Hydrogen as an energy carrier: Production pathways, thermochemical constraints, and electrolysis-based green hydrogen prospects

Hydrogen is widely recognized as a key energy carrier for enabling deep decarbonization across power generation, transportation, and energy-intensive industrial sectors. However, large-scale deployment from renewable energy remains constrained by the efficiency and cost of current production routes. This review provides a comprehensive assessment of hydrogen production technologies, encompassing both thermochemical processes and electrolysis-based green hydrogen systems. Conventional pathways, including steam methane reforming (SMR) and coal and biomass gasification, are evaluated from thermodynamic and kinetic perspectives, with emphasis on reaction energetics, process efficiency, and carbon emissions. The thermodynamics and kinetics of water electrolysis are examined in detail, highlighting the roles of Gibbs free energy, overpotentials, temperature, and kinetically sluggish oxygen evolution reactions. Recent advances in alkaline, proton exchange membrane (PEM), and solid oxide electrolyzer cells (SOECs) are critically assessed with respect to efficiency, durability, scalability, and compatibility with variable renewable electricity. Emerging electrocatalysts, particularly layered double hydroxide (LDH)-based materials, are discussed as effective routes for accelerating oxygen evolution kinetics and reducing system-level electrolysis losses. By explicitly linking thermodynamic and kinetic constraints with system-level loss analysis and catalyst development, this review presents a unified framework for evaluating hydrogen production technologies beyond conventional efficiency comparisons. The analysis demonstrates that efficiency improvements in hydrogen production are fundamentally constrained by irreversible anodic kinetics rather than thermodynamic minimum work, positioning LDH-based oxygen evolution catalysts as the most impactful lever for near-term cost and performance gains in water electrolysis. • Fossil-based hydrogen supplies >95% via SMR and gasification, emitting ∼830 Mt CO 2 yr −1 . • Reaction enthalpies quantify reforming and gasification thermodynamic limits. • Electrolysis efficiency is limited by OER activation losses. • LDH catalysts reduce anodic overpotential and improve electrolyzer efficiency. • A unified thermodynamic–kinetic framework evaluates hydrogen pathway scalability.