Redox Flow Battries

Redox flow batteries (RFB)                                          

Redox flow batteries (RFB) represent one class of electrochemical energy storage devices. The name “redox” refers to chemical reduction and oxidation reactions employed in the RFB to store energy in liquid electrolyte solutions which flow through a battery of electrochemical cells during charge and discharge.

During discharge, an electron is released via an oxidation reaction from a high chemical potential state on the negative or anode side of the battery. The electron moves through an external circuit to do useful work.  Finally, the electron is accepted via a reduction reaction at a lower chemical potential state on the positive or cathode side of the battery. The direction of the current and the chemical reactions are reversed during charging.

The total difference in chemical potential between the chemical states of the active elements on the two sides of the battery determines the electromotive force (emf or voltage) generated in each cell of the battery.  The voltage developed by the RFB is specific to the chemical species involved in the reactions and the number of cells that are connected in series.  The current developed by the battery is determined by the number of atoms or molecules of the active chemical species that are reacted within the cells as a function of time.  The power delivered by the RFB is the product of the total current and total voltage developed in the electrochemical cells.  The amount of energy stored in the RFB is determined by the total amount of active chemical species available in the volume of electrolyte solution present in the system.

The separation of power and energy is a key distinction of RFB’s, compared to other electrochemical storage systems. As described above, the system energy is stored in the volume of electrolyte, which can easily and economically be in the range of kilowatt-hours to 10’s of megawatt-hours, depending on the size of the storage tanks.  The power capability of the system is determined by the size of the stack of electrochemical cells.  The amount of electrolyte flowing in the electrochemical stack at any moment is rarely more than a few percent of the total amount of electrolyte present (for energy ratings corresponding to discharge at rated power for two to eight hours).  Flow can easily be stopped during a fault condition.  As a result, system vulnerability to uncontrolled energy release in the case of RFB’s is limited by system architecture to a few percent of the total energy stored.  This feature is in contrast with packaged, integrated cell storage architectures (lead-acid, NAS, Li Ion), where the full energy of the system is connected at all times and available for discharge.