Hydrogen (H₂) is the lightest and most abundant element in the universe, an odorless and colorless gas. It is not an energy source itself, but an energy carrier produced from various feedstocks. Hydrogen delivers a high energy density by mass, making it a versatile resource across a wide spectrum of modern applications, from chemical manufacturing to providing fuel for transportation and aerospace.
Hydrogen as a Critical Industrial Feedstock
Today, the majority of globally produced hydrogen is consumed as a chemical building block, or feedstock, for large-scale industrial processes. The single largest consumer is the fertilizer industry, where hydrogen is used to produce ammonia (NH₃). This occurs through the Haber-Bosch process, combining nitrogen from the air with hydrogen under high temperature and pressure using a catalyst. Ammonia forms the base for synthetic fertilizers, supporting global food production.
Hydrogen is also utilized in the petroleum refining sector for two primary processes: hydrotreating and hydrocracking. Hydrotreating uses hydrogen to remove contaminants like sulfur, nitrogen, and metals from crude oil fractions. This process, called desulfurization, is necessary to meet environmental standards for cleaner fuels.
Hydrocracking is a process where hydrogen breaks down heavier petroleum molecules into lighter, more valuable products such as gasoline, diesel, and jet fuel. In both refining applications, hydrogen improves the quality and yield of usable fuels. Hydrogen is also a key reactant in the synthesis of methanol, a foundational chemical used to create products including plastics and paints.
Powering Transportation Through Fuel Cells
Hydrogen is widely used in electrochemical devices called fuel cells, which generate electricity without combustion, producing only water vapor and heat. In a Polymer Electrolyte Membrane (PEM) fuel cell, hydrogen gas is fed to the anode. A catalyst splits the hydrogen molecules into protons and electrons. The protons travel through a membrane to the cathode, while the electrons are forced through an external circuit, creating an electric current to power a motor.
This principle is the basis for Fuel Cell Electric Vehicles (FCEVs), used in passenger cars, buses, and heavy-duty trucks. FCEVs offer advantages over battery-electric vehicles for long-haul and commercial fleets. Refueling takes only a few minutes, minimizing vehicle downtime and matching the speed of pumping gasoline. For heavy-duty applications, the weight of the fuel cell stack and storage tanks is often lighter than the large battery packs required for comparable range.
Fuel cells are deployed in stationary power applications, particularly in Combined Heat and Power (CHP) systems for buildings and industrial sites. These systems simultaneously generate electricity and capture waste heat for heating or cooling, achieving high energy efficiency. Hydrogen also functions as a flexible energy carrier to store intermittent renewable energy, such as solar and wind power. Excess electricity splits water into hydrogen via electrolysis, and the stored hydrogen can be converted back into electricity via fuel cells to balance the electrical grid.
Specialized High-Energy Applications
Hydrogen is suited for applications demanding high energy release or efficient heat transfer. In the aerospace sector, liquid hydrogen (LH₂) is used as a cryogenic propellant combined with liquid oxygen. This combination provides the highest specific impulse, a measure of rocket engine efficiency. LH₂ must be stored at approximately -423°F and is the fuel for powerful launch vehicles like NASA’s Space Launch System (SLS) for the Artemis missions.
Liquid hydrogen is being explored as a clean-burning fuel for future jet engines, either through direct combustion in modified gas turbines or via fuel cells powering electric motors. Hydrogen offers about three times the energy density per unit of mass compared to kerosene jet fuel, potentially reducing the required fuel weight for long-haul flights. Although this requires new aircraft designs for larger fuel tanks, it helps eliminate carbon dioxide emissions from air travel.
Hydrogen is also used as a coolant in large industrial turbo generators, typically those over 60 megawatts. Hydrogen gas is utilized because its thermal conductivity is seven to ten times greater than air. Its low density also reduces friction losses, known as windage. Circulating hydrogen through the generator’s casing efficiently dissipates heat, allowing the machine to operate at maximum efficiency.
Role in Metallurgy and Material Processing
Hydrogen’s properties as a strong reducing agent are important in metallurgy, particularly in the production of “green steel.” This involves the Direct Reduction of Iron (DRI) process, where hydrogen gas is used instead of carbon monoxide to strip oxygen from iron ore. This chemical reaction produces only metallic iron and water vapor, bypassing the carbon dioxide emissions associated with blast furnaces.
Hydrogen is used to create an oxyhydrogen flame for welding and cutting applications requiring high temperatures and a clean flame. The combustion of pure hydrogen and oxygen can reach temperatures near 2800°C, suitable for precision work. This carbon-free flame is valuable for processing materials like glass and certain metals where carbon contamination must be avoided.
In the electronics and semiconductor industry, hydrogen is used to maintain the ultra-pure, controlled environments necessary for microchip fabrication. It serves as a reducing agent in processes like epitaxy and annealing, which grow and repair crystalline layers on silicon wafers. Hydrogen is also used for cleaning residual materials in advanced manufacturing techniques, such as reacting with tin deposits in Extreme Ultraviolet (EUV) lithography.