In 2009, the U.S Department of Energy (DOE) proposed on-board hydrogen storage system performance targets that have become widely accepted. So far, researchers haven’t been able to successfully demonstrate a material that is capable of simultaneously meeting all of the requirements and criteria set out by the DOE.

A European research team has now reported on a new concept for hydrogen storage using nanoconfined reversible chemical reactions. They demonstrate that nanoconfined hydride has a significant hydrogen storage potential. Research at the Nano Energy-Materials research group at Interdisciplinary Nanoscience Center (iNANO) at Aarhus University in Denmark, led by Flemming Besenbacher and Torben R. Jensen, focuses on the utilization of nanoporous materials as scaffolds for preparation and confinement of nanosized metal hydrides. This bottom-up approach limits the particle size of the hydride to the average pore size of the scaffold material, which allows for the direct production of smaller particles than obtainable mechanically. Furthermore, particle growth and agglomeration may be hindered by the compartmentalization of the nanoparticles within the scaffold material. Nanoconfinement may also mediate improved re-hydrogenation properties of complex metal hydrides.

“Nanoconfinement of metal hydrides is receiving increasing interest in the field of hydrogen storage and this principle has already been applied to a number of promising hydrogen storage materials,” the researchers explain. In their work published in the online edition of ACS Nano, the team introduces an alternative bottom-up approach where nanoparticles of hydrides are synthesized or melt infiltrated in a nanoporous inert scaffold material, which has several advantages:

1. increased surface area of the reactants
2. nanoscale diffusion distances
3. increased number of grain boundaries, which facilitate release and uptake of hydrogen and enhance reaction kinetics

Lithium borohydride (LiBH4) and magnesium hydride (MgH2) nanoparticles are embedded in a nanoporous carbon aerogel scaffold with a maximum pore size of 21 nm and react during release of hydrogen and form magnesium diboride. The hydrogen desorption kinetics is significantly improved compared to bulk conditions, and the nanoconfined system has a high degree of reversibility and stability and possibly also improved thermodynamic properties. The purpose of this work was to further develop the concept of nanoconfinement by investigating a system of higher complexity. Lithium borohydride and magnesium hydride  have been studied intensively in the past due to their high theoretical hydrogen densities.
“However” explains Nielsen, “the use of lithium borohydride as a solid-state hydrogen storage material is hampered by its unfavorable high thermal stability; that is, release of hydrogen takes place at temperatures above 400°C and, importantly, uptake of hydrogen only occurs under extreme conditions. Similarly, application of the abundant and cheap metal magnesium is also impeded by unfavorable thermodynamic properties.” Jensen adds that, “fortunately, both the kinetic and thermodynamic properties of potential hydrogen storage materials can be significantly improved by combining exothermic and endothermic chemical reactions. A more favorable total enthalpy change may be obtained by the introduction of a new dehydrogenated state which may facilitate hydrogenation. This concept is referred to as reactive hydride composites (RHC), and it helps to preserve a high gravimetric hydrogen storage capacity.”

By studying the effect of nanoconfinement on the hydrogen storage properties of a lithium-borohydride/magnesium-hydride system, the team (which included scientists from the Institute of Material Research in Germany and Lund University in Sweden) found that it possesses a high degree of reversible stability and improved hydrogen desorption kinetics as compared to the bulk. Furthermore, the concept of nanoconfined chemical reactions may develop to become an important tool within the emerging area of nanotechnology for the improvement of the properties and reaction yield of a wide range of chemical reactions. This new scheme of nanoconfined chemistry may have a wide range of interesting applications in the future, for example, within the merging area of chemical storage of renewable energy.

Source: original article

Despite the use of materials for the most inexpensive, easy to manufacture and flexible, large-scale commercialization of these batteries confronts two major obstacles. The electrolyte is very corrosive, causing a deficiency in sustainability. It is also very colorful, preventing light from entering and effectively limiting the photo-voltage of 0.7 volts. Moreover, platinum is an expensive material, non-transparent and rare.

Benoit Marsan and his team have been working for years to develop an electrochemical solar cell. With the help of Professor Livain Breau, also from the Department of Chemistry at University of Quebec at Montreal (UQAM), researchers have developed an electrolyte consisting of new molecules, whose concentration could be increased. The gel formed is transparent, non-corrosive and can increase the photo-voltage. Therefore, the battery is more stable and better performance. The platinum cathode has also been replaced with cobalt sulfide. This material is much less expensive than platinum. It is also more efficient, more stable and easier to produce in the laboratory.

The work conducted by Professor Benoit Marsan has solved two problems that hindered growth of the solar cell industry for 20 years: increasing the efficiency of solar cells while reducing their fabrication cost. The scientific community was excited about this work published in the Journal of American Chemical Society. Several researchers contend that the work of Professor Marsan represents a breakthrough towards the production of efficient and affordable solar cells.

