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Depiction of the essential functioning of the lithium-air battery. Ions of Li combine with oxygen from the air to form particles of Li oxides, which attach themselves to carbon fibers on the electrode as the battery is being used. During recharging, the Li oxides separate again into Li and oxygen and the process can begin again. Credit: Mitchell, Gallant and Shao-Horn; MIT.

An elusive piece of the alternative energy puzzle has been storage: How does one save it for when it’s needed or carry it around to use where it’s needed? In both cases, energy density is the key parameter and in the latter case, weight matters, too.

We’ve been following MIT associate professor Yang Shao-Horn and her work on lithium-air batteries in posts about alternative catalyst materials and carbon nanotube electrodes. Her group appears to have taken a leap forward in increasing the energy density using aligned carbon nanofiber electrodes that can store four times as much energy on a weight basis than current-technology Li-ion battery electrodes.

The lightweight advantage of lithium-air batteries (or any metal-air battery) comes from replacing a solid electrode like those in typical Li-ion batteries with a porous carbon electrode. Energy is stored when Li ions react with air flowing through the porosity to form Li oxides. The more porous the carbon, the more efficiently Li oxides are stored.

As the battery is used, particles of lithium peroxide form as small dots on the sides of carbon nanofibers (top), and become larger toroidal shapes as the battery discharges (bottom), as seen in these SEM images. Credit: Mitchell, Gallant, and Shao-Horn; MIT

A press release reports that Shao-Horn’s group used chemical vapor deposition to fabricate an electrode of vertically aligned arrays of carbon nanofibers with 90 percent void space, a big increase over the 70 percent void space the group reported achieving last year.

“We were able to create a novel carpet-like material-composed of more than 90 percent void space-that can be filled by the reactive material during battery operation,” Shao-Horn says in the press release. That means, according to Robert Mitchell, a graduate student and paper’s first author, that “the carpet-like arrays provide a highly conductive, low-density scaffold for energy storage.”

The gravimetric energy, which is the amount of power that can be stored for a given weight, for these very-low-density electrodes is one of the highest reported to date and demonstrates that “tuning the carbon structure is a promising route for increasing the energy density of lithium-air batteries,” said another graduate student and coauthor, Betar Gallant.

An unexpected finding is that the orderly “carpet” structure of the fibers makes them relatively easy to observe in a scanning electron microscope, and the performance of the electrodes can be monitored at intermediate states of charge. Being able to directly observe the process may shed some light on other vexing issues, such as the degradation observed after many charge–discharge cycles.

These latest results will be published in the August issue of the journal Energy and Environmental Science (see “All-carbon-nanofiber electrodes for high-energy rechargeable Li-O₂ batteries,” doi: 10.1039/C1EE01496J).

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Zinc oxide nanostructures are synthesized in parallel microfluidic channels (held by the metal frame) by flowing reactants through the tubing. The microfluidic structure creates the device and also becomes the final packaged functional LED device.
Photo: Jaebum Joo; MITnews

Just when I thought I could stroll back into the macro world, two new papers were published that complement our recent posts on PCMMs, supercapacitors, nanoporous materials and other nanostructured materials.

First, out of MIT comes “Face-selective electrostatic control of hydrothermal zinc oxide nanowire synthesis,” by Jaebum Joo, et. al. (see Nature Materials, doi:10.1038/nmat3069). Using hydrothermal synthesis, the group grew zinc oxide nanowires with controlled morphologies and functional properties. Morphologies synthesized ranged from platelets to needles with aspect ratios that spanned three orders of magnitude (~0.1-100 are reported).

The article abstract says a classical thermodynamic model was used to explain the growth inhibition mechanism “by means of the competitive and face-selective electrostatic adsorption on non-zinc complex ions at alkaline conditions.” An online story from MITnews clarifies that “the key turns out to be the electrostatic properties of the zinc oxide material as it grows from a solution.” When ions from other compounds are added to the solution (from which the ZnO is grown hydrothermally), they attach electrostatically and preferentially to the wire at only the sides or the ends, which inhibits growth in those directions (i.e., face-selective). The hydrothermal synthesis process temperature was less than 60°C, which opens up the possibility of manufacturing devices on or in polymers and plastics.

The team fabricated a functional LED array of ZnO nanowires, but ZnO also can be used in battery, sensor and other optical applications. In the MIT story Joo says this method and the ability to use it to control morpholgy could be applied to other materials, for example, titanium dioxide, a possible solar cell material. Joo also says the successful use of hydrothermal synthesis to manipulate nanostructure has “the potential for large-scale manufacturing.” (Perhaps a candidate technology for Obama’s recently announced Advanced Manufacturing Partnership?)

