Redox flow battery makes inroads

Li-ion batteries are dominating the stationary energy storage sector, but a growing number of companies are developing different types of technologies to be competitive.

The Redox Flow Battery (RFB) system is one of the main competitors approaching the market, says IDTechEx which did an in-depth analysis about them.

“While it’s clear that li-ion batteries will dominate the scene for the time being, this promising competitor has already shown its capability to gain its portion of the energy storage market, pushing the ES industry toward new horizons.”

The RFB is based on flowing two liquids, the electrolytes, across the electrode.

In this way, the electroactive species contained in the electrolyte react on the electrode surface generating electricity. Therefore, the larger the amount of electrolyte, the bigger the amount of energy stored in the battery, while the power output of an RFB is defined by the size of the electrode stack and the kinetics of the chemical reaction, IDTechEx explains.

A further advantage of RFB compared to li-ion batteries is provided by the decoupled energy and power capacity, which allows bigger customisation of this kind of system.

The vanadium redox flow battery (VRFB) initially studied by NASA, and further developed in the 1980s by a research group led by Maria Skyllas-Kazacos at New South Wales in Australia, is today the most studied and manufactured technology within the RFB technology, IDTechEx adds.

“Besides different types of RFBs, the vanadium technology (and similarly the all-iron RFB) employs the same electroactive species (vanadium) in both electrolytes, with different oxidation states.

“The low round-trip efficiency and the high cost of vanadium (directly affecting the cost of the entire system) are two of the main VRFBs’ drawbacks. The electrolyte alone accounts for ca.30% to 40% of the overall technology cost. To reduce the effect of vanadium cost on the overall system, an increasing number of companies started collaborations with vanadium mining companies.”

South African mining company Bushveld Minerals is actively promoting the adoption of vanadium flow batteries all over Africa. Outside of the African countries, Bushveld backed the merge between two VRFBs manufacturers (the English redT and the American Avalon Battery), and in 2019 announced its participation in the purchase of a consistent share of Enerox-CellCube, previously known as Gildemeister AG.

Besides the vanadium flow batteries, the zinc-bromine (ZBB) technology is the second in the rank of adopted technologies.

Technically different from a VRFB, ZBB is based on a solid zinc electrode and a liquid bromine electrolyte, which gives this technology a higher energy density than VRFBs.

With the Australian Redflow, and the America Primus power dominating the scenario, this technology is slowly acquiring its share of the flow battery market.

Developed by the American company ESS Inc, the promising all-iron RFB is characterised by the use of iron as electroactive species in both electrolytes (catholyte and anolyte). The low-cost iron electrolyte brings a large advantage in terms of cost, although other limitations, such as hydrogen evolution, affect these kinds of batteries.

Another type of flow battery is the organic flow one (ORFB), technically similar to a VRFB but employing an organic electrolyte.

While the vanadium flow batteries employ an inorganic electrolyte based on vanadium salts, the ORFB – aka ‘Metal-free Flow Battery’ – are based on organic electrolytes such as quinone, pyridine, or ‘TEMPO’ (2,2,6,6-tetramethyl-1-piperidinyloxy) molecules.

Different companies are developing this kind of flow battery, including the German Jena Batteries, the French Kemiwatt, and HIGREEW European projects.

The hydrogen-bromine flow battery is one of the future redox flow batteries approaching the market.

Dutch company Elestor is behind the development of this technology, which in 2019 received a multi-million investment from the Dutch conglomerate Koolen Industries, in addition to the EIT Innoenergy, already an early investor in Elestor.

Moreover, the company is involved in the European MELODY project, related to the development of a sustainable redox flow battery technology that can effectively reduce the costs of electricity storage to support large-scale.

The development of new redox flow battery chemistries has the main target of decreasing the capital cost of the technology, making these systems more appealing for the stationary energy storage market, targeting four hours of storage and above.

While a li-ion battery system is clearly capable of addressing this storage time, longer storage times simply mean a different type of market addressed, where Li-ion battery systems may step out from their cost-effectiveness, hence making redox flow battery and other technology more competitive.