You need charge transfer, ohmic and diffusion resistances to calculate internal resistance. Because internal resistance consists of these three components. First of all you should perform an electrochemical impedance spectroscopy analysis with a potentiostat. You can check the relevant papers given below;
Sekar, N., & Ramasamy, R. P. (2013). Electrochemical impedance spectroscopy for microbial fuel cell characterization. J Microb Biochem Technol S, 6(2).
He, Z., & Mansfeld, F. (2009). Exploring the use of electrochemical impedance spectroscopy (EIS) in microbial fuel cell studies. Energy & Environmental Science, 2(2), 215-219.