All-Liquid Electroactive Materials for High Energy Density Organic Flow Battery

Abstract

Nonaqueous redox flow batteries (RFBs) are a promising energy storage technology that enables increased cell voltage and high energy capacity compared to aqueous RFBs. Herein, we first report a novel approach to substantially increase the energy density based on the miscible liquid redox materials 2,5-di-tert-butyl-1-methoxy-4-[2′-methoxyethoxy]­benzene catholyte and 2-methylbenzophenone anolyte. This system has a high theoretical cell voltage of 2.97 V and a calculated energy density of 223 Wh L–1 that is much higher than those of previously reported nonaqueous organic RFBs. Our reported redox flow chemistry displays excellent electrochemical performance and stability under cyclic voltammetry, bulk electrolysis, and flow cell conditions. A proof-of-principle RFB delivers a coulombic efficiency of 95% and energy efficiency of 70% and represents significant progress toward high energy density RFBs.

Symmetric, Robust, and High-voltage Organic Redox Flow Battery Model Based on a Helical Carbenium Ion Electrolyte

“…Using the Randles-Sevcik equation (Equation S1), an experimental diffusion coefficient (D) of 2.10(5) × 10 -5 cm² s -1 and 2.09(4) × 10 -5 cm² s -1 were determined for the C + /C • and C •++ /C + couples, respectively. These values are higher than other known anolytes, 29,42,43 and about four times higher than recent catholytes studied, which is an advantage for our DMQA + . [28][29][30]43,44 The electron transfer rate constants (k 0 ) were determined using the Nicholson method (Equation S2, Figures S5-6), 45 where the k 0 = 2.5(5) × 10 -3 cm s -1 for C + /C • redox couple is almost similar to that of the C •++ /C + redox pair k 0 = 2.3(1) × 10 -3 cm s -1 and both are comparable to recent electrolytes developed for RFB.…”

“…These values are higher than other known anolytes, 29,42,43 and about four times higher than recent catholytes studied, which is an advantage for our DMQA + . [28][29][30]43,44 The electron transfer rate constants (k 0 ) were determined using the Nicholson method (Equation S2, Figures S5-6), 45 where the k 0 = 2.5(5) × 10 -3 cm s -1 for C + /C • redox couple is almost similar to that of the C •++ /C + redox pair k 0 = 2.3(1) × 10 -3 cm s -1 and both are comparable to recent electrolytes developed for RFB. 28,30 The persistent oxidized and reduced species of C + , and their fast and identical electronic kinetics, suggest that this electrolyte is suitable for ORFB application.…”

“…In addition to preventing cross contamination, symmetric ORFBs can access three different oxidation states of the electrolyte within the same compartment, resulting in a charged to one polarity or to the opposite polarity via one-electron cycles in opposite directions. To date, only three examples of symmetric ORFBs have been reported, (Figure 1, green box), [31][32][33] and these either possess a high OCV with low cyclability (Figure 1, example iv), or a better reliability (max 100 cycles) but a small OCV (Figure 1 (Cyclability = capacity (Q) retention >90% after n cycles) with their electrolyte couples: (i), 28 (ii), 29 (iii), 30 (iv), 31 (v) 32 and (vi) 33 .…”

Molecular redox species for next-generation batteries

“…Given all of the factors discussed above, renewed interest in RFBs has driven a broad search for new CCs across a wide range of molecular species, and recent studies have included the exploration of metal ions, 2,11 clusters, 48,49 metallocenes, 50,51 organic molecules, [52][53][54] and polymeric species. 1,55 In the following we will examine the molecular engineering approaches that have been applied to develop CCs with improved properties and thereby enhanced RFB performance.…”

“…As a result, there is a growing interest in the development of organic CCs as alternatives to metal-centred species. Organic CCs investigated to date include nitroxide radicals such as TEMPO, 55,58 carbonyls such as fluorenone and benzophenone, 53 heterocyclic aromatics such as viologens, 55 and phenothiazine, 59 and dialkoxybenzenes, 53 to name just a few. Here we examine a selection of the approaches that have been adopted in order to optimise the solubility, stability, and redox properties of these CCs for use in RFBs.…”