Increased attention is being focused on metal-organic frameworks as candidates for hydrogen storage materials. This is a result of their many favorable attributes, such as high porosity, reproducible and facile syntheses, amenability to scale-up, and chemical modification for targeting desired properties. A discussion of several strategies aimed at improving hydrogen uptake in these materials is presented. These strategies include the optimization of pore size and adsorption energy by linker modification, impregnation, catenation, and the inclusion of open metal sites and lighter metals.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.200462786

Strategies for hydrogen storage in metal--organic frameworks.

Beyond Oil and Gas: The Methanol Economy

Each generation is confronted with new challenges and new opportunities. In a restricted system like the Earth, however, opportunities discovered and exploited by a generation can cause challenges to the subsequent ones. Fossil fuels have offered astounding opportunities during the 20th century in the rich countries of the western world, but now mankind has to face the challenges arising from fossil-fuel exploitation. The proven reserves of fossil fuels are progressively decreasing, and their continued use produces harmful effects, such as pollution that threatens human health and greenhouse gases associated with global warming. Currently the world&s growing thirst for oil amounts to almost 1000 barrels a second, which means about 2 liters a day per each person living on the Earth (Figure 1). The current global energy consumption is equivalent to 13 terawatts (TW), that is, a steady 13 trillion watts of power demand. How long can we keep running this road?

The Future of Energy Supply: Challenges and Opportunities

If the fuel cell is to become the modern steam engine, basic research must provide breakthroughs in understanding, materials, and design to make a hydrogen-based energy system a vibrant and competitive force

https://physicstoday.scitation.org/doi/pdf/10.1063/1.1878333

The Hydrogen Economy

Fifty years of developments in nuclear magnetic resonance (NMR) have resulted in an unrivaled degree of control of the dynamics of coupled two-level quantum systems. This coherent control of nuclear spin dynamics has recently been taken to a new level, motivated by the interest in quantum information processing. NMR has been the workhorse for the experimental implementation of quantum protocols, allowing exquisite control of systems up to seven qubits in size. This article surveys and summarizes a broad variety of pulse control and tomographic techniques which have been developed for, and used in, NMR quantum computation. Many of these will be useful in other quantum systems now being considered for the implementation of quantum information processing tasks.

/pdf/nmr-techniques-for-quantum-control-and-computation-1xe8uegjql.pdf

NMR techniques for quantum control and computation

### Contents 

#### News

The Hydrogen Backlash

The Carbon Conundrum

Choosing a CO2 Separation Techology

Fire and ICE: Revving Up for H2

Will the Future Dawn in the North?

Can the Developing World Skip Petroleum?

#### Review

Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Techologies S. Pacala and R. Socolow

#### Viewpoints

Sustainable Hydrogen Production J. A. Turner

Hybrid Cars Now, Fuel Cell Cars Later N. Demirdoven and J. Deutch

See related Editorial. 

S een up close, hydrogen looks like a recipe for success. Small and simple—one proton and one electron in its most common atomic form—hydrogen was the first element to assemble as the universe cooled off after the big bang, and it is still the most widespread. It accounts for 90% of the atoms in the universe, two-thirds of the atoms in water, and a fair proportion of the atoms in living organisms and their geologic legacy, fossil fuels.

To scientists and engineers, those atoms offer both promise and frustration. Highly electronegative, they are eager to bond, and they release energy generously when they do. That makes them potentially useful, if you can find them. On Earth, however, unattached hydrogen is vanishingly rare. It must be liberated by breaking chemical bonds, which requires energy. Once released, the atoms pair up into two-atom molecules, whose dumbbell-shaped electron clouds are so well balanced that fleeting charge differences can pull them into a liquid only at a frigid −252.89° Celsius, 20 kelvin above absolute zero. The result, at normal human-scale temperatures, is an invisible gas: light, jittery, and slippery; hard to store, transport, liquefy, and handle safely; and capable of releasing only as much energy as human beings first pump into it. All of which indicates that using hydrogen as a common currency for an energy economy will be far from simple. The papers and News stories in this special section explore some of its many facets.

