Carboxyl-Functionalized TEMPO Catholyte Enabling High-Cycling-Stability and High-Energy-Density Aqueous Organic Redox Flow Batteries

Abstract

Aqueous organic redox flow batteries (AORFBs) employing synthetically tailorable organic electroactive compounds have received significant attention for energy storage technologies. There have been many efforts in developing electroactive materials for AORFBs with anion-exchange membranes. On the contrary, electroactive compounds that are compatible with cationexchange membranes in AORFBs are less studied. Here, we report an electroactive 4-carboxylic-2,2,6,6-tetramethylpiperidin-N-oxyl (4-CO 2 Na-TEMPO) molecule for neutral AORFBs. The compound exhibits a good solubility of 1.5 M in an aqueous sodium-based solution, which is 3 times more than that of the pristine 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-OH-TEMPO). When paired with a 1,10-bis(3-sulfonatopropyl)-4,4′-bipyridinium (SPr) 2 V anolyte, the resulting RFB operating through a cation-exchange membrane achieved an open-circuit voltage of 1.19 V and a high energy density of 14.7 W h L −1 . In the long-term cycling study, the RFB features a stable capacity retention of 99.94% per cycle over 400 cycles with nearly 100% Coulombic efficiency.

Isopropyl alcohol and copper hexacyanoferrate boost performance of the iron tris‐bipyridine catholyte for near‐neutral pH aqueous redox flow batteries

“…Unfortunately, many of the core structures of organic and organometallic compounds with highly favorable redox properties (RFB anolytes: quinone, 26 alloxaxine, 27 phenazine, 28 and other heterocyclic aromatics; catholytes: nitroxide radicals 29 and metallocenes 30 ) are hydrophobic and thus sparingly soluble in water. There are three general strategies for improving the energy density (solubility) of a given active compound in RFBs: (i) direct molecular functionalization of solubilizing (charged) substituents, 31,32 for example, sulfonate, 3,28 alcohol, 27,28 carboxyl, 3,33 ammonium, 34,35 ethylene, 36 and phosphonate 37 groups; (ii) counterion optimization into higher-soluble salts 22,[38][39][40] ; and (iii) addition of solubilizing additives. 10,41,42 The stability of Fe(bpy) 3 2+/3+ 's robust, aromatic bipyridine ligands leaves them difficult to functionalize directly, reflected in the >10 2 cost difference between the commerciallyavailable core and functionalized bipyridine structures, 43 rendering strategy (i) impractical.…”

Artificial Bipolar Redox-Active Molecule for Symmetric Nonaqueous Redox Flow Batteries

“…Based on the solvents used, organic RFBs can be divided into two types: aqueous organic RFBs (AORFBs) and nonaqueous organic RFBs (NAORFBs). Despite some appealing merits, the energy density of AORFBs is restricted by the narrow electrochemical window of water (<1.5 V). − In contrast, NAORFBs can offer broader voltage windows up to 5 V, which can provide higher energy density than aqueous RFBs. ,− In recent years, nonaqueous RFBs that are based on metal complexes (e.g., ferrocene, cobaltocene), ,− redox-active organic molecules (e.g., phenothiazine, benzothiadiazide, nitrobenzene, 2,5-di- tert -butyl-1-methoxy-4-[2-methoxyethoxy] benzene), − and redox-active polymers have been extensively studied. − However, in these systems, the use of different redox-active anolytes and catholytes gives rise to the cross-contamination issue because active materials can cross into the opposite sides, resulting in capacity deterioration of the battery …”

Multi-objective goal-directed optimization of de novo stable organic radicals for aqueous redox flow batteries

“…However, due partly to their unique and complex chemistry, relatively few stable radical-containing materials have been explored. 7,8 As a result, most studies have focused on chemical modifications of a handful of well-known stable radical scaffolds, 9,10 primarily via mechanism-based approaches that identify optimal side-chains to improve performance such as increasing solubility or limiting possible decomposition reactions. [11][12][13][14][15][16][17][18] The scarcity of radical scaffolds complicates the tuning of their physical and electrochemical properties to meet the strict demands of high-performance, low-cost RFBs.…”