Radically new water splitting technique to produce hydrogen fuel

(Nanowerk News) A University of Colorado Boulder team  has developed a radically new technique that uses the power of sunlight to  efficiently split water into its components of hydrogen and oxygen, paving the  way for the broad use of hydrogen as a clean, green fuel. The CU-Boulder team  has devised a solar-thermal system in which sunlight could be concentrated by a  vast array of mirrors onto a single point atop a central tower up to several  hundred feet tall. The tower would gather heat generated by the mirror system to  roughly 2,500 degrees Fahrenheit (1,350 Celsius), then deliver it into a reactor  containing chemical compounds known as metal oxides, said CU-Boulder Professor  Alan Weimer, research group leader.

As a metal oxide compound heats up, it releases oxygen atoms,  changing its material composition and causing the newly formed compound to seek  out new oxygen atoms, said Weimer. The team showed that the addition of steam to  the system — which could be produced by boiling water in the reactor with the  concentrated sunlight beamed to the tower — would cause oxygen from the water  molecules to adhere to the surface of the metal oxide, freeing up hydrogen  molecules for collection as hydrogen gas.

An  artist’s conception of a commercial hydrogen production plant that uses sunlight  to split water in order to produce clean hydrogen fuel. (Image courtesy  University of Colorado Boulder)

“We have designed something here that is very different from  other methods and frankly something that nobody thought was possible before,”  said Weimer of the chemical and biological engineering department. “Splitting  water with sunlight is the Holy Grail of a sustainable hydrogen economy.”

A paper on the subject was published in the Aug. 2 issue of  Science (“Efficient Generation of H2 by Splitting Water with an Isothermal Redox Cycle  “). The team included co-lead authors Weimer and Associate Professor Charles  Musgrave, first author and doctoral student Christopher Muhich, postdoctoral  researcher Janna Martinek, undergraduate Kayla Weston, former CU graduate  student Paul Lichty, former CU postdoctoral researcher Xinhua Liang and former  CU researcher Brian Evanko.

One of the key differences between the CU method and other  methods developed to split water is the ability to conduct two chemical  reactions at the same temperature, said Musgrave, also of the chemical and  biological engineering department. While there are no working models,  conventional theory holds that producing hydrogen through the metal oxide  process requires heating the reactor to a high temperature to remove oxygen,  then cooling it to a low temperature before injecting steam to re-oxidize the  compound in order to release hydrogen gas for collection.

“The more conventional approaches require the control of both  the switching of the temperature in the reactor from a hot to a cool state and  the introduction of steam into the system,” said Musgrave. “One of the big  innovations in our system is that there is no swing in the temperature. The  whole process is driven by either turning a steam valve on or off.”

“Just like you would use a magnifying glass to start a fire, we  can concentrate sunlight until it is really hot and use it to drive these  chemical reactions,” said Muhich. “While we can easily heat it up to more than  1,350 degrees Celsius, we want to heat it to the lowest temperature possible for  these chemical reactions to still occur. Hotter temperatures can cause rapid  thermal expansion and contraction, potentially causing damage to both the  chemical materials and to the reactors themselves.”

A  laboratory model of a multi-tube solar reactor at the University of Colorado  Boulder that can be used to split water in order to produce clean hydrogen fuel.  (Photo courtesy University of Colorado Boulder)

In addition, the two-step conventional idea for water splitting  also wastes both time and heat, said Weimer, also a faculty member at  CU-Boulder’s BioFrontiers Institute. “There are only so many hours of sunlight  in a day,” he said.

The research was supported by the National Science Foundation  and by the U.S. Department of Energy.

With the new CU-Boulder method, the amount of hydrogen produced  for fuel cells or for storage is entirely dependent on the amount of metal oxide  — which is made up of a combination of iron, cobalt, aluminum and oxygen — and  how much steam is introduced into the system. One of the designs proposed by the  team is to build reactor tubes roughly a foot in diameter and several feet long,  fill them with the metal oxide material and stack them on top of each other. A  working system to produce a significant amount of hydrogen gas would require a  number of the tall towers to gather concentrated sunlight from several acres of  mirrors surrounding each tower.

