Key Takeaways & Executive Findings
- •• • Under 1 sun, the PNMF evaporator delivers 2.71 kg m−2 h−1 with ~90% solar-thermal conversion efficiency sustained over 24 cycles; this rate exceeds conventional hydrophilic melamine foam (PMF) which fails via salt crystallization within 1 h in seawater, directly enabling continuous operation without frequent maintenance. • • In 10 wt% NaCl simulated seawater, no NaCl crystals appear on the PNMF surface after 12 h, and the evaporation rate remains stable at 2.62–2.87 kg m−2 h−1 over a 20-day period; this salt resistance eliminates the need for periodic washing or brine discharge, reducing operational downtime and labor costs in remote or offshore installations. • • Under natural autumn sunlight (average irradiation <0.4 kW m−2, temperatures <24 °C), a portable PNMF-based device achieves a maximum hourly evaporation rate of 0.52 kg m−2 h−1 and cumulative freshwater production of ~3 kg m−2 over 11 h, with purified water salinity below 0.14‰; this demonstrates reliable performance under low-intensity, cool conditions where many high-efficiency evaporators underperform. • • The sandwich wettability design—hydrophobic top, hydrophilic middle, tunable hydrophobic bottom—enables directional water transport and Marangoni-driven salt backflow, preventing accumulation on the evaporation surface; this structural mechanism, combined with self-floatability from hydrophobic edges, minimizes heat loss and ensures long-term durability without complex auxiliary systems.
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Abstract
Interfacial solar-driven vapor generation offers a sustainable route to freshwater, yet practical deployment is constrained by salt crystallization, high material costs, and thermal losses. This work reports a sandwich wettability structure (PNMF) comprising a polypyrrole-coated hydrophobic top layer, a hydrophilic melamine foam interlayer, and a tunable hydrophobic bottom layer. The PPy coating absorbs broadband solar radiation and retains heat in situ; the hydrophilic interlayer supplies water through interconnected microporous channels, forming confined water clusters that reduce evaporation enthalpy. The bottom layer's central hydrophobicity regulates water transport to balance supply and evaporation, while its hydrophobic edges provide self-floatability and minimize heat loss. Under 1 sun, the PNMF evaporator achieves 2.71 kg m−2 h−1 with ~90% solar-thermal conversion efficiency over 24 cycles. In 10 wt% NaCl simulated seawater, no salt crystals formed after 12 h, and the evaporation rate remained stable at 2.62–2.87 kg m−2 h−1 over 20 days. Under natural autumn sunlight (average irradiation <0.4 kW m−2, temperatures <24 °C), a portable device produced approximately 3 kg m−2 over 11 h, with purified water salinity below 0.14‰. The simple, low-cost design addresses salt accumulation and durability bottlenecks, offering a scalable pathway for decentralized freshwater production.
1. Introduction
Interfacial solar-driven vapor generation has attracted intense research interest as a low-carbon method for seawater desalination and wastewater purification. Existing evaporators, however, face a persistent trilemma: high evaporation rates often come at the cost of salt fouling, complex fabrication, or poor durability under real-world conditions. Hydrophilic structures, while effective at water supply, suffer rapid salt crystallization—as evidenced by PMF surfaces forming salt deposits after just 1 h in seawater. Hydrophobic designs mitigate fouling but frequently compromise water transport and require costly materials or intricate engineering.
The PNMF sandwich wettability structure resolves this bottleneck by decoupling water supply, evaporation, and salt rejection. A PPy-coated hydrophobic top layer absorbs broadband solar energy and retains heat; a hydrophilic melamine foam interlayer with interconnected microporous channels supplies water and forms confined clusters that lower evaporation enthalpy; and a bottom layer with tunable hydrophobicity regulates water transport while its hydrophobic edges provide self-floatability. This architecture achieves 2.71 kg m−2 h−1 under 1 sun, maintains stable performance over 20 days in 10 wt% NaCl, and produces freshwater at <0.14‰ salinity under natural sunlight, offering a cost-effective, durable pathway to decentralized desalination.
