Ester-Substituted Bispyridinylidenes: Double Concerted Two-Electron Bipolar Molecules for Symmetric Organic Redox Flow Batteries

Abstract

Organic redox-active molecules are promising materials for charge storage in redox-flow batteries (RFBs); however, the development of all-organic RFBs is hindered by material crossover, limited energy density, and poor stability of active materials. Here, ester-substituted bispyridinylidenes are reported as the first examples of intrinsic bipolar molecules that exhibit basically concerted double two-electron redox activity at a potential difference of 1.01 V. All three oxidation states of the pentylester derivative exhibited excellent temporal stability and good solubility in the electrolyte. Testing this active material in symmetric cells, which alleviates crossover issues, revealed good cyclability (fade of 0.025% and 0.35% per cycle for static and flow cells, respectively), capacities of up to 89% of the theoretical value, and Coulombic efficiencies above 99%. Considering previous evidence for active material solubility limits of ∼2 M, and the benefits of a symmetric design, such double concerted multielectron bipolar active materials will be key to developing energy dense organic RFBs.

Stabilization of Naphthalene Diimide Anions by Ion Pair Formation in Nonaqueous Organic Redox Flow Batteries

“…Despite its conceptual simplicity, the role of ion pairs in the context of a two-electron redox process has been studied only in a few studies 26−30 and remains an underexplored territory in RFBs. 31,32 In this study, we delve into the utilization of ion pair formation to alter the redox behavior of naphthalene diimide (NDI) derivatives for nonaqueous organic RFBs. Our previous research revealed a remarkable redox stability of NDI, achieved in part by spin-pairing the singly reduced radicals into π−π stacked dimers in aqueous media.…”

“…In aqueous environments, incorporating proton coupling has proven effective in reducing the barrier for redox reactions and, thus, accelerating the redox reaction rate. , For example, in acidic solutions, anthraquinone and benzoquinone derivatives were reported to exhibit a single two-electron transfer reaction, highlighting the importance of proton-coupled electron transfer (PCET) in aqueous RFBs. ,− This concept can be extended to other Lewis acid cations such as lithium (Li + ) or potassium (K + ), particularly in nonaqueous media where protons are not available. , While concerted electron/cation transfer is less likely compared to PCET due to the higher mass of the cations, their ionic nature and high effective charge should facilitate the dissipation of negative charge in the reduced redox species. Despite its conceptual simplicity, the role of ion pairs in the context of a two-electron redox process has been studied only in a few studies − and remains an underexplored territory in RFBs. , …”

Designing the next generation of symmetrical organic redox flow batteries using helical carbocations

“…Along this path, open-shell compounds, acting as ambiphilic radicals with three stable states, have achieved significant success when appropriately managing their inherent reactivity [51,61,62,69] . The search for additional ambipolar compounds can be rooted in redox-active ligands featuring multiple stable states commonly found in homogeneous catalysis [63,64,70] or in organophotocatalysis characterized by three well-defined redox states [71] .…”

“…n corresponds to n-type molecules, p for p-type molecules, while Ab means for ambiphilic molecules. Examples of SORFBs based on strategy (A) could be found in the references [52][53][54][55][56][57][58] , strategy (B) at [49,50,59,60] and strategy (C) at [51,[61][62][63][64] .…”

Development of Modular Nitrenium Bipolar Electrolytes for Possible Applications in Symmetric Redox Flow Batteries

“…Even though the use of organic solvents offers an increased potential window, discovery of suitable electrolytes that allow access to extreme redox potentials (< −2 V for reduction − and >+1 V for oxidation, ,− vs Fc/Fc + ) has been challenging due to the general scaling relationship of stability to energy density (i.e., the more extreme the potential, the more reactive the charged species). Therefore, developing electrolytes that balance stability and energy density to optimize the cell voltage remains a significant aim of the community.…”