The Dual Role of Bridging Phenylene in an Extended Bipyridine System for High-Voltage and Stable Two-Electron Storage in Redox Flow Batteries

Abstract

Aqueous organic redox flow batteries (AORFBs) are regarded as a promising solution for grid-scale and sustainable energy storage, but some long-standing problems such as low energy density and cycling stability should be resolved. Herein, a highly soluble bipyridine modified with a bridging phenylene group and two quaternary ammonium terminals, namely, [(NPr)2PV]·4Cl, was synthesized and used as an ultralow-potential and two-electron storage anolyte for AORFBs. The phenylene group, which is linked but not coplanar with the two pyridinium redox centers, can thus prevent their communication and result in an exceptionally low redox potential (−0.77 V vs standard hydrogen electrode, 2e–). Moreover, the introduction of a phenylene group can warrant a certain degree of large π-conjugation effects and mitigate the intramolecular Coulombic repulsion between the two positively charged pyridinium centers, thus helping to enhance the electrochemical stability. When paired with 4-trimethylammonium-TEMPO as the catholyte, [(NPr)2PV]·4Cl enabled an exceptionally high cell voltage up to 1.71 V. The AORFB delivers outstanding battery performances, specifically, ∼89% energy efficiency, ∼100% Coulombic efficiency, and ∼99.94% capacity retention per cycle during a long-term cycling process. The two overlapped single-electron reductions of [(NPr)2PV]·4Cl from the initial cationic form to the monoradical form and then to the quinoid form during the charging process were clearly verified by a series of spectroscopic techniques, including no-deuterium nuclear magnetic resonance and electron paramagnetic resonance. This work presents a significant improvement for the construction of high-voltage AORFBs by virtue of the designability, diversity, and tunability of multiredox organic molecules.

Constructing Self‐Adapting Electrostatic Interface on Lithium Metal Anode for Stable 400 Wh kg−1 Pouch Cells

“…This means that the N + on the PBM is difficult to be reduced by gaining electrons, thereby reducing the reduction potential of the PBM. [ 49–53 ] On the basis of these observations, we postulate that PBM interfaces are electrochemically stable even during lithium plating. Besides, the PBM has a Young's modulus of 0.76 GPa (Figure S8, Supporting Information), which ensures PBM a flexibility that can adapt to volume changes during Li plating/stripping.…”

Reversible Redox Chemistry in Pyrrolidinium‐Based TEMPO Radical and Extended Viologen for High‐Voltage and Long‐Life Aqueous Redox Flow Batteries

“…Interestingly, it is a controversial question that whether a two-electron or two single-electron redox possesses proceed for phenylene bridging bipyridinium derivatives. [37,38] Herein, we propose a simple and effective method to clarify this question. As revealed by the CV curves in Figure S7a, Supporting Information, the one broad redox couple of [PyrPV]Cl 4 was gradually split into two overlapped couples when increasing the concentration of [PyrPV]Cl 4 from 2 to 100 mM in a 2 m NaCl aqueous solution, unambiguously confirming the two single-electron reductions.…”

High‐Power Near‐Neutral Aqueous All Organic Redox Flow Battery Enabled with a Pair of Anionic Redox Species

“…Briefly, 1,4‐di‐(pyridin‐4‐yl)benzene (PhPy) was reacted with diethyl(3‐bromopropyl)phosphonate in acetonitrile at 95 °C for 48 h, followed by a simple hydrolysis of the ester resulting in the desired product in 77 % overall yield. The PhPy precursor can be synthesized economically by a single‐step Suzuki–Miyaura reaction of 1,4‐dibromobenzene and 4‐pyridineboronic acid pinacol ester with a very high yield (>90 %) [18] . PSS‐TEMPO was made in a single step by using the commercially available 4‐hydroxy TEMPO.…”