What is a Redox Flow Battery ?


15 May 2017

This video shows the basic structure of an all Vanadium Redox Flow Battery (VRFB).

What this video does not explain is that whilst the membrane acts a separator between the solutions it also must allow protons (H+, Hydrogen ions) to pass through it – we’ll see why that is necessary later. The reason why the invention of the all Vanadium redox flow battery is a great step forward is due to the fact that if there is a small amount of diffusion of any of the solution across the membrane it will not cause the battery to stop working as Vanadium is being used as the active species on both sides.

The video describes the process during charging and then somewhat glosses over what happens during discharge – this gives us the opportunity to look at this in a little more detail.

The electrons move in the opposite direction to the conventional electrical current (they are negatively charged) so during the discharge cycle they enter the electrolyte solution on the positive side of the battery.

The reaction on the left  (positive) side of the battery is

VO2+ + 2H+ + e -> VO2+ + H2O

Vanadium in a 5+ oxidation state (Yellow) is being reduced to Vanadium in a 4+ Oxidation state (Blue).

You can see that for this to work there needs to be two Hydrogen Ions supplied along with the electron that has come from the electrical circuit – in the process one of the oxygens is liberated from the VO2+ complex and a molecule of water is produced. In an acidic solution there is a surfeit of H+ ions which can be supplied to this reaction, but over time these will get used up, the left hand electrolytes will cease to be acidic and the reaction will stop. By providing a semi-permeable membrane between the two cells through which the H+ concentration on the left side can be continuously replenished, the left hand side reaction can be kept going indefinitely. Fortunately the H+ ion, being just a single proton, is the very smallest ion that can be created, and so it is practically possible to make membranes that pass it alone.

On the other side of the battery the Vanadium is being oxidised, going from a V2+ oxidation state (Purple) in solution to a V3+ state  (Green) and liberating an electron which passes into the electrical circuit.

As a reminder for those who need to brush up on their school chemistry – oxidation is a process that might be something like a metal burning in oxygen – a metal oxide is formed in which an electron or two is moved from the metal atom (making it a positive ion) and onto the oxygen (making it a negative ion) – the metal gives up an electron and its oxidation state increases. The metal need not be passing its electron to oxygen, it could be some other species, like Chlorine, that has the ability to take electrons from the metal, so oxidation is a much more general term than just for describing reactions with oxygen.

Both reduction and oxidation happen at the same time, albeit in different parts of the battery – hence the term Redox in the name.

The left side of the battery has a potential of 1.0V and the right side -0.26V, connect both sides up with wires and you will get 1.26V generated. Put an electrical load, like a house, in the middle and you can extract energy from the battery. If you forget to switch the flow pumps on this process will stop after a while when all the electrolyte in the cell has been converted, switch the pumps on and you can run until you have completely converted all the electrolyte in your storage tanks. Depending on the size of your tanks that could be in a very long time.

Flow batteries are therefore typically described in terms of both power (eg in MegaWatts, MW) and capacity (eg in MW.hours) characteristics. For example this chinese VRFB is specified as 100MW/5000 MWh – i.e. it will be able to supply 100MW for 50 hours. That is a truly colossal battery !

Other articles will explain how recent developments in the electrolyte composition have greatly increased energy storage density and the operational temperature limits, plus descriptions of already operating VRFBs for utility level energy storage.

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