KCA/Na2SiO3/PNIPAm hydrogel with highly robust and strong solar modulation capability for thermochromic smart window

•A hydrogel with strong solar modulation for thermochromic smart window is developed.•Controlling the Na2SiO3 content leads to lower LCST of PNIPAm from 25 to 28 °C.•Introducing carrageenan enhances visible light transmittance of PNIPAm up to 87.38%.•The smart window achieves temperature drop of 12.3 °C and keeps long-term stable. Thermochromic smart windows have strong competitive advantages in the realm of building energy conservation. However, achieving thermochromic smart window with a lower critical solution temperature (LCST), high solar modulation (ΔTsol), and excellent visible light transmittance (Tlum) remains a great challenge. Here, a novel composite hydrogel based on polydiisopropylacrylamide (PNIPAm) doped with carrageenan (KCA) and Na2SiO3 was developed for high-performance smart window. The lower LCST of PNIPAm was precisely tuned in a range from 25 °C to 28 °C by controlling the content of added Na2SiO3. The inclusion of KCA diminishes agglomeration within the gel system and enhances both Tlum and ΔTsol. As a result, the KCA/Na2SiO3/PNIPAm exhibits a high Tlum (87.37 %), ΔTsol (69.65 %), a lower LCST (27.2 °C), and a high atmospheric window emissivity (0.962). The smart window achieves temperature drop of 12.3 °C under direct sunlight, and keeps highly stable even after 100 cycle tests high and low-temperature durability due to its robust solar modulation capability. This work offers valuable insights for developing high-performance and robust thermochromic smart windows and holds promising applications in building energy conservation.