Hybrid nanofiber hydrogels for highly efficient solar desalination

Abstract

Interfacial solar vaporization has emerged as an energy-efficient and carbon-neutral approach to global freshwater scarcity. However, the practical application of existing interfacial solar vapor generators (ISVGs) remains constrained by their poor stability in concentrated brine. This is attributed to the intrinsic kinetic mismatch between mass transport and evaporation in ISVGs, and the absence of rational structural design principles to regulate evaporation efficiency. An efficient structural design in nature is the porous network formed by nanofibers, which gives biological tissues like cartilage and plant tissues exceptional strength, permeability, and multifunctionality. Inspired by this, we have developed a series of self-assembled aramid nanofiber (ANF)-based hydrogels with tunable components and microstructures for designing highly salt-tolerant ISVGs. We first developed a hybrid nanofibrous network of ANF, polyvinyl alcohol (PVA), and polypyrrole (PPy). ANF provides a rigid framework, while PVA enhances load transfer between nanofibers. The templating effect of ANF network enables polymerization of PPy along nanofibers, forming conductive pathways at an ultralow percolation threshold. This allows hydrogels with high conductivity without compromising porosity or mechanical properties. Besides, the incorporation of PPy as a high-performance photothermal converter, combined with the precisely tunable composition and microstructure, this hybrid nanofiber network also provides a versatile material platform for the engineering of hydrogel-based ISVGs. Next, composite hydrogels with tunable nanofiber networks were engineered for ISVGS with high evaporation rates and salt resistance. This nanofiber-based hydrogel was used to investigate the correlation between compositional variables, microstructural characteristics, and key performance parameters including water transfer, thermal conductivity, and evaporation enthalpy. Theoretical modelling reveals the relationship between microstructure and evaporation performance. The optimized evaporators achieve a stable evaporation rate of 2.85 kg m-2 h-1 in 20% brine. Conventional solar evaporators are static, which cannot match the dynamic solar trajectory in the real application, leading to loss of solar energy. A kirigami-engineered composite hydrogel membrane was developed for 3D solar-tracking evaporator arrays. The hybrid nanofiber network ensures excellent structural robustness, enabling dynamic 3D structures. Periodic triangular cuts allow the formation of conical arrays via uniaxial stretching, adjusting surface tilt angles to track solar trajectories. This design achieves an ~80% higher evaporation rate than static devices and sustains a stable rate of 3.4 kg m-2 h-1 in saturated brine by promoting localized salt crystallization. In summary, this hybrid fiber network exhibits unique advantages in development of ISVGs. The mechanistic insights into its microstructure, thermodynamic properties, and kinetic processes, along with their impacts on evaporation performance, provide insights for designing other ISVGs. Furthermore, our proposed sun-tracking evaporator significantly enhances solar energy utilization efficiency, representing a crucial step toward practical applications.