SERS spectroscopy improves understanding of CO2 conversion to electrofuel

Scientists have long sought ways to convert abundant CO₂ to useful products such as chemicals and fuel. As early as in 1869, they were able to electrocatalytically convert CO₂ to formic acid. Over the past two decades, the rise of CO₂ in the Earth’s atmosphere has significantly accelerated research in CO₂ conversion using renewable energy resources, including solar, wind, and tidal.

Recent research in electrocatalytic CO₂ conversion points the way to using CO₂ as a feedstock and renewable electricity as an energy supply for the synthesis of different types of fuel and value-added chemicals such as ethylene, ethanol, and propane. But scientists still do not understand even the first step of these reactions--CO₂ activation, or the transformation of the linear CO₂ molecule at the catalyst surface upon accepting the first electron. Knowing the exact structure of the activated CO₂ is essential because its structure dictates both the end product of the reaction and its energy cost. This reaction can start from many initial steps and go through many pathways, giving typically a mixture of products. If scientists figure out how the process works, they will be better able to selectively promote or inhibit certain pathways, which will lead to the development of a commercially viable catalyst for this technology.

Columbia Engineering researchers announced today that they solved the first piece of the puzzle--they have proved that CO₂ electroreduction begins with one common intermediate, not two as was commonly thought. They applied a comprehensive suite of experimental and theoretical methods to identify the structure of the first intermediate of CO₂ electroreduction: carboxylate CO₂⁻ that is attached to the surface with C and O atoms. Their breakthrough, published online today in PNAS, came by applying surface-enhanced Raman scattering (SERS) instead of the more frequently used surface enhanced infrared spectroscopy (SEIRAS). The spectroscopic results were corroborated by quantum chemical modeling.

They decided to use SERS rather than SEIRAS for their observations because they found that SERS has several significant advantages that enable more accurate identification of the structure of the reaction intermediate. Most importantly, the researchers were able to measure the vibrational spectra of species formed at the electrode-electrolyte interface along the entire spectral range and on an operating electrode (in operandi). Using both quantum chemical simulations and conventional electrochemical methods, the researchers were able to get the first detailed look at how CO₂ is activated at the electrode-electrolyte interface.

Understanding the nature of the first reaction intermediate is a critical step toward commercialization of the electrocatalytic CO₂conversion to useful chemicals. It creates a solid foundation for moving away from the trial-and-error paradigm to rational catalyst design. "We expect our findings and methodology will spur work on how to make go faster and at a lower energy cost not only electrocatalytic but also photocatalytic CO₂ reduction," says Ponisseril Somasundaran, LaVon Duddleson Krumb Professor of Mineral Engineering, Department of Earth and Environmental Engineering. "In the latter case, a catalyst reduces CO₂ using direct sunlight. Even though these two approaches are experimentally different they are microscopically similar--both start with activation of CO₂ upon electron transfer from a catalyst surface. At this point, I believe that both these approaches will dominate the future."

SOURCE: Washington University in St. Louis; https://source.wustl.edu/2018/09/enabling-internet-of-photonic-things-with-miniature-sensors/