Development of a Graphite Coating and Its Transfer from Batch to Coil Process for Application in PEMFC for Hydrogen-Powered Vehicles ()
1. Introduction
Because of the current discussion on reducing greenhouse gas emissions worldwide, a new Federal Climate Protection Act was passed in the Federal Republic of Germany in mid-2021. It stipulates that Germany must reduce greenhouse gas emissions by 65 percent until 2030 compared to emissions in the year 1990. Additionally, there are plans to become greenhouse gas neutral by 2045. To achieve these ambitious goals, a large number of developments and technologies must be made available in a wide range of areas [1]. One possibility in the mobility sector, in addition to all-electric power trains, is the development of fuel cell powered vehicles based on PEMFCs. The PEMFC is already established in the fuel cell market and, according to economic analyses, will increase its market potential even further [2].
The specific advantages, such as the use of a solid electrolyte, high current density, and good dynamic behavior, make them a promising solution in hydrogen-powered vehicles [3]. Weight and space are critical in passenger cars, so metallic bipolar plates (BPPs) can have specific advantages over other materials. The fuel cell serves as a converter from chemical to electrical energy and consists of a cell stack, the individual cells of which are composed of a membrane electrode assembly (MEA) consisting of two gas diffusion layers and a catalyst-coated membrane as well as an anodic and cathodic bipolar plate [4] as shown in Figure 1.
In contrast to electrolysis, where water is converted into hydrogen and oxygen, in a PEMFC a reverse reaction takes place by feeding the process media hydrogen and oxygen [5] [6]. In this process, besides electricity, which is the primary goal to be achieved with the fuel cell, water vapor and heat are also yielded as by-products [5] [7]. In a PEMFC, the solid electrolyte (i.e. the membrane) spatially separates the reaction steps of the anodic partial reaction (oxidation/electrode discharge, Equation (1)) from the cathodic partial reaction (reduction/electrode uptake, Equation (2)) [5].
(1)
(2)
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Figure 1. Schematic partial section of a PEMFC.
From this follows the total reaction in a PEMFC (redox reaction/cell reaction, Equation (3)).
(3)
The oxidation of hydrogen on the anode side creates an excess of H+ ions in the fuel cell, which results in an acidic environment [5] [8]. Due to relatively low material costs and good formability, thin foils made of stainless steel are suitable for the bipolar plates [9]. However, the stainless steel requires surface modification due to its passivation layer, which reduces its electrical conductivity. Due to the aggressive environment in the fuel cell, a corrosion-resistant layer is also required [10]. The Surface coating is necessary, on the one hand, to increase the service life, which is affected by corrosion processes. On the other hand, the coating can increase the power density that is lowered by the electrical contact resistances. In addition, coating defects caused by forming after coating must not affect the fuel cell system, otherwise the fuel cell would be poisoned by leaching out alloy components. The formability of the coating places high demands on the coating system [11] [12] [13] [14].
A variety of technologies is available for coating materials. The process used here is physical vapor deposition (PVD) [15]. This process can be implemented using the four different techniques of evaporation, sputtering, arc evaporation, and ion plating [16]. For the coatings in the processes carried out here, cathodic arc evaporation was used. This process had to be designed in such a way that the coating of the bipolar plate could be evolved from a single-component coating to a coil coating process, allowing mass production of the bipolar plate in a cost-effective manner [17]. In a roll-to-roll coating process, in which the coil is subsequently formed, joined and separated (so-called pre-coating), the coating must, after these production steps, still achieve a performance comparable to coatings applied to prefabricated bipolar plates (post-coating).
There are many studies dealing with the topic of coatings on BPP for PEMFC and their characterization. Extensive research examines the fields of fuel cell functions, cell reactions as well as simulation methods of media flows, consideration of interfacial reactions, design of flowfield geometries or influences of cyclic operations in in-situ experiments [18] [19] [20] [21]. In this paper, we study the production aspects of a carbon based coating for metallic bipolar plates. The knowledge gained in this work contributes to transferring a carbon-based coating to a roll-to-roll process, thus contributing to upscaling of BPP production for commercial use of fuel cell vehicles.
