Janus Absorber For Solar Desalination

Water is the source of life and it is an important foundation for human survival and development.However,with the rapid growth of population,the increase in the level of urbanization and industrialization,and the intensification of water pollution,human society is facing a severe shortage of freshwater resources.About two-thirds of the world's population(4 billion)is living in varying degrees of water shortage.However,70.8%of Earth's ground area is covered with water,only 3%of which is fresh water.Thus,seawater and brine have become the first choice to obtain freshwater in water-scarce areas.Main desalination technologies are membrane and thermal methods,and they are very mature large-scale and centralized water supply solutions.However,both of the solutions consume fossil energy,which will inevitably aggravate the greenhouse effect and environmental pollution.Solar desalination consumes only sunlight.Its mechanism can be summarized as light-distillation.It is a green and environmentally friendly desalination technology and is expected to replace membrane and thermal methods.However,the photothermal utilization efficiency of the traditional solar still is only 30-40%.In order to increase the photothermal utilization efficiency,traditional optimization methods include blacking seawater or the bottom of seawater tanks.Traditional solar still uses light to increase the temperature of entire seawater.But what we need is only vapor.Heating the entire water will waste most of the solar energy.Interface photothermal conversion can effectively solve this problem.Through the application of nano-materials and designing structures,the light absorber with a high absorbance is prepared and it will float on the surface of seawater during solar desalination.Heating is localized on the surface seawater,which will increase the light utilization efficiency.The raw materials for the absorbers are very rich,such as gold nanoparticles,graphite,metal and graphene oxide.The emergence of relevant scientific research also provides many options for solar desalination.However,most of the studies neglected the influence of salt on the absorber during seawater desalination,and focused on improving efficiency instead of the stability.During the desalination process,along with water evaporation,salt can be crystallized and deposited on solar absorbers,which will block the channels for vapor escape and eventually damage the long-term stability.Here we demonstrate a flexible Janus absorber by electrospinning which can enable stable and efficient solar desalination.With hydrophilic polyacrylonitrile(PAN)bottom layer for pumping water and hydrophobic polymethylmethacrylate(PMMA)upper layer for solar absorption,the water evaporation is confined in the hydrophilic PAN layer,preventing salts from accumulating on the surface or in the absorbing layer.Under the same illumination condition,the Janus absorber demonstrates stable water output(1.3kg m-2 h-1,over 16 days).In addition,as the PMMA absorbing layer is not in direct contact with water,which lowers the thermal conduction losses,72%solar steam efficiency is demonstrated under one sun.Our work provides an efficient,portable,and practical solar steam generator for stable solar desalination.