The “Redox” Principle

But how does a redox flow cell work? Why is it better than conventional accumulators? And what differentiates nanoFlowcell® from simple redox flow batteries?

The principle of the flow battery, also known as the redox flow cell

Redox flow batteries (red for reduction = electron absorption, ox for oxidation = electron release), also known as flow batteries or liquid batteries, are based on a liquid electrochemical storage medium. The principle of the redox flow battery was patented in 1976 for the American space agency NASA. Its aim was to drive the rapid development of energy storage systems for space travel. The 1976 patents have long been open and are being extensively applied. Redox flow batteries are seen as highly promising for future use as an extremely simple and effective way of storing electrical energy. The first stationary redox flow installations are already integrated into the domestic electric infrastructure, largely as buffer batteries or reserve sources for uninterrupted electricity supply in the use of solar and wind-power plants.

In contrast to lead batteries or lithium-ion batteries, redox flow batteries store energy in liquid electrolytes. The electrolyte liquids for flow cells are usually metal salts in an aqueous solution that flow in two fully independent circuits. A special membrane positioned between them divides the cell into two half cells. The membrane prevents the two electrolyte liquids from mixing, but permits the exchange of ions. The electrolyte liquids in the two half cells are now pumped past the membrane, where the actual chemical reaction takes place in the form of reduction or oxidation, and energy is released.

Because the electrolyte liquids outside of the cells are stored in separate tanks, a redox flow battery is classified as an electrochemical energy storage medium, similar to a traditional fuel cell. The energy capacity and power output of a redox flow battery can be changed independently of one another.

The larger the tank for the electrolyte liquid, the larger the energy capacity. Likewise, the concentration of the electrolyte liquid decides the amount of energy that can be transported. Storage systems based on redox flow technology can therefore be variably adapted to the respective application.

Redox flow cell in comparison

In general, energy transfer within the flow cell runs between two platform-shaped poles (plus and minus) via an ionisable liquid, very similar to the time-honoured lead-acid car battery. The disadvantage of lead-acid batteries is that, at 50 Wh/l, they are relatively poor energy carriers, while their high lead content makes them very heavy. Furthermore, after around 500 charge cycles, they start to lose a lot of their capacity due to the so-called memory effect. Modern lithium-ion battery technology, with four times the charge density (250 Wh/l) and a limit of around 1,000 charge cycles are currently seen as an acceptable interim solution. Modern flow cells, on the other hand, already offer roughly the same power density as well as greater longevity due to the absence of the memory effect.

One step ahead - nanoFlowcell®

In general terms, the nanoFlowcell® is an extremely high-performance and compact flow battery. However, nanoFlowcell AG uses specially developed electrolytes that have enabled a significant increase in the energy density of the nanoFlowcell® system compared with simple flow batteries. The higher performance of the nanoFlowcell® is founded on the special characteristics of the newly developed electrolytes - a special compound of metal salts in a high-quality electrolyte liquid. Defined nano mechanisms in the field of quantum chemistry enabled adaptation of the charge carriers within the carrier liquid to achieve a whole new level of energy density - and thus the amount of energy stored. Remarkably, charge cycling within the cell takes place virtually without losses. The internal efficiency of the nanoFlowcell® is more than 80 percent.

With nanoFlowcell®, the energy is likewise stored in liquid electrolytes held in two separate tanks and pumped through a converter in a fashion similar to a traditional redox flow cell or fuel cell.

The separation of the energy converter and the energy storage medium in the nanoFlowcell® also means that the amount of energy stored is no longer dependent on cell size. Thanks to its unrestricted scalability, its uncomplicated structure and its ease of use, the benefits of the nanoFlowcell® as a drive for electric vehicles are clearly evident.

Charging the nanoFlowcell® is not the same as for a regular flow battery through feeding it with energy, but through topping up the spent liquid electrolytes. In the nanoFlowcell® used in the QUANT models, the electrolyte tanks empty while driving and the spent electrolyte liquid is dispersed harmlessly into the atmosphere. Filling the tank of a QUANT model and the refuelling process itself is similar to that for a regular petrol or diesel vehicle.

Another aspect relevant to the environment is that nanoFlowcell® does not require rare and comparatively expensive substances - unlike conventional redox flow batteries. In contrast to conventional energy carriers such as petrol, diesel, hydrogen or lithium-ion batteries, the electrolyte liquids in the nanoFlowcell® are neither flammable nor explosive and are completely harmless to health.

The energy density of the nanoFlowcell® is currently 20 times higher than that of a lead accumulator. This means the stored energy provides a range 20 times higher than with a battery of the same weight. Compared with the present-day lithium-ion technology used in many modern electric cars, the nanoFlowcell®, with 600 Wh/l, achieves five to six times more energy density and thus correspondingly longer range. And in contrast to a lithium-ion battery, which reaches its limits after around 1,000 charge cycles, the nanoFlowcell® still suffers no memory effect even after 10,000 charge cycles, giving it a longer lifespan and making it kinder to resources.

In short, the nanoFlowcell® has exceptional performance, is compact, scalable and straightforward in terms of both structure and use. In combination, these are important characteristics that predestine the nanoFlowcell® for use in modern electric vehicles.