Scientists create 'nano-reactor' for the production of hydrogen biofuel

Combining bacterial genes and virus shell creates a highly efficient, renewable material used in generating power from water

Scientists at Indiana University have created a highly efficient biomaterial that catalyzes the formation of hydrogen -- one half of the "holy grail" of splitting H2O to make hydrogen and oxygen for fueling cheap and efficient cars that run on water.

A modified enzyme that gains strength from being protected within the protein shell -- or "capsid" -- of a bacterial virus, this new material is 150 times more efficient than the unaltered form of the enzyme.

The process of creating the material was recently reported in "Self-assembling biomolecular catalysts for hydrogen production" in the journal Nature Chemistry.

"Essentially, we've taken a virus's ability to self-assemble myriad genetic building blocks and incorporated a very fragile and sensitive enzyme with the remarkable property of taking in protons and spitting out hydrogen gas," said Trevor Douglas, the Earl Blough Professor of Chemistry in the IU Bloomington College of Arts and Sciences' Department of Chemistry, who led the study. "The end result is a virus-like particle that behaves the same as a highly sophisticated material that catalyzes the production of hydrogen."

Other IU scientists who contributed to the research were Megan C. Thielges, an assistant professor of chemistry; Ethan J. Edwards, a Ph.D. student; and Paul C. Jordan, a postdoctoral researcher at Alios BioPharma, who was an IU Ph.D. student at the time of the study.

The genetic material used to create the enzyme, hydrogenase, is produced by two genes from the common bacteria Escherichia coli, inserted inside the protective capsid using methods previously developed by these IU scientists. The genes, hyaA and hyaB, are two genes in E. coli that encode key subunits of the hydrogenase enzyme. The capsid comes from the bacterial virus known as bacteriophage P22. 

The resulting biomaterial, called "P22-Hyd," is not only more efficient than the unaltered enzyme but also is produced through a simple fermentation process at room temperature.

The material is potentially far less expensive and more environmentally friendly to produce than other materials currently used to create fuel cells. The costly and rare metal platinum, for example, is commonly used to catalyze hydrogen as fuel in products such as high-end concept cars.

"This material is comparable to platinum, except it's truly renewable," Douglas said. "You don't need to mine it; you can create it at room temperature on a massive scale using fermentation technology; it's biodegradable. It's a very green process to make a very high-end sustainable material."

In addition, P22-Hyd both breaks the chemical bonds of water to create hydrogen and also works in reverse to recombine hydrogen and oxygen to generate power. "The reaction runs both ways -- it can be used either as a hydrogen production catalyst or as a fuel cell catalyst," Douglas said.

The form of hydrogenase is one of three occurring in nature: di-iron (FeFe)-, iron-only (Fe-only)- and nitrogen-iron (NiFe)-hydrogenase. The third form was selected for the new material due to its ability to easily integrate into biomaterials and tolerate exposure to oxygen.

NiFe-hydrogenase also gains significantly greater resistance upon encapsulation to breakdown from chemicals in the environment, and it retains the ability to catalyze at room temperature.

Unaltered NiFe-hydrogenase, by contrast, is highly susceptible to destruction from chemicals in the environment and breaks down at temperatures above room temperature -- both of which make the unprotected enzyme a poor choice for use in manufacturing and commercial products such as cars.

These sensitivities are "some of the key reasons enzymes haven't previously lived up to their promise in technology," Douglas said. Another is their difficulty to produce.

"No one's ever had a way to create a large enough amount of this hydrogenase despite its incredible potential for biofuel production. But now we've got a method to stabilize and produce high quantities of the material -- and enormous increases in efficiency," he said.

The development is highly significant according to Seung-Wuk Lee, professor of bioengineering at the University of California-Berkeley, who was not a part of the study.

"Douglas' group has been leading protein- or virus-based nanomaterial development for the last two decades. This is a new pioneering work to produce green and clean fuels to tackle the real-world energy problem that we face today and make an immediate impact in our life in the near future,” said Lee, whose work has been cited in a U.S. Congressional report on the use of viruses in manufacturing.

Beyond the new study, Douglas and his colleagues continue to craft P22-Hyd into an ideal ingredient for hydrogen power by investigating ways to activate a catalytic reaction with sunlight, as opposed to introducing elections using laboratory methods.

