Question:
I'm looking for some commonly available electrodes to do a water
electrolysis. I've tried Aluminum, copper, tin and all of them seem to
disintegrate rather quickly. Any suggestions? Oh, as well, I have been
using NaCl and/or CaCl for electolytes. Could this be the problem?
Answer:
-Congratulations! You have discovered electrochemical etching. Seriously,
that is a method of machining delicate structures in difficult metals (not
too popular nowadays).
Chloride electrolytes will rapidly corrode anodes made from ordinary
metals. More "noble" materials will allow formation of chlorine at the
anode. Definitely not a nice result when you were expecting oxygen. Try
using sulfates rather than chlorides.
For electrode materials, try carbon for the anode (+). Anodes also can be
made from gold and platinum, but why spend the money? The cathode (-) can
be either carbon or stainless steel. These should resist corrosion enough -Copper is the most noble cheap metal, and it is not nobel enough to
electrolyse water. It is (more of less) fine for the cathode, where
hydrogen is evolved. The anode is what causes problems. For the anodic
cathode, I have used the lead from a mechanical pencil. First I slit
the insulation of a copper wire, cut of about half the strands to
make more room, then twist the remaining strands around the "lead".
pull the insulation back over it, seal the slit with super glue,
and seal the end where the lead emerges from the insulation with
superglue, making sure no copper is exposed. After a couple hours
(giving ~20 ml of H2 + O2), the electrode has deteriorated to
a spongy mass that easily falls apart.
If you use chlorides for electrolytes, the anode will produce Cl2
rather than O2. In basic conditions, the Cl2 will react with
HO- to yield OCl- (hypochlorite) and Cl- and H+, not yielding any
anodic gas. I used MgSO4 (epson salt) for an electrolyte. This
seamed to work well, except the cathode (copper wire) became fouled
with a colorless, waxy substance which I couldn't identify.
