Green hydrogen, produced by electrolysing water with renewable electricity, can connect variable renewable generation with hard-to-electrify sectors such as maritime transport, aviation, primary steelmaking, ammonia production and long-duration energy storage. This chapter places green, blue and grey hydrogen in a carbon-intensity and cost framework; reviews the thermodynamics and electrochemistry of water splitting; compares alkaline, proton-exchange-membrane, anion-exchange-membrane and solid-oxide electrolysers; and examines proton-exchange-membrane and solid-oxide fuel cells. Hydrogen storage and transport pathways—including compressed gas, liquid hydrogen, metal hydrides, ammonia and liquid organic hydrogen carriers—are evaluated using gravimetric capacity, volumetric density, energy penalty and infrastructure maturity. Applications in road and maritime transport, aviation, industry, stationary power and grid services are considered alongside safety, certification and digital system-integration issues. The chapter emphasizes that commercial deployment depends not only on electrochemical efficiency but also on renewable-electricity supply, critical-material use, durability, infrastructure, lifecycle emissions and credible governance.
Keywords:green hydrogen, water electrolysis, fuel cells, hydrogen storage, hard-to-abate sectors, hydrogen systems integration