Consider hydrogen's green image. As a manufactured product, hydrogen is only as clean or dirty as the processes that produce it in the first place. Turner (p. 972) describes various options for large-scale hydrogen production in his Viewpoint. Furthermore, as News writer Service points out (p. [958][1]), production is just one of many technologies that must mature and mesh for hydrogen power to become a reality, a fact that leads many experts to urge policymakers to cast as wide a net as possible.

In some places, the transition to hydrogen may be relatively straightforward. For her News story (p. 966), Vogel visited Iceland, whose abundant natural energy resources have given it a clear head start. Elsewhere, though, various technological detours and bridges may lie ahead. The Viewpoint by Demirdoven and Deutch (p. [974][2]) and Cho's News story (p. 964) describe different intermediate technologies that may shape the next generation of automobiles. Meanwhile, the fires of the fossil fuel-based “carbon economy” seem sure to burn intensely for at least another half-century or so [see the Editorial by Kennedy (p. [917][3])]. Service's News story on carbon sequestration (p. 962) and Pacala and Socolow's Review (p. [968][4]) explore strategies—including using hydrogen—for mitigating their effects.

Two generations down the line, the world may end up with a hydrogen economy completely different from the one it expected to develop. Perhaps the intermediate steps on the road to hydrogen will turn out to be the destination. The title we chose for this issue—Toward a Hydrogen Economy— reflects that basic uncertainty and the complexity of what is sure to be a long, scientifically engaging journey.

 [1]: /lookup/doi/10.1126/science.305.5686.958
 [2]: /lookup/doi/10.1126/science.1093965
 [3]: /lookup/doi/10.1126/science.305.5686.917
 [4]: /lookup/doi/10.1126/science.1100103

https://science.sciencemag.org/content/sci/305/5686/957.full.pdf

Not So Simple

I t has been almost a decade since Science devoted a special issue to nanotechnology, and even the title of that issue (“Engineering a Small World,” 29 November 1991) reflected the scarcity of actual working nanotechnology at the time. Like others, we pointed to Richard Feynman's 1959 lecture “There's Plenty of Room at the Bottom” to provide some justification for why nanotechnology would emerge as a discipline from early efforts in atomic imaging and nanofabrication. The present special issue can reflect only some of the current trends in this rapidly expanding area.

In the News section (p. [1524][1]), writer Robert Service surveys nanotechnology's near-term prospects: the role of funding infusions, such as the U.S. National Nanotechnology Initiative and its European and Japanese counterparts, and the real promise of new materials and devices. He also takes a jaundiced view of some of the prophecies of boom and doom made by the field's boosters and critics. Meanwhile, amid the nanohype, researchers are forging ahead on several exciting fronts. Short profiles of five research groups aim to give some sense of the range of work now under way throughout the world, without pretending to be exemplary or comprehensive.

Three Reviews focus mainly on nanomechanics, which builds on (or perhaps below) the advances that have been witnessed in microelectromechanical systems (MEMS). Some of the MEMS advances are stunning enough in themselves. For example, micromirror arrays developed at Lucent can be tilted to steer light beams from one channel to another for routing fiber-optic signals. Craighead (p. 1532) discusses how even smaller nanoelectromechanical systems (NEMS) will have higher resonant frequencies and lower masses that will facilitate several new applications. Complex freestanding structures can now be fabricated and used, for example, to sense adsorbed mass, or as radiofrequency devices in the 1- to 10-megahertz range. These structures can be set in motion with applied electrical fields or with piezoelectric driving of supports. Sacrificial fabrication can also be used to create channel structures of varying complexity for fluidic systems and molecular separations.

Although many nanomechanical systems have been fabricated in hard, inorganic materials, polymeric materials are now being exploited for micromechanics and nanofabrication. Quake and Scherer (p. 1536) point out that soft polymeric systems have some natural advantages in fluidics, in that the pumps and valves used in “labs on chips” normally need soft seals and seats to work seamlessly. Soft materials can be molded and assembled in layers to leave complex fluidic networks or to create diffractive optical elements.

