Power/Performance Bits: March 28

Storing solar energy as carbon monoxide

A team at Indiana University engineered a molecule that collects and stores solar energy without solar panels. The molecule uses light or electricity to convert the greenhouse gas carbon dioxide into carbon monoxide more efficiently than any other method of carbon reduction.

Burning fuel such as carbon monoxide produces carbon dioxide and releases energy. Turning carbon dioxide back into fuel requires at least the same amount of energy.

The team’s goal was to find a method that reduces the amount of energy required to drive the formation of carbon monoxide. The molecule, a nanographene-rhenium complex connected via the organic compound bipyridine, triggers a highly efficient reaction that converts carbon dioxide to carbon monoxide.

“Carbon monoxide is an important raw material in a lot of industrial processes,” said Liang-shi Li, associate professor in IU’s Department of Chemistry. “It’s also a way to store energy as a carbon-neutral fuel since you’re not putting any more carbon back into the atmosphere than you already removed. You’re simply re-releasing the solar power you used to make it.”

The new molecule employs a nanographene complex (on left) to absorb light and drive the conversion of carbon dioxide (upper center) to carbon monoxide (on right). (Source: Ben Noffke and Richard Schaugaard, Indiana University)

Essentially, Li said, the molecule acts as a two-part system: a nanographene “energy collector” that absorbs a large portion of the visible light spectrum and an atomic rhenium “engine” that produces carbon monoxide. The energy collector drives a flow of electrons to the rhenium atom, which repeatedly binds and converts the normally stable carbon dioxide to carbon monoxide.

The team plans to make the molecule more powerful, including making it last longer and survive in a non-liquid form, since solid catalysts are easier to use in the real world. They are also working to replace the rhenium atom in the molecule, a rare element, with manganese, a more common and less expensive metal.

Sugar-powered robots

Engineers at MIT, the Technical University of Denmark, and Cornell University designed a microfluidic device they call a “tree-on-a-chip,” which mimics the pumping mechanism of trees and plants. Like its natural counterparts, the chip operates passively, requiring no moving parts or external pumps. It is able to pump water and sugars through the chip at a steady flow rate for several days.

Plants are constantly pulling water up from their roots to the topmost leaves, and pumping sugars produced by their leaves back down to the roots. This constant stream of nutrients is shuttled through a system of tissues called xylem and phloem, which are packed together in woody, parallel conduits.

To make the chip, the researchers sandwiched together two plastic slides, through which they drilled small channels to represent xylem and phloem. They filled the xylem channel with water, and the phloem channel with water and sugar, then separated the two slides with a semipermeable material to mimic the membrane between xylem and phloem. They placed another membrane over the slide containing the phloem channel, and set a sugar cube on top to represent the additional source of sugar diffusing from a tree’s leaves into the phloem. They hooked the chip up to a tube, which fed water from a tank into the chip.

(Source: Anette “Peko” Hosoi, Jean Comtet, Kaare Jensen, Robert Turgeon, and Abraham Stroock)

With this setup, the chip was able to passively pump water from the tank through the chip and out into a beaker, at a constant flow rate for several days, as opposed to previous designs that only pumped for several minutes.

Anette “Peko” Hosoi, professor and associate department head for operations in MIT’s Department of Mechanical Engineering, said the chip’s passive pumping may be leveraged as a simple hydraulic actuator for small robots. Engineers have found it difficult and expensive to make tiny, movable parts and pumps to power complex movements in small robots. The team’s new pumping mechanism may enable robots whose motions are propelled by inexpensive, sugar-powered pumps.

“The goal of this work is cheap complexity, like one sees in nature,” said Hosoi. “It’s easy to add another leaf or xylem channel in a tree. In small robotics, everything is hard, from manufacturing, to integration, to actuation. If we could make the building blocks that enable cheap complexity, that would be super exciting. I think these [microfluidic pumps] are a step in that direction.”

According to Hosoi, “If you design your robot in a smart way, you could absolutely stick a sugar cube on it and let it go.”

Perovskite solar cell stability

Researchers from Aalto University, Uppsala University, and École Polytechnique Fédérale de Lausanne (EPFL) improved the long term stability of perovskite solar cells using “random network” nanotube films, films composed of single-walled carbon nanotubes.

Perovskite solar cells are promising for their ability to capture light efficiently and conduct well, but the lifetime of solar cells made of metalorganic perovskites has proven to be very short compared to cells made of silicon.

“In a traditional perovskite solar cell, the hole conductor layer consists of organic material and, on top of it, a thin layer of gold that easily starts to disintegrate and diffuse through the whole solar cell structure. We replaced the gold and also part of the organic material with films made of carbon nanotubes and achieved good cell stability in 60 degrees and full one sun illumination conditions,” explained Kerttu Aitola, a researcher at Uppsala University.

Cross-section of the solar cell in an electron microscope image. The fluff seen in the front of the image is composed of bundles of nanotubes that have become half-loose when the samples have been prepared for imaging. (Source: Aalto University / University of Uppsala / EPFL)

In the study, thick black films with conductivity as high as possible were used in the back contact of the solar cell where light does not need to get through. According to Aitola, nanotube films can also be made transparent and thin, which would make it possible to use them as the front contact of the cell, which lets light through. Additionally, nanotube films provide superior flexibility compared to the conductive plastic flexible solar cells are currently manufactured on.

The researchers conducted a 140 hour experiment, during which traditional perovskite cells with gold electrodes experienced a dramatic, irreversible efficiency loss, rendering them effectively nonoperational. The nanotube cells showed only a small linear efficiency loss, but with a lifetime the team extrapolated to 580 hours, there’s still a long ways to go to catch up to the 20-30 year lifetime of silicon solar cells.