As global efforts intensify to reduce greenhouse gas emissions, there is an urgent need for practical and cost-effective methods to capture carbon dioxide and transform it into valuable products such as transportation fuels, chemical feedstocks, and even innovative building materials. Despite ongoing attempts, achieving economic viability has remained a significant hurdle.
A new research from engineers at MIT has the potential to significantly enhance various electrochemical systems that are being developed to convert carbon dioxide into valuable products. The team has created a new design for the electrodes utilized in these systems, which boosts the efficiency of the conversion process.
“The CO2 problem is a big challenge for our times, and we are using all kinds of levers to solve and address this problem,” professor of mechanical engineering Kripa Varanasi says.
In their study, the researchers concentrated on the electrochemical conversion of CO2 into ethylene, a commonly utilized chemical that can be turned into different types of plastics as well as fuels, and which is currently derived from petroleum. The method they devised could also be utilized to produce other high-value chemical products, such as methane, methanol, carbon monoxide, among others, the scientists indicate.
With ethylene’s market price hovering around $1,000 per ton, the objective is to match or even undercut that figure. The electrochemical technique used to convert CO2 into ethylene relies on a water-based solution and a specialized catalyst, which are activated through an electric current within a gas diffusion electrode.
There are two conflicting traits of the gas diffusion electrode materials that impact their effectiveness: they must effectively conduct electricity to minimize resistance heating losses, while also being “hydrophobic” to prevent the water-based electrolyte solution from seeping through and disrupting the reactions at the electrode surface. This presents a dilemma.
Enhancing conductivity often compromises hydrophobicity and vice versa. Varanasi and his team aimed to navigate this challenge, and after months of diligent research, they succeeded. The solution, crafted by MIT doctoral students Simon Rufer and Varanasi, is both clever and straightforward. They utilized a plastic material, PTFE (commonly known as Teflon), which is recognized for its excellent hydrophobic qualities.
However, the downside of PTFE is its poor conductivity, which necessitates that electrons travel through a very thin catalyst layer, resulting in a considerable voltage drop over distance. To address this challenge, the researchers incorporated a series of conductive copper wires through the thin sheet of PTFE.
“This work really addressed this challenge, as we can now get both conductivity and hydrophobicity,” Varanasi says.
Research on innovative carbon conversion systems are primarily conducted using very small, laboratory-scale samples, usually no larger than 1-inch (2.5-centimeter) squares. To break through this limitation and demonstrate the potential for larger-scale applications, Varanasi’s team created a sheet that was ten times larger in surface area and demonstrated its effective functioning. To achieve this milestone, they conducted fundamental tests that had seemingly not been performed before, evaluating electrodes of varying sizes under the same conditions to study the relationship between conductivity and electrode dimensions.
They discovered that conductivity significantly decreased with increasing size, implying that considerably more energy, and therefore expense, would be required to facilitate the reaction. Furthermore, the larger electrodes produced a greater amount of undesirable chemical byproducts in addition to the intended ethylene.
For practical industrial applications, electrodes may need to be around 100 times larger than those used in the laboratory, so integrating conductive wires will be essential for making such systems viable, the researchers indicate.
To address this challenge, the researchers devised a model that accounts for the spatial variability in voltage and product distribution on electrodes due to ohmic losses. Using this model alongside their experimental outcomes, they determined the optimal spacing for conductive wires to counteract the decline in conductivity.
By weaving the wire through the material, they effectively segmented it into smaller subsections based on the wire spacing, thereby enhancing performance and feasibility for larger implementations.
“We split it into a bunch of little subsegments, each of which is effectively a smaller electrode,” Rufer says. “And as we’ve seen, small electrodes can work really well.”
The exceptional conductivity of copper wire compared to PTFE material creates an efficient pathway for electrons to traverse, seamlessly connecting regions where they encounter resistance from the substrate. To showcase the reliability of their innovative system, the researchers conducted an impressive 75-hour continuous test on the electrode, demonstrating remarkable stability in performance throughout.
Rufer emphasizes that this development represents “the first PTFE-based electrode which has gone beyond the lab scale on the order of 5 centimeters or smaller. It’s the first work that has progressed into a much larger scale and has done so without sacrificing efficiency.”
Moreover, the integration of the wire-weaving process into existing manufacturing operations is straightforward, allowing for implementation even within large-scale roll-to-roll production, he notes.
“Our approach is very powerful because it doesn’t have anything to do with the actual catalyst being used,” Rufer says. “You can sew this micrometric copper wire into any gas diffusion electrode you want, independent of catalyst morphology or chemistry. So, this approach can be used to scale anybody’s electrode.”
“Given that we will need to process gigatons of CO2 annually to combat the CO2 challenge, we really need to think about solutions that can scale,” Varanasi says. “Starting with this mindset enables us to identify critical bottlenecks and develop innovative approaches that can make a meaningful impact in solving the problem. Our hierarchically conductive electrode is a result of such thinking.”
Journal reference:
- Simon Rufer, Michael P. Nitzsche, Sanjay Garimella, Jack R. Lake & Kripa K. Varanasi. Hierarchically conductive electrodes unlock stable and scalable CO2 electrolysis. Nature Communications, 2024; DOI: 10.1038/s41467-024-53523-8
