As to the specific electrolytes in a potato, there are certain to be sodium (Na⁺), potassium (K⁺), and chloride (Cl^-) ions present. There are lots of more complex ions also present in living tissue--you may want to consult a biochemistry textbook--but since they can't move as fast as the smaller sodium, potassium and chloride ions, they probably aren't directly involved in conducting electricity.