Biomimetic and electrostatic self-assembled nanocellulose/MXene films constructed with sequential bridging strategy for flexible supercapacitor

Despite the two-dimensional titanium carbide MXene (Ti3C2Tx) has shown promising applications in energy storage, the inherent self-stacking and weak connectivity among nanosheets present a challenge in manufacturing robust MXene films for emerging flexible supercapacitors. Herein, inspired by the robust "soft-hard" structured exoskeletons of crustaceans, the biomimetic cellulose nanofibers (CNFs)/MXene-Al (CM-Al) film with integrated high mechanical toughness, electroconductivity and electrochemical behavior, sequential bridging with hydrogen and ionic bonds, is crafted via the facile electrostatic self-assembly strategy. The embedded CNFs guide the in-plane orientation of MXene through hydrogen bonding, forming a "soft-hard" microstructure and dramatically enhancing the mechanical toughness of CM-Al. The rigid ionic bonds established between the oxygen-containing groups on CNFs and MXene with Al3+ further elevate the mechanical strength (163.88 MPa) and act as additional electron transfer channels to impart the reinforced electroconductivity (82.63 S cm−1). Benefiting from the more active site exposure induced by the expanded MXene layer spacing after CNFs embedding and the weakened intrinsic resistance caused by the constructed ionic bonding, the assembled flexible quasi-solid-state symmetric supercapacitor with CM-Al as electrode delivers a high area capacitance (679 mF cm−2), energy density (16.2 μWh cm−2), cycling capability (85.3 % capacitance retention after 10,000 cycles) and intrinsic tolerance to various non-stretching deformations. Moreover, the selective principles for metal ions are summarized that involved a comprehensive consideration of ionic radius, charge density and basic chemical properties. This work demonstrates an accessible and feasible construction strategy for flexible composite films and provides an alternative pathway for wearable energy storage devices.