• Hydrogen is the simplest and most abundant of all chemical elements.
• Hydrogen is an ‘energy carrier’ - a substance containing energy that can be converted to useful mechanical, heat, or electrical energy.
• When extracted from water using electrolysis powered by renewable electricity, so-called “green” hydrogen may be used as a carbon-free substitute for fossil fuels.
• In certain applications, such as industrial heating and long-distance shipping, hydrogen may be the best - or even the only - low-emission alternative to the direct use of fossil fuels. In other areas, like residential heating and short-distance transport, other low-carbon options may be more suited or cost-effective.

Hydrogen (the chemical symbol H for a single hydrogen atom), is the simplest and most abundant element in the universe. It has the atomic number 1. Hydrogen gas (H₂) is one of only seven naturally occurring homonuclear diatomic (consisting of two identical atoms) molecules.

At normal room temperature and pressure, hydrogen is a non-toxic gas. It has no taste, colour, or smell, and it has the lowest density of all gases. If cooled to extremely cold temperatures (-253°C or lower) or put under pressure, it becomes a liquid or under extreme conditions, solid. It is extremely volatile and easily ignites in air, burning at over 2,000°C with a very pale blue, near-colourless flame.

When hydrogen burns, it reacts with oxygen in the air to produce water – its name even broadly translates to ‘water-maker’. As a carbon-free fuel, the combustion of hydrogen does not produce carbon dioxide (CO₂). However, at high temperatures, nitrogen in the air reacts to form nitrogen oxides (NOx). These acidifying pollutants can directly cause damage to buildings and ecosystems, particularly through acid rain, and contribute to greenhouse gases by producing ozone via photochemical reactions in the atmosphere. Fortunately, NOx emissions can be managed with established low-NOx burner technologies.

Whilst hydrogen can exist in natural geological environments (known as “white”, “geologic”, or “gold” hydrogen), it is very reactive and forms chemical bonds with many other elements. Consequently, nearly all hydrogen used today comes from substances such as natural gas (~63%), coal (~20%) and as a by-product in refineries and in the petrochemical industry (~15%).Only a very small fraction is produced from biomass and through the electrolysis of water.

As well as using hydrogen as a chemical feedstock (e.g. ammonia and methanol manufacture) or as a reducing agent in primary iron extraction for the steel industry, hydrogen has several potential energy carrier applications:

• High-grade heat for industrial processes (e.g., glass, cement, etc.)
• Replacement for fossil fuels in transport (e.g., shipping, aviation, HGVs, etc.)
• In power generation, either as a replacement for natural gas in gas turbine generation, or in hydrogen fuel cells for utility scale or distributed power generation
• Storing surplus renewable electricity to support load balancing and enhance grid stability
• Low-grade heat (e.g., domestic, commercial, and industrial)

While hydrogen has broad potential applications, several factors could constrain its use in a future low-carbon economy. These include its current relatively high costs compared to alternatives, limited availability, and high flammability. For instance, hydrogen may be the most effective - or even the only - substitute for fossil fuels in certain applications like heavy industrial processes, heavy haulage, and sea freight transportation. However, in other areas, such as residential heating and light, or personal transport, there may be more suitable or cost-effective alternatives like heat-pumps or electric vehicles. This view may differ depending on regional circumstances and may change over time as technologies and markets evolve.