti substrate surface. Many sp 2 C and sp 3 C structures result in the growth of CH x group ions on the substrate surface. However, the H ions/atoms/molecules that promoted sp 3 hybridization contribute to the growth of the CH x groups on the substrate surface. Therefore, the competition between the CH x groups and H ions/atoms/molecules, as well as the etching and growth effects on the substrate surface must be taken into consideration. There is an abundance of dangling C bonds on the substrate surface due to the small amounts of H groups in the ion beam. Incident CH x group ions grow on the substrate surface because the dangling carbon bond on the surface could not be totally saturated. Hence, with the CH 4 ratio increases, the growth effect dominates the deposition process, which leads to a smoother specimen. The AFM result corresponded to the SEM surface analysis.
Coatings 2021, 11, x FOR PEER REVIEW 5 of 16 sp 3 hybridization contribute to the growth of the CHx groups on the substrate surface. Therefore, the competition between the CHx groups and H ions/atoms/molecules, as well as the etching and growth effects on the substrate surface must be taken into consideration. There is an abundance of dangling C bonds on the substrate surface due to the small amounts of H groups in the ion beam. Incident CHx group ions grow on the substrate surface because the dangling carbon bond on the surface could not be totally saturated. Hence, with the CH4 ratio increases, the growth effect dominates the deposition process, which leads to a smoother specimen. The AFM result corresponded to the SEM surface analysis.   sp 3 hybridization contribute to the growth of the CHx groups on the substrate surface. Therefore, the competition between the CHx groups and H ions/atoms/molecules, as well as the etching and growth effects on the substrate surface must be taken into consideration. There is an abundance of dangling C bonds on the substrate surface due to the small amounts of H groups in the ion beam. Incident CHx group ions grow on the substrate surface because the dangling carbon bond on the surface could not be totally saturated. Hence, with the CH4 ratio increases, the growth effect dominates the deposition process, which leads to a smoother specimen. The AFM result corresponded to the SEM surface analysis.    Figure 5 shows the cross-section SEM images of the DLC films with different deposition conditions, which compactly and densely cover the entire specimen surface. Due to the permeation layer, the thickness of the fully dense film cannot be clearly defined (which was described in Section 2). The a-C:H layer of the fully dense film reaches its maximum thickness of ~1281 nm as shown in Figure 5d. Figure 5a-c illustrates that the average thickness of the fully dense film is ~533.7 nm, 587.1 nm and 693.9 nm, respectively. With a longer deposition time, the thickness of the fully dense a-C:H layer increases sharply. The titanium substrate can be better prevented from direct corrosion with a larger density of a-C:H coating. With the increase in time, the high-energy ion beams bombard the growing film continuously, thus promoting the DLC atoms penetrating into the specimen with more energy to increase the thickness of the fully dense film and the permeation layer [22].  Figure 5 shows the cross-section SEM images of the DLC films with different deposition conditions, which compactly and densely cover the entire specimen surface. Due to the permeation layer, the thickness of the fully dense film cannot be clearly defined (which was described in Section 2). The a-C:H layer of the fully dense film reaches its maximum thickness of~1281 nm as shown in Figure 5d. Figure 5a-c illustrates that the average thickness of the fully dense film is~533.7 nm, 587.1 nm and 693.9 nm, respectively. With a longer deposition time, the thickness of the fully dense a-C:H layer increases sharply. The titanium substrate can be better prevented from direct corrosion with a larger density of a-C:H coating. With the increase in time, the high-energy ion beams bombard the growing film continuously, thus promoting the DLC atoms penetrating into the specimen with more energy to increase the thickness of the fully dense film and the permeation layer [22].   