WHAT IS HYDROGEN?
Hydrogen
Hydrogen (molecular formula H₂) is the simplest and lightest element in the periodic table, possessing unique physical and technical properties that make it essential in various applications and research fields.
Physical properties
The molecular mass of hydrogen gas is 2.016 g/mole. At room temperature, the gas is colorless, odorless, and tasteless. The density is about 0.08988 gH2/L at 0°C and 1 atm, making it the least dense of all gases. The phase transitions are at -252.87°C (20.28 K) and -259.14°C (14.01 K), respectively. It is slightly soluble in water and other solvents.
Technical properties
Hydrogen is highly flammable and can form explosive mixtures with air. The flammability range is 4% to 75% hydrogen in air and the autoignition temperature is about 500°C (932°F). Hydrogen has high thermal conductivity (0.1805 W/m·K at 300 K), making it useful as a coolant in high-performance applications like power generators and nuclear reactors. Hydrogen has a high specific energy (energy per unit mass) of about 142 MJ/kg, more than double that of conventional hydrocarbon fuels. Hydrogen gas diffuses rapidly, with a diffusivity in air of approximately 0.61 cm²/s at room temperature, which influences its behavior in storage and transport. More information specifically about hydrogen safety can for instance be found in this article.
Hydrogen gas as an energy carrier
The gas may play a vital role in the transition to a renewable and environmentally friendly energy system, the energy transition. One of the principal reasons for this is that it can be generated using renewable electricity and water with a process called electrolysis. Certain available and developing electrolysis technologies are summarized here.
If hydrogen and carbon dioxide are available, fuels like methane, today predominantly of fossil and non-renewable origin, can be reproduced. This is demonstrated for example at the Greengas project in Aigle.