Introduction

Triboelectric nanogenerators (TENG) are intriguing mechanical energy harvesting devices that could power small portable devices  and detectors  or charge batteries[1,2]. The working principles of TENG are based on friction-related contact electrification. Most commonly, a TENG consists of two connected conductive electrode layers from which at least one is covered with polymer insulator.[3] Recently, several research groups have demonstrated that the performance of TENG can be enhanced using ferroelectric films as contacting surfaces. Interestingly, using ferroelectric films from the same material that are inversely polarized on the opposite side of TENG further increases the performance due to magnified electrostatic induction. It appears that the piezoelectric charges created on the ferroelectric layers during contacting (pressing) drive electrostatic induction and enhance the overall performance of TENG.  Therefore, as surface charges do not have a critical role in these nanogenerators, it would be more appropriate to refer to such devices as piezoelectric-electrostatic generators (PEEG).[4]

Results and Discussion

     Our present work showcases the role of piezoelectric effects in TENG-like devices based on ferroelectric films. To the best of our knowledge, until now the highest open circuit voltages of TENG have been 350V from 4 cm² and 1130 V from 9 cm². We are demons n both piezoelectric (PENG) and TENG. PVDF is a well-known ferroelectric polymer in which the piezoelectric charge density can be increased by adding BaTiO₃ nanoparticles.[5]

    Ferroelectric contacting electrodes for TENG were prepared from PVDF solution in dimethylformamide (DMF) by spin-coating in combination with immersion precipitation. BaTiO₃ nanoparticles in wide compositional range (0‑35 vol%) were ultrasonically dispersed in PVDF solution before spin-coating in order to enhance the piezoelectric performance of ferroelectric contacting layers. During immersion precipitation, the added non-solvent (methanol) gradually mixes with the initial solvent (DMF) in the polymer solution. The polymer separates as due to lower solubility in the solvent mixture forming a finely structured continuous solid phase. Samples were poled 

     The higher piezoelectric response of BaTiO₃/PVDF nanocomposites in comparison to bare PVDF is well described in literature and explained by increased number of piezoelectric dipoles in the film. For bare polarized PVDF piezoelectric V_OC 0.5 V was measured. V_OC was found to gradually increase with BaTiO₃ content until saturation, followed by a decline in voltages at higher concentrations (Figure 1). The latter can be attributed to degrading electromechanical coupling effects (i.e. deformability). 25 vol% was found to be the optimal concentration of BaTiO₃ in PVDF, resulting in V_OC 3.4 V (Figure 1, a). Prepared inversely polarized ferroelectric films were subsequently used as contacting layers in TENG-like PEEG device. Identical nanocomposite films were deposited on the contacting surfaces, followed by poling to establish inverse polarization. As it can be seen from Figure 1 b, just like in piezoelectric measurements, compositions containing BaTiO₃, until certain loading, show higher open circuit voltage in TENG regime, clearly indicating correlation between piezoelectric response and corresponding performance of nanocomposite film PEEG device.

Conclusion

     In conclusion, highly porous piezoelectric nanocomposites based on BaTiO₃/PVDF were successfully prepared by immersion precipitation. The piezoelectric response of obtained nanocomposites increases with increasing BaTiO₃ loading. The same sample films were used in TENG-like PEEG devices in order to study the correlation between piezoelectric response and PEEG performance. A clear correlation between these two parameters was observed, pointing out a promising avenue for further development of mechanical energy harvesting nanogenerators.