Ultralow-density rigid network hydrogels enable ultrafast and stable solar water desalination

Abstract

Interfacial solar vapor generation (SVG) has emerged as a promising strategy for water purification and desalination. Among SVG technologies, hydrogel-based evaporators stand out for their high energy efficiency and effective evaporation, but their performance often collapses under high irradiance due to insufficient water transport. Here we develop an ultralow-density rigid-network (ULR) hydrogel engineered for ultrafast water transport and evaporation. The ULR network maximizes water content and establishes steep osmotic-pressure gradients for rapid water supply, while the rigid, anti-shrinkage framework preserves hierarchical pores, sustaining capillary-driven flow and maintaining continuous vapor production under extreme irradiance. ULR evaporators surpass traditional hydrogels’ theoretical maximum water transport rate, achieving an evaporation rate of 25.57 kg m⁻² h⁻¹ at 10 suns for 100 h. In practical trials, a low-cost module produced 138 L m⁻² day⁻¹, yielding 12.42 L day⁻¹ of potable water. This design advances hydrogel-based SVG toward robust, affordable solutions for real-world water scarcity.

Researchers developed ultralow-density rigid-network hydrogels for stable solar desalination under intense sunlight. They reach an evaporation rate of 25.57 kg m⁻² h⁻¹ at 10 suns and deliver water production of 138 L m⁻² day⁻¹ in low-cost modules.

Introduction

The increasing scarcity of freshwater presents a critical challenge to human survival, with approximately 4 billion people worldwide experiencing water shortages^1,2. Despite global efforts, progress remains insufficient to meet Sustainable Development Goal 6 (SDG 6), which aims to ensure access to clean water for all^3,4. Over recent decades, substantial efforts have been directed toward advancing water purification technologies, including multi-stage flash distillation⁵, multi-effect distillation⁶, and reverse osmosis⁷. However, the steep energy demands, complex infrastructure, and high costs of these systems hinder their scalability and sustainable implementation, particularly in underdeveloped regions. There is an urgent need for highly efficient and sustainable water purification technologies to mitigate the growing global water crisis. SVG, which harnesses photothermal materials to localize heat at the evaporation interface, has emerged as a promising energy-efficient strategy for producing potable water^8–12. This approach has attracted significant attention, particularly in response to alleviating the global freshwater crisis in a sustainable way^13–15.

Among various SVG evaporators, hydrogel-based solar evaporators stand out for impressive evaporation properties and high energy efficiency^16–20. However, suffering from the restricted water transport capabilities within the hydrogels, the existing hydrogel-based solar evaporators often face limitations under high solar irradiation; they typically operate optimally only under low light intensities and suffer from severely deteriorated SVG performance and poor long-term stability under intense sunlight^21–23. These challenges impede the practical deployment and miniaturization of SVG devices for high-solar-intensity scenarios. In this regard, recent advancements have focused on accelerating the water transport of hydrogel-based solar evaporators through the development of highly interconnected porous networks²⁴, the design of highly ordered channels^25,26, and the employment of anti-polyelectrolyte effect in polyzwitterionic hydrogels to facilitate rapid water transport²⁷. Despite these improvements, hydrogel-based solar evaporators still fail to afford adequate water transport rates required to maintain consistent and high evaporation performance under intense solar conditions^28–30. The core issue remains the water transport rate (q_os) within the hydrogel, which is directly proportional to both the water content (c_w) and the osmotic pressure gradient (∇π_os), expressed as q_os∝∇π_os·c_w (Fig. 1)^31–33. Here, the ∇π_os is positively correlated with the polymer network density gradient induced by the water evaporation, where a high ∇π_os typically results from the drastic decrease of c_w at the light irradiation side^34,35. Consequently, the fundamental challenge in significantly enhancing water transport lies in reconciling the intrinsic trade-off between ∇π_os and c_w within hydrogel-based evaporators during the evaporation process.

Fig. 1.

Schematic illustration of water transport mechanism within different hydrogel-based solar evaporators during water evaporation process.

