A low-cost and environmentally-friendly chitosan/spent coffee grounds composite with high photothermal properties for interfacial water evaporation

The present study explores the remarkable photothermal properties and potential applications exhibited by melanoidin-enriched spent coffee grounds (SCGs). However, the inherent challenge lies in the substantial particle size of SCGs, leading to rapid sedimentation within the medium and resulting in uneven heat distribution during photothermal analysis. To overcome this obstacle, acid hydrolysis was employed to liquefy the SCGs, allowing their dispersion in water and significantly enhancing their light absorption capacity. The photothermal properties of liquefied SCGs (LSCGs) were analyzed under 808-nm near-infrared (NIR) irradiation and compared with those of SCGs. Remarkably, the photothermal conversion efficiency of the LSCGs was as high as 52%, twice that of SCGs. To further explore their potential, the LSCGs were incorporated into chitosan (CS) to create homogenous and porous CS/LSCGs composites. These low-cost and environmentally friendly CS/LSCGs composites, made entirely from waste materials, demonstrated photothermal properties under NIR irradiation. Notably, they exhibited excellent performance in multiple cycles of water evaporation experiments, achieving an impressive water evaporation efficiency of nearly 90%.