Strange. about one year, I had a homemade setup to make chlorine gas, I used a heavy concentrated NaCl solution and graphite rods (which I got from AA batteries, I think D batteries can do a better work) and worked. of course slowly, but worked. I waited about 6 hours to get something about 25ml of chlorine gas. and I never had problem with the secondary reaction. look out for the size of the electrodes. I think that can be a problem. or may be the current. because according with the faraday's law for the electrolysis, current is what matters.