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Mesoporous titania-bronze microspheres show promise as lithium-ion battery anodes. Credit: M. Paranthaman; ORNL

Conceptually, lithium-ion batteries are simple devices comprising the anode, cathode and electrolyte. The anode’s job is to grab and stash lithium ions as fast as it can and to give them up speedily when current is drawn off through the cathode. The anode needs to be able to withstand repeated charge-discharge cycles, or to just hold onto the charge without leaking until it’s needed, like in a car battery.

Even though lithium ions are tiny, lots of them are stored, which can eventually swell the anode and lead to failure. Thus, the search for anodes with large-charge storage capacities and the ability to discharge quickly is a high priority.

Two papers recently published in Advanced Materials investigate new anode materials from very different materials categories and with completely different charge storage mechanisms.

First, out of Oak Ridge National Lab, a team led by Hansan Liu, Gilbert Brown and Parans Paranthaman investigated TiO₂-B (or TiO₂(B)) the nomenclature for so-called titania bronze, which is the monoclinic polymorph of titania. (The common polymorphs of anatase and rutile are tetragonal.)

The group synthesized micrometer-sized spherical particles with mesoporous morphology. The open network of channels and pores with sizes in the 10-15 nm range, allows lithium to intercalate in the TiO₂-B structure by a pseudocapacitve process, rather than solid state diffusion one. The mesoporous structure is evenly distributed on the surface as well as through the bulk of the particles, which means that the electrolyte has good contact with the anode. Also, the grains in the microspheres are nanosize, which allows for easy electronic transport along the grain boundaries.

The microsphere morphology is good for fabricating compact, uniform electrode layers because of the spheres’ high packing density and particle mobility. However, the microsphere synthesis process is complicated, so challenges remain to be resolved in scaling up the process.

Electrochemical tests showed that the TiO₂-B, at low current rates, displays a high discharge capacity: It’s a whopping 93 percent of theoretical capacity, compared to only about 70 percent for anatase nanopowders. The authors remark in the paper, “At high current rates, the difference of reversible capacity between the two materials is even more remarkable.” They report that during high rates of charge-discharge, the capacity of anatase nanopowders is determined by double layer capacitance. The pseudocapacitance behavior of TiO₂-B, however, allows the material to maintain large capacities at high charge-discharge rates.

In an ORNL press release, Liu says, “We can charge our battery to 50 percent of full capacity in six minutes while the traditional graphite-based lithium-ion battery would be just 10 percent charged at the same current.” This improved charging and discharging, according to the release, “combined with the fact oxide materials are extremely safe and long-lasting alternatives to commercial graphite make it well-suited for hybrid electric vehicles and other high-power applications.”

In a test of 5,000 charge-discharge cycles, TiO₂-B demonstrated a capacity loss of only 10 percent. According to the paper, “The superior cycling performance can be attributed to the structure stability of TiO₂-B polymorph and the good accommodation to volume/strain changes of mesoporous structure during lithium insertion-extraction.”

The paper proposes that a TiO₂-B microsphere anode coupled with a cathode capable of handling high charge rages, such as some LiFePO₄ materials, could provide the basis for a long lifetime, rechargeable battery for high power applications.

The paper is “Mesoporous TiO₂-B Microspheres with Superior Rate Performance for Lithium-Ion Batteries,” Liu et al., Advanced Materials (doi: 10:1002/adma20110599).

Composite anode (left) with silicon (blue spheres) in a polymer binder (light brown) and carbon particles to conduct electricity (dark brown). Silicon swells and shrinks while acquiring and releasing lithium ions, and eventually contacts break among the conducting carbon particles. Polyfluorene-base material (right, purple) is conductive and binds tightly to silicon particles despite repeated swelling and shrinking. Credit: LBNL

Coincidentally, a group at Lawrence Berkeley National Lab also published an article about a new material for lithium battery anodes.

This group studied a particulate composite composed of a polyfluorene-based conducting polymer matrix and silicon. They found that incorporating a carbonyl functional group in the PF improved the performance of the anode, increasing the electrical conductivity and assisting with electron and ion transport to the silicon particles. According to the press release, “the polymer is itself conductive and continues to bind tightly to the silicon particles despite repeated swelling and shrinking.”

The group also says that the polymer composite anodes are economical and “the manufacturing process is … compatible with established manufacturing technologies.”

See “Polymers with Tailored Electronic Structure for High-Capacity Lithium Battery Electrodes,” Liu et al., Advanced Materials (doi:10.1002/adma.201102421).

