Cornell researchers recently published findings on a new application of the Cottrell equation to identify the chemical reactions occurring during carbon-dioxide reduction.
On March 27, Cornell researcher and lead author Rileigh DiDomenico grad, as well as senior author Prof. Tobias Hanrath, chemical and biomolecular engineering, published a paper in ACS Catalysis about an unprecedented use of the Cottrell equation, a fundamental electrochemical equation describing the experimental relationship between current and time.
Senior author Prof. Héctor Abruña, chemistry and chemical biology, inspired DiDomenico to apply the equation to her research in carbon dioxide reductions.
“[The Cottrell equation] is a very traditional expression. In this day and age when you want glitter and fluff, people don’t typically use it because it’s old in that context, but it is rigorous from the most fundamental principles,” Abruña said.
The electrochemical reduction of CO2 refers to the transformation of CO2 into different reduced products like carbon monoxide, methanol, ethylene and ethanol. Electrochemical describes the use of a relationship between electricity and chemical change. Voltage is used to force electrons and protons to interact with CO2 and transform it into other products, similar to how heat drives other chemical reactions.
These chemical reactions are relevant to environmental issues because they can transform CO2 into other products, such as fuels.
“CO2 can be transformed into all these different products, and there’s all these different stages it can go through,” DiDomenico said. “If you imagine CO2 and it branches out into all these different intermediates and pathways to its final product. What we are trying to do is identify what specific route was happening in the system because if we better understand what is happening we can better control the reaction.”
DiDomenico and her team studied the pathways that produced C2 products — compounds with two or more carbons like ethylene and ethanol. In their lab setup designed to create these products, the researchers changed the voltage of the system while measuring current vs. time and the gaseous outcome.
The researchers performed rigorous electrochemical analysis of their results with the Cottrell equation. One implication of the Cottrell equation is a proportionality between the current and the inverse square root of time. On a graph of this relationship, a straight line is normally generated during a reaction when a single electron is transferred to a molecule. Therefore, any deviations from this line indicate that another type of reaction — one which is not a single electron transfer — must have occurred. Graphical analysis of these deviations provides information necessary to identify the type of reaction.
In plotting the current against the inverse square root of time measured during CO2 reduction, the researchers found that during the experiment, the graph deviated from the linear Cottrell line. However, it did so in a patterned way, going from linear to nonlinear and to linear again. This pattern corresponds to a reaction sequence known as ECE. The name dictates the type of reactions occurring and their order — electron transfer, chemical step and electron transfer again. A chemical step refers to a chemical reaction occuring that does not involve the transfer of any electrons.
The graphical ECE patterning solidified previous speculations regarding electrochemical CO2 reduction that produces C2 products.
“People have suggested that it’s ECE based on studies where they’ve observed a certain intermediate, but no one has shown it this way through current vs. time data,” DiDomenico said.
This confirmation allowed the researchers to simplify the theoretical pathways for this CO2 reduction. Because they knew the reaction pathway had to contain the separate chemical step of an ECE reaction, they could remove branches which did not contain a chemical step.
“This is a very good fit to the proposed model, so it’s more than likely that it actually is a correct interpretation of the mechanistic pathways that were being proposed,” Abruña said.
In addition to refining theory, better understanding the reactions involved gives researchers the ability to construct a reaction environment favoring a particular product.
The researchers plan to utilize the Cottrell equation for a similar method of analysis with other systems of CO2 reduction.
“You could study the full product spectrum and see what products are made, how and under what conditions,” DiDomenico said. “Instead of trying to get a certain performance, it’s more about understanding how it’s happening, which should help performance studies later on.”