An old friend with high-tech potential

Activated carbon, used in various forms for thousands of years, could get a boost for emerging technologies, thanks to a better understanding of its unique electronic and chemical properties.

Yury Volfkovich and colleagues from the Frumkin Institute of Physical Chemistry and Electrochemistry in Moscow, part of the Russian Academy of Sciences, have been investigating possible electrode materials for supercapacitors and capacitive water purification. It turns out that activated carbon, beyond its many more common uses, is particularly well suited for both.

“Activated carbon has a large specific surface area and simultaneously high electronic conductivity,” explains Volfkovich. “For applications such as supercapacitors and capacitive deionization of water, both properties are needed, but high electronic conductivity is most important.”

Activated carbon is made by steaming charcoal or soaking it in a chemical then heating it to drive off all moisture. The result is a highly porous material with an exceptionally high surface area for its volume, up to 2,500 square metres per gram. This provides ample opportunity for molecules to adsorb on to carbon surfaces, giving the material its remarkable purification properties. Activated carbon also has the ability to develop and hold a large electric charge, which magnifies its performance in water purification and makes it useful as a high-capacity charge-storing capacitor.

Understanding exactly how these different electronic and chemical behaviours emerge and interact, however, has remained a challenge.

“We developed new methods to measure the surface conductivity and porosity that reveal not only the nanoporous structure of activated carbon, but also its hydrophobic-hydrophilic properties,” says Volfkovich. “This allowed us to study the electrochemical, ion-exchange, sorption, and hydrophilic-hydrophobic properties of several types of activated carbon electrodes.”

The measurements revealed that activated carbon is ‘superhydrophilic’, able to absorb a very large volume of water that is incorporated into molecular groups on the porous carbon surfaces. The researchers were also able to determine that the surface conductivity is provided by molecular surface groups in the pores.

“Even in pure water, activated carbon exhibits considerable ionic conductivity, which makes possible its use in the production of pure water by capacitive deionization,” says Volfkovich.

Armed with this knowledge, the team showed that charging activated carbon in sulfuric acid could increase the capacitance to more than 1,000 farads per gram. This has direct implications for supercapacitors, which store and release electrical charge near-instantaneously and are emerging as key technologies that could help ‘smooth’ the supply of electricity from solar panels into the grid.

“An important finding of our research is that surface groups in activated carbon affect the energy efficiency of supercapacitors,” Yury says.