Periodic Reporting for period 1 - SOLAR-MATER (Year-Round, Fire-Safe, and Sustainable Solar Management Materials)
Reporting period: 2024-11-01 to 2026-10-31
Summary of the context and overall objectives of the project
Radiative cooling is an emerging passive thermal management technology that enables surfaces to dissipate heat by emitting infrared radiation through the atmospheric transparency window (8–13 μm) directly into outer space. Unlike conventional cooling systems, radiative cooling does not require external energy input, making it a promising strategy for reducing building energy consumption. The building sector accounts for a significant proportion of global energy use, with cooling demand rapidly increasing due to climate change and urbanization. By enhancing solar reflectivity to minimize heat gain and maximizing mid-infrared emissivity to promote heat release, radiative cooling materials can achieve sub-ambient temperature reduction under direct sunlight. Recent advances in photonic structures, polymer composites, and bio-based materials have further improved optical selectivity and scalability. Integrating radiative cooling coatings or envelopes into buildings offers substantial potential to lower indoor temperatures, decrease reliance on air conditioning systems, and reduce carbon emissions, thereby contributing to sustainable and energy-efficient infrastructure development.
Building upon conventional radiative cooling strategies, thermochromic microcapsules introduce an additional dimension of dynamic thermal regulation. Unlike static radiative cooling materials that maintain constant optical properties, thermochromic systems can reversibly adjust their solar absorption and reflection in response to temperature changes. At elevated temperatures, thermochromic microcapsules transition to a high-reflectivity (light-colored) state, reducing solar heat gain and enhancing cooling performance. At lower temperatures, they switch to a darker state with increased solar absorption, enabling passive photothermal heating and mitigating overcooling. This temperature-adaptive behavior addresses one of the key limitations of traditional radiative cooling materials—excessive cooling during cold periods. Furthermore, when combined with high infrared emissivity matrices, thermochromic microcapsules can simultaneously maintain efficient heat dissipation through the atmospheric window. Such dual-mode functionality offers strong potential for year-round energy savings in buildings, improving indoor thermal comfort while minimizing reliance on active heating and cooling systems.
Despite their significant energy-saving potential, the application of radiative cooling materials in buildings raises important fire safety concerns. Many high-performance radiative cooling systems are based on polymeric matrices or porous organic structures, which are inherently flammable and may contribute to fire growth and smoke production. When applied to building envelopes, façades, or roofing systems, these materials are directly exposed to electrical faults, external flames, or high-temperature conditions, increasing the risk of ignition. Furthermore, the incorporation of functional additives—such as phase change materials or thermochromic components—can introduce additional combustible content. Therefore, enhancing flame-retardant performance is essential to ensure safe and durable implementation in real-world construction scenarios. Integrating fire-resistant design strategies, such as inorganic fillers, char-forming bio-based polymers, or synergistic flame-retardant systems, is crucial to achieving multifunctional materials that combine adaptive radiative cooling with robust fire safety. Such an approach ensures compliance with building regulations while supporting sustainable energy-efficient infrastructure.
To address the dual challenges of energy efficiency and fire safety in buildings, this research proposes the development of fire-safe solar management materials by integrating thermochromic microcapsules into radiative cooling systems while incorporating flame-retardant design strategies. The thermochromic components enable temperature-adaptive solar modulation, providing photothermal heating at low temperatures and enhanced solar reflection at high temperatures. Simultaneously, bio-based char-forming matrices and inorganic flame-retardant additives will be introduced to improve thermal stability and suppress combustion. By coupling dynamic optical regulation with robust fire resistance, this work aims to create multifunctional materials that ensure year-round energy savings, enhanced safety, and compliance with building standards, thereby advancing sustainable and resilient building technologies.
Building upon conventional radiative cooling strategies, thermochromic microcapsules introduce an additional dimension of dynamic thermal regulation. Unlike static radiative cooling materials that maintain constant optical properties, thermochromic systems can reversibly adjust their solar absorption and reflection in response to temperature changes. At elevated temperatures, thermochromic microcapsules transition to a high-reflectivity (light-colored) state, reducing solar heat gain and enhancing cooling performance. At lower temperatures, they switch to a darker state with increased solar absorption, enabling passive photothermal heating and mitigating overcooling. This temperature-adaptive behavior addresses one of the key limitations of traditional radiative cooling materials—excessive cooling during cold periods. Furthermore, when combined with high infrared emissivity matrices, thermochromic microcapsules can simultaneously maintain efficient heat dissipation through the atmospheric window. Such dual-mode functionality offers strong potential for year-round energy savings in buildings, improving indoor thermal comfort while minimizing reliance on active heating and cooling systems.
