Humidity unlocks hidden power of ions in generating static electricity

Professor Dong-Myeong Shin and his team from the Department of Mechanical Engineering under the Faculty of Engineering at the University of Hong Kong (HKU) shed light on the contribution of ions in electric charge transfer, though their contribution differs with environmental humidity.

This discovery adds new insights into contact electrification, the phenomenon where static electricity is generated when two different materials touch and then separate. This effect is important for many technologies, including photocopiers, spray painting, energy collection, and self-powered sensors.

Challenging old beliefs about charge transfer

In their study, titled “Quantifying Electron and Ion Transfers in Contact Electrification with Ionomers,” published in Advanced Functional Materials, the team presents evidence that both electrons and ions are transferred at the same time when two solid surfaces come into contact. This challenges the long-held belief that only electrons are responsible for generating static electricity.

The researchers tested three types of films—an anionic ionomer called Nafion 211, a cationic ionomer named FAA-3, and a non-ionic polymer called Nylon—by rubbing them against a fluorinated ethylene propylene (FEP) film.

They analyzed how charges decayed under heat and used advanced techniques to detect surface ions. This allowed them to measure how much electrons and ions contributed to the charge transfer.

Humidity’s surprising role in ion transfer

It was discovered that different ionomer materials displayed distinct charge-transfer behaviors, with humidity greatly amplifying the role of ions. At a relative humidity level of above 50%, ion transfer not only enhanced surface charge density but also offset the depletion usually caused by moisture.

Remarkably, even though less than 2% of the ions actually moved across the interface, their impact was surprisingly large—they played a key role in stabilizing and strengthening static charges. To understand how this happens, the team used computer simulations, which revealed that water molecules absorbed into the ionomer formed tiny channels inside the material. These channels allowed ions to move more freely toward the surface, especially in humid conditions.

Implications for technology and materials

“This is the first time we could unambiguously separate and measure the contributions of electrons and ions in solid contact electrification,” said Dr. Xiaoting Ma, co-first author of the paper.

“By using Nafion and FAA films as model systems, we gained unique insights into how positive and negative ions behave differently depending on humidity.”

Professor Shin highlighted the broader impact: “By quantifying how much electrons and ions transfer during contact, we can develop new strategies to create materials that work reliably even in humid environments. This not only advances fundamental scientific understanding but also opens up new possibilities for eco-friendly technologies such as printing, coating, environmental monitoring, and energy harvesting.”

The team also found that introducing ions onto a non-ionic polymer surface significantly improved its resistance to humidity, offering a practical strategy for engineering robust triboelectric materials.

After integrating Nafion and FAA films into triboelectric nanogenerators (TENGs), the devices showed stable output even in humid environments, successfully powering an array of 78 LEDs and maintaining performance over thousands of cycles.