Associative pyridinium electrolytes for air-tolerant redox flow batteries

Organic redox-active molecules for redox flow batteries (RFBs) promise cost and energy-density advantages over metal-based chemistries. Viologens (4,4′-bispyridinium species) show high aqueous solubility, negative potentials, and good stability under neutral conditions, and have been used as anolytes in single- and double-electron couples. Despite this, systematic understanding of their electrochemical and degradative processes under realistic flow conditions—especially their tolerance to air—is limited, as most studies have used strictly air-free conditions. Reduced bispyridinium mono- and diradicals can readily reduce dissolved O2, producing reactive oxygen species that degrade components and electrolytes. Radical-associated assemblies (π-dimers, σ-dimers, charge-transfer complexes) have been linked to capacity fade, guiding designs to avoid them. This study constructs a structural library of bispyridinium electrolytes and applies operando metrologies to elucidate redox behavior, establish a general descriptor for reversibility, and derive principles for air-tolerant RFB operation.

Prior work has demonstrated viologens as effective aqueous anolytes with tunable redox properties and highlighted cost and lifetime considerations for organic RFBs. Classical studies on viologens detailed their electrochemistry and radical stabilization, and conjugation-extended viologens have been explored to increase capacity. Substituent (N-alkyl) effects on redox behavior are known, and Hammett correlations inform electronic tuning. Previous reports often implicated radical-associated assemblies (π-dimers, σ-dimers, charge-transfer complexes) as contributors to capacity fade, encouraging designs to minimize such associations. Viologens are also established redox mediators for O2 reduction, underscoring potential parasitic reactions under air exposure. However, comprehensive operando mechanistic elucidation under representative flow conditions, particularly addressing air tolerance and the role of radical association, has been lacking.

- Molecular design and synthesis: Built a library of extended bispyridinium compounds by inserting varied aromatic cores between pyridinium rings. Extended bipyridines (1–9) were synthesized via Suzuki–Miyaura coupling from 4-pyridinyl-boronic acid and aryl dibromides (homogeneous and heterogeneous catalysis), then N-alkylated to form 3-trimethylammonium-propyl (TMAP)-functionalized bispyridiniums (10–19). - Computational screening: Density functional theory (DFT) used to assess R-group effects and calculate redox potentials. Strong linear correlations with Hammett σm constants were found; redox potential gaps are governed primarily by the core. DFT computed singlet–triplet free energy gaps (ΔEST) for 10–19 (−27.9 to +3.6 kcal mol−1). Additional DFT and CCSD(T) calculations probed π-dimerization and electron-transfer processes. - Electrochemical characterization: Cyclic voltammetry assessed reversibility and potentials in aqueous media. First redox potentials for reversible cases spanned −0.35 to −0.82 V vs SHE; notably, compounds 12 and 13 reached −0.77 and −0.82 V, respectively. - Operando spectroscopy: Coupled operando NMR and EPR experiments under flow battery-relevant conditions quantified radical speciation. From these data, comproportionation (Kc) and dimerization (Kd) equilibrium constants were determined. Spectroelectrochemistry assigned bands for monoradicals and π-dimers. - Full-cell testing: Constructed full cells with bispyridinium anolytes and 4-hydroxy-TEMPO catholytes in NaCl electrolyte. Evaluated 10, 11, 13 at 10 mM (100 mM NaCl; current 2 mA cm−2) with cutoffs 0.5–2.0 V and 1 h holds at cutoffs. For compound 11, varied concentrations (1, 5, 10 mM) with corresponding currents (0.2, 1, 2 mA cm−2) and observed capacity retention and Coulombic efficiency. Additional testing at higher concentrations: 25 mM and 50 mM (500 mM NaCl; current 5 mA cm−2) cycling under N2, air, then N2; and 250 mM cells at 20–40 mA cm−2 with one-electron operation (upper cutoff 1.65 V) to isolate monoradical/π-dimer behavior. - Online electrochemical mass spectrometry (OEMS): Monitored O2 and H2 partial pressures during charge–discharge at 1 mM and 50 mM 11 under 1% and 20% O2 in Ar, including 2 h potential holds at 1.95 V, to diagnose oxygen reduction pathways and quantify O2 consumption. - Kinetic comparisons: Compared heterogeneous electron-transfer rate constants: ko,11 = 1.98 × 10^2 cm s−1; ko,17 = 2.8 × 10−3 cm s−1; ko,O2 = 8.4 × 10−3 cm s−1, to evaluate competition between direct electrode O2 reduction and bispyridinium-mediated pathways.

