Curved carbon nanotubes improve electrocatalysts for carbon neutrality

Electrocatalysis plays a critical role in the development of clean energy, greenhouse gas removal and energy storage technologies. A study co-led by researchers at the City University of Hong Kong (CityU) found that single-walled carbon nanotubes make excellent substrates for enhancing greenhouse gas conversion through molecular curvature.

By using these nanotubes as a support to induce stress on an electrocatalyst, the efficiency of reducing carbon dioxide to methanol can be significantly improved.

This advance paves the way for the development of curved molecular electrocatalysts to efficiently convert carbon dioxide (CO₂), one of the main greenhouse gases, into useful chemicals and fuels, thereby reducing carbon emissions. The work is published in the journal Natural catalysis.

Many molecular complexes, such as cobalt phthalocyanine (CoPc), are effective CO catalysts₂ reduction reaction (CO₂RR). But above all they reduce CO₂ to toxic carbon monoxide (CO), without generating more substantial quantities of useful products, such as methanol. “Therefore, we want to explore the potential of CoPc beyond CO production,” said Professor Ye Ruquan, from the Department of Chemistry at City University of Hong Kong (CityU), who led the research.

At the same time, deformation is known to affect the properties of two-dimensional materials, which are often on the nanometer (nm) scale. “The use of curved substrates or supports to induce local deformation is well established for modulating the properties of conventional layered materials,” explained Professor Ye.

“But rationally controlling the deformation of planar molecules is challenging due to their ultrasmall size. And how deformation affects molecular properties remains poorly understood.”

With his collaborators, Professor Ye led a research team to study the reactivity of CoPc molecular catalysts at the nanoscale by adopting support-induced stress engineering. They successfully introduced controlled strain into molecules smaller than 2 nm of the catalyst using single-walled carbon nanotubes as a support.

The curvature of nanotubes due to molecular interactions induces stress on the catalytic molecules, resulting in curvature. The use of carbon nanotube substrates of different diameters allows them to adjust the angle of curvature of the CoPc molecules ranging from 96° (for carbon nanotubes with a diameter of 1 nm) to 1.5° (for carbon nanotubes carbon of 100 nm in diameter).

Compared with traditional planar molecules, curved molecules exhibited improved electrocatalytic performance. They showed greater selectivity for CO₂ reduction, favoring the production of methanol compared to carbon monoxide.

In a tandem flow electrolyzer with monodisperse CoPc on single-walled carbon nanotubes for CO₂ reduction, the team achieved a partial current density in methanol of over 90 mA cm⁻² with a selectivity greater than 60%, which means that the total CO₂The methanol efficiency is 60%. This is a significant improvement over existing methods.

Their analysis based on theoretical calculations confirmed that the curved CoPc on the single-walled carbon nanotubes improved the binding of CO, thereby achieving a consequent reduction of carbon monoxide. In contrast, large, multi-walled carbon nanotubes promote CO release.

“Our results show that carbon nanotubes are exceptional support materials for catalysts like CoPc. The large specific surface areas of carbon nanotubes easily disperse nanoparticles, thus avoiding agglomeration, and their high electronic conductivity makes them promising for electrochemical applications,” said Professor Ye.

“More importantly, we have shown that inducing molecular distortion using single-walled carbon nanotubes provides a strategy for designing high-performance molecular electrocatalysts. This advance holds promise for achieving carbon neutrality, as it can store CO₂ and renewable electricity in the form of chemical energy,” he concluded.