The Salty Science of the Aluminum-Air Battery
by Dr. Stephanie Chasteen, Dr. Dennis Chasteen, and Dr. Paul Doherty
Online Supplementary Material
The construction of this cell is borrowed largely from the Exploratorium’s Square Wheels book.
about 20 g of salt with 400 ml of warm water. The amounts are not that
important, but this will give you a solution that is about 5% salt by
Cut the copper wire into 5 sections about 10 cm long. Strip the insulation off one half of the wire, and off about 2.5 cm of the other end of the wire. This leaves a short section of insulation in the middle of the wire, holding it together. Separate the strands of the stripped half of wire, so it looks like a broom (photo, right).
Connect the final two leads to a multimeter, LED, or piezoelectric buzzer. Fill the cups with the saltwater solution. What happens?
In the TPT paper, we stated:
Actually, a cell made entirely of vinegar water (acetic acid) will work just fine, since the H+ and acetate- ions from the dissociation of the weak acid make the solution conductive. A squirt of vinegar to your saltwater cell will make the LED glow brighter. But don’t be fooled – stirring the solution does the same thing. Upon settling, the performance of the vinegar/salt cell is generally comparable to that of the saltwater cell alone.
However, you’ll probably find that vinegar/salt cells maintain their current for a longer time than saltwater cells. If you leave a vinegar/salt cell overnight, the surface of the copper will not be coated with a reddish oxide, but stays shiny and clean. The acetate in vinegar tends to dissolve the cuprous oxide coating as it forms, by forming a complex with the Cu(II). This allows better contact with the solution and thus better electron transfer over time.
Adding vinegar to the saltwater battery also changes the cell chemistry.
Adding vinegar will result in a pH of about 2-3. Equations 1 and 2 are
no longer favored at low pH, and under acidic conditions Equation 3 (the
overall reaction) becomes
In the TPT paper, we stated:
You will find you’ll get a much more stable and powerful cell if you add a teaspoon of bleach to the saltwater (or to plain water) with current and voltage around 10 mA and 1 V, respectively with only the multimeter in the circuit. So a bleach-powered cell produces about 5 times as much power! Why is that?
When bleach is added, the battery is no longer an air battery; instead of oxygen from the air, sodium hypochorite (NaOCl), the major constitutent in bleach, and hypochorous acid (HOCl), a minor constituent are reduced. The full equations for this cell can be found online. This cell potential (3.93 V) is quite a bit higher than the 3.12 V for the saltwater battery (Equation 3). So this reaction proceeds more rapidly, generating more electrons per unit time and thus greater current. We also observed a higher voltage. Another indication that the reaction proceeds more rapidly for the bleach battery is the striking abundance of white fluffy particulate as the cell is left over time – this is Al(OH)3(S).
Here is the detailed chemistry behind that statement:
In the bleach battery, sodium hypochorite (NaOCl), the major constitutent
in bleach, and hypochorous acid (HOCl), a minor constituent, are reduced,
according to equations 6 and 7:
Even though there is relatively little HOCl in bleach, the latter reaction
is more favored because of its large potential of 3.93 volts.