Stainless steel bipolar plates for proton exchange membrane fuel cells: Materials, flow channel design and forming processes

Journal of Power Sources 451 (2020) 227783

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Review article

Stainless steel bipolar plates for proton exchange membrane fuel cells: Materials, flow channel design and forming processes Yu Leng a, b, Pingwen Ming a, b, Daijun Yang a, b, Cunman Zhang a, b, * a b

New Energy Automotive Engineering Center, Tongji University, 4800 Caoangong Road, Shanghai, 200029, China School of Automotive Studies, Tongji University, 4800 Caoangong Road, Shanghai, 200092, China

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

� Performance, durability and cost are still main challenges for bipolar plates. � Bare and coating materials are two research routes for stainless steel separators. � Fine flow channel is beneficial for improving fuel cell performance. � Conventional forming methods are hard to form fine flow channel structure. � Hot pressing may be used to form stainless steel bipolar plates with fine channel.

A R T I C L E I N F O

A B S T R A C T

Keywords: Stainless steel bipolar plates Materials Flow channel design Forming processes PEMFCs

Bipolar plate (BPP) is a key component for proton exchange membrane fuel cell (PEMFC) stack. Stainless steel BPPs possess high electrical and thermal conductivity, good gas impermeability, superior mechanical properties and formability. Furthermore, stainless steel BPPs as thin as 0.1 mm or even thinner can be manufactured by plastic forming methods in mass production with relatively lower cost for PEMFCs, especially for the application of automotive. However, relatively lower corrosion resistance and higher interfacial contact resistance (ICR) may be the two main obstacles hindering full commercialization of stainless steel BPPs. In addition, formability needs to be further improved for higher performance BPPs with fine flow channel geometries while its cost should be reduced to meet the 2020 targets set by the U.S. Department of Energy (DOE). This paper tries to present a comprehensive review of major findings of researches on base and coating materials, channel structure design and forming processes of stainless steel BPPs in recent years. The optimum materials, channel geometries and manufacturing processes currently used or investigated are summarized. In the meanwhile, challenges and future research trends on materials and forming processes of stainless steel BBPs are proposed.

1. Introduction Fuel cell technology has been considered the most promising solu­ tion to the problems, such as climate change, air pollution and energy shortage caused by fossil fuels and attracted a lot of researchers in recent

decades. In particular, the PEMFC has been identified as a potential alternative power source for various applications, such as automotive, portable and stationary due to its advantages of high power density, high efficiency, quick start up capability, relatively low operating tem­ peratures and near-zero emissions [1–3]. However, PEMFCs are mainly

* Corresponding author. New Energy Automotive Engineering Center, Tongji University, 4800 Caoangong Road, Shanghai, 200029, China. E-mail address: [email protected] (C. Zhang). https://doi.org/10.1016/j.jpowsour.2020.227783 Received 15 October 2019; Received in revised form 3 January 2020; Accepted 21 January 2020 Available online 3 February 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.

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Journal of Power Sources 451 (2020) 227783

employed for research and demonstration currently and the widespread commercialization of PEMFCs is still hindered by the barriers of power density, reliability, durability and cost [4,5]. Bipolar plates, as key multifunctional components in a PEMFC, are strongly related to the issues mentioned above. The main components of a PEMFC stack are membrane electrode assemblies (MEAs) and bipolar plates while the MEA consists of a proton exchange membrane (PEM), catalyst layers (CLs) and gas diffusion layers (GDLs) [6]. A schematic view of the PEMFC structure is shown in Fig. 1. The BPPs account for 20%–30% of the cost [7,8], 60%–80% of the weight and over 80% of the whole volume in a PEMFC stack [9–12]. The critical functions per­ formed by BPPs include: (a) separating and distributing the fuel gas at the anode and oxygen/air at the cathode; (b) collecting and conducting current from the anode of one cell to the cathode of the next cell; (c) removing the reaction products and heat from the cell; (d) providing mechanical support for the whole fuel cell stack [13,14]. The re­ quirements of BPPs for transportation applications has been specified by the 2020 DOE targets (Table 1) [15]. The selection of materials, forming processes, as well as performance assessments of BPPs are usually based on the DOE targets. Due to the multifunctional characteristics and property requirements of BPPs, a great deal of candidates have been proposed and investigated over the past few decades. The BPPs are commonly divided into three categories based on the materials utilized, namely graphite BPPs, metal BPPs and composite BPPs [16]. Graphite plates were initially utilized for BPPs because of their high electrical and thermal conductivity, low density and good resistance to chemical attack [17–20]. For stationary applications where longevity is required and compactness and volume are not the priorities, graphite remains a good choice. However, graphite BPPs are brittle and porous, which limit their use in portable and especially transportation applications where compactness and volume are of great significance [21]. In order to provide sufficient mechanical strength and ensure they are impermeable, the graphite BBPs have to be made thick [22]. Besides, the process of machining graphite bipolar plates is time-consuming and expensive, which are big obstacles for mass production [23,24]. Therefore, research attention has been drawn to alternative materials such as carbon-polymer composites and metals [25]. Carbon-polymer composites are made of resin matrices with conductive fillers, such as carbon black, carbon fiber and carbon nano­ tubes distributed in them [26–29]. The carbon-polymer composite BPPs can provide better mechanical stability and impermeability than graphite BPPs. However, the balance of conductivity and mechanical strength becomes the main challenge of carbon-polymer composite BPPs. The conductivity and strength of carbon-polymer composite BPPs are both determined by the matrices and conductive fillers. Previous studies have pointed out that the increase of conductivity by improving the content of conductive filler is inevitably accompanied by decrease of mechanical strength [30]. In addition, the high filler content makes the fabrication of carbon-polymer composite BPPs more challenging [31]. Hence, a lot of researchers are focusing on investigating and developing metal BPPs for portable, especially automotive applications.

Table 1 DOE technical targets for PEMFC BPPs. Property Cost Plate weight H2 permeation Rate Corrosion at Anodea Corrosion at Cathodeb Electrical Conductivity Interfacial Contact Resistance Flexural Strength Forming Elongation

Units

2020 Targets 1

$⋅kW kg⋅kW 1 cm3⋅cm 2⋅s μA⋅cm 2 μA⋅cm 2 S⋅cm 2 Ω⋅cm2 MPa %

1

<3 <0.4 <1.3 � 10 14 <1 and no active peak <1 >100 <0.01 >25 >40

a Testing condition: pH ¼ 3 H2SO4 0.1 ppm HF, T ¼ 80 � C, peak active current < 1 � 10 6 A⋅cm 2 (potentiodynamic test at 0.1 mV⋅s 1, -0.4 V to þ 0.6 V vs Ag/ AgCl), de-aerated with Ar purge. b Testing condition: pH ¼ 3 H2SO4 0.1 ppm HF, T ¼ 80 � C, passive current < 5 � 10 8 A⋅cm 2 (potentiostatic test at þ 0.6 V vs Ag/AgCl) for > 24 h, aerated solution.

The main materials which has been investigated extensively for metal BPPs are stainless steel, titanium, nickel, aluminum, copper and their alloys [16,17,24]. Among the metal materials used for BPPs, stainless steel (SS) is believed to the most promising candidate for automotive applications when considering their physical and chemical properties, manufacturability, costs and reserves [24,32–34]. Therefore, stainless steel has attracted the attention of many researchers and become the most commonly used material for BPPs in global automotive fuel cell industry. The leaders of fuel cell vehicle (FCV) such as Toyota [35], Honda [36–39], Hyundai [40–42] and General Motors [43,44] all use stainless steel as base material for BPPs in their FCVs. Although Toyota has changed their BPP material to titanium recently in consid­ eration of corrosion resistance and weight, stainless steel may remain the best choice for BPPs now accounting for the cost and reserves of resources. Stainless steel can provide desirable characteristics, such as high electrical and thermal conductivity, good gas impermeability, excellent mechanical property, and superior formability. In addition, BBPs made with stainless steel can offer higher strength, impact toughness and better gas impermeability than graphite plates and higher electrical conductivity than composite plates [34,45,46]. Furthermore, the unique mechanical property and superior formability of stainless steel allow for fabrication of bipolar plates as thin as 0.1 mm or even thinner in mass production [16,25,47]. Although stainless steel offers many advantages, it is more suscep­ tible to corrosion in typical acid and humid environment of PEMFCs, which can adversely affect the performance and durability of fuel cell stack [16,33,34]. The corrosion of stainless steel BPPs can be observed both at the anode and the cathode. The reducing environment at the anode can lead to unwanted hybrid formation and dissolved metal ions (Fe3þ, Cr3þ, Niþ, etc.) while the oxidizing environment at the cathode can lead to metal oxides formation and increase of the corrosion rate. The corrosion products on the surface of BPPs, such as passivation ox­ ides, will increase the ICR and thus lower the output and efficiency of the fuel cell stack. In the meanwhile, the leaking metal ions may poison the proton exchange membrane and pollute the catalyst layers, both result in lower power output and shorter lifetime of the stack. Moreover, after a long time of corrosion, micro-pits may form on the thin bipolar plate and this will lead to the damage of fuel cell stack [24,34,48]. In order to overcome the shortcomings of stainless steel BPPs, a lot of work focusing on the investigation of new materials (including base and coating ma­ terials), manufacturing processes and characterization methods has been done by scientists and engineers all over the world [6,15,16,24,25, 33,34,49]. A few reviews have been published to summarize the work on ma­ terials, fabrication and characterization of metal BPPs. H. Tawfik et al. [32] summarized the investigations done before 2007 on materials, corrosion and ICR of metal BPPs. The corrosion failure modes of metal

Fig. 1. Schematic of the PEMFC structure. 2

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Journal of Power Sources 451 (2020) 227783

BPPs, namely pinhole formation, electrocatalyst poisoning, membrane ion-exchange, and passivation formation were concluded. N. De Las Heras et al. [23] reviewed the metal materials which had been studied for automotive PEMFC bipolar plates until 2009. They mainly discussed the austenitic and ferritic stainless steel, iron based amorphous alloys, titanium, aluminium alloys, nickel alloys, copper alloys and protective coating materials. The review by H. Wang et al. [24] made an effort to clarify stainless steel, Ti-based alloys and Al-based alloys that had been researched until 2010. Coatings and surface modification methods suitable for stainless steels were also discussed in their review. S. Karimi et al. [34] reviewed the researches until 2012 on materials and fabri­ cation methods for BPPs. They pointed out that future research should concentrate on depositing a chromium carbide or amorphous carbon film on stainless steel plates by a cost effective way to meet the DOE requirement for corrosion resistance and ICR. The work by L. F. Peng et al. [50] reviewed and discussed in detail the flow field design and optimization, forming process, joining process, coating process and as­ sembly process of stainless steel BPPs which had been studied until 2014. The results of corrosion resistance and characterization methods of metal materials, such as stainless steels, aluminium alloys, nickel and titanium alloys in literatures before 2010 were presented by R. A. Antunes et al. [6]. Recently, Y. X. Song et al. [51] have reviewed on composite, non-porous graphite and metal materials, the corresponding fabrication methods, and flow field layouts for BPPs of PEM fuel cells while O. A. Alo et al. [52] discussed various manufacturing methods currently available for metallic BPPs. Although some work has been done to review metal BPPs, few lit­ eratures are found to focus on the composition of materials and plastic forming processes for stainless steel BPPs while they are believed to be one of the most promising candidates for PEMFCs in automotive appli­ cation. This work aims to provide a comprehensive review of the results and major findings of current researches on base and coating materials, flow channel design and manufacturing processes of stainless steel BBPs for PEMFCs. Efforts are also made to find out the optimum materials, flow channel geometries and forming processes currently used or investigated for stainless steel BPPs. In the meanwhile, main challenges and issues existing in the three aspects of stainless steel BPPs mentioned above are discussed. Finally, future research trends on materials, flow channel structure design and forming processes for stainless steel BBPs with higher performance, better stability and durability are proposed.

