Self-Cleaning Alginate–PVA Hydrogel Evaporator with Enhanced Solar Desalination Efficiency and Long-Term Salt Resistance

A low-cost, photothermally efficient hydrogel evaporator was fabricated using sodium alginate, poly-(vinyl alcohol), graphite, and multiwalled carbon nanotubes (MWCNTs) via a simple foaming-freezing-ionic cross-linking method. The optimized hydrogel (H2) achieved a high evaporation rate of 3.1 kg/m²·h with a solar-to-vapor efficiency of 89% under 1 sun irradiation. XRD analysis showed a reduced crystallite size (9.7 ± 0.3 nm), which was attributed to the improved pore structure and water transport characteristics. SEM images and porosity measurements revealed an interconnected porous network with 85% porosity, supporting capillary flow. The hydrogel exhibited sustained performance at high salinity (>2.1 kg/m²·h at 20 wt % NaCl), and after 20 brine cycles, its efficiency loss was <3.3% with no visible salt accumulation. ICP-MS analysis confirmed >99% salt ion rejection using real seawater samples, while compression tests showed a higher compressive stress (0.233 MPa) due to MWCNT reinforcement as compared to that without MWCNTs (0.192 MPa). Statistical <i>t</i> tests confirmed significant performance differences between hydrogel variants. The fabrication cost of the system was estimated to be approximately 5.5 USD/m², notably lower than several existing systems (over 40 USD/m²). These findings underscore the hydrogel's potential for decentralized solar desalination owing to its strong photothermal response, mechanical durability, self-cleaning capability, and low production cost. Future work will explore large-scale implementation and further pore structure optimization.