Properties of Hydrogen

Hydrogen is the smallest element, with one proton and one electron. It is highly abundant and has unique and important chemical properties.

Learning Objectives

Indicate the different kinds of reactions hydrogen may participate in and discuss its basic properties

Key Takeaways

Key Points

  • Hydrogen is the lightest element and will explode at concentrations ranging from 4-75 percent by volume in the presence of sunlight, a flame, or a spark.
  • Despite its stability, hydrogen forms many bonds and is present in many different compounds.
  • Three naturally occurring isotopes of hydrogen exist: protium, deuterium, and tritium, each with different properties due to the difference in the number of neutrons in the nucleus.

Key Terms

  • diatomic: Consisting of two atoms

Physical Properties of Hydrogen

Hydrogen is the smallest chemical element because it consists of only one proton in its nucleus. Its symbol is H, and its atomic number is 1. It has an average atomic weight of 1.0079 amu, making it the lightest element. Hydrogen is the most abundant chemical substance in the universe, especially in stars and gas giant planets. However, monoatomic hydrogen is rare on Earth is rare due to its propensity to form covalent bonds with most elements. At standard temperature and pressure, hydrogen is a nontoxic, nonmetallic, odorless, tasteless, colorless, and highly combustible diatomic gas with the molecular formula H2. Hydrogen is also prevalent on Earth in the form of chemical compounds such as hydrocarbons and water.

Hydrogen has one one proton and one electron; the most common isotope, protium (1H), has no neutrons. Hydrogen has a melting point of -259.14 °C and a boiling point of -252.87 °C. Hydrogen has a density of 0.08988 g/L, making it less dense than air. It has two distinct oxidation states, (+1, -1), which make it able to act as both an oxidizing and a reducing agent. Its covalent radius is 31.5 pm.

Hydrogen exists in two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei. The orthohydrogen form has parallel spins; the parahydrogen form has antiparallel spins. At standard temperature and pressure, hydrogen gas consists of 75 percent orthohydrogen and 25 percent parahydrogen. Hydrogen is available in different forms, such as compressed gaseous hydrogen, liquid hydrogen, and slush hydrogen (composed of liquid and solid ), as well as solid and metallic forms.


The Hydrogen Atom: Many of the hydrogen atom’s chemical properties arise from its small size, such as its propensity to form covalent bonds, flammability, and spontaneous reaction with oxidizing elements.

Chemical Properties of Hydrogen

Hydrogen gas (H2) is highly flammable and will burn in air at a very wide range of concentrations between 4 percent and 75 percent by volume. The enthalpy of combustion for hydrogen is -286 kJ/mol, and is described by the equation:

[latex]2 \text{H}_2(\text{g}) + \text{O}_2(\text{g}) \rightarrow 2 \text{H}_2\text{O}(\text{l}) + 572 \text{kJ} (286 \text{kJ}/\text{mol H}_2)[/latex]

Hydrogen gas can also explode in a mixture of chlorine (from 5 to 95 percent). These mixtures can explode in response to a spark, heat, or even sunlight. The hydrogen autoignition temperature (the temperature at which spontaneous combustion will occur) is 500 °C. Pure hydrogen- oxygen flames emit ultraviolet light and are invisible to the naked eye. As such, the detection of a burning hydrogen leak is dangerous and requires a flame detector. Because hydrogen is buoyant in air, hydrogen flames ascend rapidly and cause less damage than hydrocarbon fires. H2 reacts with oxidizing elements, which in turn react spontaneously and violently with chlorine and fluorine to form the corresponding hydrogen halides.

H2 does form compounds with most elements despite its stability. When participating in reactions, hydrogen can have a partial positive charge when reacting with more electronegative elements such as the halogens or oxygen, but it can have a partial negative charge when reacting with more electropositive elements such as the alkali metals. When hydrogen bonds with fluorine, oxygen, or nitrogen, it can participate in a form of medium-strength noncovalent (intermolecular) bonding called hydrogen bonding, which is critical to the stability of many biological molecules. Compounds that have hydrogen bonding with metals and metalloids are known as hydrides.

Oxidation of hydrogen removes its electron and yields the H+ ion. Often, the H+ in aqueous solutions is referred to as the hydronium ion (H3O+). This species is essential in acid-base chemistry.

