measure weight of the electrode before and after coating. Difference will be the mass. devide with area. that will give g/cm2.

use cv curve to calculate capacitance. meaure the area of the exposed surface. get C/cm2

specific capacitance will be C/cm2/g/cm2= C/g

you can Dissolve 5 mg of substance in 1 ml solvent and  sonication 1 h

after that take 10 M of suspension  and put on surface of electrode until dry

5mg in 1000M

      ?      in 10M 

and calculate active mass of material