Atmospheric-moisture-induced polyacrylate hydrogels for hybrid passive cooling

Heat stress is being exacerbated by global warming, jeopardizing human and social sustainability. As a result, reliable and energy-efficient cooling methods are highly sought-after. Here, we report a polyacrylate film fabricated by self-moisture-absorbing hygroscopic hydrogel for efficient hybrid passive cooling. Using one of the lowest-cost industrial materials (e.g., sodium polyacrylate), we demonstrate radiative cooling by reducing solar heating with high solar reflectance (0.93) while maximizing thermal emission with high mid-infrared emittance (0.99). Importantly, the manufacturing process utilizes only atmospheric moisture and requires no additional chemicals or energy consumption, making it a completely green process. Under sunlight illumination of 800 W m−2, the surface temperature of the film was reduced by 5 °C under a partly cloudy sky observed at Buffalo, NY. Combined with its hygroscopic feature, this film can simultaneously introduce evaporative cooling that is independent of access to the clear sky. The hybrid passive cooling approach is projected to decrease global carbon emissions by 118.4 billion kg/year compared to current air-conditioning facilities powered by electricity. Given its low-cost raw materials and excellent molding feature, the film can be manufactured through simple and cost-effective roll-to-roll processes, making it suitable for future building construction and personal thermal management needs.

Coating (HPC) [1] 0.95 0.98 Single layer solvent exchange method Modified Poly(vinylidene fluoride-cohexafluoropropen e) (PVDF) and Cellulose Acetate (CA) polymeric network poly(Nisopropylacrylamid e) (pNIPAm) [2] Atmospheric moisture-induced "green" fabrication of PAAS photonic film: PAAS photonic film is fabricated under ambient-air conditions with a relative humidity of above 60% by absorbing moisture from the ambient environment for 6 hours at night (Supplementary Figure 3).The PAAS particles are activated by water vapor molecules and form a continuous film with uniform density by a hydrogen bond between water molecules.The dry PAAS powders are uniformly distributed on a polyacrylic substrate by the blade coating approach to control its thickness and these powders are placed in an open field at midnight to absorb atmospheric moisture.A corrugated paper box with holes on the sides is employed to protect the sample Critical relative humidity for PASS powders to form a continuous film: The PAAS film fabrication and regeneration requires a minimum nighttime ambient humidity threshold of 60% RH or higher.To determine this critical threshold, we conducted an experiment involving a PAAS powder sample placed in a controlled environmental chamber set to different relative humidity levels, namely 30%, 50%, 60%, 70% and 90%, all at an ambient temperature of 22℃ for a duration of 6 hours.
After the 6-hour period, we carefully peeled off the PAAS film from the substrate to assess its mechanical stability and film continuity.The findings are presented in Supplementary Figure 4.At 30% RH, no film formation occurred, and the PAAS remained in a powdered state.At 50% RH, PAAS film formation was observed, but the films were discontinuous and exhibited poor mechanical stability.However, from 60% RH and beyond, we noted continuous film formation with satisfactory mechanical stability.
Consequently, based on these observations, we conclusively determined that the threshold for successful PAAS film fabrication and regeneration is 60% RH or higher.
Supplementary Figure 4: Photos showing the continuous film formation of PAAS powders at different relative humidity.

Colorful PAAS photonic film fabrication:
To validate the versatility of our proposed atmosphericmoisture-induced fabrication method, we introduce color pigments (Rolio© mica powder) by mixing them with PAAS power powder (1:100 ratio) and then putting them in an environment-controlled chamber (RH, ~ 90%) to form a continuous film with different colors.This procedure is similar to the one described in our manuscript and the only difference is the introduction of pigment powders.The introduced pigment powders did not affect the formation of continuous film, as depicted in Supplementary Figure 5a where four colored PAAS photonic films with different colors are displayed.These films show bright colors to the naked eye which corresponds to the absorption peaks as shown in the spectra results in Supplementary Figure 5b.Moreover, compared with the pristine white PAAS photonic film, these colored PAAS films keep high reflectance over other solar wavelengths (e.g., near-infrared) while displaying unity thermal emittance over infrared wavelengths.The high infrared thermal emittance enables effective radiative heat dissipation through the atmospheric transparent window.This moisture-induced technique for colored photonic film allows for more flexibility in terms of achieving specific color effects or incorporating multiple colors.
Furthermore, a colorful appearance will be more attractive for specific applications, such as wearable electronics, automotive and cooling textiles.However, adding pigment powders to the white radiative cooler may compromise the high solar reflectivity and hence weaken the cooling performance of the film.
Therefore, other approaches that can introduce narrowband visible wavelength absorption are highly demanded by further hybrid passive cooling technologies.

