What Is Hydrogen?

Hydrogen is the lightest and most abundant element in the universe, consisting of a single proton and a single electron in its most common isotopic form. At standard temperature and pressure, it exists as a colorless, odorless, highly flammable diatomic gas (H2) with a molecular weight of approximately 2 grams per mole. On a mass basis, hydrogen carries nearly three times the energy content of gasoline, roughly 120 megajoules per kilogram compared to 44 megajoules per kilogram for conventional petroleum fuels, making it attractive as both an industrial feedstock and a potential energy carrier.

In nature, hydrogen rarely occurs as free gas on Earth because its low molecular mass allows it to escape the atmosphere. It is instead found bonded in water, hydrocarbons, and biomass. Industrially, hydrogen is one of the most widely produced chemicals: global production exceeds 90 million metric tons per year, the majority of which goes to petroleum refining and ammonia synthesis for fertilizer production. The U.S. Department of Energy's Hydrogen Program supports research across the hydrogen value chain, from production and storage to end-use conversion, with the goal of expanding hydrogen's role in clean energy systems.

Production Methods and Water Splitting

The dominant production pathway today is steam methane reforming (SMR), which reacts natural gas with steam at high temperatures to yield hydrogen and carbon dioxide. SMR is inexpensive at scale but releases CO2 unless paired with carbon capture. Electrolysis, which uses electrical current to split water molecules into hydrogen and oxygen, produces hydrogen without direct carbon emissions when powered by renewable electricity. This water-splitting approach is described by the IEA Hydrogen program as central to decarbonizing hydrogen supply, with proton exchange membrane (PEM) electrolyzers and alkaline electrolyzers being the two leading commercial technologies. Other routes under development include thermochemical water splitting, which uses concentrated solar heat or nuclear process heat to drive chemical cycles, and biological processes using microorganisms that generate hydrogen as a metabolic byproduct.

Hydrogen as an Energy Carrier

Unlike coal, oil, or natural gas, hydrogen is an energy carrier rather than a primary energy source: energy must first be invested to produce it, after which it can be stored, transported, and converted back to useful work. In fuel cells, hydrogen reacts electrochemically with oxygen to produce electricity and water, with no combustion and no local pollutant emissions. Hydrogen can also be burned directly in turbines or internal combustion engines. Its volumetric energy density is low as a gas at ambient conditions, which creates logistical challenges for storage and transport, addressed through compression, liquefaction, or chemical bonding in metal hydrides and liquid organic hydrogen carriers. The potential to store large quantities over extended periods positions hydrogen as a seasonal storage medium that could complement variable renewable electricity generation.

Safety and Handling

Hydrogen's wide flammability range (4 to 75 percent by volume in air) and low ignition energy demand careful attention to containment, ventilation, and material compatibility. Hydrogen embrittlement, in which hydrogen atoms diffuse into metal lattices and reduce mechanical strength, is a known concern for high-pressure storage vessels and pipelines. Standards for hydrogen safety and infrastructure are developed by bodies including ISO and IEC, with specific guidance covering compressed gas cylinders, dispensing equipment, and indoor hydrogen systems. The physical properties and thermodynamic characteristics of hydrogen relevant to safe handling are detailed in publications such as research published in PMC through NCBI.

Applications

Hydrogen has applications across a broad set of industrial and energy sectors, including:

• Ammonia synthesis for agricultural fertilizers
• Petroleum refining and hydrocracking
• Fuel cells for transportation and stationary power generation
• Steelmaking via direct reduction of iron ore
• Synthetic fuel and chemical feedstock production
• Long-duration grid energy storage