Sustainable Interfacial Evaporation System Based on Hierarchical MXene/Polydopamine/Magnetic Phase-Change Microcapsule Composites for Solar-Driven Seawater Desalination

Solar photothermal-driven interfacial evaporation is a promising technology with great potential for wastewater purification and seawater desalination. However, intermittent solar illumination and salt accumulation are still the major roadblocks of interfacial evaporation in practical applications. Herein, we developed a novel interfacial evaporation system based on the hierarchical MXene/polydopamine (PDA)/magnetic phase-change microcapsule composites (hereafter named "MXene/PDA@TiO2/Fe3O4@C22-HMC") integrated with natural wood. The microcapsule composites were fabricated by microencapsulating n-docosane as a phase-change material (PCM) core in a TiO2/Fe3O4 composite shell and then coating a PDA layer, followed by surface-attaching with MXene nanosheets. The obtained MXene/PDA@TiO2/Fe3O4@C22-HMC exhibits a good optical absorption ability, high heat energy-storage capacity, and good hydrophilicity. This enables the MXene/PDA@TiO2/Fe3O4@C22-HMC-based evaporator to gain a high water evaporation rate of 2.09 kg m-2 h-1 under one-sun illumination. A combination of the microchannels in natural wood and the tiny gap between the microcapsules results in a rapid water transportation within the evaporation system, which effectively resists salt accumulation during the evaporating process. As a result, there was no salt crystal observed from the evaporator surface in a 10 wt % NaCl solution under three-sun illumination for 8 h. More importantly, the introduction of Fe3O4 nanoparticles into the TiO2 shell endows the MXene/PDA@TiO2/Fe3O4@C22-HMC with magnetism, greatly enhancing the reusability and separability of the developed evaporator to undertake multicycle salt accumulation and washing processes for long-term desalination use. The latent heat release of the n-docosane core offers the developed evaporator a large amount of heat energy for continuous evaporation on a semi-cloudy day, increasing the total water production by 1.17 kg m-2 compared to the conventional evaporator without a PCM. This study provides an effective solution for intermittent solar energy utilization and salt accumulation in the solar-driven interfacial evaporation systems of seawater desalination.