I use $\pu{12 V}$ battery with copper wires and tap water with $\ce{NaHCO3}$ dissolved in it. I connected the battery and everything went as expected but after 5 minutes bubbling stopped. I took out the copper electrodes and saw green deposits on the anode that were blocking the current.
I think green stuff was malachite formed by the reaction
$$\ce{2 Cu + O2 + H2O + CO2 -> Cu2(OH)2CO3}$$
because there is oxygen in the anode water in solution copper in the wire but how did the $\ce{CO2}$ get there? I know bicarbonates hydrolyze to give carbonic acid which gives $\ce{CO2}$ but doesn't that only happen at the surface? Any ideas what that green thing might be?