Radiative cooling, a process by which surfaces lose heat by emitting infrared radiation to the cold outer space, has emerged as a promising strategy to curb energy consumption associated with traditional air conditioning. Materials optimized for this purpose typically require high reflectance in the solar spectrum to avoid heating during daytime, alongside strong thermal emission capabilities in the mid-infrared range. However, incorporating vivid coloration into such materials has proven difficult, as pigments usually absorb significant amounts of sunlight, thereby diminishing the cooling effect. The newly developed bilayer ethyl cellulose coating overcomes this limitation by finely tuning its structural properties to achieve both color vibrancy and solar reflectance exceeding 97%.

The key innovation in this work is the use of controlled drying-induced self-stratification during a single casting step. By carefully adjusting the concentration of the ethyl cellulose precursor solution, researchers manipulate the formation of a hierarchically structured bilayer. The top layer is meticulously engineered to have a thickness that produces specific colors via thin-film interference—an optical phenomenon where different wavelengths of light are constructively or destructively interfered as they reflect through a thin film. This approach eliminates the need for traditional coloring agents that absorb sunlight and reduce reflectance.

Beneath the colorful top layer lies a highly porous and scattering bottom layer that plays a critical role in the cooling performance. This bottom layer ensures exceptionally high reflectance across the solar spectrum, effectively minimizing solar absorption and unwanted heating during the day. Simultaneously, it exhibits strong emissivity in the long-wave infrared region, allowing thermal radiation to pass freely to outer space. This unique bilayer architecture thus harmonizes color customization with the stringent optical requirements of passive radiative cooling.

Experimental evidence demonstrated that the bilayer ethyl cellulose coating can achieve up to 9°C sub-ambient cooling under a solar irradiance of 800 W/m². This performance metric is particularly impressive considering the coating’s colored appearance, which traditionally compromises cooling efficiency. By outperforming commercially available colored paints and fluorescence-based colored coatings during field tests conducted in the humid subtropical climate of Hong Kong, the material proves its robustness and applicability in real-world, challenging environmental conditions.

Beyond exceptional cooling, the simplicity of the one-step phase-separation fabrication facilitates scalable manufacturing, which is crucial for practical deployment. Unlike multi-step or lithography-based methods common in layered thin-film materials, this approach drastically reduces complexity and costs. Moreover, the use of sustainable, biomass-derived ethyl cellulose aligns with growing demands for environmentally friendly materials in building and urban infrastructure applications.

Developing colored passive cooling materials has long been constrained by compromises between aesthetics and functionality. Traditionally, the incorporation of dyes and pigments introduced significant solar absorption, defeating the purpose of passive cooling by causing heat gain during sunlight exposure. The presented bilayer coating circumvents these issues by exploiting intrinsic optical effects rather than pigment-based coloration. This paradigm shift provides designers and architects with an unprecedented level of freedom to apply colored surfaces without sacrificing energy-saving benefits.

The hierarchical porous structure responsible for the high solar reflectance simultaneously contributes to durability and weather resistance. Porosity creates multiple scattering events that reflect incident sunlight efficiently, while the intrinsic nature of ethyl cellulose confers mechanical flexibility and environmental tolerance. These properties are vital for coatings intended for external applications on buildings, vehicles, and infrastructure where prolonged exposure to sunlight, moisture, and mechanical stresses is inevitable.

An additional merit of the approach is its broad spectrum tunability, achieved simply by controlling the top layer’s thickness during the drying process. This control enables the full color gamut via thin-film interference, from subtle pastels to vibrant hues, without any trade-off in radiative cooling performance. The seamless integration of color functionality with passive cooling technology could substantially accelerate the adoption of energy-saving surface coatings in urban areas, potentially mitigating urban heat island effects and reducing electricity demand for air conditioning.

Field testing in the subtropical climate of Hong Kong serves as a rigorous benchmark, given the region’s high humidity and intense solar radiation, which typically deteriorate radiative cooling efficacy. The bilayer ethyl cellulose coating’s superior performance in these conditions attests to its practical viability and reliability. Furthermore, the comparison with conventional colored paints and fluorescence-enhanced coatings underscores the material’s competitive advantages in both energy efficiency and appearance.

