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As the global community works to tackle the pressing issue of greenhouse gas emissions, researchers are actively exploring efficient and cost-effective methods to capture carbon dioxide and transform it into valuable products such as transportation fuels, chemical feedstocks, and construction materials. Despite various initiatives, achieving economic feasibility has remained a significant challenge.
Recent research conducted by engineers at MIT promises to accelerate advancements in the development of electrochemical systems intended to convert carbon dioxide into commercially useful items. The researchers have introduced a novel design for the electrodes utilized in these systems, which enhances the overall efficiency of the conversion process.
The research findings are set to be published in the journal Nature Communications, featuring contributions from MIT doctoral student Simon Rufer, mechanical engineering professor Kripa Varanasi, and three other colleagues.
“Addressing the CO2 issue poses a considerable challenge in contemporary times, and we are employing various strategies to tackle this problem,” Varanasi notes. He emphasizes the necessity of locating practical methods to extract carbon dioxide, whether it originates from sources like power plant emissions or is captured directly from the atmosphere or oceans. After extraction, it is crucial to ensure the carbon dioxide is repurposed effectively.
A range of systems has been established to convert the captured gas into useful chemical products. Varanasi asserts, “The capability to achieve this is not the issue—our focus is on optimizing efficiency and cost-effectiveness.”
In their latest study, the team concentrated on the electrochemical process that converts CO2 into ethylene, a chemical widely utilized in producing plastics and fuels, traditionally derived from petroleum. Importantly, the methodology developed could also be adapted for creating other valuable chemical products, including methane, methanol, and carbon monoxide, according to the researchers.
Currently, ethylene’s market price hovers around $1,000 per ton, placing pressure on these systems to either match or undercut that value. The electrochemical conversion of carbon dioxide into ethylene involves the use of a water-based solution and a catalyst, where an electric current is facilitated within a device known as a gas diffusion electrode.
The performance of gas diffusion electrodes is influenced by two primary characteristics: the materials must exhibit strong electrical conductivity to minimize energy loss due to resistance heating, while also being hydrophobic to prevent the water-based electrolyte from leaking and disrupting the chemical reactions at the electrode surface.
This presents a challenge, as there is an inherent trade-off: enhancing conductivity typically diminishes hydrophobicity, and vice versa. To tackle this issue, Varanasi and his team embarked on extensive experimentation. Ultimately, they developed a straightforward yet ingenious solution.
The researchers implemented a plastic material known as PTFE (polytetrafluoroethylene, commonly referred to as Teflon) for its excellent hydrophobic qualities. However, due to its poor conductivity, electrons would have to navigate a very thin catalyst layer, resulting in significant voltage drops over greater distances. To address this limitation, the team interwove conductive copper wires through the PTFE sheet.
“This development effectively resolves the challenge of achieving both conductivity and hydrophobicity,” Varanasi explained.
Traditionally, research on carbon conversion systems has been conducted on small, lab-scale samples measuring less than 1 inch (2.5 centimeters) on a side. In an effort to demonstrate the potential for scaling up, Varanasi’s group successfully produced a sheet that was ten times larger, showcasing remarkable performance.
To reach this outcome, they conducted fundamental tests that had not been previously undertaken, examining the relationship between conductivity and electrode size under identical conditions. Their findings revealed that conductivity significantly decreased as size increased, indicating that larger electrodes would require substantially more energy, thus elevating costs.
“This finding aligned with our expectations, yet it was an area that had not been systematically studied before,” Rufer remarked. In addition to reduced conductivity, larger electrodes also generated more unintended chemical byproducts in addition to the desired ethylene.
For implementation in real-world industrial contexts, electrodes are likely to need dimensions that are up to 100 times larger than those used in laboratory settings. Therefore, incorporating conductive wires is essential for the practicality of such systems, the researchers emphasized. They also developed a model that identifies spatial variation in voltage and product distribution on electrodes resulting from ohmic losses. This model, combined with their experimental results, enabled the calculation of optimal conductive wire spacing to offset conductivity loss.
By weaving wires through the material, they effectively segmented the material into smaller subsections based on the wire configuration. “We are essentially transforming it into a series of smaller electrodes,” Rufer explained. “Our findings indicate that smaller electrodes can operate very efficiently.”
The embedded copper wire, with its superior conductivity compared to PTFE, creates an efficient pathway for electron flow, mitigating resistance in areas where electrons would ordinarily encounter more impediments.
In a bid to validate the durability of their system, the researchers operated a test electrode for 75 consecutive hours, observing minimal performance degradation. Overall, Rufer stated, “Our system represents the first PTFE-based electrode to successfully scale beyond lab dimensions of 5 centimeters or smaller without compromising efficiency.”
The method for incorporating the wire can be seamlessly adapted into existing manufacturing processes, including large-scale roll-to-roll techniques, according to Rufer.
“Our strategy is remarkably powerful as it is not contingent on the specific catalyst utilized,” Rufer noted. “The micrometric copper wire can be integrated into any gas diffusion electrode, irrespective of its catalyst characteristics. This versatility allows for the scaling of anyone’s electrode systems.”
“To address the CO2 challenge effectively, we must consider solutions capable of scaling to process gigatons of carbon dioxide annually,” Varanasi stated. “Approaching the issue from this vantage point enables us to identify key barriers and foster innovative solutions that could significantly advance our efforts. Our hierarchically conductive electrode emerges from this strategic thinking.”
The research team included MIT graduate students Michael Nitzsche and Sanjay Garimella, along with Jack Lake PhD ’23. The project received support from Shell, through the MIT Energy Initiative.
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