candidate materials are ranked based on the objectives. Finally, further information such as recyclability or availability about the top-ranked materials is to be sought. The whole process is illustrated by Fig. 2 and a detailed view of the Ashby approach can be found in the work by Ashby [54]. As for stainless steel BPPs, minimizing the corrosion current density (Icorr) and the interfacial contact resistance are defined as the main objectives while the targets of Icorr and ICR defined by DOE are considered as the constraints. Then base stainless steel materials and coating materials for BPPs are screened with the results of Icorr and ICR performance from reported literatures, respectively. Based on the Ashby charts, the materials used or investigated for stainless steel BPPs are discussed and analyzed utilizing the Ashby chart in the following par­ agraphs. It should be noted that the values of Icorr and ICR are obtained at different test conditions and this is ignored when drawing all of the Ashby charts in the present paper. 2.1. Bare materials for stainless steel BBPs Most of the researches have been focused on austenitic and ferritic stainless steels. Bare austenitic stainless steels which have been inves­ tigated for BPPs mainly include SS201, SS219, SS304(L), SS310S, SS316 (L), SS317L and SS904L [23,24,32–34]. The chemical composition re­ quirements [56] of those stainless steels are listed in Table 2. Among the stainless steels mentioned above, SS316L attracts the attention of most researchers because of their superior comprehensive properties and relatively low cost. When compared to SS201, SS219 and SS304 (L), the corrosion resistance of SS316L is relatively higher because the addition of Mo can attribute to the pitting and crevice resistance [57]. As for SS316 and SS316L, the latter is of better intergranular corrosion resis­ tance. The reason for this phenomenon is that lower carbon content can inhibit the formation of chromium carbides and chromium-depleted zone at grain boundaries which will reduce the stability of the passive layer [58–60]. Generally, the corrosion resistance of SS310S is better than that of SS316L for much higher Cr content. The contents of alloying elements Cr and Mo in SS317L are higher than that of SS316L. There­ fore, the corrosion resistance of SS317L is superior to that of SS316L. Normally, SS904L is of the best corrosion resistance among the currently researched austenitic stainless steel grades because of its higher Cr, Mo and lower C contents. However, high alloying elements and low carbon contents will lead to higher cost. Although SS317L and SS904L possess higher corrosion resistance as compared to SS316L, their utilization for volume production are restricted when cost is taken into consideration. Therefore, the research on SS316L for volume production of stainless steel BPPs is the hottest among all austenitic stainless steel materials. Typical ferritic stainless steels studied for BPPs are SS430, SS434, SS436, SS441, SS444 and SS446 [23,24,32–34]. The chemical compo­ sition requirements of ferritic stainless steels are listed in Table 2. Although the formability of ferritic stainless steels is relatively poor because of the body-centered cubic (bcc) structures, their corrosion resistance is as good as or even better than austenitic stainless steels [61]. Besides, the costs of ferritic stainless steels are generally lower. Therefore, ferritic stainless steels also attract the attention of many researchers. The Icorr and ICR values, as well as their test methods for different bare stainless steels are summarized in Table 3. It shows that the con­ ditions and compacting pressures for testing Icorr and ICR, respectively, differ from each other in different researches. The differences of testing methods are not taken into consideration when presenting the values of Icorr and ICR in the Ashby chart, as shown in Fig. 3. There seems to be no obvious distinction of the corrosion resistance of different austenitic stainless steels, such as SS304, SS304L, SS316, SS316L, SS317L and SS904L obtained by various researchers, which is contrary to the con­ clusions mentioned in the above sections. This phenomenon is probably caused by different testing conditions, such as composition and con­ centration of the solution, with or without bubbling. The different

2. Materials The biggest challenge of BPPs made with bare stainless steel is that they are susceptible to corrosion in PEMFC operating environment. Furthermore, the ICR between stainless steel BPPs and the GDLs in­ creases significantly due to corrosion products such as iron oxides. In the meanwhile, materials and manufacturing cost must be considered if full commercialization of stainless steel BPPs are to be realized. According to the study by J. M. Huya-Kouadio et al. [53], the base and coating ma­ terials account for 55% and 18% of cost of the BPP made with SS316, respectively. To solve these problems and achieve the targets of design, manufacturing, performance and durability for stainless steel BPPs, two routes have been proposed by researchers. One is to investigate novel stainless steels possessing high corrosion resistance, low ICR and supe­ rior formability as BPP or base materials, the other is to develop new corrosion resistant coating materials for stainless BPPs [24,34]. The Ashby approach described by Ashby [54] and M. C. L. de Oli­ veira et al. [55] is utilized for analyzing the base and coating materials investigated for stainless steel BPPs in the present paper. Normally, there are four steps involved in the materials selection process according to the method mentioned above. The first step is translation and identifi­ cation of properties of the desired materials which include expressing the functions, labeling the constraints, defining the objectives and recognizing the free variables. The next step is to screen all the materials available and eliminate materials out of the constraints. Then, the 3

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Journal of Power Sources 451 (2020) 227783

Fig. 2. Process of materials selection utilizing the Ashby approach. Table 2 Chemical composition requirements (wt %)a of typical stainless steel grades for BPPs. Grade

C

Si

Mn

P

S

Cr

Ni

Mo

N

Cu

Other Elements

SS304 SS304L SS310S SS316 SS316L SS317 SS317L SS904L SS430 SS434 SS436 SS441 SS444 SS446 Poss470FC

0.07 0.03 0.08 0.08 0.03 0.08 0.03 0.02 0.12 0.12 0.12 0.03 0.025 0.02 0.02

0.75 0.75 1.50 0.75 0.75 0.75 0.75 1.00 1.00 1.00 1.00 1.00 1.00 1.50 0.40

2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 1.00 1.00 1.00 1.00 0.75–1.5 1.00 0.20

0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.040 0.040 0.040 0.040 0.040 0.040 0.040

0.030 0.030 0.030 0.030 0.030 0.030 0.030 0.035 0.030 0.030 0.030 0.030 0.030 0.030 0.030

17.5–19.5 17.5–19.5 24.0–26.0 16.0–18.0 16.0–18.0 18.0–20.0 18.0–20.0 19.0–23.0 16.0–18.0 16.0–18.0 16.0–18.0 17.5–18.5 17.5–18.5 23.0–27.0 25.0–32.0

8.0–10.5 8.0–10.5 19.0–22.0 10.0–14.0 10.0–14.0 11.0–15.0 11.0–15.0 23.0–28.0 0.75 – – 1.0 1.0 0.6 0.5

– – – 2.00–3.00 2.00–3.00 3.0–4.0 3.0–4.0 4.00–5.00 – 0.75–1.25 0.75–1.25 – 1.75–2.5 – –

0.10 0.10 – 0.10 0.10 0.10 0.10 0.10 – – 0.03 – – – 0.02

– – – – – – – 1.00–2.00 – – – – – – 2.0

– – – – – – – – – – Nb/Ti Nb/Ti Nb/Ti – Nb/Ti

a Maximum, unless range or minimum is indicated. Where dashes ( ) appear in this table, there is no requirement and the element need not be determined or reported.

surface state and composition of the stainless steels before testing may be another important reason. Although the results of Icorr and ICR pre­ sented in the Ashby chart show that there is no obvious regularity among different stainless steels, it can be concluded that none of the bare stainless steels studied meets the current DOE targets. To achieve the DOE targets, the corrosion resistance and conduc­ tivity of the bare stainless steels must be improved. M. P. Brady et al. [47,62,63] developed a Fe–20Cr–4V ferritic stainless steel with lower Cr and V contents as compared to conventional Fe–27Cr–6V. The cost of Fe–20Cr–4V could be reduced while its ductility be improved. Through pre-oxidation and nitridation, the corrosion resistance and ICR of Fe–20Cr–4V were approaching the DOE targets. In the patent by JFE Steel Corp. [64], a ferritic stainless steel with excellent corrosion resis­ tance and conductivity was revealed. As compared with conventional ferritic stainless steels, the contents of C, Si, Mn and N was much lower. By immersing the stainless steel in a mixed acid of hydrofluoric and nitric with a ratio larger than 2.5, a contact resistance of below 10 mΩ⋅cm2 was achieved. In the patents by Nippon Steel Corp. [65,66], a low carbon ferritic stainless steel with 0.5–1.0 wt% B and other micro­ alloying elements, such as niobium and titanium were invented. Through dispersion and expose of M2B boride based metallic pre­ cipitates on the surface of ferritic parent phase with a passive film, the ferritic stainless steel could possess higher corrosion resistance and a

contact resistance equaling to that of gold-plated stainless steel. In recent years, a non-coated ferritic stainless steel named Poss470FC, which had been developed by POSCO corporation was utilized as the material for BPP of Hyundai Motor’s fuel cell vehicle NEXO [67]. This kind of ferritic stainless steel possessed superior corrosion resistance and conductivity, both of which achieved the 2020 DOE targets when they were treated with relatively simplified chemical surface treatment. The chemical composition of Poss470FC [37] developed by POSCO is shown in Table 2. The first step of the surface treatment was removing the first passive film formed in bright-annealing or annealing-pickling processes in a solution of 10–20 wt % H2SO4. Then the second passive film with high corrosion resistance and low ICR was formed in a mixed solution of 10–20 wt % HNO3 and 1~10 wt % HF after washing with water. The invention by POSCO pointed out that corrosion resistance and conduc­ tivity of stainless steel BPP could achieve the 2020 DOE through composition optimization and proper surface modification techniques, even without coatings. This may be one of the trends of developing high performance and high durability stainless steel BPPs for PEMFCs in the future. 2.2. Coating materials for stainless steel BPPs Another way of improving the corrosion resistance and conductivity 4

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Journal of Power Sources 451 (2020) 227783

Table 3 Properties and testing methods of bare stainless/plain carbon steel materials for BPPs. Materials

Corrosion current density Icorr (μA⋅cm 2)

Testing conditions

Interfacial contact resistance ICR (mΩ⋅cm2)

Compacting pressure (N⋅cm

AISI1020

634

0.5 M H2SO4 25 � C 0.5 M H2SO4 2 ppm HF 80 � C 1 M H2SO4 2 ppm HF 70 � C 1 M H2SO4 2 ppm HF 70 � C 0.001 M H2SO4 0.00015 M HCl 15 ppm HF 25 � C 0.5 M H2SO4 5 ppm HF 70 � C 0.05 M H2SO4 2 ppm HF 70 � C 0.05 M H2SO4 pH ¼ 2.3 2 ppm HF 80 � C 0.5 M H2SO4 2 ppm HF 80 � C

403.8

140

[68]

~53

150

[69]

~130

150

[70]

~700

150

[70]

~96 (Before corrosion test) ~302 (After corrosion test)

220

[71]

124.4

150

[72]

~90

140

[73]

~80 (Before polarization) ~1000 (After polarization)

150

[74]

~75 (Before corrosion test) ~127 (After corrosion test)

150

[75]

1 M H2SO4 2 ppm HF 70 � C 1 M H2SO4 2 ppm HF 70 � C

~150

140

[46]

~130

150

[70]

0.5 M H2SO4 2 ppm HF 80 � C 0.5 M H2SO4 2 ppm HF 80 � C 0.5 M H2SO4 2 ppm HF 80 � C 0.5 M H2SO4 2 ppm HF 80 � C 0.5 M H2SO4 2 ppm HF 70 � C 0.5 M H2SO4 2 ppm NaF 70 � C 1 M H2SO4 2 ppm HF 70 � C 1 M H2SO4 2 ppm HF 70 � C 1 M H2SO4 2 ppm HF 70 � C 0.05 M H2SO4 0.01 M NaCl 2 ppm HF 70 � C 1 M H2SO4 2 ppm HF 70 � C 1 M H2SO4 2 ppm HF 70 � C

~400

150

[76]

~375 (Before Test) ~375 (Anode After Test) ~480 (Cathode After Test) ~370

150

[77]

150

[78]

~400

150

[79]

150

140

[80]

82 (Before Test) 83 (Anode After Test) 87.5 (Cathode After Test) ~148

140

[81]

140

[46]

~134

140

[46]

~100

140

[46]

199.96

140

[82]

~145

140

[83]

~110

140

[83]

37,000 (purged with Air) 105,000 (purged with Hydrogen) SS201 SS219 SS304

~10 (at þ0.6V) (purged with Air) ~140 (at 0.1V) (purged with Hydrogen) ~8 (potentiostatic test at 0.1V) ~8 (at þ0.6V) (purged with Air) ~60 (at 0.1V) (purged with Hydrogen) ~-3.0 (potentiostatic test at 0.1V) 3.26 (at þ0.7V) (purged with Air) 0.48 (purged with Hydrogen) 145 0.2 (potentiostatic test at þ0.6V)

SS304L

~60 (purged with Air) ~100 (purged with Hydrogen)

SS310

~1 (saturated with Ar) ~0.8 (potentiostatic test at þ0.6V) (as polished)

SS316

35.21 (bubbled with Air) 24.43 (bubbled with Hydrogen þ Ar) ~50 (potentiostatic test at þ0.6V) ~35 (potentiostatic test at 0.1V) ~15 (at þ0.6V) (purged with Air) ~50 (at 0.1V) (purged with Hydrogen)

SS316L

~10 (at þ0.6V) (purged with Air) ~60 (at 0.1V) (purged with Hydrogen) ~11 (potentiostatic test at þ0.6V) ~12 (potentiostatic test at 0.1V) 17.7 (at þ0.6V) (purged with Air) 46.1 (at 0.1V) (purged with Hydrogen) 11.26 (at þ0.6V) (purged with Air) ~30 (at 0.1V) (purged with Hydrogen) 21 (purged with Air) 43.1 (purged with Hydrogen) 2.8 (potentiostatic test at þ0.6V) ~30 (bubbled with Air) ~70 (bubbled with Hydrogen þ Ar) ~10 (potentiostatic test at þ0.6V) 38.2 (bubbled with O2) 26.4 (at þ0.6V) 72.5 (at 0.1V) 40 (purged with Air) 30 (purged with Hydrogen)

SS317L

~17 (at þ0.6V) (purged with Air) ~21 (at 0.1V) (purged with Hydrogen)

SS904L

~9.5 (at þ0.6V) (purged with Air) ~9.4 (at 0.1V) (purged with Hydrogen)

SS349

~10 (at þ0.6V) (purged with Air) ~15 (at 0.1V) (purged with Hydrogen)

SS430

15.8 (bubbled with Air) 6030 (potentiostatic test at þ0.6V)

SS434

~90 (at þ0.6V) (purged with Air) ~180 (at 0.1V) (purged with Hydrogen)

SS436

~20 (at þ0.6V) (purged with Air) ~63 (at 0.1V) (purged with Hydrogen)

2

)

Reference

(continued on next page)

5

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Journal of Power Sources 451 (2020) 227783

Table 3 (continued ) Materials

Corrosion current density Icorr (μA⋅cm 2)

Testing conditions

Interfacial contact resistance ICR (mΩ⋅cm2)

Compacting pressure (N⋅cm

SS441

~60 (at þ0.6V) (purged with Air) ~300 (at 0.1V) (purged with Hydrogen)

~125

140

[83]

SS444

90 (at 0.1V) (purged with Hydrogen) 20 (at þ0.6V) (purged with Air)

1 M H2SO4 2 ppm HF 70 � C 1 M H2SO4 2 ppm HF 70 � C 1 M H2SO4 2 ppm HF 70 � C 1 M H2SO4 2 ppm HF 70 � C 0.01 M H2SO4 pH ¼ 3 80 � C 1 M H2SO4 2 ppm HF 70 � C

~90

150

[84]

~95

140

[83]

~170

150

[84]

77

140

[85]

~400

150

[63]

~20 (at þ0.6V) (purged with Air) ~56 (at 0.1V) (purged with Hydrogen) SS446

20 (at þ0.6V) (purged with Air) 7 (at 0.1V) (purged with Hydrogen)

SS446 M

4 (at þ0.6V) (purged with Air) ~10 (at 0.1V) (purged with Hydrogen)

Fe–20Cr–4V

~500 (at þ0.84 V) (purged with Air) ~20 (at þ0.14 V) (purged with Hydrogen)

2

)

Reference

~ Approximate values obtained from literatures.

