just give my little opinions~
i agree with Borek.
you better use NaOH or H2SO4.
When NaCl solution is dilute, H2O will be decomposed during electolysis because the OH- in H2O more likely to carry out oxidation than Cl-.
However, when electrolysis has been carrying for some times, the NaCl solution become concentrated because water is decomposed in to O2 and H2.
At that time, Cl- will more likely to carry out oxidation than OH- because of the concentration effect
So Cl2 will form.
**P.S. If i have get something wrong, please notice me. THANKS