dmmIt’s easy to use chemistry to generate electricity, once you understand the basics. With this experiment, you’ll use aluminum foil, salt, air, and a chemical from an aquarium to create an air battery. This experiment is for advanced students.


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The first thing you should do is dig out your digital multimeter.  We’ll be using this to find out just how much voltage your battery cell generates (and this will also tell you how many of these batteries you need to make to power a LED or motor.)


Here’s what you need:


  • salt
  • bowl of water
  • activated charcoal (from an aquarium supply store)
  • aluminum foil
  • paper towel
  • 2 alligator clip leads


 
Download Student Worksheet & Exercises


Here’s what you do:


1. Make a saturated salt solution (dissolve as much salt as you can into water in a bowl). It’s better to have an over-saturated solution (so you still see undissolved bits at the bottom).


2. Rip off a square of aluminum foil and set it on your table.


3. Soak a sheet of paper towel in your salt solution, then gently fold in half (without tearing!) and lay on top of the foil.


4. Sprinkle a 1/2″ thick layer of activated charcoal on the damp towel.


5. Lay one alligator clip lead on the carbon (make sure the metal is exposed) in the center of the layer so it makes good contact with the carbon. Clip a second clip lead to the bottom layer of foil. Note that the two clip leads should not touch! And be sure the top clip lead is completely surrounded by the charcoal and has no chance of touching the aluminum foil.


6. Fold your layers over in thirds. Clip the two leads sticking out to your digital multimeter and check the voltage. If it’s higher than 2.5V, then try attaching an LED or motor.  If it’s less than 2.5 volts, you’ll need to make a second (or third) battery and hook them up together to generate enough power to light stuff up.


7. Squish the battery to make good contact between the carbon, salt, and foil. Your voltage should change when you do this.


8. Use your digital voltmeter to measure your voltage again. Make a second battery and hook the new one’s positive to the first one’s negative terminal.  The two wires left are your leads to connect to the multimeter (or LED).


How does it work?

Most homemade batteries light up LEDs and a few flashlight bulbs, but dimly. It takes a lot more current to get a motor spinning. So what gives?


This ‘aluminum air battery’ uses a chemical reaction between the foil and air (well, specifically the oxygen in the air). The combination of oxygen and foil produces aluminum oxide and energy. If you build your battery well, you can see the energy in your turning motor shaft, but the oxide layer will be invisible to your eye. Your battery should last between 4 – 10 minutes, depending on how well you built it. You can get a larger amount of voltage by using larger wires (with more surface area contacting the charcoal). What do you have that would be a larger electrode for the battery?


The more salt you use, the better your air battery will work! (You’ll notice there’s a point, though, where no matter how much more salt you add, you can’t increase the voltage… due to the saturation point of the water. Have you tried changing the temperature of the water to increase the capacity?)


Exercises


  1. How many air batteries does it take for your LED to light up?
  2. Which electrode is positive? Which is negative? (Hint: Use the DMM to figure this out.)
  3. What is the electrolyte in this experiment?
  4. What could you use instead of an exposed alligator clip lead to make this battery last longer?

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