Fig. 3. Bare stainless steel investigated for BBPs.

of conventional stainless steels is to develop proper coatings for them. The coatings should have good corrosion resistance, high conductivity, good adhesion with the stainless steel substrate, and a compatible co­ efficient of thermal expansion with that of the substrate [86–88]. A lot of coatings have been investigated by researchers. Based on the materials utilized, coatings are normally classified into two categories [24,34,89]: carbon based and metal based coatings. The former includes graphite, amorphous carbon, diamond-like carbon and organic self-assembled monopolymers, while the latter comprises noble metals, metal ni­ trides, metal carbides, metal borides and conductive metal oxides. Meanwhile, various methods for depositing coatings or conducting surface modification [6,34,50,90], including physical vapor deposition (PVD), chemical vapor deposition (CVD), magnetron sputtering (MS), arc ion plating (AIP), electrodeposition (ED), dip coating (DC), pack cementation (PC), chemical passivation (CP), ion implantation (II), plasma surface diffusion alloying (PSDA), micro-arc alloying (MAA)

have been researched in recent years. In the aforementioned techniques, PVD, MS, AIP are the most commonly used. Corrosion resistant coatings and the corresponding coating methods for stainless steels are listed in Table 4. The performance of stainless steels with different coating materials are also screened and analyzed by the Ashby chart, as shown in Fig. 4. Similar to that of the bare stainless steels, most of the researches are focused on SS316L with corrosion resistant and conductive coatings. Typically, coatings researched for SS316L include noble metals (Au, Ag), non-noble metals (Cr, Ni), metal nitrides (Cr–N, Ti–N and Zr–N), metal carbides (Cr–C), metal oxides (ZrO2, SnO2: F), amorphous carbon (a-C) and multilayer coatings, as shown in Fig. 4 (a). According to the results presented, both non-noble metal and metal oxide coatings fail to fulfill the requirements for corrosion resistance and ICR. However, many coatings like noble metals, metal nitrides, metal carbides, and especially amorphous carbon and multilayer coatings can make SS316L meet the 6

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Journal of Power Sources 451 (2020) 227783

Table 4 Properties and testing methods of coated stainless/plain carbon steel materials for BPPs. Base materials

Coating materials

Coating/surface treatment method

Corrosion current density Icorr. (μA⋅cm 2)

Testing conditions

Interfacial contact resistance ICR (mΩ⋅cm2)

Compacting pressure (N⋅cm 2)

Reference

AISI 1020

Cr

0.0578

0.5 M H2SO4 25 � C

11.8

140

[68]

SS304

CrN

Electrical Discharge Machining (EDM) (2A) þ Low Temperature (700 � C) Chromization (LTC) Physical Vapor Deposition (PVD)

0.22 (purged with Hydrogen) 0.064 (purged with Air)

~19 (Before Test) ~20 (After Test)

220

[106]

Cr

Electroplating (EP) þ Heat treatment

11

150

[107]

NbC

Plasma Surface Diffusion alloying (PSDA)

140

[108]

Plasma Surface Diffusion alloying (PSDA)

0.5 M H2SO4 2 ppm HF 80 � C 0.05 M H2SO4 2 ppm HF 70 � C

8.47

Nb

~10 (purged with Hydrogen) 10.3 (at þ0.6V) (purged with Air) 0.058 (purged with Hydrogen) 0.051 (purged with Air) 0.384 (purged with Hydrogen) 0.461 (purged with Air)

0.001 M H2SO4 0.00015 M HCl 15 ppm HF 25 � C 0.05 M H2SO4 2 ppm HF 70 � C

140

[109]

Nb–N

Plasma Surface Diffusion alloying (PSDA)

0.127 (purged with Hydrogen) 0.071 (purged with Air)

0.05 M H2SO4 2 ppm HF 70 � C

140

[110]

Carbon nanotube (CNT)

Compression molding Process (CMP) þ Chemical Vapor Deposition (CVD) Unbalanced Magnetron Sputtering (UMS)

0.17 (at 0.1V) (purged with Hydrogen) 0.3 (at þ0.6V) (purged with Air) 0.278 (duty cycle: 40%; bias voltage: 75V)

1 M H2SO4 2 ppm HF 70 � C

10.53 (Before Test) 39.61 (Anode After Test) 48.9 (Cathode After Test) 9.26 (Before Test) 18.02 (Anode After Test) 19.14 (Cathode After Test) 9.7

200

[111]

8.4 (duty cycle: 40%; bias voltage: 75V)

140

[112]

Polypyrrole þ Graphene

Electrodeposited (EP)

9.25

19

140

[113]

TiN nanoparticles

Electrophoretic Deposition (EPD)

~0.9 (saturated with Ar)

~15

150

[74]

Cr–N–C

Magnetron sputtering ion plating (MSIP)

0.61 (at þ0.6 V) (bubbled with Air)

2.64

140

[114]

CrN

Cathode arc ion plating (CAIP)

23

150

[115]

TiN

Cathode arc ion plating (CAIP)

10

150

[115]

Cr–N

Nitridation

0.1 (purged with Hydrogen) 0.3 (purged with Air) 1 (purged with Hydrogen) 2.5 (purged with Air) 1 (bubbled with Air)

8.4

120

[116]

Au (1 μm)

Magnetron Sputtering or Cathodic Arc Physical Vapor Deposition (PVD) Magnetron Sputtering or Cathodic Arc Physical Vapor Deposition (PVD) Magnetron Sputtering or Cathodic Arc Physical Vapor Deposition (PVD) Magnetron Sputtering or Cathodic Arc Physical Vapor Deposition (PVD) Low Temperature Carburization (LTC)

~4.3

140

[117]

~998

140

[117]

~160

140

[117]

~5.1

140

[117]

~73 (As received) ~73 (After corrosion test)

150

[75]

6.4 (Cr0.64N0.36)

120

C film

SS310

Zr (0.5 μm) ZrN (0.5 μm) ZrN þ Au (0.5 μm þ 10 nm) Carbon

SS316L

Cr–N

~0.16 ~0.065 ~0.073 ~0.1 ~3 (bubbled with Hydrogen) ~0.3 (bubbled with Air) 3.97 (anode potentiostatic) 1.5 (cathode potentiostatic)

0.5 M H2SO4 5 ppm HF 25 � C 0.3 M H2SO4 2 ppm HF 25 � C 0.05 M H2SO4 2 ppm HF 80 � C 0.5 M H2SO4 5 ppm HF 70 � C 0.1 M H2SO4 2 ppm HF 80 � C 0.1 M H2SO4 2 ppm HF 80 � C 0.5 M H2SO4 5 ppm HF 25 � C 0.01 M H2SO4 pH ¼ 2 80 � C 0.01 M H2SO4 pH ¼ 2 80 � C 0.01 M H2SO4 pH ¼ 2 80 � C 0.01 M H2SO4 pH ¼ 2 80 � C 0.5 M H2SO4 2 ppm HF 80 � C

[92] (continued on next page)

7

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Journal of Power Sources 451 (2020) 227783

Table 4 (continued ) Base materials

Coating materials

Coating/surface treatment method

Corrosion current density Icorr. (μA⋅cm 2)

Testing conditions

Pulsed Bias Arc Ion Plating (PBAIP)

0.59 (at þ0.6V) (Cr0.64N0.36)

Cr (40 μm)

Pack Cementation (PC)

~0.88 (5 h) (at þ0.6V)

Cr–C film (Cr0.23C0.77)

Pulsed Bias Arc Ion Plating (PBAIP)

Cr–C þ Cr–N

Lower Temperature Chromizing Process (LTCP)

C–Cr film

Pulsed Bias Arc Ion Plating (PBAIP)

0.3 (at þ0.6V) (bubbled with Air) 0.63 (at 0.1V) (bubbled with Hydrogen) ~3.2 (bubbled with Air) (at þ0.6V) ~0.35 (bubbled with Hydrogen) (at 0.1V) ~0.1

0.5 M H2SO4 5 ppm HF 25 � C 0.5 M H2SO4 80 � C 0.001 M H2SO4 5 ppm HF 70 � C 0.5 M H2SO4 2 ppm HF 25 � C

Nitridation

Plasma Nitriding (PN)

~0.2 (bubbled with Hydrogen)

Ni–Cr (60 nm)

Ion Implantation (II)

SnO2:F

Low-Pressure Chemical Vapor Deposition (LPCVD)

Ag

Ion Implantation (II)

a-C

Close Field Unbalanced Magnetron Sputter Ion Plating (CFUBMSIP)

~6.70 (at þ0.6V) (bubbled with Air) ~5 (at 0.1V) (bubbled with Hydrogen) ~10 (at þ0.6V) (bubbled with Air) ~17 (at 0.1V) (bubbled with Hydrogen) 2.0 (at þ0.6V) (bubbled with Air) 1.1 (at 0.1V) (bubbled with Hydrogen) ~0.3 (bubbled with Air) ~0.3 (bubbled with Hydrogen)

Cr þ Mo2C

Magnetic Sputtering (MS)

0.091 (purged with Air) 0.23 (purged with Hydrogen)

0.5 M H2SO4 2 ppm HF 70 � C

TiN

Magnetic Sputtering (MS)

0.99 (bubbled with Air)

0.5 M H2SO4 2 ppm HF 70 � C

ZrN

Magnetic Sputtering (MS)

0.209 (bubbled with Air)

0.5 M H2SO4 2 ppm HF 70 � C

CrN þ Cr

Pulsed Bias Arc Ion Plating (PBAIP)

C þ CrN CrN þ TiN

Closed Field Unbalanced Magnetron Sputtering Ion Plating (CFUBMSI) Magnetron Sputtering (MS)

0.1 (purged with Air) 0.032 (purged with Hydrogen) ~1 (at þ0.6V) (purged with Air)

Ti þ TiN

Physical Vapor Deposition (PVD)

Al–Cr – Mo–N

Magnetron Sputtering (MS)

0.095 (bubbled with O2) 0.93 (at þ0.6V) 0.52 (at 0.1V) 0.02 (purged with Air) 0.03 (purged with Hydrogen)

0.5 M H2SO4 5 ppm HF 70 � C 0.5 M H2SO4 2 ppm HF 80 � C 1 M H2SO4 2 ppm HF 70 � C 0.5 M H2SO4 2 ppm HF 70 � C 0.5 M H2SO4 2 ppm NaF 70 � C

C þ CrN

Cathode Arc-Ion Plating (CAIP)

a-C þ ZrC

Closed Field Unbalanced Magnetron Sputtering Ion Plating (CFUBMSIP)

0.07 (purged with Air) 0.12 (purged with Hydrogen) 0.49 (purged with Air) ~0.3 (purged with Hydrogen)

0.1 M H2SO4 2 ppm HF 80 � C 0.5 M H2SO4 5 ppm HF 70 � C

0.3 (at þ0.6V) 0.18 (at 0.1V)

Interfacial contact resistance ICR (mΩ⋅cm2)

Compacting pressure (N⋅cm 2)

Reference

~88 (5 h)

150

[118]

2.8 (S6)

120

[96]

~22

150

[119]

0.5 M H2SO4 5 ppm HF 25 � C pH ¼ 3 H2SO4 2 ppm HF 25 � C 0.5 M H2SO4 2 ppm HF 80 � C

6.86

150

[95]

~10

150

[120]

~50 (NiCr2)

150

[121]

1 M H2SO4 2 ppm HF 70 � C

~150

150

[122]

