The virtually infinite reservoirs offered by seas and oceans give hope for a solution to the scarcity of clean water — one of the most serious current challenges worldwide, projected to become even more critical as years go by. Such a wide issue can naturally be tackled from different angles, the better use of our current resources being a relevant example to which everyone can contribute. Simultaneously, researchers constantly look for more and more effective purification processes, such as desalination and the removal of pollutants, to increase the amount of available water.

Evaporation is a natural solution to detach water from unwanted contaminants effectively, and thermal processes have been widely exploited in the past for this aim. However, unsustainable energy consumption and, in turn, additional pollution usually emerge as unavoidable and unacceptable side effects — calling for alternative, more efficient approaches. Simultaneously, a versatile method capable of preserving high efficiencies in extreme environments as various as seawater, brackish groundwater and even industrial wastewater is intensively sought.

Credit: AMERICAN CHEMICAL SOCIETY

Now, Panpan Zhang et al. (ACS Nano http://doi.org/b66c; 2017) report on the use of graphene-based membranes to generate clean water by efficiently exploiting solar thermal energy. The researchers thermally treat an ethanol-mediated suspension of graphene oxide by exploiting freeze-casting, freeze-drying and thermal annealing processes. As a result, they obtain long-range ordered assemblies of vertically aligned graphene sheets bridged by twisted carbon fibres, with controlled overall thickness in the millimetre range and displaying advantageous mechanical properties. The structure of a typical sample is shown (scale bar 10 μm), visualized by means of scanning electron microscopy.

The researchers examine the thermal behaviour of the fabricated devices under different illumination conditions, confirming the exceptional absorbance of graphene oxide over a spectral region ranging from ultraviolet to infrared. When in contact with water, elevated rates of evaporation ∼1.2 kg m−2 h−1 are induced under one-sun illumination. This is possible in view of solar thermal conversion efficiencies ∼85% and of the high temperatures achieved in turn. However, another crucial property is the geometrical structure of the device, facilitating the escape of steam through the ∼30-μm-wide interlayer spaces. The relevance of this latter aspect is confirmed by the lower efficiencies reported for non-structured graphene-oxide-based films and foams.

The researchers test the device in seawater and in various acidic and basic solutions, demonstrating that the produced steam is neutral, effectively desalinated and purified from metal ions. Moreover, they discuss several ways to improve the device performance even further. In particular, they demonstrate that the characteristic evaporation rates can be enhanced to ∼1.6 kg m−2 h−1 after O2-plasma treatment, changing the character of the as-fabricated device from hydrophobic to hydrophilic and improving its contact with water, in turn. The energy transfer within the device can also be made more efficient by introducing a thermally insulating layer preventing dissipation in bulk water.