Nickel Tetra-(4-Sulfonatophenyl) Porphyrin/Ionic Liquid Supramolecular Assemblies for Applications in Symmetrical Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) are a promising technology as a grid-level energy storage system and have attracted a growing amount of attention. In these devices, electrochemical storage is carried out through the reduction and oxidation of chemical species. The peculiarity of RFBs is that active species are in solutions, with the reaction occurring at the solid-liquid interface. In the present work, we propose the use of nickel tetra-(4-sulfonatophenyl)-porphyrin (NiTPPS) as an innovative bipolar redox-active molecule (BRM) for aqueous organic redox flow batteries (AORFBs). Thanks to its distinctive redox properties, this single yet complex molecule serves as both an anolyte and a catholyte. This symmetry allows RFBs to use identical components, offering simplified storage and reduced crossover benefits. To increase NiTPPS stability in aqueous solution, we explored the ionic liquid (IL) 1-butylpyridinium tetrafluoroborate (BupyBF₄) as a supporting electrolyte for AORFBs (Huang et al., 2019). Using an IL was also advantageous in broadening the water electrochemical potential window. Actually, by DFT calculations and aggregation studies, carried out by UV-vis spectroscopy, it was observed that the insertion of the metal atom enhances the chemical and electrochemical stability of the porphyrin macrocycle. In addition, the use of BupyBF₄ as a supporting electrolyte improved the resolution of redox processes, avoiding problems associated with water electrolysis and demetalation of the electroactive species.