Single Nanoporous MgHPO4·1.2H2O for Daytime Radiative Cooling

Calcium-Salt-Enhanced Fiber Membrane with High Infrared Emission and Hydrophilicity for Efficient Passive Cooling

Radiative cooling fabrics have gained significant attention for their ability to enhance comfort without consuming extra energy. Nevertheless, sweat accumulation on the skin and diminishing cooling efficiency usually exist in the reported polymer cooling membranes. Herein, we report a universal method to obtain a calcium (Ca)-salt-enhanced fiber membrane with high infrared emission and hydrophilicity for efficient passive cooling and flame retardancy. The modification by Ca salts (including CaSiO3, CaSO3, and CaHPO4) with strong infrared emission results in an improvement in hygrothermal management ability, especially for moisture absorption and perspiration regulation in hot and humid environments. As an example, the CaSiO3@PMMA fiber membrane exhibits exceptional reflectivity in the solar spectrum (∼94.5%), high emittance in the atmospheric window (∼96.7%), and superhydrophilicity with a contact angle of 31°. Under direct sunlight, the CaSiO3@PMMA membrane exhibits an obvious temperature drop of 11.7 °C and moisture management achieves an additional cooling of 8.9 °C, as further confirmed by the ability to reduce the rate of ice melting. Additionally, the composite membrane provides notable flame retardancy and UV resistance. This work paves a new path in developing new materials with perspiration management and flame retardancy for zero energy consumption cooling in hot and humid 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.

Porous Structure of Polymer Films Optimized by Rationally Tuning Phase Separation for Passive All-Day Radiative Cooling

Passive all-day radiative cooling (PARC) films with porous structures prepared via nonsolvent-induced phase separation (NIPS) have attracted considerable attention owing to their cost-effectiveness and wide applicability. The PARC performances of the films correlate with their porous structures. However, the porous structure formed using the NIPS process cannot be finely regulated. In this study, we prepared polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) films with porous structures optimized by rationally tuning the phase separation, which was achieved by adjusting the proportions of two good solvents with varying solubility parameters. The optimized PVDF-HFP film with a hierarchically porous structure exhibited a high solar reflectance of 97.7% and an infrared emissivity of 96.7%. The film with excellent durability achieved an average subambient cooling temperature of approximately 5.4 °C under a solar irradiance of 945 W·m-2 as well as a temperature of 11.2 °C at nighttime, thus demonstrating all-day radiative cooling. The results indicate that the proposed films present a promising platform for large-scale applications in green building cooling and achieving carbon neutrality.

Radiative Cooling for Energy Sustainability: From Fundamentals to Fabrication Methods Toward Commercialization

Radiative cooling, a technology that lowers the temperature of terrestrial objects by dissipating heat into outer space, presents a promising ecologically-benign solution for sustainable cooling. Recent years witness substantial progress in radiative cooling technologies, bringing them closer to commercialization. This comprehensive review provides a structured overview of radiative cooling technologies, encompassing essential principles, fabrication techniques, and practical applications, with the goal of guiding researchers toward successful commercialization. The review begins by introducing the fundamentals of radiative cooling and the associated design strategies to achieve it. Then, various fabrication methods utilized for the realization of radiative cooling devices are thoroughly discussed. This discussion includes detailed assessments of scalability, fabrication costs, and performance considerations, encompassing both structural designs and fabrication techniques. Building upon these insights, potential fabrication approaches suitable for practical applications and commercialization are proposed. Further, the recent efforts made toward the practical applications of radiative cooling technology, including its visual appearance, switching capability, and compatibility are examined. By encompassing a broad range of topics, from fundamental principles to fabrication and applications, this review aims to bridge the gap between theoretical research and real-world implementation, fostering the advancement and widespread adoption of radiative cooling technology.

