Aluminium/air cell

Aluminium-air cells are attractive in principle because of the high thermodynamic electrode potential and theoretical capacity of aluminium. Because of corrosion, however, the only commercial power sources based on this couple are either reserve systems or are mechanically rechargeable, i.e. the anodes are replaced after discharge. A typical cell may be written as... 

The neutral electrolyte, often sea water, can be replaced by aqueous NaOH providing that the anode is alloyed with magnesium and tin. Alkaline aluminium-air cells have significantly greater power densities. 

Aluminium-air cells were first developed for portable applications such as mooring lights, and for recharging nickel-cadmium and lead-acid storage batteries. They have been fabricated in many unusual designs, e.g. the concentric rope battery which has an aluminium core surrounded by a separator and then the oxygen cathode. The rope may be several hundred metres long and can provide 0.03 W/m for a period of 6 months on immersion in the sea. 

The high specific energy and voltage, and the feature of mechanical rechargeability of aluminium/air cell, as well as high abundance and low cost of aluminium, lead to the viability and attractiveness of the aluminimn/air cell for electric vehicles application. 

Neutral aluminium/air cells have been developed at the Serbian Academy of Science and Arts in the Yugoslavia and Bulgaria Academy of Science in Bulgaria. A 24 W, 10-cell saline or sea water was constructed with gallium-tin-magnesium aluminium alloy anode [23]. 

Fig. 10. Schematic representation of an aluminium-air cell with wedge shaped anodes.

The E°cell of an aluminium-air battery is 2.73 volts and it involves all electron process. The free energy change (DG°) of the battery in kJ. Calculated ... 

A number of cylindrical and flat magnesium-based cells have been developed on a commercial scale, mainly for military applications where high discharge currents and low unit weight are important. However, for most of these applications, magnesium batteries have now been replaced by various lithium/organic systems. There are no commercial aluminium-based Leclanchd cells. Magnesium and aluminium are both exploited as anodes in metal-air cells which are considered below. 

The pros and cons of aluminium and magnesium anodes were discussed in Section 3.5. The corrosion problem is even more serious in metal-air cells since the electrolyte may be saturated with oxygen. 

The first metal/air cell mentioned in the literature was a zinc/air cell constructed by Smee in 1840 [11]. In 1878, Maich [12], modified a Leclanch6 cell by replacing the conventional manganese dioxide cathode by a mixture of platinum and carbon powder. Ever since the work of Maich, various metals have been studied for use in this type of system in different applications. Among others, zinc, iron and aluminium have been considered to electric vehicles applications. It is, however, in the field of electric vehicles that most of the research has been conducted. 

The most common type of cell in this category is the zinc air battery, though aluminium/air and magnesium/air cells have been commercially produced. In all cases the basis of operation is the same. Such cells are sometimes called zinc fuel cells. 

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. 

A further point is that the method does not stand very good comparison with metal air batteries. If the user is prepared to use quantities of water, and is prepared to dispose of water/metal hydroxide mixtures, then systems such as the aluminium/air or magnesium/air battery are preferable. With a salt-water electrolyte, an alumininm/air battery can operate at 0.8 V at quite a high current density, producing three electrons for each aluminium atom. The electrode system is much cheaper and simpler than a fuel cell. 

Aluminium-air batteries are lightweight and produce a higher voltage than a lead-acid battery. They are expensive and are not true secondary cells because the aluminium anode has to be replaced from time to time. 

In saline aluminium—air batteries, to prevent clogging of cells by an aluminium hydroxide precipitate it is necessary to pump the electrolyte continuously up and down the cell or stir the electrolyte by gas injection. 

The ultracentrifuge is made up of an aluminium rotor several inches in diameter and it is rotated at high speed in an evacuated chamber. The solution to be centrifuged is kept in a small cell within the rotor near its periphery. The rotor can be driven electrically or by an oil or air turbine. 

Sample quantity 5 mg Sample cell an aluminium open-cup cell, 5 mm in diameter, 2.5 mm in depth Atmosphere air, nitrogen or oxygen gas TG-DTA is, however, performed predominantly in air unless otherwise noted Pressure atmospheric pressure Gas flow rate 20 cmVmin Heating rate 2.5 K/min. 

The whole result of the TG-DTA measurements performed each in air at atmospheric pressure with the aluminium open-cup cell at the value of 0 of 2.5 K/min for the sawdusts of fifteen wood species and, characteristics of the oxidatively-heating process of the sawdust of an average wood species charged in the draft cell, into which air is supplied, and subjected to the adiabatic oxidatively-heating test started from a T, below about 180 °C... 

The whole result of the TG-DTA measurements performed each in air at atmospheric pressure with the aluminium open-cup cell at the value of 0 of 2.5 K/min for the sawdusts of fifteen wood species... 

Sample containers are thin-walled cells of either aluminium or vanadium. Vanadium is required if diffraction measurements are to be included. Solids can be simply wrapped in aluminium foil. Liquids, however, must be held in sealed cans. Indium wire provides a good seal, because it is ductile, easily created and, more importantly, it has a very low coefficient of thermal expansion. Thus a can that is sealed at room temperature will remain sealed after having been cycled to 20K. If the compounds are air or moisture sensitive (solid or liquid) they are loaded into the cans in a glove-box. Gases are more problematic, one successful method is have a suitable volume of the gas attached above the cell. The whole is lowered into the cryostat where the gas is first liquefied and allowed to fill the cell, subsequently the temperature is lowered to freeze the liquid. 

After a sufficient length of time in operation, the electrolytic cell used to produce aluminium must be renewed and the waste materials it contains treated to produce solid phases that can either be safely dumped or recycled. A fluidized bed reactor is suitable for the combustion of the waste products of the cell, using humidified air. 

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