A Solution-Processed Inorganic Emitter with High Spectral Selectivity for Efficient Subambient Radiative Cooling in Hot Humid Climates

Anti-UV Passive Radiative Cooling Chiral Nematic Liquid Crystal Films for Thermal Management

Passive radiative cooling (PRC) can spontaneously dissipate heat to outer space through atmospheric transparent windows, providing a promising path to meet sustainable development goals. However, achieving simultaneously high transparency, color-customizable, and thermal management of PRC anti ultraviolet (anti-UV) films remains a challenge. Herein, a simple strategy is proposed to utilize liquid crystalline polymer, with high mid-infrared emissive, forming customizable structural color film by molecular self-assembly and polymerization-induced pitch gradient, which guarantees the balance of transparency in visible spectrum and sunlight reflection, rendering anti-UV colored window for thermal management. By performing tests, temperature fall of 5.4 and 7.9 °C are demonstrated at noon with solar intensity of 717 W m-2 and night, respectively. Vivid red-, green-, blue-structured colors, and colorless films are designed and implemented to suppress the solar input and control the effective visible light transmissivity considering the efficiency function of human vision. In addition, temperature rise of 11.1 °C is achieved by applying an alternating current field on the PRC film. This study provides a new perspective on the thermal management and aesthetic functionalities of smart windows and wearables.

Annual Energy-Saving Smart Windows with Actively Controllable Passive Radiative Cooling and Multimode Heating Regulation

Smart windows with radiative heat management capability using the sun and outer space as zero-energy thermodynamic resources have gained prominence, demonstrating a minimum carbon footprint. However, realizing on-demand thermal management throughout all seasons while reducing fossil energy consumption remains a formidable challenge. Herein, an energy-efficient smart window that enables actively tunable passive radiative cooling (PRC) and multimode heating regulation is demonstrated by integrating the emission-enhanced polymer-dispersed liquid crystal (SiO2@PRC PDLC) film and a low-emission layer deposited with carbon nanotubes. Specifically, this device can achieve a temperature close to the chamber interior ambient under solar irradiance of 700 W m-2, as well as a temperature drop of 2.3 °C at sunlight of 500 W m-2, whose multistage PRC efficiency can be rapidly adjusted by a moderate voltage. Meanwhile, synchronous cooperation of passive radiative heating (PRH), solar heating (SH), and electric heating (EH) endows this smart window with the capability to handle complicated heating situations during cold weather. Energy simulation reveals the substantial superiority of this device in energy savings compared with single-layer SiO2@PRC PDLC, normal glass, and commercial low-E glass when applied in different climate zones. This work provides a feasible pathway for year-round thermal management, presenting a huge potential in energy-saving applications.

Thermal-Rectified Gradient Porous Polymeric Film for Solar-Thermal Regulatory Cooling

Solar-thermal regulation concerning thermal insulation and solar modulation is pivotal for cooling textiles and smart buildings. Nevertheless, a contradiction arises in balancing the demand to prevent external heat infiltration with the efficient dissipation of excess heat from enclosed spaces. Here, a concentration-gradient polymerization strategy is presented for fabricating a gradient porous polymeric film comprising interconnected polymeric microspheres. This method involves establishing an electric field-driven gradient distribution of charged crosslinkers in the precursor solution, followed by subsequent polymerization and freeze-drying processes. The resulting porous film exhibits a significant porosity gradient along its thickness, leading to exceptional unidirectional thermal insulation capabilities with a thermal rectification factor of 21%. The gradient porous film, with its thermal rectification properties, effectively reconciles the conflicting demands of diverse thermal conductivity for cooling unheated and spontaneously heated enclosed spaces. Consequently, the gradient porous film demonstrates remarkable enhancements in solar-thermal management, achieving temperature reductions of 3.0 and 4.1 °C for unheated and spontaneously heated enclosed spaces, respectively, compared to uniform porous films. The developed gradient-structured porous film thus holds promise for the development of thermal-rectified materials tailored to regulate solar-thermal conditions within enclosed environments.

