Si estos problemas se resuelven podríamos ver grandes aplicaciones para las baterías de aluminio-aire en el futuro, incluidos su uso en vehículos eléctricos o incluso camiones.
Las baterías de zinc-aire tienen 3,5 veces más energía potencial que las de iones de litio, y las aluminio-aire almacenan 21 veces más, pero aún queda mucho por investigar antes de fabricarlas en grandes cantidades, por lo que en principio se destinarán a pequeños aparatos, como audífonos.
La densidad energética de las baterías de aluminio ya alcanza los 1.300 Wh/kg, y se espera llegara a 2.000 Wh/kg.
La batería de aluminio-aire pertenece a una familia de baterías bastante desconocida, las bateríass de metal-aire. Constan de un metal en el ánodo y un electrolíto (una disolución de potasa), y el cátodo, en lugar de tener un compuesto químico oxidante, tienen una membrana en contacto con el aire que permite la difusión y reacción con el oxígeno de este.
Ello da como resultado la formación de un hidróxido del metal y de la corriente eléctrica mientras aún quede metal en el ánodo. Al no necesitar un compuesto químico oxidante en el cuerpo de la batería, se reduce su volumen y peso con lo que aumenta su densidad energética de acumulación. De hecho la baterías de aluminio-aire es la que tiene mayor capacidad teórica.
Pero no todo son ventajas ya que la batería de aluminio-aire tiene problemas de corrosión y estabilidad, aunque se van haciendo avances en solucionar, pero no suficientes como para hallar aplicaciones comerciales. Las únicas existentes son las aplicaciones militares, donde los costes no tienen tanta importancia.
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Aluminium battery
Aluminium batteries or aluminum batteries are commonly known as aluminium-air batteries or Al-air batteries, since they 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 previous problems with cost, shelf-life, start-up time and byproduct removal, which have restricted their use to mainly military applications.
An electric vehicle with aluminium batteries could have potentially ten to fifteen times the range of lead-acid batteries with a far smaller total weight.
Al-air are primary batteries, i.e. non-rechargeable, and can also be considered to be fuel cells. Once the aluminium anode is consumed by its reaction with atmospheric oxygen at a cathode immersed in a water-based electrolyte to form hydrated aluminium oxide, the battery will no longer produce electricity.
However, it may be possible to mechanically recharge the battery with new aluminium anodes made from recycling the hydrated aluminium oxide. In fact, recycling the formed aluminium oxide will be essential if aluminium-air batteries are to be widely adopted.
Electrochemistry
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. Cell voltage with saltwater electrolyte is around only 0.7 V. The use of potassium hydroxide electrolyte leads to a cell voltage of 1.2 V.
Commercialization
Aluminium as a "fuel" for vehicles has been studied by Yang and Knickle. They concluded the following: The Al/air battery system can generate enough energy and power for driving ranges and acceleration similar to gasoline powered cars…the cost of aluminum 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.
There are some technical problems still to solve though to make Al-air batteries suitable for powering electric vehicles. Anodes made of pure aluminium are corroded by the electrolyte, so the aluminium is usually alloyed with tin or other proprietary elements.
The hydrated alumina that is created by the cell reaction forms a gel-like substance at the anode and reduces the electricity output. This is an issue that is being addressed in the development work on Al-air cells. For example, additives have been developed which form the alumina as a powder rather than a gel. Also alloys have been found to form less of the gel than pure aluminium.
Modern air cathodes consist of a reactive layer of carbon with a nickel-grid current collector, a catalyst (e.g. cobalt), and a porous hydrophobic PTFE film that prevents electrolyte leakage. The oxygen in the air passes through the PTFE then reacts with the water to create hydroxide ions. These cathodes work well but they can be expensive.
Traditional Al-air batteries had a limited shelf life because the aluminium reacted with the electrolyte and produced hydrogen when the battery was not in use – although this is no longer the case with modern designs. The problem can be avoided by storing the electrolyte in a tank outside the battery and transferring it to the battery when it is required for use.
These batteries can be used as reserve batteries in telephone exchanges, as a backup power source. Al-air batteries could be used to power laptop computers and cell phones and are being developed for such use.
Companies
The French company Métalectrique demonstrated at the "European Research and Innovation Exhibition", Paris, June 2007, an aluminium can battery and have since made a 6-pack "trash power" battery for the 3rd world, where electricity is scarce but discarded aluminium is often plentiful.
Aluminum based batteries
Different types of aluminium batteries had been investigated:
Aluminium-chlorine battery was patented by United States Air Force in the 1970s and designed mostly for military applications. They use aluminium anodes and chlorine on graphite substrate cathodes. Required elevated temperatures to be operational.
Aluminium-iodine secondary cell has been investigated by some Chinese researchers.
Aluminium-sulfur batteries worked on by American researchers with great claims, although it seems that they are still far from mass production. It’s unknown if they are rechargeable.