Scattering effects of the PAAS photonic film:
The PAAS photonic film strongly scatters the incident light due to its intrinsic porosity.The scattering feature is demonstrated in Supplementary Figure 7, where a red laser is employed on a regular white paper (Supplementary Figure 7a) and the PAAS photonic film (Supplementary Figure 7b).The laser beam source with a diameter of ~ 2 mm.The PAAS photonic film with a diameter of 3 cm is placed on top of white paper.First, the red laser beam is illuminated on the white paper with a negligible scattering effect, while the shining area of the red laser beam is approaching 3 times that of the white paper.Supplementary Figure 7. Photos demonstrating the red-laser scattering effect of PAAS photonic film.Apart from the backscattering benefits of the PAAS film, the diffused capability of the PAAS photonic film is another advantage compared to the shiny reflective photonic metamaterials which cause twinkling in human eyes.
Recycling procedure of the PAAS photonic film: The recycling properties of PAAS photonic film can significantly expand its lifespan and simultaneously increase the carbon downdraw for its overall lifetime.
The following photos illustrate its recycling process: The recycling of the PAAS photonic film involves freezing the used film with liquid nitrogen.The extremely low temperature of liquid nitrogen makes the PAAS film very brittle and easy to shatter.Next, the brittle used film is transferred to a blender for crushing it into powders The crushed powder is then mixed with fresh PAAS powder at a ratio of 4:1 (used/fresh) and spread homogeneously on a substrate.Finally, the mixed powder is placed inside a humidity and temperature-controlled chamber (T = ~25°C, RH = 90%) for 6 hours to obtain a continuous film.
Supplementary Figure 9.The recycling process of the used PAAS photonic film.
Experimental setup for controlled humidity chamber: The chamber walls are made of acrylate sheets (Supplementary Figure 12).One sidewall has three ports: moisture is connected to a humidifier, moisture is out, and dry air is connected to a dehumidifier.DI water is used for the humidifier.Anhydrous calcium sulfate (98%) impregnated with cobalt chloride (2%) is used as a desiccant for the dehumidifier.The "ON" and "OFF" of the humidifier and dehumidifier are controlled by a controller to maintain the relative humidity inside the chamber according to our setting.The temperature and relative humidity inside the chamber are recorded by temperature and humidity sensors.
Supplementary Figure 12.Picture of the environmental chamber setup.
Thermal stability of PAAS photonic films: A piece of PAAS film is heated at a predetermined temperature (indicated in Supplementary Figure 13) at 5°C min -1 .After reaching the desired temperature, the sample is kept at a constant temperature for 30 mins.Finally, the sample is cooled to room temperature at 10°C min -1 .The final state of the sample is shown in Supplementary Figure 11.From the figure, it can be observed that after 200 • C the PAAS photonic film would undergo physical and chemical changes visible from its color from white to dark.
Supplementary Figure 13.Photos of PAAS photonic film color after thermal treatment at various temperatures.