Figure 5 shows the cross-section SEM images of the DLC films with different deposition conditions, which compactly and densely cover the entire specimen surface. Due to the permeation layer, the thickness of the fully dense film cannot be clearly defined (which was described in Section 2). The a-C:H layer of the fully dense film reaches its maximum thickness of ~1281 nm as shown in Figure 5d. Figure 5a-c illustrates that the average thickness of the fully dense film is ~533.7 nm, 587.1 nm and 693.9 nm, respectively. With a longer deposition time, the thickness of the fully dense a-C:H layer increases sharply. The titanium substrate can be better prevented from direct corrosion with a larger density of a-C:H coating. With the increase in time, the high-energy ion beams bombard the growing film continuously, thus promoting the DLC atoms penetrating into the specimen with more energy to increase the thickness of the fully dense film and the permeation layer [22]. In Figure 6, the carbon element concentration distribution and cross-section morphology of the a-C:H coatings are evaluated by EPMA. The element diffusivity of carbon is clearly seen by EPMA mapping. During the ion beam bombardment process, the energetic atomic particles interact with film atoms, drive them into the titanium substrate and produce a graded interface to improve adhesion. Then, the film atoms grow out from the graded interface to form the dense DLC film. As shown in Figure 6b,d,f,h, the total thickness of the DLC film, including the dense film and penetration layer, is ~3 µ m, 4 µ m, 4 µ m and 12 µ m, respectively. The carbon concentration on the surface of the DLC coating deposited at CH4/H2 = 1:0 is obviously higher than that of CH4 = 50%. It may be caused by the reaction between the abundance of the H ions and C-C bonds formed on the surface, which reduces the film formation rate. In Figure 6, the carbon element concentration distribution and cross-section morphology of the a-C:H coatings are evaluated by EPMA. The element diffusivity of carbon is clearly seen by EPMA mapping. During the ion beam bombardment process, the energetic atomic particles interact with film atoms, drive them into the titanium substrate and produce a graded interface to improve adhesion. Then, the film atoms grow out from the graded interface to form the dense DLC film. As shown in Figure 6b,d,f,h, the total thickness of the DLC film, including the dense film and penetration layer, is~3 µm, 4 µm, 4 µm and 12 µm, respectively. The carbon concentration on the surface of the DLC coating deposited at CH 4 /H 2 = 1:0 is obviously higher than that of CH 4 = 50%. It may be caused by the reaction between the abundance of the H ions and C-C bonds formed on the surface, which reduces the film formation rate. In Figure 6, the carbon element concentration distribution and cross-section morphology of the a-C:H coatings are evaluated by EPMA. The element diffusivity of carbon is clearly seen by EPMA mapping. During the ion beam bombardment process, the energetic atomic particles interact with film atoms, drive them into the titanium substrate and produce a graded interface to improve adhesion. Then, the film atoms grow out from the graded interface to form the dense DLC film. As shown in Figure 6b,d,f,h, the total thickness of the DLC film, including the dense film and penetration layer, is ~3 µ m, 4 µ m, 4 µ m and 12 µ m, respectively. The carbon concentration on the surface of the DLC coating deposited at CH4/H2 = 1:0 is obviously higher than that of CH4 = 50%. It may be caused by the reaction between the abundance of the H ions and C-C bonds formed on the surface, which reduces the film formation rate.

Raman Analysis
Three key Raman parameters were used to evaluate the bonding state of the DLC film: the intensity ratio of D peak to G peak (I D /I G ), the full width at half maximum of G peak (FWHM) and the position of G peak. Figure 7 shows the Raman curves of the a-C:H coatings deposited at different conditions. The G peak and D peak are identified at approximately 1560 cm −1 and 1360 cm −1 , identifying with the graphite band (arising from breathing modes of sp 2 atoms in rings) and the disordered band (arising from bond stretching of sp 2 atoms in rings and chains), respectively [25]. The G peak position and I D /I G are regarded to have a linear relationship with the graphite crystallite size [26]. FWHM (G) has a greater sensitivity to structure disorder, which originates from the bond length and bond angle. For DLC coatings with sp 3 content (>20%), a shift of the G peak position to higher wavenumbers is consistent with an increase in sp 3 [27]. A smaller I D /I G and higher FWHM (G) can result in a higher coating density [28]. As shown in Figure 7, as the I D /I G fraction decreases, the G peak position shifts toward a higher wavenumber. With increasing methane fraction and deposition time, the FWHM (G) increases, which indicates the increasing of the sp 3 C-C bond ratio and the density of the DLC coating. Table 2 shows the detailed Raman parameters. Contrasted with CH 4 = 50%, it seems that the effect of increasing CH 4 to the precursor gas has a consistent effect of increasing the sp 3 C fraction, which may be because CH 4 promotes the formation of the C-H bond. The a-C:H film with a low sp 3 ratio ordinarily indicates low internal stress. High values of compressive stress should be prevented in film growth, nevertheless, it is also found that a moderate compressive stress could be favorable for the anti-corrosion property. Particularly, compressive residual stress can prevent the propagation of delamination through the film layer, which leads to the shrinkage of the open corrosion path in the films.

