In our relentless pursuit of energy-efficient cooling solutions, we often find inspiration in the most unexpected places. In this Behind the Paper story, we explore an innovation that came from an everyday product that many parents are familiar with— baby diapers. In fact, the story of this discovery began when the author Dr. Qiaoqiang Gan was changing his second daughter’s diaper. He noticed a white powder inside, the same water-locking material found in various products like baby diapers and agricultural applications for keeping soil moisture. This substance was intriguing not for its intended use, but rather for its optical property: an exceptional whiteness that indicates high solar reflectance (ability to reflect sunlight). Usually, white materials are excellent candidates for rejecting solar heating.

Driven by curiosity, Dr. Gan collected some of this material from his 2-year-old daughter’s diaper, and placed it outdoors overnight, in the hopes of observing radiative cooling in action. To his astonishment, the material became cooler than the ambient, and radiative cooling indeed took place. Even more surprising was that the white powder spontaneously self-fabricated into a continuous film overnight, and a totally green method for scalable fabrication.

Figure 1 | Fabrication and working principle of sodium polyacrylate (PAAS) photonic film. (a) The transfiguring procedures of the mechanical-robust and free-standing PAAS photonic film from the regular commercial inclusions (PAAS dry powders), generally used as the baby diaper filler, to (b) radiative and evaporative cooling films. (c) The cooling mechanism of the passive radiative and evaporative sodium PAAS photonic film. It absorbs atmospheric moisture during the nighttime and evaporates under direct sunlight. The porous structures backscatter the sunlight, and its infrared molecular vibrations enable high mid-infrared emittance for efficient radiation heat dissipation.

This incredible transformation was facilitated by the moisture in the air at night, which bonded the absorbent powder, sodium polyacrylate (PAAS), into a white film (see Figure 1). This film exhibited not only an excellent resistance to solar absorption but also a remarkable capacity to emit infrared radiation. Moreover, it possessed a unique feature during the day— it could evaporate the water it had absorbed during the night, providing efficient evaporative cooling. Once the water had evaporated into the atmosphere, the remaining white solid film displayed the hardness like a golf ball. What's most impressive is that this entire manufacturing process relied solely on atmospheric moisture without any need for additional chemicals or energy consumption. This represents a green manufacturing process that leaves behind zero carbon emissions. For long-time applications, it reduces energy consumption, featuring an even negative carbon emission.

But how does it all work? Figure 1 illustrates the working principle of the PAAS photonic film. Its porous structure allows it to absorb moisture from its surroundings, particularly in areas with high relative humidity at nighttime. This harvested water then evaporates, carrying heat away from the film when it's exposed to sunlight during the day. The porous structure also scatters sunlight, reducing the solar heating effect. Simultaneously, the film dissipates heat to outer space through the atmospheric window via thermal radiation, achieving hybrid passive radiative cooling. Intriguingly, the PAAS photonic film can regenerate itself by absorbing moisture when ambient humidity increases, forming a perfect daily water cycle that facilitates heat absorption and release.

Now, let's see a practical demonstration of the PAAS photonic film’s radiative cooling and evaporative cooling abilities. We conducted a direct comparison with standard roof shingles under identical outdoor conditions, as depicted in Figure 2. During noontime, when the sun blazed with an average solar intensity of 800 Wm⁻², the traditional roof shingle soared to a scorching 40°C above the ambient temperature. In contrast, the PAAS photonic film showed an impressive sub-ambient temperature drop of 5°C, highlighting its superior hybrid cooling performance. This advantage becomes particularly apparent on partly cloudy days when conventional radiative cooling materials struggle to cope with varying conditions. As night falls, the PAAS film absorbs moisture and undergoes a regeneration process. Then, as the day returns, the re-energized PAAS photonic film comes back into action, providing self-sustained and energy-free cooling. This feature underscores not only its environmental sustainability but also its potential to offer a cost-effective and efficient solution to combat rising temperatures and reduce energy consumption for cooling.

Figure 2 | (a) The outdoor cooling experimental setup under direct sunlight. (b) Temperature variations of the sodium polyacrylate (PAAS) photonic film and roof shingle. Insets are photos of the sky in different weather conditions. The red arrow points out the maximum temperature of the shingle. (c) Energy-savings map across the United States with the sodium polyacrylate (PAAS) cooling film. (d) Estimated average annual electricity and predicted average annual CO₂ emission reduction per capita and globally.

 The potential for energy-saving and carbon emission reduction of the PAAS photonic film is substantial. Using an energy estimation model, we assessed the impact of integrating this film into building rooftops. The results, as shown in Figure 2b in a simulated energy-saving map of over 100 US cities, indicate that up to 104.14 GJ of cooling energy can be conserved for a typical midrise apartment building in a tropical climate zone, equating to an 18.7% reduction in baseline cooling energy consumption. Furthermore, through energy conversion based on the EnergyPlus model simulation, the film's application results in an average saving of 2293.2 kWh of electricity for a mid-rise apartment building. This translates to an 887.9 kg reduction in CO2 emissions and a substantial 14.8 kg reduction per capita. On a global scale, this leads to an estimated annual reduction of 118.4 billion kg in total CO₂ emissions, representing roughly a 0.33% reduction from current global emissions.