Dihydrogen

Dihydrogen is the simplest molecule — two hydrogen atoms bonded by a covalent bond — and probably the most strategic of the 21st century. Its current centrality comes from two uses: ~75 Mt produced annually as chemical feedstock (mainly for Haber-Bosch and petroleum refining, which consumes H₂ to hydrocrack heavy hydrocarbons into gasolines and to desulfurise diesels), and an emerging role as a clean energy carrier in industrial decarbonisation.

Production: today ~95 % of H₂ is called "grey", from steam methane reforming (CH₄ + H₂O → CO + 3 H₂ at 800 °C over Ni catalyst), emitting ~10 kg of CO₂ per kg of H₂ produced. "Blue" H₂ adds a CO₂ capture-storage step on the co-product, lowering the footprint to ~1-2 kg CO₂/kg. "Green" H₂ comes from water electrolysis (2 H₂O → 2 H₂ + O₂), with a theoretical consumption of ~40 kWh/kg H₂ — the reference pathway for decarbonisation, provided it is powered by low-carbon electricity (renewable or nuclear). In 2024, its production remains marginal (~1 Mt/yr worldwide) but is growing exponentially: European targets of 10 Mt of green H₂ by 2030.

Energetically, H₂ has an exceptional mass-specific calorific value (120 MJ/kg, or ~3× that of gasoline) but a very low volumetric density (8 g/L at 700 bar compressed vs 730 g/L for gasoline), which complicates onboard storage. Its fuel cell (reverse electrolysis, H₂ + ½ O₂ → H₂O + electricity) reaches ~60 % efficiency vs ~25 % for an internal-combustion engine, but the full chain "electricity → electrolysis → storage → fuel cell" drops to ~30 %, compared to ~85 % for a lithium-ion battery. This is why H₂ is today favoured for uses where batteries do not fit (steelmaking, heavy chemistry, long-haul transport) rather than as a rival to passenger EVs.