refuel with hydrogen

Researchers are engaged in the development of ways to process hydrogen gas in an amount that will allow long journeys in vehicles powered by fuel cells

By Sunita Stiefel, John Petrovich and George Thomas

One day, in the late summer of 1783, Jacques Charles conducted an amazing operation in Paris. He took off to a height of a thousand meters above the ground in a balloon made of silk sacks covered with rubber, which he filled with hydrogen gas light from the air. Terrified farmers destroyed the balloon shortly after it landed, but Charles began a quest that scientists are still conducting two centuries later: to harness the power of hydrogen, the lightest element in the universe, for transportation purposes.

Whether they burn the hydrogen in the engine, or whether they use it to generate electricity in fuel cells, the possibility of using hydrogen to power the vehicles of the future is a magical possibility for several reasons. It can be produced in local factories from raw materials and diverse energy sources (eg renewable energy sources, nuclear reactors or mineral fuel). The gas is non-toxic and can actually be used as a non-polluting energy carrier in many types of machines. When hydrogen burns it does not release carbon dioxide, which is a powerful greenhouse gas. If you feed hydrogen to an array of fuel cells - a battery-like device that produces electricity from hydrogen and oxygen - it is able to drive an electric car or truck and emit only water and heat as byproducts. Vehicles powered by fuel cells can be twice as efficient as today's cars. Because of this, hydrogen can help reduce the oppressive environmental and social problems, including air pollution and the health risks it causes, global climate change and dependence on the import of foreign oil.

However, the obstacles holding back fueling cars with hydrogen are huge obstacles. In a kilogram-for-kilogram comparison, hydrogen contains three times more energy than gasoline, but today it is still impossible to store the gas in the same simple and compressed way as it is possible to store ordinary liquid fuel. One of the most challenging technical problems is how to efficiently and safely store in a vehicle a large enough amount of hydrogen to allow the driving distance and performance that drivers are used to. The researchers must therefore find a "miracle solution" for storage that will meet all the requirements: contain enough hydrogen to travel the currently accepted minimum distance between refuelings - about 500 kilometers; not to steal space from the passengers or from the luggage; release the hydrogen at a flow rate that will allow acceleration on a highway; operate in a practical temperature range; Allow refueling or charging within a few minutes and meet the price competition. The hydrogen storage technologies available today are far from these goals.

Researchers all over the world, from the automotive industry, from the government and from academia, invest a lot of effort to overcome these limitations. The Agreement for the Implementation of the Use of Hydrogen of the International Energy Agency, signed in 1977, now centers around it the largest international group focused on hydrogen storage, involving more than 35 researchers from 13 countries. The International Hydrogen Economy Partnership, created in 2003, now includes 17 governments committed to advancing hydrogen and fuel cell technologies. In 2005, the US Department of Energy established the National Hydrogen Storage Project, which includes three "Centers of Excellence" and a broad basic and applied research center conducted in many industrial, university and federal laboratories. In 2006 alone, the project awarded more than 30 million dollars to finance about 80 research projects.

infrastructural obstacles

One obstacle to widespread adoption of hydrogen fuel cells in cars and trucks is the sheer size of the task. The vehicles in the US alone consume 1.45 billion liters of fuel every day (about 530 billion liters per year), which is about two-thirds of the US's national oil consumption. More than half of this oil comes from other countries. Undoubtedly, the USA will have to invest considerable capital in converting the domestic automobile industry to the production of vehicles powered by fuel cells, and in adapting the refining system and the wide distribution network of the fuel to a system that will handle huge amounts of hydrogen. The new vehicles themselves will have to become cheap and durable enough to compete with conventional technology without compromising on performance. They also have to deal with safety assurance and long-standing negative public opinion - people still remember the Hindenburg airship disaster in 1937 and associate it with hydrogen, even though there is reliable evidence that the ship's inflammable sheath was actually the decisive factor in igniting the flame.

