I prepared platinum coated graphite and I made LSV test ,I found out that the current density was too high arround -8 mA/cm2,I want to know the reason behind that and what to do to fix it?
When you say it is too high what did you compare it to?
Graphite is highly conductive due to the vast electron delocalisation within the carbon layers. Also you probably enhanced the surface area, hence the current output, by coating your platinum surface with this material.
This may be a possible reason for a higher current density when comparing your modified electrode to a bare one.