It's 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 freshwater into the salt water to even out the salt concentration. That force can be harvested to produce energy, an approach termed "osmotic power."

But the generation of osmotic power is highly dependent on how quickly ions can cross the membrane—the thicker (and more robust) the membrane, the slower the ions will flow. Theoretically, the most efficient osmotic power 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 (MoS 2 ). In the paper that resulted, they describe a two-dimensional MoS 2 membrane containing a single nanopore, which was used to separate reservoirs containing two solutions with different concentrations of salt in order to generate osmotic power.

Understanding osmotic current

Not all ions can be transported through the nanopores of MoS 2 membranes. Surface charges present around the pore limit ion diffusion, resulting in a selective ion transport that causes a measurable net osmotic current. In fact, the size of the osmotic current 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 site of the nanopore. As the pore size increases, more negative charges accumulate at the surface. This should repel negatively charged ions from the pore while allowing positively charged ions to cross. The result is a net positive current across the membrane.

The researchers also found that the conductance of the nanopore increases with increasing pH. They think this could be due to an increase in accumulation of negative surface charges in the nanopores. Similarly, increasing the pH increases the generated voltage and current, underlining the importance of the nanopore surface charge to ion movements.

Effect of size and thickness

In addition to understanding the influence of the solution, the researchers were interested in understanding how the size of the nanopore influenced ion passage and the resulting power generation. They found that the pore's selectivity for specific ions decreases as pore size increases. This is what you might expect—as the charges that line the pore get further apart, their influence over the middle of the pore goes down. This had an effect on energy generation, too, as small pores exhibit better voltage behavior.

The thickness of the membrane is also a critical factor in determining the amount of power that can be generated. As noted above, theoretical predictions indicate that thinner membranes will result in the largest power generation. For this, the researchers performed simulations of multilayer MoS 2 membranes, which revealed that the power generation rapidly decreases as the number of MoS 2 layers increases. We still haven't confirmed this with real-world experiments, though.

How much power can we actually generate from this type of system? The scientists did calculations modeling a single-atom-thick MoS 2 membrane with 30 percent of its surface containing 10nm pores. 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 maxes out at about 1,000 W/m2.

That high power density suggests that MoS 2 membranes have some serious potential as a source of renewable energy. While it would be very difficult to scale up an atomically thin membrane, there's still the possibility of using it to harvest a small bit of energy for electronic devices.

Nature, 2016. DOI: 10.1038/nature18593 (About DOIs.)