Producing complex movements in devices is still a challenge, especially for potential applications in biological systems. Jager et al . (p. [1540][2]) review how conducting polymers could solve some problems. Bimorph actuators produce movement through the differential expansion of a layer of one material pressed against a dissimilar material. In polypyrrole-gold bimorphs, an applied electrical field creates a volume change in the polymer that produces motion (and can even peel a newly fabricated device from its supports). These actuators could find use as cell manipulators or as microrobots, all within a cellular milieu.

Nanotechnologists are now creating devices the performance of which will be tested and improved by new experiments and by more accurate theoretical treatments. Although there is still plenty of “room at the bottom,” researchers have done much more than stake out claim sites: They are digging in.

 [1]: /lookup/doi/10.1126/science.290.5496.1524a
 [2]: /lookup/doi/10.1126/science.290.5496.1540

Taking the Initiative

The search for fresh water—to drink, to bathe in, to irrigate crops—is a problem as old as civilization. Across the ages, cities have thrived where the supply is abundant and collapsed in the face of drought. Remarkably, despite the technological progress characterizing the modern era and the fact that most of Earth's surface is covered by oceans, the availability of fresh water remains a pressing concern throughout the world. In this special section, we highlight some of the diverse contemporary scientific and engineering projects dedicated to obtaining and maintaining freshwater resources.

We begin with a Review by Oki and Kanae (p. [1068][1]) on current understanding of the natural water cycle, and in particular the role of climate change in determining freshwater abundance. A related Review in this week's issue by Peterson et al. (p. [1061][2]) focuses more specifically on the Arctic region. Even in regions of abundant fresh water, contamination and disease can disable local communities. Schwarzenbach et al. (p. 1072) present a Review of chemical contamination problems. They note that although effective strategies have been implemented to remediate large-scale pollutants, a substantial amount of work looms ahead to cope with the numerous different compounds that are present at lower concentrations. In particular, collaborative efforts among chemists, geologists, and toxicologists are increasingly important to characterize the complex interactions of these compounds in the environment. Fenwick's Perspective (p. [1077][3]) follows with an optimistic view of the ongoing effort to combat waterborne diseases accompanying development programs in Africa, highlighting the role of drug donations from the pharmaceutical industry.

It may seem strange to many people that modern societies cannot fulfill their water needs simply by purifying ocean water. Tal offers a Perspective (p. [1081][4]) about the multipronged strategy—of which desalination is an increasing component—that Israel has adopted to procure fresh water in an arid environment. Bohannon highlights the resource issues specific to the Gaza region in an accompanying News story (p. 1085). Another News story by Service (p. 1088) focuses more broadly on current developments in desalination technology. Additionally, two related stories by Stone and Jia (p. 1034) and Bagla (p. 1036) in the magazine's News Focus section describe large-scale engineering efforts in China and India to supply fresh water across extensive geographical areas.

Inevitably, water resource management is a political problem as well as a scientific one (see the related Editorial by Kennedy and Hans on on p. 1019). It is clear that ensuring adequate supply will necessitate continuing collaborations across a great range of disciplines. The approaches described here offer a measure of hope for the future.

 [1]: /lookup/doi/10.1126/science.1128845
 [2]: /lookup/doi/10.1126/science.1122593
 [3]: /lookup/doi/10.1126/science.1127184
 [4]: /lookup/doi/10.1126/science.1126011

http://science.sciencemag.org/content/sci/313/5790/1067.full.pdf

A Thirsty World

T hink about coherent systems, and odds are that the first example to cross your mind will be a laser: a device that emits monochromatic light homogenized so that the photons propagate with their phase in lockstep. The well-defined wavefront of the laser beam makes it possible to focus the light to spot sizes on the order of the wavelength of the light without the aberrations usually observed with the random wavefront of incoherent light. The well-defined phase also provides a medium capable of storing and carrying information more efficiently than the intensity-modulated signals typical of incoherent sources. These useful properties of coherence have made lasers almost ubiquitous in science and technology, and that “almost” is shrinking by the day.

In general, coherence comes about when individual components of a system act in unison, either in phase or with a fixed phase difference between the components. More accurately, the system can be described as a local oscillator whose wavefunction contains phase and amplitude terms that define how the system evolves in space and time. The collection of articles in this special issue describes the recent progress and developments of coherence in optical, atomic, and semiconductor systems, and how manipulating that coherence may lead to useful applications and to experimental systems for studying the elements of quantum mechanics.

