CNT–PVDF freestanding sheets for direct solar evaporation toward continuous desalination applications

Typically, solar desalination membranes are fabricated through non-scalable techniques and consist of multiple layers (a photo-absorber layer deposited on a hydrophilic substrate), resulting in multistage evaporation processes, influencing the efficiency of the process. Moreover, the classical approach is to test such developed membranes in a batch-mode setup, in which the system is stagnant; motivating the need to investigate their applicability in continuous flow setups to expand on the commercial outreach criteria. Herein, we investigate freestanding, thin, and nanoporous composite membranes for direct solar-assisted evaporation experiments in an in-house-built continuous flow thin-film evaporation device. Our membrane fabrication methodology had three notable advantages: (1) the utilization of blackbody materials, (2) the achievement of variable membrane properties, and (3) the utilization of a single composite freestanding membrane. We show that evaporation mass flux (assisted with 1 sun of direct solar irradiation) increased as mean pore size and membrane thickness decreased, while volume porosity, solar absorption, and wettability increased. Out of all of the membranes investigated, the membrane modified with the least amount of PVDF exhibited the highest evaporation flux (0.55 L m⁻² h⁻¹) and the highest solar efficiency (~ 38.4%).