0.5 M H2SO4 2 ppm HF 80 � C

~150

150

[76]

0.5 M H2SO4 2 ppm HF 80 � C

7 (Before Test) 27.5 (Anode After Test) 27.5 (Cathode After Test) (Graphite~7) ~7 (before polarization) ~8 (after polarization) 1.544 (before polarization) 7.418 (after polarization) 18.86 (before polarization) 109.2 (after polarization) 8.4

150

[77]

150

[2]

150

[94]

150

[94]

140

[123]

~2.75

150

[98]

23.5

150

[124]

150

140

[80]

31 (Before Test) 32.5 (Anode After Test) 34 (Cathode After Test) 12

140

[81]

150

[100]

3.63 (before polarization) 3.92 (after cathode polarization)

140

[72]

(continued on next page)

8

Journal of Power Sources 451 (2020) 227783

Y. Leng et al.

Table 4 (continued ) Base materials

Coating materials

Coating/surface treatment method

Corrosion current density Icorr. (μA⋅cm 2)

Testing conditions

Compacting pressure (N⋅cm 2)

Reference

140

[101]

140

[101]

140

[101]

150

[125]

150

[126]

~15

140

[127]

5.8 (CrMoN-2A) 7.8 (CrMoN-4A) 6.8 (CrMoN-6A)

140

[127]

H2SO4 pH ¼ 3 0.1 ppm HF 80 � C 1 M H2SO4 2 ppm HF 70 � C

~1.3 (Before Test) ~1.67 (After Test)

140

[128]

~90

150

[122]

1 M H2SO4 2 ppm HF 70 � C

~210

150

[122]

0.001 M H2SO4 0.00015 M HCl 15 ppm HF 25 � C 0.05 M H2SO4 0.01 M NaCl 2 ppm HF 70 � C 1 M H2SO4 2 ppm HF 70 � C

~15.5 (Before Test) ~72 (After Test)

220

[106]

14.89

140

[82]

~240

150

[129]

1 M H2SO4 2 ppm HF 70 � C

~70

150

[84]

a-C þ Cr

Closed Field Unbalanced Magnetron Sputtering Ion Plating (CFUBMSIP)

0.76 (at 0.84 V vs SHE) (purged with Air)

H2SO4 pH ¼ 3 0.1 ppm HF 80 � C

a-C þ Ti

Closed Field Unbalanced Magnetron Sputtering Ion Plating (CFUBMSIP)

0.35 (at 0.84 V vs SHE) (purged with Air)

H2SO4 pH ¼ 3 0.1 ppm HF 80 � C

a-C þ Nb

Closed Field Unbalanced Magnetron Sputtering Ion Plating (CFUBMSIP)

0.54 (at 0.84 V vs SHE) (purged with Air)

H2SO4 pH ¼ 3 0.1 ppm HF 80 � C

CrN þ Cr2N

Electrochemical Nitridation (EN)

~0.145 (bubbled with Air) ~0.136 (bubbled with Hydrogen)

0.5 M H2SO4 2 ppm HF 25 � C

CrN þ (Ti, Cr)N þ Ti CrN þ Cr

Arc ion plating (AIP)

0.12

Closed Field Unbalanced Magnetron Sputtering Ion Plating (CFUBMSIP) Closed Field Unbalanced Magnetron Sputtering Ion Plating (CFUBMSIP)

1.406 (bubbled with Air) 2.648 (bubbled with Hydrogen) 0.0345 (CrMoN-2A) 0.0163 (CrMoN-4A) 0.0564 (CrMoN-6A) (bubbled with Air) 0.119 (CrMoN-2A) 0.0588 (CrMoN-4A) 0.0572 (CrMoN-6A) (bubbled with Hydrogen) 0.180 (bubbled with Air) 0.006 (potentiostatic test at þ0.6V) ~20 (at þ0.6V) (bubbled with Air) ~14 (at 0.1V) (bubbled with Hydrogen) ~10 (at þ0.6V) (bubbled with Air) ~4 (at 0.1V) (bubbled with Hydrogen) 0.17 (purged with Hydrogen) 0.069 (purged with Air)

0.5 M H2SO4 70 � C 0.5 M H2SO4 5 ppm HF 25 � C 0.5 M H2SO4 5 ppm HF 25 � C

CrMoN þ CrN þ Cr

a-C

Closed Field Unbalanced Magnetron Sputtering Ion Plating (CFUBMSIP) Low-Pressure Chemical Vapor Deposition (LPCVD)

SS317L

SnO2: F

SS349

SnO2: F

Low-Pressure Chemical Vapor Deposition (LPCVD)

SS430

Nb (900 μm) (Nb: 50 μm)

Roll Bonding (RB)

W

Plasma Surface Diffusion Alloying (PSDA)

2.07 (bubbled with Air) 2.50 (potentiostatic test at þ0.6V)

SS441

SnO2: F (0.6 μm)

Low-Pressure Chemical Vapor Deposition (LPCVD)

SS444

SnO2: F

Low-Pressure Chemical Vapor Deposition (LPCVD)

~50 (at þ0.6V) (bubbled with Air) ~200 (at 0.1V) (bubbled with Hydrogen) 85 (at þ0.6V) (bubbled with Air)

Interfacial contact resistance ICR (mΩ⋅cm2) 3.82 (after anode polarization) 2.3 (before polarization) 7.6 (after polarization at 1.1 VSHE) 34 (after polarization at 1.6 VSHE) 5.2 (before polarization) 14.5 (after polarization at 1.1 VSHE) 23.4 (after polarization at 1.6 VSHE) 5.48 (before polarization) 17.8 (after polarization at 1.1 VSHE) 26.1 (after polarization at 1.6 VSHE) 7 (before polarization) 13 (after polarization at þ0.6V) 14 (after polarization at 0.1V) 4.9

(continued on next page)

9

Y. Leng et al.

Journal of Power Sources 451 (2020) 227783

Table 4 (continued ) Base materials

SS446

Coating materials

Coating/surface treatment method

CrN/Cr2O3 (130 nm)

Low-temperature Nitridation (LTN)

MN(CrN/Cr2N) (4.8 nm) þMO (7.2 nm)

Electrochemical Nitridation (EN)

(Cr, Fe)2N1

Thermal Nitridation (TN)

x

Cr–N þ Cr–O

Pre-oxidation þ Nitridation (PON)

Fe–27Cr

Cr-Nitride

Fe–27Cr–6V

Cr-Nitride

Fe–20Cr–4V

Cr–O þ Cr–N

High Temperature Nitridation (HTN) High Temperature Nitridation (HTN) Pre-Oxidation (PO) þ High Temperature Nitridation (HTN)

Ni–50Cr

CrN þ Cr2N (~1

μmþ 3–5 μm)

Thermal nitridation (TN)

Corrosion current density Icorr. (μA⋅cm 2) 46 (at 0.1V) (bubbled with Hydrogen) 2 (at þ0.6V) (bubbled with Air) 3 (at 0.1V) (bubbled with Hydrogen) 0.7 (purged with Air) (at þ0.6V) 2.5 (purged with Hydrogen) (at 0.1V) 7 (at þ0.6V) (purged with Air) 0.06 (at 0.1V) (purged with Hydrogen) ~5 (at þ0.6V) (purged with Air) ~4 (at 0.1V) (purged with Hydrogen) ~1.6 (at þ0.6V) ~0.1 (at þ0.6V) ~6 (at þ0.84 V vs SHE) (purged with Air) ~3 (at þ0.14 V vs SHE) (purged with Hydrogen) ~3 (at þ0.6V) (purged with Air) ~1.2 (at 0.1V) (purged with Hydrogen)

Testing conditions

Interfacial contact resistance ICR (mΩ⋅cm2)

Compacting pressure (N⋅cm 2)

Reference

H2SO4 pH ¼ 3 80 � C

6 77

140

[85]

1 M H2SO4 2 ppm HF 70 � C

18 180

140

[130]

1 M H2SO4 2 ppm HF 70 � C

~12

150

[131]

H2SO4 pH ¼ 3 80 � C

~8

150

[132]

H2SO4 pH ¼ 3 80 � C H2SO4 pH ¼ 3 80 � C 1 M H2SO4 2 ppm HF 70 � C

~8

150

[104]

~6

150

[104]

~15

150

[63]

1 M H2SO4 2 ppm HF 70 � C

~10

150

[133]

~ Approximate values obtained from literatures.

Fig. 4. Coating materials investigated for stainless steel and carbon steel BBPs (a) Coatings for SS316L; (b) Coatings for SS316; (c) Coatings for SS304 and SS310; (d) Coatings for ferritic stainless steel and carbon steel.

DOE targets. A. Kumar et al. [91] studied a 10 nm-Au/SS316L developed by Daido Steel (Japan). The results showed that the 10 nm-Au coated SS316L achieved a corrosion resistance of 0.98 μA⋅cm 2 in the simulated

cathode environment with a solution of 0.5 M H2SO4 and 2 ppm HF at 80 � C, and the ICR of 6.3 mΩ⋅cm2 at the compact pressure of 60 N⋅cm 2. Although a small amount of Au was used and the corrosion resistance and ICR met the DOE targets for the 10 nm-Au/SS316L, it was still not 10

Y. Leng et al.

Journal of Power Sources 451 (2020) 227783

suitable for practical volume production for BPPs when cost was taken into consideration. Therefore, a variety of researches have been focused on non-noble metal coatings. In the work by B. Wu and Y. Fu et al. [92], a Cr–N film was prepared on SS316L by pulsed bias arc ion plating (PBAIP). Chromium nitride films with different stoichiometric ratios were obtained by controlling the N2 flow rate. Their test results showed that Cr0.64N0.36 coated SS316L possessed the best performance and met the DOE targets. However, the test for corrosion resistance was per­ formed at room temperature with a solution of 0.5 M H2SO4 and 5 ppm HF. In a former work by Yu Fu et al. [93], CrxN coated SS316L through PBAIP was introduced. The corrosion current densities of all specimens were tested at room temperature (25 � C) and 70 � C, respectively, with a solution of 0.5 M H2SO4 and 5 ppm HF. The SS316L coated with Cr0.49N0.51→Cr0.43N0.57 gradient film tended to have the best perfor­ mance, with a corrosion current density of about 0.1 μA⋅cm 2 at 25 � C and an ICR of 7.9 mΩ⋅cm2 at the compact pressure of 120 N⋅cm 2. A TiN coated SS316L by magnetron sputtering was described in the work by Pan Yi et al. [94]. The SS316L with a TiN coating about 3 μm in thickness had a corrosion current density of 0.099 μA⋅cm 2 in a simulated cathodic environment with a solution of 0.5 M H2SO4 and 2 ppm HF at 70 � C and the ICR of 1.544 mΩ⋅cm2 at the compact pressure of 150 N⋅cm 2. Cr–C film and amorphous carbon coated SS316Ls published in articles which can be reached are summarized in the present paper and all of them meet the DOE targets. The corrosion resistance and con­ ductivity of Cr–C film coated SS316Ls were also studied by Yu Fu and Bo Wu et al. [95]. In another work by them [96], the stoichiometric ratio of chromium and nitride was changed to obtain different ratio of sp3/sp2 carbon atom content and improve the conductivity. The SS316L with a Cr0.23C0.77 film had the lowest ratio of sp3/sp2 carbon atom content, which assisted it to achieve the lowest ICR of 2.8 mΩ⋅cm2 at the compact pressure of 120 N⋅cm 2. Kai Feng et al. [78] prepared an amorphous carbon coated SS316L utilizing close field unbalanced magnetron sputter ion plating (CFUBMSIP). The corrosion current densities of a-C coated SS316L in a 0.5 M H2SO4 and 2 ppm HF solution at 80 � C was 0.06 μA⋅cm 2 (both in simulated anodic and cathodic environment) and ICR at a compacting pressure of 150 N⋅cm 2 was about 7 mΩ⋅cm2, respectively. Further study [77] was carried out to simplify the depo­ sition process and lowering the cost in their later work. Recently, an effort has been made to enhance the stability and property of a-C through doping of niobium by Kun Hou et al. [97]. The influence of Nb doping on carbon atom sp2/sp3 ratio was also clarified. The corrosion current density of SS316L with Nb-doped a-C coating (S2) was 0.389 μA⋅cm 2 in the simulated cathodic environment with a solution of pH ¼ 3.0 H2SO4 and 5 ppm HF at 80 � C, while the ICR at a compacting pressure of 140 N⋅cm 2 was 3.81 mΩ⋅cm2. Although corrosion resistance and ICR of all the coated SS316Ls described above have achieved the DOE targets, the performance is obtained in short term ex-situ test while durability of them is not taken into consideration. One of the limitations in short term tests is that they cannot reveal the effects of micro coating defects on corrosion resistance and ICR values. However, almost all kinds of coatings may have defects or imperfections to some extent, which will probably affect the long term corrosion resistance and ICR values. Normally, defects and im­ perfections may emerge in the following three processes: (1) coating process; (2) delivery process; and (3) stack assembling process. There­ fore, a lot of researchers believe that multilayer coating is the most promising way to solve the problems caused by defects or imperfections in different coatings. In recent years, many researches have been conducted on multilayer coated SS316Ls, most of which have met the DOE targets. The multilayer coatings studied include Cr–N þ Cr, Cr–C þ Cr–N, Cr–N þ Ti–N, Ag þ PTFE, AlN þ TiN, Cr þ Mo2C, C þ Al–Cr–N, Cr–N þ (Ti, Cr)–N þ Ti, Cr–Mo–N þ Cr–N þ Cr, C þ metal, C þ metal nitride, and C þ metal carbide (Fig. 4 (a)). Among them, coatings with carbon, especially amorphous carbon surface on the top of metal or metal nitride/carbide layer attract the attention of most researchers.