Raman Analysis
Three key Raman parameters were used to evaluate the bonding state of the DLC film: the intensity ratio of D peak to G peak (ID/IG), the full width at half maximum of G peak (FWHM) and the position of G peak. Figure 7 shows the Raman curves of the a-C:H coatings deposited at different conditions. The G peak and D peak are identified at approximately 1560 cm −1 and 1360 cm −1 , identifying with the graphite band (arising from breathing modes of sp 2 atoms in rings) and the disordered band (arising from bond stretching of sp 2 atoms in rings and chains), respectively [25]. The G peak position and ID/IG are regarded to have a linear relationship with the graphite crystallite size [26]. FWHM (G) has a greater sensitivity to structure disorder, which originates from the bond length and bond angle. For DLC coatings with sp 3 content (>20%), a shift of the G peak position to higher wavenumbers is consistent with an increase in sp 3 [27]. A smaller ID/IG and higher FWHM (G) can result in a higher coating density [28]. As shown in Figure 7, as the ID/IG fraction decreases, the G peak position shifts toward a higher wavenumber. With increasing methane fraction and deposition time, the FWHM (G) increases, which indicates the increasing of the sp 3 C-C bond ratio and the density of the DLC coating. Table 2 shows the detailed Raman parameters. Contrasted with CH4 = 50%, it seems that the effect of increasing CH4 to the precursor gas has a consistent effect of increasing the sp 3 C fraction, which may be because CH4 promotes the formation of the C-H bond. The a-C:H film with a low sp 3 ratio ordinarily indicates low internal stress. High values of compressive stress should be prevented in film growth, nevertheless, it is also found that a moderate compressive stress could be favorable for the anti-corrosion property. Particularly, compressive residual stress can prevent the propagation of delamination through the film layer, which leads to the shrinkage of the open corrosion path in the films.

TEM Analysis
The microstructure of the DLC film-coated titanium was evaluated by TEM. Before the experiment, FIB-SEM was used to prepare a sample with a thickness of about 0.1 µm for research. As shown in Figure 8a,b, the schematic diagram and TEM micrographs of the DLC film deposited on titanium at CH 4 /H 2 = 1:0, 12 h are presented. The local nano-crystals and selected area electron diffraction pattern (SAED) of the amorphous DLC film are also shown in Figure 8c-f. From Figure 8c,e, part A, the amorphous phase microstructure of the pure DLC layer was confirmed by TEM analysis, which is consistent with previous studies [29,30]. As we can see in the high-resolution TEM images taken with TiC rings (Figure 8d, part B), a little TiC nanocrystalline (marked by yellow circles) was surrounded by an amorphous matrix, which suggests that atoms in the DLC have penetrated the metal substrate. The interplanar spacing can be calculated from the diffraction pattern, where d values could be calculated by the following formula: where r is the radius of the diffraction ring, λ is the electron beam wavelength and L is the distance of the photo graphic plate from the specimen (the product λL = A, called the camera constant). As shown in Figure 8f, polycrystalline rings of cubic TiC corresponding to (111), (220) and (222) crystal planes are displayed. The average lattice constant of cubic TiC acquired from the electron diffraction pattern is 0.250 nm, agreeing well with the result obtained from the TEM micrograph. As shown in Figure 8f, there are no other crystalline diffraction characteristics, indicating that only the cubic TiC phase exists in the DLC coating with an amorphous hydrocarbon matrix [31]. The nanocrystalline TiC phase has a stable and strong chemical bond energy combining titanium and carbon atoms together, which can be seen from the penetration layer of the DLC coatings. Because of the densification and chemical inertness of the DLC coating, it can protect metal substrate from corrosive solution well.

