Low-cost, long-lasting water splitter made of silicon and nickel

This image shows two electrodes connected via an external
voltage source splitting water into oxygen (O2) and
hydrogen (H2). The illuminated silicon electrode (left) uses light energy
to assist in the water-splitting process and is protected from the
surrounding electrolyte by a 2-nm film of nickel.Illustration: Guosong Hong, Stanford University

Stanford researchers have developed an inexpensive device that uses light to split water into oxygen and clean-burning hydrogen. The goal is to supplement solar cells with hydrogen-powered fuel cells that can generate electricity when the sun isn't shining or demand is high.

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 journalScience.

"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.

Source: http://news.stanford.edu/news/2013/november/nickel-water-splitter-111213.html

Major leap towards graphene for solar cells

Surprising result: Graphene retains its properties even when coated with silicon

Graphene has extreme conductivity and is completely transparent while being inexpensive and nontoxic. This makes it a perfect candidate material for transparent contact layers for use in solar cells to conduct electricity without reducing the amount of incoming light  - at least in theory. Whether or not this holds true in a real world setting is questionable as there is no such thing as "ideal" graphene - a free floating, flat honeycomb structure consisting of a single layer of carbon atoms: interactions with adjacent layers can change graphene's properties dramatically. Now, Dr. Marc Gluba and Prof. Dr. Norbert Nickel of the HZB Institute for Silicon Photovoltaics have shown that graphene retains its impressive set of properties when it is coated with a thin silicon film. These findings have paved the way for entirely new possibilities to use in thin-film photovoltaics.

"We examined how graphene's conductive properties change if it is incorporated into a stack of layers similar to a silicon based thin film solar cell and were surprised to find that these properties actually change very little," Marc Gluba explains.

To this end, they grew graphene on a thin copper sheet, next transferred it to a glass substrate, and finally coated it with a thin film of silicon. They examined two different versions that are commonly used in conventional silicon thin-film technologies: one sample contained an amorphous silicon layer, in which the silicon atoms are in a disordered state similar to a hardened molten glas; the other sample contained poly-crystalline silicon to help them observe the effects of a standard crystallization process on graphene's properties.

Even though the morphology of the top layer changed completely as a result of being heated to a temperature of several hundred degrees C, the graphene is still detectable.

"That's something we didn't expect to find, but our results demonstrate that graphene remains graphene even if it is coated with silicon," says Norbert Nickel. Their measurements of carrier mobility using the Hall-effect showed that the mobility of charge carriers within the embedded graphene layer is roughly 30 times greater than that of conventional zinc oxide based contact layers. Says Gluba: "Admittedly, it's been a real challenge connecting this thin contact layer, which is but one atomic layer thick, to external contacts. We're still having to work on that." Adds Nickel: "Our thin film technology colleagues are already pricking up their ears and wanting to incorporate it."

The researchers obtained their measurements on one square centimeter samples, although in practice it is feasible to coat much larger areas than that with graphene.

This work was recently published in Applied Physics Letters Vol. 103, 073102 (2013).
Authors: M. A. Gluba, D. Amkreutz, G. V. Troppenz, J. Rappich, and N. H. Nickel

Source: http://www.helmholtz-berlin.de/pubbin/news_seite?nid=13825&sprache=en&typoid=1

‘White graphene’ halts rust in high temps

Rice University researchers have
discovered that sheets of
hexagonal boron nitride (h-BN)
as little as one atom thick
can protect metals in harsh
environments at up to 1,100 degrees
Celsius. The top image shows
uncoated nickel oxidized after
exposure to high temperature in
an oxygen-rich environment. The second
shows nickel exposed to the same conditions
with a 5-nanometer coat of h-BN.
The third shows electron microscope
images of two, three, four and many-layer
h-BN films. The bottom image of
an h-BN sheet shows the hexagonal
arrangement of nitrogen (bright)
and boron atoms. Images by Zheng Liu

Rice U. researchers find nano-thin films of hexagonal boron nitride protect materials from oxidizing

Atomically thin sheets of hexagonal boron nitride (h-BN) have the handy benefit of protecting what’s underneath from oxidizing even at very high temperatures, Rice University researchers have discovered.

One or several layers of the material sometimes called “white graphene” keep materials from oxidizing – or rusting — up to 1,100 degrees Celsius (2,012 degrees Fahrenheit), and can be made large enough for industrial applications, they said.

The Rice study led by materials scientists Pulickel Ajayan and Jun Lou appears in the online journal Nature Communications.

Oxidation prevention is already big business, but no products available now work on the scale of what the Rice lab is proposing. The researchers see potential for very large sheets of h-BN only a few atoms thick made by scalable vapor deposition methods.

“We think this opens up new opportunities for two-dimensional material,” said Lou, an associate professor of mechanical engineering and materials science. “Everybody has been talking about these materials for electronic or photonic devices, but if this can be realized on a large scale, it’s going to cover a broad spectrum of applications.”

Lou said ultrathin h-BN protection might find a place in turbines, jet engines, oil exploration or underwater or other harsh environments where minimal size and weight would be an advantage, though wear and abrasion could become an issue and optimum thicknesses need to be worked out for specific applications.

It’s effectively invisible as well, which may make it useful for protecting solar cells from the elements, he said. “Essentially, this can be a very useful structural material coating,” Lou said.

The researchers made small sheets of h-BN via chemical vapor deposition (CVD), a process they said should be scalable for industrial production. They first grew the thin material on nickel foil and found it withstood high temperature in an oxygen-rich environment. They also grew h-BN on graphene and found they could transfer sheets of h-BN to copper and steel with similar results.

“What’s amazing is that these layers are ultrathin and they stand up to such ultrahigh temperatures,” Ajayan said. “At a few nanometers wide, they’re a totally non-invasive coating. They take almost no space at all.”

Lead authors are Rice postdoctoral researcher Zheng Liu and graduate student Yongji Gong. Co-authors are Rice graduate student Lulu Ma and Senior Faculty Fellow Robert Vajtai; Wu Zhou, a Wigner Fellow, and Juan Carlos Idrobo, a staff scientist at Oak Ridge National Laboratory; Jingjiang Yu of Agilent Technologies; Jeil Jung, a research fellow at the National University of Singapore and a postdoctoral researcher at the University of Texas at Austin; and Allan MacDonald, the Sid W. Richardson Foundation Regents Chair Professor at the University of Texas at Austin. Ajayan is the Benjamin M. and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials Science and of chemistry at Rice.

The Army Research Office, the Office of Naval Research, the Welch Foundation, the Korean Institute of Machinery and Materials, the National Science Foundation, Oak Ridge National Laboratory and the Department of Energy supported the research.

Source: http://news.rice.edu/2013/10/04/white-graphene-halts-rust-in-high-temps/