Weimer said the new design began percolating within the team  about two years ago. “When we saw that we could use this simpler, more effective  method, it required a change in our thinking,” said Weimer. “We had to develop a  theory to explain it and make it believable and understandable to other  scientists and engineers.”

Despite the discovery, the commercialization of such a  solar-thermal reactor is likely years away. “With the price of natural gas so  low, there is no incentive to burn clean energy,” said Weimer, also the  executive director of the Colorado Center for Biorefining and Biofuels, or C2B2.  “There would have to be a substantial monetary penalty for putting carbon into  the atmosphere, or the price of fossil fuels would have to go way up.”

Source: University of Colorado at Boulder

Read more: http://www.nanowerk.com/news2/green/newsid=31656.php#ixzz2bET11D4M

Hybrid ribbons a gift for powerful batteries: Vanadium oxide – graphene material works well for lithium-ion storage

Hybrid ribbons a gift for powerful batteries: Vanadium oxide – graphene material works well for lithium-ion storage

The Rice University lab of materials scientist Pulickel Ajayan determined that the well-studied material is a superior cathode for batteries that could supply both high energy density and significant power density. The research appears online this month in the American Chemical Society journal Nano Letters. The ribbons created at Rice are thousands of times thinner than a sheet of paper, yet have potential that far outweighs current materials for their ability to charge and discharge very quickly. Cathodes built into half-cells for testing at Rice fully charged and discharged in 20 seconds and retained more than 90 percent of their initial capacity after more than 1,000 cycles. “This is the direction battery research is going, not only for something with high energy density but also high power density,” Ajayan said. “It’s somewhere between a battery and a supercapacitor.”

Hydrothermal processing of vanadium pentoxide and graphene oxide creates graphene-coated ribbons of crystalline vanadium oxide, which show great potential as ultrafast charging and discharging electrodes for lithium-ion batteries. Credit: Ajayan Group/Rice University

The ribbons also have the advantage of using relatively abundant and cheap materials. “This is done through a very simple hydrothermal process, and I think it would be easily scalable to large quantities,” he said. Ajayan said vanadium oxide has long been considered a material with great potential, and in fact vanadium pentoxide has been used in lithium-ion batteries for its special structure and high capacity.

But oxides are slow to charge and discharge, due to their low electrical conductivity. The high-conductivity graphene lattice that is literally baked in solves that problem nicely, he said, by serving as a speedy conduit for electrons and channels for ions.

The atom-thin graphene sheets bound to the crystals take up very little bulk. In the best samples made at Rice, fully 84 percent of the cathode’s weight was the lithium-slurping VO2, which held 204 milliamp hours of energy per gram. The researchers, led by Rice graduate student Yongji Gong and lead author Shubin Yang, said they believe that to be among the best overall performance ever seen for lithium-ion battery electrodes. “One challenge to production was controlling the conditions for the co-synthesis of VO2 ribbons with graphene,” Yang said.

The process involved suspending graphene oxide nanosheets with powdered vanadium pentoxide (layered vanadium oxide, with two atoms of vanadium and five of oxygen) in water and heating it in an autoclave for hours. The vanadium pentoxide was completely reduced to VO2, which crystallized into ribbons, while the graphene oxide was reduced to graphene, Yang said.

The ribbons, with a web-like coating of graphene, were only about 10 nanometers thick, up to 600 nanometers wide and tens of micrometers in length. “These ribbons were the building blocks of the three-dimensional architecture,” Yang said. “This unique structure was favorable for the ultrafast diffusion of both lithium ions and electrons during charge and discharge processes. It was the key to the achievement of excellent electrochemical performance.”

In testing the new material, Yang and Gong found its capacity for lithium storage remained stable after 200 cycles even at high temperatures (167 degrees Fahrenheit) at which other cathodes commonly decay, even at low charge-discharge rates. “We think this is real progress in the development of cathode materials for high-power lithium-ion batteries,” Ajayan said, suggesting the ribbons’ ability to be dispersed in a solvent might make them suitable as a component in the paintable batteries developed in his lab.

More information: pubs.acs.org/doi/abs/10.1021/nl400001u

Journal reference: Nano Letters