TEM Analysis
The microstructure of the DLC film-coated titanium was evaluated by TEM. Before the experiment, FIB-SEM was used to prepare a sample with a thickness of about 0.1 µm for research. As shown in Figure 8a,b, the schematic diagram and TEM micrographs of the DLC film deposited on titanium at CH4/H2 = 1:0,12 h are presented. The local nanocrystals and selected area electron diffraction pattern (SAED) of the amorphous DLC film are also shown in Figure 8c-f. From Figure 8c,e, part A, the amorphous phase microstructure of the pure DLC layer was confirmed by TEM analysis, which is consistent with previous studies [29,30]. As we can see in the high-resolution TEM images taken with TiC rings (Figure 8d, part B), a little TiC nanocrystalline (marked by yellow circles) was surrounded by an amorphous matrix, which suggests that atoms in the DLC have penetrated the metal substrate. The interplanar spacing can be calculated from the diffraction pattern, where d values could be calculated by the following formula: where r is the radius of the diffraction ring, λ is the electron beam wavelength and L is the distance of the photo graphic plate from the specimen (the product λL = A, called the camera constant). As shown in Figure 8f, polycrystalline rings of cubic TiC corresponding to (111), (220) and (222) crystal planes are displayed. The average lattice constant of cubic TiC acquired from the electron diffraction pattern is 0.250 nm, agreeing well with the result obtained from the TEM micrograph. As shown in Figure 8f, there are no other crystalline diffraction characteristics, indicating that only the cubic TiC phase exists in the DLC coating with an amorphous hydrocarbon matrix [31]. The nanocrystalline TiC phase has a stable and strong chemical bond energy combining titanium and carbon atoms together, which can be seen from the penetration layer of the DLC coatings. Because of the densification and chemical inertness of the DLC coating, it can protect metal substrate from corrosive solution well.

XPS Characterization
To further understand the element composition, the bonding structure of the a-C:H film using different parameters was analyzed by X-ray photoelectron spectroscopy (XPS). In order to obtain a reliable conclusion, Ar + bombardment was used to remove the metal oxide contamination from the environment [32]. The XPS spectrum of C1s of a-C:H film is presented in Figure 9 after removing the metal oxide contamination. The binding energy of C1s can be deconvoluted into three parts. The peak located at 285.1 ± 0.1 eV, 282 ± 0.1 eV and 289.1 ± 0.1 eV is attributed to C-C, Ti-C and O-C=O, respectively, which are the main functional groups on the surface of a-C:H coatings [33]. The Ti-C bond is observed in the XPS spectrum, demonstrating the strong adhesion force between the DLC film and the substrate. The oxygen percentage of DLC coatings deposited at the condition of CH4 = 50%, 6 h; CH4 = 50%, 12 h; CH4 = 100%, 6 h; and CH4 = 100%, 12 h is ~35.1%, 31.8%, 31.7% and 23.3%, respectively, indicating that it gradually decreases with the deposition time. It is well known that a-C:H film with less C=O and C-O bonds suggest a denser coating [34].

XPS Characterization
To further understand the element composition, the bonding structure of the a-C:H film using different parameters was analyzed by X-ray photoelectron spectroscopy (XPS). In order to obtain a reliable conclusion, Ar + bombardment was used to remove the metal oxide contamination from the environment [32]. The XPS spectrum of C1s of a-C:H film is presented in Figure 9 after removing the metal oxide contamination. The binding energy of C1s can be deconvoluted into three parts. The peak located at 285.1 ± 0.1 eV, 282 ± 0.1 eV and 289.1 ± 0.1 eV is attributed to C-C, Ti-C and O-C=O, respectively, which are the main functional groups on the surface of a-C:H coatings [33]. The Ti-C bond is observed in the XPS spectrum, demonstrating the strong adhesion force between the DLC film and the substrate. The oxygen percentage of DLC coatings deposited at the condition of CH 4 = 50%, 6 h; CH 4 = 50%, 12 h; CH 4 = 100%, 6 h; and CH 4 = 100%, 12 h is~35.1%, 31.8%, 31.7% and 23.3%, respectively, indicating that it gradually decreases with the deposition time. It is well known that a-C:H film with less C=O and C-O bonds suggest a denser coating [34].

