Pop Bottle and Carbon Paper Aluminum Air Fuel Cell

This simple foil and saltwater cell can be wired in series or in parallel to provide a scalable Al/Air fuel cell.

You can't read the numbers very well but the meter is showing .999 ( I think I caught that ). These pictures will probably get updated with better shots.

The body of the cell is a plastic water or soft drink bottle. A hole is cut into the bottle and piece of carbon paper is taped over it. The bottle is filled with an electrolyte (in this case saltwater) and an aluminum electrode is inserted into ti.

The carbon paper has a wax backing and provides a water proof (hydrophobic) barrier but is porous enough to allow air to pass. This air is absorbed by the electrolyte solution and the dissolved O2 is converted back to water by the fuel cell. This allows the creation of a very efficient yet simple cell design.

According to Wikipedia Aluminum has a potential energy density of 3.5 kW-h/kg and is one of the most readily available scrap materials around.

Step 1: Bill of Materials

This is a really simple project, at least for an individual cell. Once you begin scaling up either in volume or quantity new challenges will present themselves.

For each cell you'll need:

A soda or water bottle. Try to go with smaller bottles as you work out the size and fabrication issues.
A sheet of carbon paper. Depending on what size bottles you use a single sheet will provide multiple electrodes.
Tape - I'm using Scotch brand cellophane tape which works okay. You'll want something that secures the carbon paper to the bottle and provides an easy water tight seal. I'm experimenting with other solutions.
Aluminum - I'm using aluminum foil, the electrode could easily be cut from an aluminum can. If you use a can you must scrape the anti-oxidant from the surface with a wire brush or something.
A replacement power brush from Ace Hardware used to tap the carbon electrode. Any number of alternatives can be used but I like the power brush electrodes so...nyahh!

Electrolyte - I'm using salt. Other alternatives which will produce higher power outputs but which have not been tested in this apparatus include Hydrogen Peroxide ( 3% solution from the drug store ), Chlorine bleach, vinegar (with or without salt) and Lye.

You'll need some scissors, an X-Acto knife or both for cutting a hole in the side of the bottle and trimming the carbon paper.

Okay, got everything? Then we're off...

Step 2: Create the Gas Exchange Opening

Carefully peel the label from the bottle. Get the paper off but don't worry about the glue, it will come in handy as we cut it out.

Place a business card centered on the glue where it will stick nicely. Center it visually as best you can. Use a marker to outline the card.

Use the scissors or an X-Acto knife (razor blade whatever) and cut out this shape which will also contain most of the glue. We'll the remaing bits in the next step.

Okay now we just have to cut up some carbon paper and Bob's your uncle....

Step 3: Making the Carbon Electrode

Cut a strip from the carbon paper large enough to generously overlap the hole. As you can see I cut a 4" strip which left me about 1/2" overlap at the top and bottom of the hole.

Now cut the strip down to a length which also generously overlap the hole.

At this point you can remove the protective paper, which is exactly the size of the carbon paper and verify that it fits smoothly around the bottle as though it were a label. The paper should flow smoothly over the hole.

You will notice the carbon paper has two sides, a dull grayish side and a shiny black side. The shiny black side goes towards the inside for now. I believe performance of this cathode can be substantially improved however putting the shiny side out creates an adhesion problem with the tape.

Now place the carbon paper carefully in place and secure it with the scotch tape. Smooth the tape to make sure a good tight seal exists.

Okay all that remains is to connect the electrode and charge it with saltwater.

Step 4: Assembling the Fuel Cell

If you have a replacement power brush from Ace Hardware this step is easy. If you have chosen another method to connect your cells now is the time to figure out how to connect to the carbon paper. Copper foil or tape could be used. I didn't have much luck with a simple wire lead but there is room for experimentation.

Take a strip of aluminum foil and fold it so that it can be inserted into the mouth of the bottle as shown.

Fill a glass or bowl with two cups of warm water and dissolve a generous amount of salt into it. Stir the salt until is is completely dissolved or until no more will dissolve.

We are now ready to charge this fuel cell and begin power operations....

Step 5: Charging and Operating

Charging is a simple matter, remove the aluminum foil strip and fill the bottle to at least the top of the carbon electrode with salt water. Insert the aluminum foil and attach a meter to measure voltage on the open circuit.

Initial voltage came in about 1V but rapidly dropped into the expected range of 500-600 mV.

The use of alternate electrolytes can increase output voltage to the maximum of 1.2V per cells, current is increased by the amount of aluminum surface area which is exposed to the electrolyte.

Multiple cells may be wired in series to increase voltage, in parallel to increase current or a combination of both to scale up.