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Topic Name: UCLA Researchers Solve Decade-Old Mystery Using Molecular Dynamics Simulations

Category: Environmental Fluid Mechanics

Research persons: Vidvuds Ozolins

Location: UCLA Henry Samueli School of Engineering and Applied Science, United States

Details

Environmentally friendly hydrogen gas fueled vehicles can dramatically reduce greenhouse gas emissions and lessen the country’s dependence on sources of fossil fuel. Though several hydrogen vehicles exist on the market today, there is still much room for improvement in the way that hydrogen is stored on-board the vehicle. With current technologies, hydrogen gas storage tanks have to be as large as or larger than the trunk of a car to carry enough gas to travel only one to two hundred miles.

While liquid hydrogen is denser and takes up less space, it is very expensive and difficult to produce. It also reduces the environmental benefits of hydrogen vehicles. Widespread commercial acceptance of these vehicles will require finding the right material that can store hydrogen gas at high volumetric and gravimetric densities in reasonably sized light-weight fuel tanks.

Researchers at the UCLA Henry Samueli School of Engineering and Applied Science, with the use of molecular dynamics simulations, have solved a decade old mystery that could one day lead to commercially practical designs of storage materials for use in hydrogen gas fueled vehicles. The study appears on the Proceedings of the National Academy of Sciences (PNAS) web site on February 27.

In 1997, it was discovered that adding a small amount of titanium to a well-known metal hydride, sodium alanate, not only lowers the temperature of hydrogen release from the material but also allows for an easy refueling and storage of high density hydrogen at reasonable pressures and temperatures. In fact, the weight percent of stored hydrogen was instantly doubled in comparison with other inexpensive materials.

“Nobody really understood what the titanium did. The chemical processes and the mechanisms were really a mystery,” said Vidvuds Ozolins, associate professor of material science and engineering, a member of the California NanoSystems Institute, and lead author of the study.

With computers and the power of basic physics, chemistry and quantum mechanics, Ozolins’ group decided to take a step back and analyze the sodium alanate in its pure form, without added titanium. The group analyzed the atomic processes occurring in the material and what happens to the chemical bond between the hydrogen and the material at the temperatures of hydrogen release. The computation gave the researchers information that would have been very difficult to obtain experimentally.

The computation suggested a reaction mechanism that is essential for the extraction of hydrogen from the material which involves diffusion of aluminum ions within the bulk of the hydride. By comparing the calculated activation energies to the experimentally determined values, Ozolins’ group found that aluminum diffusion is the key rate limiting process in materials catalyzed with titanium. Thus, titanium facilitates processes in the material that are essential for turning on this mechanism and extracting hydrogen at lower temperatures.

“This method and this knowledge can now be used to analyze other materials that would make for better storage systems than sodium alanate. We are still on the fundamental end of the study. But if we can figure this out computationally, the people with the technology in engineering can figure out the rest,” said Hakan Gunaydin, a UCLA graduate student in Ozolins’ lab and another one of the study’s authors.

“Sodium alanate in itself is a prototypical complex hydride with a reasonable storage density and very good kinetics. Hydrogen goes in and comes out quickly but it wouldn’t be practical for a car simply because it doesn’t contain enough hydrogen. So that’s why we are so interested in understanding how the hydrogen comes out, what happens exactly and how we can take this to other materials,” said Ozolins.

What Ozolins’ group, along with UCLA chemistry and biochemistry professor Kendall Houk, also a member of the California NanoSystems Institute, hopes to do now is to apply the methods and lessons learned to those materials that would make for a commercially practical hydrogen gas storage system. They hope their findings will one day facilitate the design and creation of an affordable and environmentally friendly hydrogen vehicle.

