Europium Macrocyclic Complexes With Rigid High-Symmetry Coordination Geometry for High-Voltage and Durable Neutral Aqueous Redox Flow Batteries.

Neutral aqueous redox flow batteries (ARFBs) are promising for grid-scale energy storage because of their intrinsic safety, low corrosiveness, and environmental compatibility, yet achieving both high energy density and long-term stability remains challenging. Herein, we report a coordination-geometry-regulated europium macroheterocyclic complex in which a Eu3+/Eu2+ redox centre is efficiently stabilized by a rigid square-antiprismatic coordinated 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA) ligand, namely Eu(DOTA). This rigid macrocyclic framework efficiently preserves the structural integrity of the complex during extended redox cycling. Theoretical calculations and spectroscopic characterization reveal that the highly symmetric Eu-O/Eu-N coordination shell homogenizes Eu-ligand interactions, significantly elevating the kinetic barrier for the first water-coordination event. Thus, Eu(DOTA) resists water-induced coordination changes and structural rearrangement, while its organized solvation shell is associated with reduced membrane crossover. Consequently, the Eu(DOTA)-based negolyte exhibits a high aqueous solubility (up to 2.2 M) and a high operating voltage of 1.43 V in neutral Eu-Fe ARFBs. At 1.0 M, the battery delivers 23.8-24.4 Ah L-1 and sustains 2000 cycles (103 days), with decay rates of 0.0070% and 0.0023% per cycle over the initial 360 and subsequent 1640 cycles, respectively. These results establish a coordination-environment-guided strategy for designing high-voltage, long-lived neutral ARFBs.