Colloidal inorganic nano- and microparticles for passive daytime radiative cooling

Compared to traditional cooling systems, radiative cooling (RC) is a promising cooling strategy in terms of reducing energy consumption enormously and avoiding severe environmental issues. Radiative cooling materials (RCMs) reduce the temperature of objects without using an external energy supply by dissipating thermal energy via infrared (IR) radiation into the cold outer space through the atmospheric window. Therefore, RC has a great potential for various applications, such as energy-saving buildings, vehicles, water harvesting, solar cells, and personal thermal management. Herein, we review the recent progress in the applications of inorganic nanoparticles (NPs) and microparticles (MPs) as RCMs and provide insights for further development of RC technology. Particle-based RCMs have tremendous potential owing to the ease of engineering their optical and physical properties, as well as processibility for facile, inexpensive, and large area deposition. The optical and physical properties of inorganic NPs and MPs can be tuned easily by changing their size, shape, composition, and crystals structures. This feature allows particle-based RCMs to fulfill requirements pertaining to passive daytime radiative cooling (PDRC), which requires high reflectivity in the solar spectrum and high emissivity within the atmospheric window. By adjusting the structures and compositions of colloidal inorganic particles, they can be utilized to design a thermal radiator with a selective emission spectrum at wavelengths of 8-13 μm, which is preferable for PDRC. In addition, colloidal particles can exhibit high reflectivity in the solar spectrum through Mie-scattering, which can be further engineered by modifying the compositions and structures of colloidal particles. Recent advances in PDRC that utilize inorganic NPs and MPs are summarized and discussed together with various materials, structural designs, and optical properties. Subsequently, we discuss the integration of functional NPs to achieve functional RCMs. We describe various approaches to the design of colored RCMs including structural colors, plasmonics, and luminescent wavelength conversion. In addition, we further describe experimental approaches to realize self-adaptive RC by incorporating phase-change materials and to fabricate multifunctional RC devices by using a combination of functional NPs and MPs.

Facile "Synergistic Inner-Outer Activation" Strategy for Nano-Engineering of Nature-Skin-Derived Wearable Daytime Radiation Cooling Materials

Natural skin-derived products, as traditional wearable materials are widely used in people's daily life due to the products' excellent origins. Herein, a versatile daytime-radiation cooling wearable natural skin (RC-skin) consisting of the collagen micro-nano fibers with the on-demand double-layer radiation cooling structure is nano-engineered through the proposed facile "synergistic inner-outer activation" strategy. The bottom layer (inner strategy) of the RC-skin is fabricated by filling the skin with the Mg₁₁ (HPO₃ )₈ (OH)₆ nanoparticles by soaking. The superstratum (outer strategy) is constituted by a composite coating with an irregular microporous structure. The RC-skin harvests the inherent advantages of natural building blocks including sufficient hydrophobicity, excellent mechanical properties, and friction resistance. Owing to the subtle double-layer structure design, the solar reflectance and the average emissivity in the mid-infrared band of RC-skin are ≈92.7% and ≈95%, respectively. Therefore, the RC-skin's temperature in the sub-ambient is reduced by ≈7.5 °C. Various outdoor practical application experiments further substantiate that RC-skin has superior radiation cooling performances. Collectively, RC-skin has broad-application prospects for intelligent wearing, low-carbon travel, building materials, and intelligent thermoelectric power generation, and this study also provides novel strategies for developing natural-skin-derived functional materials.

Hierarchical Superhydrophobic Poly(vinylidene fluoride-co-hexafluoropropylene) Membrane with a Bead (SiO2 Nanoparticles)-on-String (Nanofibers) Structure for All-Day Passive Radiative Cooling

Passive all-day radiative cooling has been proposed as a promising pathway to cool objects by reflecting sunlight and dissipating heat to the cold outer space through atmospheric windows without any energy consumption. However, most of the existing radiative coolers are susceptible to contamination, which may decrease the optical property and gradually degrade the outdoor radiative cooling performance. Herein, we prepared a hierarchical superhydrophobic fluorinated-SiO₂/PVDF-HFP nanofiber membrane by a facile and scalable technology of electrospinning and electrostatic spraying. Due to the synergistic effects of the efficient scattering of nanofibers/micropores and the phonon polarization resonance of SiO₂ nanoparticles, the membrane achieves up to 97.8% average solar reflectance and 96.6% average atmospheric window emittance. The membrane displays sub-ambient temperature drop values of 11.5 and 4.1 °C in daytime and nighttime outdoor conditions, respectively, exhibiting remarkable radiative cooling performance. Importantly, the unique bead (SiO₂ nanoparticles)-on-string (nanofibers) structure forms hierarchical roughness that endows the surface with a superior self-cleaning property. In addition, the obtained membrane exhibits remarkable flexibility and mechanical stability, which are of significant importance in cooling vehicles, buildings, and large-scale equipment.

