High-Efficiency Photothermal Water Evaporation under Low-Intensity Sunlight Using Wood Biochar Monolith

Utilizing energy directly from the sun, solar water evaporation drives the global hydrological cycle and produces freshwater from saline water in the oceans and on land. As water is a poor solar absorber, a photothermal material is needed to facilitate the conversion of photons to thermal energy and increase the efficiency of solar desalination. However, the current photothermal materials are less efficient and expensive to be manufactured. Inspired by nature, we created a new photothermal material called a wood biochar monolith (WBM) by carbonizing wood using the pyrolysis process at 1000 °C and subsequently steaming at high pressure. Under low light intensity (193 W/m²), the light to vapor efficiency of maple WBM is more than 100%. The outstanding performance of WBM is attributed to (1) the facilitated water transport in the hierarchical, open-pore network preserved from the wood precursor in WBM and (2) the reduced evaporation enthalpy of confined water in WBM and the high broadband sunlight absorptivity of WBM. Moreover, the high evaporation rate causes the temperature of WBM to be lower than that of the surrounding water, enabling thermal energy harvesting by WBM from water and making a light-to-vapor efficiency of >100% feasible. This discovery offers opportunities for developing low-cost, high-performance water desalination or humidification devices deployable in remote areas with nonconcentrated natural sunlight.