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This is a story that was in the queue before the database crashed.

Device structure does energy-free desalination before water is split:

With renewable energy becoming cheaper, there's a growing impetus to find ways to store it economically. Batteries can handle short-term fluxes in production but may not be able to handle longer-term shortfalls or seasonal changes in power output. Hydrogen is one of several options being considered that has the potential to serve as a longer-term bridge between periods of high renewable productivity.

But hydrogen comes with its own issues. Obtaining it by splitting water is pretty inefficient, energy-wise, and storing it for long periods can be challenging. Most hydrogen-producing catalysts also work best with pure water—not necessarily an item that's easy to obtain as climate change is boosting the intensity of droughts.

A group of researchers based in China has now developed a device that can output hydrogen when starting with seawater—in fact, the device needs to be sitting in seawater to work. The key concept for getting it to work will be familiar to anyone who understands how most waterproof clothing works.

[...] On the outside, there's seawater, with its standard collection of salts. On the inside, there's a concentrated solution of a single salt—potassium hydroxide (KOH) in this case—that's compatible with the hydrogen-producing electrolysis process. Submerged in the KOH solution is a set of electrodes that produce hydrogen and oxygen on either side of a separator, keeping the gas streams pure.

So what happens once the hardware starts operating? As the water inside the device is split, producing hydrogen and oxygen, the reduced water levels increase the concentration of the KOH solution (which had started out much more concentrated than seawater). This makes it energetically favorable for water to move across the membrane from the seawater to dilute the KOH. And, because of the pores, that's possible, but only if the water moves in vapor form.

As a result, the water briefly exists in the vapor stage while inside the membrane and then quickly returns to liquid once it's inside the device. All the complex mixture of salts in the seawater is left behind outside the membrane, and a constant supply of fresh water is provided to the electrodes that split it. Critically, all of this takes place without the energy use normally involved in desalination, making the overall process more energy-efficient than cleaning up water for use in a standard electrolyzer.

[...] Finally, the team suggested that this might be useful for things in addition to hydrogen production. Instead of seawater, they immersed one of the devices into a dilute lithium solution and found that 200 hours of operation increased the lithium concentrations by more than 40-fold due to water moving into the device. There are plenty of other contexts, like purifying contaminated water, where this sort of concentration ability could be useful.

Journal Reference:
Xie, H., Zhao, Z., Liu, T. et al. A membrane-based seawater electrolyser for hydrogen generation. Nature (2022). https://doi.org/10.1038/s41586-022-05379-5