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News on Hydrogen Discoveries

by LiveModern Webmaster last modified Jul 31, 2013 01:09 AM
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by Glenn Meyers last modified Jul 30, 2013

For those scientists, researchers and sustainability innovators wanting to know more about the status of hydrogen today, read the two following AAAS releases below. We need to learn much more about discoveries such as these. Congratulations to all who worked on these projects.




 

 

For those scientists, researchers and sustainability innovators  wanting to know more about the status of hydrogen today, read the two following AAAS releases below. We need to learn much more about discoveries such as these. Congratulations to all who worked on these projects.

How does hydrogen metallize?

This image shows the predicted optical absorption of a 1 μm of hydrogen in a high pressure diamond anvil cell for different crystal structures at a pressure of 300 GPa (3 million times normal atmosphere—similar to the pressure in the center of the Earth). At these pressures hydrogen no longer forms molecules, but instead forms in sheets, as shown in the figure. Scientists use optical absorption to look for metallization in hydrogen, based on the assumption that metallic hydrogen would be opaque as most metals are. But the team's analysis shows that it may very well actually be transparent. Absorption units on the graph (AU) are in factors of 10, meaning 2 AU lets just 1% of the incident light pass through the structure (quite dark!). The graphite structure is an ideal structure that is not expected to be observed in reality. The proposed high-pressure forms, phase 3 (at low temperatures) and phase 4 (at room temperature), are both predicted to be transparent in the near infrared and optical frequencies of light, although phase 4 is poor metal. The Cmca structure is a similar structure, but is predicted to be a better metal and opaque, and to form at higher pressures. Graph is courtesy of Ronald Cohen. Credit: Courtesy of Ronald Cohen, Carnegie Institution for Science

This image shows the predicted optical absorption of a 1 μm of hydrogen in a high pressure diamond anvil cell for different crystal structures at a pressure of 300 GPa (3 million times normal atmosphere—similar to the pressure in the center of the Earth). At these pressures hydrogen no longer forms molecules, but instead forms in sheets, as shown in the figure. Scientists use optical absorption to look for metallization in hydrogen, based on the assumption that metallic hydrogen would be opaque as most metals are. But the team’s analysis shows that it may very well actually be transparent. Absorption units on the graph (AU) are in factors of 10, meaning 2 AU lets just 1% of the incident light pass through the structure (quite dark!). The graphite structure is an ideal structure that is not expected to be observed in reality. The proposed high-pressure forms, phase 3 (at low temperatures) and phase 4 (at room temperature), are both predicted to be transparent in the near infrared and optical frequencies of light, although phase 4 is poor metal. The Cmca structure is a similar structure, but is predicted to be a better metal and opaque, and to form at higher pressures. Graph is courtesy of Ronald Cohen.
Credit: Courtesy of Ronald Cohen, Carnegie Institution for Science

Washington, D.C.— Hydrogen is deceptively simple. It has only a single electron per atom, but it powers the sun and forms the majority of the observed universe. As such, it is naturally exposed to the entire range of pressures and temperatures available in the whole cosmos. But researchers are still struggling to understand even basic aspects of its various forms under high-pressure conditions.

Experimental difficulties contribute to the lack of knowledge about hydrogen’s forms. The containment of hydrogen at high pressures and the competition between its many similar structures both play a part in the relative lack of knowledge.

At high pressures, hydrogen is predicted to transform to a metal, which means it conducts electricity. One of the prime goals of high pressure research, going back to the 1930s, has been to achieve a metallic state in hydrogen. There have been recent claims of hydrogen becoming metallic at room temperature, but they are controversial.

New work from a team at Carnegie’s Geophysical Laboratory makes significant additions to our understanding of this vital element’s high-pressure behavior. Their work is published in two papers by Proceedings of the National Academy of Sciences and Physical Review B.

New theoretical calculations from Carnegie’s Ronald Cohen, Ivan Naumov and Russell Hemley indicate that under high pressure, hydrogen takes on a series of structures of layered honeycomb-like lattices, similar to graphite. According to their predictions the layers, which are like the carbon sheets that form graphene, make a very poor, transparent metal. As a result, its signature is difficult to detect.

