Ingenious cellulose-based materials were applied in smart piezoelectric and triboelectric nanogenerators for highly efficient energy harvesting.

Flexible polymer dielectrics for capacitive energy storage that can function well at elevated temperatures are increasingly in demand for continuously advancing and miniaturizing electrical devices. However, traditional high-resistance polymer dielectrics composed of aromatic backbones have a compromised band gap (E_g) and hence suffer from low breakdown strength and a huge loss at high temperatures. Here, based on the density functional theory (DFT) calculations, rigid and non-coplanar alicyclic segments are introduced into the polyimide backbone to overcome the incompatibility of a high glass transition temperature (T_g) and large E_g. Thanks to the large optical E_g (∼4.6 eV) and high T_g (∼277 °C), the all-alicyclic polyimide at 200 °C delivers a maximum discharge energy density (U_e) of 5.01 J cm⁻³ with a charge–discharge efficiency (η) of 78.1% at 600 MV m⁻¹, and a record U_e of 2.55 J cm⁻³ at η = 90%, which is 10-fold larger than that of the state-of-art commercial polyetherimides (PEIs). In addition, compared with aromatic polyimides, the all-alicyclic polyimide possesses a better self-clearing characteristic due to a smaller ratio of carbon to hydrogen and oxygen, which facilitates its long-term reliability in practical applications.

Flexible and eco-friendly dielectric materials with high energy density and breakdown strength have promising applications in energy storage devices. Here, cellulose nanofibril/boron nitride nanosheet nanocomposite films were fabricated from 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibrils (TOCNs) and hexagonal boron nitride nanosheets (BNNSs) in aqueous dispersions. The obtained TOCN/BNNS nanocomposites were further cross-linked by immersion in aqueous CaCl₂ solution to improve their properties. The maximum discharged energy density of the finally prepared composite film was 3.9 J cm⁻³ with only 4 vol% of BNNSs at 380 MV m⁻¹, achieving greater enhancement than the neat TOCN film with a low energy density of 0.1 J cm⁻³ at 50 MV m⁻¹. It is much higher than the previously reported highest energy density of biopolymer-based composites (0.81 J cm⁻³) and also higher than that of the commercially available best dielectric polymer BOPP (2 J cm⁻³). Moreover, the nanocomposite films exhibited excellent mechanical flexibility and significantly improved thermal stability (100 °C increase in thermal decomposition temperature).