A novel electrode design from MIT enhances the effectiveness of electrochemical processes that convert carbon dioxide into ethylene and similar products.
In the face of escalating greenhouse gas emissions, scientists are exploring efficient and cost-effective methods to capture carbon dioxide and transform it into valuable products like fuels for transportation, raw materials for chemicals, and even construction materials. However, achieving economic viability in these endeavors has been challenging.
Recent studies conducted by engineers at MIT may accelerate advancements in various electrochemical systems aimed at transforming carbon dioxide into useful commodities. The research team introduced a new electrode design that significantly increases the efficiency of this conversion.
The results will be published in the journal Nature Communications, authored by MIT doctoral student Simon Rufer, mechanical engineering professor Kripa Varanasi, and a team of three colleagues.
“Addressing the CO2 issue is one of the major challenges of our era, and we are exploring numerous solutions,” Varanasi states. It is crucial to develop feasible methods to extract the gas, either from emissions of power plants or by capturing it directly from the atmosphere or oceans. Once CO2 is captured, it also requires a practical way to be utilized.
Various systems have been created to convert this captured carbon dioxide into valuable chemical products, according to Varanasi. “It’s not that we lack the ability – we can achieve it. The real question is how to enhance efficiency and reduce costs.”
This study centers on the electrochemical transformation of CO2 into ethylene, a versatile chemical used in producing many plastics and fuels, typically derived from petroleum today. However, the technique they devised can also be adapted to manufacture other valuable chemical products, such as methane, methanol, and carbon monoxide, according to the researchers.
The current market price for ethylene is around $1,000 per ton, so the aim is to match or lower that cost. The electrochemical process of changing CO2 into ethylene involves a catalyst and a water-based solution interacting with an electric current in a device known as a gas diffusion electrode.
The gas diffusion electrode materials possess two conflicting requirements that impact their performance: they must conduct electricity well to minimize energy loss through resistance heating, yet they also need to repel water (“hydrophobic”) to prevent the electrolyte solution from saturating the electrode surface and diminishing the reactions occurring there.
However, enhancing one property typically compromises the other. Varanasi and his colleagues explored ways to break this trade-off and, after extensive experimentation, they succeeded.
The solution crafted by Rufer and Varanasi is both clever and simple. They utilized a plastic material called PTFE (Teflon), known for its excellent hydrophobic characteristics. Nonetheless, PTFE’s poor electrical conductivity necessitates that electrons travel through a very thin catalyst layer, resulting in a significant voltage decrease over distance. To overcome this challenge, the researchers incorporated conductive copper wires throughout the thin PTFE sheet.
“This work effectively tackled the issue, allowing us to achieve both conductivity and hydrophobicity,” Varanasi remarks.
Typically, research on carbon conversion systems is conducted on small lab samples, usually measuring less than 1 inch (2.5 centimeters) square. To showcase the feasibility of scaling up, Varanasi’s team created a sheet that was ten times larger and demonstrated its effective performance.
To reach that milestone, they had to perform some foundational tests, which seemingly had never been conducted before, using electrodes of varying sizes under identical conditions to examine the correlation between conductivity and electrode size. They discovered that conductivity decreased sharply with size, meaning significantly more energy and cost would be required to facilitate the reaction.
“That’s precisely what we anticipated, but it was a subject that had not been thoroughly examined before,” Rufer explains. Additionally, the larger electrodes produced more undesirable chemical byproducts aside from the desired ethylene.
For practical industrial applications, electrodes would likely need to be approximately 100 times larger than laboratory models, indicating that the addition of conductive wires will be vital for creating feasible systems, according to the researchers. They also devised a model that takes into account the spatial variation in voltage and product distribution on the electrodes, stemming from ohmic losses. Combining this model with the experimental data allowed them to determine the ideal spacing for conductive wires to mitigate conductivity reduction.
By integrating the wire into the structure, it essentially segments the material into smaller subsections based on the wire arrangement. “We divided it into several tiny subsegments, each acting like a mini-electrode,” Rufer states. “And as we have noted, smaller electrodes can function exceptionally well.”
Because copper wire provides much greater conductivity than the PTFE, it serves as an efficient route for electrons, connecting areas where they encounter more resistance.
To validate the durability of their system, the researchers operated a test electrode continuously for 75 hours, showing minimal performance variations. Overall, Rufer comments, their design represents the first PTFE-based electrode that effectively scales beyond lab sizes of around 5 centimeters or smaller. This is the first research that has transitioned to a significantly larger scale while maintaining efficiency.
The process of integrating the wire can be easily adapted into existing manufacturing techniques, even in large-scale roll-to-roll processes, he adds.
“Our technique is powerful because it doesn’t depend on the type of catalyst used,” Rufer notes. “You can incorporate this micrometric copper wire into any gas diffusion electrode, regardless of the catalyst’s morphology or chemistry. Therefore, this approach is suitable for scaling up any electrode.”
“Considering the need to manage gigatons of CO2 each year to address the CO2 crisis, we must prioritize solutions that can scale effectively,” Varanasi emphasizes. “Approaching problems with this mindset helps pinpoint critical issues and encourages innovative strategies that can significantly contribute to solving these challenges. Our hierarchically conductive electrode exemplifies such strategic thinking.”
The research team comprised MIT graduate students Michael Nitzsche and Sanjay Garimella, alongside Jack Lake, PhD ’23. The project received support from Shell through the MIT Energy Initiative.
