Polymer metallic bipolar plates

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

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. 

Materials and Coatings for Metallic Bipolar Plates in Polymer Electrolyte Membrane Fuel Cells 361... 

Yoon, W., X. Huang, P. Fazzino, K. Reifsnider, M. Akkaoui. 2008. Evaluation of coated metallic bipolar plates for polymer electrolyte membrane fuel cells. /. Power Sources 179(1) 265-273. 

FIGURE 6.11 Relation between the current density-potential curve against the RHE (reversible hydrogen electrode) of stainless steel (left) and anode and cathode polarization curve of a polymer electrolyte fuel cell (right). (With kind permission from Springer Science+Business Media, Polymer Electrolyte Fuel Cell Durability. Influence of metallic bipolar plates on the durability of polymer electrolyte fuel cells. 2009. pp. 243-256. Scherer, J., Munter, D., and Strobel, R.)... 

Influence of Metallic Bipolar Plates on the Durability of Polymer Electrolyte Fuel Cells... 

Abstract This chapter describes the behavior and stability of metallic bipolar plates in polymer electrolyte fuel ceU application. Fundamental aspects of metallic bipolar plate materials in relation to suitability, performance and cell degradation in polymer electrolyte fuel cells are presented. Comparing their intrinsic functional properties with those of carbon composite bipolar plates, we discuss different degradation modes and causes. Furthermore, the influence and possible improvement of the materials used in bipolar plate manufacturing are described. 

Beside this, corrosive attack to the metallic bipolar plate is further supported by the common PEFC operating temperatures of 80-100°C. Elevated temperatures in particular increase the chemical attack on the polymer membrane. Especially if the membrane tends to dry out, the increasing formation of peroxides results in inaeasing attack on the membrane polymer (Cleghom and Kolde 2007 Liu et al. 2001 Endoh et al. 2004). In the case of fluorinated polymers, membrane degradation results in continuous release of fluoride ions, which also support corrosive attack on the stainless steel (Ningshen and Kamachi Mudali 2002). 

The electrolyte is a perfluorosulfonic acid ionomer, commercially available under the trade name of Nafion . It is in the form of a membrane about 0.17 mm (0.007 in) thick, and the electrodes are bonded directly onto the surface. The electrodes contain very finely divided platinum or platinum alloys supported on carbon powder or fibers. The bipolar plates are made of graphite-filled polymer or metal. 

The effect of these impacts can be greatly reduced if Pt is recovered and reused, as all heavy metals should be. A strategy for supply of Pt to the fuel cell industry is discussed by Jaffray and Hards (2003). In terms of weight, the stack breakdown on components is shown in Fig. 6.6 for two conventional material choices for the bipolar plates, graphite or aluminium. Recently, bipolar plates made of conducting polymers have been developed (Middleman et al., 2003), with thickness and weight reduction as a consequence. 

Table 9 Opportunities and risks of injection-molded polymer bipolar plates compared to metal plates...

For bipolar plates, a total (bulk and contact) resistance value of 10 mJi cm is usually specified [72]. This would result in 0.02 cm (two plates, coolant between), which at 1.5 A cm would result in 30 mV voltage loss. Bipolar plates are made of carbon, carbon/polymer composite, or metal. Both the resistance and weight/volume aspects demand thin bipolar plates. This has to be combined with low H2 permeability (<2 x 10 cm cm s ) and good mechanical integrity. 

From a materials durability point of view, carbon/polymer composite materials are to be preferred. However, metal-based bipolar plates enable the use of very thin plates, thus leading to an increase in volumetric power density. Major car manufacturers such as Honda and Toyota are using metal-based bipolar plates in... 

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