Mobile electrolyte alkaline fuel cell

The Alkaline Fuel Cell, AFC, is the most efficient low-temperature fuel cell presently available with a very high power density, therefore ideally appropriate for mobile applications. The AFC shows a similar performance as the PEFC, but with a much more demanding process control which is complicated because of the requirements of fuel purity (no CO2) and of the corrosive liquid electrolyte. Efficiencies of more than 60 % have been achieved with clean hydrogen and oxygen and noble electrode materials. The AFC was demonstrated to also work with a hydrogen-air system, in a 1 kW stack in Japan and in a 6 kW Russian system with more than 5(XX) h lifetime [34]. PEFC and AFC may play a... 

Figure 5.3 Diagram of an alkaline fuel cell with mobile electrolyte. The electrolyte also serves as the fuel cell coolant. Most terrestrial systems are of this type. 

The majority of alkaline fuel cells are of this type. The main advantage of having the mobile electrolyte is that it permits the electrolyte to be removed and replaced from time to time. This is necessary because, as well as the desired fuel cell reactions of equations 5.1 and 5.2, the carbon dioxide in the air will react with the potassium hydroxide electrolyte... 

To summarise then, the main advantages of the mobile electrolyte-type alkaline fuel cell are as follows ... 

This mobile electrolyte system was used by Bacon in his historic alkaline fuel cells of the 1950s and in the Apollo mission fuel cells. It is almost universally used in terrestrial systems, but the Shuttle Orbiter vehicles use a static electrolyte, as described in the next section. 

The first large group of fuel cells, the low temperature ones (50 - 250 °C), is characterized by its more focused application on mobile devices and the automotive industry. Within the large group of low-temperature cells, there can also be a division, considering the type of electrolyte used. This classification results in three cell types proton exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC) and phosphoric acid fuel cell (PAFC). 

Figure 1.5 shows the schematic of mobile electrolyte AFC. Most alkaline fuel cells are of this t) pe with KOH as the electrolyte. 

Union Carbide Corp. (UCC) developed AFCs for terrestrial mobile applications starting in the late 1950s, lasting until the early 1970s. UCC systems used liquid caustic electrolytes the electrodes were either pitch-bonded carbon plates or plastic-bonded carbon electrodes with a nickel current collector. UCC also built fuel cell systems for the U.S. Army and the U.S. Navy, an alkaline direct hydrazine powered motorcycle, and the Electrovan of General Motors. Finally, Professor Karl V. Kordesch built his Austin A-40 car, fitted with UCC fuel cells with lead acid batteries as hybrid. It was demonstrated on public roads for three years. The years of research and development are very well summarized in reference (5) Brennstofflyatterien. 

In an alkaline electrolyte fuel cell the overall reaction is the same, but the reactions at each electrode are different. In an alkali, hydroxyl (OH ) ions are available and mobile. At the anode, these react with hydrogen, releasing energy and electrons, and producing water. 

There are two types of mobile applications for fuel cells. The first is where the fuel cell provides the motive power for the vehicle, for example, in combination with electric traction for cars and buses. For such fuel cell vehicles, the PEM fuel cell (and for special applications the alkaline (electrolyte) fuel cell (AFC)) is the preferred option. The second... 

The pH of the electrolyte is effective on the reaction kinetics at the individual electrodes and the electrode potential at which oxidation or reduction takes place [26]. Electrolyte is typically a strong acid or a strong base, such as sulfuric acid or potassium hydroxide, which include highly mobile hydronium or hydroxide ions, respectively [20]. Typically, operation of fuel cell in alkaline media can develop the electrooxidation of the catalyst-poisoning carbon monoxide species on the anode and the kinetics of ORR is improved at the cathode [26]. However, in membrane-based fuel cells, due to the potential of carbonate formation resulting in clogging the membrane, the long-term stability is restricted and limits the use of these alkali-compatible membranes for liquid fuel cell operations [26]. 

Figure 11.20 Schematic of a hydrogen/oxygen fuel cell. In a commercial alkaline cell of this type the electrolyte is phosphoric acid and it is operated at above room temperature. At even higher temperature the electrolyte is a molten carbonate and the species that migrates is COg . At sufficiently high temperature a solid oxide can be used and the mobile ion is O,...

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