Chemistry researchers modify solar technology to produce less harmful greenhouse gases

The paper titled "Methyl Termination of p-Type Silicon Enables Selective Photoelectrochemical CO2 Reduction by a Molecular Ruthenium Catalyst," published in ACS Energy Letters, delves into the utilisation of a process known as methyl termination. This process involves the use of a basic organic compound consisting of one carbon atom bonded to three hydrogen atoms. The researchers explain how this modification of the silicon surface, a crucial element in solar cells, enhances its ability to convert carbon dioxide into carbon monoxide using sunlight.

The research received support from the Centre for Hybrid Approaches in Solar Energy to Liquid Fuels (CHASE), an Energy Innovation Hub funded by the DOE Office of Science. It was guided by artificial photosynthesis, a process that imitates how plants harness sunlight to transform carbon dioxide into energy-rich molecules. Carbon dioxide plays a significant role as a greenhouse gas, contributing to the issue of climate change. Through the conversion of carbon dioxide into carbon monoxide, a less harmful greenhouse gas and a precursor to more advanced fuels, the researchers aim to reduce the environmental consequences of carbon dioxide emissions.

"One challenge with solar energy is that it's not always available when we have the highest need for it," said Gabriella Bein, the paper's first author and a Ph.D. student in chemistry. "Another challenge is that renewable electricity, like that from solar panels, doesn't directly provide the raw materials needed for making chemicals. We aim to store solar power as liquid fuels for future use.

The researchers utilised a ruthenium molecular catalyst in conjunction with a chemically modified silicon photoelectrode to efficiently convert carbon dioxide into carbon monoxide using light energy. This process avoids the production of unwanted byproducts like hydrogen gas, resulting in a more efficient conversion of carbon dioxide into other substances.

Jillian Dempsey, a co-author of the paper and Bowman and Gordon Grey Distinguished Term Professor, highlighted the remarkable findings of their experiments. In a solution filled with carbon dioxide, they achieved an impressive 87% efficiency in producing carbon monoxide. This indicates that the system utilising the modified silicon photoelectrodes is on par with, or even surpasses, systems employing traditional metal electrodes like gold or platinum.

Furthermore, the silicon photoelectrode required 460 millivolts less electrical energy to initiate a reaction compared to solely relying on electricity. Dempsey found this to be a noteworthy development, as the method utilises direct light harvesting to help reduce or balance the energy needed for the chemical reaction that transforms carbon dioxide into carbon monoxide.

"What's interesting is normally silicon surfaces make hydrogen gas instead of carbon monoxide, which makes it harder to produce it from carbon dioxide," said Dempsey, who is also deputy director of CHASE. "By using this special methyl-terminated silicon surface, we were able to avoid this problem. Modifying the silicon surface makes the process of converting CO2 into carbon monoxide more efficient and selective, which could be really useful for making liquid fuels from sunlight in the future."

Bein and Dempsey worked together on the research alongside Professor Alexander Miller, Eric Assaf, a former graduate student in the department, Senior Research Scientist Renato Sampaio, Madison Stewart, an undergraduate chemistry major, and Senior Research Scientist Stephen Tereniak.

CHASE comprises seven distinct institutions, with its headquarters at UNC-Chapel Hill. In 2020, it secured a substantial $40 million funding from the Department of Energy to expedite groundbreaking research on harnessing sunlight to produce fuels.

Journal Reference: Gabriella P. Bein, Madison A. Stewart, Eric A. Assaf, Stephen J. Tereniak, Renato N. Sampaio, Alexander J. M. Miller, Jillian L. Dempsey. Methyl Termination of p-Type Silicon Enables Selective Photoelectrochemical CO2 Reduction by a Molecular Ruthenium Catalyst. ACS Energy Letters, 2024; 1777 DOI: 10.1021/acsenergylett.4c00122