Phosphoric acid fuel cell bipolar plates

Fig. 12.5 UTC s phosphoric acid fuel cell bipolar plate...

From a cross-flow point of view it may be of interest to mention the phosphoric acid fuel cell with the so-called DiGas system (Fig. 9), which is an air-cooled cross-flow configuration for use in utility-power stations [39]. The process air stream is diverted into two types of channels into individual cells with relatively small cross-sectional area, and into cooling plates (approximately one for every five cells) with a lai ge cross-section. Bipolar plates were molded from a mixture of graphite and phenolic resin, with a Pt-on-carbon cathode and a Pt anode combined with colloidal PTFE on a graphite-paper backing. 

Bipolar graphite plates having special channels for reactant supply and distribution over the entire electrode surface, which are now widely used in polymer electrolyte membrane fuel cell stacks, were for the first time used in phosphoric acid fuel cells. 

In addition to loss of the platinum, the carlxm support that anchors the platinum crystallites and provides electrical coimectivity to the gas-diffusion media and bipolar plates is also subject to degradation. In phosphoric acid fuel cell, graphitized carbons are the standard because of the need for corrosion resistance in high-temperature acid environments [129], but PEM fuel cells have not employed fully graphitized carbons in the catalyst layers, due in large part to the belief that the extra cost could be avoided. Electrochemical corrosion of carbon materials as catalyst supports will cause electrical isolation of the catalyst particles as they are separated from the support or lead to aggregation of catalyst particles, both of which result in a decrease in the electrochemical active surface area of the catalyst and an increase in the hydrophUicity of the surface, which can, in turn, result in a decrease in gas permeability as the pores become more likely to be filled with liquid water films that can hinder gas transport. 

Polymers are nsed in fnel cells. Those of particular interest are the polymer electrolyte membrane (PEM) and the phosphoric acid fuel cell (PAFC) designs. The latter design contains the liquid phosphoric acid in a Teflon bonded silicon carbide matrix. In March 2005 Ticona reported that it had bnilt the first fnel cell prototype made solely with engineering thermoplastics. They claimed that this approach rednced the cost of the fuel by at least 50% when compared with fuel cells fabricated from other materials. The 17-cell unit contains injection moulded bipolar plates of Vectra liquid crystal polymer and end plates of Fortron polyphenylene sulfide (PPS). These two materials remain dimensionally stable at temperatures up to 200 "C. The Vectra LCP bipolar plates contain 85% powdered carbon and are made in a cycle time of 30 seconds. 

Carbon in various forms is commonly used in phosphoric acid fuel cells and proton exchange membrane fuel cells (PEMFCs) as a catalyst support, gas-diffusion media (GDM), and bipolar plate material (Dicks 2006). Among these carbon materials, carbon black is used as a catalyst support for PEMFC apphcation because of its unique pn tolies... 

The catalysts and electrode materials used in PAFCs are also similar to those in acidic H2/air fuel cells. Carbon-supported Pt is used as the catalyst at both anode and cathode, porous carbon paper serves as the electrode substrate, and graphite carbon forms the bipolar plates. Since a liquid electrolyte is used, an efficient water removal system is extremely important. Otherwise, the liquid electrolyte is easily lost with the removed water. An electrolyte matrix is needed to support the liquid phosphoric acid. In general, a Teflon -bonded silicon carbide is used as the matrix. 

Additionally, the chemical resistance of the bipolar plate can be characterized by measuring the corrosion current under a potential typical for fuel cell operation and using 85 % phosphoric acid as an electrolyte. The relevance, detailed parameters, and development targets of this corrosion test are still subject to technical discussions and depend on the anticipated application of the plate. The accepted corrosion current under reference conditions is lower for long... 

The carbon or graphite type also mainly determines the properties of the bipolar plate like porosity, phosphoric acid uptake, and hydrophobicity, both regarding the surface and the bulk. For the HT-PEM fuel cell technology, preferred configurations are hydrophobic structures with low phosphoric acid uptake. 

HT-PEM fuel cells operate with phosphoric acid doped polymer membrane as electrolyte. The acid is physically adsorbed to the membrane. The phosphoric acid distribution within the fuel cell components, such as membrane, catalyst layers, microporous layer, gas diffusion layers, and bipolar plates, is known to be a critical parameter for performance and life time of this type of fuel cells [10]. There are no defined specifications about phosphoric acid uptake of the bipolar plate because its impact on the fuel cell performance strongly depends on several parameters and always has to be considered in a context of the overall fuel cell design. 

Fig. 19.9 Phosphoric acid uptake in different bipolar plates after >3000 h HT-PEM fuel cell stack operation. The samples based on PPS and PVDF contained the same type of synthetic graphite, whereas the sample called...

An important aspect of the development of fuel cell stacks is to make them more compact, and a key to that is to develop thinner bipolar plates preferably from metal. Especially low temperature automotive PEMFC stacks have reached impressive power densities with metallic bipolar plates. However, because of the free phosphoric acid and the elevated temperature, research and demonstration of HT-PEMFCs has so far been done almost exclusively with plates of graphite and its composite materials. 

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