Redox-active low transition temperature mixtures of propylene glycol and iodide salts as electrolytes for hybrid electrochemical capacitors

• Redox-active low transition temperature mixtures form via iodide–OH hydrogen bonding • Cation type affects transport properties in order of: K⁺ > NH₄⁺ > Na⁺ > Li⁺ • KI:propylene glycol capacitor shows specific energy 12.8 Wh·kg⁻¹ at room temperature • 6.0 Wh kg -1 of specific energy is available during a 60 s discharge at 0°C • KI:propylene glycol-based system retains 74 % capacitance after 10,000 cycles This work reports the first synthesis and systematic investigation of redox-active low-transition-temperature mixtures based on propylene glycol and iodide salts containing different monovalent cations (Li⁺, Na⁺, K⁺, NH 4 ⁺), and evaluates their application as electrolytes for hybrid electrochemical capacitors. It is shown that unlike iodide–ethylene glycol systems classified as deep eutectic solvents, these liquids undergo vitrification rather than classical crystallization. Extensive hydrogen-bonding interactions enable them to remain liquid down to −90°C while providing efficient ion transport and electroactive iodide species. Comprehensive spectroscopic, thermal, and ionic transport characterization reveals a strong dependence of these properties on the cation type. At 25°C, mixtures with larger, weakly solvated cations showed the highest conductivity (3.1 mS cm⁻¹ for K⁺) and the lowest viscosity (49.1 mPa·s for NH₄⁺), following the general trend K⁺ ≳ NH₄⁺ > Na⁺ > Li⁺. Electrochemical evaluation over 0–80°C showed that the KI:propylene glycol-based capacitor exhibits the most balanced performance, delivering a room-temperature capacitance and specific energy of 40.9 F g⁻¹ and 12.8 Wh kg⁻¹, respectively, and 6.0 Wh kg⁻¹ during a 60 s discharge at 0°C, outperforming Li⁺- and Na⁺-based analogues (2.7–3.3 Wh kg⁻¹). Long-term cycling tests showed good stability for all alkali metal-based electrolytes, exemplified by the K⁺-based system (∼74 % capacitance retention, ∼81 % coulombic efficiency over 10,000 cycles at 1.5 V), while NH₄⁺-containing mixtures degraded due to ammonium ion decomposition.These results establish these mixtures as a new, sustainable class of redox-active electrolytes and provide quantitative guidelines for electrolyte design through cation selection.