Native Wood Polyphenol‐Driven Construction of Self‐Grown Photothermal Interfaces for Salt‐Resistant Solar Evaporation

Wood‐based solar evaporators offer aligned water‐transport channels, low thermal conductivity, and renewability. However, most designs rely on externally deposited photothermal components that may compromise interfacial stability and obstruct transport pathways. Here, an endogenous construction strategy is developed by leveraging native polyphenols in eucalyptus wood to create a Janus evaporator with a self‐grown Fe–polyphenol (FeP) photothermal interface. Top‐protected bottom delignification selectively preserves polyphenolic sites in the top region as Fe coordination anchors while reconstructing the bottom region into a vertically porous, thermally insulating scaffold. Subsequent ascorbic acid (AA)‐assisted interfacial evolution enables in situ formation of the FeP interface. The hydrated FeP/AA‐delignified eucalyptus wood (FeP/AA‐DEW) exhibits an AM1.5G‐weighted solar absorptance of 82.05 ± 2.47% and a steady evaporation rate of 2.83 kg m −2 h −1 under standardized 1‐sun conditions. Boundary‐dependent experiments show that the high evaporation flux arises from absorbed solar energy, reduced equivalent evaporation enthalpy, and environmental heat exchange. The evaporator maintains 2.56 ± 0.04 kg m −2 h −1 in 20 wt.% NaCl, stable operation over 14 d, and 12.10 kg m −2 cumulative water loss during 8 h of outdoor operation. This work provides a native‐chemistry‐driven route for durable biomass‐based solar evaporation.