Sci-Toys.com has a neat demo on an easy-to-build fuel cell. Platinum wire electrodes in water are first used to separate hydrogen and oxygen from water via electrolysis. Then, with the trapped gases on the electrodes, the platinum wire acts as a catalyst to reverse the process, making water out of oxygen and hydrogen and creating electricity. They have some really nice diagrams showing the formation of the ions and flow of electrons. I wont reproduce those here, but take a look at them as a resource for the electrochemistry. Sci-Toys.com sells platinum coated wire for the electrodes.
Those of you out there who wear soft contact lenses have a handy source of platinum, making it easy to run this experiment.
In our house, we always have an excess of the lens disinfecting containers: the containers last ~3 months, the solution runs out more quickly and you get a new container with each bottle. The black plastic thingy at the bottom of the lens holder is a platinum coated piece of plastic, the catalyst for the lens disinfection reaction. Two of these will be our electrodes. Note these are not great electrodes, the resistance across the face of the platinum-coated thingies is 1000 to 2000 ohms. But they are good enough to demonstrate the reaction.
Step 1: Get the Platinum Electrodes
Here are two the two platinum electrodes and two pieces of plastic straw cut to hold the platinum pieces. Cut the straws for a nice wedge fit in a glass.
Step 2: Build the Electrode Holder.
Here are the electrodes mounted in the straws. The straws act as a support and insulating separator.
Step 3: Electrodes Mounted in a Glass.
Here is the electrode assembly suspended in a glass.
Step 4: Hooking Up the Electrodes and 9V Battery Holder.
A nine volt battery will power the whole electrolysis. Note the clips and wire connections to the nine volt battery holder. Tape is used to keep the wrapped wire connections from shorting. Make sure the clips get a good grab on the electrodes.
Step 5: Add a Multimeter to Measure the Voltage.
Tape the clips to the side of the glass for stability. Insert your multimeter probes into the clips so that you can monitor the voltage. Set the multimeter scale as shown, DC voltage, two places after the decimal point. Most inexpensive meters are way too twitchy below this DC voltage setting. Note we have zero volts before the water is added.
Step 6: Add the Water.
Add tap water until you just about cover the electrodes. Notice that we are already at 0.4V by just adding the water. This is due to residual gas on the electrodes.
Step 7: Start the Electrolysis.
Connect the nine volt battery and start the electrolysis reaction. Note the meter: 9.13V. If you get this, then everything is working.
Step 8: Making Bubbles.
After a few minutes (try four to six) you will begin seeing bubbles form on the electrodes.
Step 9: Make Electricity.
Disconnect the 9V battery. You should see a voltage of ~2V which will begin slowly decaying as the gas on the electrodes is used up.
Step 10: Reuse!
But wait, those lens cases you just discarded make awesome screw-top storage case for small parts, or a container for your next blinky LED project. Just hack off the lens holder part with a pair of wire cutters. Voila!