Interrogation of 2,2′-Bipyrimidines as Low-Potential Two-Electron Electrolytes

Abstract

As utilization of renewable energy sources continues to expand, the need for new grid energy storage technologies such as redox flow batteries (RFBs) will be vital. Ultimately, the energy density of a RFB will be dependent on the redox potentials of the respective electrolytes, their solubility, and the number of electrons stored per molecule. With prior literature reports demonstrating the propensity of nitrogencontaining heterocycles to undergo multielectron reduction at low potentials, we focused on the development of a novel electrolyte scaffold based upon a 2,2′bipyrimidine skeleton. This scaffold is capable of storing two electrons per molecule while also exhibiting a low (∼−2.0 V vs Fc/Fc + ) reduction potential. A library of 24 potential bipyrimidine anolytes were synthesized and systematically evaluated to unveil structure−function relationships through computational evaluation. Through analysis of these relationships, it was unveiled that steric interactions disrupting the planarity of the system in the reduced state could be responsible for higher levels of degradation in certain anolytes. The major decomposition pathway was ultimately determined to be protonation of the dianion by solvent, which could be reversed by electrochemical or chemical oxidation. To validate the hypothesis of strain-induced decomposition, two new electrolytes with minimal steric encumbrance were synthesized, evaluated, and found to indeed exhibit higher stability than their sterically hindered counterparts.

Organic Four‐Electron Redox Systems Based on Bipyridine and Phenanthroline Carbene Architectures

“…Note, viologen anolyte materials are typically only cycled by one electron over the dication/radical cation oxidation states [40] . More recently there has been interest in harnessing two electrons from the viologen system, [41, 42] or finding other multi‐electron redox systems at highly negative or positive redox potentials [43–45] . Note, simple pyridinium/carbene hybrid molecules such as V (Figure 1) show significant decomposition after 25 cycles for two‐electron cycling in the symmetrical H‐cell [24a] …”

“…[40] More recently there has been interest in harnessing two electrons from the viologen system, [41,42] or finding other multi-electron redox systems at highly negative or positive redox potentials. [43][44][45] Note, simple pyridinium/carbene hybrid molecules such as V (Figure 1) show significant decomposition after 25 cycles for two-electron cycling in the symmetrical H-cell. [24a] To assess the electrochemical stability, we performed repetitive CV measurements for 2 a-2 c (Figure 12A).…”

Organische Vier‐Elektronen Redox‐Systeme basierend auf Bipyridin‐ und Phenanthrolin‐Carben‐Architekturen

“…[40] In jüngster Zeit ist das Interesse daran gewachsen, zwei Elektronen aus dem Viologensystem zu nutzen, [41,42] oder andere Mehrelektronen-Redoxsysteme mit stark negativen (oder positiven) Redoxpotentialen zu finden. [43][44][45] Es sei darauf hingewiesen, dass einfache Pyridinium/Carben-Hybridmoleküle wie V (Abbildung 1) nach 25 Zyklen eine erhebliche Zersetzung für zwei-Elektronen-Zyklen in der symmetrischen H-Zelle zeigen. [24a] Um die elektrochemische Stabilität zu bewerten, führten wir für À Anions des n-Bu 4 NPF 6 Elektrolyten führt.…”

Imide-Based Multielectron Anolytes as High-Performance Materials in Nonaqueous Redox Flow Batteries

“…Thus, it is essential to design molecules with high molar solubility in the solvent/electrolyte mixture. Techno-economical models have presented guidelines of around 100 Ah/L for the volumetric capacity of redox-active materials to present a viable alternative to aqueous technologies. , Recent work focuses on small molecules with single redox events. ,,,,,,− However, molecules with multiple redox events would increase the volumetric capacity. ,− Importantly, this should not go at the cost of other parameters such as solubility.…”