Conversion of CO2 Into Multi-Walled Carbon Nanotubes Using a Sustainable Electrochemical Technique

Carbon dioxide (CO₂) is a significant greenhouse gas that is released into the atmosphere through various human activities. To reduce humanity's impact on the environment, scientists and policymakers worldwide are actively seeking ways to decrease CO₂ emissions and find practical uses for it. One approach that has gained attention is the electrochemical method, which involves converting CO₂ into other carbon-based substances such as carbon monoxide, alcohols, and hydrocarbons. This method holds promise for reducing atmospheric CO₂ levels while also producing valuable carbon-containing materials.

Environmental researchers from Doshisha University, Japan, led by Prof. Takuya Goto, have conducted a study published in Electrochimica Acta on July 10, 2023. Their research presents a method for converting CO₂ into multi-walled carbon nanotubes (MWCNT) using molten salts and sustainable electrochemistry. The study, available online since April 22, 2023, involved contributions from Dr. Yuta Suzuki from the Harris Science Research Institute and Mr. Tsubasa Takeda from the Department of Science of Environment and Mathematical Modeling.

The research team employed a sustainable electrochemical method that involved using a LiCl-KCl melt to convert CO₂ into multi-walled carbon nanotubes (MWCNT). They saturated the molten salts with CO₂ gas and utilized a partially immersed nickel (Ni) substrate as the electrode. During the process, the supplied CO₂ was electrochemically transformed into solid carbon. This environmentally friendly conversion took place through a reduction reaction that occurred at the interface of the Ni electrode, LiCl-KCl melt, and CO₂.

Professor Goto explains that the researchers investigated the electrochemical reduction of CO₂ on a nickel (Ni) electrode in a LiCl-KCl melt at a temperature of 723 K. They observed the formation of a super meniscus at the three-phase interface of the Ni electrode, LiCl-KCl melt, and CO₂ gas under high polarization. This specific region facilitated the direct electrochemical reduction of CO₂, leading to the formation of solid carbon. The solid carbon was obtained not only in the wetted area of the Ni electrode but also within the bulk molten salt through the electrochemical process.

After electrodeposition, the carbonaceous material obtained was examined using electron microscopy techniques and elemental analysis. The analysis revealed that the material consisted of commercially valuable multi-walled carbon nanotubes (MWCNTs) with diameters ranging from 30 to 50 nm. The research team then manipulated the applied voltage and extended the reaction time, resulting in noticeable changes in the MWCNTs. Increasing the electrolysis time from 10 minutes to 180 minutes caused an increase in the height of the generated MWCNTs. Prof. Goto highlights that they investigated the relationship between the applied potential, electrolytic time, and the morphology and crystallinity of the electrodeposited carbon. Based on their experimental findings, the researchers proposed a model to explain the formation of the MWCNTs on the nickel electrode.

The proposed model for the generation of multi-walled carbon nanotubes (MWCNTs) from CO₂ consists of three stages. In the first stage, CO₂ is reduced to carbon atoms at the interface of the nickel (Ni) electrode, LiCl-KCl melt, and CO₂. During the second stage, the electrodeposited carbon atoms combine with the Ni electrode's surface to form Ni-C compounds, such as NiC. In the final stage, when the solubility limit of carbon in the Ni-C compounds is reached, MWCNTs begin to grow in a cylindrical shape from the edges of the Ni-C compounds formed in the second stage. This model explains the process of MWCNT formation from CO₂ using the electrochemical method.

In conclusion, the study presents a novel and sustainable method for converting CO₂ into commercially valuable carbonaceous materials. The electrochemical process utilized in the study is environmentally friendly as it does not rely on fossil fuels. The use of high-temperature molten salts is particularly noteworthy because it allows for the direct conversion of CO₂ gas into multi-walled carbon nanotubes (MWCNTs). This research offers promising insights into the development of eco-friendly processes for utilizing CO₂ and producing valuable carbon-based materials.

Professor Goto concludes optimistically, stating that their findings demonstrate the potential for converting CO₂ into functional carbonaceous materials. By incorporating non-consumable oxygen-evolving anodes, this technique could contribute to the advancement of carbon recycling technology. This technology holds the potential to address global environmental challenges and play a crucial role in carbon pricing economies. Furthermore, as the material production process does not rely on fossil fuels, it aligns with the goal of achieving a sustainable society in the near future. Prof. Goto's remarks highlight the significance of this research in paving the way for a more sustainable and environmentally conscious approach to carbon utilization and material production.

We certainly hope his visions will be realized soon!