Researchers from the University of Illinois at Chicago and the Joint Center for Artificial Photosynthesis have determined how electrocatalysts can convert carbon dioxide to carbon monoxide using water and electricity. The discovery can lead to the development of efficient electrocatalysts for large-scale production of synthesis gas — a mixture of carbon monoxide and hydrogen.
"The electrochemical reduction of carbon dioxide to fuels is a subject of considerable interest because it offers a means for storing electricity from energy sources such as wind and solar radiation in the form of chemical bonds," said Meenesh Singh, assistant professor of chemical engineering and lead author on the study.
During his postdoctoral research at the University of California, Berkeley, Singh studied artificial photosynthesis and was part of a team that developed artificial leaves that were capable of converting carbon dioxide into fuels when exposed to direct sunlight.
In the latest research, Singh developed a state-of-the-art multiscale model that unites a quantum-chemical analysis of reaction pathway, a microkinetic model of the reaction dynamics and a continuum model for transport of species in the electrolyte to learn how carbon dioxide can be electrochemically reduced through a catalyst. In this case, it was silver and then made into carbon monoxide.
The most plausible reaction pathway is usually identified from the quantum-chemical calculation of the lowest free-energy pathway. But this approach can be misleading when coverages of absorbed species differ significantly. It is essential, therefore, to integrate the effects of electronic states of a catalyst at the atomic-level with dynamics of species in the electrolyte at the continuum level for accurate prediction of electrocatalytic reaction pathways.
"This multiscale model is one of the biggest accomplishments in electrochemistry,” said Singh.
To understand how electrocatalysts in fuel cells or electrochemical cells work, scientists needed to probe the electronics and quantum levels first — this can be challenging for the presence of an electric field. Singh and Jason Goodpaster, assistant professor of chemistry at the University of Minnesota, spent one year to individually produce and benchmark the models and integrate them into a multiscale framework for full-scale simulation of the electrochemical reactions.
This is the first time scientists have predicted quantitatively from first principles, Singh said, and the current density of carbon monoxide and hydrogen as a function of applied potential and pressure of carbon dioxide.
"Once you recognize how these reactions are occurring on electrocatalysts, you can control the structure of the catalyst and operating conditions to produce carbon monoxide efficiently," Singh said.
Since they are product gases, carbon monoxide and hydrogen can be readily separated from synthesis gas and converted into fuels like methanol, dimethyl ether, or a mix of hydrocarbons.
Electrocatalysts like gold, silver, zinc, palladium and gallium are known to yield mixtures of carbon dioxide and hydrogen at various ratios depending on the applied voltage. Gold and silver exhibit the highest activity towards carbon dioxide reduction and since silver is more abundant and less expensive than gold, "Silver is the more promising electrocatalyst for large-scale production of carbon monoxide," he said.