Granular Interfaces in TENGs: The Role of Close-Packed Polymer Bead Monolayers for Energy Harvesters.

Over the last decade, triboelectric nanogenerators (TENGs) are proposed as a viable alternative to address the impetus for affordable and clean energy. Here, a novel, cost-effective granular-based TENG comprising two electrodes covered with HCP monolayers of monodisperse polymer (PMMA, PS, and MF-resin) beads with diameters ranging between 0.5 and 10 µm is proposed. These monolayers are attained in <20 s by employing a solvent-free particle rubbing assembly technique on fluorocarbon-coated substrates. The performance of the proposed granular-based TENG is characterized using contact-separation (CS) experiments by changing the bead sizes (topography effects) and the polymer material (mechanical properties). These findings show that when identical polymer material is utilized, large beads charged negatively, and the small beads positively, coinciding with bulk polymer film reports. In addition, the MF particles always charge positively and show the highest charging due to their relatively higher Young's modulus. The results elucidate that a specific pair's surface charge density is enhanced when one of the electrodes is covered with the smaller bead with the highest Young's modulus, highlighting that mechanical properties dominate and that a substantial difference in size benefits the output. The stable performance of the TENG devices after 10 000 cycles corroborates its robustness.