Structurally and electrochemically tunable pyrylium platforms: A new class of redox anolyte for non-aqueous organic redox flow battery operating at a high-current density

Abstract

Non-aqueous all-organic redox flow batteries (NORFB) are a proficient candidate for large-scale energy storage applications. The higher operating potential window of non-aqueous solvents, diversity of redox-active organic molecules (ROMs), and independent scaling of power and energy, etc., captivate NORFB to be a potential choice for grid energy storage applications. Besides, the invention of new redox-active motifs significantly expands the development of NORFBs; thus, it is highly demanded. But, the low operational current density and slower redox kinetics are the bottlenecks for the further development of NORFB. Herein, we introduced a new class of anolyte material based on pyrylium salt (2,4,6-triphenylpyrylium tetrafluoroborate (TPT)) and demonstrated for the first time its applicability as a potential anolyte in NORFB. The substitutions at the 2, 4, and 6th positions of pyrylium salt prominently tune the structural and electrochemical behavior, thus providing a versatile platform to explore them in flow battery chemistry. TPT exhibited a one-electron reduction process with a high electron transfer rate constant of 9.94 × 10−3 cm s−1 and diffusion coefficient of 5.83 × 10−6 cm2 s−1, which is higher for the non-aqueous medium. By coupling with N-decylphenothiazine (DPTZ) catholyte, a mixed electrolyte flow cell was demonstrated with a capacity of 1.08 Ah/L and coulombic efficiency of 97 % at a high current density of 40 mA cm−2.