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Friday, November 29, 2013

Hydrogen fuel from sunlight? Low-cost, long-lasting water splitter made of silicon and nickel

Nov. 14, 2013 ? Stanford University scientists have created a silicon-based water splitter that is both low-cost and corrosion-free. The novel device -- a silicon semiconductor coated in an ultrathin layer of nickel -- could help pave the way for large-scale production of clean hydrogen fuel from sunlight, according to the scientists.
Their results are published in the Nov. 15 issue of the journal Science.
"Solar cells only work when the sun is shining," said study co-author Hongjie Dai, a professor of chemistry at Stanford. "When there's no sunlight, utilities often have to rely on electricity from conventional power plants that run on coal or natural gas."
A greener solution, Dai explained, is to supplement the solar cells with hydrogen-powered fuel cells that generate electricity at night or when demand is especially high.
To produce clean hydrogen for fuel cells, scientists have turned to an emerging technology called water splitting. Two semiconducting electrodes are connected and placed in water. The electrodes absorb light and use the energy to split the water into its basic components, oxygen and hydrogen. The oxygen is released into the atmosphere, and the hydrogen is stored as fuel.
When energy is needed, the process is reversed. The stored hydrogen and atmospheric oxygen are combined in a fuel cell to generate electricity and pure water.
The entire process is sustainable and emits no greenhouse gases. But finding a cheap way to split water has been a major challenge. Today, researchers continue searching for inexpensive materials that can be used to build water splitters efficient enough to be of practical use.
Silicon solution
"Silicon, which is widely used in solar cells, would be an ideal, low-cost material," said Stanford graduate student Michael J. Kenney, co-lead author of the Science study. "But silicon degrades in contact with an electrolyte solution. In fact, a submerged electrode made of silicon corrodes as soon as the water-splitting reaction starts."
In 2011, another Stanford research team addressed this challenge by coating silicon electrodes with ultrathin layers of titanium dioxide and iridium. That experimental water splitter produced hydrogen and oxygen for eight hours without corroding.
"Those were inspiring results, but for practical water splitting, longer-term stability is needed," Dai said. "Also, the precious metal iridium is costly. A non-precious metal catalyst would be desirable."
To find a low-cost alternative, Dai suggested that Kenney and his colleagues try coating silicon electrodes with ordinary nickel. "Nickel is corrosion-resistant," Kenney said. "It's also an active oxygen-producing catalyst, and it's earth-abundant. That makes it very attractive for this type of application."
Nickel nanofilm
For the experiment, the Dai team applied a 2-nanometer-thick layer of nickel onto a silicon electrode, paired it with another electrode and placed both in a solution of water and potassium borate. When light and electricity were applied, the electrodes began splitting the water into oxygen and hydrogen, a process that continued for about 24 hours with no sign of corrosion.
To improve performance, the researchers mixed lithium into the water-based solution. "Remarkably, adding lithium imparted superior stability to the electrodes," Kenney said. "They generated hydrogen and oxygen continuously for 80 hours -- more than three days -- with no sign of surface corrosion."
These results represent a significant advance over previous experimental efforts, added Dai. "Our lab has produced one of the longest lasting silicon-based photoanodes," he said. "The results suggest that an ultrathin nickel coating not only suppresses corrosion but also serves as an electrocatalyst to expedite the otherwise sluggish water-splitting reaction.
"Interestingly, a lithium addition to electrolytes has been used to make better nickel batteries since the Thomas Edison days. Many years later we are excited to find that it also helps to make better water-splitting devices."
The scientists plan to do additional work on improving the stability and durability of nickel-treated electrodes of silicon as well as other materials.
Other authors of the study are Ming Gong and Yanguang Li (co-lead authors), Justin Z. Wu, Ju Feng and Mario Lanza, all formerly or currently affiliated with the Dai Lab at Stanford.
Support was provided by the Precourt Institute for Energy and the Global Climate and Energy Project at Stanford and the National Science Foundation.

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Solar cells utilize thermal radiation

Nov. 13, 2013 ? Thermal radiation from the sun is largely lost on most silicon solar cells. Up-converters transform the infrared radiation into usable light, however. Researchers have now for the first time successfully adapted this effect for use in generating power.