Note for Greenhouse Gas
Greenhouse gases are components of the atmosphere that contribute to the greenhouse effect. Without the greenhouse effect the Earth would be uninhabitable; in its absence, the mean temperature of the earth would be about −19 °C (−2 °F, 254 K) rather than the present mean temperature of about 15 °C (59 °F, 288 K). Greenhouse gases include in the order of relative abundance water vapour, carbon dioxide, methane, nitrous oxide, and ozone. Greenhouse gases come from natural sources and human activity.
When sunlight reaches the surface of the Earth, some of it is absorbed and warms the surface. Because the Earth's surface is much cooler than the sun, it radiates energy at much longer wavelengths than the sun does, peaking in the infrared at about 10µm. The atmosphere absorbs these longer wavelengths more effectively than it does the shorter wavelengths from the sun. The absorption of this longwave radiant energy warms the atmosphere; the atmosphere also is warmed by transfer of sensible and latent heat from the surface. Greenhouse gases also emit longwave radiation both upward to space and downward to the surface. The downward part of this longwave radiation emitted by the atmosphere is the "greenhouse effect." The term is a misnomer, as this process is not the mechanism that warms greenhouses.
The major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect on Earth (not including clouds); carbon dioxide, which causes 9–26%; methane, which causes 4–9%, and ozone, which causes 3–7%. It is not possible to state that a certain gas causes a certain percentage of the greenhouse effect, because the influences of the various gases are not additive. (The higher ends of the ranges quoted are for the gas alone; the lower ends, for the gas counting overlaps.) Other greenhouse gases include, but are not limited to, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons, perfluorocarbons and chlorofluorocarbons (see IPCC list of greenhouse gases).
The major atmospheric constituents (nitrogen, N2 and oxygen, O2) are not greenhouse gases. This is because homonuclear diatomic molecules such as N2 and O2 neither absorb nor emit infrared radiation, as there is no net change in the dipole moment of these molecules when they vibrate. Molecular vibrations occur at energies that are of the same magnitude as the energy of the photons on infrared light. Heteronuclear diatomics such as CO or HCl absorb IR; however, these molecules are short-lived in the atmosphere owing to their reactivity and solubility. As a consequence they do not contribute significantly to the greenhouse effect.
Late 19th century scientists experimentally discovered that N2 and O2 did not absorb infrared radiation (called, at that time, "dark radiation") and that CO2 and many other gases did absorb such radiation. It was recognized in the early 20th century that the known major greenhouse gases in the atmosphere caused the earth's temperature to be higher than it would have been without the greenhouse gases.

Note for Fossil Fuel
Fossil fuels or mineral fuels are fossil source fuels, that is, hydrocarbons found within the top layer of the earth’s crust.
They range from very volatile materials with low carbon:hydrogen ratios like methane, to liquid petroleum to nonvolatile materials composed of almost pure carbon, like anthracite coal. Methane can be found in hydrocarbon fields, alone, associated with oil, or in the form of methane clathrates. It is generally accepted that they formed from the fossilized remains of dead plants and animals by exposure to heat and pressure in the Earth's crust over hundreds of millions of years. This is known as the biogenic theory and was first introduced by Mikhail Lomonosov in 1757. There is an opposing theory that the more volatile hydrocarbons, especially natural gas, are formed by abiogenic processes, that is no living material was involved in their formation.
It was estimated by the Energy Information Administration that in 2005 86% of primary energy production in the world came from burning fossil fuels. With the remaining Non-fossil being hydro 6.3%, nuclear 6.0%, and other (geothermal, solar, wind, and wood and waste) 0.9 percent
Fossil fuels are non-renewable resources because they take millions of years to form and reserves are being depleted much faster than new ones are being formed. Concern about fossil fuel supplies is one of the causes of regional and global conflicts. The production and use of fossil fuels raise environmental concerns. A global movement toward the generation of renewable energy is therefore under way to help meet increased energy needs.
The burning of fossil fuels produces around 21.3 billion tonnes (= 21.3 gigatons) of carbon dioxide per year, but it is estimated that natural processes can only absorb about half of that amount so there is a net increase of 11.6 billion tonnes of atmospheric carbon dioxide per year(one tonne of atmospheric carbon is equivalent to 44/12 or 3.7 tonnes of carbon dioxide). Carbon dioxide is one of the greenhouse gases that enhances radiative forcing and contributes to global warming causing the average surface temperature of the Earth to rise in response which climate scientists agree will cause major adverse effects, including on biodiversity and, over time, cause sea level rise.
Fossil fuels are of great importance because they can be burned (oxidized to carbon dioxide and water), producing significant amounts of energy. The use of coal as a fuel predates recorded history. Semisolid hydrocarbons from seeps were also burned in ancient times, but these materials were mostly used for waterproofing and embalming.  Commercial exploitation of petroleum, largely as a replacement for oils from animal sources (notably whale oil) for use in oil lamps began in the nineteenth century. Natural gas, once flared-off as an un-needed byproduct of petroleum production, is now considered a very valuable resource. Heavy crude oil, which is very much more viscous than conventional crude oil, and tar sands, where bitumen is found mixed with sand and clay, are becoming more important as sources of fossil fuel. Oil shale and similar materials are sedimentary rocks containing kerogen, a complex mixture of high-molecular weight organic compounds which yields synthetic crude oil when heated (pyrolyzed), have not yet been exploited commercially.
Prior to the latter half of the eighteenth century windmills or watermills provided the energy needed for industry such as milling flour, sawing wood or pumping water, and burning wood or peat provided domestic heat. The wide-scale use of fossil fuels, coal at first and petroleum later, to fire steam engines enabled the Industrial Revolution. At the same time gas lights using natural gas or coal gas were coming into wide use. The invention of the internal combustion engine and its use in automobiles and trucks greatly increased the demand for gasoline and diesel oil, both made from fossil fuels. Other forms of transportation, railways and aircraft also required fossil fuels. The other major use for fossil fuels is in generating electricity.

