Microwave expanded graphite felt coated with sulfonated polystyrene as light absorber and self-floating in photothermal water desalination

Porous supports based on microwave expanded graphite felt (Grph) coated with sulfonated polystyrene (S-PSty) were prepared and tested in photothermal seawater desalination. Self-floating supports of Grph@S-PSty were analyzed by thermogravimetric analysis (TGA), FTIR and UV/Vis spectroscopies, and their morphology determined by SEM. It was observed that supports consisted of woven graphite fibers of approximately 20 μm in diameter well coated with the S-PSty, where micropores in the range of 1 to 8 μm were formed due to microwave irradiation exposure, and thermal conductivity of support was almost three times lower than original graphite felt. The UV/Vis analysis showed that supports have a wide absorption light range in the UV/Vis-NIR spectrum with more than 92% of light absorption. Pure water evaporation efficiency using Grph@S-PSty supports was studied as a function of the amount of S-PSty coating the graphite felt and different power density of incident light. The best support was that prepared with a S-PSty solution at 0.5% (wt/v) expanded 2.5 seconds under microwave irradiation achieving 1.57 kg h⁻¹ m⁻² of evaporation rate and 99.0% of efficiency under 1 sun of illumination, therefore, this support was tested in water desalination (3.5% NaCl) with multiple evaporation cycles, achieving 1.57 kg h⁻¹ m⁻² of evaporation rate and 99% of efficiency, demonstrating that this material can be used for desalination at low cost. The system was also tested in the evaporation of NaCl aqueous solution at different concentrations to determine the stability at very high salt concentrations, achieving the same evaporation rate and efficiency at NaCl concentrations as high as 12% without salt build-up on the top surface, demonstrating the material viability under extreme conditions. Novelty statement: Self-floatable substrates based on graphite felt and sulfonated polystyrene were prepared. Graphite@S-PSty supports were used in photothermal water evaporation and desalination. Thermal efficiency of 99% and 1.57 kg h⁻¹ m⁻² of evaporation rate were reached. Very low salt accumulation was observed in the long-term evaporation.