Why is it so difficult to store enough hydrogen in vehicles? At room temperature and atmospheric pressure, hydrogen is a gas whose energy density is 3,000 times less than that of liquid fuel. A 75-liter fuel tank containing hydrogen gas at atmospheric pressure will propel a normal vehicle for a distance of only 150 meters. Therefore, engineers must increase the density of stored hydrogen to create an efficient and useful storage system in vehicles.

A minimum driving range of 500 kilometers for refueling is one of the main operational goals of a joint project for the American government and industry - FreedomCAR and Fuel Partnership - which aims to develop advanced technologies for future vehicles. The engineers who calculate such calculations assume that a liter of fuel is equivalent, in terms of energy, to a quarter of a kilogram of hydrogen. But since hydrogen is more efficient, a vehicle that today needs 75 liters of fuel to travel at least 500 kilometers, will only need 8 kilograms of hydrogen if it moves using fuel cells. There will be models that will need less hydrogen to travel this distance, or more, depending on the type of vehicle and its size. In the tests conducted so far, 60 initial models of hydrogen cars from several manufacturers have reached distances of 160 to 300 kilometers per tank.

The researchers, who aim to reach practical performance by 2010 (the year in which several companies expect to bring the first right-hand fuel cell cars to market), are comparing the performance of various storage technologies to a goal of "six percent weight". That is, a storage system in which the hydrogen will occupy 6% of its total weight. In a system whose total weight is 100 kilograms (reasonable weight for vehicles), the weight of the stored hydrogen will be 6 kilograms. Although 6% does not sound like a lot, this goal is very difficult to achieve. The best system today contains less than 2%, and is constructed of storage materials that operate at relatively low pressures. Keeping the total volume of the system within the acceptable limits of fuel tanks on the market will be even more difficult, because most of its volume will be allocated to tanks, valves, piping, regulators, sensors, insulation and everything else required to store 6 kilograms of hydrogen. An efficient system should therefore release hydrogen at a fast enough rate so that the combination of the fuel cells and the electric motor will be able to provide the power and acceleration that the driver expects.

contain hydrogen

There are several hundred prototype models of fuel cell powered vehicles today, and most of them store the hydrogen gas in high-pressure cylinders, similar to diving balloons. Composite carbon fiber coating technology has created strong and lightweight vessels that can safely contain hydrogen at pressures between 350 and 700 atmospheres [see box above]. However, simply increasing the pressure does not increase the energy density by the same ratio. Even at a pressure of 700 atmospheres, the highest energy density that can be achieved today in high pressure tanks (which store 39 grams per liter) is about 15% of the energy content of fuel in the same volume. The high pressure vessels available today can only contain about 3.5% to 4% of hydrogen by weight. The Ford company recently presented a prototype of a multi-purpose SUV called the Edge, powered by an integrated system of a charged hybrid engine and fuel cells. The vehicle carries a fuel tank that stores 4.5 kilograms of hydrogen at a pressure of 350 atmospheres and reaches a total maximum range of 320 kilometers.

The use of high pressure tanks is conceivable in some transportation applications, such as buses and other large vehicles, whose physical size allows the storage of enough hydrogen, but not in private cars. Also, the price of these containers today is 10 times higher than the competitive price in the car market.

Liquifying stored hydrogen can improve its energy density, as it compresses more hydrogen per unit volume than any other method known today. Like any gas, hydrogen condenses to a liquid at a low enough temperature. At atmospheric pressure, hydrogen condenses at a temperature of minus 253 degrees Celsius. The material density of liquid hydrogen is 71 grams per liter, and its energy density is about 30% that of gasoline. The material density that such systems achieve depends on the valves and insulation.

However, liquid hydrogen has significant disadvantages. First, its low boiling point requires deep-cooling equipment and special precautions for safe handling. Also, since the tanks operate at a very low temperature, their insulation must be excellent. Apart from that, liquefying hydrogen consumes more energy than compressing it at high pressure. This increases the price of fuel and reduces the overall energy efficiency of the low-temperature freezing process.

