Research Laboratory news

Aenert. Research Laboratory news
Being one of the main pathways to carbon neutrality or carbon negativity, the electrochemical reduction of CO₂ offers great promise for fuel production. The knowledge that copper is capable of performing this process has been around for more than 50 years. So far copper (Cu) is the only metal material in use. However, despite the fact that copper-based materials have been investigated for decades, scientists have yet to gain a fundamental understanding of the functioning of the material as an electrocatalyst and large-scale deployment of copper-based electrocatalysts for CO₂ reduction.

Now (2023), scientists at Berkeley Lab have gained new insight into the mechanisms of copper electrocatalysts by means of observing copper nanoparticles as they convert CO₂ and water into renewable fuels and chemicals, such as ethylene, ethanol, and propanol.

This was achieved by combining a new imaging technique called operando 4D electrochemical liquid-cell STEM (scanning transmission electron microscopy) with a soft X-ray probe and investigate the same sample environment, copper nanoparticles in liquid. This is a revolutionary approach because formerly, the two techniques typically could not be performed by the same instrument. This technique enables the scientists to map out atomic or molecular regions in a variety of materials.

The core of the new technique is an electrochemical “liquid cell” sample holder which is several times thinner than a human hair and the is compatible with both STEM and X-ray instruments. The cell’s ultrathin design enables reliable imaging of delicate samples as well as protects them from electron beam damage. A special electrode further made it possible to conduct X-ray experiments with the electrochemical liquid cell. Combining the two allows researchers to comprehensively characterise electrochemical reactions in real time and at the nanoscale.

The scientists used the new electrochemical liquid cell to observe copper nanoparticles develop into active nanograins during CO₂ electrolysis. To their surprise they found that copper nanoparticles combined into larger metallic copper “nanograins” within seconds of the electrochemical reaction.

To find out more, the scientists used the same electrochemical liquid cell, but this time during RSoXS (resonant soft X-ray scattering) experiments, to analyse whether copper nanograins could lead to CO₂ reduction. Soft X-rays are ideal for studying how copper electrocatalysts evolve during CO₂ reduction. By using RSoXS, researchers could monitor multiple reactions between thousands of nanoparticles in real time, and accurately make out chemical reactants and products. The experiments proved that metallic copper nanograins could serve as active sites for CO₂ reduction.

During CO₂ electrolysis, the copper nanoparticles were found to change their structure during a process called “electrochemical scrambling.” The copper nanoparticles’ surface layer of oxide degrades, creating open sites on the copper surface for CO₂ molecules to attach. As CO₂ binds to the copper nanograin surface, electrons are transferred to CO₂, causing a reaction that simultaneously produces ethylene, ethanol, and propanol along with other multicarbon products.

Further experiments showed that all of the 7-nanometer copper nanoparticles participated in CO₂ reduction, whereas the larger nanoparticles did not. Moreover, the team learned that only metallic copper can efficiently reduce CO₂ into multicarbon products.

The new study also proved the team’s findings from 2017, that 7-nanometer-sized copper nanoparticles required low inputs of energy to start CO₂ reduction. As an electrocatalyst, the 7-nanometer copper nanoparticles needed a low driving force which was about 300 millivolts less than typical bulk copper electrocatalysts.

For a long time, scientists have tried to convert CO₂ into fuel. In 2020, a synthetic protocol to the fixation of carbon dioxide was designed by converting it directly into aviation jet fuel with the help of new iron-based catalysts. A Fe-Mn-K catalyst was prepared by the so-called Organic Combustion Method. The catalyst exhibited a carbon dioxide conversion through hydrogenation to hydrocarbons in the aviation jet fuel range of 38.2% and with an attendant low carbon monoxide (5.6%) and methane selectivity (10.4%). Also light olefins such as ethylene, propylene, and butenes were produced, important raw materials for the petrochemical industry. Since this carbon dioxide is extracted from air, and re-emitted from jet fuels when combusted in flight, the result is a carbon-neutral fuel.

Image: SEM images of Fe–Mn–K catalyst. a The Fe–Mn–K catalyst precursor; b the used Fe–Mn–K catalyst



Source: Benzhen Yao, Tiancun Xiao, Ofentse A. Makgae, Xiangyu Jie/ Transforming carbon dioxide into jet fuel using an organic combustion-synthesized Fe-Mn-K catalyst/ Nature Communications 11(1), December 2020/ DOI:10.1038/s41467-020-20214-z/ Open Source Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

In 2017, an efficient and stable Na–Fe₃O₄/HZSM-5 catalyst was created, which was capable of directly converting CO₂ to gasoline-range (C5–C11) hydrocarbons with a selectivity up to 78% of all hydrocarbons. This was realised with the help of a multifunctional catalyst providing three types of active sites (Fe₃O₄, Fe₅C₂ and acid sites), which started a tandem reaction. The appropriate proximity of three types of active sites was found to play a crucial role in the successive and synergetic catalytic conversion of CO₂ to gasoline. The multifunctional catalyst also showed a remarkable stability for 1,000 h on stream.

Image: Structural characterization of Na–Fe₃O₄ catalyst. (a,c) TEM images of fresh (a) and spent (c) Na–Fe₃O₄ catalyst. Scale bar, 100 nm. (b,d) HRTEM images of fresh (b) and spent (d) Na–Fe₃O₄ catalyst. Scale bar, 10 nm



Source: Jian Wei, Qingjie Ge, Ruwei Yao, Zhiyong Wen/ Directly Converting CO₂ into a Gasoline Fuel/ Nature Communications 8(1):15174, May 2017/ DOI:10.1038/ncomms15174/ Open Source Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

There new findings have beneficial implications for future CO₂ conversion into fuels: The copper nanograins could boost the energy efficiency and productivity of some catalysts designed for artificial photosynthesis. Currently, researchers plan to use the copper nanograin catalysts in the design of future solar fuel devices.

After more than 50 years, science was finally able to show how copper electrocatalysts excel in CO₂ reduction. The research brings mankind steps closer to turning CO₂ into new, renewable solar fuels through artificial photosynthesis. The ability of the new electrochemical liquid cell to record real-time movies of a chemical process opens up opportunities to study many other electrochemical energy conversion processes.

By the Editorial Board