Wetting‐Induced Blackening Enables High‐Efficiency Solar Evaporation in Fully Biomaterial‐Based Porous Hydrogels Without Photothermal Additives

Abstract Solar‐driven interfacial evaporation (SDIE) is a promising strategy for sustainable water purification, but most systems still rely on external photothermal materials, many of which are costly or raise environmental concerns. Here, a fully biomass‐based evaporator composed of sodium alginate (SA) and coffee grounds (CO) is developed, achieving efficient solar evaporation without any photothermal additives. The key lies in a wetting‐induced blackening effect: upon water absorption, the porous hydrogel darkens significantly, increasing average solar absorbance from 71.9% to 94.4%, enabling excellent light‐to‐heat conversion. The evaporator features a radially aligned microstructure formed via directional freeze‐drying and tunable surface wettability through Fe ³ ⁺‐tannin complexation, allowing precise water transport control. As a result, it achieves a high evaporation rate of 2.2 kg m ⁻² h ⁻¹ with 91.2% solar‐to‐vapor efficiency under 1.0 sun irradiation. Furthermore, trapezoidal surface patterning induces Marangoni convection, enhancing salt resistance and light harvesting at low solar angles. Outdoor tests confirm stable freshwater output and excellent water quality. This work offers a sustainable and high‐performance SDIE solution based entirely on biomass materials, demonstrating great potential for green desalination.