Selection of solar absorbers: Three categories of solar-absorbing materials including carbon materials (such as carbon nanotubes, carbon nanoparticles, and graphene), plasmonic metals (such as Al, Au, Ag, Cu) and ceramic nanomaterials (such as transition metal nitrides and carbides) and semiconducting nanomaterials (such as TiO_2-x, MoS₂) were developed. The present research focuses on the preparation of high-light-absorption photothermal absorbers with a low-cost and a superior chemical stability.

1) Structural design of solar steam generation system: The solar-vapor conversion efficiency (η) can be calculated by

Based on Eq. (1), the solar steam conversion efficiency is directly proportional to water evaporation enthalpy and water evaporation rate. It is proved that the latent heat of water vaporization in a porous nanostructure is lower than the standard value. However, it is still unclear how to obtain a low water evaporation enthalpy by regulating the porous structure of the solar photothermal materials. It is crucial to obtain hierarchical nanostructures and evaluate its water vaporization latent heat. In order to increase the specific surface area of solar photothermal materials, porous foams and gels are designed and prepared, but the preparation of these artificially hierarchical materials is a time-consuming and costly process. It is thus imperative to explore naturally hierarchical plants that are low-cost and easy to obtain.

2) Efficiency and mechanism of VOCs photodegradation in water: Solar-driven steam generation is utilized to produce fresh water by removing salts, heavy metals, micro-organisms, and most organic pollutants in raw water. However, it also accelerates the volatility of the volatile organic compounds (VOCs) and enrich them in the distilled water if the water source is contaminated by VOCs because of their similar boiling point with water, which poses a serious threat to the atmospheric environment and human-being health. VOCs such as alcohols, aldehydes, ketones, olefins, and aromatic compounds tend to migrate and accumulate in different environmental media such as soil, water, and air. The World Health Organization and China's sanitary standards for drinking water quality both have established limit values for VOCs. Consequently, it is crucial to efficiently remove VOC during the solar-driven interfacial evaporation.

3) The relative merits of different electricity generation systems: Solar-driven steam-electricity system is mainly divided into integrating electricity generation system and self-powered electricity generation system. The integrating electricity generation module integrates a solar water evaporation module and an electricity generation module by rationally connecting the solar water evaporation module with photovoltaic device harvesting light energy, thermoelectric module recovering heat energy from water vapor, membrane structure utilizing salt concentration gradients and triboelectric nanogenerator based on steam condensation process. The electricity generation capacity of an integrated electricity generation system typically depends on the electrical output performance of the power generation module itself. Though the integrated electricity generation system can be commercially produced and has a stable performance, it has some problems such as complex structure, high cost and non-adaptability of different modules, which limit its practical application. The self-powered electricity generation system does not connect solar water evaporation module with other auxiliary equipment for electricity generation. It generates electricity directly by a hydrovoltaic effect, and its evaporation-electricity generation performance mainly depends on the microstructure of the photothermal material, such as the size of micro-/nano-pore channels and the zeta potential. The currently reported output currents range from nanoamps to microamps, and exploring current output in the milliamp range is a further research direction.