2. Experiments
2.1. Coating Processes
The layer systems used here are based on carbon thin film and are generally amorphous or nano crystalline, unless they are deposited as diamond. The bonding properties in the layer systems are composed of graphitic (sp2) and diamond-like (sp3) bonding components. In the case of the predominantly diamond-like bonds, these coatings are referred to as diamond-like-carbon (DLC), which are very hard and are mainly used as wear protection for components. Because of their poor conductivity, the focus in fuel cell coatings is on producing a high graphitic content and thus graphite-like-carbon (GLC). The coating system, consisting of a corrosion-resistant chromium layer and a conductive carbon layer, is applied to the base material, AISI 316L stainless steel, as shown in Figure 2.
The coating was carried out in an industrial size PVD coater from the company Metaplas, type MZR 373. The standard GLC coating procedure is as follows: first, the chamber and the samples are heated to approx. 300˚C, and this temperature is maintained during the subsequent process steps. Next, plasma cleaning is performed, followed by the deposition of an intermediate metallic layer and the deposition of a carbon layer. The coating is carried out at a pressure of 1 × 10−3 mbar.
2.2. Batch Processes
In the batch process, samples of plain sheets and prefabricated bipolar plates were coated. After installation of the samples in holders manufactured for this purpose, the coating chamber was loaded and the samples were coated according to the following process sequence:
After an initial bake-out of the coating chamber and the samples, a plasma cleaning was performed to remove the passivation layer of the stainless steel. Then the chromium intermediate layer was applied, followed by the carbon coating. The coating times of the individual process steps were very short, about 2 minutes each. Since the samples were coated in a plant-specific, twofold rotational movement, considerable differences in coating thickness occur here due to the coating dynamics, which were in the range of approx. 80 - 120 nm.
2.3. Coil Processes
For the coil processes, the batch coating chamber was converted by installing an external coil module. A coil coating line is simulated with the take-up and take-off roll installed in the coil module, and a drum roll installed inside the coating chamber. Analogous to the sequence of steps in the batch process, the process steps in coil coating are carried out one after the other.
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Figure 2. Schematic diagram of the graphite-like-carbon layer system.
This means that after the first bake-out step of the line, the steel coil must be rewound after each coating step. For example, in the first process step of plasma cleaning, a certain length of the material is unwound and then rewound again to subsequently carry out the chromium coating and then the carbon coating. For the coil and batch coatings, material thicknesses of 0.1 mm were investigated and the coil speeds during the coating was approx. 2.3 m/min.
3. Characterization
X-ray photoelectron spectroscopy (XPS, PHI Quantum 2000) of the overview spectrum was acquired by a focused monochromatic X-ray source (Al-Kα, 1486.68 eV) with a beam diameter of 100 µm and a power of 100 W from an area of about 1.4 × 0.4 mm2. The depth profile was measured using an Ar ion beam with an energy of 1 kV scanning over the area of 3 × 3 mm2.
The ablation rate is 1.4 nm/min for SiO2. In this case, the spectra were recorded with a beam diameter of 200 µm and a power of 50 W. Contact resistance measurements were taken by four-wire measurement between two gold-coated copper dies with an area of 2 × 2 mm2 and a pressure of about 50 N/cm2. Corrosion measurements were done with a potentiostat (Methrom Autolab) using the three-electrode electrochemical measurement method with a 0.05 mol/l H2SO4 electrolyte at room temperature with a polarization speed of 0.5 mV/s and a measurement area of approximately 1.48 cm2.
