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New hybrid material rejects ions from seawater


‘Water, water, everywhere, nor any drop to drink.’ These famous lines, written by poet Samuel Taylor Coleridge in 1797, stillperfectly summarise the salty, undrinkable nature of seawater.

But with the world’s freshwater stocks under greater stress than ever, more and more regions feel they have no choice but look to the sea to meet their water needs. And while large-scale desalination plants are being built across the planet, many communities and individuals rely on much smaller devices to remove the salt from seawater –portable solar evaporators powered solely by sunlight. Writing in in Carbon [DOI: 10.1016/j.carbon.2021.09.066],a group of researchers from China’s Hubei University say that they have designed a passive evaporator that removes ions from seawater, while also acting as a thermal storage material.

To build an effective solar water evaporator, you need to find a material that displays excellent broadband light absorption and efficient sunlight-to-heat conversion. The shape and structure of your device should minimise heat loss, while allowing the generated water vapour to escape and reach the collector. The Hubei team chose to focus their efforts on MXene – a relatively young class of 2D materials known to have excellent optical properties. After synthesising their laminar MXene film, they put it through a hydrothermal process in order to grow nanosheets of another material – Molybdenum disulphide (MoS2) – onto and between each layer of the MXene so that it was uniformly covered. MoS2 conferred the film with improved photothermal properties, and because it added surface roughness to the MXene, it also reduced the amount of light loss through reflection. The researchers say that the resulting MoS2-MXene film absorbed of more than 90 % of the solar spectrum, and that within 60 seconds of illumination under 1 sun (100 mW/cm2), the film reached a temperature of 65 °C. Now confident that the material was effective at transforming light into heat, they began to examine its evaporation performance.

In their first evaporator test, the team placed a flat MoS2-MXene film onto the top of folded piece of lint-free paper that was partly submerged within a brine bath. Within 30 mins, its weight had increased by 0.75 kg/m2 – it sped up water’s natural evaporation rate by 9.3 times. To demonstrate that the film could also be packed away compactly, the researchers rolled samples of the film up, to form cylinders measuring 1, 2, and 3 cm in height. These were again placed onto the paper structure to test their performance. This time, the evaporation rate was 2.5 kg/m2/h (for the 2 cm-tall cylinder); 1.67 times higher than the flat evaporator.

For their final test, they combined a MoS2-MXenecylinder with a paraffin (PF)-coated MoS2-MXene film, in a structure that resembled a top hat. This achieved an evaporation rate of 3.12 kg/m2/h. The hydrophobic MoS2-MXene-PF layer had an additional benefit – it acted as a collector for the salt crystals, separating them from the water, and it absorbed and stored heat, extending the evaporator’s ‘active’ time. The results, they say “showed an excellent desalination performance with 99.5% ions reject [sic], which was in full compliance with WHO drinking water standards.”


This story was written for Materials Today and was published in late October 2021:

Original research paper: Zhenzhen Guo, Wei Zhou, NailaArshad, Zexian Zhang, Di Yan, Muhammad Sultan Irshad, Li Yu, Xianbao Wang. “Excellentenergy capture of hierarchical MoS2 nanosheets coupled with MXene for efficientsolar evaporators and thermal packs,” Carbon,186 (2022) 19-27. DOI: 10.1016/j.carbon.2021.09.066