Power/Performance Bits: May 15

Aluminum batteries; shrinking optical synthesizers; assessing solar tech.


Aluminum battery materials
Scientists from ETH Zurich and Empa identified two new materials that could boost the development of aluminum batteries, a potential low cost, materially abundant option for temporary storage of renewable energy.

The first is a corrosion-resistant material for the conductive parts of the battery; the second is a novel material for the battery’s positive pole that can be adapted to a wide range of technical requirements.

The first is a corrosion-resistant material for the conductive parts of the battery. The electrolyte fluid in aluminum batteries is extremely aggressive and corrodes stainless steel, and even gold and platinum. The team turned to titanium nitride, a ceramic material that exhibits sufficiently high conductivity. Titanium and nitrogen are both abundant elements, and the material is easy to manufacture.

The team successfully made aluminum batteries with conductive parts made of titanium nitride in the laboratory. The material can easily be produced in the form of thin films, also as a coating over other materials such as polymer foils.

Maksym Kovalenko, Professor of Functional Inorganic Materials at ETH Zurich, believes it would also be possible to manufacture the conductors from a conventional metal and coat them with titanium nitride, or even to print conductive titanium nitride tracks on to plastic. “The potential applications of titanium nitride are not limited to aluminum batteries. The material could also be used in other types of batteries; for example, in those based on magnesium or sodium, or in high-voltage lithium-ion batteries,” said Kovalenko.

Functional scheme of the aluminum batteries by the ETH Zurich and Empa researchers. (Source: Walter M et al. Advanced Materials 2018, edited)

The second material is one that could be used for the battery’s positive electrode, typically made of graphite in an aluminum battery. Instead, the team tried polypyrene, a hydrocarbon with a polymeric molecular structure, which rivals graphite in terms of the amount of energy a battery is able to store.

Samples of the material in which the molecular chains congregate in a disorderly manner were particularly effective, with the large amount of space between molecular chains allowing relatively large ions of the electrolyte fluid to penetrate and charge the electrode material easily.

One of the other advantages of electrodes containing polypyrene is that the scientists were able to influence its properties, such as the porosity. The material can therefore be adapted to the specific application.

Additionally, both titanium nitride and polypyrene are flexible materials, making such a battery suitable for use in pouch cells.

Shrinking optical synthesizers
Scientists from the University of California, Santa Barbara, working under a DARPA program, developed miniature, energy-efficient components for a chip-based optical frequency synthesizer.

Setting the absolute frequency of light emitted from a laser with precision is a difficult task, and while current optical frequency synthesizers are useful for a range of applications from chemical detection using sensitive laser spectroscopy to LIDAR, they are large, expensive, and power hungry.

“The development of optical frequency synthesis has significantly enhanced our ability to accurately and precisely measure time and space,” said Gordon Keeler, the DARPA program manager leading the Direct On-Chip Digital Optical Synthesizer (DODOS) program. “However, our ability to leverage the technology has been limited. Through DODOS, we’re creating technologies that will enable broader deployment and unlock numerous applications. The goal is to shrink laboratory-grade capabilities down to the size of a sugar cube for use in applications like LIDAR, coherent communications, chemical sensing and precision metrology.”

Combining a pair of frequency combs, several miniature lasers and other compact optoelectronic components, the researchers were able to replicate the capabilities of a tabletop optical frequency synthesizer on three microchips, each less than 5 mm x 10 mm in size.

Comparing a bulk comb generator with integrated comb generators. (Source: UC Santa Barbara)

“We took something that occupied a whole optical bench, weighed 50 pounds and used a kilowatt of power and made it orders of magnitude more efficient by integrating the key elements onto silicon photonic integrated circuits,” said John Bowers, a professor of electrical and computer engineering at UCSB and director of the campus’s Institute for Energy Efficiency.

The team developed electrical integrated circuits to control the synthesizer, which can tune over 50 nanometers and deliver a frequency stability of 7 x 10-13 after one second of averaging, matching that of the input reference clock.

The two miniaturized frequency combs were created by circulating laser light generated with single-color “pump” lasers around optical racetracks fabricated on silicon chips. Doing so correctly can produce many additional colors, yielding a spectrum that looks like a hair comb where each “tooth” is an individual color or frequency. This is a significant departure from the tabletop version of an optical frequency synthesizer, which uses fiber optics, specialized mirrors and large mechanical components built by hand to achieve a similar effect.

The DODOS program is entering its final phase, during which researchers will work to integrate the individual components with electronics and fabricate a compactly packaged device suitable for use in future military and commercial optical systems.

Which solar cell where?
Researchers at MIT delved into basic questions of solar panel installations to assess what the best-performing type of cells are for which conditions: off-the-shelf, low cost panels or new types that can deliver more efficiency but also cost more.

Ultimately, the team found that for household-scale rooftop systems in relatively dry locations, the more efficient but more costly panels would be better, but for grid-scale installations or for those in wetter climates, the established, less efficient but cheaper panels are better.

The study compared two basic varieties of solar cells: standard designs that use a single type of photovoltaic material, and tandem cell designs that combine two cell types to capture more of the energy in sunlight. For the tandem cells, both two-junction cells (where cells are wired in series) and four-junction cells (where each cell is separately wired) were tested.

The team factored in not just the efficiency of the cells, but also associated installation and operational costs over time in a measurement called the levelized cost of electricity (LCOE). Installations were deployed in three different environments, arid (Arizona), temperate (South Dakota), and humid (Florida), to account for how the amount of water vapor in the air affects solar cell operation.

“For residential systems, we showed that the four-terminal tandem system [the most efficient solar cell available] was the best option, regardless of location,” said Sarah Sofia, an MIT graduate student. It really shows the importance of having a high energy yield in a residential system.”

For utility-scale installations, the cell with the lowest production costs is the best deal, given that the costs of the installation and the control systems can be spread over many more panels, and because space tends to be less constrained in such installations.

The researchers see assessments like this as a tool to guide research priorities and compare other solar technologies such as thin-films. Additionally, because the materials they studied for the four-terminal case are already commercialized, the team says four-junction tandem systems for residential applications could potentially be brought to market quickly.

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