26 Feb 2016
Splitting water is a two-step process, and in a new study, researchers have performed one of these steps (reduction) with 100% efficiency. The results shatter the previous record of 60% for hydrogen production with visible light, and emphasize that future research should focus on the other step (oxidation) in order to realize practical overall water splitting. The main application of splitting water into its components of oxygen and hydrogen is that the hydrogen can then be used to deliver energy to fuel cells for powering vehicles and electronic devices.
The researchers, Philip Kalisman, Yifat Nakibli, and Lilac Amirav at the Technion-Israel Institute of Technology in Haifa, Israel, have published a paper on the perfect efficiency for the water reduction half-reaction in a recent issue of Nano Letters.
“I strongly believe that the search for clean and renewable energy sources is crucial,” Amirav toldPhys.org. “With the looming energy crisis on one hand, and environmental aspects, mainly global warming, on the other, I think this is our duty to try and amend the problem for the next generation.
“Our work shows that it is possible to obtain a perfect 100% photon-to-hydrogen production efficiency, under visible light illumination, for the photocatalytic water splitting reduction half-reaction. These results shatter the previous benchmarks for all systems, and leave little to no room for improvement for this particular half-reaction. With a stable system and a turnover frequency of 360,000 moles of hydrogen per hour per mole of catalyst, the potential here is real.”
When an H2O molecule splits apart, the three atoms don’t simply separate from each other. The full reaction requires two H2O molecules to begin with, and then proceeds by two separate half-reactions. In the oxidation half-reaction, four individual hydrogen atoms are produced along with an O2 molecule (which is discarded). In the reduction half-reaction, the four hydrogen atoms are paired up into two H2 molecules by adding electrons, which produces the useful form of hydrogen: H2 gas.
In the new study, the researchers showed that the reduction half-reaction can be achieved with perfect efficiency on specially designed 50-nm-long nanorods placed in a water-based solution under visible light illumination. The light supplies the energy required to drive the reaction forward, with the nanorods acting as photocatalysts by absorbing the photons and in turn releasing electrons needed for the reaction.
The 100% efficiency refers to the photon-to-hydrogen conversion efficiency, and it means that virtually all of the photons that reach the photocatalyst generate an electron, and every two electrons produce one H2 molecule. At 100% yield, the half-reaction produces about 100 H2 molecules per second (or one every 10 milliseconds) on each nanorod, and a typical sample contains about 600 trillion nanorods.
One of the keys to achieving the perfect efficiency was identifying the bottleneck of the process, which was the need to quickly separate the electrons and holes (the vacant places in the semiconductor left after the electrons leave), and remove the holes from the photocatalyst. To improve the charge separation, the researchers redesigned the nanorods to have just one platinum catalyst instead of two. The researchers found that the efficiency increased from 58.5% with two platinum catalysts to 100% with only one.
Going forward, the researchers plan to further improve the system. The current demonstration requires a very high pH, but such strong basic conditions are not always ideal in practice. Another concern is that the cadmium sulfide (CdS) used in the nanorod becomes corroded under prolonged light exposure in pure water. The researchers are already addressing these challenges with the goal to realize practical solar-to-fuel technology in the future.
“We hope to implement our design rules, experience and accumulated insights for the construction of a system capable of overall water splitting and genuine solar-to-fuel energy conversion,” Amirav said.
“The photocatalytic hydrogen generation presented here is not yet genuine solar-to-fuel energy conversion, as hole scavengers are still required. CdS is unfortunately not suitable for overall water splitting since prolonged irradiation of its suspensions leads to photocorrosion. We have recently demonstrated some breakthrough on this direction as well. The addition of a second co-catalyst, such as IrO2 or Ru, which can scavenge the holes from the semiconductor and mediate their transfer to water, affords CdS-based structures the desired photochemical stability. I believe this is an important milestone.”
More information: Philip Kalisman, et al. “Perfect Photon-to-Hydrogen Conversion Efficiency.” Nano Letters. DOI: 10.1021/acs.nanolett.5b04813
Genesis Nanotechnology, Inc. ~ “Great Things from Small Things”
11 Jan 2016
One day soon, a start-up somewhere – possibly in Israel – will come up with a system to manufacture precisely-formed nanoparticles that, when joined with other particles, will change the way electronics, clothing, computers and almost everything else can be used.