Despite their significant energy-saving potential, the application of radiative cooling materials in buildings raises important fire safety concerns. Many high-performance radiative cooling systems are based on polymeric matrices or porous organic structures, which are inherently flammable and may contribute to fire growth and smoke production. When applied to building envelopes, façades, or roofing systems, these materials are directly exposed to electrical faults, external flames, or high-temperature conditions, increasing the risk of ignition. Furthermore, the incorporation of functional additives—such as phase change materials or thermochromic components—can introduce additional combustible content. Therefore, enhancing flame-retardant performance is essential to ensure safe and durable implementation in real-world construction scenarios. Integrating fire-resistant design strategies, such as inorganic fillers, char-forming bio-based polymers, or synergistic flame-retardant systems, is crucial to achieving multifunctional materials that combine adaptive radiative cooling with robust fire safety. Such an approach ensures compliance with building regulations while supporting sustainable energy-efficient infrastructure.
To address the dual challenges of energy efficiency and fire safety in buildings, this research proposes the development of fire-safe solar management materials by integrating thermochromic microcapsules into radiative cooling systems while incorporating flame-retardant design strategies. The thermochromic components enable temperature-adaptive solar modulation, providing photothermal heating at low temperatures and enhanced solar reflection at high temperatures. Simultaneously, bio-based char-forming matrices and inorganic flame-retardant additives will be introduced to improve thermal stability and suppress combustion. By coupling dynamic optical regulation with robust fire resistance, this work aims to create multifunctional materials that ensure year-round energy savings, enhanced safety, and compliance with building standards, thereby advancing sustainable and resilient building technologies.
Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far
This work focused on the preparation of high-performance thermochromic microcapsules. In this project, ethylenediaminetetraacetic acid (EDTA) was employed as a chelating intermediary. Initially, EDTA coordinates with Ba2+ ions, enabling their controlled localization on the microcapsule surface (Figure 1). Upon the subsequent addition of sulfate ions, the extremely low solubility product of BaSO4 drives the displacement of Ba2+ from the EDTA–Ba complex, resulting in the in situ formation of finely dispersed BaSO4 nanoparticles. These particles uniformly anchor onto the surface of the thermochromic microcapsules, forming a coherent shell layer. Scanning electron microscopy (SEM) observations confirmed the presence of a homogeneous and compact BaSO4 nanoparticles, while X-ray photoelectron spectroscopy (XPS) analysis verified the successful chemical formation of the BaSO4 shell.
Biomass-based flame-retardant chain extenders were synthesized from commercially available bio-sourced substances, including tannin, lysine, and phytic acid, which functioned as char-forming and dehydrating agents. The synthesis involved a classical esterification reaction, in which the hydroxyl groups of tannin were reacted with the carboxyl groups of lysine in the presence of a catalyst and a water-removing agent under controlled heating. The reaction was carried out in a stirred reactor equipped with a reflux condenser and a vacuum-assisted dewatering system to efficiently remove water and drive the reaction to completion. Subsequently, the negatively charged phytic acid was introduced, which readily chelated with the positively charged lysine through electrostatic interactions, forming stable flame-retardant chain structures. The chemical structure and bonding of the resulting products were characterized using nuclear magnetic resonance (NMR) spectroscopy to identify functional groups and confirm ester formation, and Fourier-transform infrared (FTIR) spectroscopy to analyze characteristic chemical bonds. Elemental composition was further verified to ensure successful incorporation of the biomass-derived flame-retardant components.
Calcium alginate microspheres were first prepared by dropping sodium alginate solution into a calcium chloride solution. The obtained calcium alginate microspheres, together with BaSO4-modified thermochromic microcapsules, were then incorporated into a sodium alginate solution, followed by freeze-drying to construct the final solar management material (SA/TCM@Ba-Ca) (Figure 4). The cross-sectional morphology of the composite was characterized by scanning electron microscopy (SEM). Combined with energy-dispersive X-ray spectroscopy (EDS), the results confirmed the uniform distribution of calcium ions throughout the structure. The solar reflectivity and infrared emissivity of the material were systematically investigated at different temperatures. EnergyPlus simulations further demonstrated that the material exhibits promising energy-saving potential on a global scale. In outdoor experiments, compared with pure sodium alginate, the calcium alginate composite containing thermochromic microcapsules achieved a temperature reduction of nearly 10 °C. Infrared thermal imaging provided clear confirmation of this cooling effect. The flame-retardant properties of the composite were also thoroughly evaluated. The peak heat release rate decreased by more than 50%, the peak smoke release was effectively suppressed, and the material did not ignite under an alcohol lamp flame, instead forming a protective char layer. Flame retardancy was evaluated by studying the thermal degradation behavior under a nitrogen atmosphere, along with the morphology, graphitization degree, and crystalline structure of the resulting char residues. Solar performance—including solar absorptivity, solar reflectivity, and infrared emissivity—was characterized using UV–vis–NIR spectroscopy and a mid-IR integrating sphere coupled with FTIR analysis.
