A Porous Interfacial Photothermal Layer of Fused Core–Shell Carbon‐Polymer Nanospheres for Directional Salt Crystallization and Zero‐Liquid‐Discharge

ABSTRACT

Interfacial solar evaporation offers a sustainable route for freshwater production; however, effective zero‐liquid‐discharge (ZLD) operation under hypersaline conditions remains fundamentally challenging because high salinity suppresses water activity and vapor pressure while intensifying coupled heat, mass, and salt management. Herein, we report a hierarchical hydrogel evaporator featuring a thin, porous interfacial photothermal layer composed of fused core–shell carbon‐polymer nanospheres, fabricated via interfacial precipitation polymerization. The surface‐confined photothermal layer ensures effective thermal confinement and abundant liquid‐vapor interfaces, while the underlying hydrogel matrix sustains continuous water replenishment, delivering an exceptional evaporation rate of 6.2 kg m ⁻² h ⁻¹ under one‐sun irradiation without requiring bulky 3D architectures. During ZLD operation, direction‐dependent transport within the thin porous layer drives lateral advection of concentrated brine away from the active evaporation interface while suppressing through‐thickness salt accumulation, thereby enabling localized crystallization in the distal region. As a result, near‐complete spatial separation of water evaporation and salt crystallization is achieved from 20 wt.% brine. Outdoor demonstration further delivers 26.6 L m ⁻² day ⁻¹ of freshwater and 7.7 kg m ⁻² day ⁻¹ of recovered salt, underscoring the potential of this platform for practical ZLD desalination and mineral recovery.