- Singlet–triplet gap as descriptor: ΔEST correlates strongly with electrochemical reversibility. Compounds with ΔEST < −6 kcal mol−1 (10, 11, 12, 13, 17, 18) show reversible redox; −6 to 0 kcal mol−1 (15, 16) lose reversibility, likely due to reactive triplet diradicals; ΔEST ≈ 0 kcal mol−1 (non-Kekulé 14, 19) are irreversible. - Two regimes of behavior: Wide-gap compounds (10, 11, 17) exhibit closed-shell singlet character upon double reduction and lower capacity fade; narrow-gap compounds (13, 18) have thermally accessible triplet diradicals and higher capacity fade. Operando NMR/EPR confirm distinct spectral signatures and radical speciation across regimes. - Capacity fade tied to radicals: Across compounds, capacity fade correlates with formation/persistence of open-shell species (monoradicals or diradicals). Lower radical concentrations across SOC (low Kc, high Kd, high ΔEST) improve capacity retention. - Role of π-dimerization: Contrary to prevailing views, π-dimerization mitigates capacity fade by pairing monoradicals, reducing free radical concentration and suppressing parasitic reactions, including O2 reduction. - Concentration dependence and performance (compound 11): • At 10 mM: Coulombic efficiency ~77%; capacity fade ~0.01% per cycle. • At 5 mM: Coulombic efficiency ~74%; capacity fade ~9.64% per cycle. • At 1 mM: Coulombic efficiency ~18%; onset of alternative charging at ~0.82 V vs 4-hydroxy-TEMPO, large pH increase (7→12), and near-complete capacity loss without evidence of molecular decomposition. - Oxygen reduction and air tolerance: • OEMS (1 mM 11): strong O2 consumption during charging; no H2 evolution, implicating O2 reduction (peroxide pathway, E = −0.065 V vs SHE) mediated by 11^3+ as a parasitic process leading to OH− formation. • At higher concentrations (25–50 mM 11): capacity stable under N2; under air, 25 mM loses capacity while 50 mM remains nearly unchanged; N2 sparging restores capacity for 25 mM. OEMS shows >5× lower O2 consumption per mole 11 at 50 mM vs 1 mM; O2 consumption depends on bispyridinium concentration, not Henry’s law partitioning. • Mechanistic suppression: (1) Faster direct electron transfer to bispyridinium vs O2 at the electrode; (2) π-dimerization lowers the fraction of mediator monoradicals. Kinetic data (ko,11 ≫ ko,O2) support suppression of O2 reduction at sufficient concentration. Similar behavior for compound 17 indicates generality. - Long-term cycling in air: A 250 mM 11 cell cycled for 386 total cycles, including 331 in air at 20–40 mA cm−2, with stabilized fade of ~1.41% per day over 200 cycles (0.021% per cycle) at 30 mA cm−2 and no substantial capacity loss upon current density changes. Initial capacity increased by ~12.3% over first ~130 cycles in air before slow fade; analogous stable performance observed for 17. - Electrochemical metrics: Reversible first redox potentials span −0.35 to −0.82 V vs SHE; 12 and 13 reach −0.77 and −0.82 V (most negative for unsubstituted cores).

The study addresses the central question of how to achieve air-tolerant, stable cycling with pyridinium-based (viologen-like) electrolytes by uncovering mechanistic determinants of reversibility and degradation. The singlet–triplet free energy gap provides a predictive descriptor for the onset of irreversibility across structural variants, enabling identification of two performance regimes. Capacity fade is mechanistically linked to open-shell species: elevated monoradical concentrations (from high Kc and low Kd) and thermally accessible diradicals (narrow ΔEST) promote parasitic pathways including O2 reduction and side reactions. Crucially, π-dimerization—previously implicated in capacity fade—is shown to be protective by pairing spins and reducing mediator availability for O2 reduction. Together with favorable electron-transfer kinetics from the electrode to bispyridinium species (ko for 11 >> ko for O2), this reduces both indirect (mediated) and direct oxygen reduction pathways at higher concentrations, enabling stable cycling in air. The framework unifies molecular electronic structure (ΔEST), supramolecular association (Kc, Kd), and interfacial kinetics to guide electrolyte design and operating conditions. These insights overturn the prevailing view of π-dimers as detrimental and generalize across homocyclic and heterocyclic cores (e.g., 11 and 17).

An extensive library of associative bispyridinium electrolytes reveals: (1) the singlet–triplet energy gap as a universal descriptor predicting electrochemical reversibility; (2) two distinct regimes of performance (wide-gap closed-shell vs narrow-gap diradical); and (3) enhanced cycling and air stability mediated by π-dimerization, which suppresses radical-mediated parasitic reactions. Air-tolerant operation is enabled by controlling intra- and intermolecular radical pairing (high ΔEST, low Kc, high Kd) and leveraging faster electron transfer to bispyridinium species than to oxygen. These findings challenge the notion that π-dimerization intrinsically causes capacity fade and instead promote molecular designs with Kekulé cores that favor closed-shell reduced states and strong π-dimerization of monoradicals. The resulting principles can guide development of pyridinium and broader organic RFB electrolytes with robust air tolerance and long cycle life.

Air tolerance and low fade are strongly concentration-dependent, relying on sufficient radical association (π-dimerization) to suppress mediated O2 reduction; at low electrolyte concentrations (e.g., 1 mM), parasitic O2 reduction dominates, causing rapid capacity loss and pH drift. The demonstrated stability focuses on selected representatives (notably compounds 11 and 17) and specific aqueous NaCl/4-hydroxy-TEMPO full-cell configurations and one-electron operation conditions. Some oxygen reduction persists (evidenced by pH increases), though its impact is minimized at higher concentrations.