Novel Nanotube Applications for Power Generation

As it does for sectors such as global defense and transportation, nanotube technology holds great promise for the energy sector—and, in particular, for power generation. This July, researchers from the Massachusetts Institute of Technology (MIT) said carbon nanotubes showed potential as an innovative approach to storing solar energy, and Rice University scientists claimed they were closer to developing a unique wire that could transmit power with few losses.

As compared with converting the sun’s heat into electricity or storing that heat in a heavily insulated container, MIT researchers Jeffrey Grossman, postdoc Alexie Kolpak, and their coauthors say that storing the heat in chemical form has significant advantages—mainly because the chemical material can be stored for long periods without losing any stored energy. One problem associated with this approach is that the chemicals needed to perform this conversion and storage either degraded within a few cycles or included the rare and expensive element ruthenium.

But last year, the scientists used fulvalene diruthenium—a new material they made using carbon nanotubes, tiny tubular structures of pure carbon combined with a compound called azobenzene—to store solar energy. This July, they said in an article published in Nano Letters that the new chemical system is cost-effective and highly efficient at storing energy in a given amount of space—about 10,000 times higher in volumetric energy density, Kolpak says—making its energy density comparable to that of lithium-ion batteries. By using nanofabrication methods, “you can control [the molecules’] interactions, increasing the amount of energy they can store and the length of time for which they can store it — and most importantly, you can control both independently,” she says.

Thermo-chemical storage of solar energy uses a molecule whose structure changes when exposed to sunlight and that can remain stable in that form indefinitely. Then, when nudged by a stimulus—a catalyst, a small temperature change, a flash of light—it can quickly snap back to its other form, releasing its stored energy in a burst of heat. In the journal article “Azobenzene—Functionalized Carbon Nanotubes as High-Energy Density Solar Thermal Fuels,” Grossman describes it as creating a rechargeable heat battery with a long shelf life, like a conventional battery. The system also simplifies the process by combining energy harvesting and storage in a single step—though it would require the added step of using a thermoelectric device to generate electricity.

The researchers are now working to optimize the energy barrier separating the two stable states the molecule can adopt: Too low a barrier, and the molecule would return too easily to its “uncharged” state, failing to store energy for long periods; if the barrier were too high, it would not be able to easily release its energy when needed. “The barrier has to be optimized,” Grossman says.

At Rice University in Houston, Texas, meanwhile, scientists say they have achieved a pivotal breakthrough that involves development of armchair quantum wire—a weave of metallic nanotubes that promises to carry electricity with negligible loss over long distances. In another article in Nano Letters, Rice chemist Andrew R. Barron says the wire could ideally replace the nation’s copper-based grid, which loses electricity at an estimated 5% per 100 miles of transmission.

The prime hurdle for the so-called “miracle cable” is the manufacture of massive amounts of metallic single-walled carbon nanotubes. Dubbed armchairs for their unique shape, the nanotubes can’t yet be made alone; they grow in batches with other kinds of nanotubes and must be separated out—a difficult process given that a human hair is 50,000 times larger than a single nanotube (Figure 7).

7. Nanocable. Rice University researchers have developed a carbon nanotube wire that promises to transmit electricity with negligible losses. These images show a single carbon nanotube before and after amplification. Courtesy: Barron Lab/Rice University

Rice researchers demonstrated that “you could do this—but in the first demonstration, we got only one tube to grow out of hundreds or thousands,” Barron said. Subsequent experiments raised the yield, but tube growth was minimal. In other attempts, the catalyst would literally eat—or “etch”—the nanotubes, he said. The process has taken years, but the payoff is clear because up to 90% of the nanotubes in a batch can be amplified to significant lengths, he added. While initial growth took place at 1,000C/1,832F, the researchers found the amplification step required lowering the temperature by 200 degrees, in addition to adjusting the chemistry to maximize the yield.

Barron’s team continues to fine-tune the process and hopes that by summer’s end they can begin amplifying armchair nanotubes with the goal of making large quantities of pure metallics. Their work was inspired by late Rice professor, nanotech pioneer, and Nobel laureate Richard Smalley and conducted in collaboration with Rice chemist James Tour. “Increasing the Efficiency of Single Walled Carbon Nanotube Amplification by Fe–Co Catalysts Through the Optimization of CH4/H2 Partial Pressures,” appeared in the July Nano Letters.

—Sonal Patel is POWER’s senior writer.