Nevertheless, one automaker is bringing this technology to the road. This year BMW presented a car called "Hydrogen 7", which includes an internal combustion engine that can burn gasoline and travel a distance of 500 kilometers or burn liquid hydrogen and travel a distance of 200 kilometers. The car will be sold in a limited edition to select customers in the United States and other countries, who have access to local hydrogen fueling stations.

chemical compaction

In their search for promising ways to increase energy density, scientists may be able to exploit the chemistry of hydrogen itself. In a gaseous or liquid state of aggregation, each hydrogen molecule contains two atoms bonded together. But when the hydrogen atoms are chemically bonded to certain other elements, they can be compressed to an even greater density than in liquid hydrogen. The main goal of current research in the field of hydrogen storage is to find the materials capable of performing this trick.

Some researchers are focusing on a group of substances called reversible metal hydrides, which were discovered by chance in 1969 at the Philips Eindhoven laboratories in the Netherlands. The researchers there discovered that if an alloy of samarium and cobalt is exposed to compressed hydrogen gas, it absorbs the hydrogen, a bit like a sponge that absorbs water. When the pressure was removed, the hydrogen was released from the alloy, or in other words, the process was reversible.

Following this discovery, vigorous research began. In the USA, it was led by scientists James Reilly from the Brookhaven National Laboratory and Jerry Sandrock from the Inco Research and Development Center in Safran, New York, who developed hydride alloys with improved hydrogen absorption capacity. Their initial work formed the basis for the nickel-metal-hydride (Ni-MH) batteries that are commonly used today. The density of hydrogen in these alloys can be very high, 150% more than the density of liquid hydrogen, because the hydrogen atoms are stored within the crystalline metallic lattices between the metal atoms.
Many of the properties of the metal hydrides are well suited for use in vehicles. It is possible to achieve a density that exceeds that of liquid hydrogen even at relatively low pressures of 10 to 100 atmospheres. The metal hydrides are also inherently stable, so they do not require additional energy for continued storage, although heat is required to release the gas stored in them. Their Achilles heel is their excessive weight for practical storage in cars. So far, metal hydride researchers have been able to achieve a maximum hydrogen capacity of 2% of the total weight of the material. This capacity translates into a hydrogen storage system weighing 450 kilograms (for a driving distance of 500 kilometers) - undoubtedly too heavy for today's cars, which weigh about 1,400 kilograms.

Research today in the field of metal hydrides focuses on materials with a high natural hydrogen capacity. The researchers then adapt these hydrides to meet the requirements of hydrogen storage systems: operation at a temperature of about 100°C, pressures of 10 to 100 atmospheres and a release rate that will support high acceleration. In many cases, materials with a useful capacity of hydrogen are a little too stable, so the temperature required to release the hydrogen from them is considerably higher than desired. Magnesium, for example, forms magnesium hydride whose hydrogen content by weight is 7.6%, but it must be heated to a temperature of 350°C for the hydrogen to be released. If you want the system to actually use the excess heat emitted from the fuel cell array (about 80°C) to release the hydrogen from the metal hydride, the threshold temperature must be lower.

Destabilized hydrides

Chemists John J. Oho and Gregory L. Olson of the Hughes Research Laboratories (HRL) in Malibu, California, as well as researchers elsewhere, are exploring a sophisticated approach to overcoming the temperature problem. Their "destabilized hydrides" combine several substances to change the course of the chemical reaction so that the compounds formed will release the gas at lower temperatures.

