Energy harvesting from flowing water/droplet via natural resources, such as rain drop, river water or seawater, based on water-solid interfacial phenomena is raising interest recently. Enhancing the energy harvesting performance obtained in such a way is greatly concerned. To address this challenge, a systematic experimental investigation is performed into the energy harvesting performance of a NaCl solution droplet flow over graphene supported by silicon dioxide (SiO2) substrate. It is shown that when the electrodes are placed in contact with the graphene film, the induced voltage increases by a factor of approximately three times compared to that when the electrodes are placed at a distance of 1.5 mm above the film surface. Furthermore, the magnitude of the induced voltage increases as the tilt angle of the graphene/SiO2 substrate increases to 45°, but decreases as the angle is further increased to 60° due to the reduction in the droplet contact area with the graphene surface. The effect of NaCl concentration on induced voltage is also investigated. The induced voltage is proportional to the increase of the concentration until the critical NaCl concentration of 0.6 M. Over the critical NaCl concentration, the induced voltage dramatically decreases since the cations (Na+) at the water-graphene interface are greatly screened by the large amount of anions (Cl−), and reduces the amount of electrons transferred to the graphene as a result. Finally, the magnitude of the induced voltage increases with a reducing temperature of NaCl solution due to the decrease in the degree of freedom at the water-graphene interface. Overall, these results provide a useful source of reference for the on-going development of water-based energy harvesting platform.
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