Modifying Solar Technology to Produce Safe Greenhouse Gas

Researchers at the University of North Carolina’s Chemistry Department are employing semiconductors to gather and convert solar energy into high-energy compounds with the potential to manufacture environmentally friendly fuels.

Modifying Solar Technology to Produce Safe Greenhouse Gas
Ph.D. student Gabriella Bein is first author of a paper published in ACS Energy Letters. Image Credit: University of North Carolina at Chapel Hill

The researchers describe how they modified the surface of silicon, a crucial component of solar cells, using a process known as methyl termination, which uses a simple organic compound of one carbon atom bonded to three hydrogen atoms to improve silicon's ability to convert carbon dioxide into carbon monoxide using sunlight.

Their study, “Methyl Termination of p-Type Silicon Enables Selective Photoelectrochemical CO2 Reduction by a Molecular Ruthenium Catalyst,” was published in ACS Energy Letters.

The Center for Hybrid Approaches in Solar Energy to Liquid Fuels (CHASE), an Energy Innovation Hub supported by the DOE Office of Science, supported the research. Artificial photosynthesis is a process that emulates how plants use sunlight to convert carbon dioxide into molecules that are rich in energy.

One of the main greenhouse gases causing climate change is carbon dioxide. By converting carbon dioxide to carbon monoxide, a building component for more complicated fuels and a less destructive greenhouse gas, the researchers stated that they could potentially reduce the environmental impact of carbon dioxide emissions.

One challenge with solar energy is that it is not always available when we have the highest need for it. Another challenge is that renewable electricity, like that from solar panels, doesn’t directly provide the raw materials needed for making chemicals. Our goal is to store solar power in the form of liquid fuels that can be used later.

Gabriella Bein, Study First Author and Ph.D. Student, University of North Carolina at Chapel Hill

The researchers combined a ruthenium molecular catalyst with a piece of chemically modified silicon, known as a photoelectrode, to facilitate the conversion of carbon dioxide to carbon monoxide using light energy while producing no unwanted byproducts, such as hydrogen gas, making the process more efficient for converting carbon dioxide into other compounds.

Jillian Dempsey, a co-author of the study and the Bowman and Gordon Gray Distinguished Term Professor, stated that when they ran experiments in a solution containing carbon dioxide, they discovered that they could produce carbon monoxide with 87 % efficiency, implying that the system using the modified silicon photoelectrodes is similar to or better than systems using traditional metal electrodes such as gold or platinum.

The silicon photoelectrode used 460 millivolts less electrical energy to initiate a reaction than would be obtained just from electricity. Dempsey referred to this as “significant” because the method employs direct light harvesting to complement or balance the energy necessary to fuel the chemical reaction that converts carbon dioxide to 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, but 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.

Jillian Dempsey, Study Co-Author and Bowman and Gordon Gray Distinguished Term Professor, University of North Carolina at Chapel Hill

Bein and Dempsey worked on the study 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 is made up of seven different institutions, with its headquarters at UNC-Chapel Hill. It got $40 million in funding from the Department of Energy in 2020 to expedite fundamental studies on how to manufacture fuels from sunlight.

Journal Reference:

Bein, G. P., et al. (2024) Methyl Termination of p-Type Silicon Enables Selective Photoelectrochemical CO2 Reduction by a Molecular Ruthenium Catalyst. ACS Energy Letters. doi:10.1021/acsenergylett.4c00122


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