Redox flow battery

Functionality, types and research
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How does a redox flow battery work, how can ageing be analysed, what role does the redox flow battery play in the energy transition and can it be used to power electric cars? What are the different types of liquid batteries? How do types of redox flow batteries differ and what are the advantages and disadvantages of this technology? We would like to answer these questions here.

What is a redox flow battery?

Redox flow batteries, also called redox flow battery, flow battery or liquid battery, provide electrical energy from liquid electrolyte solutions, often based on the heavy metal vanadium. The difference to the rechargeable battery (also called accumulator) is the spatial separation between the two stores of the redox flow battery, each containing electrolyte liquids of different concentrations, and the energy converter, whose cells consist of a membrane and two electrodes.

These cells process the electrolyte liquid in a chemical reaction that provides usable electrical energy. This reaction is reversible, so that with the help of electrical energy, for example from renewable sources, the electrolyte fluid returns to its original initial concentration and can thus provide electrical energy again in the next step. A sustainable cycle is created that can be used to store electricity.

How the redox flow battery works

Redox is a compound word and stands for reduction-oxidation. Reduction means taking up electrons, oxidation means giving up electrons. The redox flow battery, essentially consists of three components. The first component is the cell, consisting of membrane and two electrodes, similar to the fuel cells. The other two components are the tanks for storing the electrolyte fluids. These two electrolyte liquids of different concentrations or valences are each fed via a pump in separate circuits to one electrode each. At each electrode, one of the electrolyte liquids reacts and ions are released. These ions pass through the partially permeable membrane and are taken up again by the other electrolyte liquid.

The released electrons do not travel through the membrane, but through the electrode to the external electric circuit to the other electrode. The state of the two electrolytes changes accordingly. The electrolyte liquid that flows through and is partly unreacted is then returned to the same reservoir. This changes the value and the concentration. The level of DC voltage at a cell is dependent on the material composition of the both electrolytes (redox pair) and is between 1.1 V for hydrogen-bromine redox pairs up to 1.5 V for bromine-polysulphide in no-load operation.

Image: Schematic representation of how a redox flow battery works.

The electrical DC current and the load-dependent cell voltage are to be adjusted by the power electronics to a constant voltage level corresponding to the load. Due to the low self-consumption of the system technology and the efficient electrochemical reaction, overall efficiencies on the AC side of up to 75% are achieved.

Measure cell voltage

Individual cells can be connected to form a stack, i.e. a stack of individual cells arranged in series. This increases the usable voltage because all the voltages of each cell add up. Because the released electrons in each individual cell always move to the next cell, the weakest cell is the limiting factor for the performance. So if a cell is defective and does not pass electrons to the next cell, the cell voltage drops to 0 V and the liquid battery no longer supplies any electrical power.

Therefore, the voltage at each individual cell should be recorded and implemented in the safety chain of the plant. Because if problems at cells can be detected in time by cell voltage measurement, countermeasures can be taken by the plant management and irreversible damage and repair costs can be avoided. One solution for monitoring the cell voltages of redox flow batteries is the single cell voltage meter DiLiCo cell voltage, which monitors each individual cell for over- and undervoltage and sends faulty cells to the system control.

The service life and power of redox flow batteries determine the tank and membrane size. The amount of electrolyte liquid, i.e. the "fuel", determines the usable time for the production of electrical energy by the redox flow battery. The size of the electrodes and the membrane determine the amount of electricity produced and thus the power.

Image: The measurement device DiLiCo cell voltage for single cell voltage measurement.

The storage capacity can therefore be easily reduced or increased by adjusting the tank size. This is a significant advantage over accumulators because the storage unit and the converter unit can also be positioned spatially separate from each other.

Types of redox flow batteries

Redox flow batteries differ primarily in the composition of the electrolytes and the solvents. Due to the different compositions, more than 50 variants are described in the literature. The electrolyte consists of a solvent and salts dissolved in it. This composition determines the cell voltage and thus the energy density.

The higher the cell voltage, the greater the power of the battery with the same membrane and electrode area. The highest cell voltage is achieved by zinc-bromine redox pairs with 1.8 V. Inorganic or organic acids, but also simple saline solutions are used as solvents. The following table shows different redox pairs of redox flow batteries.

Vanadium redox systems are the most advanced so far and are available on the market through some suppliers mainly for stationary energy storage. Units with a capacity of 15 MW and 60 MWh have already been realised in Japan, for example.

The Fraunhofer-Institut für Chemische Technologie operates a 2 MW plant for research purposes directly with a wind turbine, for example to test energy self-sufficient stand-alone solutions. However, most redox flow battery types are primarily in the development stage and are not in commercial use.

Redox couple Cell voltage
Hydrogen bromine 1.1 V
Iron-chrome 1.2 V
Vanadium/polyhalide 1.3 V
Vanadium/Vanadium 1.4 V
Bromine polysulphide 1.5 V
Zinc-Bromine 1.8 V

Table: Different redox pairs and their cell voltage.

Where are redox flow batteries used?

Mainly redox flow batteries are used for stationary applications to cover peak loads and load balancing or for uninterruptible power supply. Large-scale battery projects focus primarily on coupling with wind power and photovoltaic systems for use as power storage. Especially the unlimited number of cycles within the lifetime of about 20 years, make redox flow battery therefore particularly flexible in daily use compared to the limited number of cycles of batteries.

The redox flow battery is currently not an alternative in the field of mobility due to its lower volume- and mass-related energy density compared to accumulators made of lithium. However, the redox flow battery does not have the problem of long refuelling times for battery vehicles and the resulting limited grid capacities at high charging powers, because the system can be quickly recharged by replacing the electrolytes.

The efficiency is lower than that of lithium ion batteries and the space requirement is too large. Although reports of research approaches for mobile use keep surfacing, concrete implementations are lacking so far.

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What are the advantages and disadvantages of redox flow batteries?

  • Independently scalable performance and storage capacity
  • High efficiency (70 -90%)
  • Long life (approximately 20 years) and unlimited number of cycles
  • Resistant to deep discharge, low self-discharge
  • Fast reactivity
  • Electrolyte not flammable or explosive
  • Low energy density compared to lithium ion batteries
  • Lower permissible operating temperatures minimise the range of applications
  • Strong price fluctuations for vanadium
  • Undesired transition of the electrolytes through the membrane (so-called crossover)

What is the current status of redox flow battery research?

The reduction of investment costs through automated manufacturing processes and new, more cost-effective materials are just as much in the focus of investigations as the increase of power density and increase of larger usable temperature ranges, which are being investigated in many research projects. New catalysts for increasing the exchange current density and thus increasing efficiency are also the subject of research.

Predestined for investigations on redox flow batteries to analyse new components and the associated ageing effects is the use of DiLiCo current density. This measuring device consists of a large number of current and temperature sensors over the entire surface and is positioned between two cells. There, it measures the current production between two cells over the entire surface, providing an insightful view into the cell and enabling statements on the condition with regard to the evaluation of ageing effects, operating strategies and the influence of new components compared to previous versions of redox flow batteries.

Image: The measurement device DiLiCo current density for the insight into the cell.