ICR Test
The key to improving the output power efficiency of PEMFC is ensuring that the metallic bipolar plate has a high conductivity. Therefore, subtracting the resistance of the substrate from the resistance of the a-C:H film-coated titanium can obtain the value of the ICR [35]. Table 3 shows the ICR result of DLC film at 150 N/cm 2 . The DLC film deposited at the condition of CH4 = 50% and 6 h shows the lowest resistance of 7 mΩ·cm 2 . It meets the Department of Energy's (DOE) target (10 mΩ·cm 2 , year 2020). If the CH4 ratio decreases to 50%, the ICR will increase because of its good conductive performance from the high content of sp 2 C bonds. The ICR identifies with the Raman spectroscopy.  Figure 10 presents the surface morphology on the uncoated and DLC film-coated titanium after the corrosion test. The bare substrate was destroyed; meanwhile, some cracks appeared on the surface as shown in Figure 10a. The area of localized corrosion becomes larger as CH4 = 100% decreases to CH4 = 50%. During the 168 h corrosion test in a high temperature acid solution, there exists pitting corrosion, which has been marked with red circles. It was caused by the sulfuric acid solution penetrating into the DLC coating and contacting with the bare Ti substrate. Moreover, the pitting phenomenon also resulted in the acceleration of corrosion in the local region, while causing the film to peel off.

ICR Test
The key to improving the output power efficiency of PEMFC is ensuring that the metallic bipolar plate has a high conductivity. Therefore, subtracting the resistance of the substrate from the resistance of the a-C:H film-coated titanium can obtain the value of the ICR [35]. Table 3 shows the ICR result of DLC film at 150 N/cm 2 . The DLC film deposited at the condition of CH 4 = 50% and 6 h shows the lowest resistance of 7 mΩ·cm 2 . It meets the Department of Energy's (DOE) target (10 mΩ·cm 2 , year 2020). If the CH 4 ratio decreases to 50%, the ICR will increase because of its good conductive performance from the high content of sp 2 C bonds. The ICR identifies with the Raman spectroscopy.  Figure 10 presents the surface morphology on the uncoated and DLC film-coated titanium after the corrosion test. The bare substrate was destroyed; meanwhile, some cracks appeared on the surface as shown in Figure 10a. The area of localized corrosion becomes larger as CH 4 = 100% decreases to CH 4 = 50%. During the 168 h corrosion test in a high temperature acid solution, there exists pitting corrosion, which has been marked with red circles. It was caused by the sulfuric acid solution penetrating into the DLC coating and contacting with the bare Ti substrate. Moreover, the pitting phenomenon also resulted in the acceleration of corrosion in the local region, while causing the film to peel off.

Corrosion Behavior of DLC Film
In Figure 11, the Raman spectrum of a-C:H coatings are shown, in which the films are treated with different deposition parameters after the potentiostatic corrosion. The G peak position shifted toward a higher wavenumber and the sp 3 C ratio increased compared with the Raman analysis before the corrosion test. We speculate that a small amount of the film was peeled off from the surface of the sample. The anti-corrosion behavior of a-C:H coatings deposited at 12 h with smoother curves is better than that of a-C:H coatings deposited at 6 h, according to the Raman spectrum analysis. Moreover, the peak intensity of a-C:H coatings become stronger at CH 4 = 100%.
