Jan Baptista van Helmont (1577 to 1644) was one of the first to reject the basic elements ofAristoteles. He discovered that air is not an element and that another "air" with different properties exists. He called it, based on the Greek word "chaos" which means "empty space", according to the Dutch spelling "gas".
In the Middle Ages Paracelsus (1493 to 1541) is reported to have noted that a gas is yielded when iron is dissolved in "spirit of vitriole". Turquet De Mayerne (1573-1655) noted that this gas was inflammable. However, hydrogen was first separated and identified in the second half of the eighteenth century; Robert Boyle (1627 to 1691) produced "factitious air" from diluted sulfuric acid and iron. He showed that facticious air only burns when air is present and that a part of the air disappears during the reaction. He also noticed that the combustion products are heavier than the starting material. Therefore, he also rejected the elements of Aristotoles but his findings were not adequately acknowledged. The German chemist Georg Ernst Stahl (1659 to 1734) was the doctor of King Friedrich Wilhelm of Preussen and published in 1697 the phlogiston theory. According to this theory all inflammable materials contain phlogiston, a hypothetical substance, which is liberated during combustion.
According to the phlogiston theory (Greek: phlogistos, burned) a material is more flammable and burns more violently the larger the content of phlogiston. For the first time it was possible to describe several chemical reactions by means of the phlogiston theory. Lead, for example is composed of lead oxide and phlogiston and the phlogiston is liberated during combustion leaving behind lead oxide.
The phlogiston theory was very useful and was therefore still defended when an obvious weakness of the theory appeared and an arbitrary assumption was necessary. After Boyle's observation that the mass of a substance increases during combustion, for example, the mass oflead oxide is greater than the mass oflead, a negative mass was attributed to phlogiston.
Water (H2O) was considered in all theories to be a basic element in the sense of Aristotoles. Still, in 1756, the Scot Joseph Black saw gases as a form of the element air, therefore he called the gas, known today as carbon dioxide, quick air because it reacts with magnesium to give magnesium carbonate.
Henry Cavendish (1731-1810) proved that there were different types of air, one of which was "inflammable air" and that a number of metals, when dissolved in acid, produced various amounts of this gas. He assumed that the metal was the source of the inflamable air. This was wrong, however, in accordance with the phlogiston theory. He published  in 1766 precise values for the specific weight and density. During the late 1770s, he performed experiments with electrical discharges in a hydrogen-oxygen mixture, thereby producing water. Cavendish's discovery stimulated the search for new gases and the Swedish scientist Carl Wilhelm Scheele and the English scientist Joseph Priestley independently found a gas which is a component of the air, this was called fireair. 1781 Cavendish burnt his inflammable air with the fireair and obtained nothing else but water.
On 5th June 1783 the Montgolfier Brothers gave the first public demonstration of a model hot-air balloon and in September - in the presence of King Louis XVI and Marie Antoinette - they flew a balloon carrying a sheep, a duck and a cockerel to demonstrate that it was possible to survive in the sky. Some weeks later Pilatre de Rozier, a science teacher, and the Marquis d'Arlandes, an infantry officer, became the first human air travellers when, in a hot-air balloon, they flew for 9km (5.5 miles) over Paris. Jacques Alexandre Cesar Charles (1746-1823) realized that hydrogen was lighter than air, he built the first balloon made of paper and filled with 25 m3 hydrogen gas and, on 27th August 1783, the balloon ascended to a height of nearly 914 m. The hydrogen was produced by the reaction of iron with sulfuric acid. Upon landing outside Paris, it was destroyed by terrified peasants. On December 1st, 1783, he, along with Aine Roberts, ascended to a height of549 m in the newly constructed balloon "La Charliere". Charles is best known for his formulation in 1787 of one of the basic gas laws, known as Charles's law, which
Fig. 2.1 Antoine Laurent Lavoisier (1743-1794) with his wife and secretary Marie-Anne Paulze (1758-1836), Painting by Louis David instates that, at constant pressure, the volume occupied by a fixed weight of gas is directly proportional to the absolute temperature.
