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April 2014

Size Matters If You're a Miniature Electrode

Despite widely held assumptions, carbon nanotubes' biggest benefits come from their small sizes

Ralph L. House

A graphical depiction of a transistor made from a single-walled carbon nanotube, coated with a generic charge-storage material, in this case LixMnO2. This deceptively simple device has called into question commonly held beliefs about the systems that power our modern devices, such as lithium-ion batteries. Reprinted with permission from Corso et al. (2014) Nano Letters 14(3):1329-1336. Copyright 2014, American Chemical Society.

Common beliefs about the role of carbon nanotubes in lithium-ion batteries and other electrochemical composites have been turned on their head. A team of researchers led by Philip Collins at Nanostructures for Electrical Energy Storage (NEES) showed that one single-walled carbon nanotube connects to other materials with a surface resistivity of only 30 mΩ cm2, about four times smaller than a multi-walled carbon nanotube, and a staggering 50 times smaller than typical films of platinum or graphite.

Carbon nanotubes have many beneficial attributes when used in composites, such as high strength, high conductivity, and low mass density, but being defect-free, they were predicted to have unusually poor surface resistivities, just like pristine graphite. As researchers created electrochemical composites with carbon nanotube additives, they found power enhancements instead of decreases. The improved power rates imply excellent charge transfer, exactly the opposite of what electrochemists expected. This enhancement may result from a number of factors; for example, the curvature of the tubular surfaces, contaminants, and disorder, or perhaps a side effect of carbon nanotubes' unusual electronic properties. After all, the way to make graphite electrochemically active is to mechanically and chemically introduce defects.

Because they are edge-free, and in some cases defect-free, single-walled carbon nanotubes offered a model system for studying this further. Collins' group coated single-walled nanotubes with a generic charge-storage cathode material and built transistors (see figure) to characterize the impedance of carbon-metal oxide interfaces.

Remarkably, they found no discernible difference between devices made of pristine or damaged single-walled nanotubes. They determined that the material itself (single/multiwalled nanotubes or platinum) did not make any difference, and the whole improvement in interfacial resistance resulted from rules of electrochemistry that only kick in at the nanoscale. The unmatched size of single-walled nanotube surfaces, especially compared to any other type of conductor, takes maximum advantage of these rules. When randomly dispersed in an active material, carbon nanotubes can create a percolative network through a large volume of material, yet still maintain a disproportionately small surface resistivity. This combination helps explain the improvements observed in carbon nanotube composites.

By revealing the rules that matter, combinations can now be designed and optimized to make better batteries and more efficient solar cells, taking full advantage of the carbon nanotube additives.

Acknowledgments

This work was supported by the Nanostructures for Electrical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. The work was done at the University of California, Irvine.

More Information

Corso, BL, I Perez, T Sheps, PC Sims, OT Gül, and PG Collins. 2014. “Electrochemical Charge-Transfer Resistance in Carbon Nanotube Composites.” Nano Letters 14(3):1329-1336. DOI: 10.1021/nl404349g

About the author(s):

  • Ralph L. House is a research associate specializing in the use of multiple spectroscopic techniques to analyze the steps leading to the generation of solar fuels. Ralph also leads the construction of an electrochemical bioreactor and is the Center for Solar Fuels (UNC Energy Frontier Research Center) liaison for external outreach and collaboration.

Honey, I Shrunk the Electrode

Tiny metal-lined carbon tubes speed electrons and overturn basic scientific assumption

By revealing the rules that matter, combinations can now be designed and optimized to make better batteries and more efficient solar cells, taking full advantage of the carbon nanotube additives.

In designing batteries, scientists often seek materials that don't impede electron flow. They balance resistance with other desirable properties. Composites made from carbon nanotubes, hollow fibers smaller than a red blood cell and lined with reactive molecules, have potential, but the composite's resistivity was troubling. Scientists precisely measured the resistivity of single-walled carbon nanotubes lined with manganese dioxide. The surface resistivity was less than assumed, an astounding 50 times less than platinum films used in popular energy storage devices. The nanotubes create a percolative network through the large volume of material and maintain a disproportionately small surface resistivity. The network helps explain the reduced resistivity, topples earlier assumptions, and could guide battery, solar cell, and new material design. Researchers at Nanostructures for Electrical Energy Storage, led by the University of Maryland, did the work.

More Information

Corso, BL, I Perez, T Sheps, PC Sims, OT Gül, and PG Collins. 2014. “Electrochemical Charge-Transfer Resistance in Carbon Nanotube Composites.” Nano Letters 14(3):1329-1336. DOI: 10.1021/nl404349g

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.