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LI Yeran, LIU Xing, WANG Yifan, BI Shengjie, LI Jing, WANG Jingbo, DUO Yongchao, ZHU Zhengtao, JIN Xin, WANG Wenyu (2025). Sandwich-structured wettability foam for highly efficient, cost-effective, salt-resistant, and durable solar desalination. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3437-6
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Frequently Asked Questions
What is the primary failure mechanism of conventional hydrophilic evaporators under high-salinity conditions, and how does the PNMF structure prevent it?
Conventional hydrophilic evaporators, such as PMF, suffer from rapid salt crystallization on the evaporation surface: PMF showed visible salt deposits after only 1 h in seawater. This occurs because capillary water transport delivers dissolved ions to the evaporative interface faster than they can diffuse back to bulk water. The PNMF structure prevents this by combining a hydrophobic top layer with a hydrophilic middle layer, which establishes directional water transport and localized evaporation. This configuration accelerates salt backflow to bulk water via the Marangoni effect, preventing accumulation. Empirically, no NaCl crystals were observed on PNMF after 12 h in 10 wt% NaCl, and the evaporation rate remained stable at 2.62–2.87 kg m−2 h−1 over 20 days.
How does the PNMF evaporator perform under low solar irradiance and cool ambient temperatures, and what are the implications for off-grid deployment?
Under natural autumn sunlight with average irradiation below 0.4 kW m−2 and temperatures below 24 °C, a portable PNMF-based device achieved a maximum hourly evaporation rate of 0.52 kg m−2 h−1 and cumulative water production of approximately 3 kg m−2 over 11 h. The purified water salinity remained below 0.14‰ throughout. These results demonstrate that the PNMF structure maintains effective desalination even when solar input is less than half of the standard 1 sun, making it suitable for off-grid, decentralized applications in temperate or variable climates where high-intensity sunlight is not consistently available.
What is the long-term durability of the PNMF evaporator, and what degradation rates or performance losses were observed?
The PNMF evaporator exhibited stable evaporation rates within 2.62–2.87 kg m−2 h−1 over a 20-day continuous test in 10 wt% NaCl simulated seawater. No salt accumulation was observed on the surface, and the solar-thermal conversion efficiency remained at ~90% over 24 cycles. These data indicate negligible degradation in performance, with no reported decline in evaporation rate or structural integrity. The hydrophobic top and bottom layers, along with the mechanically robust melamine foam interlayer, contribute to this durability, suggesting a service life suitable for extended field operation without frequent maintenance.
What are the scalability and cost barriers for manufacturing the PNMF structure, and how does it compare to existing solar desalination technologies?
The PNMF structure utilizes cost-effective melamine foam (MF) as the backbone, coated with polypyrrole (PPy) via a simple fabrication process. The MF network is commercially available, mechanically strong, and highly porous, while PPy is an inexpensive photothermal polymer. The design avoids expensive nanomaterials or complex lithography. The sandwich wettability is achieved through tunable hydrophobic treatments, which are amenable to roll-to-roll or dip-coating processes. While exact cost-per-square-meter figures are not provided in the text, the use of low-cost materials and scalable coating methods positions PNMF as a cost-competitive alternative to conventional evaporators that rely on precious metals, carbon nanotubes, or graphene. The self-floatability and salt resistance further reduce balance-of-system costs by eliminating pumps or periodic cleaning.
How does the sandwich wettability design control water transport to balance supply and evaporation, and what happens if the bottom layer hydrophobicity is not optimized?
The bottom layer of the PNMF structure has tunable hydrophobicity in the central region, which regulates the water transport rate. This control is critical: if water supply is too high, excess water accumulates at the evaporation surface, increasing heat loss and reducing efficiency; if too low, the evaporator dries out, causing salt precipitation and performance decline. The central hydrophobic region restricts water flow to match the evaporation rate, while the hydrophilic middle layer ensures a continuous supply. The hydrophobic edges of the bottom layer provide self-floatability, minimizing thermal contact with bulk water. Without this optimization, the evaporator would either flood or dry out, leading to salt crystallization or reduced evaporation rates, as observed in the hydrophilic PMF control.
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