Hydrogen Isotopes

Hydrogen naturally exists as three isotopes, denoted 1H, 2H, and 3H. 1H occurs at 99.98 percent abundance and has the formal name protium. 2H is known as deuterium and contains one electron, one proton, and one neutron (mass number = 2). Deuterium and its compounds are used as non-radioactive labels in chemical experiments and in solvents for 1H-NMR spectroscopy. 3H is known as tritium and contains one proton, two neutrons, and one electron (mass number = 3). It is radioactive and decays into helium-3 through beta decay with a half life of 12.32 years.

Binary Hydrides

Hydrides are compounds in which one or more hydrogen anions have nucleophilic, reducing, or basic properties.

Learning Objectives

Discuss the properties of hydrides.

Key Takeaways

Key Points

  • Binary hydrides are a class of compounds that consist of an element bonded to hydrogen, in which hydrogen acts as the more electronegative species.
  • Free hydride anions exist only under extreme conditions. Instead, most hydride compounds have hydrogen centers with a hydridic character.
  • Hydrides can be classified as ionic, covalent or interstitial, each of which possess different properties.

Key Terms

  • hydride: A compound of hydrogen with a more electropositive element.

Compounds with Anionic Hydrogen

A hydride is the anion of hydrogen (H), and it can form compounds in which one or more hydrogen centers have nucleophilic, reducing, or basic properties. In such hydrides, hydrogen is bonded to a more electropositive element or group.


Lithium Hydride, LiH: This is a space-filling model of a crystal of lithium hydride, LiH, a binary halide.

Hydride compounds often do not conform to classical electron -counting rules, but are described as multi-centered bonds with metallic bonding. Hydrides can be components of discrete molecules, oligomers, polymers, ionic solids, chemisorbed monolayers, bulk metals (interstitial), and other materials. While hydrides traditionally react as Lewis bases or reducing agents by donating electrons, some metal hydrides behave as both acids and hydrogen- atom donors.

Applications of Hydrides

Hydrides are commonly used as reducing agents, donating electrons in chemical reactions. Hydrides can be used as strong bases in organic syntheses, and their reaction with weak Bronsted acids releases dihydrogen (H2).

Hydrides such as calcium hydride are used as dessicants, or drying agents, to remove trace water from organic solvents. In such cases, the hydride reacts with water, forming diatomic hydrogen and a hydroxide salt:

[latex]\text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{Ca}(\text{OH})_2[/latex]

The dry solvent can then be distilled or vac-transferred from the “solvent pot.”

Hydride complexes are catalysts and catalytic intermediates in a variety of homogeneous and heterogeneous catalytic cycles. Important examples include hydrogenation, hydroformylation, hydrosilylation, and hydrodesulfurization catalysts. Even certain enzymes, like hydrogenase, operate via hydride intermediates. The energy carrier NADH reacts as a hydride donor or hydride equivalent.

Free hydride anions exist only under extreme conditions and are not invoked for homogeneous solutions. Instead, many compounds have a hydrogen center with a hydridic character. Hydrides can be characterized as ionic, covalent, or interstitial hydrides based on their bonding types.

Ionic Hydrides

Ionic, or saline, hydride is a hydrogen atom bound to an extremely electropositive metal, generally an alkali metal or an alkaline earth metal (for example, potassium hydride or KH). These types of hydrides are insoluble in conventional solvents, reflecting their non-molecular structures. Most ionic hydrides exist as “binary” materials that involve only two elements, one of which is hydrogen. Ionic hydrides are often used as heterogeneous bases and reducing reagents in organic synthesis.

Covalent Hydrides

Covalent hydrides refer to hydrogen centers that react as hydrides, or those that are nucleophilic. In these substances, the hydride bond, formally, is a covalent bond much like the bond that is made by a proton in a weak acid. This category includes hydrides that exist as discrete molecules, polymers, oligomers, or hydrogen that has been chem-adsorbed to a surface. A particularly important type of covalent hydride is the complex metal hydride, a powerful (reducing) soluble hydride that is commonly used in organic syntheses (for example, sodium borohydride or NaBH4). Transition metal hydrides also include compounds that can be classified as covalent hydrides. Some are even classified as interstitial hydrides and other bridging hydrides. Classical transition metal hydrides feature a single bond between the hydrogen center and the transition metal.

Interstitial or Metallic Hydrides

Interstitial hydrides most commonly exist within metals or alloys. Their bonding is generally considered metallic. Such bulk transition metals form interstitial binary hydrides when exposed to hydrogen. These systems are usually non-stoichiometric, with variable amounts of hydrogen atoms in the lattice.

Isotopes of Hydrogen

Hydrogen has three naturally occurring isotopes: protium, deuterium and tritium. Each isotope has different chemical properties.