Superhydrophobic Porous Coating of Polymer Composite for Scalable and Durable Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) technology provides an eco-friendly cooling strategy by reflecting sunlight reaching the surface and radiating heat underneath to the outer space through the atmospheric transparency window. However, PDRC materials face challenges in cooling performance degradation caused by outdoor contamination and requirements of easy fabrication approaches for scale-up and high cooling efficiency. Herein, a polymer composite coating of polystyrene, polydimethylsiloxane and poly(ethyl cyanoacrylate) (PS/PDMS/PECA) with superhydrophobicity and radiative cooling performance was fabricated and demonstrated to have sustained radiative cooling capability, utilizing the superhydrophobic self-cleaning property to maintain the optical properties of the coating surface. The prepared coating is hierarchically porous which exhibits an average solar reflectance of 96% with an average emissivity of 95% and superhydrophobicity with a contact angle of 160°. The coating realized a subambient radiative cooling of 12.9 °C in sealed air and 7.5 °C in open air. The self-cleaning property of the PS/PDMS/PECA coating helped sustain the cooling capacity for long-term outdoor applications. Moreover, the coating exhibited chemical resistance, UV resistance, and mechanical durability, which has promising applications in wider fields.

Ordered-Porous-Array Polymethyl Methacrylate Films for Radiative Cooling

Passive radiative cooling is a spontaneous pattern of reflecting sunlight and radiating heat into the cold outer space through transparent atmosphere windows. In this work, an ordered-porous-array polymethyl methacrylate (OPA-PMMA) film with the properties of excellent radiative cooling is designed and studied. An ultra-high emissivity of 98.4% in the mid-infrared region (3-25 μm) and a good solar reflectance of 85% in the ultraviolet and near-infrared solar spectra (0.2-2.5 μm) were achieved. The surface temperature of the OPA-PMMA film is 16 °C lower than that of the smooth-surface PMMA films and is 8.6 °C lower than that of the commercial white paint in the outdoor test. The structure of the OPA plays an important role in improving solar reflectivity and emissivity. The films are fabricated using a one-step low-cost process that can be applied for large-scale production. It is vital for promoting radiative cooling as a viable energy technology for buildings, fabric, or equipment that need a cooling environment.

Aerogel-Functionalized Thermoplastic Polyurethane as Waterproof, Breathable Freestanding Films and Coatings for Passive Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) is an emerging sustainable technology that can spontaneously radiate heat to outer space through an atmospheric transparency window to achieve self-cooling. PDRC has attracted considerable attention and shows great potential for personal thermal management (PTM). However, PDRC polymers are limited to polyethylene, polyvinylidene fluoride, and their derivatives. In this study, a series of polymer films based on thermoplastic polyurethane (TPU) and their composite films with silica aerogels (aerogel-functionalized TPU (AFTPU)) are prepared using a simple and scalable non-solvent-phase-separation strategy. The TPU and AFTPU films are freestanding, mechanically strong, show high solar reflection up to 94%, and emit strongly in the atmospheric transparency window, thereby achieving subambient cooling of 10.0 and 7.7 °C on a hot summer day for the TPU and AFTPU film (10 wt%), respectively. The AFTPU films can be used as waterproof and moisture permeable coatings for traditional textiles, such as cotton, polyester, and nylon, and the highest temperature drop of 17.6 °C is achieved with respect to pristine nylon fabric, in which both the cooling performance and waterproof properties are highly desirable for the PTM applications. This study opens up a promising route for designing common polymers for highly efficient PDRC.

Flexible Daytime Radiative Cooling Enhanced by Enabling Three-Phase Composites with Scattering Interfaces between Silica Microspheres and Hierarchical Porous Coatings

Daytime radiative cooling has attracted considerable attention recently due to its tremendous potential for passively exploiting the coldness of the universe as clean and renewable energy. Many advanced materials with novel photonic micro/nanostructures have already been developed to enable highly efficient daytime radiative coolers, among which the flexible hierarchical porous coatings (HPCs) are a more distinguished category. However, it is still hard to precisely control the size distribution of the randomized pores within the HPCs, usually resulting in a deficient solar reflection at the near-infrared optical regime under diverse fabrication conditions of the coatings. We report here a three-phase (i.e., air pore-phase, microsphere-phase, and polymer-phase) self-assembled hybrid porous composite coating, which dramatically increases the average solar reflectance and yields remarkable temperature drops of ∼10 and ∼ 30 °C compared to the ambient circumstance and black paint, respectively, according to the rooftop measurements. Mie theory and Monte Carlo simulations reveal the origin of the low reflectivity of as-prepared two-phase porous HPCs, and the optical cooling improvement of the three-phase porous composite coatings is attributed to the newly generated interfaces possessing the high scattering efficiency between the hierarchical pores and silica microspheres hybridized with appropriate mass fractions. As a result, the hybrid porous composite approach enhances the whole performance of the coatings, which provides a promising alternative to the flexible daytime radiative cooler.