Destabilized hydrides are part of a group of hydrogen-containing substances called complex hydrides. For a long time, chemists have believed that many of these compounds are ineffective for refueling vehicles, because they are not reversible - once the hydrogen has been released and the compound has broken down, a recycling process is needed to bring it back to a state containing hydrogen. However, chemists Borislav Bogdanovich and Manfred Schweikardi of the Max Planck Institute for Coal Research in Mülheim, Germany stunned the hydride research community in 1996 when they demonstrated how the complex hydride sodium allanate becomes a reversible hydride when a small amount of titanium is added to it. This research has led to a burst of activity over the past decade. For example, the lithium borohydride compound that HRL scientists destabilized by adding magnesium hydride stores hydrogen in a reversible bond at a rate of about 9% of its weight and releases it at a temperature of about 200°C. This is a noteworthy improvement, although the operating temperature is still high too much and the hydrogen release rate is too slow for automotive applications. Nevertheless, this is a promising study.

Despite the limitations of these metal hydrides, many automakers see them as the most practical approach to low-pressure hydrogen storage in the near to medium future. For example, the engineers of Toyota and Honda are planning a system in a "hybrid approach" that combines solid metal hydride at moderate pressure (much less than 700 atmospheres), and they expect it to achieve a travel range of more than 500 kilometers per refueling. General Motors employs teams of storage experts, including Scott Jorgensen, who support the research being done on a wide variety of metal hydride systems all over the world (including Russia, Canada and Singapore). General Motors is also collaborating with Sandia National Laboratories in the US in a four-year study with a budget of 10 million dollars to build a prototype of a complex metal hydride system.

hydrogen carriers

There are other options for hydrogen storage, which have the promising ability to perform well in cars, but they suffer from a disadvantage in the area of ​​refueling. In general, these chemical hydrides need industrial recycling of the used compound. The cycle phase must be done outside the vehicle. That is, after the vehicle's hydrogen reservoir has been used up, the by-products must be collected at a service station and recycled at a chemical plant. More than 20 years ago, Japanese researchers tested this approach using, for example, the decalin-naphthalene system. When you heat the compound Decalin (with the formula C10H18), it turns in a chemical reaction into naphthalene (a pungent smelling compound with the formula C10H8) and releases five hydrogen molecules while changing the nature of its chemical bonds. The hydrogen gas simply bubbles out of the liquid declin as it turns into naphthalene. Exposure of naphthalene to hydrogen under moderate pressure reverses the process. The naphthalene reacts with the hydrogen and turns into declin (from which the hydrogen content released is 7.2% by weight). Research chemists Alan Cooper and Guido Paz of Air Products and Chemicals in Allentown, Pennsylvania, are looking at a similar technique, which uses organic (hydrocarbon-based) liquids. Other scientists, including S. Thomas Otrey and his colleagues at the US Pacific Northwest National Laboratory and chemistry professor Larry J. Sandon of the University of Pennsylvania, are working on new liquid hydrogen carriers, such as aminoboranes, that can store large amounts of hydrogen and release it at moderate temperatures.

designed materials

Another approach to the hydrogen storage problem focuses on light materials with a very large surface area, to which hydrogen molecules adhere (or adsorb). As you might expect, the amount of hydrogen that can be stored on any surface is proportional to the surface area of ​​the material. Recent developments in nanoscale engineering have created a variety of new materials with a large surface area, some reaching more than 5,000 square meters per gram of material (or a surface area of ​​12 dunams in one teaspoon of powder). Carbon-based materials are particularly interesting because they are light, may be cheap and it is possible to create a variety of nanoscale structures from them: carbon nano-tubes, nano-rays (ray-like tubes), fullerenes (ball-like molecules) and aerogels (highly porous solids). Activated carbon, a relatively cheap material, can store up to 5% by weight of hydrogen.

However, all these carbon structures have a common limitation. The hydrogen molecules form very weak bonds with the carbon atoms, and this means that the materials with the huge surface area must be held at temperatures close to that of liquid nitrogen, i.e. minus 196 degrees Celsius. In contrast to hydride research, where researchers struggle to lower the hydrogen bond energy, carbon researchers are looking at ways to increase the bond energy by changing the structure or by adding small amounts of metals that may change the surface properties. These researchers examine theoretical simulations of carbon structures to discover promising systems for further study.