Materials in Radiative Cooling Technologies

Radiative cooling (RC) is a carbon-neutral cooling technology that utilizes thermal radiation to dissipate heat from the Earth's surface to the cold outer space. Research in the field of RC has garnered increasing interest from both academia and industry due to its potential to drive sustainable economic and environmental benefits to human society by reducing energy consumption and greenhouse gas emissions from conventional cooling systems. Materials innovation is the key to fully exploit the potential of RC. This review aims to elucidate the materials development with a focus on the design strategy including their intrinsic properties, structural formations, and performance improvement. The main types of RC materials, i.e., static-homogeneous, static-composite, dynamic, and multifunctional materials, are systematically overviewed. Future trends, possible challenges, and potential solutions are presented with perspectives in the concluding part, aiming to provide a roadmap for the future development of advanced RC materials.

High-Efficiency Thermal-Shock Resistance Enabled by Radiative Cooling and Latent Heat Storage

Radiative cooling technology is well known for its subambient temperature cooling performance under sunlight radiation. However, the intrinsic maximum cooling power of radiative cooling limits the performance when the objects meet the thermal shock. Here, a dual-function strategy composed of radiative cooling and latent heat storage simultaneously enabling the efficient subambient cooling and high-efficiency thermal-shock resistance performance is proposed. The electrospinning and absorption-pressing methods are used to assemble the dual-function cooler. The high sunlight reflectivity and high mid-infrared emissivity of radiative film allow excellent subambient temperature of 5.1 °C. When subjected the thermal shock, the dual-function cooler demonstrates a pinning effect of huge temperature drop of 39 °C and stable low-temperature level by isothermal heat absorption compared with the traditional radiative cooler. The molten phase change materials provide the heat-time transfer effect by converting thermal-shock heat to the delayed preservation. This strategy paves a powerful way to protect the objects from thermal accumulation and high-temperature damage, expanding the applications of radiative cooling and latent heat storage technologies.

Anti-Environmental Aging Passive Daytime Radiative Cooling

Passive daytime radiative cooling technology presents a sustainable solution for combating global warming and accompanying extreme weather, with great potential for diverse applications. The key characteristics of this cooling technology are the ability to reflect most sunlight and radiate heat through the atmospheric transparency window. However, the required high solar reflectance is easily affected by environmental aging, rendering the cooling ineffective. In recent years, significant advancements have been made in understanding the failure mechanisms, design strategies, and manufacturing technologies of daytime radiative cooling. Herein, a critical review on anti-environmental aging passive daytime radiative cooling with the goal of advancing their commercial applications is presented. It is first introduced the optical mechanisms and optimization principles of radiative cooling, which serve as a basis for further endowing environmental durability. Then the environmental aging conditions of passive daytime radiative cooling, mainly focusing on UV exposure, thermal aging, surface contamination and chemical corrosion are discussed. Furthermore, the developments of anti-environmental aging passive daytime radiative cooling materials, including design strategies, fabrication techniques, structures, and performances, are reviewed and classified for the first time. Last but not the least, the remaining open challenges and the insights are presented for the further promotion of the commercialization progress.

Stepless IR Chromism in Ti3 C2 Tx MXene Tuned by Interlayer Water Molecules

Real-time control over infrared (IR) radiation of objects is highly desired in a variety of areas such as personal thermal regulation and IR camouflage. This requires the dynamic modulation of IR emissivity in a stepless manner over a wide range (>50%), which remains a daunting challenge. Here, an emissivity modulation phenomenon is reported in stacked 2D Ti3 C2 Tx MXene nanosheets, from 12% to 68% as the intercalation/discharging of water molecules within the interlayers. The intercalation of water molecules dynamically changes the electronic properties and the complex permittivity in IR frequencies of Ti3 C2 Tx . This emissivity modulation is a stepless and reversible process without the assistance of any external energy input. Further, intercalating cellulose nanofibers into the Ti3 C2 Tx interlayers makes this dynamic process highly repeatable. Last, a sweat-responsive adaptive textile that can improve thermal comfort of human body under changes in metabolic rates and environmental conditions is demonstrated, showing great potential of this mechanism in passive on-demand radiation modulation.

