Electric cars are finally becoming a reality. In recent years we can regularly see Nissan Leafs and Teslas driving around the city. As the result, the search intensifies for a battery that would give an electric car a similar drive range to the existing gasoline powered cars, while not costing as much as an airplane ride.. Many older battery manufacturing technologies are being resurrected and re-considered. Media reports about new battery technology “breakthroughs” that promise “x times” more energy compared to existing batteries usually do not mention that the present research about the product is usually only a tiny improvement to a development that was done in the 1960’s or earlier. For example flow batteries continue to be mentioned in the media as a revolutionary new idea. A recent article “GE Flow Battery Aims For 240-Mile EV Range… And Beyond” highlights GM development as a novelty, while in fact only the redox material was changed without any particular changes to the flow battery technology.
But what is a flow battery anyway? It is any electrochemical battery power source consisting of two parts, which are interacting to release energy. First part called “cathode” contains an oxidizer (a material that strives to take away electrons, for example manganese dioxide in a primary battery). The second part is called an “anode,” and contains fuel (material that likes to giveaway electrons, like zinc for example). If these materials mix together, they would just burn, releasing heat, but no useful work. But just like building a dam on a river allows to direct the energy flow to the turbines to generate electricity, restricting the reaction in a particular way allows one to use the flow of electrons between the materials to do the useful work of powering our device of choice. In batteries, we allow electrons to flow only through the external load, but not directly from material to material. The same way, on other side, the same number of ions is flowing between materials in opposite direction to keep overall charge neutrality between cathode and anode.
Once reaction has finished in a primary battery, materials have to be discarded because they usually lose their electric and ionic conductivity, which are both needed for the above scheme. But in rechargeable batteries, materials don’t change their mechanical and electric properties too much; and for that reason the flow of electrons can be forced in the opposite direction by power applied using an external energy source. Material can recover their original state so the cathode can absorb electrons In most batteries anode and cathode are solid materials, which are fixed to current collectors; and capacity of the battery is determined by the size of the current collectors. On the other hand, fuel cells (where anode uses actual fuel like hydrogen or methanol) has their cathode as oxygen from the air. The capacity is determined by the size of the tank with fuel, since fuel is being constantly pumped to anode current collector as it is being used up. Fuel cells do provide a longer range of capacity, but you will need a larger tank for the fuel. Most fuels are expensive (hydrogen for example) and not easy to get. The problem with a fuel cell is that it is not rechargeable, so you have to go to fuel station to pump the fuel once it runs out. What if you have source of electricity but no station with hydrogen cylinders? That is where the flow battery comes in.
In a flow battery both oxidizer and the fuel are not attached to the current collectors, but dissolved in a solvent (in most cases water). Both are stored in separate tanks. If you use up the small amount of material that is close to current collectors, the pump brings provides more for the tank and dumps the old solution into another tank. The flow battery is rechargeable because current direction and direction of pumping can be reversed to recover cathode and anode materials in original corresponding oxidizing and reducing states. This way you have both of both worlds – cheap tanks of any size like in a fuel cell, the ability to quickly refill the battery if external supply is available, and the ability to recharge the battery.
Fig. 1 Vanadium redox flow battery
But wait there's more...in Part 2 we will learn more about the history of flow batteries and future development.