Improving the efficiency of electrochemical carbon capture and conversion systems

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Dyes are used to detect the concentration of carbon dioxide in water. There is a gas-absorbing material on the left, and the paint indicates that carbon dioxide is concentrated near the catalyst. Credit: Varanasi Research Group

The new design can accelerate the reaction rates in electrochemical systems to remove carbon from power plant waste.

Carbon dioxide capture and conversion systems from power plant waste can be an important tool in preventing climate change, but most are relatively inefficient and expensive. Now researchers WITH have developed a method that will significantly increase the performance of systems that use catalytic surfaces to increase the rate of carbon separation electrochemical reactions.

Such catalytic systems are an attractive choice for carbon capture because they can produce useful, valuable products such as transportation fuels or chemical raw materials. This product can help subsidize the process by offsetting the cost of reducing greenhouse gas emissions.

In these systems, a stream of gas, usually carbon dioxide, passes through water to deliver carbon dioxide for an electrochemical reaction. The movement with water is slow, which slows down the rate of conversion of carbon dioxide. The new design allows the carbon dioxide flow to remain concentrated in the water near the catalyst surface. According to researchers, this concentration can almost double the performance of the system.

The results are described in today’s issue Cell Reports Physical Science MIT postdoc Sami Khan SM ’15 PhD ’19, MIT engineering professors Kripa Varanasi and Yang Shao-Horn, who are now associate professors at Simon Fraser University, and recent graduate Jonathan Hwang PhD ’19.

“Separation of carbon dioxide is a challenge of our time,” says Varanasi. There are a number of approaches, including geological sequestration, ocean storage, mineralization, and chemical transformation. The electrochemical transformation is particularly promising when it comes to the production of marketable products from this greenhouse gas, but there is still a need for development to be economically viable. “The goal of our work was to understand what the biggest shortage was during this period and to improve or reduce that shortage,” he says.

The glass cap was found to promote the delivery of carbon dioxide to the catalytic surface and the desired chemical transformations. In these electrochemical systems, the flow of carbon dioxide-containing gases is either pressurized or mixed with water by foaming in a vessel equipped with electrodes of a catalyst material such as copper. A voltage is then applied to promote chemical reactions that produce carbon compounds that can be converted into fuel or other products.

Such systems have two difficulties: The reaction can proceed so rapidly that it uses up the supply of carbon dioxide to the catalyst faster than it is filled; and if this happens, a rival reaction – the splitting of water into hydrogen and oxygen – can take up most of the energy involved in the reaction.

Efforts to optimize these reactions by injecting tissue into the catalyst surfaces to increase the surface area of ​​the reactions did not live up to expectations, as the supply of carbon dioxide to the surface could not keep up with the increasing reaction rate and thus switched to hydrogen production. extra time.

The researchers solved these problems using a gas-absorbing surface placed close to the catalyst material. This material is a special textured “gas”, a superhydrophobic material that repeats water, but allows a smooth layer of gas called plastron to remain close to the surface. It blocks the incoming carbon dioxide flow against the catalyst to maximize the desired carbon dioxide conversion reactions.

Surface that attracts gas to a special tissue

On the left, a bubble hits a surface that attracts a special tissue gas and spreads to the surface, while on the right, a bubble hits and bounces on an untreated surface. The treated surface is used in the new work to keep the carbon dioxide close to the catalyst. Credit: Varanasi Research Group

Using dye-based pH, the researchers visualized the carbon dioxide concentration gradients in the test cell and showed that the enhanced carbon dioxide concentration came from the plastron.

Carbon Dioxide Concentration Level in Water

Here, dyes are used to detect the concentration of carbon dioxide in the water. Green indicates where carbon dioxide is more concentrated, and blue indicates where it is depleted. The green area on the left indicates that carbon dioxide remains near the catalyst due to the gas-absorbing material. Credit: Varanasi Research Group

In a series of laboratory experiments using this correction, the carbon conversion reaction rate nearly doubled. As time went on, the reaction decreased rapidly in previous experiments. The system produced high ethylene, propanol and ethanol, which are potentially automotive fuels. Meanwhile, the evolution of rival hydrogen was drastically reduced. While the new work makes it possible to fine-tune the system to produce any product mixture, in some applications the optimization of hydrogen production as a fuel may also be the desired outcome.

“Important is metric selectivity,” says Khan, referring to the ability to create valuable compounds that can be produced with a certain material, texture, and tension mixture and adjust the configuration to the desired result.

By concentrating carbon dioxide near the catalyst surface, the new system also produced two new potentially useful carbon compounds, acetone and acetate, which had not previously been significantly detected in such electrochemical systems.

In this initial laboratory work, a strip of a hydrophobic, gas-absorbing substance was placed next to a single copper electrode, but in future work a practical device could be made using a set of dense layer plates.

Compared to previous work on electrochemical carbon reduction with nanostructured catalysts, Varanasi says, “we outperform them all because of how we deliver carbon dioxide, which changes the game despite being the same catalyst.”

“This is a completely innovative way to feed carbon dioxide in electrolysis,” says Ifan Stephens, a professor of material engineering. Imperial College London, who was not involved in this study. “The authors are converting the concepts of fluid mechanics used in the oil and gas industry into the production of electrolytic fuels. I think that such cross-fertilization from different areas is very exciting. ”

Stephens adds, “Carbon dioxide reduction has great potential as a way to produce platform chemicals such as ethylene from waste electricity, water and carbon dioxide. Ethylene is currently formed by the decomposition of long-chain hydrocarbons from residual fuels; production emits large amounts of carbon dioxide into the atmosphere. This method could potentially lead to a more efficient reduction in carbon dioxide and, as a result, distract our society from our trust in fossil fuels. ”

Reference: Sami Khan, Jonathan Hwang, Yang-Shao Horn and Kripa K. Varanasi, “Catalyst-proximal plastrons increase the activity and selectivity of carbon dioxide electroreduction”, January 25, 2021, Cell Reports Physical Science.
DOI: 10.1016 / j.xcrp.2020.100318

The study was supported by the Italian energy firm Eni SpA through the MIT Energy Initiative and an NSERC PGS-D graduate student scholarship from Canada.

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