Double-network agar/sodium alginate hydrogel-based photothermal evaporator with layered structure for solar desalination

Interface solar evaporators generate fresh water through solar evaporation, presenting significant potential to address global water scarcity. However, current evaporators struggle to balance sustainability, high evaporation rates, and robust mechanical properties, resulting in limited applications. In this study, agar/sodium alginate hydrogels were synthesized using a dual-network strategy, with the internal water state adjusted to optimize their properties. Additionally, the incorporation of nanocellulose significantly enhanced the mechanical properties of the evaporator (improved 65% compressive strength), thereby extending its service life in harsh environments. Moreover, nanocellulose acted as an efficient green dispersant for carbon nanotubes (CNTs), facilitating photothermal conversion and improving solar energy absorption. Notably, we employed a freeze–thaw process to induce lateral aggregation of sodium alginate, resulting in layered water channels that optimized water transport and evaporator performance. This unique design resulted in the development of evaporators that exhibited a high evaporation rate of 3.286 kg·m⁻²·h⁻¹ in a 3.5 wt% NaCl solution, achieving a photothermal conversion efficiency of 98.65% under 1 kW·m⁻² light intensity. Moreover, even in a 20 wt% NaCl solution, the evaporation rate remained at 3.02 kg·m⁻²·h⁻¹. This work combined highly hydrophilic natural materials with a specialized water delivery channel design to develop hydrogel-based solar evaporators with high evaporation rates and excellent mechanical properties, offering strong potential for long-term interfacial evaporation applications.