Billions of dollars are being spent on ways to capture the atmospheric CO2 that is contributing to climate change. It can then be sequestered, or converted into industrial materials that reduce the use of fossil fuels. Processes exist to convert CO2 into fuel or chemical building blocks, but at present they are not efficient enough to compete on price with non-renewable sources. One electrochemical process uses gas diffusion electrodes (GDEs) with a catalyst to convert carbon dioxide gas into ethylene, which can be used as a fuel or as a chemical precursor for plastics and other materials.
The basic tradeoff in GDE design is that you must choose between conductivity of the electrode material and its hydrophobic properties. A more conductive material will transfer electricity more efficiently, converting more CO2, but if the electrodes absorb water, output will be reduced and the risk of corrosion of the electrodes that will further degrade performance.For example, carbon paper is conductive but is prone to flooding with the electrolyte fluid that is used to bring the CO2 to the electrode. Polytetrafluoroethylene (PTFE) is very hydrophobic but has low conductivity. (PTFE is better known by its brand name, Teflon.)
Researchers at MIT have investigated why GDEs do not scale effectively. They focused on an electrode structure based on expanded PTFE, or ePTFE, which is porous. This material is then coated with a thin layer of copper to serve as a catalyst for the process.
Prior research has shown that such a structure can be efficient in experimental GDEs that typically range in size from 1 to 5 square centimeters. Conductivity declines rapidly as the electrode size increases, however, which in turn reduces the efficiency of the output due to loss of energy from increased resistance.
Carbon Capture: Just Add Copper
In an attempt to increase the conductivity for larger GDEs, the researchers came up with a method to “weave” copper wire through the electrodes. They named this new approach the Hierarchically Conductive Gas Diffusion Electrode (HCGDE for short).
In this design, the researchers ran copper wire with a 75 micrometer diameter through the electrode, matching the copper catalyst layer to eliminate the risk of electrolysis between dissimilar metals. The wire runs along the catalyst layer, then pierces the ePTFE membrane to connect to the current collector in the back of the electrode. According to the researchers’ paper, this design can be produced in volume, either with roll-to-roll manufacturingor standard sewing equipment. Roll-to-roll production can drive down manufacturing costs dramatically, as this is a process routinely used in printing a variety of materials.
The team used a simulation model to determine the optimum spacing of the wire on the catalyst layer. They determined that the best results would be achieved when the wires were run less than 10 millimeters apart in order to maintain sufficient conductivity while minimizing resistance losses.
The researchers then created three electrodes based on their HCGDE design in three sizes: 5 cm2, 14 cm2, and a whopping 50 cm2, fully 10 times as large as other experimental electrodes, as well as electrodes without the woven copper wires. Using a wire spacing of 4 mm, the increased conductivity came at the expense of less than 2 percent of the electrode material.
The test results show that the largest electrode was effective at producing ethylene from CO2, reaching about 75 percent Faradaic efficiency (FE) while requiring a lower voltage than the traditional GDE design. Faradaic efficiency is a theoretical limit of how much product can be produced with a given amount of energy (electrical current); a higher number means more efficient production. Furthermore, the electrical efficiency was similar for all three sizes. To test the stability of the technology, the researchers then ran the 50 cm2 unit for 75 hours. This showed stable results, although salt precipitation did degrade performance over time.
Scaling Up Carbon Capture Efforts
According to Kripa Varanasi, a professor of mechanical engineering at MIT, these results demonstrate that the technology works even when increasing the electrode size to be 10 times larger. They also expect that the performance will be maintained with even larger electrodes.
“There are numerous processes with high energy demands that are challenging to decarbonize, making it essential to develop effective methods for capturing and converting CO2. Tackling the CO₂ challenge is one of the defining problems of our time. To address it, we must process gigatons of CO2 annually, requiring scalable and cost-effective solutions,” Varanasi says. “By adopting this mindset, we can pinpoint critical bottlenecks and design innovative approaches that deliver meaningful results.”
“Capturing and storing carbon dioxide remains prohibitively expensive for many emitters,” says Eve Pope, a technology analyst with IDTechEx, a tech analysis company. “Breakthroughs in technologies that can utilize carbon dioxide, displacing petroleum-based incumbents, can unlock additional revenue streams to economically incentivize the capture of CO2.” Pope says that IDTechEx’s forecasts for these kinds of “CO2-derived drop-in chemicals” could exceed US $1 billion in revenue before 2040.
If HCDGE technology can indeed be scaled to industrial levels, it could provide a cost-competitive alternative for fossil fuels, both as a fuel to reuse carbon dioxide already in the atmosphere, and as a precursor for materials used in durable goods that could sequester that carbon for long periods of time.
Alfred Poor is a technology speaker and writer with a focus on health tech. He was the editor of “Health Tech Insider,” a website that covered wearable and mobile devices for health and medical applications. He also helps tech company executives make a better impression in video meetings and online presentations. A graduate of Harvard College, he is the author or co-author of 15 books and is widely quoted in major media outlets.