After the potentiostatic corrosion test for 168 h, the concentration of the dissolved metal ion in the sulfuric acid corrosion solution was measured by ICP. Table 4 shows the results of the metal specimen and a-C:H coatings. Contrary to the bare specimen (about 0.58 ppm), the metal ions dissolved in the liquid have significantly reduced (0.34 ppm~0.49 ppm), which indicated that the a-C:H coating plays a key role in protecting the titanium substrate. The hydrogen ions in an acid environment would form a strong corrosion layer to accelerate the corrosion [36]. It is obvious that the concentration of Ti 4+ in a-C:H coatings formed in a 100% CH4 atmosphere, as 0.34 ppm for 12 h is lower than that of other conditions. The higher density of a-C:H coatings with CH 4 = 100% has an important effect on preventing the corrosive solution from reacting with the metal substrate. Even the DLC film deposited at CH 4 /H 2 = 1:0, 6 h, is lower than that at CH 4 /H 2 = 1:1, 12 h. The smoother surface of a-C:H coatings has a higher anti-corrosion ability due to the smaller exposed area in the corrosive environment. The high oxygen content of a-C:H coatings deposited at CH 4 = 50% can accelerate corrosion in an acid solution as well. The conclusion is in agreement with the AFM and SEM results. In Figure 11, the Raman spectrum of a-C:H coatings are shown, in which the films are treated with different deposition parameters after the potentiostatic corrosion. The G peak position shifted toward a higher wavenumber and the sp 3 C ratio increased compared with the Raman analysis before the corrosion test. We speculate that a small amount of the film was peeled off from the surface of the sample. The anti-corrosion behavior of After the potentiostatic corrosion test for 168 h, the concentration of the dissolved metal ion in the sulfuric acid corrosion solution was measured by ICP. Table 4 shows the results of the metal specimen and a-C:H coatings. Contrary to the bare specimen (about 0.58 ppm), the metal ions dissolved in the liquid have significantly reduced (0.34 ppm~0.49 ppm), which indicated that the a-C:H coating plays a key role in protecting the titanium substrate. The hydrogen ions in an acid environment would form a strong corrosion layer to accelerate the corrosion [36]. It is obvious that the concentration of Ti 4+ in a-C:H coatings formed in a 100% CH4 atmosphere, as 0.34 ppm for 12 h is lower than that of other conditions. The higher density of a-C:H coatings with CH4 = 100% has an important effect on preventing the corrosive solution from reacting with the metal substrate. Even the DLC film deposited at CH4/H2 = 1:0, 6 h, is lower than that at CH4/H2 = 1:1, 12 h. The smoother surface of a-C:H coatings has a higher anti-corrosion ability due to the smaller exposed area in the corrosive environment. The high oxygen content of a-C:H coatings deposited at CH4 = 50% can accelerate corrosion in an acid solution as well. The conclusion is in agreement with the AFM and SEM results. As shown in Figure 12, the polarization curve of Ti substrate and a-C:H coatings in 0.5 mol/L sulfuric acid solution purged with air at 90 °C is displayed. The high active peak of uncoated titanium can be clearly seen due to active dissolution and formation of nonconductive oxide scale. Compared with metal substrate, DLC films coated on titanium show a higher corrosion potential and a more stable passivation zone under simulated fuel cell conditions. Under the condition of gas ratio CH4 = 100% and 12 h, the maximum corrosion potential of ~0.15 V is obtained, which is much higher than the uncoated substrate of ~−0.34 V. It is known that a high corrosion potential usually means an excellent  As shown in Figure 12, the polarization curve of Ti substrate and a-C:H coatings in 0.5 mol/L sulfuric acid solution purged with air at 90 • C is displayed. The high active peak of uncoated titanium can be clearly seen due to active dissolution and formation of non-conductive oxide scale. Compared with metal substrate, DLC films coated on titanium show a higher corrosion potential and a more stable passivation zone under simulated fuel cell conditions. Under the condition of gas ratio CH 4 = 100% and 12 h, the maximum corrosion potential of~0.15 V is obtained, which is much higher than the uncoated substrate of~−0.34 V. It is known that a high corrosion potential usually means an excellent anticorrosion performance due to the noble electrochemical behavior [37]. Compared with the bare specimen (where the corrosion electric current density is~1.8 µA/cm 2 ), the corrosion current density of DLC film-coated titanium is decreased to the minimum (of~0.23 µA/cm 2 , at the condition of CH 4 100% and 12 h) under the actual operation environment where the cathode potential is about 0.6 V. All the corrosion current densities of the DLC films coated on titanium meet the DOE's target (2020) of 1 µA/cm 2 (0.5 mol/L sulfuric acid solution with 5 ppm HF at 70 • C). The smaller corrosion current density usually implies a smaller corrosion rate. The electrochemical performance of different DLC films coated on titanium is similar on the polarization curves. The DLC films on the substrate can protect titanium from reacting with the sulfuric acid solution to prevent metal corrosion. DLC film using the condition of gas ratio CH 4 100% and a longer deposition time of 12 h has a more stable electrochemical property and a better anti-corrosion performance because of the inherent microstructure characteristic, which can be contrasted by the SEM analysis. The electrochemical behavior is not carried out in the anode environment because the corrosion is relatively weak compared with the environment of the cathode [35].
can protect titanium from reacting with the sulfuric acid solution to prevent metal corrosion. DLC film using the condition of gas ratio CH4 100% and a longer deposition time of 12 h has a more stable electrochemical property and a better anti-corrosion performance because of the inherent microstructure characteristic, which can be contrasted by the SEM analysis. The electrochemical behavior is not carried out in the anode environment because the corrosion is relatively weak compared with the environment of the cathode [35].