One day, but not yet, according to Richard Robinson, a visiting scholar at Hebrew University’s Institute of Chemistry. Based at Cornell University, Robinson is in Israel to do research in the area of nanotechnology, where scientists manipulate very tiny atomic particles to create surprising and unique effects that are far different than anything observed in physics until now.
“We know a lot about the principles of nanotechnology now, but there is still a lot to do at the research stage, which is one reason why nanotech hasn’t yet made its presence known to a large extent in the greater society,” Robinson told The Times of Israel . “Nevertheless nanotechnology is already having a major impact in certain applications, like lighting.”
In fact, one of the first commercially successful nano-based products to emerge came from the very Hebrew University lab where Robinson is doing research. Using unique quantum materials, Qlight developed semiconductor nanocrystals that can emit and provide extra brilliance to light, such as enhancing the color of display screens.
Last year the company was acquired by Merck, the German chemical and technology company. Qlight’s technology, said Merck CEO Karl-Ludwig Kley, is “far superior to anything currently on the market, and that will help us retain and expand our position as market leader.”
There will likely be many more such announcements and pronouncements in the future, and many of them are set to be based on technology developed in Israel, said Robinson. “Israel is ahead of the curve on nanotechnology research,” said Robinson.
And there’s plenty more research that needs to be done. “Over the past 20 years or so we have essentially been rewriting the textbooks on physics, because the laws that apply to ‘normal’ particles do not apply to nano-sized particles,” he added.
In other words, certain things happen when five nanometer-sized particles are combined with six nanometer-sized particles. “We’re still observing, categorizing and recording the reactions of these particles sizes with each other and others, in different kinds of materials, and their combinations,” said Robinson.
At home in Cornell, Robinson does a lot of work in materials, controlling their size, shape, composition and surfaces, and assembling the resulting building blocks into functional architectures. Among the applications Robinson’s lab is targeting are new materials for printable electronics and electrocatalysis. His group is also pioneering a new method to probe phonon transport in nanostructures.
On practical example of how nanotech will affect energy is to allow for a much more efficient production method for solar energy. In a solar energy system, the sun’s rays hit photovaltic cells that capture the energy and convert it into direct current (DC) electricity, which is then converted to alternating current (AC), for use in home electric systems or for transfer to the grid. But it turns out that the PV cells being used don’t capture as much of the sun’s rays as they can because of fluctuations in the wavelength of the rays due to time of day or time of year; only about 25% of the rays are captured on average.
PV cells are designed to capture the sun at its strongest in midday, but they can’t capture rays at other times of the day. Using nanomaterials that respond to specific wavelengths PV technology can be much more efficient, tripling the usable “bounty” from the sun, said Robinson.
Eventually, said Robinson, nanotech will live up to the hype that has surrounded it for the past two decades.
“The manufacturing process for nanoparticles is not yet precise. In order for nanotech to be fully commercialized, we need a way to produced nanoparticles on a mass basis with the right size needed for each application,” Robinson said. “We’re not there yet, but it’s on the way – and with all the nanotech research here in Israel, it may just be an Israeli start-up that develops it.”
This post is part of a series examining the connections between nanotechnology and the top 10 trends facing the world, as described in the Outlook on the Global Agenda 2015. All authors are members of the Global Agenda Council on Nanotechnology.
In the 2015 World Economic Forum’s Global Risks Report survey participants ranked Water Crises as the biggest of all risks, higher than Weapons of Mass Destruction, Interstate Conflict and the Spread of Infectious Diseases (pandemics). Our dependence on the availability of fresh water is well documented, and the United Nations World Water Development Report 2015 highlights a 40% global shortfall between forecast water demand and available supply within the next fifteen years. Agriculture accounts for much of the demand, up to 90% in most of the world’s least-developed countries, and there is a clear relationship between water availability, health, food production and the potential for civil unrest or interstate conflict.
The looming crisis is not limited to water for drinking or agriculture. Heavy metals from urban pollution are finding their way into the aquatic ecosystem, as are drug residues and nitrates from fertilizer use that can result in massive algal blooms. To date, there has been little to stop this accretion of pollutants and in closed systems such as lakes these pollutants are being concentrated with unknown long term effects.
