Reversible octahedral - tetrahedral coordination transformation in CoCl2-based thermochromic films for early fire-warning and energy-efficient smart windows

Designing reversible thermochromic material that integrate fire warning capabilities and flame retardancy is of great significance for optimizing fire safety systems. In this work, cobalt chloride (CoCl 2 ) is introduced as both a color developer and flame-retardant component into the PVA (Polyvinyl alcohol) matrix, combined with flame-retardant synergists (tetrakis (hydroxymethyl)phosphonium sulfate (THPS), Al(OH) 3 , Na-MMT, or Ca-MMT) to fabricate a series of novel reversible thermochromic PVA/CoCl 2 films with excellent flame retardancy. The composites films exhibit remarkable reversible thermochromic performance, transitioning between pink and blue within the temperature range of 30–150°C, and the underlying thermochromic mechanism based on coordination configuration transformation is systematically elucidated. By integrating with a color recognition system, the films enable remote and intelligent early fire warning. Moreover, the films demonstrate superior flame-retardant properties, achieving UL-94 V-0, significantly improved LOI value, and a reduction of up to 91.8% in heat release and 87.4% in smoke release during combustion. Owing to their rapid color response to temperature fluctuations, the films also hold significant potential for smart window applications. Consequently, this work offers a promising strategy for the development of multifunctional intelligent materials for fire protection and energy-saving applications.