Fig. 2. Critical factors that effect on the real output charge density.

a Capacitance of the capacitor made of dielectrics with different thickness (by depositing electrode, electrode area: 3.2 × 3.2 cm). b The schematic of contact status when using a rigid electrode (up) and soft gel electrode (down). c The capacitance of main TENG devices with six contact optimization compared to the one with deposited electrode (regarded as fully sufficient contact; dielectric thickness: 4 μm). d Electric scheme of CE-TENG when exciting by a voltage source. e Output charge density when applying different bias voltage (operation frequency: 1 Hz, relative humidity: 5%, dielectric thickness: 4 μm). f Experimental and theoretical charge density of CE-TENG with different dielectric thickness. g Output charge density of CE-TENG when varying atmospheric humidity (dielectric thickness: 4 μm PEI, bias voltage: 400 V). h Output charge density of CE-TENG with different contact status (dielectric thickness: 4 μm, bias voltage: 350 V). i The actual contact area and actual charge density of main TENG devices with six contact optimization (electrode area: 10 cm², dielectric thickness: 4 μm).