In this work, we introduce a concept of anti-shrinkage ULR hydrogels to achieve high-efficiency solar water generation across a broad range of sunlight intensity. The ultralow-density network of hydrogels enables the generation of sufficient ∇π_os and maintains relatively high c_w within hydrogels, leading to a fast osmosis water supply. Furthermore, the inherent anti-shrinkage rigid network resists the excessive shrinkage of hydrogel matrixes, thus maintaining a highly porous structure under intense lighting conditions and providing additional capillary action for water transport and extensive channels for salt migration (Fig. 1). Our network engineering hydrogel-based solar evaporators surpass the theoretical maximum water transport rate of traditional hydrogels calculated by the Flory-Rehner theory and achieve a water evaporation rate of 25.57 kg m⁻² h⁻¹ under 10 suns over 100 h. Practically, our cost-effective ULR hydrogel-based evaporator system achieved an exceptional water production rate of 138 L m⁻² day⁻¹, yielding 12.42 L of fresh water per day in field trials, demonstrating its real-world viability for decentralized water generation. This ULR network engineering not only provides insights into hydrogel-based evaporators with rapid water transport capabilities under high-intensity sunlight but also plays a crucial role in advancing decentralized drinking water systems for underdeveloped regions, enhancing water security, and ensuring access to clean water for vulnerable communities worldwide.

Results

Design and SVG properties of ULR hydrogels

The ULR hydrogels were synthesized readily by pouring the mixture of sodium alginate (SA) and carbon nanotubes (CNT) onto the dried low-molecular-weight chitosan (LMWCS) membrane (Fig. 2a). The interface allows LMWCS to diffuse into the CNT-SA dilute solution, reacting with SA due to the strong electrostatic complexation^36–38. The detailed fabrication process was in the experimental section. The simplicity of this reaction-diffusion method avoids complex chemical processes, facilitating the large-scale production of hydrogel-based evaporators (Fig. 2b). Fourier Transform Infrared spectroscopy confirmed the electrostatic complexation between the SA and CS, as indicated by the merging of the CS amide II band at ~1570 cm⁻¹ and the SA carbonyl asymmetric band at ~1610 cm⁻¹, accompanied by the appearance of a new absorption feature at ~1582 cm⁻¹ (Supplementary Fig. 1). Rheology tests validated the formation of crosslinked polymeric networks (Supplementary Fig. 2). With the addition of minimal CNT, the hydrogels maintained a high saturated water content (> 96 wt%) (Supplementary Fig. 3) Optimizing CNT content at 0.75 wt% yielded the best evaporation properties (Supplementary Fig. 4). Moreover, we tailored the network density by varying the SA concentrations (0.25, 0.5, 0.75, 1 wt% for ULR-1, ULR-2, ULR-3, and ULR-4 hydrogels, respectively). As shown in the scanning electron microscopy (SEM) images, the ultralow network density can impart the hydrogel with a highly interconnected porous structure (Fig. 2c), which is beneficial to water transport during the evaporation process. Increased electrostatic complexation correspondingly decreased the hydrogels’ saturated water content from 96.96 to 92.26 wt% (Fig. 2d). Notably, our ULR-1 hydrogel-based evaporator exhibits one of the lowest network densities, corresponding to the highest water content (i.e., the lowest polymer mass fraction) among the previously reported works (Supplementary Fig. 5).

Fig. 2. Fabrication, characterization, and SVG properties of ULR hydrogels.

a Synthesis process of ULR hydrogel. b Photograph of large-scale ULR hydrogel. c SEM images of cross-sectional morphology of the ULR-1 hydrogel. The inset shows a magnified view of the region. d Water contents of ULR hydrogels with different concentrations of SA. e Evaporation rates and energy efficiencies of different ULR hydrogel-based evaporators. f Evaporation rates of the ULR-1 hydrogel-based evaporator and common hydrogel-based evaporator under different solar intensities. The red × markers indicate the operational failure of conventional hydrogel evaporators at illumination intensities above 6 suns. g Morphological changes of ULR-1 hydrogels and common hydrogels before and after 10 suns’ irradiation. h Durability test of the ULR-1 hydrogel-based evaporator on continuous solar desalination for 100 h under 10 suns. Insets: The mass curves of the ULR-1 hydrogel-based evaporator at the 1st hour and 100th hour evaporation. i Comparison of evaporation rates of previously represented works under different solar intensities. Error bar in (d–f): standard deviation.