Note for Liquid Hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. It is a common liquid rocket fuel for rocket applications. In the aerospace industry, its name is often abbreviated to LH2 or LH2. Hydrogen is found naturally in the molecular H2 form, hence the H2 part of the name.
To exist as a liquid, H2 must be pressurized and cooled to a very low temperature, 20.27 K (−423.17 °F/−252.87°C). One common method of obtaining liquid hydrogen involves a compressor resembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a practical form of storing hydrogen. As in any gas, storing it as liquid takes less space than storing it as a gas at normal temperature and pressure. Once liquified it can be maintained as a liquid in pressurized and thermally insulated containers.
In rocket engines, liquid hydrogen is frequently used as a coolant to cool the engine nozzle (regenerative cooling) and other parts before being mixed with the oxidizer (often liquid oxygen (LOX)) and burned. The resulting exhaust of such LH2 - LOX engines is very clean water with traces of ozone and hydrogen peroxide.
Liquified hydrogen can be used as a fuel in an internal combustion engine. Various concept hydrogen vehicles have been built using this form of hydrogen (see BMW H2R).
Liquid hydrogen is also used to cool neutrons to be used in neutron scattering, since neutrons and hydrogen nuclei have similar masses, kinetic energy exchange per interaction is maximum (elastic collision).
Hydrogen has one of the highest gravimetric energy densities of all available fuels, which means it has very high energy content per unit weight (143 MJ/kg, 40 percent more than other rocket fuels).
As one of the lightest fuels available, one liter of hydrogen weighs only 0.07 kg. That is a density of 70.8 kg/m³ (at 20 K).
Producing "zero emissions", the byproducts of its combustion are mainly water vapor.
In terms of volumetric energy density, liquid hydrogen requires much more volume than other fuels to store the same amount of energy. Four liters of liquid hydrogen are needed to match the same energy content of one liter of gasoline.
Liquid Hydrogen requires complex storage technology such as the special thermally insulated containers and requires special handling common to all cryogenic substances. Same as Liquid oxygen.
Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away. (Typically it will evaporate at a rate of 1% per day.)
Hydrogen will leak into the chemical structure of the containers and weaken them.
These reasons account for the switch to solid propellants in ballistic missiles. The solid fuel remains stable for years inside a missile in contrast with LH2, which delayed the launch by requiring a last refuel.

Note for Molecular Dynamics
Molecular dynamics (MD) is a form of computer simulation wherein atoms and molecules are allowed to interact for a period of time under known laws of physics, giving a view of the motion of the atoms. Because molecular systems generally consist of a vast number of particles, it is impossible to find the properties of such complex systems analytically; MD simulation circumvents this problem by using numerical methods. It represents an interface between laboratory experiments and theory, and can be understood as a "virtual experiment". MD probes the relationship between molecular structure, movement and function. Molecular dynamics is a multidisciplinary method. Its laws and theories stem from mathematics, physics, and chemistry, and it employs algorithms from computer science and information theory. It was originally conceived within theoretical physics in the late 1950's, but is applied today mostly in materials science and biomolecules.
Before it became possible to simulate molecular dynamics with computers, some undertook the hard work of trying it with physical models such as macroscopic spheres. The idea was to arrange them to replicate the properties of a liquid. J.D. Bernal said, in 1962: "... I took a number of rubber balls and stuck them together with rods of a selection of different lengths ranging from 2.75 to 4 inches. I tried to do this in the first place as casually as possible, working in my own office, being interrupted every five minutes or so and not remembering what I had done before the interruption." Fortunately, now computers keep track of bonds during a simulation.
Molecular dynamics is a specialized discipline of molecular modeling and computer simulation based on statistical mechanics; the main justification of the MD method is that statistical ensemble averages are equal to time averages of the system, known as the ergodic hypothesis. MD has also been termed "statistical mechanics by numbers" and "Laplace's vision of Newtonian mechanics" of predicting the future by animating nature's forces and allowing insight into molecular motion on an atomic scale. However, long MD simulations are mathematically ill-conditioned, generating cumulative errors in numerical integration that can be minimized with proper selection of algorithms and parameters, but not eliminated entirely. Furthermore, current potential functions are, in many cases, not sufficiently accurate to reproduce the dynamics of molecular systems, so the much more demanding Ab Initio Molecular Dynamics method must be used. Nevertheless, molecular dynamics techniques allow detailed time and space resolution into representative behavior in phase space.
There is a significant difference between the focus and methods used by chemists and physicists, and this is reflected in differences in the jargon used by the different fields. In chemistry and biophysics, the interaction between the particles is either described by a "force field" (classical MD), a quantum chemical model, or a mix between the two. These terms are not used in physics, where the interactions are usually described by the name of the theory or approximation being used and called the potential energy, or just "potential".
Beginning in theoretical physics, the method of MD gained popularity in materials science and since the 1970s also in biochemistry and biophysics. In chemistry, MD serves as an important tool in protein structure determination and refinement using experimental tools such as X-ray crystallography and NMR. It has also been applied with limited success as a method of refining protein structure predictions. In physics, MD is used to examine the dynamics of atomic-level phenomena that cannot be observed directly, such as thin film growth and ion-subplantation. It is also used to examine the physical properties of nanotechnological devices that have not or cannot yet be created.
In applied mathematics and theoretical physics, molecular dynamics is a part of the research realm of dynamical systems, ergodic theory and statistical mechanics in general. The concepts of energy conservation and molecular entropy come from thermodynamics. Some techniques to calculate conformational entropy such as principal components analysis come from information theory. Mathematical techniques such as the transfer operator become applicable when MD is seen as a Markov chain. Also, there is a large community of mathematicians working on volume preserving, symplectic integrators for more computationally efficient MD simulations.