Al-Fe-O, Al-Cu-O and Al-Fe-OH batteries were proposed by some researchers for military hybrid vehicles. Corresponding practical energy densities claimed are 455, 440, and 380 Wh/kg.
Battery specifications Energy/weight 1300 W·h/kg (theoretical 3.5 kW·h/kg thus 37% eff.): (molAl/0.027 kg)(3 mole−/molAl)(9.5e4 C/mole−)(A·s/C)(h/3600 s)(W/V·A)(1.2 V/cell)(kW/1e3 W) = 3.5 kW·h/kg·cell less the weight of oxygen and nonconsumables (electrolyte, and positive electrode) Energy/size N/A Power/weight 200 W/kg Nominal Cell Voltage.
Métalectrique SAS.
We design and fabricate Aluminium-Air fuel cells (batteries). Our current main project, the MAA power cell (pat pending) could revolutionise the entire power industry. We have so far been sponsored by several French government groups. Enquiries from other investors are still welcome.
Imagine: 2000 km non-stop in an electric car! Introducing our new MAA power cells… long-lasting power, zero CO2.
Technology
Aluminum-air energy!
•Aluminium has been a sought-after accumulator of energy for over 50 years, because of its high stored energy density, its light weight and its recyclability. The original Zaromb cell (1960) stored 15 times the energy of a comparable lead-acid battery.
•Aluminium as a solid fuel can be safely stored, until needed, for an unlimited amount of time. No pressurised or combustible components are required.
•Aluminium is an abundant and fully recyclable metal. All of the aluminium is easily recovered from the aluminium hydroxide waste of the cell, using only a fraction of the energy required for the original bauxite process. If we use only recycled aluminium, it is actually cheaper than using fossil fuels.
Advantages over hydrogen-only fuel cells
•Safety – No need to store explosive compressed gas.
•Refuel by cartridge exchange – No need for costly infrastructure
•Simplicity and huge reduction in cost.
•Available now – Not in 20 years time.
What is special about our aluminium-air cells?
Previous al-air systems had heat, corrosion and byproduct removal problems. We have spent 10 years developing the novel chemistry and technology to fix those problems, allowing..
•Immediate startup – No warm-up period.
•Steady, reliable power take-off due to minimisation of the aluminium hydroxide byproduct.
•Cool running due to minimisation of the hydrogen byproduct.
•No special alloys or purity. We can use any grade of aluminium, even recycled, which makes it much cheaper to refuel.
•Lightweight design and parts minimisation for reliability.
Our technology was verified at the University of Nantes. A report of which is available to investors.
Combined hydrogen and aluminium-air technologies
Since we produce a small (ie safe) amount of hydrogen, we can use that in a small hydrogen cell to produce even more power. The waste water from the hydrogen cell can be used in the al-air cell.
How Green is our technology?
•There are no polluting emissions.
•50% of the aluminium in use is derived from recycled aluminium and 55% of aluminium produced from bauxite uses electricity from hydroelectric stations. (ref. www.world-aluminium.org)
•All byproducts are recycled. See the diagram..
•In the event of accidental spillage, there are no toxic byproducts. In fact the main byproduct, aluminium hydroxide, is used in medicines and cosmetics and is present naturally in the soil.
Markets
Portable refridgeration
• Refridgeration transport by road releases many tonnes of carbon dioxide (dry ice) into the atmosphere. Similar units for air transport are limited by current battery technology to 2 hours duration. We can replace both with Al-Air systems to give at least 40 hours duration with zero emissions.
• Portable fridges in Africa, used to transport vital vaccines have to be taken to very remote areas and rely on either very expensive, limited solar power or unsafe, unreliable kerosene power. We can provide a safer, cheaper, long-duration alternative.
Electric vehicles
• We are developing a power unit for an electric car. Our system increases the range to 2000 km, so removing the only remaining barrier to widespread electric vehicle use. For refueling stops you would plug-in a new unit and leave the old one at the garage to be refueled later. This idea requires no new infrastructure and is a lot quicker and safer than recharging a traditional battery.
• We are also developing a smaller power unit suitable for electric wheelchairs, scooters, quads and boats.
Other markets
• Hearing aid batteries: Al-Air has 4 times the specific energy density of Zinc-Air, which currently dominate this market – worth $400m annually.
• Back-up interior power systems for homes / offices: power outages are prevalent in the developing world but you can’t always just start up a noisy, smelly internal combustion engine. We can provide quiet, clean and safe power.
• Long-life remote power for hand-held devices, phones, computers, military communications, patient monitors and recorders, nerve and muscle stimulators, and drug infusion pumps
• Personal robots are probably the next big thing in tech but they’ll need batteries that last a long time if they are to be useful.
www.ectechnic.co.uk/ALUMAIR.HTML
www.exploratorium.edu/ti/conf/nsta2009/julie/Al-air.pdf
