Energy – Anytime, Anywhere

A research team from the Department of Mechanical Engineering has developed a novel moisture-activated electricity generator (MEG) that generates electricity in an environmentally-friendly way, just utilising moisture from the air. And, unlike most current MEGs, the new model can work for prolonged hours even in non-humid conditions.

“Motivated by the need for an ‘anytime, anywhere’ energy source, we initiated this work by turning to atmospheric moisture, an abundant and continuously available resource, and leveraging Hong Kong’s characteristically humid climate as a practical testbed,” said Professor Dong-Myeong Shin, leader of the research team. “Beyond simply using humidity as the stimulus, a central advantage of MEGs is to deliver direct current output.”

Previous models have repeatedly been hampered by two major barriers to real-world deployment: a pronounced performance drop under low-humidity conditions and a short electricity power generation time.

“These shortcomings directly motivated our attempt to redesign MEGs capable of sustaining stable electricity generation even at low relative humidity, and we successfully got meaningful results,” said Professor Shin. “Building on this foundation, our ongoing follow-up work is focussed on one of the most challenging aspects in this field, long-term current-output stability, by developing a new hydrogel architecture, with device-level performance testing currently underway.”

The primary active material in the device is a cationic hydrogel, in which the positively charged moieties are immobilised on the polymer backbone, while only the counter anions remain mobile and participate in ion transport. Mr Eunjong Kim, PhD candidate and first author of the paper, noted, “On this basis, we artificially imposed a salt concentration gradient within the hydrogel, thereby sustaining non-equilibrium ion redistribution and enabling ion migration to persist over extended operation times.”

Driving forces

“Specifically, the device was engineered such that three driving forces operate concurrently: one, asymmetric moisture absorption, which establishes a directional water-uptake flux; two, intrinsic anion transport within the cationic hydrogel matrix; and three, diffusion of free ions induced by the internal salt-concentration gradient.”

By synchronising these driving forces, the designed MEG addresses the two most significant limitations of conventional MEGs – suppressed output under low relative humidity and insufficient long-term operational stability – and thereby enables more reliable electricity generation in practical environments.

The new generator, at the single-device level, delivers a power density of 13.8 milliwatts per square metre (mW/m2) at 30 per cent relative humidity (RH), and increases to approximately 42mW/m2 at 80 percent RH – as compared to other MEG units which have a limited lifespan of under 16 hours. “However, a single unit typically cannot meet the voltage requirements of practical electronics,” said Mr Kim. “To address scalability, we developed a space-efficient, stackable module architecture similar in concept to LEGO-style assembly, and this enabled systematic series and parallel integration.”

Just six of these units stacked were sufficient to power an electronic wristwatch, and stable operation was maintained even after 135 days. At a larger scale, an 80-unit series module generated approximately 40 volts, enabling the operation of a commercial smart-window film under ambient air conditions, thereby demonstrating the feasibility of practical deployment through modular integration.

Broad coverage

Using a global relative humidity map, the team estimated the geographic coverage over which the proposed MEG could operate effectively, and projected that – under operating conditions of RH at or above 30 percent – the device would be functional across approximately 97 per cent of the Earth’s surface. This significant measure underlines just how useful atmospheric moisture could be as a highly accessible energy resource for practical deployment.

“This broad coverage suggests that MEGs could serve as a viable power source for implementing sustainable self-powered systems in real-world environments,” said Professor Shin. “Importantly, because the generator provides a directly usable electrical output without requiring additional power-managing components, it offers a pathway to eliminate the bulky external power unit that often dominates the volume of small electronic systems.

“This feature is particularly attractive for miniaturised platforms, enabling compact integration as an energy source for a wide range of applications – from wearable electronics to the Internet of Things networks operating under ambient conditions.”

While the present device can supply direct power for substantially longer durations at low RH than many conventional ambient energy harvesters, further improvements are still required before it can be commercialised as a practical replacement for portable batteries. A key priority is the development of charge-transport strategies that mitigate corrosion and parasitic interfacial reactions.

“In particular, progressive metal-electrode corrosion can reduce the effective electrical contact area over time, which inevitably leads to a gradual decline in current output during prolonged operation,” explained Mr Kim. “Additionally, device engineering must account for real-world variables, such as surface contamination and environmental swings, that can alter moisture uptake and interfacial charge transfer under practical conditions.”

The team is concentrating their efforts now on improving electrical output stability, environmental and operational safety and manufacturability, together with robust packaging and encapsulation strategies. “If electrical performance stability and long-term reliability continue to be improved, MEGs are widely viewed as a promising platform with a comparatively high potential for real-world utilisation and eventual commercialisation,” concluded Professor Shin.