The second paper, published in the same issue of Nature Materials, is from a group at Samsung Electronics in Korea. In an interesting departure from the more common PCMM chalcogenide approach, they looked at tantalum oxide-based bilayer structures for nonvolatile memory devices. (See “A fast, high-endurance and scalable non-volatile memory device made from asymmetric Ta₂O_5-x/TaO_2-x bilayer structures,” Nature Materials, doi:10.1038/nmat3070, by Myoung-Jae Lee, et. al.)

(Quick note: Based on the abstract and online images, this appears to be a material property- and performance-oriented paper, and it is not known whether the material is nanostructured.)

Like others researching nonvolatile memory materials, they are looking for “a material or device structure that satisfies high-density, switching-speed, endurance, retention and most importantly power — consumption criteria” (from the abstract). The paper describes an asymmetric passive switching device with an impressive cycling endurance of over 10¹² and switching times of 10 ns. They were able to demonstrate a significant reduction of switching current and, therefore, power consumption.

The paper’s abstract postulates that there may be another benefit: “[B]y combining two such devices, each with an intrinsic Schottky barrier, we eliminate any need for a discrete transistor or diode in solving issues of stray leakage current paths in high-density crossbar arrays.”

Samsung’s published interest in nonvolatile memory materials combined with IBM’s recent proof-of-concept chalcogenide device seems to comprise pretty strong evidence that the leap from lab to prototype is underway, and Si-based flash memory may soon be just that, a memory.

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President Barack Obama announces Advanced Manufacturing Initiative at Carnegie Mellon University. Credit: Pete Souza; Official White House Photo.

Last week President Obama unveiled a new initiative to invest in emerging technologies and create new manufacturing jobs and increase the nation’s global competitiveness. During a visit to Carnegie Mellon University in Pittsburgh, Pa., Obama introduced the Advanced Manufacturing Partnership, which, according to the White House press release, will invest more than $500 million to leverage existing programs and proposals to meet these goals.

The press release said that AMP’s initial investments will target manufacturing for critical national security industries, advanced materials development, robotics, improving energy efficiency of manufacturing processes and accelerating the product development timeline for manufactured goods.

“Today, I’m calling for all of us to come together- private sector industry, universities, and the government- to spark a renaissance in American manufacturing and help our manufacturers develop the cutting-edge tools they need to compete with anyone in the world,” said Obama in the press release. “With these key investments, we can ensure that the United States remains a nation that ‘invents it here and manufactures it here’ and creates high-quality, good paying jobs for American workers.”

AMP is a response to the first of four recommendations made by the President’s Council of Advisors on Science and Technology in their just-released report, “Ensuring Leadership in Advanced Manufacturing (pdf).” The report cites an erosion of domestic leadership in manufacturing and the heavy investment of other nations to fill that void, the advantages of having R&D and manufacturing located in the United States, the essential role of an advanced manufacturing competence in national security and that, historically, federal investment in new technologies has cleared the way for fledglings to become major new industries.

The PCAST report concludes that individual companies cannot go it alone: “Private investment must be complemented by public investment to overcome market failures. Key opportunities include investing in the advancement of new technologies with transformative potential, supporting shared infrastructure, and accelerating the manufacturing process through targeted support for new methods and approaches.”

To create an environment conducive to innovation and to overcome market failures, the PCAST report recommended a four-point plan:

• Launch an advanced manufacturing initiative;
• Improve tax policy;
• Support research; and
• Strengthen the workforce.

AMP is the administration’s response to the first of these, and as recommended by PCAST, is a government, industry and academic partnership. It will be led by Andrew Liveris, CEO of Dow Chemical and Susan Hockfield, president of MIT, and will work closely with the White House’s National Economic Council, Office of Science and Technology Policy, as well as with PCAST.

The first team has been picked already. From industry it will be Allegheny Technologies, Caterpillar, Corning, Dow Chemical, Ford, Honeywell, Intel, Johnson & Johnson, Northrop Grumman, Proctor & Gamble and Stryker. Participating universities are MIT, Carnegie Mellon, Georgia Tech, Stanford, UC-Berkeley and University of Michigan. Government players are DARPA, DOE, DOD, and the Commerce Department.

The White House press release gives examples of how several partnerships that are in place will modify their programs to support AMP goals. Several of the named agencies have a long history as important, strategic investors in materials science and engineering such as NSF, NASA and NIST. For example, NIST, a Commerce Department agency, issued a press release outlining its programs that will support the AMP initiative including robotics, nanomanufacturing, advanced materials design through the Materials Genome Initiative and an advanced manufacturing technology consortium scheduled for launch in FY2010.