However, the optimization of performance of this designed composite materials is not carried out due to the early termination. Meanwhile, the mechanisms of flame retardancy and solar management were not systematically investigated. The common underlying mechanisms linking flame retardancy and solar management were not elucidated by analyzing elemental composition, chemical bonding, and physical structure.
Based on above work performed, a multifunctional composite material was developed that enables photothermal conversion at low temperatures and radiative cooling at high temperatures, while simultaneously exhibiting flame-retardant properties. This design significantly reduces building energy consumption and enhances fire safety performance.
Biomass-based flame-retardant chain extenders were synthesized from commercially available bio-sourced substances, including tannin, lysine, and phytic acid, which functioned as char-forming and dehydrating agents. The synthesis involved a classical esterification reaction, in which the hydroxyl groups of tannin were reacted with the carboxyl groups of lysine in the presence of a catalyst and a water-removing agent under controlled heating. The reaction was carried out in a stirred reactor equipped with a reflux condenser and a vacuum-assisted dewatering system to efficiently remove water and drive the reaction to completion. Subsequently, the negatively charged phytic acid was introduced, which readily chelated with the positively charged lysine through electrostatic interactions, forming stable flame-retardant chain structures. The chemical structure and bonding of the resulting products were characterized using nuclear magnetic resonance (NMR) spectroscopy to identify functional groups and confirm ester formation, and Fourier-transform infrared (FTIR) spectroscopy to analyze characteristic chemical bonds. Elemental composition was further verified to ensure successful incorporation of the biomass-derived flame-retardant components.
Calcium alginate microspheres were first prepared by dropping sodium alginate solution into a calcium chloride solution. The obtained calcium alginate microspheres, together with BaSO4-modified thermochromic microcapsules, were then incorporated into a sodium alginate solution, followed by freeze-drying to construct the final solar management material (SA/TCM@Ba-Ca) (Figure 4). The cross-sectional morphology of the composite was characterized by scanning electron microscopy (SEM). Combined with energy-dispersive X-ray spectroscopy (EDS), the results confirmed the uniform distribution of calcium ions throughout the structure. The solar reflectivity and infrared emissivity of the material were systematically investigated at different temperatures. EnergyPlus simulations further demonstrated that the material exhibits promising energy-saving potential on a global scale. In outdoor experiments, compared with pure sodium alginate, the calcium alginate composite containing thermochromic microcapsules achieved a temperature reduction of nearly 10 °C. Infrared thermal imaging provided clear confirmation of this cooling effect. The flame-retardant properties of the composite were also thoroughly evaluated. The peak heat release rate decreased by more than 50%, the peak smoke release was effectively suppressed, and the material did not ignite under an alcohol lamp flame, instead forming a protective char layer. Flame retardancy was evaluated by studying the thermal degradation behavior under a nitrogen atmosphere, along with the morphology, graphitization degree, and crystalline structure of the resulting char residues. Solar performance—including solar absorptivity, solar reflectivity, and infrared emissivity—was characterized using UV–vis–NIR spectroscopy and a mid-IR integrating sphere coupled with FTIR analysis.
However, the optimization of performance of this designed composite materials is not carried out due to the early termination. Meanwhile, the mechanisms of flame retardancy and solar management were not systematically investigated. The common underlying mechanisms linking flame retardancy and solar management were not elucidated by analyzing elemental composition, chemical bonding, and physical structure.
Based on above work performed, a multifunctional composite material was developed that enables photothermal conversion at low temperatures and radiative cooling at high temperatures, while simultaneously exhibiting flame-retardant properties. This design significantly reduces building energy consumption and enhances fire safety performance.
Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)
The project advances the state of the art by demonstrating the integration of dynamic solar modulation and flame retardancy within a single system. It establishes a unified structure–property relationship connecting combustion chemistry and optical thermal regulation. The material shows strong potential to reduce building cooling energy demand while improving fire safety, directly addressing energy efficiency and building safety challenges. EnergyPlus simulations indicate global applicability across diverse climate zones. The use of bio-based polymers and neutral-condition inorganic modification supports environmentally responsible material design aligned with green transition goals.