We evaluated the SVG properties of different ULR hydrogels. ULR-1 hydrogel, with the lowest network density, demonstrates a superior evaporation rate of 2.67 kg m⁻² h⁻¹ under one sun and a high energy conversion efficiency of 93.3% due to its good photothermal capabilities (Fig. 2e and Supplementary Figs. 6–8). Polymer–water interactions within the network modulate the hydrogen-bonding state of water (Supplementary Fig. 9), enriching intermediate water that is associated with accelerated evaporation and is consistent with cluster-type evaporation frameworks reported in the literature^19,39–41. The ULR-1 hydrogel-based evaporators enable high-efficiency solar water generation with a high and stable evaporation rate under high-intensity irradiation. It can sustain the original morphology and an excellent water evaporation rate of ~25.57 kg m⁻² h⁻¹ under 10 suns (Fig. 2f, g and Supplementary Fig. 10). This rate remained consistent even when scaled to 30 × 30 × 0.2 cm³ (Supplementary Fig. 11). However, increasing the polymer network density (φ_p) significantly reduced the evaporation rate under similar conditions, underscoring the critical role of network density in maintaining high performance (Supplementary Fig. 12). In contrast, the common hydrogels (polyacrylamide (PAM)/polypyrrole (PPy)) failed entirely under such intense sunlight, leading to drastically reduced SVG performance (Fig. 2f, g and Supplementary Fig. 13). The common hydrogels cannot work at all since 6 suns’ intensity due to seriously destroyed morphology. Further, our ULR-1 hydrogels demonstrated ultrahigh stability over 100 h of operation and exhibited significant salt rejection capabilities, enhancing their utility for long-term solar desalination (Fig. 2h and Supplementary Fig. 14). The ultrahigh evaporation rates and long-term stability under the high-intensity sunlight of our ULR-1 hydrogels are superior to previously reported representative works (Fig. 2i)^{21–23,25,28,42–48}. To highlight the versatility of the ULR networks beyond their chemical composition, we fabricated other hydrogel-based solar evaporators using different polyelectrolytes and photothermal agents. These hydrogel-based solar evaporators also performed robustly under 10 suns, achieving an ultrahigh evaporation rate of 19.43–25.57 kg m⁻² h⁻¹ (Supplementary Fig. 15). Moreover, to boost commercial viability, we constructed hydrogel-based solar evaporators using economical, readily available carbon black, which also  exhibited a high evaporation rate of 23.5 kg m⁻² h⁻¹ under the same conditions (Supplementary Fig. 16).

Mechanism underlying SVG properties under intense sunlight

The distinct design of the ULR networks endowed the hydrogel-based evaporators with excellent SVG performance under high-intensity sunlight. The key to these properties is understanding the water transport behavior within ULR networks. As for the hydrogel-based evaporator system, the q_os are proportionally hinged on the c_w within hydrogel matrixes and the swelling stress gradient (∇P). Here, the swelling stress P is equal to π_os, which correlates directly with φ_p within the hydrogels. Under intense irradiation, evaporation induces a polymer chain density gradient-essentially osmotic pressure gradient ∇π_os, which drives water transport while concurrently reducing c_w. The trade-off between ∇π_os and c_w within the traditional hydrogel-based evaporators constrains the rapid osmosis water supply, limiting stable evaporation performance under high-intensity sunlight.

To elucidate the impact of φ_p on water transport, we quantitatively analyzed the relationship between the q_os and φ_p according to the Flory-Rehner theory (Supplementary Note 3, 4)⁴⁹. Simulations revealed the distribution of polymer networks and water in hydrogels under varying q_os conditions. The parameters used for simulations are listed in Supplementary Fig. 17 and Table 2. The h represents the thickness of hydrogel samples during the evaporation, which is lower than that of the initial thickness (Supplementary Fig. 18). Initially, the φ_p of the hydrogel is equal to that on the non-irradiation side (φ_o). However, during the evaporation, φ_p(h) increases from the non-irradiation side to the irradiation side, peaking at the surface (denoted as φ_p(h_sur)) in Fig. 3a. This gradient indicates a corresponding ∇P, promoting water transport from less to more irradiated regions. Interestingly, φ_p(h_sur) increases with increasing of q_os, reaching a maximum when φ_p(h_sur) increases to 100%, indicating that the higher chain density gradient is required to provide higher ∇P for faster water transport (Fig. 3b).

