Experiencing Technical Difficulties (resonant) wrote,
Experiencing Technical Difficulties

Aluminum-air batteries for home use

A while ago, I was considering making a voltage converter that would let you charge a cellphone by dissolving a used aluminum can in salt water with a carbon cathode.

When I was in Albania and Kosovo, I saw that most businesses had a refrigerator-sized emergency diesel generator out front.

Aluminum-air batteries could be used to replace these generators.

Gasoline and diesel fuel give off about 46 MJ/kg when burned. When used in a generator, they produce about 15 MJ/kg in electricity.
A sheet of aluminum gives off about 31 MJ/kg when oxidized. When used in an aluminum-air battery, it produces about 30 MJ/kg in electricity.

A gasoline or diesel generator is expensive, is noisy, and requires regular maintenance.
An aluminum-air battery is very cheap to make. It is silent. It does require maintenance, but cleaning the cathodes and changing the salt-water electrolyte isn't harder than changing a generator's oil.

If you spill gasoline or diesel fuel, you get a smelly mess that will pollute groundwater and that can cause a fire or explosion.
If you drop a sheet of aluminum on the ground, it makes a loud clanging sound and might hurt your toes. It will not burn, explode, or poison you under normal conditions.

Gasoline and diesel fuels are mostly made from fossil fuels. Only a small percentage comes from renewable resources like used cooking oil.
Aluminum is mostly made using hydroelectric or geothermal power, from readily-available bauxite ore that is as common as dirt.

An aluminum-air battery capable of replacing a diesel generator would be a big tank of salt water, with a sandwich in it made of sheets of aluminum metal, carbon fibre, copper mesh, and polyester cloth. There might be a small water pump to keep the salt water stirred up, to keep oxides from clogging the electrodes and to keep the fluid uniformly oxygenated. A solid-state inverter would convert the electricity to the desired voltage and frequency.

I'm going to order some pieces of TPS61222 ($0.55 each) or TPS61202 ($1.00 each) and make a prototype.

EDIT: OK, it looks like the key factor is going to be the carbon cathode. It needs a large surface area, and needs to be heavily oxygenated. When ItsRainingJen and I went out shopping together, I picked up some carbon brushes (from electric motors) at Active Surplus, thinking I could use those as cathodes. They do work, but the current quickly dropped by a factor of three after only a few minutes. That indicates I'll need a pump like in a fishtank to either bubble air through the electrolyte, or else spray it into the air so it drips freshly-oxygenated fluid onto the cathodes.

Aluminum-air battery with carbon cathode

Anode = sheet of aluminum foil
Cathode = carbon brush from an electric motor, picked up at Active Surplus for 50 cents
Separator = paper coffee filter
Electrolyte = saturated solution of table salt in filtered water

Resistance of wiring was under 0.1 ohms.
Voltage was 0.4 V, versus theoretical 0.7 V
Current started at over one amp, dropping quickly to 0.4
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