Membranes represent a significant portion of the final costs of redox flow batteries (RFB) used in energy storage systems. It is therefore important to identify the membrane technologies that could simultaneously ensure high performance and low-cost production, as this will be crucial to bringing redox flow storage closer to wider adoption in commercial projects.
With this in mind, researchers from Finland's Aalto University and Tianjin University in China have conducted a review of the different kinds of membranes that could be used in non-aqueous redox flow batteries (NARFBs). They describe NARFBs as a second-generation technology with a much higher energy density and temperature window, compared to aqueous devices.
“NARFBs are typically composed of two electrolytes, often distinctly divided as anolyte and catholyte, two electrodes, and one membrane,” they explained. “Application of non-aqueous electrolyte provides a wide selection of solvents, thus, more flexible design of the device, although NARFB works on the similar principles as the aqueous RFBs.”
The electrochemical performance and long-term stability in such storage systems are ensured by three main components – the electrolytes, the electrodes, and the membrane. The latter is used to prevent a problem known as “crossover,” which occurs when the redoxmers migrate to the wrong side of the battery, thus causing capacity losses that can reach up to 50%.
Their analysis included commercial membranes such as dense, ceramic and porous products, as well as intrinsic dense membranes and intrinsic porous devices. The researchers also looked at composite membranes – a category they have identified as the most promising future technology.
“An ideal membrane in NARFBs should have high ionic conductivity and selectivity, low swellability, low-cost, and high mechanical and chemical stability in organic solvent,” the researchers said, adding that dense ceramic membranes feature high ionic selectivity and low swellability, although they have poor mechanical stability and low ionic conductivity. They are also expensive.
Porous membranes are said to exhibit high ionic conductivity and low swellability at low costs. They also exhibit high chemical and mechanical stability, but they irremediably suffer from crossover issues.
“Modified composite membranes achieve high ionic selectivity, low swelling ability, high ionic conductivity, meanwhile, exhibit chemical and mechanical stability simultaneously because of excellent chemical stability and mechanical strength of the porous substrate,” the academics explained. “Intrinsic composite membranes also have a potential of achieving high ionic selectivity and conductivity, low swelling ability, chemical and mechanical stability simultaneously because of the possibilities in monitoring their component materials.”
The Chinese-Finnish research team described all membrane technologies in “Membranes in non-aqueous redox flow battery: A review,” which was recently published in the Journal of Power Sources. Scientists from the Rotterdam University of Applied Sciences and the University of Warwick published a similar review, conducted on all membrane technologies for RFBs, in September. In addition, another research team from Aalto University recently conducted a comprehensive analysis of all redox flow battery (RFB) and hybrid RFP technologies.
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