Dyes can be used to reveal the concentration levels of carbon dioxide in water. On the left side of the container is the gas-attracting material, with the dye showing that the carbon dioxide stays concentrated next to the catalyst. Photo: Varanasi Research Group.
Dyes can be used to reveal the concentration levels of carbon dioxide in water. On the left side of the container is the gas-attracting material, with the dye showing that the carbon dioxide stays concentrated next to the catalyst. Photo: Varanasi Research Group.

Systems for capturing and converting carbon dioxide from power plant emissions could be important tools for curbing climate change, but most are relatively inefficient and expensive. Now, researchers at Massachusetts Institute of Technology (MIT) have developed a method for significantly boosting the performance of systems that use catalytic surfaces to enhance the rates of carbon-sequestering electrochemical reactions.

Such catalytic systems are an attractive option for carbon capture because they can produce useful, valuable products, such as transportation fuels or chemical feedstocks. This output can help to subsidize the carbon-capture process, offsetting the costs of reducing greenhouse gas emissions.

In these systems, a stream of gas containing carbon dioxide typically passes through water to deliver carbon dioxide for the electrochemical reaction. The movement through water is sluggish, which slows the rate of conversion of the carbon dioxide. The new method ensures that the carbon dioxide stream stays concentrated in the water right next to the catalyst surface. This concentration, the researchers have shown, can nearly double the performance of the system.

The method is reported in a paper in Cell Reports Physical Science by MIT postdoc Sami Khan, who is now an assistant professor at Simon Fraser University in Canada, along with MIT professors of mechanical engineering Kripa Varanasi and Yang Shao-Horn, and recent graduate Jonathan Hwang.

"Carbon dioxide sequestration is the challenge of our times," Varanasi says. There are a number of approaches, including geological sequestration, ocean storage, mineralization and chemical conversion. When it comes to making useful, saleable products out of this greenhouse gas, electrochemical conversion is particularly promising, but it still needs improvements to become economically viable. "The goal of our work was to understand what's the big bottleneck in this process, and to improve or mitigate that bottleneck," he says.

The bottleneck turned out to involve the delivery of carbon dioxide to the catalytic surface that promotes the desired chemical transformations. In these electrochemical systems, the stream of carbon dioxide-containing gases is mixed with water, either under pressure or by bubbling the gases through a container outfitted with electrodes of a catalyst material such as copper. Applying a voltage to the container promotes chemical reactions that convert the carbon dioxide into carbon compounds, which can then be transformed into fuels or other products.

These electrochemical systems face two challenges. The first is that the reaction can proceed so fast that it uses up the supply of carbon dioxide reaching the catalyst more quickly than it can be replenished. If that happens, a competing reaction – the splitting of water into hydrogen and oxygen – can take over and sap much of the energy being put into the reaction.

Previous efforts to optimize these reactions by texturing the catalyst surfaces to increase the surface area for reactions had failed to deliver on expectations, because the carbon dioxide supply to the surface couldn't keep up with the increased reaction rate, thereby switching to hydrogen production over time.

The researchers addressed these problems through the use of a gas-attracting material placed in close proximity to the catalyst. This specially textured material is both 'gasphilic' and superhydrophobic, causing it to repel water while allowing a smooth layer of gas called a plastron to stay close to its surface.

The plastron keeps the incoming flow of carbon dioxide right up against the catalyst, so that the desired carbon dioxide conversion reactions can be maximized. By using dye-based pH indicators, the researchers were able to visualize carbon dioxide concentration gradients in the test cell and show that the enhanced concentration of carbon dioxide emanates from the plastron.

In a series of lab experiments using this setup, they found that the rate of the carbon conversion reaction nearly doubled. It was also sustained over time, whereas in previous experiments the reaction quickly faded out. The system could produce high rates of ethylene, propanol and ethanol – a potential automotive fuel. Meanwhile, the competing hydrogen evolution reaction was sharply curtailed.

In some applications, however, the desired outcome might be to optimize for hydrogen production, which can also be done. "The important metric is selectivity," Khan says, referring to the ability to generate valuable compounds that will be produced by a given mix of materials, textures and voltages, and to adjust the configuration according to the desired output.

By concentrating the carbon dioxide next to the catalyst surface, the new system also produced two other potentially useful carbon compounds – acetone and acetate – that had not previously been detected in any electrochemical systems at appreciable rates.

In this initial laboratory work, a single strip of the hydrophobic, gas-attracting material was placed next to a single copper electrode, but in future work a practical device might be made using a dense set of interleaved pairs of plates, Varanasi suggests.

Compared to previous work on electrochemical carbon reduction with nanostructured catalysts, "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," says Varanasi.

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.