Alkaline electrolyte fuel cells

A fuel cell produces electricity directly from the electrochemical reaction of hydrogen, from a hydrogen-containing fuel, and oxygen from the air. Hydrogen is industrially produced by steam reformation of naphtha oil, methane and methanol. High-purity hydrogen has been mainly used as a fuel for low-temperature fuel cells such as polymer or alkaline electrolyte fuel cells (Lin and Rei, 2000). 

Alkaline electrolyte fuel cells are relatively inexpensive and show high efficiency. They are preferred when pure oxygen is supplied. Unfortunately, alkaline electrolytes react with the slightly acidic carbon dioxide of the atmosphere. This reaction changes the chemical nature of the electrolyte. After a few hours of operation with air as the source of oxygen, the alkaline fuel cell no longer functions efficiently. 

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. 

Figure 1.4 Electrode reactions and charge flow for an alkaline electrolyte fuel cell. Electrons flow from anode to cathode, but conventional positive current flows from cathode to anode. 

Historically, the major alkaline electrolyte fuel cells have operated at well above ambient pressure and temperature. The pressures and temperatures, together with information about the electrode catalyst, is given for a selection of important alkaline fuel cells in Table 5.1. 

Table 5.1 Operating parameters for certain alkaline electrolyte fuel cells. [Data from Warshay and Prokopius, 1990 and Strasser, 1990.] The pressure figures are approximate, since there are usually small differences between each reactant gas...

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. 

Methanol fuel works in both the alkaline FC considered in Chapter 5 and the PEMFC of Chapter 4. However, the reaction between the product carbon dioxide and the electrolyte of the alkaline (electrolyte) fuel cell (AFC) is a major problem, which would appear to be insoluble. The anode reaction of the alkaline DMFC is... 

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 reaction mechanisms of this fuel cell, in acid and alkaline electrolytes, are shown in Table 42.2. The major differences, electrochemically, are that the ionic conductor in the acid electrolyte is the hydrogen ion (or, more correctly, the hydronium ion, HjO ) and the OH or hydroxyl ion in the alkaline electrolyte. The only by-product of a hydrogen/oxygen fuel cell is water in the acid electrolyte water is produced at the cathode and, in the alkaline electrolyte fuel cell, it is produced at the anode. 

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