Aluminium–air battery
Aluminium–air batteries produce electricity from the reaction of oxygen in the air with aluminium. They have one of the highest energy densities of all batteries, but they are not widely used because of problems with high anode cost and byproduct removal when using traditional electrolytes. This has restricted their use to mainly military applications. However, an electric vehicle with aluminium batteries has the potential for up to eight times the range of a lithium-ion battery with a significantly lower total weight.
Imagem: Tim Evanson · BY-SA · Openverse
The anode oxidation half-reaction is Al + 3OH− → Al(OH)3 + 3e− -2.31 V. The cathode reduction half-reaction is O2 + 2H2O + 4e− → 4OH− +0.40 V. The total reaction is 4Al + 3O2 + 6H2O → 4Al(OH)3 +2.71 V. About 1.2 volts potential difference is created by these reactions and is achievable in practice when potassium hydroxide is used as the electrolyte. Saltwater electrolyte achieves approximately 0.7 volts per cell. The specific voltage of the cell can vary depending upon the composition of the electrolyte as well as the structure and materials of the cathode. Other metals can be used in a similar way, such as lithium-air, zinc-air, manganese-air, and sodium-air, some with a higher energy density. However, aluminium is attractive as the most stable metal.
Imagem: Tim Evanson · BY-SA · Openverse
Aluminium (Al) has been widely used as an anode material in metal-air batteries due to its high energy density, recyclability, and abundance. However, challenges with Al anodes include corrosion and passivation. Impurities in commercially available aluminium lead to the formation of layers that impair performance. Corrosion reactions produce hydrogen and form aluminium hydroxides, while the formation of an oxide film upon exposure to air or water further limits functionality. Improving Al anode performance involves optimizing grain size and crystal orientation, as finer grain structures enhance corrosion resistance and electrochemical activity. The study done by Fan and Lu examined the relation between the grain size and the anode performance. In this study, aluminium anodes with finer grain sizes were created using a method called Equal Channel Angular Pressing (ECAP). As the number of extrusion passes increased, the grains became smaller and more uniform. However, the process had limitations due to heat from deformation causing some grain growth. The results showed that refining the grain size improved the anode's electrochemical activity, reduced corrosion, and increased polarization and charge-transfer resistance. Tests confirmed that the anode with fine grains performed better than one with larger grains. The fine-grain structure also provided better anti-corrosion properties and enhanced battery performance in a 4 mol/L NaOH solution. At a current density of 10 mA/cm2, the fine-grain anode showed a 41.5% increase in capacity density and a 55.5% increase in energy density compared to the coarse-grain anode. Besides microstructure optimization, processing of the anodes can also impact the performance. Anodes fabricated using laser sintering show increased capacity compared to non-sintered samples, which highlights the importance of processing of the anode in terms of the anode performance.
Imagem: Electrolux Design Lab · BY-NC · Openverse
Issues
Aluminium as a "fuel" for vehicles has been studied by Yang and Knickle. In 2002, they concluded: The Al/air battery system can generate enough energy and power for driving ranges and acceleration similar to gasoline powered cars...the cost of aluminium as an anode can be as low as US$ 1.1/kg as long as the reaction product is recycled. The total fuel efficiency during the cycle process in Al/air electric vehicles (EVs) can be 15% (present stage) or 20% (projected), comparable to that of internal combustion engine vehicles (ICEs) (13%). The design battery energy density is 1300 Wh/kg (present) or 2000 Wh/kg (projected). The cost of battery system chosen to evaluate is US$ 30/kW (present) or US$ 29/kW (projected). Al/air EVs life-cycle analysis was conducted and compared to lead/acid and nickel metal hydride (NiMH) EVs. Only the Al/air EVs can be projected to have a travel range comparable to ICEs. From this analysis, Al/air EVs are the most promising candidates compared to ICEs in terms of travel range, purchase price, fuel cost, and life-cycle cost.
