Foaming photothermal inks for direct-ink writing: hierarchical design and enhanced solar-powered interfacial evaporation

Solar-powered interfacial evaporation stands out as a burgeoning technology with significant potential for applications in thermal distillation and desalination with minimal carbon footprints. State-of-the-art photothermal materials rely on aerogels, hydrogels and sponges/foams as their primary building blocks. Nevertheless, the inherent limitations of conventional three-dimensional (3D) structures and associated fabrication methodologies hinder the maximization of crucial performance-enhancing strategies. Despite notable progress, this technology still confronts enduring performance roadblocks. Herein, we present the development of monolithic, self-supporting and robust 3D matrices through the direct-ink writing technique employing “foaming” photothermal inks. The adoption of a prefoaming strategy enables the incorporation of a progressively wide concentration range of photothermal composites, enhancing the 3D printability of photothermal inks. For the first time, our work achieved direct-ink writing of porous photothermal materials, resulting in the prompt formation of intricate and hierarchical 3D matrices designed for efficient solar-powered interfacial evaporation and desalination. The rapid prototyping of multiscale hierarchical structures synergistically improves mass flows, reduces the energy demand for evaporation and expands actual evaporation areas, representing an accomplishment seldom realized in the majority of 3D photothermal monoliths using conventional fabrication methodologies. As a result, a high water evaporation rate of 2.94 kg m-2 h-1 is attained under 1 sun. Gratifyingly, our work highlights the principal benefits of employing direct-ink writing and foaming inks. This methodology enables the direct fabrication of hierarchically porous structures, boosting solar evaporation while concurrently minimizing material consumption. The remarkable cost-effectiveness of 1755.4 g h-1 $-1 surpasses the majority of 3D interfacial steam generators developed thus far. © The Royal Society of Chemistry 2024.