Solar Desalination with Monolithic Foams Containing a Combination of Graphite and SnSe Featured with a Hierarchical Structure and Bimodal Pores for Spontaneous Salt Rejection

Abstract

The increasing interest in solar water evaporation technology is due to its potential to replace conventional desalination systems. In this study, we suggest a new solar desalination system that has a hierarchical photothermal structure and is structurally monolithic. The overall membrane is made up of a macroscale porous melamine foam frame that acts as a porous insulator and a mass transfer layer. The mass transfer layer is designed with a bimodal porous structure of melamine foam, with large pores coated with a composite of graphite and SnSe (G-SnSe) and small pores. This simple structure provides many benefits. First, it combines two photothermal mechanisms, thermal vibration in graphite and non-radiative relaxation in SnSe, to achieve synergies in the conversion of light to heat energy. Second, compared to the single materials, graphite or SnSe alone, the nanocomposite of G-SnSe has a higher effective surface area, resulting in increased solar absorption. Finally, the melamine foam structure with bimodal pores enhances seawater wicking and has a high salt rejection capacity due to its volumetric porosity bimodal configuration of up to 35%. The monolithic melamine foam with a hierarchical photothermal structure minimizes interference with water transport due to the resistance-free presence of a continuous interfacial layer.The monolithic structure evaporaters (MSE) achieved a high evaporation rate of 1.303 kg m-2h-1 under 1 sun condition due to increased capillarity by the small-scale pores in the photothermal layer. Furthermore, spontaneous salt rejection was observed during solar desalination, attributed to the presence of intrinsic large-scale pores in melamine foam for draining the concentrated salt water from the surface. The feasibility of outdoor solar desalination was established, with a high evaporation rate of 1.259 kg m-2h-1. The mechanical stability of MSE was achieved by incorporating a small amount of binder (less than 3% of total photothermal materials), maintaining efficient performance for up to 30 days without degradation. The proposed strategy in the simple monolithic structure with bimodal porosity provides a general toolkit for designing high-performance solar desalination devices due to high evaporation rate and spontaneous salt rejection capability.In summary, the solar water evaporation technology is becoming increasingly popular as a possible replacement for traditional desalination systems. We propose a novel monolithic solar desalination system with a hierarchical photothermal structure, which is composed of a macroscale porous melamine foam as a skeleton which has both roles : an insulator and a mass transfer layer. The bimodal porous structure of melamine foam with large pores coated with a composite of graphite and SnSe provides many benefits, including increased solar absorption and a high salt rejection capacity. The MSE structure achieves a high evaporation rate and spontaneous salt rejection, making it a feasible option for outdoor solar desalination. The proposed strategy provides a general toolkit for designing high-performance solar desalination devices.