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A group of École Polytechnique Fédérale de Lausanne (EPFL) researchers has demonstrated that light can be used to optimize the production of osmotic power, or blue energy, at river estuaries. The researchers created a system that reproduced the conditions that occur at estuaries, where electrically charged salt ions move from the salty seawater to the fresh river water. They shined light on their system, which combined water, salt, and a membrane just three atoms thick, and found that the system produced twice as much power under the effect of light as it did in the dark.
Reproducing the conditions that occur at estuaries, EPFL scientists shined light on a system combining water, salt, and a membrane just three atoms thick to generate more electricity. Courtesy of EPFL.
The EPFL system contained two liquid-filled compartments, at different salt concentrations, separated by a molybdenum disulfide (MoS2) membrane. In the middle of the membrane was a nanopore between 3 and 10 nm in diameter. Every time a salt ion passed through the nanopore from the high- to the low-salt concentration solution, an electron was transferred to an electrode, generating an electric current.
To improve the system’s potential to generate power, the researchers used low-intensity laser light. Light released embedded electrons and caused them to accumulate at the membrane’s surface, which increased the surface charge of the material. The light also caused the nanopore to allow more positively charged ions to pass through the membrane.
By using light to increase the surface charge of MoS2 membranes, the researchers found that they could double the osmotic power generated by a single nanopore at a neutral pH. The increased surface charge at the pore rim enhanced the ion selectivity and led to a larger osmotic voltage, while the increased surface charge of the membrane enhanced the surface conductance, leading to a larger osmotic current. The combination of these effects could efficiently boost energy generation using membranes containing arrays of nanopores of varying sizes.
Under the effect of light, the system produces twice as much power as it does in the dark. Courtesy of EPFL.
The researchers believe that a system of mirrors and lenses could be used to direct light onto membranes at river estuaries. Similar systems are used in solar collectors and concentrators, a technology already widely employed in photovoltaics, they said. “Essentially, the system could generate osmotic power day and night,” said researcher Michael Graf. “Output would double during daylight hours.”
Before the technology can be used for real-world applications, the ultrathin membrane would need to be mechanically stabilized. This could be done by using a silicon wafer containing a dense array of silicon nitride membranes, which are easy and cheap to manufacture, said the researchers.
The research was published in Joule (https://doi.org/10.1016/j.joule.2019.04.011).READ MORE