Flow Batteries: Please Pour Me Electrons!

Scientists from the Fraunhofer Institute in Germany are carrying out serious development work in the field of electric batteries, alternative to the classical ones. With redox flow technology, the process of storing electricity is really radically different ...

The batteries, which are charged with liquid as fuel, are poured into a car with a gasoline or diesel engine. It may sound utopian, but for Jens Noack of the Fraunhofer Institute in Pfinztal, Germany, this is actually everyday life. Since 2007, the development team in which he is involved has been developing this exotic form of rechargeable battery in full swing. In fact, the idea of a flow-through or so-called flow-through redox battery is not difficult, and the first patent in this area dates back to 1949. Each of the two cell spaces, separated by a membrane (similar to fuel cells), are connected to a reservoir containing a specific electrolyte. Due to the tendency of substances to chemically react with each other, protons move from one electrolyte to another through the membrane, and electrons are directed through a current consumer connected to the two parts, as a result of which an electric current flows. After a certain time, two tanks are drained and filled with fresh electrolyte, and the used one is “recycled” at the charging stations.

While this all looks great, unfortunately there are still many obstacles to the practical use of this type of battery in cars. The energy density of a vanadium electrolyte redox battery is in the range of only 30 Wh per kilogram, which is roughly the same as that of a lead acid battery. To store the same amount of energy as a modern 16 kWh lithium-ion battery, at the current level of redox technology, the battery will require 500 liters of electrolyte. Plus all the peripherals, of course, the volume of which is also rather big - a cage necessary to provide a power of one kilowatt, like a beer box.

Such parameters are not suitable for cars, given that the lithium-ion battery stores four times more energy per kilogram. However, Jens Noack is optimistic, because developments in this area are just beginning and the prospects are promising. In the laboratory, the so-called vanadium polysulfide bromide batteries achieve an energy density of 70 Wh per kilogram and are comparable in size to the nickel metal hydride batteries currently used in the Toyota Prius.

This reduces the required volume of tanks in half. Thanks to a relatively simple and inexpensive charging system (two pumps pump new electrolyte, two suck out used electrolyte), the system can be charged in ten minutes to provide a range of 100 km. Even fast charging systems like the one used in the Tesla Roadster last six times longer.

In this case, it is not surprising that many automotive companies turned to the Institute's research, and the state of Baden-Württemberg allocated 1,5 million euros for development. However, it will still take time to reach the automotive technology phase. “This type of battery can work very well with stationary power systems, and we are already making experimental stations for the Bundeswehr. However, in the field of electric vehicles, this technology will be suitable for implementation in about ten years,” Noack said.

Exotic materials are not required for the production of flow-through redox batteries. No expensive catalysts such as platinum used in fuel cells or polymers such as lithium ion batteries are required. The high cost of laboratory systems, reaching 2000 euros per kilowatt of power, is due solely to the fact that they are one-of-a-kind and are made by hand.

Meanwhile, the institute's specialists are planning to build their own wind farm, where the charging process, that is, the disposal of the electrolyte, will take place. With redox flow, this process is more efficient than electrolyzing water into hydrogen and oxygen and using them in fuel cells - instant batteries provide 75 percent of the electricity used for charging.

We can envision charging stations that, along with conventional charging for electric vehicles, serve as buffers against the peak load of the power system. Today, for example, many wind turbines in northern Germany have to be turned off despite the wind, otherwise they would overload the grid.

As far as security is concerned, there is no danger. “When you mix two electrolytes, a chemical short circuit occurs that gives off heat and the temperature rises to 80 degrees, but nothing else happens. Of course, some liquids are not safe, but so are gasoline and diesel. Despite the potential of flow-through redox batteries, researchers at the Fraunhofer Institute are also hard at work developing lithium-ion technology ...

text: Alexander Bloch

Redox flow battery

A redox flow battery is actually a cross between a conventional battery and a fuel cell. Electricity flows due to the interaction between two electrolytes - one connected to the positive pole of the cell and the other to the negative. In this case, one gives positively charged ions (oxidation), and the other receives them (reduction), hence the name of the device. When a certain level of saturation is reached, the reaction stops and charging consists in replacing the electrolytes with fresh ones. Workers are restored using the reverse process.