Construction and function of the vanadium redox flow battery

The vanadium redox flow battery is an electrochemical flow reactor with series-connected electrochemical cells. Each individual cell is internally divided into two half-cells by a membrane. The half-cells are each supplied in parallel with fluid, the energy carrier - also known as electrolyte. On charging, energy is supplied causing the fluid to be chemically altered in an oxidation/ reduction reaction. On discharge the chemical energy of the fluid is released in the reverse reaction and electrical energy can be drawn from the electrodes. When in use the liquid energy carriers are continuously pumped in a circuit between reactor and storage tanks. Electrical energy is supplied to the reactor via a charger and provided to the load via an inverter. The liquid energy carrier contains sulphuric acid with dissolved vanadium salts in a range of oxidation states

At the electrodes the following reactions take place:

At the positive electrode:           VO₂⁺ + 2H⁺ + e^- = VO²⁺ + H₂O

At the negative electrode:          V²⁺ = V³⁺ + e^-           

Overall:                                       VO₂⁺ + 2H⁺ + V²⁺ = VO²⁺ + H₂O + V³⁺

Thermodaynamic properties

• Cell voltage: 1.0 V discharged to 1.6 V charged

• Practical specific energy: 15 to 25 Wh/ kg depending on the vanadium concentration in the electrolyte

• Direct reading of state of charge (SOC) via open circuit voltage (OCV) due to electrode reactions in liquid solution

Special features of the technology

• Energy is stored in liquid energy carriers. The energy storage is given by the mass of liquid
• Power is governed by the size and number of electrochemical reactor cells
• Electrochemical reactor cells are connected in series and/ or parallel to define the limiting voltage and current
• Direct reading of the state-of-charge (SOC)
• High safety and reliability, through the relatively low specific energy
• The energy carriers have an almost unlimited lifetime and can be reused after a reconditioning process

These special features allow a high degree of modularity in the battery configuration and properties:
Modularity of design arises through the independence of energy and power. Modularity of behaviour is achieved through temperature management, pump speed control and activating/ deactivating modules to meet the load.