Artificial photosynthesis offers clean source of hydrogen

Devices made using conventional semiconductor technologies could make hydrogen using just fresh or saltwater and sunlight

The idea of a hydrogen economy, where currently oil and its derivatives underpin much of every day life but could be superseded using hydrogen as a fuel, has been around for decades.

It is delayed, however, by two factors: the lack of a distribution structure for hydrogen and the difficulty in making the gas.

There are two ways of producing hydrogen: decompose water into its constituent gases, which requires electricity; or make it from natural gas, which does not reduce reliance on fossil fuels.

Researchers have been trying for many years to develop a method to use the sun’s energy to power water decomposition, mimicking the natural process of photosynthesis whereby green plants convert solar into chemical energy, but nature’s tricks have, as ever, proved difficult to copy.

photosynthesis
A gallium nitride nano-cityscape, colorised and at 52,000 times magnification Image: Faqrul A. Chowdhury, McGill University

Zetian Mi, a specialist in computer and electrical engineering, began working on this problem while at McGill University in Montréal. Now at the University of Michigan, Mi has published a paper describing a device capable of this artificial photosynthesis and water decomposition in Nature Communications.

The device is made from silicon and gallium nitride, a semiconductor often used in LEDs. Mi and his colleagues built a “nano -sized cityscape” of gallium nitride towers on a silicon wafer substrate. When light strikes the towers, the photons knock electrons out of the crystal structure, which then become mobile, as do the positively charged holes they leave behind. It is these mobile charge carriers that split hydrogen away from water molecules.

“When this specially engineered wafer is hit by photons, the electric field helps separate photogenerated electrons and holes to drive the production of hydrogen and oxygen molecules efficiently,” said Faqrul Alam Chowdhury, a doctoral student at McGill involved in the research.

Previous direct solar water splitters have achieved just over one per cent stable solar-to- hydrogen efficiency in fresh or saltwater. Mi’s team, however, achieved efficiency of over three per cent.

“Although the three per cent efficiency might seem low, when put in the context of the 40 years of research on this process, it’s actually a big breakthrough,” Mi said. “Natural photosynthesis, depending how you calculate it, has an efficiency of about 0.6 per cent.” The team’s goal as they continue research is to reach five per cent efficiency, which they see as the threshold for commercialisation, and then continue to improve performance, aiming to reach a target of 20 to 30 per cent.

One way to improve performance might be to use the silicon wafer backing of the gallium nitride towers to help capture light and funnel charge carriers into the gallium nitride.

Mi conducts similar research to strip carbon dioxide of its oxygen to turn the resulting carbon into hydrocarbons, such as methanol and syngas. This research path could potentially remove carbon dioxide from the atmosphere, like plants do.