Electrolyte is added so that the reaction will go at a nice rate using a low current.
Acetic acid is a weak acid, and household vinegar is only 3% acetic acid. It doesn't work;
the electrolysis is just too slow.
Sulfuric acid works well. The trouble with a lot of other acids is that they undergo side reactions. For example, hydrochloric acid is easy to get (it's in brick-cleaning solutions) but you'll get chlorine
gas (which is very poisonous!) from positive electrode. Try a 1.0 M solution of Na2SO4. The precise concentration doesn't matter. That's safer
than sulfuric acid but still works well. You can buy sodium sulfate as 'Glauber's salt' in a drugstore.
You should use an inert metal or you'll drive side reactions that involve the metal. Platinum wire is a favorite
electrode material for this reason. You might try nichrome wire; it's a lot cheaper than platinum.
As you found out, copper won't do.
You were seeing the hydrogen come off on the negative electrode (4 H2O(l) + 4e- = 2 H2(g) + 4 OH-(aq)) but instead of oxygen at the positive electrode (2 H2O(l) = O2(g) + 4 H+(aq) + 4e-), you were getting Cu(s) = Cu2+(aq) + 2e- at the other
electrode. The latter reaction is easier, and when the positive electrode is made of copper, it occurs preferentially.
You should set up five six-volt lantern batteries in series to supply the current. Use anything less, and the reaction occurs
You might try doing sulfuric acid with water in one setup, and sulfuric acid with household hydrogen peroxide in the other;
that way you can show how the composition of water and H2O2 differ.
Author: Fred Senese firstname.lastname@example.org