Lawrence Livermore team shows carbon nanotube porins are fastest known proton conductors; potential application for PEM fuel cells
Lawrence Livermore National Laboratory (LLNL) researchers have shown that 0.8-nm-diameter carbon nanotube porins, which promote the formation of one-dimensional water wires, can support proton transport rates exceeding those of bulk water by an order of magnitude.
The transport rates in these nanotube pores also exceed those of biological channels and Nafion—one of the most common and commercially available membranes for proton exchange membrane (PEM fuel cells). Carbon nanotubes are the fastest known proton conductor. The research appears in the journal Nature Nanotechnology. Practical applications include proton exchange membranes (PEMs); proton-based signaling in biological systems; and the emerging field of proton bioelectronics (protonics).
|a, Schematics showing CNTPs inserted into a liposome. b,c, Molecular models showing water molecules in the inner pores of two CNTPs of different diameters: 1.5 nm (b), and 0.8 nm (c). Tunuguntla et al. Click to enlarge.
A nanometer is one billionth of a meter. By comparison, the diameter of a human hair is 20,000 nanometers.
The cool thing about our results is that we found that when you squeeze water into the nanotube, protons move through that water even faster than through normal (bulk) water.—Aleksandr Noy, an LLNL biophysicist and a lead author of the paper
(Bulk water is similar to what you would find in a cup of water that is much bigger than the size of a single water molecule).
The idea that protons travel fast in solutions by hopping along chains of hydrogen-bonded water molecules dates back 200 years to the work of Theodore von Grotthuss and still remains the foundation of the scientific understanding of proton transport. In the new research, LLNL researchers used carbon nanotube pores to line up water molecules into perfect one-dimensional chains and showed that they allow proton transport rates to approach the ultimate limits for the Grotthuss transport mechanism.
The possibility to achieve fast proton transport by changing the degree of water confinement is exciting. So far, the man-made proton conductors, such as polymer Nafion, use a different principle to enhance the proton transport. We have mimicked the way biological systems enhance the proton transport, took it to the extreme, and now our system realizes the ultimate limit of proton conductivity in a nanopore.—Aleksandr Noy
Of all man-made materials, the narrow hydrophobic inner pores of carbon nanotubes (CNT) hold the most promise to deliver the level of confinement and weak interactions with water molecules that facilitate the formation of one-dimensional hydrogen-bonded water chains that enhance proton transport.
Earlier molecular dynamic simulations showed that water in 0.8-nm diameter carbon nanotubes would create such water wires and predicted that these channels would exhibit proton transport rates that would be much faster than those of bulk water.
Ramya Tunuguntla, an LLNL postdoctoral researcher and the first author on the paper, said that despite significant efforts in carbon nanotube transport studies, these predictions proved to be hard to validate, mainly because of the difficulties in creating sub-1-nm diameter CNT pores.
However, the Lawrence Livermore team along with colleagues from the Lawrence Berkeley National Lab and UC Berkeley was able to create a simple and versatile experimental system for studying transport in ultra-narrow CNT pores.
They used carbon nanotube porins (CNTPs), a technology they developed earlier at LLNL, which uses carbon nanotubes embedded in the lipid membrane to mimic biological ion channel functionality. The key breakthrough was the creation of nanotube porins with a diameter of less than 1 nm, which allowed researchers for the first time to achieve true one-dimensional water confinement.
Other Livermore and Berkeley researchers include Frances Allen, Kyunghoon Kim and Allison Belliveau. The work was funded by the Department of Energy’s Office of Basic Energy Sciences.
Ramya H. Tunuguntla, Frances I. Allen, Kyunghoon Kim, Allison Belliveau & Aleksandr Noy (2016) “Ultrafast proton transport in sub-1-nm diameter carbon nanotube porins” Nature Nanotechnology doi: 10.1038/nnano.2016.43