First-of-its-kind electrode from MIT converts CO2 into usable fuels, plastics
Given global warming and targets to reduce greenhouse gas emissions, researchers are practically and economically exploring the possibilities of carbon dioxide capture and conversion into useful products, such as transportation fuels, chemical feedstocks, and building materials.
Nevertheless, many of these endeavors still need to be more economical.
New research conducted by engineers at MIT could significantly enhance various electrochemical systems currently being developed for converting carbon dioxide into valuable commodities.
The team has developed a new design for the electrodes used in these systems, which will improve the efficiency of the conversion process.
The findings were published today in Nature Communications in a paper by MIT doctoral student Simon Rufer, Professor of Mechanical Engineering Kripa Varanasi, and three other colleagues.
Converting CO2 into plastics
In this study, the team focused on the electrochemical conversion of carbon dioxide into ethylene. This widely used chemical can be transformed into various plastics and fuels derived from petroleum.
Their developed method could also produce other high-value chemical products like methane, methanol, and carbon monoxide.
Ethylene sells for about $1,000 per ton, so the objective is to meet or exceed that price.
The electrochemical process for converting CO2 into ethylene involves a water-based solution and a catalyst material, which interact with an electric current in a device known as a gas diffusion electrode.
Two competing characteristics of the gas diffusion electrode materials influence their performance: They must be good electrical conductors to prevent wasted energy due to resistance heating and be hydrophobic (water-repelling) to avoid interference from the water-based electrolyte solution. Unfortunately, there is a tradeoff; enhancing conductivity typically reduces hydrophobicity and vice versa.
Varanasi and his team aimed to navigate this conflict and, after months of work, succeeded in doing so.
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They developed a straightforward solution using PTFE (commonly known as Teflon), which is recognized for its excellent hydrophobic properties. However, PTFE lacks conductivity, necessitating electrons to travel through a very thin catalyst layer, resulting in a significant voltage drop over distance.
The researchers wove conductive copper wires throughout the thin PTFE sheet to address this challenge.
New electrode design
Research on potential carbon conversion systems has mostly been conducted on small, lab-scale samples, typically measuring less than 1 inch (2.5 centimeters) on each side.
To demonstrate scalability, Varanasi’s team created a ten times larger sheet that showcased its effective performance.
To achieve this, they conducted tests that had never been done before. They ran experiments under identical conditions using electrodes of different sizes to analyze the relationship between conductivity and electrode size.
They found that conductivity dropped significantly with increased size, indicating that more energy—and therefore cost—would be needed to drive the reaction.
The researchers indicate that real-world industrial applications would require electrodes perhaps 100 times larger than the laboratory versions, making the addition of conductive wires essential for practical implementation.
The researchers also created a model that accounts for the redistribution of voltage and products on the electrodes due to the ohmic losses.
Together with the experimental data they had examined, this model enabled them to estimate the optimal distances between the conductive, easily connecting wires to prevent the drop in conductivity.
In this manner, copper wire embedded in the material subdivided the material into smaller sections according to the distribution of the wires.
Because copper wire is significantly more conductive than PTFE, it acts as a superhighway for electrons, bridging areas with greater resistance.
To demonstrate the robustness of their system, the researchers continuously ran a test electrode for 75 hours, experiencing little change in performance.
