Multifunctional ionic bonding-strengthened (Ti₃C₂T_x MXene/CNF)-(BNNS/CNF) composite films with Janus structure for outstanding electromagnetic interference shielding and thermal management

Flexible multifunctional polymer-based electromagnetic interference (EMI) shielding composite films play a pivotal role in 5 G communication technology, smart wearables, automotive electronics, and aerospace. In this work, (Ti₃C₂T_x MXene/cellulose nanofibers (CNF)-(hydroxy‑functionalized BNNS (BNNS-OH)/CNF) composite films (TBCF) with Janus structure are prepared via vacuum-assisted filtration of BNNS-OH/CNF and Ti₃C₂T_x/CNF suspension by one after another. Then ionic bonding-strengthened TBCF (ITBCF) is obtained by Ca²⁺ ion infiltration and cold-pressing technique. The Janus structure endows ITBCF with the unique “conductive on one side and insulating on the other” property. When the mass ratio of Ti₃C₂T_x and BNNS is 1:1 and the total mass fraction is 70 wt.%, the electrical conductivity (σ) of the Ti₃C₂T_x/CNF side of ITBCF reaches 166.7 S/cm, while the surface resistivity of the BNNS-OH/CNF side is as high as 304 MΩ. After Ca²⁺ ion infiltration, the mechanical properties of ITBCF are significantly enhanced. The tensile strength and modulus of ITBCF are 73.5 MPa and 15.6 GPa, which are increased by 75.9% and 46.2% compared with those of TBCF, respectively. Moreover, ITBCF exhibits outstanding EMI shielding effectiveness (SE) of 57 dB and thermal conductivity (λ) of 9.49 W/(m K). In addition, ITBCF also presents excellent photothermal and photoelectric energy conversion performance. Under simulated solar irradiation with a power density of 120 mW/cm², the surface stabilization temperature reaches up to 65.3 °C and the maximum steady state voltage reaches up to 58.2 mV.