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.
Highlights
Electricity generated by solar and wind has increased exponentially since the 1990s in order to minimize dependence on fossil fuels
In applications related to electrochemistry, naphthalene diimide (NDI) has been investigated as a sensor for organic molecules[33,34] and as cathode for lithium ion based batteries.[35,36,37]
We previously evaluated core-unsubstituted NDI with a propyldimethylamino sidechain, here called 2H-NDI2- forming (2H-NDI), and found that the molecule’s strong self-associative behavior has a significant effect on its aqueous electrochemistry on the cyclic voltammetry (CV)
Summary
Electricity generated by solar and wind has increased exponentially since the 1990s in order to minimize dependence on fossil fuels. Two previous articles present the use of NDI in AORFBs. Firstly, an NDI molecule with a glycine sidechain was coupled with 4-OH-TEMPO in a flow battery that delivered good performance, albeit with a limited solubility of the NDI of 30 mM and 40 mM in 1 M KCl and NaCl electrolytes respectively.[40] Secondly, an NDI-polymer was demonstrated together with 2,7-AQDS in a redox flow battery that used a single mediator redox targeting reaction as mode of operation.[41] The battery showed a high degree of capacity retention, which further underlines the electrochemical stability of naphthalene diimides in aqueous solution. It is identified that if a TEMPO-based species had been used instead of BTMAP-Fc, a 0.41 V higher voltage would have been achieved throughout.[2,12,40]
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