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TEM image of a Ti0₂ nanocrystal after hydrogenation reveals engineered disorder on the crystal’s surface, a change that enables the photocatalyst to absorb infrared light. Credit S. Mao et al; Science Express.

By tinkering with the outer layer of titanium oxide nanocrystals, researchers at Lawrence Berkeley National Lab have figured out a way to turn the material into a tough and more effective photocatalyst for environmental and energy applications. They claim this is the first time durability and efficiency have been combined in a photocatalyst.

Samuel Mao, an investigator with the Advanced Energy Technologies Department of the lab’s Environmental Energy Technologies Division, says they were trying to improve hydrogen production from organic materials in water when they had the idea to introduce disorder in nanophase TiO₂ and hopefully expand its light-absorption ability.

The groups work (doi:10.1126/science.1200448), “Increasing solar absorption for photocatalysis with black, hydrogenated titanium dioxide nanocrystals” is published in Science Express, and may offer a path for generating hydrogen from organic compounds found in natural and polluted water sources.

Mao shows how the disorder-engineered titanium dioxide nanocrystals turned the material from white to black. Credit: Roy Kaltschmidt, Berkeley Lab Public Affairs

Mao’s group used hydrogenation to engineer disorder in the TiO₂. They had a hint the nanocrystals might be effective over a wider spectrum of light when they saw that the material had turned from white to black post hydrogenation.

After 22 days of lab test using a full-spectrum solar light simulator with methanol serving as a sacrificial reagent, they report that, ”We found that one hour of solar irradiation generated 0.2 0.02 mmol of H₂ using 0.02 g of disorder-engineered black TiO₂ nanocrystals (10 mmol hour^–1 g^–1 of photocatalysts). This H₂ production rate is about two orders of magnitude greater than the yields of most semiconductor photocatalysts. The energy conversion efficiency for solar hydrogen production, defined as the ratio between the energy of solar-produced hydrogen and the energy of the incident sunlight, reached 24% for disorder-engineered black TiO₂ nanocrystals,” which they attribute to the nanocrystals new ability to absorb light from the infrared part of the spectrum.

The group also demonstrated similar effects when they substituted phenol and methylene blue for the methanol.

According to an LBL news release, the group says this is the first time a TiO₂-based photocatalyst is able to convert infrared, visible and ultraviolet light. “The more energy from the sun that can be absorbed by a photocatalyst, the more electrons can be supplied to a chemical reaction, which makes black titanium dioxide a very attractive material,” says Mao in the release.

Theoretical physicist Peter Yu explains in the release that, “by introducing a specific kind of disorder, mid-gap electronic states are created accompanied by a reduced band gap,” says Yu, who also is a professor in the University of California at Berkeley’s Physics Department. “This makes it possible for the infrared part of the solar spectrum to be absorbed and contribute to the photocatalysis.”

Mao and his group say they are now tackling how to reach similar energy conversion levels in water containing more commonplace organic compounds.

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Apparently more folks are jumping on the titanium dioxide’s anti-smog bandwagon. A few days ago I wrote about studies underway in Netherlands where they are testing TiO₂-coated concrete roadway pavers for their ability to remove NOx emissions in the air.

Two days ago I learned that concrete roof tiles treated with titanium dioxide are now being marketed in the U.S. for their anti-NOx benefits. The MonierLifeTiles company, part of the Australia-based Boral corporate group, claims that in one year, a 2000 square foot roof of the new tiles “destroy the same amount of nitrogen oxides as a car produces from being driven 10,800 miles.”

The company doesn’t provide any references on these numbers, but the implication, of course, is that a consumer could theoretically offset the NOx emissions of his or her car assuming they drove around 10,000 miles a year. According to the company’s website, the Fraunhofer Institute for Building Physics did confirm the TiO₂-treated tiles ability to degrade NO molecules (see below).

Now, apparently Monier-Boral have been selling these tiles for some time outside the U.S., so this isn’t exactly a new product. The company signaled (sort of - I’ll explain below) that it is now interested in the U.S. market when it teamed up with KB Home to outfit a model house in a new KB community, Alamosa, in West Lancaster, Calif., near LA.

The model home also features solar panel-battery-LED lighting system produced by a Chinese company, BYD – the same BYD that is manufacturing electric vehicles). USA today has a brief write up on the model house. (Sunpluggers has a more detailed story about the house’s systems but doesn’t mention the roof tiles.)