The team of Kai Feng and Zhuguo Li et al. [98] prepared a C þ CrN multilayer coating on SS316L by CFUBMSIP and investigated the corrosion resistance and ICR. The results showed that the corrosion current density of C þ CrN coated SS316L in a simulated cathode environment with a solution of 0.5 M H2SO4 and 2 ppm HF solution at 80 � C was about 0.1 μA⋅cm 2, while the ICR at a compacting pressure of 150 N⋅cm 2 was about 2.75 mΩ⋅cm2. In order to further improve the corrosion resistance, their team prepared a C þ Al–CrN multilayer coated SS316L utilizing the same deposition method [99]. The multi­ layer coating consisted of a 0.9 μm carbon layer and a 1.4 μm Al–Cr–N sub-layer, making the total thickness of about 2.3 μm. By doping aluminium in the previous Cr–N sub-layer, the corrosion current density was reduced to 0.01 μA⋅cm 2 at the same test condition, while the ICR remained 4.88 mΩ⋅cm2 at the same compacting pressure of 150 N⋅cm 2. Besides, their research pointed out that the atom ratio of aluminium and chromium should be no higher than 2:4 in order to achieve high corrosion resistance and low ICR. S. H. Lee et al. [100] also studied the performance of C þ CrN multilayer coating which was prepared by cathode arc-ion plating (CAIP) on SS316L. The multilayer consisted an out layer of 1.22 μm CrN and a sub-layer of 7 nm carbon layer. Elec­ trochemical tests showed that the corrosion current density of the multilayer coated SS316L were about 0.07 μA⋅cm 2 and 0.12 μA⋅cm 2 in a simulated cathodic and anodic environment, respectively. However, it should be noted that the test was conducted at 80 � C in a solution of 0.1 M H2SO4 and 2 ppm HF in which the concentration of H2SO4 was much lower than that of the work by K. Feng and Z. G. Li et al. In addition, the ICR at a compacting pressure of 150 N⋅cm 2 was 12 mΩ⋅cm2, a little higher than the DOE target. The team of L. F. Peng, P. Y. Yi, and X. M. Lai et al. [72,101,102] developed a variety of multilayer coatings with an amorphous carbon surface layer for SS316L by utilizing CFUBMSIP deposition technique. First, they prepared an a-C þ Zr–C coated SS316L whose corrosion current density in a simulated cathodic environment and ICR at a compacting pressure of 140 N⋅cm 2 were 0.49 μA⋅cm 2 and 3.63 mΩ⋅cm2, respectively. In their later work, three multilayer coat­ ings, including a-C þ Cr, a-C þ Ti, and a-C þ Nb for SS316L were investigated and they all met the DOE targets. Among them, the a-C þ Ti coated SS316L achieved the lowest corrosion current density of 0.07 μA⋅cm 2 in a simulated cathodic environment at 0.84 V vs SHE. All the specimens were tested at 80 � C in a pH ¼ 3.0 H2SO4 and 0.1 ppm HF solution, of which the HF concentration was much lower than that of other reported literatures. They also prepared and studied a set of a-C þ TiCx þ Ti multilayer coatings with different a-C nanolayer for SS316L by varying magnetron sputtering process. The corrosion resistance of the a-C þ TiCx þ Ti multilayer (under 60 V/20 min þ300 V/39 min) coated SS316L in the simulated cathodic environment with a solution of pH ¼ 3.0 H2SO4 and 0.1 ppm HF at 80 � C was 0.085 μA⋅cm 2, while the ICR at a compacting pressure of 140 N⋅cm 2 was 1.85 mΩ⋅cm2. A compre­ hensive review on carbon-based coatings has been published by the team of L. F. Peng, P. Y. Yi, and X. M. Lai et al. [103] recently. In this review, pure amorphous carbon films, metal doped amorphous carbon films, metal carbide films, as well as coating evaluation method are introduced for metallic BBPs of PEMFC. Other stainless steels, such as SS316 (Fig. 4 (b)), SS310 (Fig. 4 (c)), SS304 (Fig. 4 (c)), ferritic stainless steels and plain carbon steels (Fig. 4 (d)) with different coatings have also been investigated. The coatings which have met the DOE targets for SS316 include Au, Cr–N–C and Au þ ZrN. For coated SS304, most of them fail to meet the DOE target for ICR although they achieve the target for corrosion resistance. This reveals that further work are needed to reduce the ICR of coated SS304 in the future. Among those coated ferritic stainless investigated, the Cr–N coated Fe–Cr–V stainless steel [62,63,104] and a PL-TaN/Ta coated SS430 [105] have been reported to approach or achieve the DOE targets. Both corrosion resistance and ICR of the coated SS430 in most re­ searches need to be improved to meet the DOE targets in the future. An interesting phenomenon was observed when contrasting the bare (Fig. 3) and coated (Fig. 4 (d)) plain carbon steels [68,69] for BPPs by 11

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Journal of Power Sources 451 (2020) 227783

Ashby approach. As for the bare plain carbon steels, both the corrosion current density and ICR are much higher than the DOE targets. However, the corrosion current density and ICR of coated carbon steels are approaching the DOE targets. This provides us another possible solution for solving the corrosion and cost problems for metallic BPPs. By developing high performance coatings with fewer defects, base metal materials with lower corrosion resistance and cost, such as SS304 or even plain carbon steels may be utilized for manufacturing BBPs in volume production. In summary, all kinds of bare conventional stainless steel materials fail to meet the corrosion resistance and ICR requirements for BPPs set by DOE. Therefore, Coatings are needed before they can be utilized in a PEMFC. The Ashby charts revealed that the most promising coatings for stainless steel BPPs include Cr–N, Ti–N, Cr–C, amorphous carbon and multilayer coatings. For coating methods, physical vapor deposition, such as arc-ion plating and especially magnetron sputtering are believed to be the most suitable techniques, especially for multilayer coatings. In addition to depositing coatings, another way of achieving the DOE tar­ gets for stainless steel BPPs is to develop novel stainless steel grades and proper surface modification methods, such as the non-coated ferritic stainless steel with superior corrosion resistance and conductivity developed by POSCO. Therefore, there are normally two possible ways of achieving the goal set by DOE. One is to develop bare stainless steel materials with higher corrosion resistance and conductivity through composition and process optimization, the other is to research on high corrosion resistant and conductive coatings with fewer defects or im­ perfections. For both ways of developing new materials for stainless steel BPPs, the formability of bare stainless steel has to be improved, which will be discussed in detail in the following sections.

Fig. 5. Schematic of flow channel geometries (a) machined graphite bipolar plate; (b) stamped/hydroformed stainless steel bipolar plate.

performance and manufacturability should be taken into consideration when designing channel height, channel width, rib width, draft angle, channel radius and rib radius for thin stainless steel BPPs. Channel height may affect fuel cell performance and manufactur­ ability of stainless steel bipolar plate in different aspects [137–139]. As channel height decreases, the mass transfer towards the catalyst layers and water removal ability may be improved, and thus enhancing elec­ trochemical reaction rate and fuel efficiency. However, the reduced channel height can cause excessive pressure drop and power loss due to GDL intrusion. On the other hand, the increase in channel height may not only lead to a lower efficiency of mass transfer and water removal, but also result in a larger volume of fuel cell or the stack, and thus a lower power density. In addition, a larger channel height requires better formability of the stainless steel materials for bipolar plates, which may increase the difficult and cost of manufacturing. Therefore, there exist a lower and an upper limit within which fuel cells are of higher perfor­ mance while the bipolar plate can be manufactured with better quality and lower costs. Studies on the effects of channel height of different flow field pat­ terns have been conducted extensively by numerical analysis and/or experiments. X. D. Wang et al. [140,141] investigated the effects of channel height with vary or constant channel height to width ratios (h/w) on mass transfer and fuel cell performance by numerical simu­ lation. When the channel height decreased from 2.0 mm to 0.5 mm with a constant width of 1.0 mm, fuel cell performance was improved remarkably at operating voltages lower than 0.7 V. They owed the improvement of fuel cell performance to enhanced water removal ability and higher oxygen utilization efficiency at lower operating voltages. However, the channel height had a negligible effect on cell performance when operating voltages were higher than 0.7 V. This was due to little oxygen consumption and liquid water production. When channel height decreased from 1.533 mm to 0.307 mm with a constant h/w of 1, the same effect of channel height on fuel cell performance was observed as with the constant width of 1.0 mm. On the other hand, as channel height decreased, the pressure drop increased significantly due to smaller cross-sectional area. Their studies indicated that a smaller channel height would contribute to a better fuel cell performance and higher pressure drop. H. C. Chiu et al. [142] also developed a two phase three dimensional model to channel height and other geometric parameters of parallel and serpentine flow fields on transport phenomena and fuel cell performance. As channel height decreased from 1.5 mm to 0.75 mm, the cell performance remained almost the same when operated at higher

3. Flow channel design and forming process For stainless steel BBPs as thin as 0.1 mm or even thinner, the flow channel geometries not only play a critical role in distributing reactant gases, conducting electricity and heat, removing the water produced and providing sufficient mechanical support for the MEA [134,135], but also have a great impact on their manufacturability, quality and yield. Therefore, the flow channel structure of stainless steel BPPs is deter­ mined by both the requirements for fuel cell performance and plastic forming process utilized. In the following sections, flow channel geom­ etries and plastic forming processes for thin stainless steel BBPs are discussed and analyzed. Challenges on structure design and forming process are pointed out as well. In the meanwhile, future research trends on flow channel design and forming process for higher performance stainless steel BPPs are also suggested. 3.1. Flow channel structure design As mentioned above, flow channel geometries of BPPs have remarkable influences on reactant gas distribution, conductivity, heat and water management, mechanical stability, and thus the performance, reliability and durability of fuel cell [136]. The flow channel geometries of bipolar plates produced by different processes with different materials vary from each other. For instance, there is big difference between the flow channel geometries of machined graphite BPP and stamped/hy­ droformed stainless steel BPP, as shown schematically in Fig. 5 (a) and (b), respectively. For graphite BPP manufactured through machining, channel height (h), channel width (w) and rib width (ws) are the pa­ rameters of its flow channel geometries. However, the structure of stainless steel BPP produced by plastic forming process is more complex than that of the machined graphite BPP. Besides channel height, channel width and rib width, draft angle (α), channel radius (r) and rib radius (R) are also included in the geometries of stamped/hydroformed stainless steel BPP. Although draft angle, channel and rib radius are not neces­ sarily the intended design parameters for fuel cell performance, they have a great impact on formability. Therefore, both of fuel cell 12

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Journal of Power Sources 451 (2020) 227783

performance with a channel height and h/w of 1.0 mm and 1, respec­ tively. The researches by H. C. Chiu et al. [142], N. J. Cooper et al. [145] and K. B. S. Prasad et al. [153] had similar findings as presented by S. W. Cha et al. and X. D. Wang et al. Based on the above research results, it can be concluded that channel height in interdigitated flow field had a smaller influence on fuel cell performance even at lower operating voltages. Besides, excessive smaller channel height may even result in poor cell performance. The effects of channel height on interdigitated flow field pattern is contrary to that in parallel or serpentine flow field patterns. Another aspect that may affect the real channel height of BPP in fuel cell stack is the intrusion of GDL into channel. The results by I. Nitta et al. [154] indicated that GDL intrusion varied from about 0.07 mm to 0.2 mm as compression thickness was changed from 0.08 mm to 0.23 mm, while channel width had a smaller effect on GDL intrusion depth. A. R. Kim et al. [155] studied the influence of w/ws ratio on GDL intrusion depth. They found that as w/ws ratio changed from 1.1 to 1.5, GDL intrusion depth increased from about 0.18 mm to 0.23 mm. Their study also proved that larger GDL intrusion would cause higher pressure drop. In another work by A. R. Kim et al. [156], both the experimental and numerical results showed that the penetration depth of GDL with an initial thickness of 0.38 mm increased from about 0.04 mm to 0.12 mm when clamping pressure was changed from 0.2 MPa to 1.2 MPa. At the same time, the total pressure drops increased from about 5 kPa to 6.5 kPa. In addition to channel height, channel width, rib width and w/ws ratio are also critical geometrical parameters having great impacts on fuel cell performance. Generally, a larger channel width can provide more sites for chemical reaction. However, rib width may be too small if the width of channel is excessively large. Although water removal ability under the ribs can be improved with smaller rib widths, the ability of electron and heat conductivity may reduce while the stress under the ribs and ununiformity of stress distribution may increase, which can degrade the fuel cell performance and durability. The experimental results by Y. G. Yoon et al. [157] indicated that cell performance was improved especially at low operating voltage when rib width decreased from 3.0 mm to 0.5 mm while channel width remained 1.0 mm. The numerical results by W. Sun et al. [158] showed that current density was increased by 7% as rib width decreased from 1.0 mm to 0.5 mm with a constant channel width of 1.0 mm. And a further decrease of rib width from 0.5 mm to 0.25 mm might lower the current density slightly. In the meanwhile, water removal ability was improved as w/ws ratio increased from 1 to 4. J. The investigation by Scholta et al. [159] proved that fuel cell ohmic resistance tended to be low when w/ws ratio is smaller than 1. They also found that the optimized dimensions for both channel and rib width were in the range of 0.5 mm–1.0 mm if channel to rib width ratios were below 1. In the work by D. H. Ahmed et al. [160], an optimized channel to rib width ratio of 1.3–1.4 at high operating current density was suggested. The research by K. D. Baik et al. [161] showed that cell performance was improved for less GDL intrusion and lower contact resistance as w/ws ratio decreased from 3.74 to 1.48, and then degraded as w/ws ratio further decreased from 1.48 to 0.2. Y. Kerkoub et al. [162] found that cell performance was improved by 120%, 45% and 23% for serpentine, interdigitated and parallel flow fields, respectively, as w/ws ratio decreased from 2.66 to 0.25 with a fixed pitch of 2.0 mm. The research results by S. W. Cha et al. [143] and X. D. Wang et al. [141] indicated that fuel cell performance was improved as channel width decreased from 1.533 mm to 0.1 mm with equal channel height and rib width. However, X. D. Wang et al. also pointed out that pressure drop would increase significantly as channel width were smaller than 0.3 mm. K. S. Choi et al. [163] found that better fuel cell performance was obtained when channel width and the w/ws ratio decreased from 1.75 mm to 1.0 mm and 7 to 1, respectively. The work by H. C. Chiu et al. [142] and N. J. Cooper et al. [145] also proved that channel with smaller width and height could enhance cell perfor­ mance when they were in the range of 0.75 mm–1.5 mm, for both