Learning Objectives

Discuss the chemical properties of hydrogen’s naturally occurring isotopes.

Key Takeaways

Key Points

  • Protium is the most prevalent hydrogen isotope, with an abundance of 99.98%. It consists of one proton and one electron. It is typically not found in its monoatomic form, but bonded with itself (H2) or other elements.
  • Deuterium is a hydrogen isotope consisting of one proton, one neutron and one electron. It has major applications in nuclear magnetic resonance studies.
  • Tritium is a hydrogen isotope consisting of one proton, two neutrons and one electron. It is radioactive, with a half-life of 12.32 years.

Key Terms

  • diatomic: Consisting of two atoms.
  • isotope: Forms of an element where the atoms have a different number of neutrons within their nuclei. As a consequence, atoms of the same isotope will have the same atomic number, but a different mass number.

Properties of Isotopes of Hydrogen

Hydrogen has three naturally occurring isotopes: 1H (protium), 2H (deuterium), and 3H (tritium). Other highly unstable nuclei (4H to 7H) have been synthesized in the laboratory, but do not occur in nature. The most stable radioisotope of hydrogen is tritium, with a half-life of 12.32 years. All heavier isotopes are synthetic and have a half-life less than a zeptosecond (10-21 sec). Of these, 5H is the most stable, and the least stable isotope is 7H.


Protium: Protium, the most common isotope of hydrogen, consists of one proton and one electron. Unique among all stable isotopes, it has no neutrons.


1H is the most common hydrogen isotope with an abundance of more than 99.98%. The nucleus of this isotope consists of only a single proton (atomic number = mass number = 1) and its mass is 1.007825 amu. Hydrogen is generally found as diatomic hydrogen gas H2, or it combines with other atoms in compounds —monoatomic hydrogen is rare. The H–H bond is one of the strongest bonds in nature, with a bond dissociation enthalpy of 435.88 kJ/mol at 298 K. As a consequence, H2 dissociates to only a minor extent until higher temperatures are reached. At 3000K, the degree of dissociation is only 7.85%. Hydrogen atoms are so reactive that they combine with almost all elements.


2H, or deuterium (D), is the other stable isotope of hydrogen. It has a natural abundance of ~156.25 ppm in the oceans, and accounts for approximately 0.0156% of all hydrogen found on earth. The nucleus of deuterium, called a deuteron, contains one proton and one neutron (mass number = 2), whereas the far more common hydrogen isotope, protium, has no neutrons in the nucleus. Because of the extra neutron present in the nucleus, deuterium is roughly twice the mass of protium (deuterium has a mass of 2.014102 amu, compared to the mean hydrogen atomic mass of 1.007947 amu). Deuterium occurs in trace amounts naturally as deuterium gas, written 2H2 or D2, but is most commonly found in the universe bonded with a protium 1H atom, forming a gas called hydrogen deuteride (HD or 1H2H).

Chemically, deuterium behaves similarly to ordinary hydrogen (protium), but there are differences in bond energy and length for compounds of heavy hydrogen isotopes, which are larger than the isotopic differences in any other element. Bonds involving deuterium and tritium are somewhat stronger than the corresponding bonds in protium, and these differences are enough to make significant changes in biological reactions. Deuterium can replace the normal hydrogen in water molecules to form heavy water (D2O), which is about 10.6% denser than normal water. Heavy water is slightly toxic in eukaryotic animals, with 25% substitution of the body water causing cell division problems and sterility, and 50% substitution causing death by cytotoxic syndrome (bone marrow failure and gastrointestinal lining failure). Consumption of heavy water does not pose a health threat to humans. It is estimated that a 70 kg person might drink 4.8 liters of heavy water without serious consequences.

The most common use for deuterium is in nuclear resonance spectroscopy. As nuclear magnetic resonance (NMR) requires compounds of interest to be dissolved in solution, the solution signal should not register in the analysis. As NMR analyzes the nuclear spins of hydrogen atoms, the different nuclear spin property of deuterium is not ‘seen’ by the NMR instrument, making deuterated solvents highly desirable due to the lack of solvent-signal interference.


Isotopes of Hydrogen: The three naturally occurring isotopes of hydrogen.


3H is known as tritium and contains one proton and two neutrons in its nucleus (mass number = 3). It is radioactive, decaying into helium-3 through beta-decay accompanied by a release of 18.6 keV of energy. It has a half-life of 12.32 years. Naturally occurring tritium is extremely rare on Earth, where trace amounts are formed by the interaction of the atmosphere with cosmic rays.

Heavier Synthetic Isotopes

4H contains one proton and three neutrons in its nucleus. It is a highly unstable isotope of hydrogen. It has been synthesized in the laboratory by bombarding tritium with fast-moving deuterium nuclei. In this experiment, the tritium nuclei captured neutrons from the fast-moving deuterium nucleus. The presence of the hydrogen-4 was deduced by detecting the emitted protons. Its atomic mass is 4.02781 ± 0.00011 amu. It decays through neutron emission with a half-life of 1.39 ×10−22 seconds.

5H is another highly unstable heavy isotope of hydrogen. The nucleus consists of a proton and four neutrons. It has been synthesized in a laboratory by bombarding tritium with fast-moving tritium nuclei. One tritium nucleus captures two neutrons from the other, becoming a nucleus with one proton and four neutrons. The remaining proton may be detected and the existence of hydrogen-5 deduced. It decays through double neutron emission and has a half-life of at least 9.1 × 10−22 seconds.

6H decays through triple neutron emission and has a half-life of 2.90×10−22 seconds. It consists of one proton and five neutrons.

7H consists of one proton and six neutrons. It was first synthesized in 2003 by a group of Russian, Japanese and French scientists at RIKEN’s RI Beam Science Laboratory, by bombarding hydrogen with helium-8 atoms. The helium-8’s neutrons were donated to the hydrogen’s nucleus. The two remaining protons were detected by the “RIKEN telescope”, a device composed of several layers of sensors, positioned behind the target of the RI Beam cyclotron.


Hydrogenation reactions, which involve the addition of hydrogen to substrates, have many important applications.

Learning Objectives

Discuss hydrogenation reactions.

Key Takeaways

Key Points

  • Hydrogenation reactions typically have three components: hydrogen, the substrate, and catalysts, which are usually required to facilitate the reaction at lower temperatures and pressures.
  • There are two classes of catalysts with different mechanisms of hydrogenation: heterogeneous and homogenous.
  • Hydrogenation reactions are not limited to the conversion of alkenes to alkanes, but span a variety of reactions where substrates can effectively be reduced.
  • Incomplete hydrogenation reactions have significant health implications and have been correlated with circulatory diseases.

Key Terms

  • hydrogenation: The chemical reaction of hydrogen with another substance, especially with an unsaturated organic compound.
  • substrate: The compound or material which is to be acted upon.

Hydrogenation Reactions

Hydrogenation refers to the treatment of substances with molecular hydrogen (H2), adding pairs of hydrogen atoms to compounds (generally unsaturated compounds). These usually require a catalyst for the reaction to occur under normal conditions of temperature and pressure. Most hydrogenation reactions use gaseous hydrogen as the hydrogen source, but alternative sources have been developed. The reverse of hydrogenation, where hydrogen is removed from the compounds, is known as dehydrogenation. Hydrogenation differs from protonation or hydride addition because in hydrogenation the products have the same charge as the reactants.


Hydrogenation: Hydrogen can be added across a double bond—such as the olefin in maleic acid shown—by utilizing a catalyst, such as palladium.

Hydrogenation reactions generally require three components: the substrate, the hydrogen source, and a catalyst. The reaction is carried out at varying temperatures and pressures depending on the catalyst and substrate used. The hydrogenation of an alkene produces an alkane. The addition of hydrogen to compounds happens in a syn addition fashion, adding to the same face of the compound and entering from the least hindered side. Generally, alkenes will convert to alkanes, alkynes to alkenes, aldehydes and ketones to alcohols, esters to secondary alcohols, and amides to amines via hydrogenation reactions.

Catalysts of Hydrogenation

Generally, hydrogenation reactions will not occur between hydrogen and organic compounds below 480 degrees Celsius without metal catalysts. Catalysts are responsible for binding the H2 molecule and facilitating the reaction between the hydrogen and the substrate. Platinum, palladium, rhodium, and ruthenium are known to be active catalysts which can operate at lower temperatures and pressures. Research is ongoing to procure non-precious metal catalysts which can produce similar activity at lower temperatures and pressures. Nickel-based catalysts, such as Raney nickel, have been developed, but still require high temperatures and pressures.


Heterogeneous Catalysis: The hydrogenation of ethylene (C2H4) on a solid support is an example of heterogeneous catalysis.

Catalysts can be divided into two categories: homogeneous or heterogeneous catalysts. Homogeneous catalysts are soluble in the solvent that contains the unsaturated substrate. Heterogeneous catalysts are found more commonly in industry, and are not soluble in the solvent containing the substrate. Often, heterogeneous catalysts are metal-based and are attached to supports based on carbon or oxide. The choice of support for these materials is important, as the supports can affect the activity of the catalysts. Hydrogen gas is the most common source of hydrogen used and is commercially available.

Hydrogenation is an exothermic reaction, releasing about 25 kcal/mol in the hydrogenation of vegetable oils and fatty acids. For heterogenous catalysts, the Horiuti-Polanyi mechanism explains how hydrogenation occurs. First, the unsaturated bond binds to the catalyst, followed by H2 dissociation into atomic hydrogen onto the catalyst. Then one atom of hydrogen attaches to the substrate in a reversible step, followed by the addition of a second atom, rendering the hydrogenation process irreversible. For homogeneous catalysis, the metal binds to hydrogen to give a dihydride complex via oxidative addition. The metal binds the substrate and then transfers one of the hydrogen atoms from the metal to the substrate via migratory insertion. The second hydrogen atom from the metal is transferred to the substrate with simultaneous dissociation of the newly formed alkane via reductive elimination.

Industrial Uses of Hydrogenation Reactions

Heterogeneous catalytic hydrogenation is very important in industrial processes. In petrochemical processes, hydrogenation is used to saturate alkenes and aromatics, making them less toxic and reactive. Hydrogenation is also important in processing vegetable oils because most vegetable oils are derived from polyunsaturated fatty acids. Partial hydrogenation reduces most, but not all, of the carbon-carbon double bonds, making them better for sale and consumption. The degree of saturation of fats changes important physical properties such as the melting range of the oils; an example of this is how liquid vegetable oils become semi-solid at various temperatures.


Partial hydrogenation in margarine: Margarine is a semi-solid butter substitute created from vegetable oil, which is typically unsaturated and therefore liquid at room temperature. The process of partial hydrogenation adds hydrogen atoms and reduces the double bonds in the fatty acids, creating a semi-solid vegetable oil at room temperature.

Incomplete hydrogenation of the double bonds has health implications; some double bonds can isomerize from the cis to the trans state. This isomerization occurs because the trans configuration has lower energy than the cis configuration. The trans isomers have been implicated in contributing to pathological blood circulatory system conditions (i.e.,atherosclerosis and heart disease).

The Hydrogen Economy

The hydrogen economy refers to using hydrogen as the next important source of fuel.

Learning Objectives

Describe the hydrogen economy.

Key Takeaways

Key Points

  • The current hydrocarbon economy is becoming impractical because of increasing demand and diminishing resources. The hydrogen economy could act as a replacement because of its higher energy density and its smaller negative impact on the environment.
  • The hydrogen economy is limited because it is difficult to transport and store hydrogen. In addition, the dangers associated with hydrogen limit its practical application.
  • Hydrogen can be generated via several processes, but is predominately accomplished by steam reforming fossil fuels. This process requires a large input of energy and releases carbon dioxide.

Key Terms

  • hydrocarbon economy: Referring to the current global economy which is based on fossil fuels as the main energy source.
  • hydrogen economy: A hypothetical future economy in which the primary form of stored energy for mobile applications and load balancing is hydrogen (H2). In particular, H2 replaces fossil fuels used to power automobiles.

Introduction: The Hydrogen Economy

The hydrogen economy refers to a hypothetical future system of delivering energy through the use of hydrogen (H2). The term was first coined by John Bockris at a 1970 talk at the General Motors (GM) Technical Center. Advocates of this proposed system promote hydrogen as a potential fuel source. Free hydrogen does not occur naturally in quantities of use, like other energy sources, but it can be generated by various methods. As such, hydrogen is not a primary energy source, but an energy carrier. The feasibility of a hydrogen economy depends on issues including the use of fossil fuel, the generation of sustainable energy, and energy sourcing.

Comparing Hydrogen Energy to Other Sources

As a potential energy source, the hydrogen economy stands to eliminate or reduce the negative effects of using hydrocarbon fuels, the currently dominant energy source that releases high amounts of carbon into the atmosphere. In the current hydrocarbon economy, transportation is fueled by petroleum, the use of which ultimately results in the release of carbon dioxide (a greenhouse gas ) and many pollutants into the atmosphere. In addition, the supply of raw materials that are essential for a hydrocarbon economy is limited, and the demand for such fuels is increasing each year.

