Continued development of redox flow batteries for storing renewable energy
For the implementation of the energy transition, technologies for storing renewable energy also need to be developed further. One solution to this problem is redox flow batteries. To advance their development, Wevo has conducted a range of studies, some in cooperation with partners such as the Fraunhofer Institute for Chemical Technology (ICT).
As part of the energy transition, the German government wants to reduce Germany’s greenhouse gas (GHG) emissions by 65 per cent by 2030 compared with 1990. According to the Climate Protection Act (Klimaschutzgesetz), Germany is set to be GHG neutral by 2045. In order to hit the carbon-reduction targets in the energy sector and in industry transport, buildings and agriculture, renewable energy storage technologies will also need to play an important role.
This is because innovative storage technologies are prerequisite for ensuring sufficient energy is available even when the sun and wind are not. An alternative to the lithium-ion batteries that have predominantly been used to date are the so-called redox flow batteries.
Vanadium electrolyte: challenge for materials
In redox flow batteries, the energy is stored in an electrolyte, often based on vanadium. This places very high demands on the materials used, making design of the battery complex. Specially developed, highly chemical-resistant potting compounds, adhesives and sealants from Wevo are suitable for use in this aggressive environment – and they also provide multiple other benefits for this battery technology.
Convincing individual product properties
The results of the study were presented at the International Flow Battery Forum 2022 in Brussels – an international conference for flow batteries – and they met with great interest. This has resulted in a number of promising projects in which the use of Wevo materials in different types of flow batteries is being further tested. Such as in a joint research project with the Hamburg University of Applied Sciences (HAW) and the DECHEMA Research Institute. Research is being conducted here into the use of Wevo’s sealants and adhesives in tubular vanadium redox flow batteries. In a further step, the resistance of these materials in organic electrolytes – in addition to vanadium electrolytes – will be investigated.
Wevo materials on the test bench
Fraunhofer ICT campus in Pfinztal near Karlsruhe
The durability of Wevo products in redox flow batteries was investigated for the first time in cooperation with the Fraunhofer ICT. For this purpose, test specimens based on polyurethane, epoxy resin and silicone were exposed for 135 days to different oxidation levels of the electrolyte – to V2+, V3.5+ and to the particularly aggressive oxidation level V5+.
Testing chemical stability
The high degree of chemical resistance was initially demonstrated by the very slight change in the weight of the test specimens. It was further confirmed by the Shore hardness test, which Wevo subsequently carried out in its own laboratory. These results were not expected, especially in the case of the silicone elastomer. There were no significant changes, and no swelling of the test specimens was detected.
Surface condition of the test specimens
Optical analysis of the test specimens comprised examining the condition of their surface. To this end, the research team scratched a cross-shaped pattern into the test specimens with a cutter knife prior to exposing them to the electrolyte. The clearer the cross pattern and the sharper the contours following exposure, the more resistant the polymer is to the electrolyte.
As can be clearly seen in the light microscope images ( graphic bottom right), the cross patterns have hardly changed, particularly those scratched into the polyurethane resin and silicone – even those exposed to the vanadium electrolyte in oxidation level V5+. The contours are still clearly visible, which can be attributed to very high resistance to the electrolytes. In addition, the light microscope images show that the colour of the test specimens has not changed significantly, which also means high chemical resistance.
Condition of the electrolytes
The condition of the electrolytes following specimen exposure also played an important role. As the majority of the electrolytes changed very little, we deduced that there had been no significant interaction between the components of the resins and the electrolytes and consequently no additional oxidation or reduction processes of the vanadium electrolyte itself. This also indicates a very high level of resistance of the materials in the electrolytes in oxidation levels V2+ and V3.5+ ( graphic left).
The slight green discoloration in the V2+ electrolyte is due to possible oxidation with atmospheric oxygen during measurement. The green or blue discoloration in the V5+ electrolyte to which silicone and epoxy resin were exposed indicates certain reduction processes, which are not unusual for this redox stage of the electrolyte. The very slight discoloration of the polyurethane resin can therefore be regarded as especially positive and indicates exceptionally high resistance to the electrolyte. These results were essentially confirmed by additional quantitative analyses, such as titration and UV/VIS spectroscopy, conducted with the DECHEMA Research Institute.
The Wevo products tested have the potential to replace the rubber and fluoroelastomer gaskets currently in use, enabling automated manufacture and new degrees of freedom in design. They also seal the components contained in the battery securely and positively – protecting the components and terminal rails against corrosion. In addition, the materials can be used for other applications throughout the battery system, such as protecting and insulating the current collectors, pumps, flow meters, sensors and power electronic components or coating the electrolyte tanks.
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Image sources: Image 1: Stefan Hohloch, Image 2: Stefan Holoch, Image 3: Stefan Hohloch, Image 4: Fraunhofer ICT, Image 5: designsstock
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