Hydrogen: Characteristics
Huge energy per kilo, awkward bulk per litre
What this tab covers The physical and chemical properties that set its grade, price and uses, with the commercial grades and the standards that define them.Hydrogen is the lightest and most energy-dense gas by mass (about 120 MJ/kg LHV) but very bulky by volume. Purity matters most for fuel cells and electronics, while refineries accept lower grades. Low density, embrittlement of steel and a wide flammability range drive the cost of storage and transport.
Properties that matter commercially
| Property | Value / specification | Why it matters | Source |
|---|---|---|---|
| Energy content | about 120 MJ/kg LHV, 33.3 kWh/kg | sets value as fuel and electrolysis efficiency | physical data (indicative) |
| Density | 0.09 kg/m3 at 0 C; 71 kg/m3 as liquid at -253 C | makes transport costly; drives conversion to ammonia | physical data (indicative) |
| Purity for fuel cells | at least 99.97% with ppm limits on CO and sulphur (ISO 14687) | CO and sulphur poison PEM catalysts | ISO 14687 (indicative) |
| Flammability | 4-75% by volume in air | drives safety zoning and leak detection | physical data (indicative) |
Commercial grades
- Industrial 99.9% (refining, chemicals)
- High purity 99.999% (electronics, glass float baths)
- Fuel cell grade 99.97% to ISO 14687
- Liquid hydrogen (aerospace)