Rapid Water Harvesting and Nonthermal Drying in Humid Air by N-Doped Graphene Micropads

We demonstrate a novel nanotextured graphene micropad that can rapidly harvest water from air to generate microscale water droplets with the desired size in designated positions on demand by simply applying a negative electric bias of -1.5 to -15 V. More interestingly, the water droplets can be reversibly dried nonthermally with the pad at ambient temperature in humid air (∼85% RH) by applying a positive electric bias of +1.5 to +15 V. The harvesting and drying rates on the glass are 2.7 and 1.5 μm³/s under biases of -15 and +15 V, respectively, but no apparent harvesting or drying activities are observed without the bias. The energy consumption is minimal as there is no Joule current due to the insulative substrate. It is shown that substrate wettability and ions play an important role in enabling the fast water harvesting and nonthermal drying. Molecular modeling is developed to understand the harvesting and drying mechanisms at the atomic scale. The water harvesting/drying approach may be useful for many technological applications such as micro/nanolithography, 3D printing, MEMS, and biochemical and microfluid devices.