Coffee grounds-based hydrogel as a high-performance and durable evaporator for solar-driven freshwater generation

Abstract

Solar-driven desalination has emerged as a promising technology to mitigate the water crisis. However, challenges in developing cost-effective, eco-friendly, and durable solar evaporators from readily available materials still exist. Herein, a high-performance evaporator was developed by directly employing natural coffee grounds (CGs), a common biomass waste with huge annual discharge. The original CG particles adhere firmly to calcium alginate (CA) skeleton through a one-step freeze-drying and ionic crosslinking, thus showing outstanding mechanical properties and stability of the CG-CA composite. The formed three-dimensional honeycomb framework as well as the strong intervention of water jointly reduce the reflectance of incident photons, this enables CG-CA to display broad sunlight absorption in the wet state. As for the employed CG, no special treatment or functional processing is required. Combined with rational control of the morphology, the enlarged side surface of hydrogel acts as a cold evaporation surface to acquire energy from the surrounding, delivering an improved seawater evaporation rate of 1.59 kg m−2 h−1 with a clean water production rate of 0.62 kg m−2 h−1 under one sun illumination. The exploited solar evaporator derived from biomass waste CG suggests a practical, sustainable, and eco-friendly approach for efficient solar desalination, also for effective waste recycling.