SOME INFO TO How to Make / DIY GRAPHENE / GRAPHITE PLASTIC
You need a really ULTRAFINE powder of graphite or better a intercalation graphite. The polystyrene is only like binder and for this reason you need to make the right concentration of graphite powder/polystyrene (add more graphite). And to finish,... if you got the possibility to using a presses machine. I use a press machine and I press the electrodes with couples of bars. The graphite in polystyrene cannot conduct very well if it ist not good compressed!
ALTERNATIVE: Try to make some bioplastic from milk! In warm milk you add some acid and graphite powder! But dont forget to press the material.
Simiral binder: polyurethane