Looking forward, the integration of this innovative coating into building envelopes, vehicle exteriors, and outdoor equipment offers a new class of multifunctional materials. These could help reduce reliance on energy-consuming cooling systems, thus lowering carbon emissions and operational costs. Additionally, the bio-based nature of ethyl cellulose aligns with circular economy principles, potentially easing concerns related to material sustainability and end-of-life disposal.

In conclusion, the development of one-step-processed bilayer ethyl cellulose coatings represents a significant stride in the field of passive radiative cooling technology. By combining controlled self-stratification, thin-film interference coloration, and hierarchical porosity, researchers have crafted a material that not only achieves sub-ambient temperature reduction but also meets aesthetic and environmental standards. This discovery could catalyze widespread adoption of radiative cooling surfaces, marking a pivotal advancement in sustainable urban design and climate resilience strategies.

The synergy between advanced optical engineering and biomaterial science in this study highlights the transformative potential of interdisciplinary research. As passive cooling materials evolve, such innovations will be critical in addressing the escalating challenges posed by global warming and energy consumption. This promising work paves the way for future explorations into multifunctional coatings that harmonize energy efficiency, aesthetics, and sustainability in unprecedented ways.

Further research may delve into the long-term durability under diverse climatic conditions, large-scale manufacturability, and integration with existing building materials. Moreover, exploring other biomass-derived polymers for similar applications could widen the gamut of sustainable materials in this arena. Given the urgent demand for accessible cooling technologies worldwide, breakthroughs such as this bring us closer to cost-effective, scalable solutions that mitigate climate change effects while enriching built environments.

The implications of this study resonate beyond cooling surfaces. The principle of exploiting self-stratification and thin-film interference for property tuning can inspire innovations across photonics, energy harvesting, and sensor technologies. Ultimately, the intersection of green chemistry, nanostructured materials, and functional design embodied by this research embodies the forward path toward a sustainable and resilient technological future.

Subject of Research: Biomass-derived bilayer ethyl cellulose coatings for passive radiative cooling with full-color tunability through thin-film interference.

Article Title: One-step-processed bilayer ethyl cellulose for full-colour sub-ambient daytime radiative cooling.

Article References:
Liu, Y., Blagojevic, N., Xuan, Q. et al. One-step-processed bilayer ethyl cellulose for full-colour sub-ambient daytime radiative cooling. Nat Energy (2026). https://doi.org/10.1038/s41560-026-02039-0

DOI: https://doi.org/10.1038/s41560-026-02039-0

At the heart of this breakthrough lies an intricate coupling between a thermoradiative diode (TRD) and a heat engine, forming a self-sustaining thermodynamic system that can autonomously generate a positive photon chemical potential. Under typical circumstances, photon chemical potential—a thermodynamic parameter dictating the energy carried by infrared photons—defaults to zero or negative values in passive emitters, restricting the amount of thermal energy radiated. Generating a positive photon chemical potential conventionally demands external energy inputs, thereby negating the passivity of the system. The novel theoretical model circumvents this limitation by employing a cleverly integrated heat engine, which recycles the waste heat to energize the TRD, effectively “charging” the emitted photons with additional chemical potential without continuous external power.

Detailed theoretical calculations reveal that this paradigm-shifting arrangement can reach radiative cooling power densities up to 485 watts per square meter at ambient temperatures. This immense figure not only bests the standard blackbody radiation limit near 459 W/m² but also signals a decisive leap in passive cooling efficiency. The enhanced output suggests potential applications spanning from building climate control to thermal management in electronic devices, promising significant energy savings and emissions reductions compared to electrically driven air conditioning systems.

One insightful element of this research is the exploration of different embodiments of the coupling between the TRD and various heat engine types. While initial conceptualization employed an idealized Carnot engine, the researchers have studied practical alternatives such as thermoelectric generators (TEGs). These devices convert temperature gradients directly into electrical energy and, when paired strategically with the TRD, can further improve system efficiency. This adaptability is crucial for real-world deployments where simplicity, cost, and robustness are paramount concerns.