"Incorporating this material into a solar-powered system is the next step," Douglas said.

Indiana University

Nanotechnology World Association

Physicists conduct most precise measurement yet of interaction between atoms and carbon surfaces

An illustration of atoms sticking to a carbon nanotube, 
affecting the electrons in its surface.David Cobden and students

Physicists at the University of Washington have conducted the most precise and controlled measurements yet of the interaction between the atoms and molecules that comprise air and the type of carbon surface used in battery electrodes and air filters — key information for improving those technologies.

A team led by David Cobden, UW professor of physics, used a carbon nanotube — a seamless, hollow graphite structure a million times thinner than a drinking straw — acting as a transistor to study what happens when gas atoms come into contact with the nanotube’s surface. Their findings were published in May in the journal Nature Physics.

Cobden said he and co-authors found that when an atom or molecule sticks to the nanotube a tiny fraction of the charge of one electron is transferred to its surface, resulting in a measurable change in electrical resistance.

“This aspect of atoms interacting with surfaces has never been detected unambiguously before,” Cobden said. “When many atoms are stuck to the minuscule tube at the same time, the measurements reveal their collective dances, including big fluctuations that occur on warming analogous to the boiling of water.”

Lithium batteries involve lithium atoms sticking and transferring charges to carbon electrodes, and in activated charcoal filters, molecules stick to the carbon surface to be removed, Cobden explained.

“Various forms of carbon, including nanotubes, are considered for hydrogen or other fuel storage because they have a huge internal surface area for the fuel molecules to stick to. However, these technological situations are extremely complex and difficult to do precise, clear-cut measurements on.”

This work, he said, resulted in the most precise and controlled measurements of these interactions ever made, “and will allow scientists to learn new things about the interplay of atoms and molecules with a carbon surface,” important for improving technologies including batteries, electrodes and air filters.

Co-authors were Oscar Vilches, professor emeritus of physics, doctoral students Hao-Chun Lee and research associate Boris Dzyubenko, all of the UW. The research was funded by the National Science Foundation.

Source: http://www.washington.edu/news/2015/05/28/physicists-conduct-most-precise-measurement-yet-of-interaction-between-atoms-and-carbon-surfaces/

Not all diamonds are forever

Images taken by Rice University scientists show that some diamonds are not forever.

Rice University researchers see nanodiamonds created in coal fade away in seconds

The Rice researchers behind a new study that explains the creation of nanodiamonds in treated coal also show that some microscopic diamonds only last seconds before fading back into less-structured forms of carbon under the impact of an electron beam.

The research by Rice chemist Ed Billups and his colleagues appears in the American Chemical Society’s Journal of Physical Chemistry Letters.

A series of images shows a small nanodiamond (the dark spot in the lower right corner) reverting to anthracite. Rice University scientists saw nanodiamonds form in hydrogenated coal when hit by the electron beam used in high-resolution transmission electron microscopes. But smaller diamonds like this one degraded with subsequent images. The scale bar is 1 nanometer. Courtesy of the Billups Lab

Billups and Yanqiu Sun, a former postdoctoral researcher in his lab, witnessed the interesting effect while working on ways to chemically reduce carbon from anthracite coal and make it soluble. First they noticed nanodiamonds forming amid the amorphous, hydrogen-infused layers of graphite.

It happened, they discovered, when they took close-ups of the coal with an electron microscope, which fires an electron beam at the point of interest. Unexpectedly, the energy input congealed clusters of hydrogenated carbon atoms, some of which took on the lattice-like structure of nanodiamonds.

“The beam is very powerful,” Billups said. “To knock hydrogen atoms off of something takes a tremendous amount of energy.”

Even without the kind of pressure needed to make macroscale diamonds, the energy knocked loose hydrogen atoms to prompt a chain reaction between layers of graphite in the coal that resulted in diamonds between 2 and 10 nanometers wide.

But the most “nano” of the nanodiamonds were seen to fade away under the power of the electron beam in a succession of images taken over 30 seconds.

“The small diamonds are not stable and they revert to the starting material, the anthracite,” Billups said.

