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Redox-Flow Batteries, Flow Batteries, RF Batteries, RFB, VRFB

What is a Flow Battery? How does a Redox-Flow battery work?

Most common type (50% of market in 2020): Vanadium Redox Flow Battery also VRFB or VFB.Characteristic for this type of battery is that the power (size of the reactor) can be scaled independently of the capacity (electrolyte volume), because the electroactive materials in liquid form (electrolytes) can be stored in external tanks.

Historic development of the Redox Flow Battery

RF batteries have been developed since the 1970s. However, a method for storing electrical energy in liquids was patented as early as 1949 by Walther Kangro of the Technical University of Braunschweig. In the 1980s and 1990s, various prototypes were built and tested in the USA and Asia. Commercial installations with power outputs of up to 200 kW and capacities of 800 kWh are available. The vanadium redox flow battery developed in the 1980s is considered to be the best studied and most installed redox flow battery-system. In recent years few european flow-battery manufacturer have appeared.

Most important Issues to be solved in the world of Redox Flow Batteries

Research efforts are currently focused mainly on the reduction of system and maintenance costs and the search for new electrolyte systems for higher energy densities, electrode optimization for higher performance, membrane development for lower maintenance costs and electrical system development. The flow battery is rather a complex machine than a simple battery and has to be well engineered to competitive in the battery market. 

Architecture and scientific work principle of the Redox-Flow Battery

The special characteristic of RF batteries is that at least one redox-active substance is present as flowing media (e.g. liquids, gases and suspensions). This enables a decoupling of the scaling of energy and power, as is the case with fuel cells. The capacity of an RF battery is determined by the size of the electrolyte tanks, its power by the size of the energy converter.

In contrast to the other battery types, redox flow batteries use two liquid electrolytes solution with the two active materials; the anolytes and the catholytes which are converted at the electroactive high surface carbon-anode and carbon-cathode. The electrolyte solution located near the anode is called anolyte. Analogously, the electrolyte solution on the cathode side is called the catholyte. The electrolytes are separated from each other by a so-called ion-selective membrane. The membrane allows to move the charge-compensating ions and anolyte and catholyte maintain in the cathodic half-cell and anodic-half cell respectively.

The redox reactions can take place both in single cells and in fuel cell-like energy converters, the so-called cell-stacks. Unlike other batteries, RFB’s require powerful pumps to circulate the liquid electrolytes through anodic and cathodic carbon-mesh.

A redox flow battery cell is usually constructed as follows: The flow frames, which are responsible for the distribution of the electrolytes, are separated by the ion-selective membrane. The flow frames contain felts made of graphite or other carbon-types, which serve as electrodes with a high surface-area. In addition, insulation plates are added for electrical insulation between the half-cells, graphite plates as current-collectors for the termination of the half-cells and end plates for mechanical stability. Together with fluid technology, mechanical pumps and, if necessary, thermal management, actuators, sensors, battery management systems and other components, the cell forms the redoxflow battery.

The Biggest Advantages of Redox-Flow Batteries

1.

Theoretically, Vanadium-metal can be easily recycled as the anodic side and the cathodic side of the battery are both pure Vanadium-solutions which makes this type of battery promising for stationary energy storage (SES)

2.

High overcharge and deep discharge tolerance

3.

Long service life 10 to 20 years and cycle stability > 10,000 cycles possible

4.

Very Low Self-discharge

Biggest Challenges of the Redox-Flow- Battery Technology

1.

  • Low energy density of the full battery is 2O Wh/L which corresponds to an gravimetric energy density of approx. 50 Wh/L. This is 10 times less in energy density than Lithium Ion Batteries.

2.

  • The low energy density the complex machinery makes the RFB expensive for many applications