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?

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    $\begingroup$ Tap water leaves room for imagination due to possible side reactions occurring with the dissolved salts and even organics. Ideally you want to use distilled water for any electrochemical experiment. $\endgroup$
    – andselisk
    Apr 1 '21 at 6:42
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    $\begingroup$ If sodium bicarbonate is dissolved in water, it produces $\ce{HCO3-}$ ions which are in equilibrium with $\ce{CO2}$ according to the following equilibrium $$\ce{2HCO3- <=> CO3^{2-} + CO2 + H2O}$$ This is the origin of the $\ce{CO2}$ engaged in your equation. $\endgroup$
    – Maurice
    Apr 1 '21 at 9:57
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    $\begingroup$ So are there $\ce{CO2}$ molecules everywhere in the solution? $\endgroup$
    – Ferid9
    Apr 1 '21 at 10:00
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    $\begingroup$ Yes. CO2 molecules are everywhere in a $\ce{NaHCO3}$ solution. $\endgroup$
    – Maurice
    Apr 1 '21 at 10:54
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