Adjusting Channel Size within PVA-Based Hydrogels via Ice Templating for Enhanced Solar Steam Generation

Solar steam generation has attracted interest in recent years as a clean source for power generation and water treatment. To achieve a high rate of steam generation, it is important for the solar evaporator to not only have high photothermal conversion efficiency but also have a large effective surface area for water evaporation. Herein, we report the use of PVA-based composite hydrogels as the solar evaporator, which inner channel sizes were adjusted through an ice-templating method. Although PVA hydrogels have been actively pursued as a promising material for solar steam generation, the influence of inner channel sizes to material performance has not yet been well investigated. Up to this point, the inner channel sizes of most reported PVA-based solar evaporators have been in the range of several to a few tens of micrometers. In this study, we have narrowed the channel size down to submicrometer range by suppressing growth of ice crystals while freezing the hydrogels. Strong capillary force induced by the narrow channels facilitated transport of water over long distances. Therefore, even when the hydrogel was molded into a pillar structure with length of several centimeters, water could be rapidly transported from one end to another, allowing effective evaporation at much increased heights compared to hydrogels with larger channel sizes. The increase in surface area for water evaporation led to a steam generation rate of 6.35 kg m–2 h–1 per water surface area covered by the evaporator under 1 sun illumination (100 mW cm–2), which is significantly higher than previously reported values.