It is found experimentally that the potential of an electrochemical cell with applied current density varies as follows:
To produce the passage of current through a cell it is necessary to apply a potential difference that exceeds the equilibrium f.e.m by at least the overpotential of the cell. Furthermore, the potential of the cell will decrease as it generates current, because under such conditions it no longer works reversibly and therefore does less work than the maximum. When the current is high and close to the limiting value of one of the electrodes, a sharp drop in the work potential is observed.
I know this is what is observed, but I can't explain why. Why does this happen?