Architecting spent coffee grounds for highly efficient solar water evaporation

Solar-driven photothermal water evaporation offers a promising alternative for fresh water production, necessitating efficient and sustainable photothermal materials. Spent coffee grounds, an abundant biowaste, are a potential biomass-based evaporation material for this application, though limited by relatively low evaporation efficiency and the need for additional materials. Here, we present a hydrothermal-treated spent coffee grounds (DA-CGs) that contains carbon dots with strong photothermal conversion abilities, serving as an efficient solar evaporation material. Additionally, this material can also reduce water evaporation enthalpy by promoting intermediate water formation, while their hierarchical structures enhance sunlight trapping and expands the liquid-vapor interface. These features collectively contribute to their high efficiency in solar-driven water evaporation. The DA-CGs evaporator demonstrated a high evaporation rate of 2.45 kg m⁻² h⁻¹ for pure water under 1 sun and an outdoor daily water yield of 7.14 kg m⁻² when purifying seawater. These advantages highlight its potential for large-scale water purification and provide valuable insights into utilizing biowaste for efficient solar water evaporation.