Power/Performance Bits: June 9

Building foam batteries out of trees

A method for making elastic high-capacity batteries from wood pulp was unveiled by researchers in Sweden and the US. Using nanocellulose broken down from tree fibers, a team from KTH Royal Institute of Technology and Stanford University produced an elastic, foam-like battery material that can withstand shock and stress.

“It is possible to make incredible materials from trees and cellulose,” says Max Hamedi, researcher at KTH and Harvard University. One benefit of the new wood-based aerogel material is that it can be used for three-dimensional structures.

A 3D structure enables storage of significantly more power in less space than is possible with conventional batteries, he says. “Three-dimensional, porous materials have been regarded as an obstacle to building electrodes. But we have proven that this is not a problem. In fact, this type of structure and material architecture allows flexibility and freedom in the design of batteries.”

A close-up of the battery developed at KTH’s Wallenberg Wood Science Center. (Source: Max Hamedi/Wallenberg Wood Science Center)

The process for creating the material begins with breaking down tree fibers, making them roughly one million times thinner. The nanocellulose is dissolved, frozen and then freeze-dried so that the moisture evaporates without passing through a liquid state. Then the material goes through a process in which the molecules are stabilized so that the material does not collapse.

The finished aerogel can then be treated with electronic properties. “We use a very precise technique, verging on the atomic level, which adds ink that conducts electricity within the aerogel. You can coat the entire surface within.”

In terms of surface area, Hamedi compares the material to a pair of human lungs, which if unfurled could be spread over a football field. Similarly, a single cubic decimeter of the battery material would cover most of a football pitch, he says.

“You can press it as much as you want. While flexible and stretchable electronics already exist, the insensitivity to shock and impact are somewhat new.”

Hamedi says the aerogel batteries could be used in electric car bodies, as well as in clothing, providing the garment has a lining.

Single-molecule diode

Columbia University researchers designed a new technique to create a single-molecule diode, and, in doing so, they have developed molecular diodes that perform 50 times better than all prior designs.

“Our new approach created a single-molecule diode that has a high (>250) rectification and a high “on” current (~0.1 micro Amps),” says Latha Venkataraman, associate professor of applied physics at Columbia Engineering. “Constructing a device where the active elements are only a single molecule has long been a tantalizing dream in nanoscience. This goal, which has been the ‘holy grail’ of molecular electronics ever since its inception with Aviram and Ratner’s 1974 seminal paper, represents the ultimate in functional miniaturization that can be achieved for an electronic device.”

Molecule used to create the first single-molecule diode. Diodes are fundamental building blocks of integrated circuits; they allow current to flow in only one direction. (Source: Latha Venkataraman/Columbia)

Brian Capozzi, PhD student and lead author of the paper, said “A well-designed diode should only allow current to flow in one direction—the ‘on’ direction—and it should allow a lot of current to flow in that direction. Asymmetric molecular designs have typically suffered from very low current flow in both ‘on’ and ‘off’ directions, and the ratio of current flow in the two has typically been low. Ideally, the ratio of ‘on’ current to ‘off’ current, the rectification ratio, should be very high.”

In order to overcome the issues associated with asymmetric molecular design, the team focused on developing an asymmetry in the environment around the molecular junction by surrounding the active molecule with an ionic solution and used gold metal electrodes of different sizes to contact the molecule.

Because this new technique is so easily implemented, it can be applied to all nanoscale devices of all types, including those that are made with graphene electrodes.