Sorption-enhanced hydrogen production with in-situ CO2 capture on hollow Ca-based sorption-catalysis bifunctional materials: A review

Hydrogen is a versatile energy carrier with growing importance in low-carbon energy and industrial systems, and it can be produced through diverse thermochemical, electrochemical, and biological pathways. Among these pathways, the reforming of carbon-containing feedstocks, including methane, alcohols, and biomass-derived volatiles, remains an important route for hydrogen production; however, the accompanying CO 2 formation lowers H 2 purity and increases the burden of downstream carbon separation. Calcium-based sorption-catalysis bifunctional materials, integrating CaO-based CO 2 sorption sites with catalytic active metals such as Ni, Fe, and Co, offer an effective strategy for in situ CO 2 capture and sorption-enhanced reforming. However, conventional CaO-based materials suffer from sintering, pore blockage, and active-site degradation during repeated cycles. Hollow-structured calcium-based materials, with internal cavities, tunable shells, and hierarchical pores, may contribute to buffer the volume variation of CaO/CaCO 3 conversion, shorten gas diffusion pathways, and improve the exposure of sorption-catalytic active sites, thereby enhancing cyclic stability and reforming efficiency. Unlike previous reviews that are primarily organized around reaction systems, feedstocks, sorbent classes, or general stabilization strategies, this review adopts hollow structural design as the central framework for linking synthesis, cavity-shell architecture, sorption-catalysis performance, cyclic deactivation and regeneration, and scale-up. Hard-template, soft-template, and spray pyrolysis/combustion routes are compared, and the effects of cavity and shell geometry, porosity, defects, interfaces, and active-phase/stabilizer distribution on CO 2 sorption, catalytic performance, and cyclic stability are evaluated. The covered applications include CO 2 capture, sorption-enhanced reforming, water-gas shift, reforming of biomass- and waste-derived volatiles, and integrated capture-utilization through reverse water-gas shift, methanation, and methane dry reforming. Particular attention is given to CaO sintering, shell cracking, cavity collapse, metal agglomeration, carbon deposition, and corresponding mitigation and regeneration strategies. Finally, priorities in controllable synthesis, green scale-up, in situ characterization, mechanistic validation, and reactor compatibility are identified. Overall, this review serves as a reference for the rational design, performance evaluation, and scale-up of Ca-based bifunctional materials for calcium looping, sorption-enhanced hydrogen production, and integrated CO 2 capture and utilization.