Increased performance of an all-organic redox flow battery model via nitration of the [4]helicenium DMQA ion electrolyte

Abstract

Symmetrical non-aqueous organic redox flow batteries are highly promising systems for electrochemical energy storage. A higher solubility and energy density of a previously described robust ROM was obtained by straightforward nitration of its core.

Building Bridges: Unifying Design and Development Aspects for Advancing Non-Aqueous Redox-Flow Batteries

“…Their first study involved the plain carbenium ion, which is able to reduce to a neutral radical, and oxidize to a dicationic radical state with a ∆E of 2.12 V, paired with high diffusion coefficients (9.4 and 9.9 × 10 −6 cm 2 s −1 , respectively). Although the dicationic radical of the plain carbenium could not be isolated, a follow-up study with a nitro-substituted carbenium exhibited increased stability to all redox-states, including the doubly reduced form, allowing for an impressive 3.02 V OCV (Figure 13) [114]. The updated design also yielded a large increase in solubility from 32 to 154 mM with the nitro-substituent, leading in turn to higher energy densities also (9.25 W h L −1 versus the previous 1.84 W h L −1 , with 12.5 W h L −1 upon double reduction).…”

“…A1 = N-methylphthalimide [96,97], A2 = quinoxaline [98], A3 = anthraquinone [99], A4 = benzophenone [100], A5 = viologen [101,102], A6 = azobenzene [103,104]. B1 = N-ferrocenylphthalimide [105], B2 = 1,4-diaminoanthraquinone [106,107], B3 = thianthrene [108], B4 * = 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) in ionic liquid [109], B5 = 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide [110,111], B6 = 1,5-diphenyl-3-isopropyl-6-oxo-verdazyl [36], B7 = 1,2,4-benzotriazin-4-yl [112], B8 = dimethoxyquinacridinium [113,114], C1 = 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) [96], C2 = dialkoxybenzene [98,[115][116][117][118][119], C3 = ferrocene [120][121][122], C4 = phenothiazine [99], C5 = triarylamine [123].…”

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

“…One promising recent design strategy involves the direct π-conjugation of typical anode and cathode active species, [29][30][31]42 where such electronic coupling causes synergistic benefits in cell potentials compared to the parent molecules. 29,30 Most intrinsically bipolar molecules are based on oneelectron redox events (one-electron oxidation and one-electron reduction), 14,18,[26][27][28][29][30][31][32][33][34][35]38,39 which severely limits energy densities (the product of cell voltage, active material concentration, and number of electrons transferred). Indeed, a previous analysis has suggested that for nonaqueous organic RFBs to be cost-effective, active material concentrations as high as 4−5 M would be needed (assuming one-electron processes and cell voltages of 3 V).…”

“…), such that the low and high redox potential units are discrete and largely have the properties of their respective unbridged parent structures. The other is intrinsic, which features a conjugated framework with electronic communication between atoms associated with reduction and those associated with oxidation (including cases where those atoms are largely identical). ,,− Intrinsically bipolar molecules therefore offer advantages in term of atom economy but are more challenging to design than artificial alternatives. One promising recent design strategy involves the direct π-conjugation of typical anode and cathode active species, − , where such electronic coupling causes synergistic benefits in cell potentials compared to the parent molecules. , …”

“…Most intrinsically bipolar molecules are based on one-electron redox events (one-electron oxidation and one-electron reduction), ,,− ,, which severely limits energy densities (the product of cell voltage, active material concentration, and number of electrons transferred). Indeed, a previous analysis has suggested that for nonaqueous organic RFBs to be cost-effective, active material concentrations as high as 4–5 M would be needed (assuming one-electron processes and cell voltages of 3 V) .…”