A dual-selective thermal emitter with enhanced subambient radiative cooling performance

Radiative cooling is a zero-energy technology that enables subambient cooling by emitting heat into outer space (~3 K) through the atmospheric transparent windows. However, existing designs typically focus only on the main atmospheric transparent window (8-13 μm) and ignore another window (16-25 μm), under-exploiting their cooling potential. Here, we show a dual-selective radiative cooling design based on a scalable thermal emitter, which exhibits selective emission in both atmospheric transparent windows and reflection in the remaining mid-infrared and solar wavebands. As a result, the dual-selective thermal emitter exhibits an ultrahigh subambient cooling capacity (~9 °C) under strong sunlight, surpassing existing typical thermal emitters (≥3 °C cooler) and commercial counterparts (as building materials). Furthermore, the dual-selective sample also exhibits high weather resistance and color compatibility, indicating a high practicality. This work provides a scalable and practical radiative cooling design for sustainable thermal management.

A Novel Multifunctional Photonic Film for Colored Passive Daytime Radiative Cooling and Energy Harvesting

Passive daytime radiative cooling (PDRC) materials with sustainable energy harvesting capability is critical to concurrently reduce traditional cooling energy utilized for thermal comfort and transfer natural clean energies into electricity. Herein, a versatile photonic film (Ecoflex@BTO@UAFL) based on a novel fluorescent luminescence color passive radiative cooling with triboelectric and piezoelectric effect is developed by filling the dielectric BaTiO3 (BTO) nanoparticles and ultraviolet absorption fluorescent luminescence (UAFL) powder into the elastic Ecoflex matrix. Test results demonstrate that the Ecoflex@BTO@UAFL photonic film exhibits a maximum passive radiative cooling effect of ∽10.1 °C in the daytime. Meanwhile, its average temperature drop in the daytime is ~4.48 °C, which is 0.91 °C higher than that of the Ecoflex@BTO photonic film (3.56 °C) due to the addition of UAFL material. Owing to the high dielectric constant and piezoelectric effect of BTO nanoparticles, the maximum power density (0.53 W m-2 , 1 Hz @ 10 N) of the Ecoflex@BTO photonic film-based hybrid nanogenerator is promoted by 70.9% compared to the Ecoflex film-based TENG. This work provides an ingenious strategy for combining PDRC effects with triboelectric and piezoelectric properties, which can spontaneously achieve thermal comfort and energy conservation, offering a new insight into multifunctional energy saving.

Hierarchically structured passive radiative cooling ceramic with high solar reflectivity

Passive radiative cooling using nanophotonic structures is limited by its high cost and poor compatibility with existing end uses, whereas polymeric photonic alternatives lack weather resistance and effective solar reflection. We developed a cellular ceramic that can achieve highly efficient light scattering and a near-perfect solar reflectivity of 99.6%. These qualities, coupled with high thermal emissivity, allow the ceramic to provide continuous subambient cooling in an outdoor setting with a cooling power of >130 watts per square meter at noon, demonstrating energy-saving potential on a worldwide scale. The color, weather resistance, mechanical robustness, and ability to depress the Leidenfrost effect are key features ensuring the durable and versatile nature of the cooling ceramic, thereby facilitating its commercialization in various applications, particularly building construction.

A solution-processed radiative cooling glass

Passive daytime radiative cooling materials could reduce the energy needed for building cooling up to 60% by reflecting sunlight and emitting long-wave infrared (LWIR) radiation into the cold Universe (~3 kelvin). However, developing passive cooling structures that are both practical to manufacture and apply while also displaying long-term environmental stability is challenging. We developed a randomized photonic composite consisting of a microporous glass framework that features selective LWIR emission along with relatively high solar reflectance and aluminum oxide particles that strongly scatter sunlight and prevent densification of the porous structure during manufacturing. This microporous glass coating enables a temperature drop of ~3.5° and 4°C even under high-humidity conditions (up to 80%) during midday and nighttime, respectively. This radiative "cooling glass" coating maintains high solar reflectance even when exposed to harsh conditions, including water, ultraviolet radiation, soiling, and high temperatures.