As a potential fuel, hydrogen is appealing because it has a high energy density by weight. This results in a 38% efficiency for a combustion engine, compared to 30% when gasoline was used as a fuel. In addition, it provides an environmentally clean source of energy that does not release pollutants. However, there are several obstacles for the use of hydrogen as a fuel, including the purity requirement of hydrogen and difficulties that arise with its storage.

Hydrogen production is a large and growing industry. Globally, 50 million metric tons of hydrogen (equivalent to 170 million tons of oil) were produced in 2004. There are two primary uses for hydrogen today. Half of the hydrogen produced is used to synthesize ammonia in the Haber process. The other half is used to convert heavy petroleum sources into lighter fractions which can be used as fuels. Currently, global hydrogen production is 48% from natural gas, 30% from oil, 18% from coal and 4% from water electrolysis.


The Hydrogen Economy: The hydrogen economy could possibly revolutionize the current energy infrastructure by transferring fuel demands from fossil fuels onto hydrogen.

Methods of Producing Hydrogen

Hydrogen production is mostly accomplished by steam reforming from hydrocarbons, but alternative methods are being developed. Steam reforming is conducted at high temperatures and possesses efficiencies up to 80%. The process involves methane and water and is highly exothermic:

[latex]\text{CH}_4 + \text{H}_2\text{O} \rightarrow \text{CO} + 3\text{H}_2 + 191.7 \text{kJ}/\text{mol}[/latex]

In a second stage, additional hydrogen is generated at a lower temperature:

[latex]\text{CO}+\text{H}_2\text{O} \rightarrow \text{CO}_2 + \text{H}_2 - 40.4\text{kJ}/\text{mol}[/latex]

Other ways of producing hydrogen from fossil fuels include partial oxidation and plasma reforming. Hydrogen can also be produced from water splitting. Fuel cells are electrochemical devices capable of transforming chemical energy into electrical energy. Fuel cells require less energy input than other alternatives and perform water electrolysis at lower temperatures, both of which have the potential of reducing the overall cost of hydrogen production. Water can also be split through thermolysis, but this requires high temperatures and catalysts. In addition, hydrogen can be produced via enzymes and bacteria fermentation, but this technology has not yet been prepared for main scale commercialization. Other methods include photoelectrocatalytic production, thermochemical production, and high temperature and pressure electrolysis.

Obstacles to Adoption of Hydrogen as a Fuel

One major obstacle in the hydrogen economy is its transport and storage. Although H2 has high energy density based on mass, it has very low energy density based on volume. This is a problem because at ambient conditions molecular hydrogen exists as a gas. To be a suitable fuel, hydrogen gas must be either pressurized or liquified to provide enough energy. Increasing the gas pressure will ultimately improve the energy density by volume, but this requires a greater amount of energy be expended to pressurize the gas. Alternatively, liquid hydrogen or slush hydrogen (a combination of liquid and solid hydrogen) can be used. Liquid hydrogen, however, is cryogenic and boils at 20 K, therefore a lot of energy must be expended to liquify the hydrogen.

Storing hydrogen in tanks is ineffective because hydrogen tends to diffuse through any liner material intended to contain it, which ultimately leads to the weakening of the container. Hydrogen can be stored as a chemical hydride or in some other hydrogen-containing compound. These compounds can be transported relatively easily and then decomposed into hydrogen gas. Current barriers to practical storage stem from the fact that high temperatures and pressure are needed for the compound to form and for the hydrogen to be released. Hydrogen can be adsorbed onto the surface of a solid storage material and then be released upon necessity; this technology is still being investigated.

Hydrogen has one of the widest explosive/ignition mix ranges with air. This means that any leak of hydrogen from a hydrogen:air mixture will most likely lead to an explosion if it comes into contact with a spark or flame. This limits the use of hydrogen as a fuel, especially in enclosed areas such as tunnels or underground parking. Pure hydrogen-oxygen flames burn in the UV range and are invisible, so a flame detector is needed to detect if a hydrogen leak is burning. Hydrogen is also odorless, so leaks cannot be detected by smell.

Although the hydrogen economy is supposed to create a smaller carbon footprint, there are many concerns regarding the environmental effects of hydrogen manufacturing. The main source of hydrogen is fossil fuel reforming, but this method ultimately leads to higher emissions of carbon dioxide than using the fossil fuel in an internal combustion engine. Other issues include the fact that hydrogen generation via electrolysis requires a greater energy input than directly using renewable energy, and the possibility of other side products.