The gallium nitride nanowires used the light energy to free electrons and the positively charged spaces they leave behind, known as holes. The holes split water molecules, separating the protons (hydrogen) from the oxygen. Then, at the metal catalysts, the electrons split the carbon dioxide, producing carbon monoxide and sometimes drawing in the free hydrogen to make methane. Processes are under development to separate the oxygen from the other gases.

“Our technology sheds light on how to build distributed syngas production from air, water and sunlight,” said Baowen Zhou, co-corresponding author of the study with Mi and a former postdoctoral research fellow in Mi’s lab at McGill University and U-M.

By changing the ratio of gold to chromium oxide in the nanoparticles, Mi’s team was able to control the relative amounts of hydrogen and carbon monoxide produced in the reaction. This is important because the ratio of hydrogen to carbon monoxide affects how easy it is to produce a type of fuel or chemical.

“What is surprising is the synergy between gold and chromium oxide to make the CO2 reduction to syngas efficient and tunable. That was not possible with a single metal catalyst,” Mi said. “This opens up many exciting opportunities that were not previously considered.”

Mi’s tunable syngas setup uses standard industrial manufacturing processes, and is scalable. While Rashid used distilled water in this experiment, seawater and other electrolyte solutions are also expected to work, and Mi has used them in related water-splitting studies.

“The semiconductor we use as the light absorber is based on silicon and gallium nitride, which are the most commonly produced semiconductors, and we use very little material for the gallium nitride. Each nanowire is about one micrometer in thickness,” Mi said.

Mi’s next goal is to increase the efficiency of the device, which currently stands at 0.89%. When 10% of the light energy is converted to chemical energy, he hopes that the technology could see the technology be adopted for renewable energy, similar to solar cells.

The project was supported through the Emission Reduction Alberta ERA, based at McGill University in Canada, former home of Mi. The co-authors all have current or former ties to McGill. Rashid is currently a postdoctoral researcher in electrical and computer engineering at the University of Waterloo in Canada. Zhou is currently an associate professor of mechanical engineering at Shanghai Jiao Tong University.

A licensing agreement for intellectual property developed in this study is in the process of being negotiated in order to bring the technology to market and make a positive environmental impact. The two companies are NS Nanotech Inc. and NX Fuels Inc, both co-founded by Mi. The University of Michigan and Mi have a financial interest in these companies.

Study abstract: Tunable green syngas generation from CO2 and H2O with sunlight as the only energy input (DOI: 10.1073/pnas.2121174119)