Vanadium Redox
Flow Technology


Vanadium redox flow technology was developed by the US NASA in the 1970s. With its international team of experts, VoltStorage has now taken resource-saving storage technology to a new level in recent years. The characteristic feature of vanadium redox flow technology is the storage of electrical energy in a vanadium-based electrolyte liquid. This storage technology is very climate-friendly because it does not require any rare materials or conflict raw materials, is fully recyclable, and also has a high level of operational reliability and a long lifetime.

Operating principle


The storage medium is a vanadium-based liquid electrolyte. It is stored in two separate tanks takes on different oxidation states through a redox process. This process makes storing energy possible.


The electrolyte flows through the battery cells in two separate circuits. When the battery is charged, the electrolyte is reduced in the negative half cell and oxidized in the positive half cell. When discharging, this process is reversed again.


A special pump system flows the electrolyte into and through the battery cells. This makes it possible that not all the electrolyte has to be kept in the cells, enabling the decoupling of energy capacity and power. The pumping system is only activated on demand.

Positive electrolyte

Discharged, the positive electrolyte circuit contains vanadium ions with an oxidation state +4. In this state the electrolyte exhibits a blue colour. During charging, the vanadium ions oxidize and take on the oxidation state +5. When the storage system is fully charged, the positive electrolyte circuit only contains vanadium +5. The electrolyte then has a yellow colour.

Negative electrolyte

Discharged, the negative electrolyte circuit contains vanadium ions with an oxidation state of +3. In this state the negative electrolyte exhibits a blue-green colour. During charging, the vanadium ions reduce and take on the oxidation state +2.
When the storage system is fully charged, the negative electrolyte circuit only contains vanadium +2. The electrolyte then has a purple colour.

Battery stack

The positive and negative electrolytes are pumped through special battery cells for the charging and discharging processes. Each battery cell consists of two half-cells separated by a selective membrane. The membrane is ion-permeable so that during charging and discharging, freed-up ions can migrate through the membrane into the other electrolyte circuit. However it is selective, so that the vanadium ions can not pass. Inside of each half cell, the electrolytes undergo redox reactions which ensure that electrical energy is converted into chemical energy and stored.

Pumping system

A pump system moves the positive and the negative electrolytes within each circuit, supplying the battery cells with fresh electrolyte from the storage tanks.

Redox-Flow Batteriezelle von VoltStorage

Bipolar plate

The bipolar plate electrically connects the cells of a redox-flow battery stack. It thus provides the electrical conductivity, which is increased from a material standpoint by using particularly conductive and corrosion-resistant materials, such as graphite.

Ion exchange membrane

The transparent and thin ion exchange membrane separates the positive and negative half cells. The membrane ensures selective ion exchange between the half-cells, preventing transfer of vanadium ions between half cells, which is necessary for efficient charging and discharging.

Graphite felt

The electroactive area is located in the centre and is completely filled with a graphite felt. The fine-meshed felt has a very large surface area and thus improves the electrochemical redox process while at the same time allowing the electrolyte to flow through. Direct contact with the ion exchange membrane results in a charge exchange between the differently charged half cells.

Cell frame with electrolyte channels

The cell frame of the redox flow battery cells from VoltStorage contain many electrolyte-guiding channels. These channels have been specially designed to ensure uniform supply of electrolyte to the electroactive area of the battery cells.

Vanadium redox flow battery cell

The cells of a vanadium redox flow battery each consist of two half cells. Each half cell contains a frame with specially designed channels to ensure evenly distributed electrolyte supply to the entire electroactive area. The electroactive area is located in the centre of the half cell and is completely filled with graphite felt. The half cells are separated by an ion exchange membrane. Each battery cell is enclosed by two bipolar plates.

technological benefits


With the vanadium redox flow technology, we rely on a proven CO2-reducing battery technology. A study by the Technical University of Munich has shown that VRF batteries produced by VoltStorage result in up to 37% less CO2 emissions than the production of comparable lithium batteries.


The vanadium redox flow technology does not require any rare raw materials. The vanadium used in the storage medium is obtained as a by-product of iron production – and thus without overexploitation of nature and the associated impact on our ecosystems.

100% non flammable

Operational safety plays a key role in energy storage. The vanadium redox flow technology offers a decisive advantage in this respect: The vanadium-based electrolyte consists exclusively of non-combustible components, and is mostly pure water.

VoltStorage Battery Stack and Engineer

Pioneering innovations

VoltStorage continuously strives to challenge the status quo and set new standards in storage technology. With the development of the resource-saving vanadium redox flow storage technology, VoltStorage is making an important contribution to the development of groundbreaking solutions for Long Duration Batteries. You can find out more about VoltStorage’s current research and development in the Innovation Lab.