Materials For Energy

Keeping the heat up for super-efficient solar cells

By Gopal R. Rao

(From Chemistry World)

Scientists have found a way to siphon off the 'hot' electrons that are responsible for much of the energy lost in current solar cells. The research could lead to the development of far more efficient photovoltaic materials - theoretically doubling the proportion of solar energy that can be converted into electric power. 

In current silicon solar cells, only around a third of the energy that hits the cell is converted into electricity. Much of the energy delivered by the incoming photons is lost as heat - so-called hot electrons are generated, which cool extremely rapidly. Thus, their energy cannot easily be put to good use.

Now a research team has used a combination of nanomaterials and titanium dioxide to capture the hot electrons. The authors suggest that this is the first time it has been demonstrated that hot electrons can be taken out from a nanomaterial.

Interestingly, hot electrons are known to stay hot for longer in nanomaterials, giving just enough time - hundreds as opposed to only a few picoseconds - to capture them. The team deposited semiconducting lead selenide nanocrystal quantum dots on titanium dioxide, an electron conductor, and used optical second harmonic generation to 'watch' the hot electrons moving across the interface between the two materials.

Credit: University of Texas at Austin

Though the authors have not built an actual solar cell incorporating these materials, they estimate that their method might eventually make it possible to increase efficiency to 66 per cent.

Hot-Electron Transfer from Semiconductor Nanocrystals
Science 18 June 2010: Vol. 328. no. 5985, pp. 1543 - 1547. DOI: 10.1126/science.1185509

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Gopal R. Rao, Ph.D.
Web Science Editor
Materials Research Society (MRS)

Electric Vehicle Batteries: Where do the Materials Come From?

By Russell Chianelli

Electric vehicles are a promising technology for reducing pollution, especially in crowded cities.  Currently, the power train in electric cars consists of the storage of power in batteries with lithium anodes and power generation by an electric motor.  The electric motor requires rare earth elements such as neodymium.  Where do we get the Li and the neodymium?  We present some articles on the subject and welcome comments regarding the  trade and environmental issues associated.

The first article, The Battle Over Rare Earth Metals, deals with the supply of lithium from around the world.  A huge deposit exists in Bolivia, and there are other deposits in China.  When considering "green technologies," such as electric vehicles, we must also realize that raw materials such as Li, though abundant, may come from unusual places such as Bolivia, and this will require new trade agreements and arrangements.  On the other hand, electric motors for electric vehicles require rare elements such as neodymium.  These metals are rare and necessary for production of the vehicles, as indicated in the article titled, Beyond Lithium: What the Rare Earth Squeeze Means for Hybrid Cars.  The production of rare earth elements can also cause environmental problems in their mining and refining. It is important to understand that many "green technologies," while promising, have hidden environmental and trade implications.

The Battle Over Rare Earth Metals 

Beyond Lithium: What the Rare Earth Squeeze Means for Hybrid Cars 

Shortage of Rare Earth Minerals May Cripple U.S. High-Tech, Scientists Warn Congress

Huge Lithium Potash Deposit Found in Mexico

NMR Observations of Li "Moss" Formation in Li Batteries

By Gopal R. Rao

Lithium ion batteries (LIBs) are widely used in various consumer electronic devices because of their large energy storage densities. There is an even larger demand for more energy from LIBs that can be met by using Li metal-containing negative electrodes instead of graphite anodes currently used. One of the current problems with Li electrodes is that after several charge/discharge cycles in non-aqueous electrolytes, dendritic Li metal, sometimes called Li “moss,” forms on the Li-metal anode. Some of these dendrites can fall off into the electrolyte. These floating Li fibers, along with those on the electrode itself, cause various problems, including short circuits and subsequent overheating. This issue needs to be resolved if Li-metal anodes are to replace current graphite anodes. Various optical and electron microscopy methods have been used to qualitatively study the formation of the Li moss to better understand the mechanisms involved, but they have proven to be unsatisfactory. A new study by Clare P. Grey, R. Bhattacharya and others of Stony Brook University, University of Cambridge, and CSIRO, Australia, now reports, in Nature Materials, the use of in situ nuclear magnetic resonance (NMR) for investigating the Li dendrite formation during electrochemical cycling.

Credit: Nature Materials

The study showed that in situ NMR can dynamically monitor the growth and stripping of these Li microstructures. Using simple calculations based on the skin depth of the metallic structures, the amount of the Li dendrites formed can be quantified using the change in the Li metal signal. Two different ionic liquids were tested for their ability to prevent dendrite formation. The results clearly showed that this technique can sensitively monitor, with accuracy, the early stages of dendrite formation in Li batteries. The method can be used to judge the efficacy of current strategies to prevent the Li “moss” formation.

In situ NMR observation of the formation of metallic lithium microstructures in lithium batteries
Nature Materials Volume: 9, Pages: 504–510 (2010). DOI: 10.1038/nmat2764

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Gopal R. Rao, Ph.D.
Web Science Editor
Materials Research Society (MRS)

What Can Be Done About The BP Oil Spill?

By Russell Chianelli

When the BP oil well, located 50 miles off the coast of the U.S. in the Gulf of Mexico and one mile below the ocean surface, blew out, I couldn’t help but think back to the March 1989 Exxon Valdez oil spill.

