Molecular crowding electrolyte for high-voltage flexible aqueous capacitors

The limited voltage window of aqueous electrolyte arises from water electrolysis, significantly constraining energy density and restricting the application of aqueous supercapacitors in various scenarios. Herein, a molecular crowding strategy that seamlessly integrates water and a crowding agent to form strong hydrogen bonds for higher water electrolysis overpotential is introduced to address this dilemma. The molecular crowding electrolyte (MCE) using polyethylene glycol 400 (PEG-400) as crowding agent in 2 mol L⁻¹ LiCl aqueous solution achieves an extraordinary voltage window of ∼2.8 V, which also enhances charge storage ability by promoting the desolvation process of the hydrated Li⁺ ions. In addition, interactions between hydroxyl groups of PEG molecules and surface terminations of Ti₃C₂ effectively protect Ti₃C₂ sheets from oxidation under positive potential. Consequently, a symmetric Ti₃C₂-based 2-electrode device demonstrates a wide voltage window of 1.5 V, with a high specific capacitance of 48.3 F g⁻¹. Furthermore, the high-viscosity MCEs could also be used to fabricate in-plane micro-supercapacitors printed on polyester fabric substrates, showing a superior specific capacitance of 18.3 mF cm⁻², good rate capability, and high stability under deformation. This work represents a stride toward electrochemically stable aqueous supercapacitors and offers insights into textile-based energy devices for wearable applications. © Science China Press 2025.