Dominant role of CaCl₂ in integrated CO₂ capture and utilization (ICCU-RWGS) in chloride-based molten salts

Integrated CO₂ capture and utilization (ICCU) in chloride-based molten salts provides a promising solar-driven pathway toward carbon-neutral syngas production, yet the mechanistic role of chloride components remains poorly understood. Here, we demonstrate that CaCl₂ governs both reaction kinetics and pathway selection by tuning the solubility equilibrium of CaO and CaCO₃. In CaCl₂-NaCl-KCl molten systems, moderate CaCl₂ contents (40–60 wt%) markedly enhance CO yield (from 2.4 to 5.8 mmol g⁻¹) and CO₂ conversion (up to 78%), whereas excessive CaCl₂ (>60 wt%) suppresses both capture and conversion. In situ spectroscopy and kinetic analysis reveal that CaCl₂-mediated dissolution of carbonate species shifts the reaction mechanism from the solid-state formate route to a homogeneous RWGS pathway, accelerating CO formation. These findings uncover a previously overlooked function of molten salts as active chemical regulators rather than passive heat carriers and establish a solubility-governed framework for designing solar-compatible ICCU systems with tunable performance and enhanced stability.