But with PV panels covering a chunk of the 1519-square-foot home’s roof, the owners may need to be driving a hybrid to brag that they are offsetting their smog contribution.

The Alamosa house has gained publicity, but it seems to me that the company is stumbling when it comes to actually marketing this line of tile. First, there is nothing about the tiles on the website they promote (www.montierlifetile.com) about the tile. Not even a press release about the Alamosa house. A little Googling leads one to the company’s European site where more info exists, but nothing helpful about sales.

Second, a few calls to some of Monier’s regional sales people in the U.S. led to a lot of unanswered voicemails. When I was finally referred to the MonierLifeTile’s national customer service number, the person at the other end of the line said she had never heard of Auranox and didn’t know what I was referring to. A message with the national sales manager has, so far, gone unanswered. Thus, I can’t tell you what the tiles cost or where they are made.

Third, the company is using untrained PR folks who either don’t know when to put a leash on the hyperbole or, worse, just make things up:

(from one of their emails, emphasis added) “So, if a homeowner has a roof with Auranox tiles he/she can have a net zero impact on the environment.”

(and this) “About 48% of all greenhouse gases and air pollution comes from homes and buildings and 18% from the entire transportation industry.”

The one thing that the company seems to be doing right is its Facebook page.

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In a lecture at the school, Brouwers said that as part of a road resurfacing project, around 1,000 square meters of the road’s surface were covered with special concrete paving stones containing the titania. For comparison purposes, another area of 1.000 square meters was surfaced with normal concrete pavers.

Researchers took measurements at heights of between a half and one-and-a-half meters. The results were significant:

“Over the area paved with air-purifying concrete the NOx content was found to 25 to 45 per cent lower than that over the area paved with normal concrete. ‘The air-purifying properties of the new paving stones had already been shown in the laboratory, but these results now show that they also work outdoors,’ said professor Brouwers. Further measurements are planned later this year.”

The titania in the pavers photocatalytically converts removes the nitrogen oxides from the air and converts it into nitrate. The idea of using titania technology on concrete for roads and construction aren’t particularly new, but this is the first time I have seen measurements taken from an actual roadway.

The concrete stones were made by paving stone manufacturer Struyk Verwo Infra. Although they are available for sale, they cost about 50% more than their non-treated counterparts. However, Brouwers argues that these materials are only a relatively small part of the cost of road construction and that the use of these pavers effectively adds only 10% to the cost of a project. Thus, they may be cost-effective response in regions where maximum NOx emissions are being exceeded.

Brouwers also says the concrete can be combined with asphalt when an asphalt surface is preferred.

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Scanning transmission electron microscope (STEM) image of PbSe quantum dots. Inset shows Pb atomic columns of single quantum dot. (Credit: John Silcox, Cornell University)

In a recent post at ScientificAmerican.com, David Bielo details many of the inefficiencies of photovoltaic cells. Although layered cells composed of various elements can convert more than 40 percent of sunlight into electricity, more simple semiconducting materials such as silicon hover around 20 percent when mass-produced. And, at best, such cells could convert only a third of incoming sunlight due to physical limits.

However, researchers at the universities of Minnesota and Texas may have reduced on of the major inefficiencies: heat loss. Nanosize crystals of semiconducting material, in this case a mixture of lead and selenium, move electrons fast enough to channel some of them faster than they can be lost as heat.

According to Bielo:

Solar cells employ semiconducting material because when a photon of sunlight of the right wavelength strikes that material, it knocks loose an electron, which can then be harvested as electrical current. But many of those loosened electrons dissipate as heat rather than being funneled out of the photovoltaic cell. Previous work in 2008 had shown that nanocrystals of semiconducting material can, in effect, slow down such “hot” electrons. As a result, these nanocrystals, also known as quantum dots, might be able to boost the efficiency of a solar cell.

In the June 18 issue of Science, researchers claim that quantum dots capture some of the “hot” electrons but they can also channel them to a typical electron-accepting material-the same titanium dioxide used in conventional solar cells. Because that transfer is so fast, fewer of the excited electrons are lost as heat, thus boosting the theoretical efficiency to as high as 66 percent.

Unfortunately, according to Bielo, that’s not all that’s required to build such a highly efficient solar cell. The next step would be to show that the captured electrons and transferred current can be carried away on a wire, as in a conventional solar cell. The challenge will be making a wire small enough to connect to a solar cell incorporating a quantum dot no bigger than 6.7 nanometers in diameter-and one that won’t lose much of the current as heat. And it would be years if not decades before such quantum dot-based solar cells might be manufactured.

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