Note for Quantum Mechanics
In physics, quantum mechanics is the study of the relationship between energy quanta (radiation) and matter, in particular that between valence shell electrons and photons. Quantum mechanics is a fundamental branch of physics with wide applications in both experimental and theoretical physics. Quantum theory generalizes all classical theories, including mechanics, electromagnetism (except general relativity), and provides accurate descriptions for many previously unexplained phenomena such as black body radiation and stable electron orbits.The effects of quantum mechanics are typically not observable on macroscopic scales, but become evident at the atomic and subatomic level.
It is currently necessary to use quantum mechanics to understand the behavior of systems at atomic length scales and smaller. For example, if Newtonian mechanics governed the workings of an atom, electrons would rapidly travel towards and collide with the nucleus. However, in the natural world the electrons normally remain in an unknown orbital path around the nucleus, defying classical electromagnetism.
Quantum mechanics was initially developed to provide a better explanation of the atom, especially the spectra of light emitted by different atomic species. The quantum theory of the atom developed as an explanation for the electron's staying in its orbital, which could not be explained by Newton's laws of motion and by Maxwell's laws of classical electromagnetism.
In the formalism of quantum mechanics, the state of a system at a given time is described by a complex wave function (sometimes referred to as orbitals in the case of atomic electrons), and more generally, elements of a complex vector space. This abstract mathematical object allows for the calculation of probabilities of outcomes of concrete experiments. For example, it allows one to compute the probability of finding an electron in a particular region around the nucleus at a particular time. Contrary to classical mechanics, one can never make simultaneous predictions of conjugate variables, such as position and momentum, with arbitrary accuracy. For instance, electrons may be considered to be located somewhere within a region of space, but with their exact positions being unknown. Contours of constant probability, often referred to as “clouds” may be drawn around the nucleus of an atom to conceptualize where the electron might be located with the most probability. It should be stressed that the electron itself is not spread out over such cloud regions. It is either in a particular region of space, or it is not. Heisenberg's uncertainty principle quantifies the inability to precisely locate the particle.
The other exemplar that led to quantum mechanics was the study of electromagnetic waves such as light. When it was found in 1900 by Max Planck that the energy of waves could be described as consisting of small packets or quanta, Albert Einstein exploited this idea to show that an electromagnetic wave such as light could be described by a particle called the photon with a discrete energy dependent on its frequency. This led to a theory of unity between subatomic particles and electromagnetic waves called wave–particle duality in which particles and waves were neither one nor the other, but had certain properties of both. While quantum mechanics describes the world of the very small, it also is needed to explain certain “macroscopic quantum systems” such as superconductors and superfluids.
Broadly speaking, quantum mechanics incorporates four classes of phenomena that classical physics cannot account for: (i) the quantization (discretization) of certain physical quantities, (ii) wave-particle duality, (iii) the uncertainty principle, and (iv) quantum entanglement. Each of these phenomena is described in detail in subsequent sections.
Predictions of quantum mechanics have been verified experimentally to a very high degree of accuracy. Thus, the current logic of correspondence principle between classical and quantum mechanics is that all objects obey laws of quantum mechanics, and classical mechanics is just a quantum mechanics of large systems (or a statistical quantum mechanics of a large collection of particles). Laws of classical mechanics thus follow from laws of quantum mechanics at the limit of large systems or large quantum numbers.
Main differences between classical and quantum theories have already been mentioned above in the remarks on the Einstein-Podolsky-Rosen paradox. Essentially the difference boils down to the statement that quantum mechanics is coherent (addition of amplitudes), whereas classical theories are incoherent (addition of intensities). Thus, such quantities as coherence lengths and coherence times come into play. For microscopic bodies the extension of the system is certainly much smaller than the coherence length; for macroscopic bodies one expects that it should be the other way round.

The study was funded by the U.S. Department of Energy and the National Science Foundation.