When galvanic cell operates or when current starts flowing in a circuit, voltage drops from electromotive force of the cell (EMF) because of internal resistance of the battery.
There are three sources of internal resistance or sources of polarization:
- Ohmic polarization
- Activation polarization
- Concentration or Diffusion polarization
Working voltage of the cell is given by following equation: $$ E = EMF - I(R_o+R_a+R_c) $$
where R represents polarizations mentioned and I is current in the cell.
EMF of the cell is created because of charge separation in electrochemical double layer on anode and cathode due to electrochemical processes of certain species or redox systems present. More charge is separated, bigger the absolute value of electric potential developed on electrode. EMF of the cell can be calculated from Nernst equation if equilibrium is established on both interfaces/electrodes.
My question is how do these three types of polarizations mentioned affect amount of charge separated on interfaces when current starts flowing in a circuit physically speaking? What processes happen on electrodes so that voltage of the cell drops from EMF value?
For diffusion polarization that is quite clear since if activity of some species like ions is different close to electrode compared to bulk of electrolyte if diffusion is slow process, from Nernst equation electrode potential is different compared expected value calculated from activity of species in the bulk.
If we look at ohmic polarization it seems that electrode potential developed is affected by ohmic resistance of conductors in the cell. Since voltage of the cell drops, this would mean that electric potential of cathode decreases while of anode increases compared to Nernst equation values. Why is it that potential of cathode drops while of anode increases?
Activation polarization represents resistance due to kinetics of electrochemical reactions. It also decreases voltage of the cell. Slower the reaction, bigger the voltage drop of the cell compared to EMF. What does activation resistance have to do with charge separation in double layer and why does it decrease cathode potential and increase anode potential?