Chemical looping and chemical looping gasification for hydrogen: Technologies, scale-up status, and data-driven process optimization

• A comprehensive review of conventional, renewable, and emerging hydrogen pathways. • Shows the superiority of chemical looping technologies in efficiency and CO₂ capture. • Identifies CLG as a route to net-zero and net-negative hydrogen production • Compares alkaline, PEM, and SOEC for large-scale green hydrogen systems. • Evaluates techno-economics, oxygen carriers, scalability, and ML-based optimization Hydrogen is widely recognized as a key energy carrier for achieving deep decarbonization across power generation, industry, and transportation sectors. However, current hydrogen production remains dominated by fossil-fuel-based processes, particularly steam methane reforming, which are associated with significant carbon emissions. In this context, chemical looping (CL) and chemical looping gasification (CLG) have emerged as promising thermochemical pathways capable of producing hydrogen and syngas with inherent CO₂ separation and high energy efficiency. This review provides a comprehensive and structured assessment of chemical looping–based hydrogen technologies, with a primary focus on CL and CLG systems, spanning reaction mechanisms, oxygen carrier materials, reactor configurations, scales of implementation, and process integration strategies. Special attention is given to biomass- and waste-based CLG routes, which offer the potential for carbon-neutral or carbon-negative hydrogen production. Decoupled and dual-loop gasification architectures are also examined as advanced configurations that enhance tar reduction, hydrogen selectivity, and operational flexibility. In addition, this work presents a comparative evaluation of major hydrogen production pathways, including ATR, conventional gasification, and looping-based systems, thereby positioning chemical looping within the broader landscape of hydrogen technology. . Unlike previous reviews, this work integrates reactor-scale developments with machine-learning-assisted optimization, techno-economic indicators, and scale-up readiness to provide a unified roadmap for chemical looping hydrogen systems. Key technical challenges related to oxygen carrier durability, reactor hydrodynamics, fuel variability, and economic viability are identified, along with future research directions. Overall, this review establishes chemical looping as a versatile and forward-looking platform for sustainable hydrogen production.