All-cellulose-based solar evaporators with improved wet mechanical integrity via mercerization

With rising demand for sustainable technologies, renewable energy-driven water purification and desalination have gained increasing attention. Here, we present a newly developed all-cellulose interfacial solar evaporator system with enhanced wet mechanical integrity through mercerization. The devices consist of a porous microfibrillated cellulose supporting layer and a cellulose nanofibril photothermal layer integrated with carbon nanotubes. Post-fabrication mercerization, an alkali treatment using sodium hydroxide, induced a crystalline transformation from cellulose I to cellulose II, resulting in improved fiber entanglement and structural integrity. Systematic characterization revealed a robust structure and outstanding performance, with solar-to-vapor energy conversion efficiencies up to 95 % and evaporation rates of 15 kg·m^-2·h^-1 under high-intensity simulated sunlight (5-10 kW·m^-2), as well as the highest efficiencies reported to date at 3 and 5 kW·m^-2. It also demonstrates >99.9 % rejection of common salts and toxic heavy-metal ions, exhibiting self-regenerating behavior, highlighting their suitability for long-term field use. These results demonstrate that mercerization, a relatively mild chemical treatment, is effective in overcoming the intrinsic wet strength limitations of cellulose-based devices, thereby enabling their practical application in sustainable water purification. Our study highlights the potential of all-cellulose, biodegradable solar evaporators for scalable, eco-friendly water treatment solutions.