Flexible and Salt Resistant Janus Absorbers by Electrospinning for Stable and Efficient Solar Desalination

DOI: 10.1002/aenm.201702884 during desalination process and block the channels for vapor escape, resulting in the reduction of energy transfer efficiency, pure water yield, and unstable performance. Therefore, long-term stability becomes critical issues that need to be addressed. Recently, Janus membrane is emerging as a novel class of materials comprised of a two-layer structure with opposing properties and different functions.[26,27] Since the first study of Janus particle by Cho and Lee in 1985,[28] various Janus such as micelles,[29] rods,[30] and sheets[31] have been fabricated, with wide applications in oil/water separation,[32] switchable ion transport,[33] interfacial mass transfer,[34] and fog collection.[35] Here we demonstrate that a flexible Janus absorber fabricated by convenient electrospinning process can enable stable and efficient solar desalination. Taking advantage of the unique structures of Janus absorbers, two functions of solar steam generation, solar absorption and water pumping, are decoupled into different layers, with upper hydrophobic carbon black nanoparticles (CB) coating polymethylmethacrylate (PMMA) layer for light absorption and water evaporation, and bottom hydrophilic polyacrylonitrile (PAN) layer for pumping water. Therefore, salt may only be deposited in the hydrophilic PAN layer and quickly be dissolved because of continuous water pumping. Under 1-sun illumination, the Janus absorber demonstrates efficient solar steam generation (72%) and stable water output (1.3 kg m−2 h−1, over 16 d, with 45 min each day), not achieved in most of previous absorbers. With unique structure design achieved by scalable process, our flexible Janus absorber provides an efficient, stable, and portable solar steam generator for direct solar desalination. Figure 1 presents the illustration of solar steam generation (Figure 1a), and structures of Janus absorber (Figure 1b) in sea water. Porous absorbers naturally float on the water surface, absorbing solar energy to generate steam, without heating the bulk water. CB coating PMMA (CB/PMMA) layer stays above the water surface due to its hydrophobic property while PAN layer is immersed in water for efficient water supply. During the solar steam generation process, the CB/PMMA layer harvests solar energy and converts light to heat, generating vapor from the interfacial region of CB/PMMA and PAN. Thus, the With recent progress in interfacial solar steam generation, direct solar desalination is considered a promising technology for providing a clean water solution through a cost effective and environmental-friendly pathway. As a high and stable water production rate is the key to enable widespread applications, salt deposition becomes a critical issue that needs to be addressed. Herein, the authors demonstrate that a flexible Janus absorber fabricated by sequential electrospinning can enable stable and efficient solar desalination. Taking advantage of the unique structure of Janus, two functions of steam generation, solar absorption and water pumping, are decoupled into different layers, with an upper hydrophobic carbon black nanoparticles (CB) coating poly methylmethacrylate (PMMA) layer for light absorption, and a lower hydrophilic polyacrylonitrile (PAN) layer for pumping water. Therefore, salt can only be deposited in the hydrophilic PAN layer and quickly be dissolved because of continuous water pumping. Janus absorber demonstrates high efficiency (72%) and stable water output (1.3 kg m–2 h–1, over 16 days) under 1-sun, not achieved in most previous absorbers. With a unique structure design achieved by scalable process, this flexible Janus absorber provides an efficient, stable and portable solar steam generator for direct solar desalination.