Ion exchange membranes as separators in all-organic redox flow-batteries

The ever-growing global population makes of first priority the transition from a fossil fuels-based society to a society based on renewable clean energies. Solar and wind energies are so far the fastest-growing sources of electricity. However, there is a shift between the peak hours of electricity production and the peak hours of energy consumption, hence, a need of high capacity electricity-storage devices is of first importance.

Lithium Ion Batteries (LIBs) are currently the most widely used storage devices on the market but concerns arose regarding their irreversible aging and fire hazard. Redox Flow-Batteries (RFBs) have great potential to overcome these drawbacks. The redox active material is dissolved in a solvent with supporting electrolyte and stored in external reservoirs. The central part of the RFB is the Membrane Electrode Assembly (MEA) composed of an Ion Exchange Membrane (IEM) sandwiched between two metal electrodes. The electrolytes (the anolyte and catholyte) are pumped at each side of the MEA and generate power through redox reactions occurring respectively at the anode and cathode electrodes. This configuration enables to decouple the stored energy and power. The energy capacity can be tuned by increasing the volume of the reservoirs and power can be tuned by using a higher membrane area. Recently, the scientific community started to develop new RFBs based on organic solvents instead of aqueous solvents. This configuration aimed to overcome the narrow electrochemical stability window of water (1.23V) limiting the maximal voltage achievable with aqueous RFBs.

As non-aqueous RFBs is a relatively new technology, mostly commercial IEMs are employed as separators. Commercial IEMs aimed to be used in aqueous environments and their use in organic solvents can lead to several issues. Organic solvents can strongly interact with IEM ensuing in extensive swelling of the membrane with the resulting effect being the crossover of the redox active species decreasing the overall performance of the battery. Another physical effect is the broad applied electrochemical potential gradient across the IEM that can lead to redox reactions of the membrane material.

This challenging project is dedicated to the development of IEMs able to withstand organic solvent environment and broad electrochemical potential gradients. It is of first importance to identify polymer candidates having enough functional groups to perform crosslinking (giving solvent stability) but on the other hand, those functional groups are generally responsible for the redox activity of the polymer. The major challenge is therefore the tradeoff between having enough freedom for polymer modification while keeping an electrochemically inert material. This project is multidisciplinary going from the design of IEM materials (organic synthesis and polymer chemistry) to the characterization of their behavior as ionic conductors in harsh environment and RFB performance testing.