According to millward:
I was wondering if anyone out there might be able to tell me how these water clocks work. All you do is fill it up with mildly salty water, and that's what it runs on. The one I've got has been keeping good time for about 5-6 days now. Once in a while a little bubble comes up, so it must be producing some kind of waste gas. I'd like to build my own circut based on whatever devices run this clock, just for fun. Would anyone know what kind of electronics run on water? And where I could get them? I didn't buy my clock through the net, but it's selling at: http://www.computergear.com/waterclock.html All the site says about how it runs is: "The fuel cell extracts electrons from the electrolyte forming a steady stream of electrical current that drives the clock." That's all very well, but it doesn't tell me what the name of the fuel cell is, where I can get them, and how I can build a curcit based on the fuel cell's power.
I would assume it's just your basic electro-chemistry at work here. You put a couple electrodes in a saline solution or other liquid with electrolytes in it (e.g. soda pop, Gatorade, etc.), and you'll get electricity being produced. You can also get the same results by sticking electrodes into a potato, lemon, etc.
Granted, the amount of current you get is very small, but it's enough for the typical low-power LCD digital clock. There's nothing special about the electronics, other than the fact that the power consumption has to be very low.
I don't remember the details of the electro-chemical reactions, but the electrolytes react chemically with the metal electrodes to produce the electricity. Depending on the specific reaction, yes, you could get some gas produced as a by-product. You also end up with compounds being deposited on the metal electrodes, or with something being eaten away from the electrodes, so eventually they'd have to be replaced too, or the reaction stops.
Anyone with a better recall of basic chemistry? (It's been too many decades since I've looked at this.)