voltages while it was improved remarkably at lower operating voltages. Their results were consistent with the findings by X. D. Wang et al. S. W. Cha et al. [143] examined the square channel with different channel sizes, ranging from 0.005 mm to 1.0 mm by computational and exper­ imental methods simultaneously. The results of their study further confirmed that fuel cell performance was able to be improved by lowering the channel height or cross-sectional area. However, the experimental results also showed that peak power density would degrade when the channel height was below 0.1 mm, which was con­ trary to the trend predicted by CFD model. N. Akhtar et al. [144] con­ ducted experiments to investigate the effect of channel height, which ranged from 0.6 mm to 1.5 mm, on performance of fuel cell with parallel flow field pattern. They also found that the shallower channels enhanced water removal and reactant gas utilization efficiency. As a result, the performance of fuel cells with smaller channel height was improved. N. J. Cooper et al. [145] also examined the influence of channel height, from 0.25 mm to 1.0 mm, in parallel flow field pattern on fuel cell performance. The results in their research were in accordance with those discussed above. In the meanwhile, other work which had been focused on water removal of flow field channel with different sizes also indicated that reduction in channel height are beneficial for water removal. S. Imanmehr and N. Pourmahmod [146] performed a parametric study to investigate the influences of channel height and other structural pa­ rameters in a simple serpentine flow field pattern on fuel cell perfor­ mance by numerical method. They obtained the same conclusion as researches discussed above when they varied the channel height from about 2.0 mm to 0.1 mm. S. Shimpalee et al. [147] examined the effects of channel height uniformity on performance and distributions of a fuel cell with serpentine flow field pattern and trapezoidal cross section shape. Their results showed that pressure drop increased with larger channel height deviation and shallower channel at the outlet. They also pointed out that shallower channel at the outlet and perfect channel height uniformity gave more uniform distribution for stationary and automotive conditions, respectively. J. P. Owejan et al. [148] studied the effects of channel geometries, including channel height of bipolar plates with wave-like flow field on water management in fuel cells by neutron radiography method. They recommended a channel depth of 0.4 mm for both anode and cathode flow field after taking reaction ef­ ficiency, GDL intrusion and manufacturing into consideration compre­ hensively. N. Jaruwasupanta and Y. Khunatorn [149] performed numerical simulation and experiments to investigate the influence of channel height on pressure drop and flow velocity. Their results indi­ cated that pressure drop and flow velocity both increased significantly as channel depth decreased from 1.2 mm to 0.8 mm. D. H. Chang and S. Y. Wu [150] examined numerically and experimentally the effects of channel height in two kinds of flow field, namely serpentine and parallel-serpentine flow field on performance of fuel cell. Pressure drop was also proven to increase as channel height changed from 0.6 mm to 0.2 mm. Besides, fuel cells with a channel height of 0.4 mm were found to possess the highest power density, which is in accordance with the research by J. P. Owejan et al. Recently, D. K. Shin et al. [151] inves­ tigated the effect of inclined channel on fuel cell performance. Their results indicated that concentration losses would be reduced while maximum power of fuel cell would be improved as channel depth decreased gradually from gas inlet to outlet. Researchers have also investigated the fuel cells with interdigitated flow field pattern to explore the effects of channel height on perfor­ mance. The results of numerical simulation by S. W. Cha et al. [152] indicated that the performance of fuel cell with interdigitated flow field pattern were relatively insensitive to channel height varying from 0.519 mm to 0.101 mm. In addition, a smaller channel height might result in degraded cell performance for poorer water removal and reactant gas distribution. This phenomenon was further proven by X. D. Wang et al. [140] who performed a numerical analysis of channel height, ranging from 1.533 mm to 0.307 mm, on fuel cell performance. Their study also pointed out that interdigitated flow field pattern possessed better 13

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Journal of Power Sources 451 (2020) 227783

parallel and serpentine flow field. Experiments have been conducted by X. Y. Zhang et al. [164] to study the effect of rib width on fuel cell performance. The results proved that fuel cell performance would be improved when rib width decreased from 2 mm to 0.5 mm while channel width remained 1 mm. However, their study also pointed out that the effect of rib width on fuel cell performance might be different when pumping power was taken into consideration. In addition, the work by D. H. Jeon [165] revealed that the ability of water removal from GDL would be reduced as channel and rib width became smaller. Apart from channel height, channel width and rib width, other channel geometries such as draft angle, channel and rib radius may also influence the performance of fuel cell [144]. The numerical results by S. Shimpalee et al. [147] indicated that cell performance at high current density for both stationary and automotive conditions was improved when draft angle decreased from 33.7� to 0� . The reason for this phe­ nomenon may be that while channel height remained the same, channel width and cross section area also became smaller as draft angle decreased. The study performed by N. Nakagaki [166] revealed that a

smaller draft angle helped to form a narrower channel pitch and thus larger contact area between the GDL and MEA, which could contribute to higher electrical conductivity, more uniform clamping pressure dis­ tribution and smaller GDL intrusion depth into channel. According to the results of previous studies shown in Table 5, fine flow channels with smaller height and width are beneficial for fuel cell performance. However, it is a big challenge to form such smaller ge­ ometries by conventional plastic forming methods, such as stamping and hydroforming. As channel and rib widths get smaller, the forming limit of channel height will decrease. For channel and rib widths smaller than 0.5 mm, the maximum channel height which can be formed is generally lower than 0.6 mm for stainless steel bipolar plates manufactured by stamping or hydroforming. Based on the research results discussed above, the suitable channel height, channel width, rib width and the ratio of channel to rib width for stainless steel bipolar plates produced by plastic forming may be in the range of 0.35 mm–0.55 mm, 0.4 mm–1.0 mm, 0.4 mm–1.0 mm and 0.6 to 1.4, respectively. The appropriate draft angle, channel and rib radius may need to be in the range of 10� –30� ,

Table 5 Research results on flow channel design for BPPs. Variables

Methods

Flow channel geometries

Results

Reference

h

Numerical simulation

(1) h ¼ 0.5–2.0 mm, w ¼ 1.0 mm; (2) h ¼ 0.3–1.7 mm, h/ w ¼ 1.0.

[140, 141]

Numerical simulation

(1) h ¼ 0.75–1.5 mm.

Numerical simulation and experiment Experiment

(1) h ¼ 0.005–1.0 mm, h/w ¼ 1.0. (1) h ¼ 0.6–1.5 mm.

Experiment Numerical simulation Numerical simulation and experiment Numerical simulation and experiment Numerical simulation

(1) h ¼ 0.25–1.0 mm. (1) h ¼ 0.1–2.0 mm. (1) h ¼ 0.8–1.2 mm.

(1) For both constant w and h/w, fuel cell performance with parallel flow field pattern was improved as channel height decreased, especially at operating voltages lower than 0.7 V; (2) Water removal ability and oxygen utilization efficiency were enhanced as channel height decreased; (3) Pressure drop would increase as channel height decreased due to smaller cross-sectional area. (1) Fuel cell performance remained almost the same when operated at higher voltages as channel height decreased; (2) Fuel cell performance would be improved remarkably at lower operating voltages as channel height decreased. (1) Fuel cell performance was improved as channel height decreased; (2) However, peak power density would degrade when the channel height was below 0.1 mm. (1) Fuel cell performance was improved as channel height decreased; (2) Water removal ability and reactant gas utilization efficiency were enhanced as channel height decreased. (1) Fuel cell performance was improved as channel height decreased. (1) Fuel cell performance was improved as channel height decreased. (1) Pressure drop and flow velocity both increased significantly as channel depth decreased.

[150]

Experiment

(1) w ¼ 1.0 mm; (2) ws ¼ 0.5–3.0 mm. (1) w ¼ 1.0 mm; (2) ws ¼ 0.25–1.0 mm; (3) w/ws ¼ 1–4.

(1) Pressure drop increased significantly as channel depth decreased. (2) Fuel cell possessed the highest power density with a channel depth of 0.4 mm. (1) The performance of fuel cell with interdigitated flow field pattern were relatively insensitive to channel height. (1) Fuel cell performance was improved as rib width decreased, especially at low operating voltages. (1) Current density was increased by 7% as rib width decreased from 1.0 mm to 0.5 mm with a constant channel width of 1.0 mm; (2) Further decrease of rib width from 0.5 mm to 0.25 mm might lower the current density; (3) Water removal ability was improved as w/ws ratio increased from 1 to 4. (1) Fuel cell ohmic resistance tended to be low when w/ws ratio is smaller than 1; (2) Optimized dimensions for both channel and rib width were in the range of 0.5 mm–1.0 mm if channel to rib width ratio were below 1. (1) Fuel cell performance was improved as w/ws ratio decreased from 3.74 to 1.48; (2) Less GDL intrusion and lower contact resistance were the main reason for improvement of fuel cell performance as w/ws ratio decreased; (3) Further reducing of w/ws ratio from 1.48 to 0.2 would degrade fuel cell performance. (1) Fuel cell performance was improved as w/ws ratio decreased. (1) Fuel cell performance was improved as channel width decreased from 1.75 mm to 1.0 mm; (2) Fuel cell performance was improved as channel to rib width ratio decreased from 7 to 1. (1) Fuel cell performance was improved as rib width decreased from 2.0 mm to 0.5 mm.

[163]

(1) Fuel cell performance at high current density was improved as draft angle decreased from 33.7� to 0� .

[147]

w/ws

Numerical simulation

Numerical simulation and experiment Experiment

Numerical simulation and experiment Numerical simulation Experiment

α

Numerical simulation and experiment

(1) h ¼ 0.2–0.6 mm. (1) h ¼ 0.1–0.5 mm.

(1) (2) (3) (1)

w ¼ 0.5–1.5 mm; ws ¼ 0.5–1.0 mm; w/ws ¼ 0.5–1.5. w/ws ¼ 0.2–3.74.

(1) (2) (1) (2) (1) (2) (1)

w/ws ¼ 0.25–2.66; pitch ¼ 2.0 mm. w ¼ 1.0–1.75 mm; w/ws ¼ 1–7. w ¼ 1 mm; ws ¼ 0.5–2.0 mm. 0–33.7� .

Optimized channel geometries based on previous researches

[142]

[143] [144] [145] [146] [149]

[152] [157] [158]

[159] [161]

[162]

[164]

(1) h ¼ 0.35–0.55 mm; (2) w ¼ 0.4–1.0 mm; (3) ws ¼ 0.4–1.0 mm; (4) w/ws ¼ 0.6–1.4 mm; (5) α ¼ 10–30� ; (6) R ¼ 0.1–0.2 mm; (7) r ¼ 0.05–0.2 mm.

14

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Journal of Power Sources 451 (2020) 227783

0.05 mm–0.2 mm and 0.1 mm–0.2 mm, respectively, when formability was taken into consideration [166–169]. The optimized ranges for different flow channel geometries are listed in Table 5. Although a lot of work has been done on flow channel structure design, much more efforts are still needed to improve the performance and formability and lower the cost of stainless steel bipolar plates. First, the real channel geometrical model of stainless steel BPP, rather than a geometrical model based on graphite BPP should be established for further optimization of its structure. Second, the effects of GDL intrusion have to be taken into consideration for more precise simulation and better flow channel geometry optimization. Moreover, structure opti­ mization of thin stainless steel BPPs should be performed on the basis of their formability.

