Engineering solutions for pollution control and clean energy

5 November 2018

Professor Dennis Leung (Croucher Fellowship 1987) is using the power of the sun – namely photocatalysis and photocatalytic oxidation – to fuel engineering innovations that can help address global problems ranging from the fouling of ships to indoor air pollution, while the eco-friendly paper-based aluminium oxide batteries he has developed could have countless applications for micro and portable electronic products.

The professor of mechanical engineering at the University of Hong Kong has developed novel materials with an ultimate goal of helping to protect the environment in these seemingly different areas of research.

Professor Dennis Leung (left)

Hong Kong has flourished over the years as one of the busiest seaports in the world. But that has come at a price, with heavy metal-based antifouling paints, used to protect vessels from the build-up of marine organisms, adding to the pollution of its waters.

In 2017, Leung and his collaborator at the City University of Hong Kong tested water and soil samples collected from the seabed near local shipyards. They found that the amount of copper exceeded the norm by as much as 60 times.

“Conventional antifouling paints largely contain heavy metals, such as copper, as the main biocidal agent, which means the surrounding seawater and sediment at shipyards are severely contaminated,” Leung said. After being ingested by aquatic species, these metals then enter the food chain, eventually affecting human health.

Controlling the growth of organisms such as barnacles, clams and algae that attach themselves to ship hulls is necessary to reduce fuel consumption in global shipping – frictional resistance from fouling increases fuel use by 30 to 40 per cent, Leung explained.

He is working on a potentially revolutionary solution: an eco-friendly nano-photocatalytic antifouling paint that both protects the marine environment and reduces fuel consumption.

Photocatalysis and photocatalytic oxidation have been found to have an antifouling effect, he explained. The new paint works by disrupting the cell wall of organisms when activated by sunlight. The paint’s ultra hydrophobic and super wetting properties also prevent organisms from attaching themselves to hull surfaces. Most significantly, unlike traditional antifouling paints, this technology can reduce or even eliminate the use of heavy metals such as copper.

“Our recent field study of nano-photocatalytic antifouling paint using a modified visible-light activated photocatalyst has successfully proven that a photocatalyst additive can reduce the use of copper in antifouling paint. The findings further reveal the potential of photocatalysis for the development of copper-free antifouling paint,” said Leung, co-principal investigator of this project, funded by the Hong Kong government’s Innovation and Technology Fund.

The paint has the added benefits of being cheap and easy to produce, and long-lasting. Following the success of the team’s initial tests, there is now high potential for further development once laboratory tests are completed.

Tackling pollution with VUV photocatalytic oxidation

Leung has also applied photocatalytic oxidation to a very different environmental problem – the challenge of indoor air pollution, which has been linked to one in six deaths globally, according to a 2017 report in The Lancet.

Indoor air pollution is particularly severe in developing countries where women spend much of their time cooking on traditional stoves that use firewood, without adequate smoke emission controls.

Current technologies usually target the reduction of particulate matter. But various pollutants co-exist in the environment and require a combination of costly cleaning processes. VUV (vacuum ultraviolet) photocatalytic oxidation has been found to be effective and low cost, and can be applied to homes, offices, and hospitals.

Photocatalytic oxidation can break down volatile organic matters and pathogenic bacteria at room temperature under light irradiation, normally ultraviolet light. Leung and his team have shown that high-energy VUV light at a 185 nm wavelength produces greater photocatalytic degradation of indoor air pollutants and pathogenic bacterial contaminants over traditional photocatalytic oxidation processes.

VUV photocatalytic oxidation technology is efficient for the purification of indoor air pollutants due to the coexistence and synergetic effect of VUV photolysis, photocatalytic oxidation and catalytic ozonation during the process, explained Leung.

Volatile organic compound (VOC) degradation under VUV photocatalysis oxidation.

VUV photocatalytic oxidation technology has also been found to be effective in degrading water pollutants in wastewater, particularly petrochemical and textile wastewater. “We are confident that this technology could meet the challenges of providing a low cost and efficient process for wastewater management,” he said.

Novel paper-based battery

Leung is also focusing on engineering solutions for renewable and clean energy. Aluminium batteries are well known for their high energy density, power output, and cost-effectiveness. He is developing novel paper-based aluminium air batteries, with the main components comprising aluminium, cellulose paper and carbon paper. The battery fabrication process is greatly simplified because it does not need bulky liquid storage or active electrolyte delivery. Additionally, the exhausted battery can be burnt and the remaining aluminium oxide recycled back to aluminium.

Leung explained the restricted electrolyte transport and ion diffusion inside the porous and tortuous paper enables direct utilisation of aluminium (Al), even at low purity, in an alkaline electrolyte with a high specific capacity of 1732 mAh/g. “Furthermore, the intrinsic flexibility and printability of paper have enabled the fabrication of flexible and printable Al-air batteries, which are more lightweight and versatile for the emerging miniwatt market, such as wearable electronics, point-of-care diagnostic assays, biosensors and smart packages.”

All in all, Leung is making engineering work for a more sustainable future, for the benefit of humans and the wider environment we live in.

Professor Dennis Leung obtained his BEng and PhD from the Department of Mechanical Engineering at the University of Hong Kong in 1982 and 1988 respectively. He worked with Hong Kong Electric for five years before joining the University of Hong Kong in 1993, where he is now Professor of Mechanical Engineering. He was the recipient of a Croucher Fellowship in 1987. He received the Outstanding Earth Champion Hong Kong Award for his contributions to environmental protection in 2008 and was named a Highly Cited Researcher by Clarivate Analytics in 2017. 

To view Professor Leung’s Croucher profile, please click here