Since the first demonstration of the laser in 1960, progress in improving the extent of the spatial and temporal coherence of laser light has been steady. Psaltis (p. [1359][1]) reviews developments in optical coherence and highlights examples of the key role coherence has played in applications in imaging, communication, and information processing and storage. Just as lasers revolutionized the field of optics, many researchers have high expectations that other coherent systems will provide similar advances in their respective fields. Exploiting the fundamental tenet of quantum mechanics whereby particles have the properties of waves, Kasevich (p. [1363][2]) reviews coherence effects in collections of cold atoms and describes how the shorter wavelengths associated with atoms and the recent accessibility of bright coherent atom sources may lead to precision measurements beyond anything currently available to the optical regime. For integration and potential applications, the ideal system would be based in semiconductors. Snoke (p. [1368][3]) describes recent work toward the formation of coherent semiconductor systems.

With coherence in the atomic and optical systems generally comprising macroscopic systems, Mabuchi and Doherty (p. [1372][4]) look at coherence effects at the quantum limit, probing the interactions between single atoms and single photons confined to high-quality cavities. In addition to providing invaluable information on the limits of quantum mechanics, such effects might also shed new light on information science in the quantum regime.

The section opens with a pair of News features by staff writers Charles Seife and Robert Service. Seife presents a smattering of common applications that scientists and engineers have found for coherent waves, whether in light, sound, or subatomic particles. Service describes efforts to create new coherent sources of the type of radiation known as “hard” x-rays—a long-sought range of high frequencies with enormous potential for scientific research, technology, and medicine.

 [1]: /lookup/doi/10.1126/science.1078823
 [2]: /lookup/doi/10.1126/science.1079430
 [3]: /lookup/doi/10.1126/science.1078082
 [4]: /lookup/doi/10.1126/science.1078446

Ruling the Waves

W hen a child sees a bird flying past or the fluttering wings of a butterfly, does it inspire thoughts about how to build airplanes? Or does it simply convey the idea that flight is possible? We are immersed in the natural world, so it is not surprising that it inspires the design of engineered structures, or that we would like to probe this world further to learn all its secrets.

The four Reviews in this issue highlight this two-sided relationship as it applies to the development of new materials. From nature we have learned how soft brittle materials like chalk are made tougher through composite structures. Some of these same design principles have been applied to the much wider range of building blocks available to the engineer (Mayer, p. [1144][1]). Advances in materials processing, particularly in the area of polymers, are also making it possible to fabricate systems with advanced optical capabilities (Lee and Szema, p. [1148][2]), inspired by living eyes and by creatures that we have recently learned can see even though they appear to lack eyes.

On the flip side, new material systems have been designed to probe and manipulate biological interactions. Two Reviews examine highly complementary questions. The first summarizes what has been learned about the interactions of cells with surfaces that possess features at different size scales (Stevens and George, p. [1135][3]). The second explores how cells sense the stiffness and strength of underlying substrates (Discher et al. , p. [1139][4]). From both we see how new materials are being used to probe cell cycles and interactions and to coax cells to grow in desired ways.

The drive toward medical applications is helping push research at the biology/materials interface. On the small scale, nanotechnology is being explored to find new methods for cancer detection and treatment (Service, p. [1132][5]). A Science of Aging Knowledge Environment (SAGE KE) feature story by Davenport explores the progress and problems of engineering tissues in the lab. This endeavor could revolutionize medicine, potentially reversing illnesses such as diabetes, heart disease, and liver failure.

As research at the interface between materials and biology increasingly overlaps, we look forward to seeing how each continues to inspire developments in the other.

 [1]: /lookup/doi/10.1126/science.1116994
 [2]: /lookup/doi/10.1126/science.1115248
 [3]: /lookup/doi/10.1126/science.1106587
 [4]: /lookup/doi/10.1126/science.1116995
 [5]: /lookup/doi/10.1126/science.310.5751.1132

https://science.sciencemag.org/content/sci/310/5751/1131.full.pdf

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