Rapid Synthesis of Asymmetric Methyl-Alkyl Carbonates Catalyzed by α-KMgPO4 in a Sealed-Vessel Reactor Monowave 50

Dimethyl-carbonate (DMC) is a green carboxymethylation agent for synthesis of the versatile long-chain alkyl carbonates through base-catalyzed transesterification with aliphatic alcohols. Herein, we demonstrated the facile preparation of a novel heterogeneous base catalyst α-KMgPO4 using commercially cheap metal salts via hydrothermal-calcination procedure. The combination of temperature programmed desorption (TPD) and FTIR measurements with CO2 pre-adsorbed revealed the presence of weak and medium base sites on α-KMgPO4. Furthermore, α-KMgPO4 catalyzed transesterification of DMC and n-octanol was performed in a sealed-vessel reactor (Monowave 50). The results show that the reaction was completed in only 10 min with the 97.5% conversion of n-octanol and >99% selectivity to asymmetric methyl-octyl carbonate under the optimal conditions. Additionally, the possible catalytic mechanism is proposed. As an extended contribution, the tribology performance of the asymmetric methyl-alkyl carbonates was further evaluated.

Beyond the Visible: Bioinspired Infrared Adaptive Materials

Infrared (IR) adaptation phenomena are ubiquitous in nature and biological systems. Taking inspiration from natural creatures, researchers have devoted extensive efforts for developing advanced IR adaptive materials and exploring their applications in areas of smart camouflage, thermal energy management, biomedical science, and many other IR-related technological fields. Herein, an up-to-date review is provided on the recent advancements of bioinspired IR adaptive materials and their promising applications. First an overview of IR adaptation in nature and advanced artificial IR technologies is presented. Recent endeavors are then introduced toward developing bioinspired adaptive materials for IR camouflage and IR radiative cooling. According to the Stefan-Boltzmann law, IR camouflage can be realized by either emissivity engineering or thermal cloaks. IR radiative cooling can maximize the thermal radiation of an object through an IR atmospheric transparency window, and thus holds great potential for use in energy-efficient green buildings and smart personal thermal management systems. Recent advances in bioinspired adaptive materials for emerging near-IR (NIR) applications are also discussed, including NIR-triggered biological technologies, NIR light-fueled soft robotics, and NIR light-driven supramolecular nanosystems. This review concludes with a perspective on the challenges and opportunities for the future development of bioinspired IR adaptive materials.

Selectively Enhancing Solar Scattering for Direct Radiative Cooling through Control of Polymer Nanofiber Morphology

Radiative cooling can alleviate urban heat island effects and passively improve personal thermal comfort. Among many emerging approaches, infrared (IR) transparent films and fabrics are promising because they can allow objects to directly radiate heat through bands of atmospheric transparency while blocking solar heating. However, achieving high solar reflectance while maintaining IR transmittance using scalable nanostructured materials requires control over the shape and size distribution of the nanoscale building blocks. Here, we investigate the scattering and transmission properties of electrospun polyacrylonitrile (PAN) nanofibers that feature spherical, ellipsoidal, and cylindrical morphologies. We find that nanofibers that have ellipsoidal beads exhibit the most efficient solar scattering, mainly due to the additive dielectric resonances of the ellipsoidal and cylindrical geometries, as confirmed through electromagnetic simulations. This favorable scattering decreases the amount of material needed to reach above 95% solar reflectance, which, in turn, enables high infrared transmittance (>70%) despite PAN's intrinsic IR absorption. We further show that these PAN nanofibers (nanoPAN) can enable cooling of surfaces with relatively low solar reflectance, which is demonstrated by covering a reference blackbody surface with beaded nanoPAN. During peak solar hours, this configuration lowers the temperature of the black surface by approximately 50 °C and is able to achieve as low as 3 °C below the ambient air temperature. More broadly, our demonstration using PAN, which is not as IR transparent as more commonly used polyethylene, provides a method for utilizing lower purity materials in radiative cooling.