Membrane fuel cell, components bipolar plate

A detailed cost analysis for a polymer electrolyte membrane fuel cell power plant of 5 kW was provided in 2006 by Kamarudin et al. According to their data, the total cost of such a plant will be about 1200 of which 500 is for the actual fuel-cell stack and 700 for the auxiliary equipment (pumps, heat exchangers, etc.). The cost of the fuel-cell stack is derived from the components as 55 /kW for the membranes, 52 /kW for the platinum, 128 /kW for the electrodes, and 148 /kW for the bipolar plates. 

PEMFCs are very clean systems and act as filters for impurities introduced from ambient air, fuel used, and even degradation products from the cell materials. Both the membrane and the catalyst are susceptible to cmitamination and poisoning. Electrode degradation of PEMFCs can occur as a result of various impurities found in the fuel feed, air stream, as well as corrosimi by-products from fuel cell components such as the bipolar plate, catalysts, or membrane. 

Direct hydrogen-fuel-cell-powered vehicles have reached a level of development where the major automotive companies have publicly announced that initiation of commercialization is imminent around 2015. The targets of performance, durability, and cost agreed upon by various organizations, including the US DOE, appear to be achievable in the specified time frame. Well-delineated pathways and strategies have been established to address the barriers of cost and durability of PEMFC stacks and achieve the automotive targets. The principal directions for reduction of cost and enhancement of durability of key fuel cell components, i.e., electrocatalysts, membranes, and bipolar plates are briefly summarized in this section. 

Trends in short- and lOTiger-term directions for key fuel cell components including electrocatalysts/supports, membranes, and bipolar plates have been elaborated in this section improvement of the performance and durability of these components will directly impact the entire automotive fuel cell system requirements, complexity, and cost. Durable catalysts with enhanced ORR activity, durable membranes that perform at very low humidity and durable bipolar plates that have low contact resistance will not only increase the power density and cost of the fuel cell stack but also simplify and lower/eliminate system component costs of the air compressor, humidification systems, recycle pumps, radiator, start-up/shutdown and freeze-start-related components, etc. A combination of advances in all the fuel cell components discussed above, system simplification, governmental policies that are sensitive to sustainable clean energy, and development of a hydrogen infrastructure will enable achieving the projected technical and cost targets needed for automotive fuel cell commercialization. 

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. 

An analysis of the individual PEM components offers evidence of almost unbroken R D see Fig. 13.10 (Jochem et al., 2007). The overall importance of the membrane is striking. Furthermore, the numbers of annual applications for bipolar plates (BPP) and the gas-diffusion layer (GDL) decrease after 2002, while the increase in membrane applications flattens out. This correlates with the equally lower number of fuel cell patents in the field of mobile applications. 

Due to their high electrical and thermal conductivity, materials made out of metal have been considered for fuel cells, especially for components such as current collectors, flow field bipolar plates, and diffusion layers. Only a very small amount of work has been presented on the use of metal materials as diffusion layers in PEM and DLFCs because most of the research has been focused on using metal plates as bipolar plates [24] and current collectors. The diffusion layers have to be thin and porous and have high thermal and electrical conductivity. They also have to be strong enough to be able to support the catalyst layers and the membrane. In addition, the fibers of these metal materials cannot puncture the thin proton electrolyte membrane. Thus, any possible metal materials to be considered for use as DLs must have an advantage over other conventional materials. 

In PEMFCs working at low temperatures (20-90 °C), several problems need to be solved before the technological development of fuel cell stacks for different applications. This concerns the properties of the components of the elementary cell, that is, the proton exchange membrane, the electrode (anode and cathode) catalysts, the membrane-electrode assemblies and the bipolar plates [19, 20]. This also concerns the overall system vdth its control and management equipment (circulation of reactants and water, heat exhaust, membrane humidification, etc.). 

A component of a fuel cell that consists of a polymer membrane electrolyte coated with (or sandwiched between) positive and negative electrodes and then placed between bipolar plates. 

The most important components in a fuel cell are the Membrane Electrolyte Assembly (MEA) and the bipolar plates. The MEA usually consists of an electrolyte membrane, which is coated with catalytically active platinum-electrodes and a gas diffusion layer of hydrophobic graphite. As the electrolyte membrane cation exchange polymers are used. A crucial break-through was reached here by the employment of fluoridated polymers. The market leader here is Nation developed by the company Dupont. 

Fig. 14.1 Functional layers in a fuel cell. Functional layers can be integrated to a component such as catalyst coated membrane (CCM), catalyst coated substrate (CCS)/gas diffusion electrode (GDE) or bipolar plate (BPP)...

Carbon constitutes the most abundant element of the different FC components. Setting aside the membrane, which is a polymer with a carbon backbone, all the other components, i.e. the CL, GDL and current collector plates (bipolar plates) are made almost entirely of graphitic carbon. The electrocatalyst support of the CL is commonly carbon black in the form of fine powder. GDLs are thin porous layers formed by carbon fibers interconnected as a web or fabric, while current collector plates are carbon monoliths with low bulk porosity. As explained above each of these components has a particular function within the fuel cell and in particular in the triple phase boundary. The structure and properties of the carbon in each of the different FC components will determine the whole performance of the cell. 

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