While current solutions such as reverse osmosis exist, and are widely used in the water desalination of seawater, the water they produce is expensive. This is because high pressures are required to force the waster through a membrane and maintaining this pressure requires around 2kWh for every cubic meter of water. While this is less of an issue for countries with cheap energy, it puts the technology beyond the reach of most of the world’s population.
Any new solution for water issues needs to be able to demonstrate precise control over pore sizes, be highly resistant to fouling and significantly reduce energy use, a mere 10% won’t make a difference. Nanotechnology has long been seen as a potential solution. Our ability to manipulate matter on the scale of a few atoms allows scientists to work at the same scale as mot of the materials that need to be removed from water — salts, metal ions, emulsified oil droplets or nitrates. In theory then it should be a simple matter of creating a structure with the correct size nanoscale pores and building a better filter.
Ten years ago, following discussions with former Israeli Prime Minister Shimon Peres, I organised a conference in Amsterdam called Nanowater to look at how nanotechnology could address global water issues. While the meeting raised many interesting points, and many companies proposed potential solutions, there was little subsequent progress.
Rather than a simple mix of one or two contaminants, most real world water can contain hundreds of different materials, and pollutants like heavy metals may be in the form of metal ions that can be removed, but are equally likely to be bound to other larger pieces of organic matter which cannot be simply filtered through nanopores. In fact the biggest obstacle to using nanotechnology in water treatment is the simple fact that small holes are easily blocked, and susceptibility to fouling means that
Fortunately some recent developments in the ‘wonder material’ graphene may change the economics of water. One of the major challenges in the commercialisation of graphene is the ability to create large areas of defect-free material that would be suitable for displays or electronics, and this is a major research topic in Europe where the European Commission is funding graphene research to the tune of a billion euros. Simultaneously there are vast efforts inside organisations such as Samsung and IBM. While defects are not wanted for electronic applications, recent research by Nobel Prize winner Andrei Geim and Rahul Nair has indicated that in graphene oxide they result in a barrier that is highly impermeable to everything except water vapour. However, precisely controlling the pore size can be difficult.
Another approach taken by researchers at MIT involves bombarding graphene sheets with beams of gallium ions to create weak spots and then etching them to create more precisely controlled pore sizes. A similar approach to water transport through defects has been taken by researchers at Penn State University.
While all of the above show that graphene has prospects for use as a filter medium, what about the usual limiting issue, membrane fouling? Fortunately another property of graphene is that it can be hydrophilic, it repels water, and protein absorption has been reported to have been reduced by over 70% in bioreactor tests. Many other groups are working on the use of graphene oxide and graphene nanoplatelets as an anti-fouling coating.
While the graphene applications discussed so far address one or two of the issues, it seems that thin films of graphene oxide may be able to provide the whole solution. Miao Yu and his team at the University of South Carolina have fabricated membranes that deliver very high flux and do not foul. Fabrication is handled by adding a thin layer of graphene to an existing membrane, as distinct from creating a membrane out of graphene, something which is far harder to do and almost impossible to scale up.
Getting a high flux is crucial to desalination applications where up to 50% of water costs are caused by pressurising water for transmission through a membrane. Performance tests reveal around 100% membrane recovery simply by surface water flushing and pure water flux rates (the amount of water that the membrane transmits) are two orders of magnitude higher than conventional membranes. This is the result of the spacing between the graphene plates that allows the passage of water molecules via nanoscale capillary action but not contaminants.
Non-fouling is crucial for all applications, and especially in oil/water separation as most of what is pumped out of oil wells is water mixed with a little oil.
According to G2O Water, the UK company commercialising Yu’s technology, the increased flux rates are expected to translate directly into energy savings of up to 90% for seawater desalination. Energy savings on that scale have the potential to change the economics of desalination with smaller plants powered by renewable energy and addressing community needs replacing the power hungry desalination behemoths currently under construction such as the Carlsbad Project. This opens the possibility of low-cost water in areas of the world where desalination is currently too expensive or there is insufficient demand to justify large scale infrastructure.
While more work is required to build a robust and cost-effective filtration system, the new ability to align sheets of graphene so that water but nothing else is transmitted may be the simple game-changer that allows the world to finally address the growing water crisis.
Author: Tim Harper is Chief Executive Officer of G2O Water.
Image: The colors of Fall can be seen reflected in a waterfall along the Blackberry River in Canaan, Connecticut REUTERS/Jessica Rinaldi