Besides the carbon-based approaches, another fascinating approach in nanoscale engineering deals with a group of materials called metal-organic materials. A few years ago, Omar Yagi, a chemistry professor who was at the time at the University of Michigan in Ann Harbor and now at the University of California in Los Angeles, invented the metal-organic framework compounds (MOFs). Yagi and his colleagues showed that it is possible to produce this type of crystalline and highly porous materials by linking inorganic compounds with organic "supports". The resulting MOF compounds have an eye-catching structure and physical properties that can be controlled to give the material different and varied desired functions. The surface area of ​​these complex structures may be very large (up to 5,500 square meters per gram), and researchers are able to build chemical sites on their surface for optimal hydrogen binding. To date, the researchers have been able to demonstrate MOF compounds capable of storing 7% by weight of hydrogen at a temperature of minus 196 degrees Celsius, and they continue to work on improving this performance.

Although the current progress in hydrogen storage methods is encouraging, finding the "perfect" approach may take a long time and require prolonged and innovative research and development efforts. For centuries, the fundamental promise—and challenge—of using hydrogen for transportation has remained virtually unchanged. The storage of hydrogen in a light and useful container allowed Jacques Charles to cross the sky in his balloon in the last decades of the 18th century. Similarly, finding a suitable tank to store hydrogen in cars will allow people to move all over the world in the coming decades of the 21st century, without polluting the skies above them.

Overview/ Hydrogen storage
One of the biggest hurdles facing the engineers of future fuel cell vehicles is how to cram enough hydrogen into the fuel tank to allow the car to travel at least 500 kilometers, as drivers demand.
Normally, the hydrogen is compressed into containers at high pressure and at ambient temperature, but the containers do not contain the necessary amount of compressed gas. Liquid hydrogen systems, which operate at very low temperatures, also suffer from considerable disadvantages.
Several alternative technologies are currently being developed for high-density hydrogen storage, but currently, none of them are up to the challenge.

The hydrogen storage challenge
A hydrogen storage system must carry enough fuel to travel at least 500 kilometers, and be small and light enough to carry in a car. If by 2010 they develop a system in which hydrogen occupies 6% of its weight and which manages to compress 45 grams of hydrogen per liter, it will probably meet the requirements of the first generation of vehicles powered by fuel cells (the black target in the first graph). None of the systems operating today is up to this. By 2015 even better performance will be required because the variety of vehicles that will be available then will be wider. It should be noted that the values ​​shown in the graph also include the volume of equipment needed to operate each system. The density of pure liquid hydrogen, for example, is 71 grams per liter, but when the tank and auxiliary equipment are included, the volume capacity drops to slightly less than 40 grams per liter. Since the hydrogen absorbers are still in an early stage of development, their capacity and price data are not available.

About the authors
Sunita Satyapal, John Petrovic and George Thomas work in the Applied Research and Development Program for Hydrogen Storage Technologies at the US Department of Energy. Stiapal, who held various positions in academia and industry, is the manager of an R&D team for applied hydrogen storage at the Ministry of Energy. Petrovich is a member (retired) of the US National Laboratory in Los Alamos, a consultant to the Department of Energy and a member of the American Ceramic Materials Society and the American International Materials Society. Thomas, who currently serves as a consultant to the Department of Energy, has 30 years of experience studying the effects of hydrogen on metals at Sandia National Laboratories in the US. The opinions presented in the article are those of the authors only; They do not reflect the positions of the US Department of Energy.

And more on the subject

The Hydrogen
Economy: Opportunities, Costs, Barriers, and R&D Needs. National Research Council and National Academy of Engineering. National Academies Press, 2004.
www.nap.edu/
catalog.php?record-id=10922

Hydrogen Program: 2006 Annual Merit Review Proceedings. US Department of Energy.
www.hydrogen
.energy.gov/annual-review06-proceedings.html

US Council for Automotive Research: www.uscar.org

International Energy Agency Hydrogen Implementation Agreement: www.ieahia.org

The International Partnership for the Hydrogen Economy: www.iphe.net