Probing Contact Electrification: A Cohesively Sticky Problem

Abstract

Contact electrification and the triboelectric effect are complex processes for mechanical-to-electrical energy conversion, particularly for highly deformable polymers. While generating relatively low power density, contact electrification can occur at the contact–separation interface between nearly any two polymer surfaces. This ubiquitousness of surfaces enables contact electrification to be an important phenomenon to understand energy conversion and harvesting applications. The mechanism of charge generation between polymeric materials remains ambiguous, with electron transfer, material (also known as mass) transfer, and adsorbed chemical species transfer (including induced ionization of water and other molecules) all being proposed as the primary source of the measured charge. Often, all sources of charge, except electron transfer, are dismissed in the case of triboelectric energy harvesters, leading to the generation of the “triboelectric series”, governed by the ability of a polymer to lose, or accept, an electron. Here, this sole focus on electron transfer is challenged through rigorous experiments, measuring charge density in polymer–polymer (196 polymer combinations), polymer–glass (14 polymers), and polymer–liquid metal (14 polymers) systems. Through the investigation of these interfaces, clear evidence of material transfer via heterolytic bond cleavage is provided. Based on these results, a generalized model considering the cohesive energy density of polymers as the critical parameter for polymer contact electrification is discussed. This discussion clearly shows that material transfer must be accounted for when discussing the source of charge generated by polymeric mechanical energy harvesters. Thus, a correlated physical property to understand the triboelectric series is provided.

Energy Interplay in Materials: Unlocking Next‐Generation Synchronous Multisource Energy Conversion with Layered 2D Crystals

“…[33] Triboelectricity is significantly less well understood, [34] with triboelectric charge harvesting being attributed to electron tunneling, adsorbed ion transfer, dipole induction, or mechanochemistry (bond cleavage). [5,[35][36][37] Regardless, it is the build-up of electric charge that occurs during the contact, and motion, between two materials at the interface.…”

“…The fundamental mechanism that causes the observed charge in the triboelectric effect is still under debate, particularly for resistive, polymeric, and nanoconfined materials. [5,35] It is currently hypothesized that, given the spatial confinement in 2D crystals, the observed charge occurs due to electron tunneling phenomena. [41] In the context of SMEC with piezoelectricity and triboelectricity-for any piezoelectric material reported in the literature, it is expected there will be a contribution from the triboelectric charge in the current measurements.…”

“…Traditional energy harvesters have focused on single energy sources including mechanical (force [ 3,4 ] and friction [ 5 ] ), electromagnetic (light and magnets [ 6 ] ), or thermal energy, and huge advances have been made in improving their efficiencies. However, focusing on a single energy source means that much of the available energy for harvesting is ignored.…”

“…Thus, to provide higher energy and power output, small-scale energy harvesters must be found that increase utilization of the total available environmental energy. Traditional energy harvesters have focused on single energy sources including mechanical (force [3,4] and friction [5] ), electromagnetic (light and magnets [6] ), or thermal energy, and huge advances have been made in improving their efficiencies.…”

Balancing charge dissipation and generation: mechanisms and strategies for achieving steady-state charge of contact electrification at interfaces of matter

“…The transfer of fragments of materials was confirmed by both physical (e.g., Atomic Force Microscopy, AFM) and chemical analyses (e.g., X-ray photoelectron spectroscopy, XPS). [44][45][46][47][48] We previously investigated the mechanism of material transfer by performing contactcharging experiments with polydimethylsiloxane (PDMS) of varying softness against polyvinyl chloride (PVC). 49 We found that softer pieces of PDMS generated larger amounts of charge by contact electrification.…”