Insights and perspectives on graphene-PVDF based nanocomposite materials for harvesting mechanical energy

A great deal of research work on sustainable and renewable energies is carried out to satisfy the incremental requirement for miniature, uninterrupted and self-reliant power supply. In the pursuit of such sources of energy, research work is focused on such devices that can harvest energy at ambient conditions and convert it into electrical energy for subsequent usage. Nanogenerator based on the piezoelectric effect is considered an exciting candidate for harvesting mechanical energy from the ambiance. Here we will discuss the working principles of piezoelectric nanogenerators, followed by introducing different types of piezoelectric materials. This review article provides recent insights into the synthesis, characterization, properties, performance, and applications of graphene, its derivatives in polyvinylidene fluoride (PVDF), and its co-polymers for efficient piezoelectric energy harvesting. PVDF and its copolymers are a class of piezoelectric polymer that can be molded to make flexible energy harvesting devices owing to their strong electroactive properties, simple processability, and good endurance. However, the electrical energy derived from the pure PVDF is minimal in real applications. Consequently, they require unique treatments such as drawing, poling, adding filler materials, etc. Adding filler materials is an advantageous option as it reduces cost, induces ease of fabrication, and displays the enhanced electrical output. This review will cover the recent advances in PVDF-based piezoelectric nanogenerator using graphene-based filler, followed by the discussion based on the addition of carbon nanotubes (CNT) in PVDF and its co-polymers. In addition, the review will cover the introduction of metal-oxide/graphene and metal-oxide/CNT-based nanocomposites in PVDF, which shows improved mechanical energy harvesting properties. The electrospinning technique to increase the piezoelectricity of the graphene-PVDF nanocomposite is also elaborated. It is evidenced that the enhancement in the piezoelectricity of PVDF is due to the nucleation of polar piezoelectric β and γ phases. At low nanofiller concentration, the electrical conductivity inside the polymer matrix increases, which increases the d33 coefficient. However, on attaining the percolation threshold, the output of the piezoelectric device reduces drastically. Theoretical studies demonstrated how the shape and size of graphene particles affect the percolation threshold. The issues arise due to the aggregation of the graphene-based particles in PVDF that affects the piezoelectric properties. It was realized that loading metal oxide nanoparticles in reduced graphene oxide enhance its dispersibility within the polymer matrix. The piezoresponse force microscopy (PFM) characterization technique which characterizes the piezoelectric properties, is also reviewed. Finally, we mention the challenges and the way forward to integrate graphene-PVDF-based mechanical energy harvesting systems towards self-sustainable applications.