|Working principle of a redox flow battery. Source: Jens Noack, Fraunhofer ICT. Click to enlarge.|
Researchers from the Fraunhofer Institute for Chemical Technology ICT in Pfinztal near Karlsruhe are developing improved redox flow batteries for automotive applications in an attempt to address storage capacity and charging time limitations of current Li-ion battery solutions for electric vehicles.
Redox flow batteries (RFB) are chemical energy storage devices, and use dissolved redox couples (an oxidizing agent and a reducing agent in an electron transfer process) held in separate external tanks; electricity is converted in a separate power module. The two fluid electrolytes containing metal ions flow through porous graphite felt electrodes, separated by a membrane which allows protons to pass through it. During this exchange of charge a current flows over the electrodes, which can be used by a battery-powered device.
|Experimental cell design for a Vanadium Redox Flow Cell. Source: Jens Noack. Click to enlarge.|
During discharge the electrodes are continually supplied with the dissolved substances from the tanks; once they are converted the resulting product is removed to the same or another tank. The devices could theoretically be recharged at a station in a few minutes. “The discharged electrolyte is simply pumped out and replaced with recharged fluid. The pumped-off electrolyte can be recharged at the gas station, for example, using a wind turbine or solar plant,” says engineer Jens Noack from ICT.
A number of different redox couples are possible; Fraunhofer has been doing some detailed work with conventional vanadium redox flow batteries for a number of years. For the new system, Noack says, “We are using different redox couples than in well-known redox flow batteries, but at this moment we can’t say what it is in detail.” Noack presented Fraunhofer’s work on conventional vanadium redox flow batteries at the just-completed 216th meeting of the Electrochemical Society in Vienna, Austria.
Redox flow batteries theoretically offer a number of advantages:
- high energy efficiency >75 % (> 95% found on lab scale)
- long calendar life, excellent cycle ability (> 10,000)
- flexible design
- fast response time
- overcharge and over discharge tolerant
- low maintenance costs
- low self discharge or no discharge depending on pumping of electrolyte
Up to now, however, redox flow batteries have had the disadvantage of storing significantly less energy than lithium-ion batteries. The vehicles would only be able to cover about a quarter of the distance—around 25 kilometers (15 miles)—which means the driver would have to recharge the batteries four times as often.
The Fraunhofer team has increased the capacity four or fivefold, to approximately that of lithium-ion batteries, according to Noack. The team has already produced the prototype of a cell, and is in the process of assembling several cells into a larger unit and optimizing them.
This work is being carried out with colleagues from the University of Applied Sciences, Ostphalia, in Wolfenbüttel and Braunschweig. They are testing electric drives and energy storage units on model vehicles that are only a tenth of the size of normal vehicles. The research team has already built a traditional redox flow battery into a model vehicle. A vehicle on a scale of 1:5 is on display on a test rig set up at the eCarTech in Munich (Hall C3, Stand 424) from 13 to 15 October.
In the coming year the researchers also want to integrate the new battery, with four times greater mileage, into a model vehicle.
Jens Noack and Jens Tübke. A Comparison of Materials and Treatment of Materials for Vanadium Redox Flow Battery. The Electrochemical Society 216th Meeting (October 2009, Vienna)