3.2. Plastic forming process Many fabrication methods for stainless steel bipolar plate, such as electroforming [170], magnetic pulse forming [171], electrochemical machining [172], electrical discharge machining [173], electroetching [174] and additive manufacturing [175] have been investigated in recent years. However, for stainless steel bipolar plate as thin as 0.1 mm or even thinner, those processes mentioned above can’t be used while plastic forming processes are believed to be the most suitable manufacturing methods for mass production. Generally, plastic forming methods which have been studied for producing thin stainless steel bi­ polar plate can be divided into the following categories: (1) stamping; (2) hydroforming; (3) rubber pad forming or flexible forming; (4) roll forming; (5) novel plastic forming methods. Recent achievements in

Table 6 Recent achievements in stainless steel BPPs manufactured by plastic forming processes. Forming methods

Materials

Thickness (mm)

BPP Size (mm � mm)

Stamping

SS304 SS304

0.05 0.1

50 � 50 50 � 54

SS304

0.1

50 � 55

ferritic SS SS316L

0.15

Hydroforming

Rubber pad forming

Roll forming

a

0.1

150 � 100

SS316L

0.051

70 � 70

SS304

0.051

SS316

0.051

150 � 150

SS304

0.1

40 � 40

SS304

0.11

SS304

0.1

40 � 40

SS304

0.1

100 � 100

SS304

0.1

66 � 66

SS304

0.1

54.8 � 54.8

SS316L

0.1

SS316

0.1

57.1 � 56.4 57.1 � 56.4

SS316L SS304L SS316L

0.1 0.1 0.1

52 � 28 100 � 27 160 � 30

Forming conditions

Loading force: 70–110 kN Loading speed: 1 kN⋅s 1 Loading force: 50–100 kN Loading frequency: 0.5 Hz Cycle: 2–20

Reference

Channel width (mm)

Rib width (mm)

0.55 0.2–0.6

0.8 1.4

0.8

0.45–0.6

0.8–1.4

0.6–1.4

30–60

0.05–0.3/ 0.05–0.3

[183]

0.5

0.586

0.5

41.8

0.3/0.3

[185]

0.1–0.15/ 0.1–0.15 0.20a/0.25a

[168]

Loading force: 1000 kN Loading force: 100–300 kN Loading speed: 0.1–1.0 mm⋅s 1 Loading pressure: 55.2–82.7 MPa Loading speed: 20.7 MPa⋅s 1 Loading pressure: 20–60 MPa Loading speed: 0.1–10 MPa⋅s 1 Loading pressure: 60–250 MPa Loading speed: 13 MPa⋅min 1 Loading pressure: 80–100 MPa Loading force: 100–250 kN Loading force: 20–100 kN Loading force: 10–40 kN Loading speed: 0.2 mm⋅s 1 Loading pressure: 15–55 MPa Loading speed: 5–30 mm⋅s 1

0.4

Loading pressure: 350–600 kN Loading speed: 5 mm⋅s 1

0.75

0.9 a

Draft angle (� )

Channel/Rib Radius mm/mm

Channel depth (mm)

0.1–0.3/0.1–0.3

0.9 a

a

[219] [179]

Pitch ¼ 1.5

10

0.15–0.98

0.46–1.33

5–20

0.13–0.26/ 0.13–0.26

[220]

0.75a

Pitch ¼ 1.5a

10a

0.20a/0.25a

[200]

0.75a

0.75a

10a

0.20a/0.25a

[221]

0.8a

1.5a

10a

0.2a/0.3a

[194]

0.5

0.8

5–20

0.1–0.2/0.1–0.3

[12]

0.4–1.2

0.8–1.6

15

0.2–0.4/0.2

[204]

0.6

0.6–2.0

10

0.3/0.3

[208]

0.4

0.8

1.2

10–30a

0.2/0.2a

[205]

0.75

1.1

1.2

10

0.2/0.2

[222]

0.75

1.1

1.2

10

0.2/0.2

[10]

0.5a 0.36 1.0–2.0

0.5a Pitch ¼ 1.16

30a 16

0.2a 0.25/0.25 0.1–0.6a

[214] [213] [212]

Dimension of flow channel geometries obtained from dies. 15

0.75a

1.2

[3,199, 200]

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Journal of Power Sources 451 (2020) 227783

typical plastic forming processes for stainless steel BPPs are summarized in Table 6.

BPPs with high performance and quality by stamping. A lot of researches have been performed to face the challenges of conventionally stamped stainless BPPs, both experimentally and numerically. Stamping conditions, such as loading method, stamping force, blank hold force, punch speed and lubrication, play an important role in stamping of stainless steel BPPs. J. Y. Koo et al. [179] examined the effects of load type and stamping force on formability of SS304 with a thickness of 0.1 mm and 0.3 mm, respectively. Their results indicated the optimized stamping force and sine wave dynamic load cycle for larger channel height of bipolar plate were 90 kN and 5, respectively. Q. H. Hu et al. [180] studied the effects of punch speed within the range of 0.5 mm⋅s 1 to 4.0 mm⋅s 1 on formability of SS304 with a thickness of 0.15 mm by numerical analysis and experiments. They pointed out that bipolar plate had the lowest thinning rate of about 17.5% and a smaller tendency of wrinkling and cracking when punch speed was in the range

3.2.1. Stamping Stamping, also known as pressing, is a plastic forming process which form sheet metal into the desired shape and geometry with a punch and die under specific force and velocity, as shown schematically in Fig. 6 (a). Thin stainless steel BPPs with appropriate flow channel geometry and excellent mechanical properties are able to be achieved by stamping [176]. Besides, the existing stamping equipment and production lines which can be used directly for stainless steel BPPs manufacture, as well as stamping process itself both contribute to relatively lower cost [7,53]. Furthermore, the high production rate and long life moulds of stamping can help to realized mass production for stainless steel BPPs [53,177, 178]. However, challenges still remain in manufacturing stainless steel

Fig. 6. Schematic of plastic forming methods for producing stainless steel BPPs (a) stamping; (b) hydroforming; (c) rubber pad forming; (d) direct roll forming; (e) indirect hot stamping; (f) direct hot stamping. 16

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Journal of Power Sources 451 (2020) 227783

of 0.1 mm⋅s 1 to 1.1 mm⋅s 1. In the work by C. Sudarsan et al. [181], the effects of punch speed ranging from 0.067 mm⋅s 1 to 6.7 mm⋅s 1 on forming limit of SS304 with a thickness of 0.2 mm was studied experi­ mentally. Testing results showed that the stamping speed had negligible influence on forming limit when it was in the range of 0.067 mm⋅s 1 to 1.67 mm⋅s 1. However, the forming limit decreased by about 13% as stamping speed was further increased from 1.67 mm⋅s 1 to 6.7 mm⋅s 1. Their research results were in accordance with that of the work by Q. H. Hu et al. In addition, other researchers, such as two-stage forming approach [182] and dynamic loading [183–185] had also been devel­ oped to improve the formability and channel dimension precision for stainless steel bipolar plates. An approach of flow field and mould design for stamped stainless steel BPPs was explained systematically by D. K. Qiu et al. [168]. In their work, response surface method (RSM) was utilized to build the formability model. Many researches [186–190] have also been conducted to address the problems, such as springback and residual stress of cold stamped stainless steel BPPs. In the patent by Honda Motor Corp. [191], a compound die and the corresponding forming method for manufacturing stainless steel BPPs with larger channel height and higher accuracy were proposed. General Motors Corp. invented a two-step forming method for fabricating stainless steel BPPs with less reduction and more uniform distribution of wall thickness by diminishing stretching deformation [192]. Toyota Boshoku Corp. made improvement on the basis of the method invented by General Motors Corp. and developed a more effective way of minimizing wall thickness and warp [193]. Although much progress has been made on stamping of stainless steel BPPs, the flow channel geometries of current cold stamped stainless steel BPPs have not meet the requirements for high performance fuel cell stack. For instance, while fine flow channels with smaller channel width, rib width and draft angle have been proven to provide better fuel cell performance, it’s difficult or unable to produce these refined channel features through conventional stamping method because of the rela­ tively lower forming limit. Therefore, tradeoff must be made between fuel cell performance and formability for stamped stainless steel BPPs. Another challenge related to stamped BPPs is that quality problems, such as excessive thinning, wrinkle and microcracks may occur during forming. In addition, flow channel dimension consistency and batch stability that may be caused by springback and die dimension precision should be guaranteed for mass production of stainless steel BPPs. Much work is still needed in the aspects mentioned above in the future.

improved significantly by this method for hydraulic pressure in the range of 80 MPa–100 MPa. In recent years, L.F. Peng and Z. T. Xu et al. [195,196] established different models for predicting and analyzing formability of sheet metal for bipolar plate in hydroforming process. Researches were also conducted to investigate the difference of BPPs formed by stamping and hydroforming, respectively. M. Koç et al. [197–199] investigated the effects of stamping and hydroforming on corrosion resistance of stainless steel bipolar plate. Their experiments revealed that stamping would degrade the corrosion resistance of stainless steel BPPs while hydroforming had smaller effect on it as compared to the non-deformed stainless steel blank. The effects of stamping and hydroforming process on contact resistance and surface morphology of stainless steel BPPs were investigated as well by M. Koç et al. [3,200]. According to their research, the hydroformed stainless steel BPPs showed better surface quality and lower ICR than the stamped ones. Hydroforming possesses some advantages over stamping as dis­ cussed above. However, its production rate is relatively lower than stamping process. Besides, it faces the same challenges as stamping for producing high performance stainless steel BPPs with fine flow channels and good dimensional accuracy. Much work needs to be performed to improve the production rate and formability in the future as well. 3.2.3. Rubber pad forming Rubber pad forming (RPF), also called flexible forming (FF), is a sheet metal forming process which utilize a rigid die and a soft rubber pad to form the designed parts, as shown in Fig. 6 (c). The rigid die and soft rubber pad can be assembled precisely and problems which may be associated with conventional stamping process can be eliminated [201, 202]. Besides, the surface quality of BBPs produced by RPF is relatively higher than stamped BPPs. At the same time, the cost and time for die manufacturing can be reduced with fewer rigid dies [203]. Therefore, rubber pad forming is also believed to be a potential way for mass production of stainless steel BPPs. One of the factors that may influence on quality of stainless steel BPPs by RPF is the rubber pad utilized. The work by L. F. Peng et al. [204] indicated that the stress distribution and thickness of the formed part remained almost the same when the shore A hardness of rubber pad was between 55 and 70. The same conclusion as L. F. Peng et al. was drawn by Y. X. Liu and L. Hua [12]. The experimental results by M. G. Jeong et al. [205] proved that forming height of flow channel increased slightly when rubber hardness decreased from 35 to 20 or rubber thickness increased from 10 mm to 60 mm. The influences of rubber hardness and thickness on dimensional accuracy of SS316 stainless steel BBPs fabricated by RPF were studied by M. Elyasi et al. [206]. Filling percentage and dimensional accuracy were proven to be improved as rubber hardness and thickness were increased from 45 to 90 and 10 mm–30 mm. Polyurethane rubbers with hardness and thickness in the range of 50–70 and 40 mm–80 mm were investigated numerically and experimentally by K. M. Younis et al. [207]. The results indicated that the forming height decreased with higher rubber hardness for all kinds of former block types when rubber thickness was 40 mm. The work by Y. X. Liu et al. [208] pointed out that the concave die forming was better for bipolar plates with channel to rib width ratio larger than 1 (w/s > 1) while convex die forming was more appropriate for bipolar plates with w/s < 1. Besides, concave die forming was also proven to have a larger peak forming load than that of convex die forming under the same condition [10,208]. Larger forming force and forming speed were also beneficial for larger forming height [205,208]. However, they might reduce the life of rubber pad. Therefore, tradeoff must be made between forming height and rubber pad life. In the work by M. Elyasi et al. [209], a semi-stamping rubber pad forming process was proposed. Their research results indicated that filling percentage and uniformity of thickness distribution could be improved through the new process. In the study by C. B. Zhang et al. [9], the rubber pad used in conventional forming process was replaced by a polymer powder with a diameter of

3.2.2. Hydroforming Hydroforming is another kind of plastic forming process to generate the desired part shapes from tubular or sheet metal materials by pres­ surized hydraulic fluid and a die, as shown in Fig. 6 (b). Like stamping, hydroforming is regarded as a potential way for mass production of metal BPPs as well. As compared to stamping, there is no punch in hydroforming process. Therefore, the cost and time involving die design and fabrication can be reduced [50]. Besides, hydroforming offers several advantages, such as better surface quality, less springback, better dimension consistency, higher drawing ratio, and the capability of forming complicated parts [3,50,194]. Therefore, a lot of studies have been carried out on hydroformed stainless steel BPPs for optimized forming process parameters and better formability. L. F. Peng et al. [167] studied the key geometrical parameters of flow channel on reaction efficiency and formability of a 0.2 mm thick hydroformed stainless steel bipolar plate by design of experiments (DOE) method, response surface method (RSM) and adoptive simulated annealing (ASA) optimization method. Their results indicated that stainless steel bipolar plate with the channel height, channel width, rib width and corner radius of 0.5 mm, 1.0 mm, 1.6 mm and 0.5 mm could contribute to higher reaction efficiency and acceptable formability. N. Mohammadtabar et al. [194] studied a double step hydroforming pro­ cess for stainless steel bipolar plate with serpentine flow field by nu­ merical simulation and experiment. The formability was proven to be 17

Y. Leng et al.

Journal of Power Sources 451 (2020) 227783

80 μm. Experimental results indicated that polymer powder forming process was more flexible than conventional rubber pad forming in producing BPPs with different geometries. Currently, rubber pad forming has not been used for real production for its relatively longer production cycle time as compared to stamping. Besides, the rubber pad may need to be replaced frequently in mass production of BPPs, which could lead to longer production time and higher cost. In addition, the current rubber pad forming process are not able to produce BPPs with fine flow channels mentioned above as well.