Source: original article

Some of the greatest challenges of modern society are related to energy consumption, dissipation, and waste. Among these, present and future technologies based on nanoscale materials and devices hold great potential for improved energy conservation, conversion, or harvesting. A prominent example is that of integrated electronics, where power dissipation issues have recently become one of its greatest challenges. Power dissipation limits the performance of electronics from handheld devices (~10^–3 W) to massive data centres (~10⁹ W), all primarily based on silicon micro/nanotechnology.

Importantly, the figures for data centre energy consumption have doubled in five recent years, with waste heat requiring drastic cooling solutions. Such challenges are also evident at the individual micro-processor (CPU) level, where the race to increase operating frequency beyond a few GHz recently stopped when typical dissipated power reached 100 W/cm² (see figure below). Such electronic power and thermal challenges have negative impacts in areas from massive database servers to new applications like wearable devices, medical instrumentation, or portable electronics. In the latter situations, there is a basic trade-off between the available functionality and the need to carry heavy batteries to power it.

Despite tremendous progress over the past three decades, modern silicon transistors are still over three orders of magnitude (>1000×) more energy inefficient than fundamental physical limits. These limits have been estimated as approximately 3kBT ≈ 10^–20 J at room temperature for a binary switch with a single electron and energy level separation kBT, where kB is the Boltzmann constant and T is the absolute temperature. In the average modern microprocessor, the dissipated power is due, in approximately equal parts, to both leakage (or sleep) power and active (dynamic) switching power.

Power dissipation is compounded at the system level, where each CPU Watt demands approximately 1.5× more for the supply, PC board, and case cooling. Such power misuse is even more evident in systems built on otherwise power-efficient processors, e.g., in the case of the Intel Atom N270 (2.5 W power use) which is typically paired up with the Intel 945GSE chipset (11.8 W power use). At the other extreme, data centres require 50%–100% additional energy for cooling, which is now the most important factor limiting their performance, not the hardware itself.

Such energy challenges for the electronics infrastructure stem not only from the power supply side which calls for new energy sources, efficient batteries, or thermoelectrics, but also from the demand side, i.e., the need for more energy-efficient computing devices. Breakthroughs in our understanding and improvement of energy efficiency in nanoelectronics will have a global effect, impacting our energy supplies, budgets, and the environment.

On a broader scale, just over half the man-made energy in the world is wasted as heat (10¹³ W), from power plants and factories to car engines and the power bricks on our laptops. Efficiently reclaiming even a small percentage of such wasted heat would itself nearly satisfy the electricity needs of our planet. The fundamental issues at hand are, in fact, a two-sided problem: on one side, there is a significant need for low-energy computing devices, which is perhaps the biggest challenge in micro/nanoelectronics today. On the other side there is the challenge of waste heat dissipation, guiding, or conversion into useful electricity. On a large scale, a transistor twice as energy-efficient could lower power use by a significant percentage of the planet power budget. Such progress is crucial to maintaining progress in a post-CMOS world, and has great environmental implications as well.

Original article: Eric Pop, Energy Dissipation and Transport in Nanoscale Devices

The phenomenon, described as thermopower waves, “opens up a new area of energy research, which is rare,” says Michael Strano, who was the senior author of a paper describing the new findings that appeared in Nature Materials. The lead author was Wonjoon Choi, a doctoral student in mechanical engineering.

A carbon nanotube (shown in the illustration made by Christine Daniloff) can produce a very rapid wave of power when it is coated by a layer of fuel and ignited, so that heat travels along the tube. Like a collection of flotsam propelled along the surface by waves travelling across the ocean, it turns out that a thermal wave — a moving pulse of heat — travelling along a microscopic wire can drive electrons along, creating an electrical current.

The key ingredient in the recipe is carbon nanotubes — submicroscopic hollow tubes made of a chicken-wire-like lattice of carbon atoms. These tubes, just a few nanometers in diameter, are part of a family of novel carbon molecules, including buckyballs and graphene sheets.

In the new experiments, each of these electrically and thermally conductive nanotubes was coated with a layer of a reactive fuel that can produce heat by decomposing. This fuel was then ignited at one end of the nanotube using either a laser beam or a high-voltage spark, and the result was a fast-moving thermal wave travelling along the length of the carbon nanotube like a flame speeding along the length of a lit fuse. Heat from the fuel goes into the nanotube, where it travels thousands of times faster than in the fuel itself. As the heat feeds back to the fuel coating, a thermal wave is created that is guided along the nanotube. With a temperature of 3 000 kelvins, this ring of heat speeds along the tube 10 000 times faster than the normal spread of this chemical reaction. The heating produced by that combustion, it turns out, also pushes electrons along the tube, creating a substantial electrical current.

Combustion waves — like this pulse of heat hurtling along a wire — “have been studied mathematically for more than 100 years,” Strano says, but he was the first to predict that such waves could be guided by a nanotube or nanowire and that this wave of heat could push an electrical current along that wire. In the group’s initial experiments, Strano says, when they wired up the carbon nanotubes with their fuel coating in order to study the reaction, “we were really surprised by the size of the resulting voltage peak” that propagated along the wire.

