Influence of ambient humidity on contact electrification between nonionic insulators

Contact electrification is an interfacial process in which two surfaces exchange electrical charges when rubbed against each other. Consequently, the surfaces may gain opposite polarity, leading to the onset of an electrostatic attraction. This principle is applied in triboelectric nanogenerators (TENGs). However, the underlying mechanisms' details are still ill-understood, especially the influence of relative humidity (RH). Using the colloidal probe technique it is convincingly shown that when two insulators touch at ambient conditions, water plays an important role in the charge accumulation process due to the presence of H+ and OH- ions. The charging process is enhanced and faster with increasing relative humidity, also beyond RH = 40\%, due to the geometrical asymmetry introduced in the system. In addition, the time constant of charging is determined, which depends heavily on the relative humidity.