Austenitic stainless steel (SS316L) of 0.1 mm thickness was used as substrate material. The specimens were cleaned with distilled water, methanol and then dried with nitrogen prior to deposition. For the deposition of the carbon-based coatings, a cathodic arc deposition system was used. The base pressure of the vacuum chamber was set at 10⁻⁴ Pa. Before coating, the native oxide layer of the steel substrates was removed by a plasma etching process. A chromium interlayer was deposited on the cleaned substrates, from a pure chromium target (99.99%), at a chamber temperature of 300˚C and pressure of approximately 10⁻¹ Pa. Afterwards, three different carbon top layer variants were deposited from a 99.99% graphite target in the deposition temperature range between 300˚C and 100˚C. Bias voltage of several hundred of volts and pressure of approximately 10⁻¹ Pa were kept constant for all carbon thin films. The total thickness, measured by X-ray reflectivity, of the deposited thin films was approximately 60 - 66 nm.

Raman spectroscopy was used to evaluate the microstructure of the deposited thin films. The measurements were done with a Renishaw inVia and an excitation laser of 514 nm wavelength. For the evaluation of the D peak and G peak, a Lorentzian and a Breit-Wigner-Fano function were used, respectively.

The interfacial contact resistance was determined similar to the procedure described from Davies et al. [4] and Wang et al. [5]. In brief, the measurement setup consits of a tensile tester for applying the compaction force and an electrical circuit attached to two gold-coated copper blocks with two GDL sheets and a sample in-between. The total resistance of the assembly is measured by an ohmmeter. By measuring the contact resistance of one GDL sample, which is also used for calibration, one can calculate the individual contact resistance of the sample. For the measurements the compaction force ranged from 25 to 200 N·cm⁻².

Corrosion resistance was investigated by potentiodynamic polarization tests in 0.5 M H₂SO₄ solution at room temperature and 80˚C. For this purpose, a Metrohm Autolab PGSTAT302N potentio-/galvanostat was used with a three-electrode setup including a graphite rod as counter electrode, a Ag/AgCl reference and the sample as working electrode. Before the potentiodynamic tests, the open circuit potential was recorded for a duration of 1 h. The measurements were performed between −0.5 to 1.5 V vs. standard hydrogen electrode (SHE) at a scan rate of 0.001 V·s⁻¹.

Raman spectra of carbon films are mainly composed of two broad peaks at around 1350 cm⁻¹ and 1580 - 1600 cm⁻¹, namely D and G peak, respectively. The G peak (graphite) is related to the graphite lattice and the bond stretching of all sp² atoms, and the D peak (defect) results from the breathing modes of carbon rings and defects in the graphite crystalline structure [33]. Figure 1 shows the recorded Raman spectra of the deposited thin films. Whilst, the films deposited at 300˚C and 100˚C exhibit similar spectra and have pronounced D and G peaks (see Figure 1(a)), the 200˚C carbon thin film only has small peaks (see Figure 1(b)).

The shape of this Raman spectrum corresponds to a more disordered structure as described by Onoprienko et al. [34] and Chung et al. [35].

The full width half maximum (FWHM) and position of the G peak are associated with disorder in the film structure, i.e. distortion of bond length and angle in six-fold carbon rings which originates from poor graphitic in-plane ordering [36] [37]. Additionally, higher I D / I G and G peak position indicates a higher sp² content in the material. According to Table 1, the carbon top layers deposited at 300˚C and 100˚C are likely to exhibit a similar microstructure. In comparison to that, the 200˚C carbon thin film has a significantly higher intensity ratio I D / I G of 1.59 and G peak position. Moreover, the FWHM of the G peak is the lowest of the samples and together with the other Raman parameters this indicates that the carbon top layer deposited at 200˚C might have a different microstructure but still a very high sp² content. This is in good agreement with the evaluation of carbon Raman spectra from Ferrari et al. [33] [36] [38].

J. Robertson [30] reported that for G peak FWHM values above 50 cm⁻¹ the graphite crystallites are less than or equal to 2 nm in size. In this region it is possible to determine the grain size L a by

I D / I G = c L a 2 (1)

where L a is in Å and c is a constant approximately equal to 0.0055 [36]. Thus, the carbon thin films in this work have a grain size of 1.34 nm, 1.70 nm and 1.32 nm, respectively. Based on the results, the deposited carbon top layers can be classified as graphite-like carbon or nano-crystalline carbon with probably up to 100% sp² content.

The interfacial contact resistance between the metallic BPP and the GDL has a significant impact on the achieved electrical power (output) of a PEMFC. Hence, a low ICR is necessary for bipolar plates and corresponding surface modifications. Figure 2 shows the ICR as a function of the applied compaction force for

the carbon-based coatings depending on the carbon deposition temperature. Up to a compaction of 100 N/cm², the contact resistance rapidly decreases because of the constant increase in the contact area between the GDL and the specimen. The bare SS316L substrate exhibits the highest ICR values which can be attributed to the native oxide layer. This underlines the need for highly conductive coatings. By applying a carbon-based coating, the ICR is reduced by more than two orders of magnitude. There are only small differences between the carbon layers deposited at different temperatures. However, the sample with a carbon top layer deposited at 200˚C has the highest resistance in the measurement range. Still, all carbon-based coatings more than fulfill the criteria, released by the U.S. department of energy, of 10 mΩ·cm² [39] and achieve values less than 2 mΩ·cm² at 150 N/cm².

The potentiodynamic polarization curves of all specimens at room temperature in 0.5 M H₂SO₄ are depicted in Figure 3. The bare SS316L substrate has the lowest

corrosion resistance and thus the highest corrosion current j corr of approx. 1.8 × 10⁻⁵ A·cm⁻² (see Table 2). At around 0 V the formation of a protective oxide layer begins, which can be seen by the strong oxidation peak. Up to a potential of 1.2 V, the stainless steel remains inert in the environment and above this potential the material enters the transpassive state. Here, the dissociation of the electrolyte (i.e. water) occurs as well. This results in a steep increase in the current signal which can also be observed for the coated samples.

The deposition of a carbon-based coating greatly improves the corrosion resistance leading to a significantly lower corrosion current j corr between 10⁻⁶ and 10⁻⁹ A·m⁻² (see Table 2). Structural properties such as surface morphology, grain size and defects affect the corrosion resistance of a material and thus the corrosion current density [40]. Consequently, the microstructure of a surface modification, i.e. a PVD coating, is an important parameter. The considerable lower current density of the 200˚C carbon top layer can be attributed to its different microstructure compared to the other carbon thin films.

At an electrolyte temperature of 80˚C, the differences between the polarization curves are less distinct except for the sample with a carbon top layer deposited at 200˚C (see Figure 3, right). In general, the increased temperature leads to a more aggressive environment and hence more corrosion. This can be seen by the clear shift towards higher current densities and thus significantly higher corrosion currents j corr (see Table 2). In the case of the 100˚C carbon top layer, the rise in j corr is particularly pronounced. In contrast, the 200˚C carbon top layer still has a very low corrosion current. Again, this is likely due to structural differences (see Figure 1) and thus better corrosion protection of the SS316L substrate.