Metallic Bipolar Plates

Some of the most common metallic bipolar plates are made of carbon-steel, stainless steel SS-316, nickel/chromium alloy, titanium, and aluminum using 

One of the major issues with the use of metallic bipolar plate is the possibility of corrosion. Corrosion occurs in a fuel cell owing to high humidity conditions and formation of oxide layers leading to increased interface contact resistance and transport metallic ions toward the electrode/catalyst sites causing degradation of electrochemical kinetics. Corrosion also leads to failure and lower durability of the bipolar plates. Most metal and metal alloys exhibit poor corrosion resistance, and in order to provide corrosion resistance, some types of surface treatment and coatings are employed. 

In a metallic plate, the base metals are coated with different coating materials such as noble metals like Au, metal oxides, metal nitrides, metal carbides, carbons, and polymers. An appropriate coating can provide corrosion resistance in a most cost-effective manner. 

Coating methods are electrodeposition, electroplating, physical vapor deposition (PVD), chemical vapor deposition, sputtering, spraying, and nitriding. 

In the PVD method, ion beams are used to form a charged molecular vapor cloud of coating materials such as gold or TiN and then forming the coating layer by settling the charge particles on the base material surface. 

---

H. Tawfik, Y. Hung, and D. Mahajan. Metal bipolar plates for PEM fuel cell—A review. Journal of Power Sources 163 (2007) 755-767. 

T. Matsuura, M. Kato, and M. Hori. Study on metallic bipolar plate for proton exchange membrane fuel cell. Journal of Power Sources 161 (2006) 74-78. 

Wind, J., A. LaCroix, A. Braeuninger, et al. 2003. Metal bipolar plates and coatings. In Handbook of fuel cells—Fundamentals, technologies, and applications, ed. W. Vielstich, H. A. Gasteiger, and A. Lamm. Vol. 3 Fuel cell technologies and applications, 294-307. New York John Wiley Sons. 

Brady, M. R, P. R Tortorelli, J. Rihl, et al. 2007. Nitrided metallic bipolar plates. Annual progress report, U.S. DoE Hydrogen Rrogram. http //www.hydrogen. energy.gov/pdfs/progress07/v b 2 brady.pdf (accessed 2008). 

Wind, J. et ah, Metal bipolar plates and coatings, in Handbook of Fuel Cells Fundamentals, Technology, and Applications, 1st ed., Vielstich, W., Lamm, A., and Gasteiger, H.A., Eds., John Wiley Sons, West Sussex, England, 2003, p. 294. 

Silva, R.R et al., Surface conductivity and stability of metallic bipolar plate materials for polymer electrolyte fuel cells, Electrochim. Acta, 51, 3592, 2006. 

It should be noted that the products of this decomposition are water, carbon dioxide, and HF. While PFSA membrane FCs have been demonstrated for many thousands of hours, the flux of HF is significant enough so that uncoated metallic bipolar plates are precluded. Hard to machine graphite bipolar plates must be used or an electrically conducting corrosively resistive coating must be developed for easily fabricated metal bipolar plates. Lifetime studies of PEM... 

IV.D.19 Cost-Effective Surface Modification for Metallic Bipolar Plates... 

Develop a low-cost metallic bipolar plate alloy that will form an electrically conductive and corrosion resistant nitride surface layer during thermal nitriding to enable use in a PEM fuel cell environment. 

Assessment of Thermal Nitridation to Protect Metal Bipolar Plates in PEM Fuel Cells , M.P. Brady, K. Weisbrod, C. Zawodzinski, I. Paulauskas, R.A. Buchanan, and L.R. Walker, submitted to Electrochemical and Solid State Letters. 

Metallic Bipolar Plate Alloys Amenable to Inexpensive Surface Modification for Corrosion Resistance and Electrical Conductivity , M.P. Brady, J.H. Schneibel, B.A. Pint, and P.J. Maziasz, United States Provisional Patent Disclosure, April 2002. 

Corrosion of the plates not only detracts from their mechanical properties but also gives rise to undesirable corrosion products, namely, heavy-metal ions, which, when depositing on the catalysts, strongly depress their activity. The corrosion processes also give rise to superficial oxide films on the metal parts, and these cause contact resistance of the surfaces. For a lower contact resistance, metallic bipolar plates sometimes have a surface layer of a more stable metal. Thus, in the first polymer electrolyte membrane fuel cell, developed by General Electric for the Gemini spacecraft, the bipolar plates consisted of niobium and tantalum coated with a thin layer of gold. A bipolar plate could also be coated with a layer of carbide or nitride. 

Chemically and thermally resistant steels contain considerable quantities of chromium (more than 20%). On the side of the oxygen, cathode metalhc chromium is oxidized to Cr203. Depending on the temperature and oxygen partial pressure, this oxide may further oxidize to volatile compounds CrOg and Cr02(0H)2, which could then settle at the cathode/electrolyte interface and hinder oxygen reduction. Such an evaporation of chromium is the basic difficulty in the use of metallic bipolar plates. 

The gases are distributed within the two compartments from channels on the bipolar plates. Bipolar plates can be made of carbon-based material or metal. The decision on the bipolar plate material and design depends strongly from the available space. By increasing the requirement of power density the metal bipolar plate are more suitable. The cell pitch of 1.2-1.5 mm can be manufactured to achieve a volumetric power density of 2 kW/1 or more. 

With respect to fuel-cell technology itself, the small portable units use commercially available membrane electrode assemblies (MEA) and gas diffusion layers (GDL). As the operating temperature of small fuel-cell stacks usually lies below 50 °C, the requirements with respect to material stability of MEA and GDL, but also of sealing gaskets and bipolar plates are comparable lower than for other applications. For example, it is well known that metallic bipolar plates show significantly lower corrosion below 50 °C than at typical operation temperature of 80 °C [6,7], so that a sufficient lifetime for portable applications can be achieved with stainless steel. 

Fig. 14.16 Selection of bipolar plates having identical flow fleld design investigated within the DECODE project, (a) Uncoated metallic bipolar plate (0.1 mm SS 316 L), (b) gold coated metallic bipolar plate (200 nm Au on 0.1 mm SS 316 L), (c) milled graphite composite bipolar plate. Pictures courtesy of Reinz Dichtungs GmbH, DANA Corporation...

In the fuel cell stack used in the Toyota Mirai vehicle, metallic bipolar plates made from titanium were used. 

From the results achieved in the DECODE project, it can be concluded that the accumulation of cationic stainless steel corrosion products in the electrolyte membrane can be suppressed by proper design of the MEA, particularly by avoiding any contact of free electrolyte membrane with liquid water originating from the coolant or from condensate accumulated in the active area. Furthermore contact of corrosion inducing contaminants such as for example chloride ions with the metallic bipolar plates must be prevented. 

Despite being sufficiently corrosion resistant in the operating time scale required for automotive applications, metallic bipolar plates are considered less likely to be used in stationary applications requiring operating times of 40,000 h and above. 

Qualitative comparison of composite and metallic bipolar plates [68]... 

A wide variety of sealing solutions have been reported such as the use of O-rings or die cut flat seals, adhesive bonding of the components, molded, dispensed or screen printed elastomers to the bipolar plate or the membrane electrode assembly, separate sealing frames, bead seals on metallic bipolar plates etc. [6, 75, 76]. 

Proper design of the flow zones is of particular importance in the case of metallic bipolar plates since the design of anode, cathode and coolant flow field cannot be optimized individually. Furthermore, when using a bead seal concept, coolant flow diversion in the bead needs to be taken into account. 

---