4. Results and Discussion
If the coating is applied before the forming process, the adhesive strength of the coating must be high enough to avoid delamination of the coating. Commercially manufactured bipolar plates have a high degree of forming due to their fine channel geometries, which thins out the base material at the channel flanks. This leads to high stresses in the base material and in the coating. The forming processes can cause shear cracks in the coating in the area of the channel flanks. However, passivation of the stainless steel base material also occurs at the defective areas. This effect is not occurring in other base materials, which are not suitably corrosion-resistant for the requirements of this type of fuel cell. For this reason, this process cannot be transferred to materials that are not resistant to corrosion without further research.
The overview spectrum in Figure 3 shows the qualitative and quantitative detection of the chemical elements in the surface region of the GLC sample. It can be seen that the largest portion of 93.1 at.% is carbon, as also shown in Table 1. Another 5.2 at.% is oxygen and the other contaminations are negligible. This result indicates that the surface mainly consists of carbon and oxygen, and the base material (SS316L with contents of iron, chromium and nickel) is not detectable.
In the depth profile of the XPS measurement (Figure 4, left), a clearly pronounced layer system can be seen. A separate carbon layer is clearly visible down
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Table 1. Atomic concentration table of the XPS measurement.
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Figure 3. Overview spectrum of the surface region of the GLC layer system from coil coating process.
to a depth of approx. 60 nm. After a small transition area of carbon and chromium, the chromium layer continues to a depth of approx. 130 nm. Plasma pre-cleaning removes the material but as can be seen from the oxygen peak, the oxygen layer is not completely eliminated. As can be seen from the transition from carbon to chromium, the carbon layer is firmly anchored, which can also be seen in the forming results (not published here). Iron is not present on the surface, which can be seen in the overview spectra as well as in the depth profile. Figure 4, right, shows the XPS measurement of a batch coating, which is comparable to the measurement of the coil coating. The coating of the coil process is even thicker than that of the batch process, and thus allows even faster coil speeds. Comparable coating in the transfer from batch to coil processes was achieved.
The carbon-based coatings significantly reduce the contact resistance of stainless steel, and the coating tests in the batch and coil processes show comparable
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Figure 4. XPS depth profile of the GLC layer system with the relevant elements from coil coating process (left) and batch coating process (right).
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Figure 5. Contact resistance measurements of the carbon/chrome layer in batch and coil process, gold layer, and the SS316L.
contact resistances, which are in the range of gold-coated stainless-steel sheets, as shown in Figure 5 (C/Cr-Batch and C/Cr-Coil). The corrosion current density of the 0.1 mm thick samples was significantly reduced by the coating. Carbon coating with an intermediate chromium layer shows an increase in corrosion current at about 1.2 V voltage shown in Figure 6 (C/Cr-Batch and C/Cr-Coil).
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Figure 6. Current density potential curves of the carbon/chrome layer in batch and coil process, gold layer, and the SS316L
Coating systems with alternative metallic interlayers can potentially further improve the corrosion properties of the coatings. However, the performances of a carbon-based coating produced in a batch or coil coating process are comparable and can reach electrical contact resistances and corrosion current densities (approx. between 0.25 and 1.0 V) comparable to gold.
5. Conclusion
The upscaling of a carbon coating processes with a chromium interlayer from batch to coil has been successfully demonstrated. Contact resistance and corrosion measurements demonstrated the good performance of the coatings. Carbon-based coatings deposited in PVD coil processes show high potential for industrial mass production of steel coil to produce bipolar plates with pre-coating. An important aspect is that the cost of producing bipolar plates can be greatly reduced by using coil processes rather than batch process. Further investigations to transfer the coatings to a steel coil line are planned. Additionally, further research in the field of alternative interlayers has the potential to increase the corrosion resistance of metallic BPP.
Acknowledgements
Maurizio Giorgio would like to thank the Fraunhofer Institute for Material and Beam Technology in Dortmund for their support, as well as the Institute for Material Science at the Technical University of Dresden. He would also like to thank all project partners from miniBIP II (Reference no. 03ETB007B) and the Fraunhofer internal research project HOKOME.