Naphthalene diimides (NDI) in highly stable pH-neutral aqueous organic redox flow batteries

Abstract

• Naphthalene diimides in aqueous organic redox flow batteries. • Core-substitution for avoiding detrimental self-association. • The importance of electrolyte composition of the battery performance. • Exploring dimerization of NDI at different redox states using DFT calculations. • Simulating the voltammetric behaviour of the system based on DFT calculations. In the pursuit of environmentally friendly energy storage, aqueous organic redox flow batteries (AORFBs) that use naphthalene diimide hold promise for successful application. In the present article, two different naphthalene diimides (NDI) are studied as negative electrolyte materials for pH-neutral aqueous organic/organometallic redox flow batteries. The two molecules, one core-unsubstituted NDI (2H-NDI) and one core-dimethylamino substituted NDI (2DMA-NDI) are coupled with a solubilized ferrocene (BTMAP-Fc) at a concentration of 50 mM in phosphate buffered potassium chloride. High energy efficiencies and coulombic efficiencies were obtained for both batteries, but a gradual capacity fade was observed while cycling. However, when changing the cation of the supporting electrolyte from potassium to ammonium, similar energy and coulombic efficiencies were obtained, but with undetectable capacity losses over 320 cycles. Finally, 2H-NDI and BTMAP-Fc at 500 mM were tested in the ammonium-based electrolyte, and while obtaining high coulombic efficiency, the energy efficiency and cycling stability decreased compared to the same system at lower concentration. It is concluded that loss of activity is mainly due to formation of electrochemically inactive compounds and that the electrolyte cation is of great importance for the outcome. Important design strategies for AORFB molecules include using supporting salts that prevent self-association and introducing sterically hindering substituents to the structures.