Tuning biphasic electrolytes for membrane-free redox flow batteries: influence of sodium thiocyanate on partition and viscosity

Membrane-free redox flow batteries (RFBs) offer a promising alternative to conventional systems by removing costly and degradation-prone ion-selective membranes. However, achieving low cross-contamination of redox-active species while maintaining low viscosity remains a key challenge. This study investigates the effect of sodium thiocyanate (NaSCN) on the partitioning of methyl viologen (MV) and TEMPO, as well as on the thermophysical properties of aqueous biphasic systems (ABS) designed for membrane-free RFBs. Four ABS were studied, comprising ammonium sulfate ((NH₄)₂SO₄) and various ionic liquids (ILs): [N 4444 ]Cl, [N 4444 ]Br, [P 4444 ]Cl, and [P 44414 ]Cl. NaSCN addition promoted anion exchange, replacing Cl − and Br − with SCN − , leading to strong segregation of IL cations and SCN − in the IL-rich phase and displacement of hydrophilic ions and water to the (NH 4 ) 2 SO 4 -rich phase. This improved MV/TEMPO separation but also increased the viscosity of the IL-rich phase, especially at NaSCN concentrations above 9 wt%. At moderate concentrations (~3.5 wt%), NaSCN enhanced species partitioning while keeping IL-rich phase viscosities below 20 mPa·s (except for [P 44414 ]Cl). The IL molecular structure, particularly alkyl chain length, critically influenced both MV/TEMPO selectivities and electrolyte viscosities. COSMO-RS simulations successfully predicted phase preferences but showed deviations in partition coefficients, especially at high NaSCN levels. These results reveal a tunable trade-off between redox species separation and viscosity, driven by IL composition and NaSCN content, and offer a foundation for designing efficient, scalable electrolytes for membrane-free RFB applications. • Anion exchange (SCN over Cl/Br) increased ABS selectivity of redox active species. • Equilibrated 3.5 wt% NaSCN resulted in optimum ABS viscosity, density and partition. • Excess NaSCN (>9 wt%) increases viscosity on IL-rich phase, by water expulsion. • Phosphonium excel over ammoniums in partition efficiency, specialty long chain ones. • COSMO-RS predicts partition of redox active materials with qualitative accuracy only.