Co(II) Complex Promoted PVDF β-Phase Crystallization: Innovations in Pressure Sensing and Energy Harvesting

Abstract

The development of self-powered piezoelectric nanogenerators (PENG) using conventional bulk oxides has been hindered by several issues, including heavy metal toxicity, limited flexibility, and complex synthetic processes. These challenges can potentially be addressed by employing discrete molecular-based metal complexes. However, research in this area remains limited due to the difficulties in controlling non-centrosymmetric point/ space groups. In this manuscript, we present a Co(II) complex with the molecular formula [Co(bpy)3] (PF6)2 (referred as Co-bpy hereafter; where bpy = bipyridine), which crystallizes in the polar P31 space group. A flexible PENG device was fabricated using a PVDF composite containing various weight/volume % of Co-bpy (3% (PVH3), 5% (PVH5), 7% (PVH7), 10% (PVH10), and 15% (PVH15)). Our detailed investigation reveals that the Co-bpy filler triggers the crystallization of the electroactive β-phase of PVDF under mild conditions, a typically challenging task with few reports in the literature. Among the composites tested, PVH10 exhibited a five-fold increase in open-circuit output voltage of 13.5 V peak-to-peak compared to pristine PVDF. This is attributed to the combined piezoelectric effect of both the Co-bpy filler and the polar β-phase of PVDF. For PVH10, we observed a power density of 1.16 µW cm⁻² and an open-circuit output current of 1.3 µA. Additionally, we demonstrated that the generated power could be stored in capacitors (3.3 µF and 10 µF) in less than 2 minutes. Furthermore, we developed highly sensitive pressure sensors capable of detecting pressures as low as 5 kPa with a mechanical sensitivity of 0.45 V/kPa.