Small fuel cells electrolyte

Metal foam (see, for example. Figure 3.5) has already been discussed in the context of heat exchangers. Micro-reactors, highly relevant to the subject of small fuel cells, have also been introduced in earlier chapters. The construction of metal foam based methanol steam micro-reformers to generate hydrogen for polymer electrolyte membrane fuel cells (PEMFCs) has been reported and in Guangzhou, Chinese researchers have looked at laminated micro-reactors in which copper-based catalysts have been supported by metal foams (see Figure 11.11 Yu et al., 2007). 

Fuel cell systems can take a number of configurations depending on the combinations of fuel and oxidant, the type of electrolyte, temperature of operation, application, etc. Table 42.1 is a summary of various types of fuel cells distinguished by the electrolyte and operating temperature. The PEM fuel cell is currently the predominant one for portable and small fuel cells as it is the only one operating near ambient conditions. ... 

Proton Exchange Membrane (PEM) These cells use a perfluorinated ionomer polymer membrane electrolyte which passes protons from the anode to the cathode. They operate at a relatively low temperature (70 to 85°C), and are especially notable for their rapid start-up time. These are being developed for use in transportation applications and for portable and small fuel cells. 

Small fuel cells can be exploited most effectively if they can stand by and operate at ambient temperatures (and can therefore start rapidly), can operate on ambient air, can respond rapidly to load changes, have a non-migrating (solid) electrolyte, and have a reasonably high power density and specific-power. The fuel cell type that best suits these criteria is clearly the PEMFC, despite a drawback related to the fact that liquid water embodied in the solid polymer tends to freeze, and thereby impedes proton conduction, when its temperature drops below the freezing point. The PEMFC can stand by under freezing conditions. 

It is clear that a system that can utilize a liquid fuel directly at the fuel cell anodes would be particularly appealing in small fuel cell applications. As mentioned earlier, at this time metiianol is the only carbonaceous species that can both serve as a practical fuel and provide reasonable electrochemical performance at the fuel cell anode. Substantial research and development efforts have been directed at direct-methanol fuel cells using PEM electrolyte for more than a decade, in the context of small, field-deployable systems. 

Design Principles An individual fuel cell will generate an electrical potential of about 1 V or less, as discussed above, and a current that is proportional to the external load demand. For practical apph-cations, the voltage of an individual fuel cell is obviously too small, and cells are therefore stacked up as shown in Fig. 27-61. Anode/ electrolyte/cathode assemblies are electrically connected in series by inserting a bipolar plate between the cathode of one cell and the anode of the next. The bipolar plate must be impervious to the fuel... 

One leading prototype of a high-temperature fuel cell is the solid oxide fuel cell, or SOFC. The basic principle of the SOFC, like the PEM, is to use an electrolyte layer with high ionic conductivity but very small electronic conductivity. Figure B shows a schematic illustration of a SOFC fuel cell using carbon monoxide as fuel. 

The stability of ceramic materials at high temperatures has made them useful as furnace liners and has led to interest in ceramic automobile engines, which could endure overheating. Currently, a typical automobile contains about 35 kg of ceramic materials such as spark plugs, pressure and vibration sensors, brake linings, catalytic converters, and thermal and electrical insulation. Some fuel cells make use of a porous solid electrolyte such as zirconia, Zr02, that contains a small amount of calcium oxide. It is an electronic insulator, and so electrons do not flow through it, but oxide ions do. 

Summing up this section, we would like to note that understanding size effects in electrocatalysis requires the application of appropriate model systems that on the one hand represent the intrinsic properties of supported metal nanoparticles, such as small size and interaction with their support, and on the other allow straightforward separation between kinetic, ohmic, and mass transport (internal and external) losses and control of readsorption effects. This requirement is met, for example, by metal particles and nanoparticle arrays on flat nonporous supports. Their investigation allows unambiguous access to reaction kinetics and control of catalyst structure. However, in order to understand how catalysts will behave in the fuel cell environment, these studies must be complemented with GDE and MEA tests to account for the presence of aqueous electrolyte in model experiments. 

Proton Exchange Membrane Fuel Cells (PEMFCs) are being considered as a potential alternative energy conversion device for mobile power applications. Since the electrolyte of a PEM fuel cell can function at low temperatures (typically at 80 °C), PEMFCs are unique from the other commercially viable types of fuel cells. Moreover, the electrolyte membrane and other cell components can be manufactured very thin, allowing for high power production to be achieved within a small volume of space. Thus, the combination of small size and fast start-up makes PEMFCs an excellent candidate for use in mobile power applications, such as laptop computers, cell phones, and automobiles. 

Alkaline Fuel Cell (AFC) The electrolyte in this fuel cell is concentrated (85 wt%) KOH in fuel cells operated at high temperature ( 250°C), or less concentrated (35-50 wt%) KOH for lower temperature (<120°C) operation. The electrolyte is retained in a matrix (usually asbestos), and a wide range of electrocatalysts can be used (e.g., Ni, Ag, metal oxides, spinels, and noble metals). The fuel supply is limited to non-reactive constituents except for hydrogen. CO is a poison, and CO2 will react with the KOH to form K2CO3, thus altering the electrolyte. Even the small amount of CO2 in air must be considered with the alkaline cell. 

Recently, the major activity in transportation fuel cell development has focused on the polymer electrolyte fuel cell (PEFC). In 1993, Ballard Power Systems (Burnaby, British Columbia, Canada) demonstrated a 10 m (32 foot) light-duty transit bus with a 120 kW fuel cell system, followed by a 200 kW, 12 meter (40 foot) heavy-duty transit bus in 1995 (26). These buses use no traction batteries. They operate on compressed hydrogen as the on-board fuel. In 1997, Ballard provided 205 kW (275 HP) PEFC units for a small fleet of hydrogen-fueled, full-size transit buses for demonstrations in Chicago, Illinois, and Vancouver, British Columbia. Working... 

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. 

Proton exchange membrane fuel cells (PEMFCs) work with a polymer electrolyte in the form of a thin, permeable sheet. The PEMFCs, otherwise known as polymer electrolyte fuel cells (PEFC), are of particular importance for the use in mobile and small/medium-sized stationary applications (Pehnt, 2001). The PEM fuel cells are considered to be the most promising fuel cell for power generation (Kazim, 2000). 

Fuel cells may become the energy-delivery devices of the 21st century. Although there are many types of fuel cells, polymer-electrolyte fuel cells are receiving the most attention for automotive and small stationary applications. In a polymer-electrolyte fuel cell, hydrogen and oxygen are combined electrochemi-cally to produce water, electricity, and some waste heat. 

Finally, there are some miscellaneous polymer-electrolyte fuel cell models that should be mentioned. The models of Okada and co-workers - have examined how impurities in the water affect fuel-cell performance. They have focused mainly on ionic species such as chlorine and sodium and show that even a small concentration, especially next to the membrane at the cathode, impacts the overall fuelcell performance significantly. There are also some models that examine having free convection for gas transfer into the fuel cell. These models are also for very miniaturized fuel cells, so that free convection can provide enough oxygen. The models are basically the same as the ones above, but because the cell area is much smaller, the results and effects can be different. For example, free convection is used for both heat transfer and mass transfer, and the small... 

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