Graphene and carbon nanotubes have a combination of excellent electrical properties and light weight that may eventually revolutionize electronics and energy storage technologies. But for now, most of their applications remain stuck in research labs, as producing them in bulk and then incorporating them into a device have both proven to be challenging. Now, some researchers at Stanford may have overcome the latter hurdle: they've managed to create a carbon nanotube ink that can easily be printed onto commercially available paper, which can then be directly incorporated into batteries and capacitors.

This isn't the first time that batteries based on a paper-nanotube combination have been demonstrated. But the earlier work involved a complex production process that required layering the nanotubes on a solid substrate and then forming paper around them. The new work dispenses with most of that complexity.

The first step in the process involves dissolving a mixture of carbon nanotubes in an aqueous solution; for that, the authors relied on sodium dodecylbenzenesulfonate, a close cousin of the detergent SDS, which is both commonly used and inexpensive. Typically, solutions such as this are applied to metallic or polymer substrates, after which elaborate means are necessary to remove the solvent and detergent; these often end up damaging the nanotube sheets. For this research, the authors simply applied the solution directly to some commercial Xerox paper.

The paper has a number of advantages aside from being cheap and easy to obtain. The primary one is that it can wick away the detergent solution that was used to carry the carbon nanotubes, along with any washes that are used to ensure that the detergent is eliminated. As a consequence, the resulting paper-nanotube sheets are evenly coated and stay intact through multiple washes—something that's not true for nanotube sheets deposited on a polymer.

The nanotube sheets also remained in contact with the paper when it was rolled up, and survived what the authors termed the "Scotch tape test"—sticking a piece of tape to the surface and peeling it off didn't damage anything. The authors were even able to pattern the nanotube layer simply by applying it with a paintbrush (one image in the paper suggests that the research team included someone adept at Chinese character calligraphy). The paper, once coated, also survived months in solution without apparent degradation.

How did the resulting material perform? The authors obtained resistance values of about 10?/square (a measure of resistance for square surfaces that's independent of units of distance). By contrast, a surface prepared with metallic nanofibers—silver, in this case—had a value of about 1?/square. So, it's fairly conductive, but not quite as good as typical conductors.

But the authors were able to demonstrate that the nanotubes have a number of distinct advantages. For one, we already know that they make excellent substrates for the production of supercapacitors. The authors showed that their material outperformed anything currently on the market when it comes to energy and power density. The long-term stability was also excellent, as they ran one device through 40,000 charge cycles with only a 0.6 percent loss of capacitance.

They also used the paper-nanotube combination as a charge collector for a lithium battery (the portion of the device that moves charges from the lithium-based storage materials to the actual electrodes). Again, the material had excellent performance and stability; more importantly, they weighed far less than the materials typically used for this purpose.

It's a little tough doing direct comparisons between these test devices and materials already on the market. For example, the authors provide a value for their capacitor in watt-hours/kg that's comparable to the value for lead-acid batteries, which would be huge. But the figure only applies to the nanotube ingredient, rather than the entire battery, so it seems like an unfair comparison.

Still, the nanotube-paper combination has a number of significant advantages: it's lightweight, simple to make, will get cheaper as we get better at making nanotubes, and it provides decent performance in energy storage devices. It's also possible that a paper that's a bit more specialized than the stock they kept for the lab printer would boost some of these properties further. Even if there are other materials that outperform it on some measures, charge storage applications require a balancing of price, weight, performance, charge density, and durability. It's possible that nanotube-coated paper will be in the sweet spot for some applications.

PNAS, 2009. DOI: 10.1073/pnas.0908858106