Energy storage is essential in view of the rapidly growing demands for low cost energy based on sustainable resources. Redox flow batteries have the capacity to store the energy generated during periods of larger production and low demand.
Redox flow batteries (RFB) involve the flow of a liquid electrolyte through an electrochemical cell. The energy is stored in the active materials contained in the electrolytes, normally contained in separate tanks. The electrolyte is pumped through a stack containing a series of single cells with two electrodes each separated by an ion-exchange membrane. The size of the electrolyte storage tanks determines the energy rating while the size of the battery stack determines the power rating of the system. Redox flow batteries often use an ion-exchange membrane similar to that of fuel cells; hence they are called regenerative fuel cells sometimes. Typically power densities are 18 W kg-1 and energy density of 16 W h kg-1, with 70-80% DC-DC efficiency and 65-72% AC-AC efficiency.
Challenges in the zinc-cerium RFB include leakage of species across the ion-exchange membrane causing mixing of the respective active components of the electrolytes, achieve long cycle life and improving energy efficiencies. The formation of zinc dendrites during the charging cycle limits the cycle life to about 1500-2000 cycles.