Batteries convert chemical energy into electrical energy. They have two electrodes—called a cathode and an anode—where chemical reactions that either use or produce electrons take place. The electrodes are connected by a solution—called an electrolyte—through which ions can move, completing an electrical circuit. In this activity, the salt provides ions that can move through the wet paper towel and transfer charge.
To generate electrical energy, this battery relies on oxidation of aluminum at the anode, which releases electrons, and a reduction of oxygen at the cathode, which uses electrons. The movement of electrons through an external circuit generates an electric current that can be used to power simple devices. A diagram of the battery and equations for the half and overall reactions are given below:
Equations for the half and overall reactions:
anode: Al(s) + 3OH−(aq) → Al(OH)3(s) + 3e−
cathode: O2(g) + 2H2O(l) + 4e− → 4OH−(aq)
overall: 4Al(s) + 3O2(g) + 6H2O(l) → 4Al(OH)3(s)
Aluminum foil provides an affordable supply of aluminum. Activated charcoal, which is mostly made of carbon, can conduct electricity and is non-reactive. It provides a highly porous surface that is exposed to oxygen in the air. One gram of activated charcoal can have more internal surface area than an entire basketball court! This surface provides a large number of sites to which oxygen can bind and participate in the cathode reaction.
This large reaction area makes it possible for the simple aluminum–air battery to generate 1 volt (1 V) and 100 milliamps (100 mA). This is enough power to run a small electrical device and provides a safe and easy way to make a powerful battery at home or in school.