Soon after, the French scientist Antoine Lavoisier (1743-1794) (Fig. 2.1) confirmed Cavendish's experiments and wrote: "It seams that the phenomena can be explained without the use of phlogiston ." According to Lavoisier's antiphlogistic theory it was not the properties of matter that were seen as elements but rather chemical elements were substanses with specific properties. From todays view phlogiston was the energy (heat) which is liberated during combustion.
At the same time, a group of French chemists started to introduce a new nomenclature in chemistry. Especially misleading names were replaced by Greek words, which described the most obvious property of a substance.
In 1787 Lavoisier presented the following proposal, formulated by this group of chemists, to the Academy of Science in Paris . The fire air should be called "oxygene" from the Greek words "oxys" for acidic and "genes" for genesis, because Lavoisier assumed that oxygen was the reason for the acidity. For the inflammable air the word "hydrogene" was proposed, based on the Greek word "hydor" meaning water former.
For use as a fuel, a property of hydrogen, that is of even greater importance than flammability is the large amount of energy released during combustion. Lavoisier and Pierre Laplace measured the heat of combustion of hydrogen in 1783-1784 using an ice calorimeter. The experiment took 11.5 h and the amount of ice melted was equivalent to about 9.7 x 107 J per kg of hydrogen. This was much higher than
values obtained for other substances and, whether for this reason or because of other uncertainties, the results were not published until 1793. The Lavoisier-Laplace value was not too far off the correct value of 1.20 x 108 J per kg hydrogen.
Nicholson and Carlisle in 1800 split water into oxygen and hydrogen by passing an electric current through it. Water was the first substance to be "electrolyzed".
From the year 1800 on the use of hydrogen was limited to water gas (a mixture of hydrogen and carbon monoxide) for illumination and as an additive to town gas (a mixture of methane, carbon monoxide and hydrogen) for heating. These gases were displaced by natural gas as a gaseous energy carrier in the middle of the twentieth century.
In 1823, Johann Wolfgang Dobereiner invented the first "pocket" lighter to light cigars . The "Dobereiner Platinum lighter" (Fig. 2.2) was the first mass produced hydrogen device and over 20 000 were sold. Hydrogen, produced by a chemical reaction of zinc with sulfuric acid, streams over a platinum sponge and reacts spontaneously with oxygen to form water at the metal surface. Subsequently, the released heat ignites also the hydrogen in the gas phase and a flame emerges. The so-called catalytic combustion on catalytic metal surfaces is one of the most important physical effects of hydrogen-metal interactions and is also the basis of the following device.
Sir William Grove (1811-1896) constructed in 1839 a "gas voltaic battery" which was the forerunner of modern fuel cells (Fig. 2.3). He based his experiment on the fact that sending an electric current through water splits the water into its component parts hydrogen and oxygen. Grove allowed the reaction to reverse -combining hydrogen and oxygen to produce electricity and water. The term "fuel cell" was coined later, in 1889, by Ludwig Mond and Charles Langer, who attempted to build the first practical device using air and industrial coal gas. However the development of the fuel cell to a powerful current source was very difficult and in 1866 the first dynamoelectric generator was demonstrated which efficiently converts every type of mechanical energy into electricity. Therefore, fuel cells lost
upper part ofthe glass cylinders the electrode was in contact with hydrogen (hy) and oxygen (ox), respectively. The generated current was again used to electrolyze water (Ostwald, 1896 ).
Fig. 2.3 Sir William Robert Grove demon strated in1839 the first fuel cell with four galvanic elements in series. Diluted sufu-ric acid was used as the electrolyte and platinum wires as the electrodes. In the upper part ofthe glass cylinders the electrode was in contact with hydrogen (hy) and oxygen (ox), respectively. The generated current was again used to electrolyze water (Ostwald, 1896 ).
their importance as electricity generators and were not further developed until the middle ofthe twentieth century.
Gustav Kirchhoff and Robert Bunsen analyzed 1861 the emitted spectrum of the sun and found hydrogen to be the major constituent of the sun .
Soon after the discovery of the metal palladium in 1803 by W.H. Wollaston , T. Graham  reported that this metal could absorb large amounts of hydrogen by forming a metal hydride.While, in those times, the discovery was more a scientific curiosity, it is now the basis for the promising technology of hydrogen storage in metal hydrides.