Billups turned to Rice theoretical physicist Boris Yakobson and his colleagues at the Technological Institute for Superhard and Novel Carbon Materials in Moscow to explain what the chemists saw. Yakobson, Pavel Sorokin and Alexander Kvashnin had already come up with a chart – called a phase diagram — that demonstrated how thin diamond films might be made without massive pressure.

They used similar calculations to show how nanodiamonds could form in treated anthracite and subbituminous coal. In this case, the electron microscope’s beam knocks hydrogen atoms loose from carbon layers. Then the dangling bonds compensate by connecting to an adjacent carbon layer, which is prompted to connect to the next layer. The reaction zips the atoms into a matrix characteristic of diamond until pressure forces the process to halt.

Natural, macroscale diamonds require extreme pressures and temperatures to form, but the phase diagram should be reconsidered for nanodiamonds, the researchers said.

“There is a window of stability for diamonds within the range of 19-52 angstroms (tenths of a nanometer), beyond which graphite is more stable,” Billups said. Stable nanodiamonds up to 20 nanometers in size can be formed in hydrogenated anthracite, they found, though the smallest nanodiamonds were unstable under continued electron-beam radiation.

Billups noted subsequent electron-beam experiments with pristine anthracite formed no diamonds, while tests with less-robust infusions of hydrogen led to regions with “onion-like fringes” of graphitic carbon, but no fully formed diamonds. Both experiments lent support to the need for sufficient hydrogen to form nanodiamonds.

Kvashnin is a former visiting student at Rice and a graduate student at the Moscow Institute of Physics and Technology (MIPT). Sorokin holds appointments at MIPT and the National University of Science and Technology, Moscow. Yakobson is Rice’s Karl F. Hasselmann Professor of Mechanical Engineering and Materials Science, a professor of chemistry and a member of the Richard E. Smalley Institute for Nanoscale Science and Technology. Billups is a professor of chemistry at Rice.

The Robert A. Welch Foundation, the Ministry of Education and Science of the Russian Federation and the Russian Foundation for Basic Research supported the research.

http://news.rice.edu/2014/05/22/not-all-diamonds-are-forever-2/#sthash.42KRKrBn.dpuf

The magic of nuclear physics

The island of stabilityThe white crosses indicate isotopes with ‘magic’ proton or neutron numbers, for example lead-208 (²⁰⁸Pb), which consists of two magic numbers—82 protons and 126 neutrons—making it especially stable. While very heavy atoms with proton numbers in the hundreds are highly unstable, nuclear physicists predict that even some of the heaviest atoms have magic numbers and form an ‘island of stability’. The as-yet-undiscovered ‘doubly magic’ nucleus with 114 protons and 184 neutrons is presumed to be located on this island.© Yuri Oganessian

Except for hydrogen, all other chemical elements originated from violent nuclear processes in stars. Powerful machines known as accelerators allow physicists to study how these elements form and also to create new elements and atomic isotopes. As such, employing accelerators to study atoms not only improves our understanding of the Universe, but also produces synthetic isotopes useful in applications that include medical diagnostics.

Shortly after the Big Bang, the Universe consisted only of hydrogen—the smallest of the chemical elements—and its electrically uncharged partner, the neutron. The diversity of chemical elements that have come into existence since then resulted from nuclear processes taking place within stars. The element helium soon followed hydrogen, forming from the fusion of two hydrogen atoms, and then came carbon, which forms upon the fusion of three helium atoms. Heavier elements, including iron—the most energetically stable element—are created at the end of the life of a star. And elements heavier than iron appear only as a product of catastrophic stellar processes such as supernovae.

Nuclear physicists are using powerful accelerators to shed light on the mechanisms underpinning the Universe’s violent beginnings. Such accelerators can create new atoms with exotic, unstable nuclei that had vital roles in the creation of heavier elements during the explosion of stars.

The dawn of synthetic nuclear physics

Nuclear physics came of age as a research field in the 1920s, when nuclear particles, such as the proton, were discovered. Initial interest focused on understanding the properties of these particles and their involvement in the fundamental nuclear processes that were also discovered at the time: fission and fusion.

