Power/Performance Bits: Oct. 24

Molecular storage; cheap grid battery; paper-based supercapacitor.


Molecular storage
Chemists at the Institut Charles Sadron and Aix-Marseille University used mass spectrometry to read several bytes of data recorded on the molecular scale with synthetic polymers, setting a new benchmark for the amount of data stored as a sequence of molecular units (monomers) that can be read.

Polymers have great potential since, to record a bit, their component monomers require 100 times less space than current hard drives. Digitally encoded polymers can also be stored at room temperature. However, there has not been an effective way to sequence polymer data.

The team used synthetic molecules, which are simpler to work with than natural molecules like DNA. Their structure was optimized for sequencing by mass spectrometry. The polymers are made up of two kinds of monomers (with phosphate groups), corresponding to 0 and 1 respectively. After every eight of these monomer “bits,” a molecular separator was added. The number of bytes represented by the complete polymer equals the number of eight-bit groups. The first step in reading the encoded information is to divide the polymer into molecular bytes by snapping it apart at the separator sites; the next is to break the phosphate bonds, for sequencing of each byte.

MS2 spectrum of an 8-byte digital polymer containing the ASCII-encoded word Sequence. (Source: Abdelaziz Al Ouahabi et al., Nature Communications)

The team of chemists managed to synthesize polymers that can store up to eight bytes. Thus, they were able to record the word “Sequence” in ASCII code, as well as the four byte word “Byte.” By successfully decoding the words using mass spectrometry, they set a new record for the length of a molecule that may be read using this technique.

Although manual analysis of the digital data takes a few hours, the team says it should be possible to reduce the time needed to a few milliseconds by developing software to perform this task. By associating short read times with current automated methods for writing data, they hope this could lead to synthetic polymer storage of several kilobytes of data.

Ultimately, the team sees potential for polymer-based molecular memories that rely on libraries of coded chains, a practice already done in the field of DNA storage, where individual chains containing about 100 coded residues and a short localization address sequence are typically used to store large quantities of information.

Cheap grid battery
Researchers at MIT developed an “air-breathing” flow battery based on sulfur that could store electricity for very long durations for about one-fifth the cost of current technologies. The battery could be used to make sporadic renewable power a more reliable source of electricity for the grid.

The battery’s cost is about $20 to $30 per kilowatt hour. The most widely used materials in lithium-ion batteries have a cost of about $100 for each kilowatt hour of energy stored.

In a flow battery, electrolytes are continuously pumped through electrodes and travel through a reaction cell to create charge or discharge. The new flow battery consists of a liquid anode (anolyte) of cheap polysulfide that contains lithium or sodium ions, and a liquid cathode (catholyte) that consists of an oxygenated dissolved salt, separated by a membrane.

Upon discharging, the anolyte releases electrons into an external circuit and the lithium or sodium ions travel to the cathode. At the same time, to maintain electroneutrality, the catholyte draws in oxygen, creating negatively charged hydroxide ions. When charging, the process is simply reversed. Oxygen is expelled from the catholyte, increasing hydrogen ions, which donate electrons back to the anolyte through the external circuit.

“What this does is create a charge balance by taking oxygen in and out of the system,” said Yet-Ming Chiang, professor of materials science and engineering at MIT.

A photo and schematic of the “air-breathing” flow battery. Courtesy of the researchers. Left photo: Felice Frankel.

The prototype is currently about the size of a coffee cup. But flow batteries are highly scalable, and cells can be combined into larger systems.

As the battery can discharge over months, the best use may be for storing electricity from unpredictable wind and solar power sources. Chiang says this could be the first technology to compete, in cost and energy density, with pumped hydroelectric storage systems, which provide most of the energy storage for renewables around the world but are very restricted by location.

“The energy density of a flow battery like this is more than 500 times higher than pumped hydroelectric storage. It’s also so much more compact, so that you can imagine putting it anywhere you have renewable generation,” said Chiang.

Paper-based supercapacitor
Researchers at the University of Korea and the Georgia Institute of Technology developed a paper-based flexible supercapacitor with the best performance so far in a textile-based supercapacitor. The device uses metallic nanoparticles to coat cellulose fibers in the paper.

By implanting conductive and charge storage materials in the paper, the technique creates large surface areas that function as current collectors and nanoparticle reservoirs for the electrodes. Testing shows that devices fabricated with the technique can be folded thousands of times without affecting conductivity.

The researchers began by dipping paper samples into a beaker of solution containing an amine surfactant material designed to bind the gold nanoparticles to the paper. Next they dipped the paper into a solution containing gold nanoparticles. Because the fibers are porous, the surfactants and nanoparticles enter the fibers and become strongly attached, creating a conformal coating on each fiber.

By repeating the dipping steps, the researchers created a conductive paper on which they added alternating layers of metal oxide energy storage materials such as manganese oxide. The ligand-mediated layer-by-layer approach helped minimize the contact resistance between neighboring metal and/or metal oxide nanonparticles. Using the simple process done at room temperatures, the layers can be built up to provide the desired electrical properties.

Image shows that the flexible metallized paper developed in this research retains its conducting properties even when crumpled and folded. (Source: Ko et al., published in Nature Communications)

The researchers demonstrated that their self-assembly technique improves several aspects of the paper supercapacitor, including its areal performance, an important factor for measuring flexible energy-storage electrodes. The maximum power and energy density of the metallic paper-based supercapacitors are estimated to be 15.1 mW/cm2 and 267.3 uW/cm2, respectively, substantially outperforming conventional paper or textile supercapacitors.

Though the research involved small samples of paper, the solution-based technique could likely be scaled up using larger tanks or even a spray-on technique, the researchers say.

The next steps will include testing the technique on flexible fabrics, and developing flexible batteries that could work with the supercapacitors. The researchers used gold nanoparticles because they are easy to work with, but plan to test less expensive metals such as silver and copper to reduce the cost.

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