At the time, I was working as an Exxon’s Corporate Research Laboratory and was designated as the lead scientist for the bioremediation project to clean the beaches in Alaska .

People remember the Exxon Valdez spill, but they don’t realize that it wasn’t the biggest tanker spill; this was the Amoco Cadiz accident in 1978 off the coast of France in Brittany . The Amoco Cadiz was about five times larger (approximately 220,000 tons vs. approximately 40,000 tons). However the largest oil spill in the world before the Gulf War was the 1979 Ixtoc Blow-Out in the Bahia de Campeche, Mexico. When PEMEX drilled a deep exploratory well, similar to the BP blowout, the sea bottom gave way and in nine months approximately 400,000 tons of oil was released. That’s more than 10 times the amount released by the Exxon Valdez. The Ixtoc Blow-Out lasted for 9 months until a relief well was drilled. Estimates ranged from 4000 barrels to 30,000 barrels per day being released, illustrating the difficulty in knowing exactly the amount being released from drilling blow-outs.

The Ixtoc Blow-Out was very similar to what we’re experiencing with the BP spill. Yet, in the Ixtoc Blow-Out very little of this huge amount of oil ever reached the shores of Padre Island, Texas, approximately 600 miles away. Why? Because of hydrocarbon-eating microbes called hydrocarbon degraders.

Hydrocarbon degraders are microorganisms that can consume oil, creating CO₂, H₂O and more of themselves. Approximately 50 percent of the petroleum goes to make more hydrocarbon degraders. The hydrocarbon degraders are then consumed by higher organisms (plankton) which provides food for marine life such as fish.

Every year 2 million to 12 million tons of oil naturally seep from the ocean floor and into the sea. In fact, many of the deposits in the Gulf of Mexico were discovered by observing these oil seeps, which is why the hydrocarbon degraders are everywhere, waiting for their “dinner” or fuel. They in turn provide “dinner” for plankton and then fish. Fishermen should be prepared for the extra catches that are coming because after every major oil spill there’s an explosion of local fish.

But before a fish explosion can happen, the microorganisms need to be able to get to the oil and digest it. Since oil and water don’t mix, adding a dispersant will accelerate the breakdown of the oil by making it more available to the microorganisms.

The best option for oil in open water is to use an EPA approved dispersant, such as COREXIT. BP has begun using dispersants. It is important to remember that the dispersants remove a surface oil slick by dispersing it in the water and this accelerates the ability of the hydrocarbon degraders to consume the oil. Removal of the surface slick also protects the beaches. In the case of the Exxon Valdez a storm came before dispersant were used and the storm covered the beaches with oil. A look at the beaches in Prince William Sound after the storm (www.materials forenergy.org) show a thick coating of fresh oil everywhere. This is not the case in the current spill because of the use of dispersants.

What would make this clean-up even more effective would be to use nutrient enhanced bioremediation. The hydrocarbon degraders like any living organism need nitrogen and phosphorous (nutrients) to make more of themselves. In the open sea the rate at which they consume the oil is limited by the availability of the nutrients. Nutrient enhanced bioremediation, the project I worked on in Valdez is the addition of nutrients to accelerate the natural rate of biodegradation. It is just like adding fertilizer to your garden plants.

The science of hydrocarbon degraders on oil spills was originally investigated by Dr. Ronald Atlas, now of the University of Louisville, who studied the Amoco Cadiz oil spill, which occurred on March 16, 1978 . The Amoco Cadiz was the largest tanker spill ever, 220,000 tons of crude oil on the beaches of Brittany, France. Nutrients (nitrogen and phosphorous) from farms above the beach enhanced the growth of the hydrocarbon degraders giving rise to the concept of nutrient enhanced bioremediation for dealing with oiled beaches.

It was this idea that Ron Atlas and myself developed for the beaches in Alaska after the Exxon Valdez oil spill. This was the largest successful bioremediation project ever attempted. The materials used were INIPOL EAP-22, an oleophilic (sticking to oil) nutrient and CUSTOMBLEND, a typical agricultural fertilizer. These nutrients were successfully used on the beaches in Alaska and not on oil in the open water. The oil on the beaches was consumed in approximately two weeks. This was in the cold waters of Prince William Sound. In the warm waters of the Gulf of Mexico this process should be even faster.

 

Bioremediation: Helping Nature’s Microbial Scavengers, R.R.Chianelli, Proceedings of the Royal Institution, vol.65, 105-126(1994). Download Bioremediation Technology Development and Application to the Alaskan Spill

Microbial Degradation of Petroleum Hydrocarbons: an Environmental Perspective, Ronald M. Atlas, Microbiological Reviews, Vol.45, No.1,180-209(1981).

The Worst Major Oil Spills in History

IXTOC I well, Bahia de Campeche, Mexico, Oil Spill (1979)

Amoco Cadiz Oil Spill (1978)

The Gulf of Mexico: Black Storm Rising

Cleanup Tool Helps Break Up Slick

Gulf Oil Spill Is Bad, But How Bad

Exxon Valdez Oil Spill Bioremediation Video