Ultra-thin flexible paper of BNNT–CNF/ZnO ternary nanostructure for enhanced solid-state supercapacitor and piezoelectric response

Cellulose nanofibers (CNFs) and boron nitride nanotubes (BNNTs) are light-weight, eco-friendly and flexible materials with diverse chemical, physical, mechanical, piezoelectric and energy storage properties. In particular, their exceptional properties have been exploited in piezoelectric and supercapacitor applications. However, it is fairly challenging to enhance the supercapacitor and piezoelectric performances of ultrathin paper. Thus, a new strategy with a BNNT/CNF–inorganic ternary nanostructure is required in which the outstanding energy storage and conversion of inorganic materials simultaneously enhance the performance of supercapacitors and their piezoelectric properties. Herein, we have synthesized ZnO nanoparticles (NPs) on CNF and BNNT surfaces through cost-effective and facile hydrothermal method to give a ternary nanostructure and explored the electrochemical and piezoelectric performance. The BNNT–CNF/ZnO ternary nanostructure displays a remarkable specific capacitance of 300 F g−1 with high energy (37.5 W h kg−1) and power density (0.9 kW kg−1) at a current density of 1 A g−1. In terms of the flexibility of BNNT–CNF/ZnO, an ultra-thin flexible solid-state symmetric supercapacitor was fabricated, which demonstrated a high specific capacitance of 94 F g−1 under 1 A g−1 current density along with noticeable cycling stability (97% over 5000 cycles) and remarkable energy density (30.3 W h kg−1 at 1.3 kW kg−1). Furthermore, we examined the piezoelectric effect by employing a specific force on the BNNT–CNF/ZnO nanostructure flexible paper with different paper thicknesses. The BNNT–CNF/ZnO paper-based device presented an effective piezoelectric coefficient (d33) of −12.6 pC N−1 with 90 μm thick paper which is higher than that of the BNNT–CNF paper (−7.9 pC N−1). Therefore, this work will pave the way for the development of innovative next-generation flexible energy storage devices to generate recycled energy.