Electrowetting-assisted generation of stable high charge densities in silica-fluoropolymer electrets

Electric nanogenerators that directly convert the energy of moving drops into electrical signals require hydrophobic substrates with a high density of static electric charge that is stable in the ‘harsh environment’ created by continued exposure to potentially saline water. Charge-Trapping Electric Generators (CTEG) that rely on stacked inorganic oxide – fluoropolymer (FP) composite electrets charged by homogeneous ElectroWetting-assisted Charge Injection (h-EWCI) seem to solve both problems, yet the reasons for this success remained elusive. We systematically measured the generated charge density at variable oxide and FP thickness, charging voltage, and charging time leading to a consistent model of the charging process. We performed thermal annealing up to 230°C to assess the stability of the injected charge. We found that charge injection is controlled by an energy barrier at the water-FP interface, followed by charge trapping at the FP-oxide interface. Protection by the FP layer prevents charge densities up to -1.7 mC m-2 from degrading and the dielectric strength of SiO2 enables charge decay times up to 48h at 230°C, suggesting lifetimes of thousands of years at room temperature. Combining a high dielectric strength oxide and a weaker FP top coating with electrically controlled charging provides a new paradigm for developing ultra-stable electret materials for applications in energy harvesting and beyond.