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| Fig. 1: Thermite welding. (Source: Wikimedia Commons) |
Aluminum can produce a great amount of heat while reacting with oxygen, so it's used widely in thermite welding process, shown in Fig. 1. Also, the technology of aluminum-hydro fuel has been around for some decades. Because of the high anode cost and the problem of byproduct removal, it is mainly used for military purpose at this point. During recent years, people have been trying to power electric cars with this technology. There are several difficulties of making electric cars powered by this method commercially favorable. First, the coherent and adherent layer of aluminum oxide forming on the surface of the aluminum particles would prevent water from reacting with the aluminum metal. [1] Second, storage systems using this approach wasn't able to meet the 2010 DOE system targets of 6 wt.% hydrogen and 45 grams hydrogen per liter. [1] Finally, the cost of producing hydrogen by this approach is dictated by the cost of aluminum metal. The November 2007 commodity price for aluminum is $2.36 per kg. [1] At this price, hydrogen from an aluminum-water hydrogen generation approach would cost approximately $21 per kg H2. [1] Even assuming high volume production, the DOE target range for hydrogen cost of $2-3 per kg H2 would not be met. [1] Currently the aluminum-hydro technology is more suitable for the purpose of non-vehicular applications.
However, in Feb. 2008, engineers in Purdue University developed an aluminum alloy that is able to split water and produce hydrogen with competitive cost compared to existing conventional fuels, which makes this technology promising for the future transportation and power generation.
There are three possible reactions of aluminum reacting with water listed below.
2Al + 6H2O → 2Al(OH)3 + 3H2
2Al + 4H2O → 2AlO(OH) + 3H2
2Al + 3H2O → Al2O3 + 3H2
All these reactions are thermodynamically favorable from room temperature past the melting point of aluminum (660°C). [1] All are also highly exothermic. From room temperature to 280°C Al(OH)3 is the most stable product, while from 280-480°C AlO(OH) is most stable. Above 480°C Al2O3 is the most stable product. [1] From the standard heat of formation of Al2O3 we obtain for the energy content of 1 kg of Al
| 1.67 × 107 J per mole of
Al2O3 2 × 0.027 kg per mole of Al |
= 3.09 × 107 J per kg of Al |
The addition of gallium-indium-tin alloy is critical for the operation of the cell because it hinders the formation of a passivating aluminum oxide layer on the pure aluminum after the water reacts with aluminum. [2] The undesired passive layer would prevent further reaction between water and pure aluminum. Since it is hard to directly store hydrogen in vehicles, the system in which the hydrogen is produced which can be collected in different tank is developed. This system contains the aluminum powder with the liquid alloy of gallium indium and tin. [2] When the water falls on the mixture of alloy the water molecules get split into the molecules of the hydrogen and oxygen. [2] Table 1 shows the energy density calculated and compared with other types of fuel. [2]
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| Table 1: Energy output from different types of fuels. |
The last of these, the energy obtained from aluminum by first splitting water with it to make hydrogen, is calculated per
| 1 kg Al 0.027 kg/mole |
× 1.5 × 0.002 kg/mole | = | 0.11 kg H2 |
A modern, commercially available electric car powered by fuel cells with a range of 400 km requires about 4 kg of hydrogen, which can be produced by 36 kg of aluminum with the aluminum-water reaction assuming a conversion yield of 100%. [3] Such an on-demand supply system only occupies a volume less than 50 L and costs around US$ 86 based on the current price of US$ 2.4/kg for primary aluminum. [3] In comparison, at least a 225 L tank with a cost of US$ 1800 are needed to feed such a car if hydrogen is stored in a conventional high-pressure tank operating at 200bar. [3]
Being the most abundant crustal metal on the earth, which can be fully recycled, aluminum is regarded as a "viable metal", the utilization of which exactly coincides with today's theme of developing sustainable energy. [3] Also, another advantage of aluminum is its light weight. With its low density of 2700 kg/m3, aluminum is the lightest among all commonly used metals. [3] The density of its different alloys is in the range of 2600 - 2800 kg/m3. [3] Such a property helps to lead to a significant reduction in the total weight of a system. Lastly, the technology provides a pollution- free solution for future fuel source.
Despite the weight reduction of using aluminum compared to other metals, the electric vehicle powered by this system would still be a lot heavier than typical gasoline powered vehicles. And as the demand of aluminum grows, the price of the materials would soar as well. Also the problem of passivating oxide layer still needs to be address with better solutions, so there's still a big room for further research regarding this topic.
The development of Al-water fuel cell provides an alternative way of hydrogen fuel usage. The hydrogen no longer needs to be stored as liquid hydrogen, instead, the hydrogen is produced whenever Al reacts with water. The outstanding energy density of aluminum greatly improves the mileage per tank of fuel, which is an essential feature for future electric vehicles. The main drawback of this technology is still the passive aluminum oxide layer forming on the interface between aluminum and water, preventing further reaction from smoothly happening. To sum up, although there're still several problems remain unsolved, the Aluminum-Hydro technology is a highly promising fuel alternative for both transportation and stationary applications.
© Ling-Hsiang Chen. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. Petrovic and G. Thomas, "Reaction of Aluminum with Water to Produce Hydrogen," U.S. Department of Energy, 2008
[2] S. Puranik and S. Gupta, "Efficient Use of Hydrogen Made by Splitting Water by Reaction with Aluminum Alloy," Int. J. Eng. Technol. Res. 2, 1911 (2013).
[3] H.Z. Wang et al., "A Review on Hydrogen Production Using Aluminum and Aluminum Alloys," Renew. Sust. Energy Rev. 13, 845 (2009).
