In a Redox flow cell, both the anode and cathode reactions take place in the solution, which is on the surface of inert electrodes acting as current collectors. The reactants flowing across these electrodes come from containers outside the electrochemical cell; they are prevented from mixing in the electrochemical cell by a specially chosen membrane: an ion selective membrane or a micro-porous separator.
In this system, the electrochemical reactor unit is decoupled from the storage unit, leading to a decoupling between power and capacity. The available power is then determined by the following:
size of the stack,
surface area of the electrodes and, 
number of unit cells.
The available capacity is determined by the volume of electrolytes in the charged state. Both reactants and reaction products are soluble, so that there are no limiting factors associated with electrochemical morphology changes.

Diagram of Redox Flow Battery Working Principle