h1

Build your own burrito battery

November 20, 2007

Up until now, the best way to MacGyver a battery has been to stick a copper wire and a galvanized nail into a lemon (or an apple, or a potato, or…). Fruit or vegetable batteries produce a respectable voltage (around a volt) but only a tiny amount of current- not enough to light a flashlight bulb.

Now there’s the “burrito” battery. It produces 400 times as much current as a lemon battery- enough to actually light a little holiday lightbulb, or run a small DC motor. All you need is some duct tape. And a piece of aluminum foil, some table salt, a paper towel, some activated charcoal, and some copper wire to use for leads. You make a “burrito” with the aluminum foil as the tortilla. Line the foil with the paper toil, moistened with salt water. Fill the burrito with activated charcoal, also moistened saturated salt water. Sink one copper lead into the charcoal and duct-tape the other to the aluminum foil and voila! You should have about 1 V, with 100 milliamperes of current. Attach several cells in parallel for more current, and in series for more voltage!

You can find a step-by-step guide for building the battery at http://www.exo.net/~pauld/activities/AlAirBattery/alairbattery.html. The authors of that page have published a nice classroom activity sheet to go along with the battery in the December 2007 issue of the Journal of Chemical Education (M. Tamez, J. H. Yu, J. Chem. Ed. 2007, 84, 1936A-1937A).

How does it work?

All batteries work by running chemical reactions that release electrons in one place, and capture electrons in another. When you run a wire between these two places, an electric current flows through it. You can tap off some of the energy of that current to light a bulb or drive a motor.

In the aluminum-air battery, the aluminum in the foil is the electron source. Aluminum on the surface of the foil reacts with hydroxide ions in the salt water to form aluminum hydroxide. Every aluminum atom that reacts releases three electrons into the foil:

Al(s) + 3OH(aq) –> Al(OH)3(s) + 3e

This is called an oxidation (a reaction that involves a loss of electrons).

Air absorbed in the nooks and crannies of the activated charcoal acts as the electron sink. Specifically, oxygen gas captures electrons using this half reaction:

O2(g) + 2H2O(l) + 4e –> 4OH(aq)

This is called a reduction (a reaction that involves a gain of electrons). Notice that the activated charcoal doesn’t actually do anything in this reaction. It just provides a nice place for the reaction to occur. It has a high surface area, so it is in contact with a lot of oxygen molecules. It is also able to shuttle electrons from the copper wire to oxygen absorbed on its surface.

The battery won’t work unless the oxidation and reduction reactions can work together. By itself, the foil will build up a positive charge as it loses electrons. It’s hard to pull negatively charged electrons from the positively charged foil, so the oxidation reaction will stop if the charge on the foil isn’t neutralized somehow.

Similarly, if you keep dumping electrons into the oxygen in the charcoal, a negative charge would build up. The reduction reaction will shut down, because you couldn’t force any more negatively charged electrons onto the negatively charged surface of the charcoal.

This is where the wet, salty paper towel comes in. The towel acts as a salt bridge that prevents charge from building up on either the foil or the charcoal. Chloride ions in the salt (Cl) move towards the foil, neutralizing the positive charge buildup there. Sodium ions (Na+) migrate towards the charcoal, neutralizing the negative charge buildup caused by the reduction.

Advertisements

4 comments

  1. […] about here? Must have been one hell of an explosion. The last time I checked, I could rip through aluminum foil with my bare hands. But hey, technology moves so fast these days that the kids can barely keep up […]


  2. are there any alternatives to activated charcoal?


  3. The charcoal just provides good electrical contact with air.



Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: