Hydrogen gas (H2)
Hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H2. With an atomic weight of 1. 00794, hydrogen is the lightest element. Besides the common H1 isotope, hydrogen exists as the stable isotope Deuterium and the unstable, radioactive isotope Tritium. Hydrogen is the most abundant of the chemical elements, constituting roughly 75% of the universe’s elemental mass. 
Hydrogen can form compounds with most elements and is present in water and most organic compounds. It plays a particularly important role in acid-base chemistry. Oxidation of hydrogen in the sense of removing its electron, formally gives H+, containing no electrons and a nucleus which is usually composed of one proton. That is why H+ is often called a proton.
H2 is easily ignited. Once ignited it burns with a pale blue, almost invisible flame. It is flammable over a wide range of vapor(air) concentrations.
Dihydrogen is an elemental molecule consisting of two hydrogens joined by a single bond. It has a role as an antioxidant, an electron donor, a fuel, a human metabolite and a member of food packaging gas.
The name derives from the Greek hydro for “water” and genes for “forming” because it burned in air to form water. Hydrogen was discovered by the English physicist Henry Cavendish in 1766.
Scientists had been producing hydrogen for years before it was recognized as an element. Robert Boyle produced hydrogen gas as early as 1671 while experimenting with iron and acids. Hydrogen was first recognized as a distinct element by Henry Cavendish in 1766. From the Greek word hydro (water), and genes (forming). Hydrogen was recognized as a distinct substance by Henry Cavendish in 1776. Diagram of a simple hydrogen atom.
The heavier elements were originally made from hydrogen atoms or from other elements that were originally made from hydrogen atoms.
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. All heavier isotopes are synthetic and have a half-life less than a (10-21 sec). Of these, 5H is the most stable, and the least stable isotope is 7H .
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. H. Monoatomic hydrogen is rare on Earth due to its propensity to form covalent bonds with most elements. Hydrogen is also prevalent on Earth in the form of chemical compounds such as hydrocarbons and water. 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. , 
Table(1): properties of Hydrogen and its isotopes
|Tritium ( T)||Deuterium (D)||Protium (H)||property|
|3.016||2.014||1.008||Atomic mass (amu)|
|Radioactive(t1/2=12.3 yrs)||stable||stable||Nuclear stability|
|6.032||4.028||2.016||Molecular mass (amu)|
|20.62||18.73||13.96||Melting point (K)|
|25.04||23.67||20.30||Boiling point (K)|
|74.14||74.14||74.14||Internuclear distance (pm)|
|446.9||443.4||435.9||Enthalpy of dissociation (kJ/mol)|
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+). These species is essential in acid-base chemistry.
Table(2): Chemical properties of Hydrogen
|H, Hydrogen gas(H2)||Chemical formula|
|It burns in air or oxygen to produce water|
H2 reacts with every oxidizing element
|Combining hydrogen and nitrogen at high pressure and temperature produces ammonia (NH3) Combined with carbon monoxide produces methanol (CH3OH)||Reactivity with gases|
|It combines readily with non-metals, such as sulfur and phosphorus. It combines readily with the halogens which include fluorine, chlorine, bromine, iodine, and astatine||Reactivity with non-metals|
|Highly Flammable, a highly combustible diatomic gas||Flammability|
|When mixed with air and with chlorine it can spontaneously explode by spark, heat or sunlight. Example: the destruction of the Hindenburg airship||Combustion|
|Common acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), acetic acid (HC2H3O2) and phosphoric acid (H3PO4||Acid compounds|
Hydrogen can be produced using a number of different processes. Thermochemical processes use heat and chemical reactions to release hydrogen from organic materials such as fossil fuels and biomass. Water (H2O) can be split into hydrogen (H2) and oxygen (O2) using electrolysis or solar energy. Microorganisms such as bacteria and algae can produce hydrogen through biological processes. 
Some thermal processes use the energy in various resources, such as natural gas, coal, or biomass, to release hydrogen from their molecular structure. In other processes, heat, in combination with closed-chemical cycles, produces hydrogen from feedstocks such as water. For example;
- Natural Gas Reforming (also called steam methane reforming or SMR):
Today, 95% of the hydrogen produced in the United States is made by natural gas reforming in large central plants. This is an important technology pathway for near-term hydrogen production.
In this method Natural gas contains methane (CH4)
that can be used to produce hydrogen
with thermal processes, such as
steam-methane reformation and partial oxidation.
Steam-methane reforming reaction
CH4 + H2O (+ heat) → CO + 3H2
Water-gas shift reaction
CO + H2O → CO2 + H2 (+ small amount of heat)
Electrolizers use electricity to split water into hydrogen and oxygen. This technology is well developed and available commercially, and systems that can efficiently use intermittent renewable power are being developed.
This reaction takes place in a unit called an electrolyzer. Electrolyzers can range in size from small, appliance-size equipment that is well-suited for small-scale distributed hydrogen production to large-scale, central production facilities that could be tied directly to renewable or other non-greenhouse-gas-emitting forms of electricity production. Different electrolyzers function in slightly different ways, mainly due to the different type of electrolyte material involved.
Direct Solar Water Splitting Processes
Direct solar water splitting, or photolytic processes use light energy to split water into hydrogen and oxygen. These processes are currently in the very early stages of research but offer long-term potential for sustainable hydrogen production with low environmental impact. For example;
- Photoelectrochemical (PEC):
In (PEC) water splitting, hydrogen is produced from water using sunlight and specialized semiconductors called photoelectrochemical materials, which use light energy to directly dissociate water molecules into hydrogen and oxygen. This is a long-term technology pathway, with the potential for low or no greenhouse gas emissions. In this method The PEC water splitting process uses semiconductor materials to convert solar energy directly to chemical energy in the form of hydrogen. The semiconductor materials used in the PEC process are similar to those used in photovoltaic solar electricity generation, but for PEC applications the semiconductor is immersed in a water-based electrolyte, where sunlight energizes the water-splitting process.
Microbes such as bacteria and microalgae can produce hydrogen through biological reactions, using sunlight or organic matter. These technology pathways are at an early stage of research, but in the long term have the potential for sustainable, low-carbon hydrogen production. For example;
In photolytic biological systems, microorganisms—such as green microalgae or cyanobacteria—use sunlight to split water into oxygen and hydrogen ions. The hydrogen ions can be combined through direct or indirect routes and released as hydrogen gas. Challenges for this pathway include low rates of hydrogen production and the fact that splitting water also produces oxygen, which quickly inhibits the hydrogen production reaction and can be a safety issue when mixed with hydrogen in certain concentrations.
Applications of H2
Hydrogen is versatile and can be utilized in various ways.
These multiple uses can be grouped into two large categories:
- Hydrogen as a feedstock. A role whose importance is being recognized for decades and will continue to grow and evolve.
- Hydrogen as an energy vector enabling the energy transition. The usage of hydrogen in this context has started already and is gradually increasing. In the coming this field will grow dramatically. The versatility of Hydrogen and its multiple utilization is why hydrogen can contribute to decarbonize existing economies.
Here is some recent researches about Hydrogen applications :
- Molecular Hydrogen as a preventive and therapeutic medical gas: initiation, development and potential of Hydrogen medicine
H2 has been accepted to be an inert and nonfunctional molecule in our body. H2 reacts with strong oxidants such as hydroxyl radical in cells, and proposed its potential for preventive and therapeutic applications. Since most drugs specifically act to their targets, H2 seems to differ from conventional pharmaceutical drugs. Owing to its great efficacy and lack of adverse effects,H2 has promising potential for clinical use against many diseases.
· Hydrogen utilization enhancement of proton exchange membrane fuel cell with anode recirculation system through a purge strategy
Proton exchange membrane (PEM) fuel cells are widely considered as potential alternative energy candidates for internal combustion engines because of their low-temperature start, high energy density, and ease of scale up. However, their low hydrogen utilization rate is one of the main reasons for the limited commercial development.
The hydrogen utilization rate is the highest when the purge time is 0.3 s and the purge period is 10 s. Simulation results show that the PEM fuel cells with an anode recirculation configuration exhibit a better performance than other configurations in terms of hydrogen utilization. Experimental results also demonstrate the feasibility of the proposed system, the performance of which is also superior to that of other hydrogen supply system.
- Medical application of Hydrogen in cancer treatment
Hydrogen gas has been recognized as one medical gas that has potential in the treatment of cardiovascular disease, inflammatory disease, neurodegenerative disorders, and cancer. As a hydroxyl radical, and due to its anti-inflammatory effects, hydrogen gas may work to prevent/relieve the adverse effects caused by chemotherapy and radiotherapy without compromise their anti-cancer potential. Hydrogen gas may also work alone or synergistically with other therapy to suppress tumor growth via inducing apoptosis, inhibiting CSCs-related and cell cycle-related factors, etc.
- Hydrogen industrial use
Hydrogen is used in the production of carbon steels, special metals and semiconductors. In the electronics industry, it is widely employed as a reducing agent and as a carrier gas. High-purity hydrogen is also used as a carrier gas in gas chromatography.
Nowadays, hydrogen is used in several industrial processes. Among other applications, it is important to point its use as raw material in the chemical industry, and also as a reductor agent in the metallurgic industry. Hydrogen is a fundamental building block for the manufacture of ammonia, and hence fertilizers, and of methanol, used in the manufacture of many polymers. Refineries, where hydrogen is used for the processing of intermediate oil products, are another area of use. Thus, about 55 % of the hydrogen produced around the world is used for ammonia synthesis, 25 % in refineries and about 10 % for methanol production. 
Gaseous hydrogen can be transported through pipelines much the way natural gas is today. Owned by merchant hydrogen producers, these pipelines are located where large hydrogen users, such as petroleum refineries and chemical plants, are concentrated such as the Gulf Coast region.
Transporting gaseous hydrogen via existing pipelines is a low-cost option for delivering large volumes of hydrogen. The high initial capital costs of new pipeline construction constitute a major barrier to expanding hydrogen pipeline delivery infrastructure. Research today therefore focuses on overcoming technical concerns related to pipeline transmission, including:
- The potential for hydrogen to embrittle the steel and welds used to fabricate the pipelines
- The need to control hydrogen permeation and leaks
- The need for lower cost, more reliable, and more durable hydrogen compression technology.
Potential solutions include using fiber reinforced polymer (FRP) pipelines for hydrogen distribution. The installation costs for FRP pipelines are about 20% less than that of steel pipelines because the FRP can be obtained in sections that are much longer than steel, minimizing welding requirements.
One possibility for rapidly expanding the hydrogen delivery infrastructure is to adapt part of the natural gas delivery infrastructure to accommodate hydrogen. Converting natural gas pipelines to carry a blend of natural gas and hydrogen (up to about 15% hydrogen) may require only modest modifications to the pipeline.3 Converting existing natural gas pipelines to deliver pure hydrogen may require more substantial modifications. Current research and analyses are examining both approaches. , 
Safety considerations and risks
Hydrogen storage can be challenging as a result of unfavourable volumetric energy density resulting in:
- High Pressures (up to 700 bar-g)
- Extremely Low Temperatures (-253C)
Indirect storage via hydrogen carriers poses challenges in volume, H2 liberation processes and cost. Furthermore hydrogen given its minimal molecular size is very leaky, diffusing through containment materials. Catastrophic rupture of 700 bar-g storage tanks will release tremendous energy.
The consequence can be limited by location and choice of tanks
– Sonic release – speed of sound 1270 m/s (2.8 x methane)
– Rapid dilution in air for free jet
– Open installations with sufficient distance away from structure/obstructions may limit gas concentrations to < 15% H2 in air
Minimum ignition energy 0.019 mJ (methane 0.29 mJ) Static electricity sparks ~1 mJ
Ignition phenomena often hard to explain and may include but are not limited to
–Equipment/component static electricity and discharges
–Sparks from dust/particles in a jet
–Shockwaves heating pockets of H2 and air above AIT
Impact of Hydrogen on the environment
Hydrogen has been portrayed by the media as a fuel that is environmentally clean because its combustion results in the formation of harmless water, however Hydrogen first must be generated. The effect of Hydrogen generation on the environment depends on the production process and the related by products.
The introduction of Hydrogen as fuel will have a beneficial effect on the environment only, if Hydrogen is generated using renewable energy, because it does not lead to carbon emission. The hydrogen fuel available in the market is generated mainly by using steam reforming of natural gas. Its generation leads to emission of greenhouse gases at the same level as in the combustion of fossil fuels.
: Pharmacology & Therapeutics. Volume 144, Issue 1, October 2014, Pages 1-11.shigeo ohta. Department of Biochemistry and Cell Biology, Institute of Development and Aging Sciences.
: International Journal of Hydrogen Energy. Kun-Yung shen, Suhan Park, Young-Bae Kim. Department of Mechanical Engineering, Chonnam National University, Gwangju, 61186, Republic of Korea