Why you can not use the mass of your electrode material? If you do not use can only calculate the capacitance, but without knowing it per area unit.

I think a controlled experiment is needed.

Then you can compare the difference in capacitance and know exactly the contribution from your active material. Hope that my answer is in a right way to answer your question. How you prepared your electrode/terminal? Normally, the capacitance is calculated based on either mass or area...

Ali, ask yourself what key value you are interested in and what it is telling you. To get the capacitance the mass is never needed. However, in order to relate this value to say other powders, the mass and BET are needed. Normalized values can be compared.