Interface-induced degradation of amorphous carbon films/stainless steel bipolar plates in proton exchange membrane fuel cells
Graphical abstract
Introduction
In the past few decades, the development of proton exchange membrane fuel cells (PEMFCs) as a promising new power source for next-generation automobiles, as well as stationary and portable devices, has been rapidly increasing [[1], [2], [3], [4], [5], [6]]. Bipolar plates (BPPs), which are a key component of PEMFCs, are required to possess an excellent electrical conductivity and a high resistance to corrosion. These properties are predominately important, given the harsh conditions under which PEMFCs are used [[5], [6], [7]]. Ultra-thin sheet metals, including stainless steel and titanium plates, owing to their high mechanical strength, easy formability, and low cost, are promising choices to replace traditional graphite sheets for BPPs fabrication [[7], [8], [9], [10], [11], [12], [13], [14]]. Regrettably, in the combined acidic (pH = 2–5), humid, and high temperature environment in which PEMFCs are employed, loss of metallic BPPs, following the release of catalyst-poisoning ions and increased electrical resistivity, can degrade the performance of PEMFCs, especially after a long-time operation [7,15]. To overcome such drawbacks, various protective coatings, including noble metal Au/Pt films, metal carbide, and amorphous carbon (a-C) films, are being attempted, supposed that they can endow metallic BPPs with good electrical conductivity, and the same time, improve their interfacial contact resistance (ICR) as well as their corrosion resistance level [3,16,17].
Among the developed coating strategies, amorphous carbon (a-C) films are a promising material for metallic BPP protection. Their electrical conductivity and corrosive properties can be modulated by adjusting their sp2/sp3 ratio, introducing a third element, or a buff layer [[18], [19], [20], [21], [22], [23]]. Additionally, considering the possibility of the industrial mass production and low material cost of a-C films, many studies have recently attempted to explore the performance of a-C film protected metallic BPPs, so as to clarify the associated degradation mechanism, especially with respect to long-time operation. For example, after a 7 h potentiostatic test, Wu et al. pointed out that the ICR and corrosion resistance of 316L stainless steel (316Lss) were greatly improved owing to a chromium-containing a-C film coating, and they attributed the ICR variation to a higher sp3/sp2 ratio [24]. Yi et al. conducted durability tests on a-C films for 24 h. Their investigation revealed that limited defects and the proper graphitization of a-C films could improve metallic BPP performance [25,26]. Additionally, based on molecular dynamics simulations, a more graphite-like and denser structure has been suggested for the a-C films [19]. Regarding the degradation mechanism a-C/metallic BPPs, the formation of a porous passive film [27], the increase in adsorbed oxygen content on defective sites and dangling bands on the film surface [28], and the self-passivating ability resulting from the oxidation of the metallic atoms in the a-C films [29], have been proposed. Until now, the effect of a-C films on the performance of PEMFCs has been adequately studied experimentally, and based on molecular dynamics simulations, a surface passivation degradation mechanism has been proposed. However, the role of the a-C/metallic substrate interface, which is still unclear, is usually neglected. It is possible that the corrosive electrolytes can induce certain passivation layers on the surface of uncoated metallic BPPs, and thus damaging the a-C/metallic substrate. Alternatively, the a-C/bufflayer interface could act as a key contributor to the degradation of a-C films on metallic BPPs, and then restricting the design of high-performance a-C films used in PEMFCs.
In this study, H-free a-C films were selected and deposited on 316Lss using the direct current magnetron sputter (DCMS) technique. The composition and structure of the a-C films were adjusted by changing the sputtering power, and the dependence of the sp2/sp3 ratio and sp2 clusters of the a-C films on interfacial electrical conductivity and corrosion resistance was systematically investigated. In addition, no extra employed buffer-layer benefited the direct observation on evolution of a-C/316Lss interface before and after electrochemical test. After electrochemical tests in a simulated corrosive PEMFC environment, the ICR and interfacial characteristics of the a-C/316Lss system were studied to clarify the degradation mechanism, which could provide new insights regarding the designing of high-performance a-C films for metallic BPPs in PEMFCs.
Section snippets
Experimental details
a-C films were prepared on p-type Si (100) wafers, quartzes, and 316Lss substrates (Φ 1.5 mm × 3 mm) using the DCMS technique with a rectangular piece of graphite (dimension, 380 mm × 100 mm × 7 mm; purity, 99.99%) as the cathode target [30]. Before fixing in the chamber, all the substrates were cleaned ultrasonically using acetone and alcohol and dried using high-purity N2. With the chamber vacuum bellowed at 3 × 10−3 Pa, Ar+ glow discharge was used to clean the substrates at −350 V bias for
Thickness, morphology and adhesion of the deposited a-C films
The deposition rate and thickness of the a-C films deposited at different sputtering powers are presented in Table 1. The average deposition rate increased monotonically from 2.4 to 5.1 nm min−1 as the sputtering power increased from 0.9 to 2.1 kW. This observation could be attributed to the change in deposition rate per unit power, because the higher sputtering power or ion energy brings about a much higher sputtering yield, resulting in a higher deposition rate [[44], [45], [46]].
Even though
Conclusion
In summary, a designed series of a-C films were deposited on 316Lss samples by a DCMS technique at different sputtering target powers. The influence of the a-C/316Lss substrate interface on the performance of PEMFCs was focused. All the a-C films greatly improved the performance of the metallic 316Lss BPPs under simulated PEMFC operational conditions. In particular, comparing with that observed using bare 316Lss samples, the use of the optimal a-C films (0.9 kW) could reduce the maximum
CRediT authorship contribution statement
Hao Li: Conceptualization, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Peng Guo: Conceptualization, Investigation, Data curation, Writing - review & editing, Visualization, Project administration. Dong Zhang: Software, Resources, Project administration, Funding acquisition. Linlin Liu: Software, Investigation. Zhenyu Wang: Software, Investigation. Guanshui Ma: Investigation, Data curation, Writing - review & editing. Yang Xin:
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
Acknowledgement
This work was financial supported by National Science and Technology Major Project (2017-VII-0012-0108), A-class pilot of the Chinese Academy of Sciences (XDA22010303), National Natural Science Foundation of China (51801226), Ningbo Science and Technology Innovation Project (2018B10014), and K.C.Wong Education Foundation (GJTD-2019-13).
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