After further development, the system now puts out energy, in proportion to its weight, about 100 times greater than an equivalent weight of lithium-ion battery. The amount of power released, he says, is much greater than that predicted by thermoelectric calculations. While many semiconductor materials can produce an electric potential when heated, through something called the Seebeck effect, that effect is very weak in carbon. “There’s something else happening here,” he says. “We call it electron entrainment, since part of the current appears to scale with wave velocity.”

The thermal wave, he explains, appears to be entraining the electrical charge carriers (either electrons or electron holes) just as an ocean wave can pick up and carry a collection of debris along the surface. This important property is responsible for the high power produced by the system, Strano says. Because this is such a new discovery, he says, it’s hard to predict exactly what the practical applications will be. But he suggests that one possible application would be in enabling new kinds of ultra-small electronic devices — for example, devices the size of grains of rice, perhaps with sensors or treatment devices that could be injected into the body. Or it could lead to “environmental sensors that could be scattered like dust in the air,” he says.

In theory, he says, such devices could maintain their power indefinitely until used, unlike batteries whose charges leak away gradually as they sit unused. And while the individual nanowires are tiny, Strano suggests that they could be made in large arrays to supply significant amounts of power for larger devices. The researchers also plan to pursue another aspect of their theory: that by using different kinds of reactive materials for the coating, the wave front could oscillate, thus producing an alternating current. That would open up a variety of possibilities, Strano says, because alternating current is the basis for radio waves such as cell phone transmissions, but present energy-storage systems all produce direct current. “Our theory predicted these oscillations before we began to observe them in our data,” he says.

Also, the present versions of the system have low efficiency, because a great deal of power is being given off as heat and light. The team plans to work on improving that efficiency.

Source: original article

The automobile fleet is contributing significantly to air quality degradation in large cities. The use of hydrogen as a fuel is interesting since combustion delivers a lot of energy without polluting. However, some obstacles, like the efficient storage of this gas, remain before it can be implemented in the transportation industry.

Figure 1: Hydrogen powered car built by BMW

Hydrogen storage solutions

The United States Department of energy (DOE) has fixed two targets for hydrogen storage solutions applied to automotive transportation. The first target requires a ratio of hydrogen weight / tank weight that is superior to 0,065 (6,5% weight). This target limits the weight of the tank. The second target requires a hydrogen volumetric density higher than 62 kg/m in order to limit the volume of the tank.

Four main solutions were proposed to solve this problem:

• Compression or liquefaction of hydrogen
• Metal hydrides
• Chemical tanks
• Adsorbent materials

Figure 2 shows the commercially available solutions. Each one is compared to the DOE targets. To satisfy both targets, a solution must be located in the grey region of the graph. Carbon nanostructures are not displayed on the graph since they are not readily available in the market at the moment.

Figure 2: Comparison of storage solutions available on the market

Carbon nanotubes

Solid state carbon may exist under three different crystaline forms named allotropes. Diamond and graphene, are the two well known allotropes. The third allotrope is called fullerene. It is a new class of material which is formed by a spherical or cylindrical wrapping of graphene sheets.

Nanotubes are fullerenes formed by graphene sheets wrapped as shown in Figure 3. The diameter of those tubes can be as small as 1 nm.

Figure 3: Representation of the carbon nanotube structures

The porosity of nanotubes is large. This property enables the adsorption of various gases including hydrogen.

Hydrogen storage in carbon nanostructures

Experimental results obtained in the evaluation of hydrogen storage capacity of carbon nanotubes vary significantly from one research team to another. Some groups (Dillon et al., Chambers et al.) obtained results indicating this material allows to reach DOE targets. Others (Ahn et al., Hirscher et al.), are indicating the contrary.

We chose to evaluate the storage capacity of nanotubes with Monte-Carlo numerical simulations. Our results indicate that carbon single-walled nanotubes can store 0.22% to 0.79% weight (3.95 to 7.94 kg/m) of hydrogen at room temperature and under a pressure of 10 MPa.

Figure 4: Simulation of the interaction between nanotubes (20,0) and hydrogen

Conclusion

Our results indicate that carbon nanostructures are falling short of the DOE targets. Such structure as thus more likely to be inappropriate for hydrogen storage in transport applications. However, we obtained interresting results by including the effect of impurities dispersed in nanotubes. We invite you to consult those papers to learn more about this subject:

P. Guay, Modélisation Monte-Carlo de l’adsorption de l’hydrogène dans les nanostructures de carbone, INRS-EMT, Montréal, 2003 (pdf)

P. Guay, B. L. Stansfield and A. Rochefort, On the Control of Carbon Nanostructures for Hydrogen Storage Applications, Carbon 42 (2004), 2187-2193 (abstract) (pdf)

A review of the literature is also available for consultation.

Author : Patrice Guay

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