Fig. 3. Mechanism of stable SVG properties under concentrated sunlight.

a Schematic of the hydrogel-based solar evaporators used for the water transport simulation. b The variation of φ_p(h) with h under different q_os. c Experimental and theoretical q_os-max of ULR hydrogels with different φ_o under h = 0.5 mm. d Experimental and theoretical q_os-max of common hydrogels with different φ_o. e Shrinking ratio of ULR hydrogels and common hydrogels under 10 suns’ irradiation. f, g SEM images of f the ULR hydrogels and g common hydrogels after 10 suns’ irradiation. h Schematic illustration of dual-force for fast water transport within the ULR hydrogels. i–k Simulated distributions of i salt, j water, and k polymer chain density in the ULR hydrogels (φ_o = 0.035).

Further, we tested the q_os-max of hydrogels with varying φ_o (Supplementary Note 5 and Fig. 19). The ULR hydrogel with lower φ_o can offer a high q_os-max, consistent with our theoretical analysis that posits the ultralow-density network facilitates the swift water supply. When the φ_o is lower than 0.035, the q_os-max of our ULR hydrogel reaches up to 25.57 kg m⁻² h⁻¹, surpassing the theoretical q_os-max of 15.6 kg m⁻² h⁻¹ (Fig. 3c). In contrast, for common hydrogel-based evaporators with low-density flexible polymer chains (φ_o = 0.028), their experimental q_os-max is much lower than that of the theoretical maximum value and that of ULR hydrogels (Fig. 3d), further demonstrating the merits of ULR networks for hydrogel-based evaporators. This is because the rigid ULR networks mitigate the excessive shrinkage of hydrogel matrixes and maintain high porosity for additional capillary action at the light-irradiation side under intense sunlight (Fig. 3e, f). In contrast, the common hydrogel-based evaporators with similar chain density display a significant shrinkage ratio during evaporation, where the porous structure is destroyed due to a lack of effective support from the intrinsic flexible networks (Fig. 3g and Supplementary Fig. 20). The deteriorated porous structure cannot work normally under high light intensity. Hence, according to the water transport behaviors, the ULR networks within hydrogel feature two distinct layers: the bottom layer driven by∇π_os and the top layer propelled with ∇π_os + capillary forces (Fig. 3h). The high ∇π_os and high c_w enable high q_os-max of the bottom layer, and the ∇π_os + capillary dual driving forces ensure the fast water transport at the top layer. This dual-force mechanism enables rapid water transport, helping to overcome the intrinsic trade-off between ∇π_os and c_w. Furthermore, we also considered the influence of the thickness on the SVG properties of the ULR hydrogel, specifically accounting for established values of φ_o. Our experiments across varying thicknesses revealed that the ULR hydrogel consistently achieves a significantly higher q_os-max than that of the common hydrogel (Supplementary Fig. 21).

Additional tests in the seawater environment confirmed that reducing the φ_o and thickness of hydrogel matrixes can also greatly increase the maximum q_os-max of hydrogel-based solar evaporators (Fig. 3c, d and Supplementary Fig. 21). Compared to the common hydrogels, the hydrogel with lower φ_o and anti-shrinkage rigid networks features enhanced abundant ion transport channels, facilitating faster ion transport and resulting in lower salt concentrations in the ULR hydrogels under the same irradiation conditions (Supplementary Fig. 22). Furthermore, through integrating the ∇π_os-driven water transport with concentration gradient-induced salt diffusion (Supplementary Notes 6–8), our simulations across ULR hydrogels illustrate the distributions of water, salt, and polymer chains density with varying thickness (Fig. 3i–k and Supplementary Figs. 23 and 24). The high water content and low salt concentration visually indicate that the ULR networks can greatly accelerate the transport of water and salt within the hydrogel-based solar evaporators.

Outdoor practical experiments and sustainable social impacts

To assess the practical viability of our solar vapor generator, we constructed a water evaporation and collection system featuring the Fresnel lens and 3D wedge collector. This system includes a water inlet, tank, and outlet (Fig. 4a). For field testing, we set up a large-scale ULR-1 hydrogel integrating with the aforementioned devices. A 110 × 110 cm² Fresnel lens concentrated sunlight onto the hydrogel-based solar evaporators, focusing on an area of ~ 900 cm². This configuration amplified the average light irradiation intensity in the focal area tenfold relative to the surrounding non-focal areas. As the steam condensed, it was collected in a beaker positioned below the setup. From 8:00 am to 4:00 pm, the system demonstrated an ultrahigh water production rate of 138 L m⁻² day⁻¹ and produced a total of 12.42 L of fresh water, achieving a collection efficiency of 69% and underscoring its scalability and potential impact on global water markets (Fig. 4b, c). Notably, water evaporation rates were observed to decrease by 20–30% when performed within an enclosed space, a phenomenon attributable to light degradation through reflection and scattering, as well as increased humidity levels above the evaporators^50,51. The purified water derived from seawater exhibited significantly reduced levels of Na⁺, Mg²⁺, K⁺, and Ca²⁺, aligning with World Health Organization (WHO) standards for drinking water (Fig. 4d). The system also proved effective in purifying wastewater, dramatically lowering concentrations of heavy metals like As⁵⁺, Sb⁵⁺, Se⁴⁺, and Cr⁶⁺ to approximately 0.1 ppb (Supplementary Fig. 25).