The PCAST report recommended that AMP funding should rise from $500 million to $1 billion over the course of four years. While touring Carnegie Mellon and seeing demonstrations of several cutting-edge technologies developed at the university, Obama said that it was important for ideas to have a place to incubate and become products that can be made in the US and sold worldwide. “And that’s in our blood. That’s who we are. We are inventors, and we are makers, and we are doers.”

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Today’s online MIT News reports on work being done by associate professor Yang Shao-Horn on a simple way to identify materials capable of being effective catalysts for oxygen reduction, which is the key electrochemical reaction in solid oxide fuel cells and in metal–air batteries. This new work could change the costly trial-and-error approach to catalyst development.

Palladium and platinum are known to have good catalytic properties, however, like all noble metals, they are pricey and scarce. If alternative and renewable energy technologies are going to become affordable, commodity-scale contributors to the energy portfolio, the materials used will need to be commodities, too.

Shao-Horn’s work builds on earlier (and ongoing) work by Jens Nørskov to understand the relationship between the electronic structure of the catalyst material and its efficacy as a catalyst. A good catalyst must bond with oxygen just enough - neither too much nor too little. Nørskov’s team was able to correlate the average energy of a catalyst’s outermost electron to the binding energy of oxygen to metal surfaces. That is, by looking only at the distribution of electrons in the orbitals that bond metal to oxygen, it is possible to predict which metal-oxide systems are likely to be effective catalysts.

Shao-Horn says in the story, the Nørskov work provides “…a theoretical framework and experimental evidence that explains why” some catalysts work better than others. Shao-Horn’s work, published June 13 in Nature Chemistry, evaluates perovskite oxide catalysts with transition metal B-site ions. (See “Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal-air batteries,” doi:10.1038/nchem.1069)

In the paper, Shao-Horn’s team propose a mechanism for the oxygen reduction reaction, where hydroxide ions are displaced by peroxide ions, followed by the formation of a surface oxide, and finally, regeneration of surface hydroxide. The study was able to show that the bonding orbital (here the sigma-star orbital) and the strength of the transition metal-oxygen covalent bond influence competition between the steps in the ORR. Transition metals investigated in the study include Cr, Mn, Fe, Co, Ni and mixed compounds.

Comparing the electronic configuration of the B-site transition metal ions and their catalytic activity yields a volcano-shaped plot with a sharp peak of high performance and steeply sloped sides signifying poor catalytic performance. The range of performance between the “base” of the volcano-plot and the peak can be a factor of at least 10,000.

This work falls more or less into the category of integrated computational materials design, whereby materials development is accelerated by understanding how structure defines properties, and through processing, the design of formulations and applications. Research advances like this one will be critical to addressing the materials problems in this century’s Energy Race.

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A pink object under the cloak becomes invisible. Credit: SMART Center.

Last week I mentioned that someone has found a new, cheap way to optically “hide” objects (other than the metamaterials route). Via Gizmag, I heard about the the work of two groups who, in parallel, are using calcite crystals to make objects seem to disappear — and its one of those things where you immediately say, “Why didn’t I think of that?”

Calcite, boron nitride, silicon carbide and other crystals (and some plastics) are known for having birefringence (aka, double refraction). Briefly put, birefringence causes a ray of light to split into two rays. The property is already used in LCDs and other optical and electronics applications.

What’s novel is the two groups — one from the SMART Center and the other collaboration between researchers at University of Birmingham (U.K.), Imperial College, London and Technical University of Denmark — is to put two prism-shaped pieces of calcite next to each other, aligning their optical axes. If the resultant wedge (the SMART group used a 38mm X 10mm X 2mm wedge) is then put over an object, it appears to disappear when viewed from either side of the wedge, and the two-dimensional effect works for macroscopic objects “larger than 3500 free-space wavelengths, inside a transparent liquid environment.  Its working color range encompassing red, green and blue light has also been demonstrated.”

The U.K./Denmark collaborative group also were able to achieve the effect in air. One of these researchers, Shuang Zhang, lead investigator from the University of Birmingham’s School of Physics and Astronomy, predicted bigger things ahead. He says:

“By using natural crystals for the first time, rather than artificial metamaterials, we have been able to scale up the size of the cloak and can hide larger objects, thousands of times bigger than the wavelength of the light. Previous cloaks have succeeded at the micron level (much smaller than the thickness of a human hair) using a nano- or micro-fabricated artificial composite material. It is a very slow process to make these structures and they also restrict the size of the cloaking area. We believe that by using calcite, we can start to develop a cloak of significant size that will open avenues for future applications of cloaking devices.”

The SMART groups work is published in Physical Review Letters, and the U.K./Denmark group’s work is published in Nature Communications.

Physics World was particularly impressed with the SMART Center’s work, naming it one of the “Top 10 breakthroughs of 2010.”

Here is a video from featuring Zhang showing how the calcite crystal can make part of a panda disappear:

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