Aug 23, 2006
Nanopores store extra charge
Researchers at Drexel University, US, and Paul Sabatier University, France, have found that the capacitance of carbon supercapacitors increased with decreasing pore size for pores less than 1 nm across. The finding could have implications for tailoring supercapacitor structure for specific applications.
"[Our finding] reverses the thinking about how pore size is important for supercapacitors," said Yury Gogotsi of Drexel University. "Following this work, decreasing pores below 1 nm can lead to smaller, lighter, more powerful supercapacitor devices. It may also have implications for understanding ionic transport in narrow channels in many other systems, including cells in human bodies."
Supercapacitors (or electrical double layer capacitors) store charge by ion absorption over the surface of highly porous materials, in this case carbon. Supercapacitors typically have a capacitance of tens of farads per gram of material, compared with traditional dielectric capacitors, which generally have capacitances in the microfarad range. Their high storage capacity arises from the small separation (∼1 nm) between the charged ions and the carbon surface, and the large surface area of the carbon surface.
Until now, scientists had believed that the supercapacitor's pores must be larger than the electrolyte ion and its solvation shell in order to provide high capacitance and to minimize the time for energy discharge. But Gogotsi and colleagues John Chmiola and Gleb Yushin from Drexel and Patrice Simon, Cristelle Portet and Pierre-Louis Taberna from Paul Sabatier University have turned this thinking on its head. They tested the capacitance of carbon materials containing pores between 0.– 2.25 nm in diameter. For titanium-carbide-derived carbon with pores below 1 nm in diameter, decreasing the pore size increased the capacitance. For pores larger than 1 nm, increasing the pore size caused the capacitance to increase, in accordance with existing theory.
"Initial work was directed at following the prior precedent of increasing the pore size of the carbon materials and determining the effect on supercapacitor performance," said Gogotsi. "However, unlike in all previous studies, we had an advantage of using carbide-derived carbon, in which we could change the pores in a fairly broad range. When we realized that we could improve performance further by looking at the lower end of the pore size spectrum, into a range thought to be previously inaccessible, we knew that we had stumbled onto something very special."
The researchers believe that decreasing the pore size to less than twice the solvated ion size reduced normalized capacitance because "compact ion layers from adjacent pore walls impinged", reducing the area available for double layer formation.
But if the pore size approached the diameter of the ion, normalized capacitance increased by 100%. The scientists reckon that the solvation shell became distorted as the ion squeezed into the pore, so that the distance between the ion centre and the surface decreased, increasing capacitance. Using pores smaller than 1 nm increased volumetric capacitance from 55 F/cm3 to 80 F/cm3.
The researchers believe that it may be possible to tailor supercapacitor structure to the application – for example, using narrower pores to give high energy density but longer discharge time for applications such as hybrid electric vehicles, but using larger pores for pulse power applications. What's more, creating carbon materials with a large volume of short narrow pores might improve both energy and power characteristics.
"Current work in our lab is directed at maximizing the volume of pores that have a diameter below 1 nm, and shows the revolutionary performance gains that we believe this approach can yield," said Gogotsi. "Also, another push is testing in electrolyte systems that allow a larger voltage window to be utilized. By combining these approaches, we feel that next-generation supercapacitors will far outperform even the high values we presented in the Science paper."
The researchers reported their work in Sciencexpress.
About the author
Liz Kalaugher is editor of nanotechweb.org.