The substrate used was copy paper. The surface area of the FNG cell was ~2.5 cm 2 . Cyanoacrylate glue was used for the device fabrication. The Mn3O4 membranes were mixed with cyanoacrylate (glue) in very low concentration, the homogeneous slurry was prepared with 1 mg: 1 ml ratio of sample to adhesive solution under constant stirring and was coated on to the paper substrate (first dielectric layer (d1)). Kapton tape was used as the second dielectric layer (d2). Aluminum and copper were used to form contacts at either ends (Schematic in Figure ). The entire arrangement was enclosed using a single layer of transparent polythene sheet to secure the device. Two separate nanogenerator devices were fabricated using either bulk Mn3O4 or Mn3O4 membranes for comparative testing. The thickness of the dielectric layer was ~ 80 µm, which is the sum of the substrate (50 µm), Mn3O4 membrane sample (2 -6 nm) and the adhesive (30 µm). 𝜀 𝑟 is the relative permittivity of the dielectric layer (3.780), 𝜀 𝑜 is the relative permittivity of air (8.854 x 10 -12 F m - 1 ) and Z is the gap between the electrodes; in this case, we use the height of the substrate to Kapton of 125 ± 10 µm. The above details were used for device calculations accordingly.