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Abstract

The performance of the Al anode is optimised by destabilising the resistive oxide film on the surface of the metal without increasing the rate of corrosion, and by using a suitable electrolyte in which the Al reaction products are readily soluble.

The destabilisation of the resistive oxide layer is done by alloying the Al metal with alloying elements of high hydrogen overpotential. Elements such as, Zn, Ga, In, Pb, Bi and Sn are found to be highly indispensible. Whereas, Fe and Cu are found to have deleterious effect on the self-discharge characteristics of the anode, resulting in a high rate of hydrogen evolution on the metal. The hydrogen evolved is suppressed by the addition of inhibitors to the electrolyte, to reduce the anodic sites of the Fe and Cu atoms on the surface of the Al metal. Trace amounts of mercuric oxide (HgO) is found to be the best inhibitor. It does not only reduce the rate of hydrogen evolution on the Al surface, it also couples with the atoms on the metal surface to produce an anodic shift in potential.

The low solubility of Al reaction products in the conventional electrolytes gives rise to the problem of cell clogging, which at the moment characterises the state-of-art in the development of Al-air batteries. This problem is alleviated by using an electrolyte mixture in
which the solubility of Al is increased by as much as 140%. The higher solubility obtained for the electrolyte mixture means that its use in aluminium cells, results in a reduction in the amount of reaction product precipitated and thus reduces the tendency of cell clogging. Moreover, the precipitate formed in the electrolyte is granular and can therefore be easily pumped or cleaned away.

One other problem encountered, in the use of existing Al-air batteries, is the the storage capability of the Al anodes. This problem is dealt with, in this work, by finding an alloy that is electrochemically active in the operating medium but behaves like the bare metal, aluminium, under atmospheric conditions. Alloy Q4 is found to possess these qualities. The alloy can be stored openly and infinitely, and can be handled anyhow without any damaging effect. The system, using this anode, can therefore be recharged conveniently and safely. For applications that demand long time use with minimal supervision, an automatic means of recharging has been validated. The anode material is fabricated into shot , to enable a continuous automatic feeding of the active anode material into the cell.

As part of the optimisation of the Al anode performance, temperatures between 40 and 45°C are found to be the optimum operating temperature range for Al cells.

Full cell discharge characteristics of the improved cell show that the aluminium anode Material used in the cell has a high limiting current density and thus, allows high currents to be drawn from the cell. Also, since aluminium is comparatively more soluble in the electrolyte used, the cell can operate at a longer discharge time, than normal, without getting clogged up. A typical cell containing about 57 g of electrolyte, is discharged at 3A for over 48 hours without any serious problems; e.g the problem of cell clogging. The energy density of the cell is calculated for, a 24 hour operation time, to be nearly 400 Wh/Kg; 3,714 Wh/Kg of Al. The cell can be operated from as low as -20°C without the use of an external heater to warm it up.

The commercial viability of the cell is highlighted. The cell, so-improved, can be used to provide power for applications both onshore and offshore. It can be used as the main power source in submersibles, life-boats, military field equipment and reconnaissance vehicles. It can also be used as emergency power source for lighting, burglar and fire alarms, computer memory banks, as starters for car engines and as back-ups for generators.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science Q Science > QD Chemistry T Technology > TP Chemical technology
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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