Fig. 4. Practical outdoor water yield experiments and analysis.

a Photograph of experimental set-up for solar desalination. b The mass of collected freshwater, outdoor solar intensity, temperature, intensity of convective flow, and humidity at different periods during outdoor solar desalination. Facula area: ~ 900 cm². c Photograph showing total freshwater was collected from 8:00 am to 4:00 pm. d Measured concentrations of four primary ions in seawater and the as-purified water after desalination under 10 suns irradiation. e The global map of long-term average solar radiation intensity. Map data obtained from the “Global Solar Atlas 2.0”, a free, web-based application developed and operated by Solargis s.r.o. on behalf of the World Bank Group, utilizing Solargis data, with funding provided by the Energy Sector Management Assistance Program. f A techno-economic analysis of our system reveals the relationship between water pricing and operational duration across different regions worldwide.

Our ULR-hydrogel water production system, built on cost-effective and easily prepared materials, offers an accessible and sustainable solution for potable water, particularly benefiting underdeveloped and remote regions where grid energy and extensive infrastructure are unavailable. The system leverages abundant solar energy and a passive water production process, eliminating the need for external power sources, thus ensuring a low-carbon and energy-efficient approach to water harvesting. To evaluate its economic viability, we conducted a comprehensive techno-economic analysis based on global long-term average solar radiation intensity (Fig. 4e and Supplementary Note 9)^52–54. This analysis estimates the payback period that the time required for the system to produce water at a lower cost than bottled or tap water. Taking Djibouti as a representative example, our system could achieve cost parity with bottled water in as little as ~10 days (Fig. 4f). Although surpassing the tap water in cost-effectiveness takes a longer period (approximately 2–3 years), the long-term benefits significantly outweigh initial investment costs, particularly in regions where water access is highly unreliable or dependent on expensive imports.

Beyond economic feasibility, the system offers profound societal and environmental benefits. Providing a decentralized, on-site water generation method, it reduces reliance on energy-intensive desalination plants and the environmental footprint associated with bottled water production, such as plastic waste and carbon emissions from transportation. The system’s ability to function without complex infrastructure makes it an ideal solution for disaster-stricken areas, refugee camps, and arid regions where conventional water supply systems are impractical or prohibitively expensive. Additionally, addressing water scarcity has cascading benefits in alleviating hunger, improving sanitation, and fostering local economic development. Reliable water access enhances agricultural productivity, supporting food security in water-stressed regions. It also reduces waterborne diseases, improving public health outcomes. Moreover, by fostering local manufacturing and maintenance opportunities, the system creates employment and empowers communities, driving economic self-sufficiency. By integrating affordability, sustainability, and societal impact, our ULR-hydrogel system directly contributes to multiple SDGs, including SDG 6 (Clean Water and Sanitation), SDG 3 (Good Health and Well-Being), SDG 2 (Zero Hunger), and SDG 8 (Decent Work and Economic Growth). This holistic approach ensures not only immediate relief in water-stressed areas but also long-term environmental resilience and social equity, reinforcing water security as a fundamental human right.

