Capillary-Marangoni synergism enabled salt-resisting sodium alginate hydrogel foam for efficient solar-driven water harvesting

Abstract

Solar-driven interfacial water evaporation is gradually becoming a sustainable freshwater supply method. However, developing scalable solar evaporators capable of stable operation in complex environments remains a critical challenge. Herein, a melamine foam (MF) based solar evaporator was fabricated through a facile “crosslinking-oxidation” approach, in which polypyrrole (PPy) was coated onto MF via a sodium alginate (SA) hydrogel. The resulting SA-MF composite features a hierarchical porous structure. Its synergistic capillary action and inherent hydrophilicity significantly enhance water transport kinetics. Notably, the SA hydrogel interface induces Marangoni convection within the interfacial water film, which promotes rapid salt ion diffusion back into the bulk water, thereby conferring exceptional salt rejection capability. Benefiting from the excellent water transport and thermal management capabilities of the alginate hydrogel matrix, this evaporator achieved an evaporation rate of 3.47 kg m⁻² h⁻¹ under 1 sun irradiation, with an evaporation efficiency of 95%. Importantly, no salt crystals are observed on the surface after 8 h of continuous operation in 25 wt% NaCl solution. Furthermore, the SA-PPy@MF evaporator exhibited robust environmental adaptability, durable mechanical resilience and long-term operational stability, thereby extending its service life and ensuring continuous operation under extreme conditions.