Water Harvesting from the Atmosphere in Arid Areas with Manganese Dioxide

Water harvesting from the atmosphere using adsorption-based technology holds great promise to solve water scarcity in arid regions. Birnessite (i.e., a layered structure MnO₂) can store water molecularly between its layers, providing a path for water adsorption. This study investigates the water sorption characteristic of birnessite from both thermodynamic and kinetic perspectives. The water vapor adsorption on birnessite follows a Type II sorption isotherm. Water molecules quickly adsorb to the interlayers at lower RH region values, while multilayer water–water interactions occur via hydrogen bonding at surfaces and result in condensed water at higher RH. Furthermore, birnessite features excellent solar absorptivity; the temperature can be raised by 87 °C under solar irradiation at a sun flux of ~900 W/m², providing energy to trigger partial desorption of interlayer water. According to the Do and Do model simulation, birnessite can harvest 0.07 kg of water per kilogram of sample (kg_H2O/kg_Sample) per cycle at RH of 23% when the dew point temperature is set to 11 °C. Finally, a device based on the concept of sorption-based atmospheric water harvesting is built to present this application. This inexpensive water adsorption material with solar absorptivity displays an applicable promise for solving water scarcity in arid regions.