Discussion

In exploring ULR hydrogel-based solar evaporator’s substantial advantages over other SVG materials and systems, our findings highlight several key insights: first, the design of ULR networks effectively overcomes the inherent limitations of water transport in conventional hydrogels during the evaporation process. By integrating capillary and osmotic pressure gradient pumping mechanisms, this ultralow-density, anti-shrinkage rigid network addresses the trade-off between water content and osmotic pressure in conventional hydrogels, significantly enhancing water transport rates during evaporation across a wide range of light intensities; second, our ULR hydrogels achieve a stable and ultrafast evaporation rate of 25.57 kg m⁻² h⁻¹ under 10 suns’ illumination. Our simple and low-cost setup allows the systems to demonstrate an impressive water production rate of 138 L m⁻² day⁻¹ clean water, making it a viable solution for individuals in water-scarce and underdeveloped regions, particularly where conventional desalination plants and extensive infrastructure are unfeasible. Looking ahead, further advancements in ULR network design could expand its applicability to a broader range of hydrogel-based systems, improving water yield and efficiency in real-world settings. Additionally, refining the water collection system could maximize freshwater recovery, further enhancing the practicality and usability of this approach. Beyond desalination, this strategy holds immense potential for extracting valuable salts and minerals from seawater and industrial brines, contributing to sustainable resource recovery and circular water management. In summary, this work fosters greater water security and economic sustainability, making clean water accessible to vulnerable communities worldwide and paving the way for a more resilient and equitable future.

Methods

Chemicals and materials

CS (food grade) with a 90% degree of deacetylation and a weight-average molecular weight (M_w) of 5000 Da was purchased from Xi’an Realin Biotechnology Co., Ltd. (Xi’an, China). SA (food grade) with a viscosity of 485 mPa·s was purchased from Qingdao Hyzlin Biology Development Co., Ltd. (Shandong, China). K-carrageenan (K-C), potassium pyroantimonate (≥ 99%), p-aminophenylarsonic acid (≥ 98%), phytic acid (50% in water), aniline (≥ 99%), and pyrrole (≥ 99%) were purchased from Aladdin Reagents Co., Ltd. (Shanghai, China). Commercial CNTs of purity > 98% were purchased from Timesnano Co., Ltd. (China). Acrylamide (AM, > 98%), N,N′-methylenebisacrylamide (MBAA, ≥ 98%), and N,N,N′,N′-methylethylenediamine (TEMED, 99%) were purchased from Aladdin Reagents Co., Ltd. (Shanghai, China). Ammonium peroxodisulfate (APS, 99.99%), potassium dichromate (≥ 99.8%), sodium selenite pentahydrate (≥ 98%), sodium chloride (≥ 99.5%), and carbon black (≥ 98%) were supplied by Sinopharm Chemical Reagent, China. Seawater was taken from the Yellow Sea (Qingdao, China). Xanthan gum (XG, food grade) was purchased from Deosen Biochemical (Ordos) Ltd. (Ordos, China). Sodium carboxymethyl cellulose (CMC-Na, food grade) was purchased from Shanghai Ever Bright Enterprise Development Co., Ltd. (Shanghai, China). All chemicals were used without further purification.

Preparation of the PPy and polyaniline (PANI)

First, the phytic acid (0.05 M) and water were stirred well. Then the pyrrole (0.45 M) or aniline (0.45 M) was added. After introducing the APS (0.05 M) into the precursor solution and reacting at normal temperatures and pressures for 8 h, the PPy and PANI were synthesized. The resultant PPy and PANI were washed with distilled water repeatedly to remove the unreacted monomers.

Fabrication of ULR hydrogels

Firstly, we prepared the concentrated LMWCS (M_w = 5000) solution and the CNT-SA dilute solution. Thereafter, the CS solution was dried inside a culture dish at normal temperature and pressure, and the CNT-SA (SA, M_w = 388 kDa) dilute solution was poured onto the dried CS membrane. The LWMCS would gradually diffuse into the CNT-SA dilute solution to react with SA due to the strong electrostatic complexation between them. And their thicknesses increased with increasing the reaction time. After the reaction for 60 min, a ULR hydrogel with a thickness of 2 mm was formed. For the CNT-K-C/CS, PPy-SA/CS, PANI-SA/CS, CNT-XG/CS, CNT-CMC-Na/CS, and carbon black-SA/CS ULR hydrogels, they were prepared using the same strategy.

Preparation of common PAM/PPy hydrogel membranes

The PAM hydrogels were prepared by thermal-induced free-radical polymerization. Briefly, 15, 20, 25, 35, and 45 wt% of AM (monomer) and MBAA (cross-linker, 0.5 wt% based on weight of AM) were added into DI water. After magnetic stirring for 10 min, APS (initiator, 0.8 wt% based on weight of AM) and TEMED (accelerator, 0.5 wt% based on the weight of AM) were added. The mixture was stirred for 20 min in an ice bath. Thereafter, the well-mixed precursor solution was injected into a laboratory-made mold consisting of two glasses separated by the spacers with different thicknesses and kept at 50 °C for 1 h. After removing the molds, the PAM hydrogels with different contents of PAM and thickness were obtained. The as-prepared PAM hydrogel was immersed into the hytic acid (0.05 M)/pyrrole (0.45 M) mixture of solution and for 1 h, and then was covered with APS solutions (0.05 M) for 1 h to form PAM/PPy hydrogel. After the reaction, PAM/PPy hydrogel was dialyzed in the deionized water for 1 day to remove excess acid and by-product.

