Reversal in Output Current Direction of 4H-SiC/Cu Tribovoltaic Nanogenerator as Controlled by Relative Humidity

Tribovoltaic nanogenerators (TVNG) represent a fantastic opportunity for developing low-frequency energy harvesting and self-powered sensing, by exploiting their real-time direct-current (DC) output. Here, a thorough study of the effect of relative humidity (RH) on a TVNG consisting of 4H-SiC (n-type) and metallic copper foil (SM-TVNG) is presented. The SM-TVNG shows a remarkable sensitivity to RH and an abnormal RH dependence. When RH increases from ambient humidity up to 80%, an increasing electrical output is observed. However, when RH rises from 80% to 98%, the signal output not only decreases, but its direction reverses as it crosses 90% RH. This behavior differs greatly from that of a Si-based TVNG, whose output constantly increases with RH. The behavior of the SM-TVNG might result from the competition between the built-in electric field induced by metal-semiconductor contact and a strong triboelectric electric field induced by solid-liquid triboelectrification under high RH. The authors also demonstrated that both SM-TVNG and Si-based TVNG can work effectively as-is even fully submerged in deionized water. This mechanism can affect other devices and be applied to design self-powered sensors working under high RH or underwater.