High-Selectivity Planar Thermal Emitter with a Stable Temperature over 1400 K for a Thermophotovoltaic System

A selective thermal emitter with superior thermal stability and perfect selective thermal emission in specific bands can facilitate the lifting of the thermophotovoltaic (TPV) energy conversion efficiency in TPV systems. Scalable planar selective thermal emitters with superior spectral selectivity and robust high-temperature stability are desired to meet the requirements of large-scale deployments of TPV systems. However, stably reradiating the available thermal photons at above 1273 K remains a significant challenge for selective thermal emitters. In this work, we demonstrated a high-selectivity planar thermal emitter based on the composite ceramic of ZrC and Al2O3. The prepared selective thermal emitter provides an emissivity of around 90% above the photon energy (0.6 eV) at 1423 K, a strong emission suppression effect below 0.6 eV, and superior thermal stability up to at least 1423 K. Therefore, the overall spectral efficiency can reach around 53%. Coupled with an InGaAs PV cell, the TPV system based on the selective thermal emitter demonstrates a predicted heat-to-electricity power conversion efficiency of 29.78% at 1423 K due to the matched spectral response of the emitter with the PV cell. Our work opens a new way forward for TPV systems based on planar selective thermal emitters.

A Janus Textile Capable of Radiative Subambient Cooling and Warming for Multi-Scenario Personal Thermal Management

Textiles with radiative cooling/warming capabilities provide a green and effective solution to personal thermal comfort in different climate scenarios. However, developing multiple-mode textiles for wearing in changing climates with large temperature variation remains a challenge. Here a Janus textile is reported, comprising a polyethersulfone (PES)-Al₂ O₃ cooling layer optically coupled with a Ti₃ C₂ T_x warming layer, which can realize sub-ambient radiative cooling, solar warming, and active Joule heating. Owing to the intrinsically high refractive index of PES and the rational design of the fiber topology, the nanocomposite PES textile features a record high solar reflectance of 0.97. Accompanied by an infrared (IR) emittance of 0.91 in the atmospheric window, sub-ambient cooling of 0.5-2.5 °C is achieved near noontime in humid summer under ≈1000 W m^-2 solar irradiation in Hong Kong. The simulated skin covered with the textile is ≈10 °C cooler than that with white cotton. The Ti₃ C₂ T_x layer provides a high solar-thermal efficiency of ≈80% and a Joule heating flux of 66 W m^-2 at 2 V and 15 °C due to its excellent spectral selectivity and electrical conductivity. The switchable multiple working modes enable effective and adaptive personal thermal management in changing environments.

Selective spectral absorption of nanofibers for color-preserving daytime radiative cooling

Passive radiative cooling is a promising solution for cooling objects without consuming energy. However, chemical colors absorb visible light and generate heat, posing a challenge in the design of a colored sub-ambient daytime radiative cooler (CSDRC) in a simple and scalable way. Herein, we used nanofibers (NF) to achieve selective spectral absorption of the daytime radiative cooler through a dope-dyeing electrospinning technique. This approach allows for the selective absorption of desired colors in the visible spectrum, while the nanofiber structure provides strong visible and near-infrared light scattering to minimize solar heating. We selected cellulose acetate (CA) with mid-infrared emittance characteristics for efficient sky cooling. Our design enabled the CA NF CSDRC to exhibit an ultra-high NIR reflectance of 99%, a high MIR emittance of 95%, and vibrant colors. These unique optical properties resulted in a reduction of the maximum ambient temperature by 3.2 °C and a cooling power of ≈40 W m^-2 at a solar intensity of 700 W m^-2. Additionally, the flexibility and deformability of the colored nanofiber cooler make it suitable for thermal management in various practical applications. Our work provides a simple and scalable solution for designing colored passive radiative cooling materials.

