Hi vpwelch,
Welcome to the forum!
Just for everyone's reference, here's a link to the experiment you named:
https://www.sciencebuddies.org/science- ... p002.shtml
When you are operating the fuel cell in reverse (electrolysis) mode, this is what happens:
--At the anode, water is broken down into oxygen gas, protons, and electrons.
--Protons and electrons are forced to take different paths to the cathode: the electrolyte membrane conducts only protons, and a wire conducts only electrons.
--At the cathode, protons and electrons combine into hydrogen.
The overall reaction is 2H2O(l) → 2H2(g) + O2(g). So, if you're producing hydrogen, you must also be producing oxygen (half as much by volume).
http://en.wikipedia.org/wiki/Electrolysis_of_water
My best guess as to what is going wrong is that the oxygen tube or storage tank is leaking. You could try switching the tubing and tanks temporarily to see if you then observe the accumulation of oxygen but not hydrogen. I'd also recommend carefully inspecting the tubing, tanks, and any other parts the oxygen would travel through for cracks or other damage. If the leak is occuring at one end of a tube, you might be able to correct it by wrapping plumber's tape around the post before connecting the pipe to it.
http://homerepair.about.com/od/plumbing ... ools_5.htm
It might also be a good idea to examine both electrodes for any signs of corrosion or degradation. One reason for using distilled water is to reduce the concentration of contaminants that could facilitate side reactions. I think it's possible that the anode could be converted into a metal oxide (rusting is an example of this) while continuing to release hydrogen, but that reaction would not go on indefinitely -- eventually you wouldn't be getting any hydrogen either.
Best wishes,
Amanda