Drivers of membrane fouling in the vanadium acetylacetonate flow battery

Vanadium acetylacetonate (V(acac)₃) disproportionation electrochemistry promises a crossover-tolerant, high-voltage flow battery, but exhibits low efficiency and short cycle life. We show that membrane fouling, rather than a parasitic side reaction, dominates early performance fade. Crossover rates through porous membranes were estimated from voltage transients with an adaptive observer while cycling flow-through reactors. For $0.1\text{M}$ V(acac)₃ and $0.3\text{M}$ TEABF₄ in acetonitrile flowed countercurrently at $5.0\text{cm}{\text{s}}^{-1}$ parallel to the separator, fresh Daramic 175 and Celgard 4650 afforded active-species mass-transfer coefficients of $3.8\mu \text{m}{\text{s}}^{-1}$ and $7.5\mu \text{m}{\text{s}}^{-1}$, respectively, which decreased and became non-Fickian as cycling progressed. At $\pm 10\text{mA}{\text{cm}}^{-2}$ from 0%–20% state of charge, voltage efficiency with Celgard fell from 96% to 60% over 27 cycles. Separator replacement restored the coulombic and voltage efficiencies, which repeated their first progression. Impedance spectra from series-connected canary cells reveal that separator resistances remain stable during open-circuit exposure to charged single electrolytes, but increase under applied current or open-circuit contact with differently charged electrolytes.