Hydrogen, the lightest and most abundant element in the universe, is a colorless, odorless gas under standard conditions. Its simple atomic structure, consisting of one proton and one electron, gives it unique chemical versatility. Today, hydrogen is gaining attention as a potential energy carrier to help decarbonize hard-to-abate sectors like heavy industry and long-haul transport. Understanding the element requires looking beyond its basic chemistry to modern classifications based on production methods and physical states, which dictate its environmental impact and practical application.
The Element’s Unique Place in Chemistry
Hydrogen is categorized as a nonmetal, but its placement on the periodic table is a subject of long-standing debate among chemists. It is conventionally placed in Group 1, above the alkali metals like lithium and sodium, because it shares the electron configuration of having one electron in its outermost shell (1s1). Like the alkali metals, hydrogen readily loses this single electron to form a positive ion (H+), or a proton, allowing it to participate in chemical reactions.
The element also exhibits properties similar to the halogens in Group 17, as it can gain one electron to achieve a stable, full outer shell, forming a negative hydride ion (H-). This dual chemical nature means hydrogen does not comfortably fit into any single group and is often treated as a unique element. Unlike metals, hydrogen exists naturally as a diatomic molecule (H2), a non-metallic gas, which solidifies its classification as a non-metal under ordinary conditions. Its three isotopes—protium, deuterium, and tritium—also have distinct physical properties.
Classifying Hydrogen by Production Method
The most common modern classification system categorizes hydrogen by “color,” which refers to the carbon intensity and source material used during its production. This system is designed to quickly communicate the environmental impact of the hydrogen being discussed. The vast majority of hydrogen currently produced globally falls into the high-emission categories.
Gray Hydrogen
Gray hydrogen is the most common and least environmentally friendly form of industrial hydrogen today. It is produced primarily from natural gas through a process called Steam Methane Reforming (SMR). In SMR, high-temperature steam reacts with methane (CH4) to yield hydrogen and carbon dioxide (CO2). The CO2 generated during this process is released directly into the atmosphere, resulting in a high carbon footprint, typically around 9 to 12 kilograms of CO2 for every kilogram of hydrogen produced.
Blue Hydrogen
Blue hydrogen is also derived from fossil fuels, like natural gas, using the SMR process. The distinction is that blue hydrogen incorporates Carbon Capture, Utilization, and Storage (CCUS) technology to mitigate emissions. CCUS aims to capture a large percentage of the CO2 byproduct—ideally 90% or more—and store it permanently underground in geological formations. While this significantly reduces the carbon footprint compared to gray hydrogen, it is not entirely carbon-free. Its effectiveness depends heavily on the capture rate and the prevention of methane leakage during the process.
Green Hydrogen
Green hydrogen is considered the cleanest form, as its production results in zero or near-zero greenhouse gas emissions. It is generated through the electrolysis of water, a process where an electrical current splits water (H2O) into hydrogen (H2) and oxygen (O2). For the hydrogen to be classified as “green,” the electricity powering the electrolyzers must come exclusively from renewable energy sources, such as wind, solar, or hydropower.
Emerging Classifications
The color-coding system extends to other production pathways that are currently being developed or are minor contributors. Pink hydrogen, for example, is produced via electrolysis, but the electricity comes from nuclear power plants. Yellow hydrogen is a similar classification where the electrolysis is powered by electricity from the general grid mix, meaning its carbon intensity varies greatly depending on the local power sources.
Classifying Hydrogen by Physical State
Beyond its chemical and production classifications, hydrogen is also categorized by its physical state, which is a practical classification governing how it is stored and transported for use. Because hydrogen has a very low energy density by volume at ambient conditions, it must be compressed or cooled to maximize the amount that can be stored.
Gaseous Hydrogen
The most common physical state for storage is compressed gaseous hydrogen. This involves storing the gas under extremely high pressure in specialized tanks, typically at 350 bar for commercial vehicles or 700 bar for light-duty vehicles. Compressing the hydrogen increases its volumetric energy density, though even at 700 bar, the density is still significantly lower than that of liquid fuels. High-pressure storage requires robust, fiber-reinforced composite tanks to ensure safety and integrity.
Liquid Hydrogen (LH2)
Liquid hydrogen (LH2) is achieved by cooling the gas to cryogenic temperatures, specifically -252.8°C. This process is energy-intensive but results in a much higher volumetric energy density compared to compressed gas, making it suitable for applications that require a large amount of fuel in a small space, such as rockets. Storing LH2 requires heavily insulated, cryogenic tanks to maintain the low temperature and manage the boil-off that occurs as the liquid slowly warms.
Solid-State Hydrogen
Solid-state hydrogen refers to methods where hydrogen is chemically or physically bound to a solid material for storage. One promising method involves the use of metal hydrides, which are compounds where hydrogen atoms are absorbed into the crystal lattice of a metal alloy. This classification offers an alternative storage approach with lower operating pressures and potentially higher volumetric density than compressed gas, though the gravimetric capacity and cost remain areas of ongoing research.