Solar steam generation test

The water evaporation performance was measured by using a solar simulator (PLS-SXE300, Beijing Perfectlight Technology Co., Ltd., China). The solar density was measured using a solar power meter (PL-MW2000). The 2 mm-thick ULR hydrogel was cut into small pieces with an exposed quadrate area of 4 cm², and each piece was put on top of a melamine sponge of the same size. The melamine sponge was then transferred to PS foam floating on brines, and exposed to simulated sunlight with different intensities for desalination tests in a plastic box. The mass change of water during evaporation was measured using a precision balance with an accuracy of 0.0001 g. Before illuminating, the evaporation rate under dark conditions was measured for 1 h. The environment temperature and humidity were maintained at ~20 °C and ~40%, respectively. The dark-condition evaporation rate was subtracted from the solar-illuminated evaporation rate. All evaporation rates were measured after a stabilization period of 30 min under the simulated sunlight.

Characterization

The morphologies of hydrogels were characterized by scanning electron microscope (SEM, FEI Quanta 250 FEG) operating at 10 kV. The samples were prepared by freeze-drying and then cryogenically fractured in liquid nitrogen. Before the characterization, the fractured surface was coated with a thin layer of gold by the sputtering method. The transmittance and reflectance spectra of the as-prepared ULR hydrogel and CNT were measured by a UV–vis NIR spectrometer (Perkin Elmer LAMBDA35) in the range of 200–2500 nm. The corresponding absorptance (A) was calculated by A = 1–R–T, where R and T are reflectance and transmittance, respectively. The contact angles of water of the ULR hydrogel were measured by contact angle analyzer (Theta, Biolin, Sweden). The thermal conductivity test was performed by using a TC3000E system (TC3000E, XIA TECH, CHN). The temperature response of the samples was measured by using an infrared camera (FLK-TI480-PRO). The concentration of ions was tracked by inductively coupled plasma mass spectrometry (PerkinElmer NexION 300X). The Raman spectra of water and hydrogels were measured by spectrometer (LabRAM HR Evolution). The evaporation behavior of samples was evaluated by differential scanning calorimetry (Discovery DSC 250, TA Instruments). Prior to measurements, the instrument was calibrated for temperature and enthalpy using an indium standard. For each test, ~10 mg of sample was loaded into an aluminum DSC pan and hermetically sealed with a perforated aluminum lid to allow water vapor to escape during heating. The sample pan was placed on the DSC sample platform, and an empty aluminum pan prepared in the same manner was used as the reference. Measurements were conducted under a nitrogen atmosphere at a heating rate of 5 °C min⁻¹ over a temperature range of 25–170 °C. The evaporation enthalpy was determined by integrating the endothermic evaporation peak using the instrument’s analysis software. The pore size distribution was detected by the Brunauer–Emmett–Teller N₂ adsorption/desorption method (ASAP 2420) and Brunauer–Joyner–Hallenda method using desorption data. The pore size distribution was also measured by a mercury intrusion method (Micromeritics AutoPore V 9620). Error bars in all figures represent standard deviations. These values were derived from triplicate experimental measurements (n = 3) and were calculated as $\sigma =\sqrt{\frac{1}{n}{\sum }_{i=1}^n{\left(x_i-\overline{x}\right)}^2}$, where $\sigma$ is the standard deviation, $x_i$ denotes the value from the $i$-th measurement, and $\overline{x}$ is the mean of the replicate measurements.

Author contributions

K.S. and G.Y. supervised the project. C.F.L., C.X.L., W.F., K.S., and G.Y. conceived and designed the experiments. C.F.L., C.X.L., M.L., and W.F. performed the experiments and analyzed the data. W.F., C.F.L., C.X.L., K.S., and G.Y. co-wrote the manuscript. W.F., C.F.L., S.S., H.J., Y.W., and P.Q. contributed to the model and theoretical calculation analysis.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewers for their contribution to the peer review of this work. A peer review file is available.

Data availability

All the data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary information. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-72288-w.