Yoshio Nishina was a pioneer of nuclear research in Japan. Following his education at the University of Tokyo and research stints in Europe in the 1920s, Nishina became a chief scientist at RIKEN in 1931. His mission was to establish a nuclear physics laboratory.

One of Nishina’s key research areas—and still the subject of ongoing research—was the study of different nuclear isotopes. Atomic cores consist of two types of particles: the proton, the number of which determines the identity of a chemical element; and the neutron, which is electrically neutral and determines an element’s isotopes. Hydrogen, for example, has one proton and three different isotopes; but heavier chemical elements, such as uranium, can have many different isotopes.

The stability of an isotope strongly depends on its mix of neutrons and protons, and this mix also influences how one element transforms into another during a nuclear reaction. A radioactive element, such as uranium, can follow many different reaction pathways to become a more stable isotope. Even the radioactive decay rates of an element can vary greatly between isotopes, ranging from billions of years to a fraction of a second.

An ideal way to study isotope properties is by synthesizing isotopes in the laboratory with the help of accelerators. These machines accelerate and smash atoms together. At sufficiently high energy, the collision of atoms creates new isotopes, or even new elements. Using the first Japanese accelerator—a cyclotron—Nishina discovered a new isotope of uranium, uranium-237, which differed from the form typically found in nature, uranium-238 (containing 92 protons and 146 neutrons).

Nishina built two cyclotrons at RIKEN but both were destroyed after the Second World War. New cyclotrons followed at RIKEN and the latest, the ninth cyclotron, is the world’s most powerful superconducting ring cyclotron (SRC). The SRC can accelerate heavy atomic nuclei to speeds of up to 70 per cent of the velocity of light and has a beam intensity that is 100 times greater than any other accelerator in the world.

The SRC is part of the nuclear research facility at RIKEN that now bears Nishina’s name—the RIKEN Nishina Center for Accelerator-Based Science (RNC). Inaugurated in 2006, the center hosts some 200 full-time scientists and collaborates with many international institutions.

Demystifying ‘magic numbers’

Experiments using the RNC’s latest generation of cyclotrons are pushing frontiers in nuclear research, particularly in the search for new isotopes with so-called ‘magic numbers’. These isotopes have a set number of protons or neutrons in their nuclei and are surprisingly stable in comparison to other isotopes. Calcium-54, for example, consists of 20 protons and 34 neutrons. This isotope is very unusual: typically, 34 neutrons do not constitute a ‘magic’ isotope; and, its magic properties were theoretically proposed shortly before their experimental discovery by researchers at RIKEN¹.

Explaining how elements as heavy as uranium could have resulted from stellar explosions hinges on understanding magic numbers and the stability of their associated elements.

The intense beams of atomic isotopes generated by RIKEN’s cyclotrons are useful in the search for very heavy elements. As heavy isotopes are accelerated and collide with each other, they re-assemble into different atoms, allowing the discovery of new chemical elements. Such research led to the discovery in 2012 of element 113 at the RNC², after more than nine years of thorough searching.

The synthesis of new chemical elements could lead to a new understanding of nuclear physics. Very heavy atoms are highly unstable and have a very short life, making it difficult to prove their existence following high-energy collisions. However, nuclear physicists predict that even some of the heaviest atoms have magic numbers and could survive for longer after a collision. They are predicted to form a so-called ‘island of stability’ within the short-lived heavy elements, which is typically the domain of elements with approximately 120 protons (Fig. 1).

Since existing accelerators lack the power necessary to reveal new elements within this island of stability, RIKEN plans to replace its synchrotrons 5 and 6 with a new superconducting accelerator. This powerful accelerator would produce beam intensities some 100 times greater than presently possible, securing RIKEN’s leadership in the study of heavy isotopes.

Nuclear research for better living

Atomic accelerators also have application beyond fundamental physics. In medicine, for example, certain radioactive isotopes are used as markers in diagnostic experiments because their distribution within the body can be precisely measured. Many of these isotopes are a by-product of uranium fission in nuclear reactors. As research reactors are being decommissioned, alternative methods to produce radioactive isotopes, including accelerators, have become increasingly important.

The use of high-energy ion beams further extends to agriculture. Naturally occurring radioactivity is one of the causes of mutations in plants. Radioactive rays can destroy DNA or cause small changes to the genome. Over many generations, such changes can accumulate and influence an organism’s evolution.

