Could a graphene aerogel get us a step closer to efficient, solar-powered wastewater treatment?
All over the world, solar photovoltaics are being used to transform sunlight into electricity. But, visible light isn’t the only energy source our sun offers. Solar thermal systems, widely used in cities such as Sao Paulo, make use of the sun’s heat. Dark panels capture infrared wavelengths of light, warming water for use in homes and businesses. Using highly-concentrated sunlight, it’s possible to heat liquids beyond their boiling point, producing steam that could then be used in power generation or chillers. Such a system could also be used to treat water in remote areas, but to date, most have relied on complex optical set-ups, or access to vacuum conditions.
A new paper, published in Carbon [DOI: 10.1016/j.carbon.2017.12.124], reports on a low-cost material that could be used to efficiently harvest solar energy for steam generation. The team started by oxidising graphite flakes to produce graphene oxide, which was then suspended in water. This was hydrothermally treated and freeze-dried to produce a porous graphene aerogel. The resulting material was hydrophobic (water-repellent), a property that had previously been shown to limit the efficiency of its water-to-steam conversion. So, it was treated with an oxygen plasma, producing a modified graphene aerogel (MGA) that was slightly hydrophilic.
After characterisation by SEM, a 10-mm tall cylinder of MGA was added to a quartz tube filled with 20 ml of potable water. It was found to float on the water-air interface, and when exposed to sunlight (1 kW m-2), the team observed the liquid water on the upper surface of MGA rapidly changing to steam. This localized heating effect was enabled by the broadband light absorbing qualities of the material – its optical reflectance was measured at just 4%. The thermal conductivity of MGA (as measured by IR camera) was determined to be 0.1456 W m-1 K-1 (at 20°C); four times lower than the water in which it was suspended.
The MGA’s low thermal conductivity limited the heating effect to the upper layers of water – in other words, the material acted as a solar concentrator – promoting the generation of steam. Capillary action also ensured that the MGA ‘sponge’ remained hydrated throughout the experiment. After 30 mins of irradiation, 0.158 g of water was found to have evaporated from the MGA. This compares with 0.037 g of water when the MGA was removed, translating to an increase in the evaporation efficiency of ~64%, compared to water alone. Though this research remains at a proof-of-concept stage, the authors conclude that “…MGA could be an excellent candidate for rapid solar steam generation.”
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Yang Fu, Gang Wang, Xin Ming, Xinghang Liu, Baofei Hou, Tao Mei, Jinhua Li, Jianying Wang, Xianbao Wan. “Oxygen plasma treated graphene aerogel as a solar absorber for rapid and efficient solar steam generation” Carbon 130 (2018) 250-256. DOI: 10.1016/j.carbon.2017.12.124