3.2.5. Hot pressing In addition to the plastic forming processes discussed above, novel methods, such as combination of electromagnetic forming [215], high-velocity impact forming [52] and hot stamping [216] have also been developed and investigated. Among them, hot stamping is believed to one of the most promising way of forming stainless steel BPPs with fine flow channel geometries mentioned in the above sections. Hot stamping or hot pressing, which has been developed in recent years, is an advanced plastic forming method for manufacturing ultrahigh strength steel sheet part. According to the process, Hot stamping can be divided into two categories, namely direct hot stamping and in­ direct hot stamping, as shown schematically in Fig. 6(e) and Fig. 6 (f). Under the condition of hot stamping, the steel is in soft state. Therefore, the formability can be improved while the stamping force needed is smaller. Besides, the springback of the hot-stamped parts can be reduced effectively. However, current researches of hot stamping are mainly focused on high or ultra-high strength steel parts with a thickness of 0.5 mm–1 mm, and few studies have been conducted on stainless steel BPPs with a thickness of 0.1 mm or even smaller based on the concept of hot stamping. M. Yokoyama et al. [217] fabricated a metallic glassy BPP with high corrosion resistance and low ICR for PEMFC by hot stamping. Later, L. R. Narayan et al. [216] from University of Windsor investigated the formability of a ferritic stainless steel BPP under heated conditions. Their research results showed that the formability could be improved with smaller springback forming temperature and force were set to be 150–200 � C and 45–50 kN, respectively. Although the heating temper­ ature they explored were within 200 � C, their work pointed out a new route for improving the formability of stainless steel and manufacturing stainless steel BBPs with fine flow channel geometries and higher dimensional accuracy. S. Esmaeili et al. [218] studied the hot metal gas forming (HMGF) for manufacturing metallic bipolar plate. Their study proved that pressure needed was much smaller when metallic bipolar plates were formed at high temperatures.

3.2.4. Roll forming Conventional stamping and hydroforming are currently regarded as the potential processes for mass production of stainless steel BPPs because they can produce efficiently thin BPPs at lower cost. However, as mentioned in the above sections, stamping and hydroforming are not able to form BPPs with fine flow channel or high aspect ratio (h/w). In addition, current stamping and hydroforming processes are still not efficient enough for producing stainless steel BPPs if full commerciali­ zation of fuel cell is to be realized. Moreover, cost for stainless needs to be further reduced according to the 2020 targets for BPPs set by DOE. In order to address the problems mentioned above, roll forming process have been proposed by some researchers in recent years. Roll forming is a type of rolling process continuously bending a long strip of sheet metal into the desired shapes, as shown schematically in Fig. 6 (d). Roll forming processes for stainless steel BPPs production can generally be divided into two types based on the directions of flow channel and rolling [210]. One type is forming flow channels along rolling direction, the other is forming flow channels which is perpen­ dicular to rolling direction. Stainless steel BPPs can be manufactured by one roller set or a series of roller sets according to design requirements. Although bending combined with tension has been known to be the deformation mode for typical roll forming, the mechanisms of defor­ mation and springback for thin stainless steel by roll forming remains unclear. The first report of producing thin stainless steel BPPs by roll forming can be found in the work by P. Zhang et al. [211]. Numerical simulation with Abaqus software as well as experiments was performed to study the deformation, thickness thinning and springback of thin stainless steel in micro roll forming process. J. H. Huang et al. [212] investigated experimentally and numerically the roll forming process for manufacturing thin stainless steel BPPs with channel aspect ratios by using a set of gear-like rollers. The experimental results in their work showed that 0.1 mm thick SS316L BPPs with channel aspect ratio up to 1.0 and thickness reduction lower than 18.7% could be made by roll forming process. In the work by B. Abeyrathna et al. [213], a series of roller sets with different geometries placed in four stations had been proposed to form stainless steel BPPs. Rollers with flow channel direc­ tion in accordance with rolling direction were utilized. The roll forming process and design method proposed by them were proven to be feasible for fabricating stainless steel BPPs. However, the maximum thickness thinning of the roll formed stainless steel in their work tended to be up to 30%. A. Bauer et al. [214] developed a set of rollers with forming parts inserted in it, which was different from the rollers mentioned above, to manufacture stainless steel BPPs. Bipolar plates with flat regions around the edges for assembling were able to be made through this method. Despite the potential for mass production of stainless steel BPPs with higher efficiency and lower cost, there are still many problems to be solved before applying roll forming process to practical production. As mentioned above, the mechanisms of deformation and springback when forming thin stainless steel BPPs is not clear at present. In addition, stainless steel BPPs with only parallel flow fields are reported to be manufactured by roll forming in all of the literatures published. Serpentine flow fields or complex flow channel geometries, such as 3D structure are not able to be made through this forming method currently. Therefore, much work still needs to be done in the future if roll forming is to be utilized for mass production of stainless steel BPPs.

4. Challenges and research trends Although a lot of progress has been made in recent years, there are still many challenges in producing stainless steel BPPs that can fulfil the requirements of higher performance, reliability, durability and lower cost for full commercialization of PEMFC. In the flowing sections, challenges in materials and forming process of stainless steel BPPs are summarized and future research trends on those aspects are discussed. 4.1. Bare stainless steel materials Bare stainless steel materials for BPPs are mainly austenitic and ferritic stainless steels in the current. For austenitic stainless steels, SS316L is believed to be the most promising material for BPPs of PEMFCs because of its superior comprehensive properties, such as good corrosion resistance, plastic formability and relatively low cost. As compared to austenitic stainless steels, the alloy elements, such as nickel and molybdenum in ferritic stainless steels are much lower. Therefore, the costs of ferritic stainless steels are generally lower than that of austenitic stainless steels, which is helpful in reducing the cost of BPPs. However, both austenitic and ferritic stainless steels face the chal­ lenge of improving the corrosion resistance of themselves. As shown in the Ashby chart of Fig. 4, none of the conventional bare stainless steels can fulfil the requirements of 2020 DOE targets. Coatings and the cor­ responding coating processes can be saved if the corrosion resistance and conductivity of bare stainless steels are high enough, thus reducing the cost of stainless steel BPPs. Improving the corrosion resistance and conductivity of stainless steels through composition optimization and appropriate surface modification methods may be the research trends in the future. The non-coated ferritic stainless steel [37] with high corro­ sion resistance and low ICR which has been utilized in Hyundai’s fuel cell vehicle NEXO is good proof for this research trend. Another 18

Y. Leng et al.

Journal of Power Sources 451 (2020) 227783

challenge that conventional stainless steels face now is the limitation of their formability in producing high performance BPPs with fine flow channel geometries. To further improve the formability of stainless steel materials, optimization of rolling process as well as composition of them may also be the research trend for stainless steel BPPs.

durability. The authors believe that hot stamping as well as optimization of stainless steel composition is a promising way to achieve the re­ quirements for stainless steel BPPs mentioned above.

4.2. Coating materials

In this work, an effort has been made to review the achievements and most important findings in recent years on bare and coating materials, flow channel design and plastic forming processes for stainless steel BPPs of PEMFCs. Although much progress has been made, challenges still remain in the way of manufacturing stainless steel BPPs with higher performance and longer lifetime, and realizing full commercialization of PEMFCs.

5. Conclusions

As revealed by the Ashby charts, Cr–N, Ti–N, Cr–C, amorphous car­ bon (a-C) and multilayer coatings are the most promising coatings for stainless steel BPPs. In the meanwhile, PVD, such as AIP and MS are believed to be the most suitable coating techniques, especially for multilayer coatings. By adding corrosion resistant and conductive coatings with appro­ priate coating processes, the electrochemical performance of conven­ tional stainless steels, such as SS316L and SS304, can meet the 2020 DOE targets. However, their durability or lifetime may still be a big challenge for them because of the defects or imperfections in the coat­ ings. The defects or imperfections which is hardly avoidable may emerge during the stages of coating, delivering, stacking or application. In most cases, they are sources where corrosion begins. Future research may focus on developing coatings which have few or even no defects and good adhesive properties with stainless steel matrix, such as multilayer coatings. Although corrosion resistance and ICR of the appropriately coated stainless steel BPPs can achieve the DOE targets, however, their cost is still a great challenge. Developing high performance and low cost coating materials may be a research trend on this issue.

(1) Among all the conventional bare stainless steel materials inves­ tigated for BPPs, SS316L seems to be the most suitable base material for the moment when considering corrosion resistance, formability and cost comprehensively. However, none of the conventional bare stainless steel fulfil the 2020 DOE targets for BPPs. Developing novel stainless steel grades through composi­ tion optimization and appropriate surface modification tech­ niques is believed to be a promising route for producing high corrosion resistant and conductive stainless steel BBPs. (2) Currently, Cr–N, Ti–N, Cr–C, amorphous carbon (a-C) and multilayer coatings are promising coatings for the conventional bare stainless steel BPPs while PVD, such as AIP and MS are considered the suitable coating methods. Although corrosion resistance and ICR of the conventional stainless steel BPP can meet the DOE requirements, their durability still needs to be verified in the future. Defects or imperfects generated during coating, delivering, stacking or application may be the source that deteriorate the durability of stainless steel BPPs. Researching on high performance coatings and the corresponding coating techniques for conventional stainless steel BPPs is believed to be another promising way for manufacturing stainless steel BPPs with better performance and durability. (3) The flow channel geometries for stainless steel BPPs are not only designed on the basis of fuel cell performance, but also limited by their forming limit under certain forming process. Based on fuel cell performance, the appropriate channel height, channel width, rib width, draft angle, channel and rib radius may be in the range of 0.35 mm–0.55 mm, 0.4 mm–1.0 mm, 0.4 mm–1.0 mm, 10� –30� , 0.05 mm–0.2 mm and 0.1 mm–0.2 mm, respectively, for stainless steel BPPs manufactured by plastic forming methods, such as stamping and hydroforming. (4) Stainless steel BBPs with fine flow channel geometries are proved to be an effective way to further improve the performance and power density of PEMFC. However, the forming processes uti­ lized for stainless steel BPPs fabrication now, such as stamping and hydroforming, are hard or unable to form fine channel structures. Novel forming processes, such as hot forming is believed to be a potential way of producing high performance stainless steel BPPs with fine flow channel geometries.

4.3. Forming processes The formability of stainless steel BPPs is not only decided by the materials utilized, but also depend on forming process for the stainless steel BPPs. The flow channel geometries are designed based on fuel cell performance while they are limited by their formability under certain forming process. Based on the results of previous researches, the appropriate channel height, channel width, rib width and the ratio of channel to rib width may be in the range of 0.35 mm–0.55 mm, 0.4 mm–1.0 mm, 0.4 mm–1.0 mm and 0.6 to 1.4, respectively, while draft angle, channel and rib radius may be in the range of 10� –30� , 0.05 mm–0.2 mm and 0.1 mm–0.2 mm, respectively, for stainless steel BPPs. In order to further improve the performance of fuel cell, fine flow channels with channel and rib widths of 0.4 mm–0.6 mm and smaller draft angle are suggested. However, conventional stamping and hydro­ forming are faced with huge challenges in manufacturing stainless steel BPPs with fine flow channels and precision channel geometries. For instance, when stamping is adopted for producing stainless steel BPPs, there are forming limits of their flow channel geometries, as shown in the work by D. Qiu et al. [168]. It can be concluded that conventional cold stamping can hardly form fine flow channel geometries with channel width, rib width and pitch length of 0.5 mm, 0.5 mm and 1.5 mm, respectively. Rubber pad forming has the same problems as conventional stamp­ ing and hydroforming in producing stainless steel BPPs with fine structures. Besides, the efficiency and cost for mass production of stainless steel BPPs may also affect its use in practical production. As a new forming method proposed in recent years, roll forming seems to be a potential process for producing stainless steel BPPs in large volume with higher efficiency and lower cost. However, deformation mechanism of thin stainless steel in roll forming process needs to be investigated and understood thoroughly. In addition, the way of forming BPPs with more complex structures, such as serpentine flow field and 3D flow channel has to be found out before roll forming can be used for practical pro­ duction of stainless steel BPPs. Future studies should focus on novel forming methods to manufac­ ture stainless steel BPPs with fine flow channels structures and precision channel dimensions for FEMFC with high performance, reliability and

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. Acknowledgements The authors would like to acknowledge the support of National Key R&D Program of China (Grant No. 2018YFB1502500).

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