Carbon spheres: microscopy image (a) shows agglomerated spheres while single spheres can be seen in images (b-d). Credit: ESRI, Swansea University.
Carbon spheres: microscopy image (a) shows agglomerated spheres while single spheres can be seen in images (b-d). Credit: ESRI, Swansea University.

Tiny spheres of carbon, which can be produced easily and greenly, could adsorb carbon dioxide from industrial processes to reduce emissions [Khodabakhshi et al., Carbon 171 (2021) 426-436,].

Carbon spheres come in all sizes from the nano- to the microscale and hold potential for a wide range of applications from energy storage and conversion to drug delivery. Porous carbon spheres are particularly attractive for gas absorption and storage because they are highly stable and have a large surface area. But most current methods for producing carbon spheres are expensive or impractical, requiring multiple steps, templates, purification, or chemical activation.

Now researchers from the Energy Safety Research Institute at Swansea University in Wales have developed a one-step means of producing porous carbon spheres using chemical vapor deposition (CVD) that does not require a catalyst, chemical activation or purification solvents. The team identified pyromellitic acid as a safe, easy-to-handle solid feedstock, which is currently produced as a by-product from oil refining but can be derived from sustainable sources.

“We have found a way to produce porous carbon spheres using benign and cheap starting materials,” says Enrico Andreoli, who led the effort. “[Our] simple process makes uniform porous carbon spheres without any catalyst, activator, or template. Unlike other CVD methods, our method can produce spheres at large scale without relying on hazardous gas or liquid feedstocks.”

The process works best at 800°C, when carbon spheres with a high density of tiny pores less than 1 nm in diameter and micropores are produced. Unlike other pyrolysis processes, the spheres self-assemble without the need for templates or catalysts. The sphere’s properties are interesting too. They have a high carbon capture capacity at both atmospheric and lower pressures, adsorbing carbon dioxide in preference to nitrogen, which is a common component of flue gases.

“The micropores in our spheres means they perform very well in capturing carbon,” explains Saeed Khodabakhshi, first author of the study. “Porosity is also useful in processes where sorption and exchange are essential, like in water purification, environmental remediation, and gas separation.”

The chemical, thermal, and electrical properties of carbon spheres make them attractive for catalysis, while their spherical shape could make them a promising addition to lubricants.

“Our carbon spheres are also being examined for potential use in batteries and supercapacitors so, in time, they could become essential to renewable energy storage, as well as for carbon capture,” adds Khodabakhshi.

While the scope for applications is wide, producing bulk amounts of carbon spheres currently requires relatively large footprint equipment, point out the researchers. However, they are confident that there are suitable reactor options.