Here’s a new idea for a nontoxic battery: light fuel-coated carbon nanotubes on fire (like a fuse) to generate electricity.

Sounds crazy but it works, according to inventor Michael Strano, the Carbon P. Dubbs Professor in Chemical Engineering at MIT. Plus it avoids toxic materials such as lithium, which can be difficult to dispose of and that have limited global supplies),

The new approach is based on a discovery announced in 2010 by Strano and his co-workers: A wire made from carbon nanotubes can produce an electrical current when it is progressively heated from one end to the other — for example, by coating it with a combustible material and then lighting one end to let it burn like a fuse.

Basically, the effect arises as a pulse of heat pushes electrons through the bundle of carbon nanotubes, carrying the electrons with it like a bunch of surfers riding a wave.

Experiments at the time produced only a minuscule amount of current in a simple laboratory setup. But now, Strano and his team have increased the efficiency of the process more than a thousandfold and have produced devices that can put out power that is, pound for pound, in the same ballpark as what can be produced by today’s best batteries. The researchers caution, however, that it could take several years to develop the concept into a commercializable product.

The new results were published in the journal Energy & Environmental Science, in a paper by Strano, doctoral students Sayalee Mahajan PhD ’15 and Albert Liu, and five others.

MPC-MIT | Experimenting With Thermopower Waves

Virtually indefinite shelf life

The improvements in efficiency, he says, “brings [the technology] from a laboratory curiosity to being within striking distance of other portable energy technologies,” such as lithium-ion batteries or fuel cells. In their latest version, the device is more than 1 percent efficient in converting heat energy to electrical energy, the team reports, which is about 10,000 times greater than that reported in the original discovery paper.

“It took lithium-ion technology 25 years to get where they are” in terms of efficiency, Strano points out, whereas this technology has had only about a fifth of that development time. And lithium is extremely flammable if the material ever gets exposed to the open air — unlike the fuel used in the new device, which is much safer and also a renewable resource.

Already, the device is powerful enough to show that it can power simple electronic devices such as an LED light. And unlike batteries that can gradually lose power if they are stored for long periods, the new system should have a virtually indefinite shelf life, Liu says. That could make it suitable for uses such as a deep-space probe that remains dormant for many years as it travels to a distant planet and then needs a quick burst of power to send back data when it reaches its destination.

In addition, the new system is very scalable for use wearable devices. Batteries and fuel cells have limitations that make it difficult to shrink them to tiny sizes, Mahajan says, whereas this system “can scale down to very small limits. The scale of this is unique.”

This work is “an important demonstration of increasing the energy and lifetime of thermopower wave-based systems,” says Kourosh Kalantar-Zadeh, a professor of electrical and computer engineering at RMIT University in Australia, who was not involved in this research. “I believe that we are still far from the upper limit that the thermopower wave devices can potentially reach,” he says. “However, this step makes the technology more attractive for real applications.”

He adds that with this technology, “We can obtain phenomenal bursts of power, which is not possible from batteries. For instance, the thermopower wave systems can be used for powering long-distance transmission units in micro- and nano-telecommunication hubs.”

The work was supported by the Air Force Office of Scientific Research and the Office of Naval Research.

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Abstract of Sustainable power sources based on high efficiency thermopower wave devices

There is a pressing need to find alternatives to conventional batteries such as Li-ion, which contain toxic metals, present recycling difficulties due to harmful inorganic components, and rely on elements in finite global supply. Thermopower wave (TPW) devices, which convert chemical to electrical energy by means of self-propagating reaction waves guided along nanostructured thermal conduits, have the potential to address this demand. Herein, we demonstrate orders of magnitude higher chemical-to-electrical conversion efficiency of thermopower wave devices, in excess of 1%, with sustainable fuels such as sucrose and NaN₃ for the first time, that produce energy densities on par with Li-ion batteries operating at 80% efficiency (0.2 MJ L⁻¹ versus 0.8 MJ L⁻¹). We show that efficiency can be increased significantly by selecting fuels such as sodium azide or sucrose with potassium nitrate to offset the inherent penalty in chemical potential imposed by strongly p-doping fuels, a validation of the predictions of Excess Thermopower theory. Such TPW devices can be scaled to lengths greater than 10 cm and durations longer than 10 s, an over 5-fold improvement over the highest reported values, and they are capable of powering a commercial LED device. Lastly, a mathematical model of wave propagation, coupling thermal and electron transport with energy losses, is presented to describe the dynamics of power generation, explaining why both unipolar and bipolar waveforms can be observed. These results represent a significant advancement toward realizing TPW devices as new portable, high power density energy sources that are metal-free.