With ion beams, this process can be sped up by introducing mutations at a faster rate. Careful selection of useful mutations in each generation can produce better plants for cultivation. Developing rice that is tolerant to salty water, or even mixtures containing up to 50 per cent sea water, is one example. Controlled mutations could become increasingly important in ensuring a sufficient food supply for an ever-increasing global population.

After more than 80 years of nuclear physics research at RIKEN, much work is still required to better understand the properties of magic isotopes, the formation of heavy elements, the interplay of protons and neutrons in the nucleus and the internal structure of protons and neutrons. Attaining this understanding will require accelerators with even higher energies and intensities than are presently available. In that quest, the RNC will continue to play an important role. The new generation of accelerators being planned will help us to understand how the elementary particles in an atom interact with each other to form the world around us.

Source: http://www.rikenresearch.riken.jp/eng/perspectives/7757.html

Proton flow battery advances hydrogen power

Researchers have developed a concept hydrogen battery based simply on storing protons produced by splitting water.

The novel concept developed by researchers at RMIT University advances the potential for hydrogen to replace lithium as an energy source in battery-powered devices.

The proton flow battery concept eliminates the need for the production, storage and recovery of hydrogen gas, which currently limit the efficiency of conventional hydrogen-based electrical energy storage systems.

Lead researcher Associate Professor John Andrews, from RMIT's School of Aerospace, Mechanical and Manufacturing Engineering, said the novel concept combined the best aspects of hydrogen fuel cells and battery-based electrical power.

"As only an inflow of water is needed in charge mode - and air in discharge mode - we have called our new system the 'proton flow battery'," Associate Professor Andrews said.

"Powering batteries with protons has the potential to be a much more economical device than using lithium ions, which have to be produced from relatively scarce mineral, brine or clay resources.

"Hydrogen has great potential as a clean power source and this research advances the possibilities for its widespread use in a range of applications - from consumer electronic devices to large electricity grid storage and electric vehicles."

The concept integrates a metal hydride storage electrode into a reversible proton exchange membrane (PEM) fuel cell.

During charging, protons produced from splitting water are directly combined with electrons and metal particles in one electrode of a fuel cell, forming a solid-state metal hydride as the energy storage. To resupply electricity, this process is reversed.

Published in the International Journal of Hydrogen Energy (January, 2014), the research found that, in principle, the energy efficiency of the proton flow battery could be as high as that of a lithium ion battery, while storing more energy per unit mass and volume.

The published paper is the first to articulate and name the proton flow battery concept, and the first to include an experimental preliminary proof of concept.

"Our initial experimental results are an exciting indicator of the promise of the concept, but a lot more research and development will be necessary to take it through to practical commercial application," Associate Professor Andrews said.

Source: http://www.rmit.edu.au/browse;ID=6wpkdgvja13n;STATUS=A

Novel Material Stores Unusually Large Amounts of Hydrogen

Formation of iridium trihydride inside a diamond anvil cell
at a pressure of 100 GPa (gigapascals). The dark area
at the center is iridium trihydride, the yellow areas are
hydrogen. Credit: Thomas Scheler/University of Edinburgh

X-ray study reveals the formation of iridium trihydride at high pressure

An international team of researchers has synthesized a new material that stores an unusually large amount of hydrogen. Performing high-pressure X-ray studies at DESY’s PETRA III and other light sources, the scientists detected the formation of previously unobserved iridium hydride from hydrogen and metallic iridium at a pressure of 55 gigapascals (GPa), corresponding to approximately 550,000 times the Earth’s atmospheric pressure. The new material can store up to three times more hydrogen than most other metal hydrides, and its synthesis may contribute to the development of high-capacity hydrogen fuel cells in cars and other applications.

The researchers from Edinburgh University (UK), Oviedo University (Spain) and DESY also showed that iridium hydride has an unexpected structure that does not occur in other known hydrides. Since a material’s structure determines its mechanical and electronic properties, the new structure could potentially lead to the discovery of unprecedented properties. The study was published in the journal Physical Review Letters.