Furthermore, the model demystifies how design parameters—especially the geometric size ratios between the TRD and heat engine components—influence overall performance. An optimal balance must be struck to maximize photon chemical potential generation and heat conversion without introducing thermodynamic losses that could sabotage the system’s radiative cooling advantage. The analysis reinforces that passive operation is achievable when system architecture aligns precisely with the underlying thermodynamic constraints, eliminating the need for external electrical power and enabling autonomous cooling.

The concept of self-sustaining radiative cooling via photon chemical potential manipulation also catalyzes a rethinking of basic thermal management strategies. Conventional wisdom has long held that the Stefan-Boltzmann blackbody limit imposed an insurmountable ceiling on passive heat dissipation. Yet this new theory elucidates pathways to transcend this boundary by infusing the emitted infrared photons with chemical potential energy harvested internally. Such fundamental progress challenges century-old thermodynamic presumptions, opening avenues for novel device engineering at the intersection of photonics, thermodynamics, and materials science.

While immediate applications may remain in the theoretical realm, the implications for future experimental designs and industrial systems are profound. Buildings equipped with radiative cooling facades based on this technology could dramatically reduce reliance on grid electricity, mitigating urban heat islands and global carbon emissions. Electronic devices, notorious for heat buildup constraining performance and longevity, might enjoy enhanced thermal regulation from miniature versions of such coupled systems, boosting efficiency and durability.

The pursuit of practical implementations will require multidisciplinary efforts encompassing advanced material synthesis, nanoscale fabrication, and system integration. Challenges such as fabricating thermoradiative diodes with optimal spectral selectivity and ensuring long-term thermal stability must be addressed. Nevertheless, this work functions as a conceptual beacon guiding such endeavors by providing rigorous theoretical underpinning and design principles.

Significantly, the ability to autonomously generate a positive photon chemical potential without external energy distinguishes this approach from other radiative cooling enhancements relying on active systems. This self-sufficiency not only reduces operational costs and complexity but also enhances scalability and environmental compatibility, critical factors for widespread adoption.

In summary, the novel theoretical framework combining thermoradiative diodes with heat engines ushers in a new epoch for passive radiative cooling technology. By transcending conventional thermal radiation limits through photon chemical potential manipulation, it offers a pathway toward more powerful, efficient, and autonomous thermal management solutions. As climate change intensifies and energy sustainability becomes paramount, such innovations promise to play pivotal roles in reshaping how humanity controls heat, cools environments, and conserves energy.

For in-depth exploration, the reader is encouraged to consult the original article titled “Photon chemical potential-driven power enhancement in passive radiative cooling: a theoretical model,” authored by X. Zhang and W. Li, published in the Journal of Photonics for Energy, Volume 15, Issue 2, 022507 (2025). This seminal study lays the conceptual foundation and offers comprehensive thermodynamic analyses illuminating the transformative potential of this approach.

Subject of Research: Passive radiative cooling enhancement through photon chemical potential manipulation in coupled thermoradiative diode and heat engine systems.

Article Title: Photon chemical potential-driven power enhancement in passive radiative cooling: a theoretical model

News Publication Date: 13-Apr-2025

Web References:
https://www.spiedigitallibrary.org/journals/journal-of-photonics-for-energy/volume-15/issue-2/022507/Photon-chemical-potential-driven-power-enhancement-in-passive-radiative-cooling/10.1117/1.JPE.15.022507.full

References:
Zhang, X. and Li, W., “Photon chemical potential-driven power enhancement in passive radiative cooling: a theoretical model,” Journal of Photonics for Energy 15(2), 022507 (2025). DOI: 10.1117/1.JPE.15.022507

Image Credits: Zhang and Li, doi 10.1117/1.JPE.15.022507

Keywords

Photonics, Energy resources, Theoretical chemistry, Physical chemistry, Environmental chemistry