Quantifying the triboelectric series

Abstract

Triboelectrification is a well-known phenomenon that commonly occurs in nature and in our lives at any time and any place. Although each and every material exhibits triboelectrification, its quantification has not been standardized. A triboelectric series has been qualitatively ranked with regards to triboelectric polarization. Here, we introduce a universal standard method to quantify the triboelectric series for a wide range of polymers, establishing quantitative triboelectrification as a fundamental materials property. By measuring the tested materials with a liquid metal in an environment under well-defined conditions, the proposed method standardizes the experimental set up for uniformly quantifying the surface triboelectrification of general materials. The normalized triboelectric charge density is derived to reveal the intrinsic character of polymers for gaining or losing electrons. This quantitative triboelectric series may serve as a textbook standard for implementing the application of triboelectrification for energy harvesting and self-powered sensing.

Fig. 1

Principle for measuring the triboelectric charge density. a Simplified model of the measurement method. The tested material contacts the liquid metal of mercury, and then separates periodically. The positive electrode of the electric meter is connected to the mercury, and the negative is connected to the copper electrode. b–e The Theoretical model under open-circuit condition. b The contact electrification causes charge transfer between materials, the charges coincide at the same plane. The system has no net charge, there is no potential difference. c Voltage is generated between the two electrodes. Suppose the polymer has a strong capacity to absorb electrons, when the materials are separated, the polymer has negative charges, mercury has positive charges. Therefore, the potential difference is created between the two parts. d The potential reaches the maximum when the gap reaches certain distance L. The copper electrode is only influenced by the electric fields of the charges on the surface of the polymer. e When the polymer approaches the mercury, voltage drops due to the combined influence of the two electric fields. Finally, they are fully contacted, there is no voltage between the two electrodes (back to b). f–i The theoretical model under a short-circuit condition. f The two materials fully contact each other, there is no potential difference. g When the materials are separated, the negative charges on the surface of the polymer induce positive charges in copper, as copper and mercury are electronically connected, the positive charges in mercury flow into the copper side. h Approximately all charges flow into copper side to equalize the potential difference when the gap reaches certain distance L. i When the sample approaches the mercury, the negative charges on the surface of the polymer induces positive charges in mercury, the positive charges flow from copper to mercury until the charges are neutralized finally at the same plane when they are fully contacted (back to f)

Fig. 2

Experimental set-up for the triboelectric series measurement. The whole measurement was set in a glove box filled with ultra-high purity of nitrogen gas at fixed temperature, pressure, and humidity. The linear motor was settled on a high-load lab jack (a). The height of the sample to contact the liquid metal can be finely adjusted by both the high-load lab jack and a linear motor. The static part has the liquid metal as the electrode. The motion part consists of the tested sample, and it is controlled by the linear motor (b). An acrylic base is attached to the end of the linear motor. A magnet is engraved into the acrylic base to attract another magnet engraved in the sample substrate. Each sample consists of an acrylic substrate with the magnet, electrode, and the tested materials (c). Both the miniature platform optical mount and the two-axis tilt and rotation platform can adjust the orientations of the sample and liquid metal level

Fig. 3

Typical measured signals. a A typical output of open-circuit voltage in two cycles of contact and separation. b Short-circuit transferred charge between the two electrodes in two cycles. c The measured charge transferred for three samples for the same material of PTFE. d Stability of the measured value over a relatively long time-period for many cycles

Fig. 4

The quantified triboelectric series. The error bar indicates the range within a standard deviation. Source data are provided as a Source Data file