Fast and Furious Bucket Brigade

Confining water in tiny straws confirms predicted rapid transport of protons along a water “wire”—vital for more efficient fuel cells.

carbon nanotubes
Proton transport is faster in water if the water is confined in, for example, a sub-nanometer carbon nanotube straw (orange). The illustration shows one such straw embedded in a lipid membrane (blue). A very tight inner channel in the carbon nanotube squeezes water molecules (red (oxygen atoms) and gray (hydrogen atoms)) into a one-dimensional chain that allows for rapid proton transport along—an order of magnitude faster than in bulk water. Image courtesy of Lawrence Livermore National Laboratory

The Science

The movement of protonshydrogen atoms with the electrons striped away—through a membrane is critical to efficiently producing electricity in a fuel cell. A decade ago, theory predicted that confinement of water into a single one-dimensional chain would achieve rapid proton transport, with the protons hopping down the chain of hydrogen-bonded water molecules. Now tiny (less than a nanometer in diameter) carbon tubes that confine water into one-dimensional wires have validated this prediction of rapid proton transport and achieved rates an order of magnitude faster than protons in bulk water and faster than state-of-the-art fuel cell membranes.

The Impact

Carbon nanotube pores that create and maintain water “wires” could allow for rapid proton transport in next-generation bioinspired artificial proton conducting membranes for more efficient fuel cells to power your car and home.

Summary

Proton transport is important in biology and energy technology. Rapid proton transport is achieved through protons hopping along water molecules similar to a bucket brigade passing a pail of water down the line. Ten years ago, simulations predicted that water confinement in hydrophobic (“water-fearing”) carbon straws could achieve faster proton hopping, but demonstrating this efficient proton transport in artificial systems has been very challenging. Scientists at Lawrence Livermore National Laboratory and the University of California-Merced have now prepared short carbon nanotubes with two different diameters embedded in synthetic lipid membranes that mimic biological ion channels. When the diameter was greater than 1 nanometer, water was not confined to one dimension and, in this case, proton transport rates were comparable to bulk water. However, when much narrower (0.8-nanometer-diameter) tubes were used, the confined water formed one-dimensional water wires that drastically boosted the proton transport rate. To prepare the narrow tubes, single-walled carbon nanotubes were sonicated to cut and purify them. The smaller diameter nanotubes readily incorporated into a lipid membrane and formed passageways across the membrane—roughly 17 nanotubes per 200-nanometer liposome (a spherical vesicle encapsulating water with a lipid bilayer). Changes in the intensity of a fluorescent dye trapped in the lipid vesicle membrane were used to monitor proton transport that was initiated by rapid acidification (increasing the proton concentration) of the solution surrounding the liposome. The proton transport in the water “wires” inside narrow carbon nanotubes was an order of magnitude faster than bulk water and faster than the state-of-the-art membrane in fuel cells.