Conclusions
The a-C:H/DLC film on titanium substrate as a protective film was formed using the IBD method by changing the CH4/H2 ratio and deposition time for PEMFC. The IBD technique has many advantages, such as lower compressive stress because of the existence of a graded interfacial, higher film-substrate adhesion and better surface characteristics of high precision parts. The chemical composition and corrosion property of the DLC coatings were analyzed. The following conclusions are drawn: The surface roughness and ID/IG fraction of the a-C:H/DLC coating deposited at CH4/H2 = 1:0 are lower, while they decrease with the increasing deposition time. The ID/IG fraction decreases to a minimum of 0.827 at CH4/H2 = 1:0, 12 h, indicating the highest sp 3 bond fraction, mainly because the CH4 promotes the formation of the C-H bond. The ICR reduced to 7 mΩ cm² at CH4/H2 = 1:1, 6 h, because of the high sp 2 C content. The thickness of the DLC film decreases with the introduction of hydrogen due to the etching effect of H ions. The cubic TiC phase found from the penetration layer has a stable and strong chemical bond energy.
The a-C:H coating deposited at CH4 = 100% is denser than that at CH4 = 50% from the SEM analysis. With the parameter of CH4 = 100%, 12 h, the a-C:H coating has the lowest

Conclusions
The a-C:H/DLC film on titanium substrate as a protective film was formed using the IBD method by changing the CH 4 /H 2 ratio and deposition time for PEMFC. The IBD technique has many advantages, such as lower compressive stress because of the existence of a graded interfacial, higher film-substrate adhesion and better surface characteristics of high precision parts. The chemical composition and corrosion property of the DLC coatings were analyzed. The following conclusions are drawn: The surface roughness and I D /I G fraction of the a-C:H/DLC coating deposited at CH 4 /H 2 = 1:0 are lower, while they decrease with the increasing deposition time. The I D /I G fraction decreases to a minimum of 0.827 at CH 4 /H 2 = 1:0, 12 h, indicating the highest sp 3 bond fraction, mainly because the CH 4 promotes the formation of the C-H bond. The ICR reduced to 7 mΩ·cm 2 at CH 4 /H 2 = 1:1, 6 h, because of the high sp 2 C content. The thickness of the DLC film decreases with the introduction of hydrogen due to the etching effect of H ions. The cubic TiC phase found from the penetration layer has a stable and strong chemical bond energy.
The a-C:H coating deposited at CH 4 = 100% is denser than that at CH 4 = 50% from the SEM analysis. With the parameter of CH 4 = 100%, 12 h, the a-C:H coating has the lowest titanium ion concentration of 0.34 ppm with the maximum thickness of about 12 µm, which indicate the best anti-corrosion behavior because the a-C:H coating is dense from the surface morphology. Compared with the bare specimen, the DLC specimen reveals the highest corrosion potential of 0.15 V and the lowest corrosion current density of 0.23 µA/cm 2 . This higher electrochemical stability implied a better anti-corrosion property. All the corrosion current densities of the DLC films meet the DOE's 2020 target of 1 µA/cm 2 . The pitting corrosion occurs in the passivation layer because of a self-excited reaction. Meanwhile, some pores are formed on the surface, which can accelerate the corrosion rate and cause the film to peel off. However, the nanocrystalline TiC phase in the penetration layer inhibits this behavior due to the enhanced adhesion.
Therefore, the a-C:H coating deposited on the Ti substrate by the IBD method at CH 4 /H 2 = 1:0 has the better corrosion inhibition property resistance. As a result, the DLC coating is helpful to decrease the rate of corrosion and can be considered as a candidate bipolar plate for PEMFCs.