The development of low-cost, large-scale energy storage technologies is required for increased reliance on non-polluting, intermittent renewable energy sources connected to our electrical grid. Our research focuses on the development of organic materials for charge storage in non-aqueous redox flow batteries (RFBs). At present, most commercial RFBs contain aqueous, vanadium-based electrolytes, which have been demonstrated on scales as large as 1.5 MW. With non-aqueous electrolytes, a wider electrochemical window allows for 2-3 times higher operating voltages, while avoiding the use of corrosive electrolytes. Most of the organic species reported as electron donors are only stable in the neutral and singly oxidized states; removal of a second electron results in an unstable dication. Here we sought to increase the stability of dication species without significantly raising molecular weight, thus preserving atom economy. Considering phenothiazine donors, we found that the addition of certain substituents led to improved stability of the dication form; specifically, electron-donating methoxy groups (Figure 1) are more effective in stabilization than non-conjugated substituents. This presentation will include the synthesis of these materials, as well as electrochemical and spectroelectrochemical analysis. Furthermore, the results of density functional theory calculations will be presented to evaluate the design of a stable two-electron donor.

Figure 1. Structural representation of 3,7-dimethoxyphenothiazine derivatives (a) and cyclic voltammograms of N-ethyl-3,7-dimethoxyphenothiazine (DMeOEPT) showing reversible first and second oxidations (b).