Electrochemical Capacitance of CNF–Ti₃C₂T_x MXene-Based Composite Cryogels in Different Electrolyte Solutions for an Eco-Friendly Supercapacitor

Cellulose nanofibrils (CNFs) are promising materials for flexible and green supercapacitor electrodes, while Ti₃C₂T_x MXene exhibits high specific capacitance. However, the diffusion limitation of ions and chemical instability in the generally used highly basic (KOH, MXene oxidation) or acidic (H₂SO₄, CNF degradation) electrolytes limits their performance and durability. Herein, freestanding CNF/MXene cryogel membranes were prepared by deep freeze-casting (at −50 and −80 °C), using different weight percentages of components (10, 50, 90), and evaluated for their structural and physico-chemical stability in other less aggressive aqueous electrolyte solutions (Na₂/Mg/Mn/K₂-SO₄, Na₂CO₃), to examine the influence of the ions transport on their pseudocapacitive properties. While the membrane prepared with 50 wt% (2.5 mg/cm²) of MXene loading at −80 °C shrank in a basic Na₂CO₃ electrolyte, the capacitance was performed via the forming of an electroactive layer on its interface, giving it high stability (90% after 3 days of cycling) but lower capacitance (8 F/g at 2 mV/s) than in H₂SO₄ (25 F/g). On the contrary, slightly acidic electrolytes extended the cations’ transport path due to excessive but still size-limited diffusion of the hydrated ions (SO₄²⁻ > Na⁺ > Mn²⁺ > Mg²⁺) during membrane swelling, which blocked it, reducing the electroactive surface area and lowering conductivities (<3 F/g).