Hydrides as hydrogen-storing and superconducting materials

Chemical compounds of metals and hydrogen, called metal hydrides, are promising candidates for industrial hydrogen storage. Once hydrogen is released from the hydride, it can be further used to produce electricity in an environmentally friendly manner such as in fuel-cell-driven vehicles. “In a broader sense, our research aims at a deeper understanding of how metal hydrides form and how the hydrogen can subsequently be extracted again,” explains Thomas Scheler, the study’s first author and former PhD student of Eugene Gregoryanz’s group at Edinburgh University.

Another important application is the potential of metal hydrides to act as superconductors, i.e. materials that conduct electric currents without electrical resistance below a critical temperature. Such behavior has been observed or predicted for the hydrides of the noble metals palladium and platinum, for instance, and may also occur in the hydrides of other chemically related noble metals such as iridium. However, iridium hydride had never been observed prior to this study, and the researchers consequently set out to synthesize it for the first time.

Synthesis of previously unobserved iridium hydride

At PETRA III’s experimental station P02 for extreme conditions research, the scientists placed a piece of iridium inside a pressure cell, known as diamond anvil cell, which they subsequently loaded with hydrogen. The research team then compressed the sample using pressures as high as 125 GPa. Simultaneously, PETRA III’s intense and bundled X-rays traversed the sample and revealed its structural changes with varying pressure.

“At 55 GPa, we observed new X-ray signals, which do not stem from metallic iridium and which became stronger and stronger with increasing pressure,” explains Gregoryanz. The researchers concluded that iridium hydride had indeed formed as a new material phase.

However, the signals of the hydride were obscured by signals from residual iridium still present in the sample. “We therefore employed laser heating of the sample, which helped with the synthesis of iridium hydride,” says DESY researcher Zuzana Konôpková. In the laser-heated sample, the hydride formed much faster without leaving residual iridium behind.
“Laser heating was only one of several experimental conditions that were crucial for the success of our experiments,” adds Scheler. “The availability of fast detectors and high X-ray energies at P02 as well as PETRA III’s extremely focused X-ray beam contributed as well.”

Iridium hydride’s unusual structure

When analyzing their data, the researchers noticed that the newly formed iridium hydride phase had an unusual structure. Normally, the formation of metal hydrides involves the widening of a metal’s inner structure with hydrogen occupying the intermediate space between metal atoms. In these hydrides, hydrogen is not bound to the metal and typical hydrogen-to-metal ratios are close to 1.

In contrast, hydrogen becomes part of the crystal lattice in iridium hydride and forms a structure not observed in any other hydride. “Our X-ray data suggest that the iridium atoms occupy the corner positions of a cube while hydrogen is located at the center of the faces in the simple cubic lattice,” says Scheler. As a result, each iridium atom is surrounded by three hydrogen atoms, resulting in an iridium trihydride phase that can store up to three times more hydrogen than most other metal hydrides.

However, the identification of iridium trihydride from the X-ray data alone remains indirect because it can only be inferred from hydrogen-induced movements of the iridium atoms. “Hydrogen itself, which is the lightest of all elements, is almost invisible to X-rays and we cannot determine its positions directly,” explains Konôpková. Therefore, the researchers performed additional theoretical calculations, which supported the presence of a simple cubic lattice, albeit distorted, in iridium hydride.

Potential applications

The present study could potentially impact future developments of hydrogen-storage and hydrogen-fuel-cell technologies. Although iridium is too rare and too expensive for routine industrial applications, the synthesis of a novel iridium hydride material with a new structure and high hydrogen content may aid the search for other high-capacity metal hydrides.

Moreover, future research has yet to uncover iridium hydride’s mechanical and electronic properties. “Our experiments revealed the material’s structure,” Gregoryanz says. “This information can now be used for theoretical predictions of its properties, including superconductivity.”


Original Publication
“High-pressure synthesis and characterization of iridium trihydride”; T. Scheler, M. Marqués, Z. Konôpková, C. L. Guillaume, R. T. Howie, and E. Gregoryanz; Phys. Rev. Lett. (2013); DOI:10.1103/PhysRevLett.111.215503


Source: http://www.desy.de/information__services/press/pressreleases/@@news-view?id=6701&lang=eng

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