Salt water, a layer of molybdenum disulphide and a pore is all you need to produce electricity.
It is possible to generate energy using nothing but the difference between fresh and salt water. When fresh and salt water are separated by a membrane that blocks the passage of certain ions, there is a force that drives the fresh water into the salt water to equalize the salt concentration. That force can be harvested to produce energy, an approach called “osmotic force.”
But the generation of osmotic power is highly dependent on how fast ions can cross the membrane: the thicker (and more robust) the membrane, the slower the ions will flow. Theoretically, the most efficient osmotic energy generation would come from an atomically thin membrane layer. But can this theoretical system be achieved here in reality?
Recently, scientists answered that question using atomically thin membranes composed of molybdenum disulfide (MoS2). In the resulting paper, they describe a two-dimensional MoS2 membrane with a single nanopore, which was used to separate reservoirs containing two solutions with different salt concentrations to generate osmotic force.
Understanding osmotic flow
Not all ions can be transported through the nanopores of MoS2 membranes. Surface charges exist around the pore limit of ion diffusion, resulting in a selective ion transport that causes a measurable net osmotic flow. In fact, the magnitude of the osmotic flow is actually determined by the surface charges present at the nanopore.
Analysis of experimental data at pH 5 revealed a negative surface charge at the nanopore site. As the pore size increases, more negative charges accumulate on the surface. This should repel negatively charged ions from the pore, while allowing positively charged ions to pass through. The result is a net positive current through the membrane.
The researchers also found that the conductivity of the nanopore increases with increasing pH. They think this may be due to an increase in the accumulation of negative surface charges in the nanopores. Similarly, increasing the pH increases the generated voltage and current, underscoring the importance of the surface charge of nanopores for ion motions.
Effect of size and thickness
In addition to understanding the influence of the solution, the researchers were interested in understanding how nanopore size affected ion passage and resulting power generation. They found that the selectivity of the pore for specific ions decreases as the pore size increases. Here’s what you’d expect: As the charges traveling along the pore get farther apart, their influence on the center of the pore decreases. This also had an effect on power generation, as small pores show better stress behavior.
The thickness of the diaphragm is also a critical factor in determining the amount of power that can be generated. As mentioned above, theoretical predictions indicate that thinner membranes will result in the greatest power generation. To this end, the researchers performed simulations of multilayered MoS2 membranes, which showed that power generation decreases rapidly with the number of MoS2 layers increases. However, we still haven’t confirmed this with real-world experiments.
How much power can we actually generate with this type of system? The scientists made calculations to model a one atom thick MoS2 membrane of which 30 percent of the surface contains pores of 10 nm. The results suggest that with the right salt gradient you could get a power density of 106 W/m2. For comparison: the amount of energy we get from the sun is about 1000 W/m at most2.
That high power density suggests that MoS2 membranes have serious potential as a source of renewable energy. While it would be very difficult to scale up an atomically thin membrane, there is still the possibility of using it to harvest a little bit of energy for electronic devices.
Nature2016. DOI: 10.1038/nature18593 (About DOIs.)