VRFBs

VRFBs, or vanadium redox flow batteries, are a type of rechargeable flow battery in which energy is stored in liquid vanadium-based electrolytes confined to external storage tanks. The electrochemical reactions occur in a cell stack separated from the tanks by a membrane or ion-exchange separator. Because the two half-cells use vanadium in different oxidation states, cross-contamination of electrolytes is minimized, allowing the same element to cycle through multiple oxidation states without mixing.

Typically, one electrolyte contains V2+/V3+ and the other VO2+/VO2+ in sulfuric acid. The electrolytes are pumped through the cell stack; ions pass through a membrane that conducts select ions while preventing mixing, enabling charge and discharge. The energy capacity is determined by the total electrolyte volume, while the power output is governed by the size and design of the cell stack.

Advantages include long cycle life, good safety characteristics due to low risk of thermal runaway, and decoupled energy and power capacity, which simplifies scaling for grid storage. The system uses a common element in both halves, reducing cross-contamination and simplifying recycling.

Disadvantages include relatively low energy density and higher upfront capital cost compared to some other chemistries. Membrane crossover and electrolyte management can cause efficiency losses and capacity fade. Ongoing research seeks cheaper membranes, higher energy density, reduced pumping energy, and longer service life.

VRFBs are used primarily for stationary energy storage applications, particularly grid-scale and renewable integration, where long discharge durations and high cycle life are valued. Several utility-scale installations and pilot projects exist worldwide, with ongoing development to lower costs and improve performance.