May 30, 2021. By Kolemann Lutz
MIT Researchers developed bubble-capturing (gasphilic) material surfaces to enhance the electrochemical reduction (CO2RR) of carbon dioxide in aqueous liquid water near copper catalyst by nearly 2X to produce biofuels and form carbon 2 products such as ethylene, propanol, and ethanol.
Rising CO2 concentrations in Earth’s atmosphere are increasing at alarming rates, increasing global temperatures and threatening ecosystems. Carbon capture and conversion via electrochemical reduction represents a low power and mass solution of synthesizing chemicals or fuels in situ to organic feedstocks such as formic acid (HCOOH), carbon monoxide (CO), methane (CH4), ethylene (C2H4) and ethanol (C2H5OH).
Most research on electrochemical reduction of carbon dioxide has focused on developing catalysts to enhance kinetics (activity) and production distribution (selectivity).
Poor solubility of CO2 in water limits CO2RR current density, and the hydrogen evolution reaction (HER) is favored in undersaturated medium close to catalyst. Copper catalysts have been studied and developed extensively as it can produce hydrocarbons with one carbon (C1) such as methane, and two carbons (C2) such as ethylene.
Recent studies have shown increased current density or selectivity toward C2 products on copper catalysts by using nanostructures, by controlling surface crystallinity, and by utilising gas diffusion electrodes. Traditionally, the main bottleneck in CO2RR has been the delivery of CO2 to the catalytic surface. To improve the carbon conversion in liquid aqueous water solutions, the special texture and chemistry of gasphilic surfaces can be used to better capture the gas bubbles.
MIT Researchers developed a gasphilic CO2 trap that adsorps in close proximity to catalyst to sustain a supersaturated CO2 concentration around catalyst, increasing gas-liquid mass transfer. By creating nano pyramidal textures on silicon, CO2 bubbles are efficiently captured to form a sheet of gas underwater known as a plastron.
This gasphilic texture is a superhydrophobic material that repels water and keeps a steady flow of carbon dioxide right up against the catalyst so that the desired carbon dioxide conversion reactions can be maximized.
After pressure is applied or CO2-gases are bubbled through mixed water in a container with electrodes of electropolished copper catalyst, a voltage was applied to promote the chemical reactions.
In a series of lab experiments, the researchers were able to nearly double the rate of carbon conversion reaction to produce high rates of ethylene, propanol, and ethanol. The hydrogen faradaic efficiency, or how good electrons or current is transferred, is suppressed from 33% to 13% on smooth copper and 62% to 33% on nanostructured copper to form C2+ products such as ethylene, propanol, and ethanol.
Future studies can evaluate high specific surface area catalyst architectures primarily used with gas diffusion electrodes (GDE) along with the plastron approach in aqueous phase systems. “In future work a practical device might be made using a dense set of interleaved pairs of plates”, mentioned by Kripa Varanasi, MIT professor of mechanical engineering.
When comparing the gasphilic plastron approach to previous work on electrochemical carbon reduction, Varanasi says, "we significantly outperform them all, because even though it's the same catalyst, it's how we are delivering the carbon dioxide that changes the game."
The electrochemical reduction of carbon dioxide can also be employed to produce useful, valuable products, such as CH4 or chemical feedstocks. As the formation of C2+ hydrocarbon products typically brings greater power and mass requirements, CO2RR is a lightweight, low power solution that holds the potential to help synthesize a myriad of carbon products to support life on planet Mars and beyond.
Sami Khan, Jonathan Hwang, Yang-Shao Horn, Kripa K. Varanasi. Catalyst proximal plastrons enhance activity and selectivity of carbon dioxide electroreduction, Cell Reports Physical Science, Volume 2, Issue 2, 2021, https://doi.org/10.1016/j.xcrp.2020.100318