“The difficulty of detection means that the line between metal and non-metal in hydrogen is probably blurrier than we’d previously supposed,” Cohen said “Our results will help experimental scientists test for metallic hydrogen using advanced techniques involving the reflectivity of light.”

Source: AAAS EurekAlert

The best of 2 worlds: Solar hydrogen production breakthrough

When light hits the system, an electrical potential builds up. The metal oxide layer acts as a photo anode and is the site of oxygen formation. It is connected to the solar cell by way of a conducting bridge made of graphite (black). Since only the metal oxide layer is in contact with the electrolyte, the silicon solar cell remains safe from corrosion. A platinum spiral serves as the cathode where hydrogen is formed. Credit: Image: TU Delft

When light hits the system, an electrical potential builds up. The metal oxide layer acts as a photo anode and is the site of oxygen formation. It is connected to the solar cell by way of a conducting bridge made of graphite (black). Since only the metal oxide layer is in contact with the electrolyte, the silicon solar cell remains safe from corrosion. A platinum spiral serves as the cathode where hydrogen is formed.
Credit: Image: TU Delft

The photo anode, which is made from the metal oxide bismuth vanadate (BiVO4) to which a small amount of tungsten atoms was added, was sprayed onto a piece of conducting glass and coated with an inexpensive cobalt phosphate catalyst.

“Basically, we combined the best of both worlds,” explains Prof. Dr. Roel van de Krol, head of the HZB Institute for Solar Fuels: “We start with a chemically stable, low cost metal oxide, add a really good but simple silicon-based thin film solar cell, and – voilà – we’ve just created a cost-effective, highly stable, and highly efficient solar fuel device.”

Thus the experts were able to develop a rather elegant and simple system for using sunlight to split water into hydrogen and oxygen. This process, called artificial photosynthesis, allows solar energy to be stored in the form of hydrogen. The hydrogen can then be used as a fuel either directly or in the form of methane, or it can generate electricity in a fuel cell. One rough estimate shows the potential inherent in this technology: At a solar performance in Germany of roughly 600 Watts per square meter, 100 square meters of this type of system is theoretically capable of storing 3 kilowatt hours of energy in the form of hydrogen in just one single hour of sunshine. This energy could then be available at night or on cloudy days.

Metal oxide as photo anode prevents corrosion of the solar cell

Van de Krol and his team essentially started with a relatively simple silicon-based thin film cell to which a metal oxide layer was added. This layer is the only part of the cell that is in contact with the water, and acts as a photo anode for oxygen formation. At the same time, it helps to prevent corrosion of the sensitive silicon cell. The researchers systematically examined and optimized processes such as light absorption, separation of charges, and splitting of water molecules. Theoretically, a solar-to-chemical efficiency of up to nine percent is possible when you use a photo anode made from bismuth vanadate, says van de Krol. Already, they were able to solve one problem: Using an inexpensive cobalt phosphate catalyst, they managed to substantially accelerate the process of oxygen formation at the photo anode.

A new record: More than 80 percent of the incident photons contribute to the current!

The biggest challenge, however, was the efficient separation of electrical charges within the bismuth vanadate film. Metal oxides may be stable and cheap, but the charge carriers have a tendency to quickly recombine. This means they are no longer available for the water splitting reaction. Now, Van de Krol and his team have figured out that it helps to add wolfram atoms to the bismuth vanadate film. “What’s important is that we distribute these wolfram atoms in a very specific way so that they can set up an internal electric field, which helps to prevent recombination,” explains van de Krol.

For this to work, the scientists took a bismuth vanadium wolfram solution and sprayed it onto a heated glass substrate. This caused the solution to evaporate. By repeatedly spraying different wolfram concentrations onto the glass, a highly efficient photo-active metal oxide film some 300 nanometers thick was created.

“We don’t really understand quite yet why bismuth vanadate works so much better than other metal oxides. We found that more than 80 percent of the incident photons contribute to the current, an unexpectedly high value that sets a new record for metal oxides” says van de Krol. The next challenge is scaling these kinds of systems to several square meters so they can yield relevant amounts of hydrogen.

Source: AAAS EurekAlert


 

 

 
 
 

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