A Thermochromic Hydrated Ionic Polymer with an Adjustable Transition Temperature for Smart Windows and Temperature Monitoring

Thermochromic materials with customizable transition temperatures are essential for energy-efficient, smart windows and temperature-monitoring applications. This study presents a hydrated ionic polymer featuring Ni²⁺ as the central ion, where reversible transitions between octahedral and tetrahedral coordination enable tunable light absorption. By adjustment of the polyvinylpyrrolidone (PVP)/poly(vinyl alcohol) (PVA) ratio, the hydroxyl content is precisely controlled, achieving a transition temperature range of 30-50 °C. The material exhibits a solar modulation of 8-23% and visible-light modulation of 10-39%, with no phase separation during operation. A two-step ligand exchange mechanism is identified: at low temperatures, H₂O coordinates with Ni²⁺, forming a transparent octahedral structure. As the temperature rises, water desorbs and Cl^- partially replaces H₂O, producing a blue tetrahedral structure. Further heating triggers −OH dissociation, allowing more Cl^- coordination, deepening the blue color. Molecular dynamics simulations reveal that the PVP/PVA ratio controls water diffusion and hydrogen bonding. Practical tests demonstrate that this polymer reduces solar-exposed temperatures by 10 °C in smart windows and serves as an effective temperature-monitoring label with excellent cycling stability over 100 cycles. This study presents a new perspective for designing thermochromic materials with customizable properties, offering new insights for energy-saving and sensing applications.