Biomimetic Robust All-Polymer Porous Coatings for Passive Daytime Radiative Cooling

Passive daytime radiation cooling (PDRC) has gained considerable attention as an emerging and promising cooling technology. Polymer-based porous materials are one of the important candidates for PDRC application due to their easy processing, free of inorganic particle doping, and multifunctionality. However, the mechanical properties of these porous materials, which are critical in outdoor services, have been overlooked in previous studies. Herein, a nonsolvent-induced phase separation (NIPS) method combined with ambient pressure drying to prepare polyethylene-polysilicate all-polymer porous coatings is developed. The coatings possess a Cyphochilus beetle-like skeleton structure with optimal skeleton size, laminated anisotropy, and high volume fraction (64 ± 1%). These structure features ensure a maximum skeleton density without optical crowding, thus enhancing light scattering and stress dispersion, and balancing optical and mechanical properties. The coatings exhibit significant mechanical robustness (only ≈70 µm thickness reduction after 1000 Taber abrasion cycles at a 750 g load without influencing optical performance), durability, optical properties (a solar reflectance of ≈95% and an average near-normal thermal emittance of ≈96%), and PDRC performance (realizing sub-ambient cooling of ≈3-6 °C at midday with different weather conditions). The work provides a new solution to improve the practicability of polymer-based porous coatings in PDRC outdoor services and other fields.

Phase Change Material Enhanced Radiative Cooler for Temperature-Adaptive Thermal Regulation

Passive radiative cooling (PRC), as an electricity-free and environmentally friendly cooling strategy, is highly desirable in improving the global energy landscape. Despite numerous efforts, most designs for PRC are so devoted to improving the cooling performance in the daytime that they neglect the triggered overcooling at night. Herein, we approached an effective design for temperature-adaptive thermal management through integrating PRC and temperature control of room-temperature phase change material. Compared with conventional radiative coolers, the developed phase change material-enhanced radiative cooler (PCMRC) can adjust its performance according to the temperature of day and night. The PCMRC achieved an average subambient temperature drop of ∼6.3 °C under direct sunlight and an average temperature rise of ∼2.1 °C above ambient temperature at night, as well as a reduced temperature difference between day and night. The temperature-adaptive PCMRC shows great promise for passive radiative cooling regulation, which can further extend the applications of passive radiative cooling.

Dual-Encapsulated Nanocomposite for Efficient Thermal Buffering in Heat-Generating Radiative Cooling

Radiative cooling has been considered an innovative passive method to resolve the problem of overheating of electronic devices. However, it is inefficient for cooling huge heat generation components. Herein, we report a dual-encapsulated nanocomposite (DEN) by integrating radiative cooling and phase-change materials (PCMs) for thermal buffering in heat-generating radiative cooling. The leak of PCMs is avoided by a simple dual-encapsulated structure with a three-dimensional (3D) interconnected cellular-like network structure and radiative cooling layer on the surface, 75% superior to the state-of-the-art single encapsulation designs. Additionally, our DEN not only shows outstanding optical properties with strong solar reflection (R̅_solar = 0.96) and IR-selective emission (ε̅_8-13 μm = 0.94 and η_ε = 1.15) but also exhibits high phase-change enthalpy (ΔH_m = 192.2 J/g, ΔH_c = 175.7 J/g), enabling remarkable radiative cooling capability and desirable thermal energy peak shaving and valley filling effect. Outdoor experiments demonstrate that DEN achieves a temperature drop up to 23 °C compared to the control group without DEN coverage when electronics generate heat. This dual-encapsulated nanocomposite provides a novel strategy and solution for outdoor passive thermal management.

Green-Manufactured and Recyclable Coatings for Subambient Daytime Radiative Cooling

Passive daytime radiative cooling, which reflects sunlight and simultaneously emits heat into space to cool surfaces without energy input, is a promising strategy for energy conservation. Integrating radiative cooling with building systems can tremendously alleviate electrical cooling, but manufacturing high-efficient and eco-friendly coatings remains an urgent and challenging task. Here, we present a simple and scale-up strategy for fabricating ultrawhite coatings consisting of porous ethyl cellulose matrix-random BaSO₄ nanoparticles utilizing green solvents. With the synergistic effect of the ideal intrinsic properties of the materials and the strong Mie scattering of the porous structure, the ultrawhite coating possesses a record solar reflectance of 98.6% and a thermal emittance of 98.1%, resulting in a subambient temperature drop of over 2.5 °C under a solar intensity of ∼920 W m^-2. Better yet, our coatings can be conveniently brushed, rolled, or sprayed onto various types of substrates, with excellent durability, self-cleaning, and cost-effectiveness, paving an attractive and viable pathway for large-scale applications in practical buildings.