There is more to solar radiation than meets the eye: sun- burn develops from unseen UV radiation, while we sense infrared radiation as heat on our skin, though invisible to us. Solar cells also 'see' only a portion of solar radiation: ap- proximately 20 percent of the energy contained in the solar spectrum is unavailable to cells made of silicon -- they are unable to utilize a part of the infrared radiation, the short-wavelength IR radiation, for generating power.

Researchers of the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, together with their colleagues at the University of Bern, Switzerland, and the Heriot-Watt University in Edinburgh, Scotland, have now for the first time made a portion of this radiation usable with the assistance of a practical up-converter. The technology that transforms infra- red into usable light has been known about since the 1960s. However, it has only been investigated in connection with solar cells since 1996. "We have been able to adapt both the solar cells and the up-converter so as to obtain the biggest improvement in efficiency so far," reports Stefan Fischer happily, a scientist at ISE. The potential is big: silicon solar cells theoretically convert about thirty percent of sunlight falling upon them into electrical power. Up-converters could increase this portion to a level of forty percent.

A ladder for light particles

However, how does the up-converter manage to utilize the infrared radiation for the solar cells? As solar radiation falls on the solar cells, they absorb the visible and near-infrared light. The infrared portion is not absorbed, however, it goes right through them. On the back- side, the radiation runs into the up-converter -- essentially a microcrystalline powder made of sodium yttrium fluoride embedded in a polymer. Part of the yttrium has been replaced by the scientists with the element erbium, which is active in the optical range and responsible in the end for the up-conversion.

As the light falls on this up-converter, it excites the erbium ions. That means they are raised to a higher energy state. You can imagine this reaction like climbing up a ladder: an electron in the ion uses the energy of the light particle to climb up the first step of the ladder. A sec- ond light particle enables the electron to climb to the second step, and so on. An ion that has been excited in this manner can "jump down" from the highest step or state. In doing so, it emits light with an energy equal to all of the light particles that have helped the elec- tron to climb on up. The up-converter collects, so to speak, the energy of several of these particles and transfers it to a single one. This has so much energy then that the solar cells "see" it and can utilize it.

Researchers had to adapt the solar cells in order to be able to employ an up-converter such as this. Normally, metal is vapour-deposited on the backside, enabling current to flow out of the solar cells -- so no light can shine through normally. "We equipped the solar cells with metal lattices on the front and rear sides so that IR light can pass through the solar cells. In addition, the light can be used by both faces of the cell -- we call this a bi-facial solar cell," explains Fischer. Scientists have applied specialized anti-reflection coatings to the front and rear sides of the solar cell. These cancel reflections at the surfaces and assure that the cells absorb as much light as possible. "This is the first time we have adapted the anti- reflection coating to the backside of the solar cell as well. That could increase the efficiency of the modules and raise their energy yields. The first companies are already trying to accomplish this by implementing bi-facial solar cells," says Fischer, emphasizing the potential of the approach.


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Blue-green algae a five-tool player in converting waste to fuel


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Tuesday, November 19, 2013

HOMEMADE SOLAR PANEL

The present economic scenario and the ever-shrinking state of the global oil reserves is not good news for everyone. But on a positive note it makes us investigate for alternative energy sources that is become the solution of cutting down present and the future energy consumption, particularly in terms of fossil fuel usage like oil. For example, it has become more and more economical to build homemade solar energy system. Building the home made solar panels that utilize deep cell storage batteries produce the electricity upon consumer demand and reduce the their individual dependence on the country’s energy grid.
Who is there in the world who does not wish to have the energy bills get reduced by 80% or more? Many of us shall want to take advantage of such a great deal. If such is the demand then follow the guide Earth4Energy that offers lot of fantastic homemade solar solutions on their site, which can be built easily, for the low cost of less than $200. Click Here!

Cheaper Chinese solar panels are not due to low-cost labor


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Friday, November 8, 2013

Laying down a discerning membrane

One of the thinnest membranes ever made is also highly discriminating when it comes to the molecules going through it. Engineers have constructed a graphene oxide membrane less than 2 nanometers thick with high permeation selectivity between hydrogen and carbon dioxide gas molecules.

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First uses of new solar energy technology: Killing germs on medical, dental instruments


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Thursday, November 7, 2013

Charge your mobile phone with formic acid?

With the technology of today it is possible to use environmental friendly formic acid in fuel cell powering your mobile phone or laptop. A physicist in Sweden has developed new catalysts to improve the capacity of these fuel cells.

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