Refitting to extract the refractive index of PAAS:
The PAAS powders are dissolved into DI water with a weight ratio of 1:500 (PAAS/water) to get a homogeneous solution.Two droplets of PAAS solution are encapsulated between two cover glass slips with a thickness of 0.15 µm for the transmittance spectrum measurement.To estimate the refractive index of PAAS, we use the method outlined by Verleur et al. [5] to extract the refractive index by refitting the transmittance spectrum.Refractive index (n + i) can be assumed to be of Lorentz-Drude oscillator form given by: Here,  ,   , Γ  , and j are the strength, resonant frequency, damping factor of kth Lorentz-Drude oscillator, and the imaginary unit, respectively.N such oscillators are assumed. ∞ is the contribution from higher frequencies.Since the incidence in the measurement is at 0 °, the transmittance spectrum is calculated for that angle of incidence.  correspond to several of the vibration bond resonance of PAAS.Consider a structure having 5-layer media: air, glass, PAAS, glass, and glass, respectively.The refractive index of PAAS is obtained by tunning 5 oscillator parameters by matching the refitted transmittance spectrum to the measured one.This can be done by minimizing the error between the refitted and measured spectra.The minimization is done in two steps: first, we use a MATLAB-based genetic algorithm to arrive at an initial guess of oscillator parameters.This brings us closer to the global optimum of the objective function, providing a reasonable fit that is further improved by using the constrained optimization function fmincon.
The final refitted spectrum is shown in Supplementary Figure 21.The refractive index is plotted in Figure 3d.
Supplementary Figure 21.Transmittance of refitted and original PAAS sample that is sandwiched between two glass slide covers.
Optimized thickness of PAAS photonic film for improved cooling performance: The thickness of the PAAS film plays a pivotal role in determining the overall performance of the cooling system, and it comes with its own set of advantages and disadvantages.
One notable disadvantage of thicker films is their potential to hinder the effective emission of thermal radiation from the underlying radiative cooling films.This could result in a decrease in the overall radiative cooling capacity, especially if the film absorbs and scatters a significant amount of the outgoing thermal radiation.However, there are several significant advantages associated with a thicker PAAS film: The primary advantage is the increased moisture absorption capacity enabled by the greater thickness.A thicker film can store a larger volume of water (as demonstrated in Supplementary Figure 24a), facilitating prolonged and continuous evaporative cooling.This attribute proves especially valuable in arid or semiarid climates, where water availability may be limited.Additionally, thicker PAAS films have a higher heat capacity, allowing them to store more thermal energy during the day.Consequently, the cooling effect can be sustained for extended periods, even after sunset.Moreover, increased thickness often imparts mechanical robustness and durability to the PAAS film, rendering it more resistant to physical damage and wear.This enhanced durability contributes to extending the operational lifespan of the cooling application.
In response to the impact of the PAAS film's thickness on its performance, we conducted an additional outdoor experiment in this revision.This experiment involved testing PAAS films with varying thicknesses to assess their cooling performance, and we carefully monitored the solar intensity and relative humidity during the experimental period (as displayed in Supplementary Figure 24b).Our findings indicated that while a thicker hydrogel film indeed enhances moisture absorption and heat storage capabilities, it also introduces certain trade-offs, such as reduced radiative cooling efficiency.After careful evaluation, we determined that a 2-mm-thick film provided the optimal cooling performance (as shown in the temperature response graph in Supplementary Figure 24c).The PAAS photonic film with a thickness of 2 mm exhibited the lowest temperature during the experimental period.This particular thickness strikes a balance between moisture absorption capacity and radiative cooling efficiency, ultimately delivering the best overall cooling performance.Consequently, this thickness value is employed and discussed in our manuscript.

Supplementary Figure 5 .
(a) Photos displaying the continuous PAAS photonic film of four different colors (red, yellow, green, and blue).(b) Spectra of the colored PAAS photonic film to demonstrate its aesthetic functionality while maintaining effective radiative cooling capabilities.

Table 1 .
Comparison of optical/thermal properties of PAAS with other literature.The values marked with an asterisk (*) indicate estimates based on the information provided in the corresponding reference.
SEM image of PAAS dry powder.With an average diameter of ~100 µm, this dry powder is the base of PAAS photonic film fabrication.Starting from the friable power of PAAS, a continuous film is fabricated for cooling purposes.