On 10th May 1898, James Dewar  used regenerative cooling to become the first to statically liquefy hydrogen. Using liquid nitrogen he precooled gaseous hydrogen, under 180 atmospheres, then expanded it through a valve in an insulated vessel, also cooled by liquid nitrogen. The expanding hydrogen produced about 20 cm3 of liquid hydrogen, about 1 % of the hydrogen used.
In 1909 the German Chemist Fritz Haber discovered a catalyzed process , which allowed the synthesis of ammonia (NH3) from the elements hydrogen and nitrogen. He received the Nobel prize in chemistry for his discovery. The Nobel prize for Fritz Haber was a subject of controversy because Haber is also the inventor of war gas (phosgene COCl2), which killed hundreds of thousands of soldiers in World War I. Conscience-stricken, Haber's wife committed suicide. Carl Bosch succeeded to scale up Haber's synthesis from the laboratory scale to industrial production. After World War I other industrialized countries also introduced ammonia synthesis and therefore the consumption of hydrogen increased rapidly.
In 1929 Bonhoeffer and P. Harteck  successfully prepared the first pure para-hydrogen sample. In 1931 Urey, Brickwedde and Murphy  investigated the visible atomic Balmer series spectra of hydrogen samples and discovered the hydrogen isotope H2, deuterium. In 1935 Oliphant, Harteck and Lord Rutherford  synthesized "superheavy hydrogen", H3, tritium, through neutron bombardment of deuterated phosphoric acid.
On 1st March 1954 the USA ignited the first hydrogen bomb on the tiny Bikini Atoll in the Marshall Islands, contaminating a passing Japanese fishing boat and showering nearby villagers with radioactive ash. The bomb was 1000 times more powerful than the one dropped on Hiroshima. Three weeks later it emerged that a Japanese fishing boat, called Lucky Dragon, was within 80 miles (129 km) of the test zone at the time. Its 23 crew were severely affected by radiation sickness. They were among 264 people accidentally exposed to radiation because the explosion and fall-out had been far greater than expected.
Hydrogen is frequently used as a process gas in the petrochemical industry. Historically, it started with the production of water gas (a mixture of CO and H2). Its strongest impact was on the Lurgi process, developed in Germany in the late 1930s. LURGI was the cable address of Metallurgische Gesellschaft founded on 5th February 1897 . The Lurgi process permits the conversion of coal into methane, from which can be synthesized other hydrocarbon fuels such as gasoline. Such processes were operated on a large scale by Germany during World War II but are not now economically competitive with hydrocarbon fuels obtained from oil feedstocks. However, for modern conditioning of crude oil to obtain the desired end products (gasoline, etc.) similar process technologies are used.
Steam reforming, hydrogen reforming or catalytic oxidation, is a method of producing hydrogen from hydrocarbons. On an industrial scale, it is the dominant method for producing hydrogen. In 2000, the total annual hydrogen production worldwide amounted to 500 x 109m3 (STP) or aproximately 45 x 106tons. The major sources are natural gas and coal, accounting for 78 % of the total production, 48 and 30 % respectively. 50% of the total hydrogen production is used for the synthesis of ammonia and 25 % for the cracking and purification of crude oil.
In 1955 Justi  (Fig. 2.4) described the utilization of hydrogen as an energy carrier medium. In 1969 an overall hydrogen energy concept using the favorable properties of hydrogen and including nonconventional energy systems was developed by Bockris, Gregory, Marchetti, Veziroglu and others .
The rapid development of solid state physics in the twentieth century also promoted the development of new solid state devices connected to hydrogen. Examples are sensors (e.g. hydrogen sensitive Pd-MOS structures ), nickel metal hydride batteries (E. Justi 1968 ), hydrogen switchable mirrors (R. Griessen 1995 ), and amorphization of bulk samples by hydrogen . The metal hydride battery was the first realization ofhydrogen-based energy storage, because a large amount of hydrogen has to be stored in one of its electrodes. The breakthrough was the discovery of a cheap metal alloy with superior hydrogen storage capacities, LaNi5 by Philips, Eindhoven . However, hydrogen storage materials for application in automobiles have to meet higher hydrogen storage capacities. It is too early to designate the ultimate hydrogen storage material but certainly, a breakthrough on the road towards high hydrogen content is the discovery by Bogdanovic in 1996 of a catalyst catalyzing light-weight alanates .
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