Vanadium Redox Flow Battery

The only rechargeable battery where you can separately decide how big the tank is and how powerful the pump — and refill it simply by swapping the liquid.

A vanadium redox flow battery stores electricity as chemical potential energy in two separate tanks of liquid electrolyte — one holding V²⁺/V³⁺ ions, the other V⁴⁺/V⁵⁺ ions dissolved in dilute sulfuric acid. Pumps circulate the liquids through a cell stack where electrons are transferred across an ion-exchange membrane. Charging pushes the vanadium into higher oxidation states; discharging reverses the process. Because all four oxidation states are of the same element, any ions that leak across the membrane do not permanently contaminate the electrolyte — a critical advantage over other flow chemistries. The decoupling of power (determined by stack size) from energy (determined by tank volume) makes the VRFB uniquely flexible for grid applications where the ratio of storage duration to discharge rate changes with the use case.

During discharge, V²⁺ in the negative half-cell is oxidised to V³⁺ at the carbon-felt electrode, releasing one electron to the external circuit. Simultaneously, VO₂⁺ (V⁵⁺) in the positive half-cell is reduced to VO²⁺ (V⁴⁺). These two half-reactions together give a standard cell voltage of 1.26 V at 50 % state of charge; real open-circuit voltages run slightly higher (~1.4 V) due to concentration and membrane effects. Charging reverses both reactions. The membrane — typically a Nafion proton exchange membrane — passes protons to balance charge without allowing large vanadium ions to cross freely. Cell stacks are built from multiple cells in series to reach useful voltages (hundreds of volts in grid installations), while energy scales with tank volume at roughly 20–25 Wh per litre of electrolyte. Round-trip energy efficiency in commercial grid-scale systems runs 70–80 %, somewhat lower than lithium-ion but adequate for daily renewable energy time-shifting.

In plain terms

Imagine a water wheel connected to a millstone. The wheel is the battery's 'power' — how fast it can spin. The water tank above is the battery's 'energy' — how long it can keep spinning. In a normal battery, the wheel and the tank are glued together inside the same box. In a flow battery, you can make the tank as big as you like without changing the wheel at all. Need to store more energy? Just add a bigger tank.

Also called: VRFB, vanadium flow battery, VFB, all-vanadium battery.

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A short history

The concept of using dissolved redox couples to store energy in liquid electrolyte dates to NASA research in the early 1970s, where Lawrence Thaller filed the foundational redox flow battery concept (NASA TM-X-71540, 1974). The all-vanadium variant — using the same element in both half-cells to prevent cross-contamination — was invented by Maria Skyllas-Kazacos and colleagues at the University of New South Wales (UNSW), Sydney, with the key patent filed in 1986 (U.S. Patent 4,786,567). An initial 1 kW stack was assembled at UNSW in 1991. The first large-scale deployment — 200 kW / 800 kWh for load-levelling at a Japanese power station — was installed by Kashima-Kita Electric Power and Mitsubishi Chemical Industries in 1995. Sumitomo Electric became the dominant commercial supplier through the 2000s and 2010s, culminating in the 15 MW / 60 MWh Hokkaido installation in 2015. Chinese manufacturers subsequently drove the technology to multi-hundred-megawatt scale, with the Dalian Rongke project — approved in 2016 and phase-one commissioned in 2022 — establishing a new record for flow battery deployment.

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