Metallic bipolar plates, corrosion-resistant

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... 

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. 

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-Resistant Coatings for Metallic Bipolar Plates... 

Bipolar plate materials have historically been metals coated with corrosion-resistant layers or graphite with a seal treatment (to lower the gas permeability). In recent years, major efforts have been made for developing RP bipolar plates. The new plates usually have molded-in gas flow channels so that they can be fabricated rapidly and cost-effectively. However, the cost of bipolar plates ( 10/plate with 400 cm ) today is still too high to be applied to automotive and other civil power applications, and the conductivity is marginal. 

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. 

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. 

Today, uncoated stainless-steel materials are still in focus of research, as bare substrates promise to be the most cost-effective solution for metal bipolar plates. Nevertheless, it becomes obvious that a surface modification method or protective coating applied to metal bipolar plates is essential to improve their corrosion resistance. 

Another way of providing higher corrosion resistance and performance stability to metallic bipolar plates is the use of conductive and corrosion-resistant coatings. 

In 2002, Schmidt et al. estimated the cost of the first PEMFCs as 20,000/kW (here and below, all prices are in U.S. dollars). Of this figure, 90% was for labor (the cells were hand-made), and only 10% was for materials. In mass production, for instance when making 1 million 50-kW power plants per year, labor cost could fall to 10/kW. In the opinion of Schmidt et al., it would be necessary to develop new membranes costing no more than 20/m, and to lower the platinum content of the electrodes to 0.25 mg/cm, in order to bring the cost of material down from 2000/kW to a desirable figure of 30/kW or at least to a temporarily acceptable value of 100 to 200/kW (also, platinum should be recovered and recycled). A very important and difficult problem is that of making cheaper corrosion-resistant metallic bipolar plates. All ancillary devices must also become much cheaper. 

Metals such as titanium, aluminum, nickel, and stainless steels have been pursued for bipolar plate applications [5,8,10-12], However, these research efforts met limited success because of the chemical instability of the metals in the fuel ceU environment, especially when in contact with the acidic electrolytic membrane. Corrosion of the metal bipolar plate leads to a release of cations, which can both lead to an increase in membrane resistance and poisoning of the electrode catalysts [12]. The oxide film formed on the surface of the self-passivating metals also results in high voltage losses across the plate/macro-diffuser interface [8,11]. 

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. 

Many different metallic materials have been proposed and discussed in the scientific and patent literature. Because of their low density, typical lightweight metals such as aluminum and titanium are often used to manufacture metallic bipolar plates (Mepsted and Moore 2003). But owing to difficulties in stamping thin aluminum and titanium foils, most of the known designs use chemically etched plates. In addition, these metals and most of their alloys exhibit only poor stability in acidic conditions, as well as a strong tendency to form very thick and nonconductive oxide layers. Thus, the use of aluminum or titanium in PEFCs requires electrochemically dense and corrosion-resistant coatings. 

One such example is the formation of protective layers for metallic bipolar plates in proton exchange membrane fuel cells (PEMFCs). Bipolar plates serve to electrically connect the anode of one cell to the cathode of the next in a fuel cell stack to achieve a useful voltage. Metallic alloys would be ideal as bipolar plates because they are amenable to low-cost/high-volume manufacturing, offer high thermal and electrical conductivities, and can be made into thin sheet or foil form (0.1-1 mm thick) to achieve high power densities [16-18], However, most metals exhibit inadequate corrosion behavior in PEMFC environments (aqueous/acidic in the 60-80°C temperature range) due to formation of passive oxide layer(s), which increase cell resistance. 

The bipolar plates are usually fabricated with non-porous machined graphite or corrosion-resistant metal plates. Distribution channels are engraved in these plates. Metallic foams can also be used for distributing the reactants. One key point is to ensure a low ohmic resistance inside the bipolar plate and at the contact with the M EA. Another point is to use materials with high corrosion resistance in the oxidative environment of the oxygen cathode. 

Metals as the material for bipolar plates have the additional advantage that thinner, lighter plates can be made. Future developments in this area probably will yield new ways to protect the metal surfaces from corrosive attack by the medium and from formation of the superficial oxide films that lead to contact resistance. All the problems associated with bipolar plates have been discussed in detail in a review by Tawfik et al. (2007). 

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