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Surface and Coatings Technology
15 September 2022
A series of NbC/α-C:H films were prepared through arc ion plating for the protection of metallic bipolar plates in proton exchange membrane fuel cells (PEMFCs). Phase characterizations show that NbC grains were randomly embedded into the α-C:H matrix and their size and content increased with the decreasing of C2H2 flow rate. Results from the kelvin probe force microscope (KPFM) and conductive atomic force microscope (CAFM) presented a uniform surface potential distribution but significant local conductivity differences. DFT calculations and potentiostatic polarization tests indicated that dissolution of NbC grains were preferred in PEMFC cathode environment due to preferential adsorption of corrosive media. On the basis of theoretical calculations and experimental results, a corrosion mechanism was presented based on percolation model to better understand the corrosion process of NbC/α-C:H films in PEMFC cathode.
As a kind of energy conversion device that transforms chemical energy directly into electrical energy, proton exchange membrane fuel cells (PEMFCs) have been considered as an essential part of future clean energy system due to its high efficiency, low noise and zero emission , . A typical PEMFCs contains bipolar plates, gas diffusion layers, catalyst layers and proton exchange membranes (generally Nafion). During the energy conversion process, hydrogen is oxidized in the anode catalyst layer and the released electrons are transferred through an external circuit to the cathode. The cathode oxygen combines with the electrons and the protons transferred through the proton exchange membrane to generate water thus complete the circuit. In this process, metallic bipolar plates should maintain high electrical conductivity and corrosion resistance to insure an ideal power output and lifetime performance. However, the environment that bipolar plates stayed in PEMFC is always acidic, moist and accompanied with a fluctuating electrode potential, causing severe corrosion and low interfacial conductivity of commonly used titanium or stainless steel bipolar plate materials , . To improve this situation, conductive and anti-corrosive films like noble metal films , carbon films , , metal nitride , , carbide films  and others ,  have been designed to protect metallic bipolar plates.
Among all those film materials, amorphous carbon (α-C) films have been extensively investigated because of their chemical inertness, mechanical hardness and electrical conductivity . The properties of α-C films are mainly affected by their microstructure and hybridization states. In recent years, progresses have been made through the modification of α-C film deposition method like chemical vapor deposition (CVD), physical vapor deposition (PVD) and deposition parameters like bias voltage, argon flow rate, deposition time as well as the choice of seed layer , . However, the great distinction in material properties between α-C and metal substrate results in their weak affinity and the widely used sputtering deposition may induce high internal stress . Those characteristics may cause film exfoliation and reduce the life time of metallic bipolar plates. Moreover, the relatively low deposition rate may not meet the increasing demand for efficient commercial production. Previous studies have proved that doping metal elements into α-C matric can significantly alleviate the problems mentioned above. The binding states and components distribution in the film are directly affected by the affinity between doping atoms and carbon atoms . In the aspect of theoretical research, through first-principles calculation, Choi et al.  revealed that residual stress in α-C network was caused by the bond angle distortion. Incorporating metal can reduce the directionality of the bond, thus weaken the residual stress. Later, they investigated the atom bonding characteristics between metal and carbon atoms and revealed that the nonbonding, anti-bonding characteristics were strongly depended on the d-orbital states of transition metal atoms . Apart from these, influence of doping atom species and doping conditions on film microstructure like morphologies, electron hybridisations of carbon atoms and size of nanocrystalline carbides have also been investigated by plenty of experimental researches , .
In all these transition metal elements, Nb has been proved to be a promising doping element in α-C film. The niobium carbide (NbC) that it formed with carbon possesses a much higher chemical stability and electrical conductivity than other metal carbide like TiC and ZrC , which are critical factors for metallic bipolar plates that directly related to the performance of PEMFC. Nedfors et al.  found that the deposition rate of nanocomposite NbC coatings prepared with reactive sputtering is way much faster than non-reactive sputtering. The film microstructure, mechanical and electrical properties as well as chemical stability were systematically investigated based on the deposition conditions and corresponding chemical compositions. Hou et al.  revealed that doping a moderate Nb into carbon matrix can significantly enhance the C-sp2/C-sp3 ratio and refine grain size, which is beneficial for the conductivity and film compactness, leading to a lower interfacial contact resistance (ICR) and corrosion current. However, the corrosion mechanism of such films in PEMFC environment is still lacking and further study is urgently needed to better understand the basic principle, which further contributes to the development of related films for better bipolar plate performance.
In this work, adsorption characteristics between film components and particles in corrosion media were studied through spin-polarized density functional theories (DFT) calculations to assess the film corrosion resistance. Potentiostatic polarization results indicate that the corrosion behavior was distinct as the NbC content increased to a certain extent and the dissolution of NbC was the dominant process during polarization test. Based on these theoretical and experimental results, a percolation model was built for the understanding of NbC/α-C:H film corrosion mechanisms in PEMFC cathode environment.
Pure titanium TA1 cut into 70mm∗60mm∗0.1mm foils were chosen as the substrates. Before film deposition, the substrates were grounded with sand papers up to 2000 grid and ultrasonic cleaned in ethanol and deionized water gradually. The films were prepared through arc ion deposition technology with a niobium target and C2H2 as the niobium and carbon sources, respectively. To reduce the generation of droplets and get a smoother film surface, a magnetic field is applied on the target surface
GI-XRD analyses were conducted to explore the phase composition of the NbC/α-C:H films and the results were shown in Fig. 1(a). All the peaks can be assigned to either titanium or NbC characteristic diffraction signals in which titanium comes from the substrate while NbC comes from the film. Specifically, the (111) and (311) peaks of NbC are partially overlapped with the titanium signals while peak (220) is relatively independent. With the increase of C2H2 flow rate, peak intensity of (220)
Through the combination of experimental researches and theoretical calculations, this work systematically analyzed the corrosion mechanism of the NbC/α-C:H films. Material characterization indicated that NbC/α-C:H films prepared through arc ion plating were composed of NbC grains randomly embedded into α-C:H matrix. The film system was highly disordered and the NbC grain size and content increased with the decreasing of C2H2 flow rate. Adsorption energies base on DFT calculations proved that
CRediT authorship contribution statement
Conceptualization, Yong Gou, Zhigang Shao and Zidong Wei; Investigation, Yong Gou and Guang Jiang and Jinkai Hao; Methodology, Yong Gou, Guang Jiang and Zhigang Shao; Validation, Zhigang Shao; Writing - review & editing, Yong Gou, Guang Jiang, Jinkai Hao, Zhigang Shao and Zidong Wei.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This work was financially supported by the National Key Research and Development Program of China (No. 2021YFB4001703), Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA21090101) and LiaoNing Revitalization Talents Program (No. XLYC1902079). In addition, we sincerely appreciate Dalian Key Laboratory of Electrolysis for Hydrogen Production's support to our work.
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© 2022 Published by Elsevier B.V.