Fuel cells cathodes, requirements

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

In another report, James and Kalinoski [4] performed an estimation of the costs for a direct hydrogen fuel cell system used in automotive applications. The assumed system consisted of an 80 kW system with four fuel cell stacks, each with 93 active cells this represents around 400 MEAs (i.e., 800 DLs) per system. The study was performed assuming that the DL material used for both the anode and cathode sides would be carbon fiber paper with an MPL. In fact, the cost estimate was based on SGL Carbon prices for its DLs with an approximate CEP value of around US 12 m for 500,000 systems per year. Based on this report, the overall value of the DLs (with MPL) is around US 42.98 per kilowatt (for current technology and 1,000 systems per year) and 3.27 per kilowatt (for 2015 technology and 500,000 systems per year). Figure 4.2 shows the cost component distribution for this 80 kW fuel cell system. In conclusion, the diffusion layer materials used for fuel cells not only have to comply with all the technical requirements that different fuel cell systems require, but also have to be cost effective. 

Alkaline fuel cells (AFC) — The first practical -+fuel cell (FC) was introduced by -> Bacon [i]. This was an alkaline fuel cell using a nickel anode, a nickel oxide cathode, and an alkaline aqueous electrolyte solution. The alkaline fuel cell (AFC) is classified among the low-temperature FCs. As such, it is advantageous over the protonic fuel cells, namely the -> polymer-electrolyte-membrane fuel cells (PEM) and the - phosphoric acid fuel cells, which require a large amount of platinum, making them too expensive. The fast oxygen reduction kinetics and the non-platinum cathode catalyst make the alkaline cell attractive. 

Stationary fuel cell operation requires a steady flow of protons through all membrane cross sections, perpendicular to the transport direction. Proton flow induces water transport from anode to cathode by electroosmotic drag [78], Taken alone, this effect would lead immediately to membrane dehydration and to a drastic increase of its ohmic resistance. However, accumulation of water on one side of the membrane inevitably causes a backflow of water. The balance between this backflow and the electroosmotic flow leads to a stationary profile of water across the membrane. 

The performance of a fuel ceU is determined by the total surface area of the catalyst particles that participate in the reactions. Ideally, aU the surface of the Pt particles is used and the Pt achieves 100% dispersion (e.g., Pt exists as individual atoms). In reality, the ideal situation does not exist and only a small fraction of the Pt atoms can participate in the fuel cell reaction. The reasons are that the Pt can not achieve 100% dispersion and the fuel cell reactions require the so-called three-phase boundaries. It can be seen from Eqs. 1 and 2 that both the anode and the cathode reactions involve protons, electrons, and reactants. So, only the Pt surface that is accessible to protons, electrons, and the reactant is active, and such regions are often called catalyst-electrolyte-reactant three phase boundaries as illustrated in Fig. 2. All the other Pt surface area is basically wasted. For an electrode composed of Pt (or... 

An earlier patent from the US Department of Energy desoibes a different approach to hnking fuel cells and gas turbine cycles (Anon, 1993). An indirectly heated gas turbine (GT) cycle is followed by a fuel cell cycle, the heated air from the turbine being used to directly heat the fuel cell cathode. The hot cathode recycled gases provide a substantial part of the heat required for indirect heating of the compressed air used in the GT. A separate combustor provides the balance of the heat needs. Hot gases from the fuel cell reduce the GT fuel needs and also the NO emissions. Residual heat from the fuel cell may be used in a steam cycle or for absorption cooling. 

Ambient air (stream 200) is compressed in a two-stage compressor with intercooling to conditions of approximately 193 °C (380 °F) and 8.33 atmospheres (122.4 psia). The majority of the compressed air (stream 203) is utilized in the fuel cell cathode however, a small amount of air is split off (stream 210) for use in the reformer burner. The spent oxidant (stream 205) enters a recuperative heat exchange before entering a cathode exhaust contact cooler, which removes moisture to be reused in the cycle. The dehumidified stream (stream 207) is again heated, mixed with the small reformer air stream, and sent to the reformer burner (stream 211). The reformer burner exhaust (stream 300) preheats the incoming oxidant and is sent to the auxiliary burner, where a small amount of natural gas (stream 118) is introduced. The amount of natural gas required in the auxiliary burner is set so the turbine shaft work balances the work required at the compressor shaft. The cycle exhaust (stream 304) is at approximately 177 °C (350 F). 

The oxygen reduction reaction (ORR) at the PEM fuel cell cathode is a multielectron, multistep reaction with a sluggish kinetics thus, a catalyst is generally required to accelerate the reaction. At present, platinum (Pt)-based catalysts are the most practical catalysts for the ORR in PEM fuel cells. The mechanism of the Pt-catalyzed ORR has been an active research area for about the past 40 years [21-24]. Yet, despite numerous studies, the detailed mechanism remains elusive. Figure 6.2 illustrates the simplified mechanism [24]. On Pt, the oxygen reduction reaction can proceed along several pathways for example, a "direct" four-electron reduchon to water, a two-electron pathway to hydrogen peroxide, and a "series" pathway with two- and four-electron reduction to water. 

Equation (6-45) may be solved numerically for a variety of boundary conditions, such as constant T at the walls, or constant or prescribed beat flux. Because of complicated three-dimensional heat transfer pathways (shown in Figure 6-23), calculation of heat fluxes and temperature profiles in a fuel cell stack requires 3-D numerical simulation. Figure 6-25 shows temperature distribution in a representative cross-section of a fuel cell obtained by 3-D numerical simulation [28]. From Figure 6-25, it is obvious that there are significant temperature variations inside a fuel cell stack. Because most heat in a fuel cell stack is produced in the cathode catalyst layer, that layer expectedly has the highest temperature. 

This analysis was done for output levels ranging from 3 to 200 kW and with the assumption that the stack operates near ambient pressure replacing an expensive compressor that provides three atmospheres of air pressure to the vehicle fuel cell cathode with a blower for the stationary system. This reduces the parasitic power required to run the fuel cell system and also reduces the ancillary costs for the system. 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

Systematic studies of cathodic oxygen reduction, unlike those of its anodic evolution, were only started in the 1950s when required for the realization of fuel cells. The large polarization of this reaction is one of the major reasons that the efficiency of the fuel cells developed so far is not very high. 

Polymer electrolyte fuel cells (PEFCs) have attracted great interest as a primary power source for electric vehicles or residential co-generation systems. However, both the anode and cathode of PEFCs usually require platinum or its alloys as the catalyst, which have high activity at low operating temperatures (<100 °C). For large-scale commercialization, it is very important to reduce the amount of Pt used in fuel cells for reasons of cost and limited supply. 

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