Alkaline fuel cells electrode structure

The monolithic structure, good mechanical strength, high surface area, and electrical conductivity of these carbon materials make them attractive as electrodes for various electrochemical applications. As hydrogen oxidation (or oxygen reduction) catalysts may be incorporated to such porous materials, one specific application to consider is the use of this type of material as alkaline fuel cell electrode. 

Alkaline Fuel Cell The electrolyte for NASA s space shuttle orbiter fuel cell is 35 percent potassium hydroxide. The cell operates between 353 and 363 K (176 and 194°F) at 0.4 MPa (59 psia) on hydrogen and oxygen. The electrodes contain platinum-palladium and platinum-gold alloy powder catalysts bonded with polytetrafluoro-ethylene (PTFE) latex and supported on gold-plated nickel screens for current collection and gas distribution. A variety of materials, including asbestos and potassium titanate, are used to form a micro-porous separator that retains the electrolyte between the electrodes. The cell structural materials, bipolar plates, and external housing are usually nickel, plated to resist corrosion. The complete orbiter fuel cell power plant is shown in Fig. 27-62. 

Alkaline fuel cells have been used extensively on early spacecraft imtil they were superseded by more reliable solar cells. The high cost of the space cells and the use of corrosive compoxmds requiring special care in handling have been held against AFCs. Current AFC development employs multi-component electrodes using Ni for structural stability and as catalyst, carbon black as electron conductor and polytetrafluoroethylene (PTFE) pore-forming... 

In Chapter 10, the authors will demonstrate the preparation techniques for ASPEM and the characterization results. The relationship between structure and properties will be discussed and compared. The double-layer carbon air cathodes were also prepared for solid-state alkaline metal fuel cell fabrication. The alkaline solid state electrochemical systems, sueh as Ni-MH, Zn-air fuel cells, Al-air fuel cells, Zn-Mn02 and Al-Mn02 cells, were assembled with anodes, cathodes and alkaline solid polymer electrolyte membranes. The electrochemical cells showed excellent cell power density and high electrode utilization. Therefore, these PVA-based solid polymer electrolyte membranes have great advantages in the applications for all-solid-state alkaline fuel cells. Some other potential applieations include small electrochemical devices, sueh as supercapacitors and 3C electronic products. 

Priestnall et al. (2002) proposed a new concept of MR fuel cells called compact mixed-reactant (CMR) fuel cells, based on the use of a structure containing porous flow-though electrodes. Such a structure can be a single cell or stacks of cells connected in series or parallel. In their initial proof-of-principle experiments, the authors used an alkaline fuel cell system with an electrolyte 0.008 M NBH4 in 10 M KOH saturated with air. 

We have already pointed out that alkaline fuel cells can be operated at a wide range of temperatures and pressures. It is also the case that their range of applications is quite restricted. The result of this is that there is no standard type of electrode for the AFC, and different approaches are taken depending on performance requirements, cost limits, operating temperature, and pressure. Different catalysts can also be used, but this does not necessarily affect the electrode structure. For example, platinum catalyst can be used with any of the main electrode structures described here. 

Several varieties of fuel cells use an electron-conducting porous DM as an interface between the catalyst layer and the current collectors. This DM is not shown in Figure 2.9, since it is not a universal feature of all fuel cells. For example, PEFCs use a carbon-based porous media for this purpose, as shown in Figure 2.14. Either a woven carbon cloth or a carbon fiber structure bonded with a graphitized thermoset resin is typically used for this purpose. Alkaline fuel cells also use a similar porous media to aid electron conduction between the porous electrodes and current collectors. 

The manufacturing process can be done using modified paper-making machines, at quite low cost. Such electrodes are not just used in fuel cells but are also used in metal/air batteries, for which the cathode reaction is much the same as for an alkali fuel cell. For example, the same electrode can be used as the cathode in a zinc air battery (e.g. for hearing aids), an aluminium/air battery (e.g. for telecommunications reserve power), and an alkaline electrolyte fuel cell. The carbon-supported catalyst is of the same structure as that shown in Figure 4.6 in the previous chapter. However, the catalyst will not always be platinum. For example, manganese can be used for the cathode in metal air batteries and fuel cells. 

The electrochemical reduction of carbon dioxide to produce fuel has often been termed by some as artificial photosynthesis. Compared to the traditional researches using alkaline solution electrolytes, porous separators, and solid metallic electrode structures, there are numerous benefits to using a cell design based on a solid polymeric ion-conduction MEA with porous catalytic electrodes. In this part, we will introduce a typical example of hydroxide-ion-conduction membrane used for electrochemical conversion of carbon dioxide in alkaline PEM cells, conducted by Valdez and coworkers [132],... 

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