Multiscale Biomimetic Evaporators Based on Liquid Metal/Polyacrylonitrile Composite Fibers for Highly Efficient Solar Steam Generation

Abstract

Solar steam generation (SSG) offers a cost-effective solution for producing clean water by utilizing solar energy. However, integrating effective thermal management and water transportation to develop high-efficiency solar evaporators remains a significant challenge. Here, inspired by the hierarchical structure of the stem of bird of paradise, a three-dimensional multiscale liquid metal/polyacrylonitrile (LM/PAN) evaporator is fabricated by assembling LM/PAN fibers. The strong localized surface plasmon resonance of LM particles and porous structure of LM/PAN fibers with interconnected channels lead to efficient light absorption up to 90.9%. Consequently, the multiscale biomimetic LM/PAN evaporator achieves an outstanding water evaporation rate of 2.66 kg m−2 h−1 with a solar energy efficiency of 96.5% under one sun irradiation and an exceptional water rate of 2.58 kg m−2 h−1 in brine. Additionally, the LM/PAN evaporator demonstrates a superior purification performance for seawater, with the concentration of Na+, Mg2+, K+ and Ca2+ in real seawater dramatically decreased by three orders to less than 7 mg L−1 after desalination under light irradiation. The multiscale LM/PAN evaporator with hierarchical structure regulates the water transportation as well as thermal management for highly effective solar-driven evaporation, providing valuable insight into the structural design principles for advanced SSG systems.

1. Introduction

With the boosting population growth and severe environmental pollution, global freshwater scarcity becomes an essential issue confronting more than three billion people [1–3]. Consequently, an urgent demand arises for the development of facile and effective water purification and desalination technologies [4–6]. To date, a variety of technologies, including thermal distillation, electrodialysis and reverse osmosis, are used for water purification and desalination [7]. However, most of these technologies rely on non-renewable fossil fuels, leading to elevated emissions [8]. In contrast, solar steam generation (SSG), which utilizes eco-friendly solar energy to generate potable water, has garnered considerable attention [9–12].

An ideal SSG system features strong light absorption, effective thermal management, as well as prompted water transportation [13, 14]. The light absorption is greatly determined by the properties and microstructure of photothermal material [15]. Thus, various photothermal materials have been developed, including plasmonic particles [16–18], semiconductors [19, 20], carbonaceous materials [21–24] and polymers [25–27]. Eutectic gallium/indium, a near-room-temperature liquid metal (LM), exhibits outstanding plasmonic effect similar to gold [28–30] and silver particles [31], enabling LM particles as potential photothermal materials utilized in SSG system [32–34]. Meanwhile, LM particles demonstrate significant potential for high-efficiency thermal management due to the high latent heat, heat flux density as well as lower density compared to traditional metals [35–37]. Compared with carbon nanomaterials such as carbon nanotube and graphene, which suffer from significant heat loss during conduction due to their high thermal conductivity [38, 39], LM particles process a relatively low thermal conductivity (26.6 W m−1 K−1) [40]. More importantly, LM particle triggers a better isolation when integrated into substrate, leading to the reduced heat loss [41].

Additionally, LM particles exhibit superior hydrophilicity compared to gold and silver particles due to the easy formation of a thin oxide film on their surface [42, 43]. There are three kinds of water clusters, including free water (FW), intermediate water (IW) and bound water (BW), according to the different interactions between water molecules [44]. Due to weaker interaction with adjacent water molecules, the evaporation rate of IW is significantly higher than that of FW [45, 46]. By incorporating LM particles with various hydrophilic materials, such as polyvinyl alcohol [47], cellulose [48] and chitosan [49], the interactions between water and evaporator were enhanced, leading to the increase of IW and the decrease in the evaporation enthalpy [50–53]. Consequently, it can be inferred that LM particle can achieve effective photothermal conversion, low heat loss and suitable water supply ability, which is beneficial to enhance the SSG performance [54]. However, the fluidic nature of LM leads to the weak compatibility with polymers, posing challenges for preparing solid LM–polymer composites.