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posted by hubie on Monday December 05, @11:25AM   Printer-friendly

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).

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  • (Score: 2) by bradley13 on Monday December 05, @01:01PM (3 children)

    by bradley13 (3053) Subscriber Badge on Monday December 05, @01:01PM (#1281245) Homepage Journal

    I'm not a chemist, but note: It removes all ocean salts, by causing water to travel across a membrane into an even saltier solution. This has zilch, zippo, nada to do with the electrolysis device - it would work anytime and anywhere. All you have to do is keep the salt concentration on the receiving side high, for example, through solar-driven evaporation.

    Now: in this particular case, the "saltier" solution contains only a single salt, Potassium Hydroxide (KOH). Maybe that's somehow more easily removed than the original salts? Any chemists know? If not, this desalination claim is bogus.

    The actual point of the original article ought to be: the KOH is less corrosive than sea water to the electrodes used for electrolysis. So the osmosis is useful for the electrolysis. Otherwise, nothing new here, publish-or-perish, move along...

    Everyone is somebody else's weirdo.
    • (Score: 2) by Thexalon on Monday December 05, @01:20PM

      by Thexalon (636) Subscriber Badge on Monday December 05, @01:20PM (#1281248)

      Otherwise, nothing new here, publish-or-perish, move along...

      It's also that those who profit from not investing in renewable power sources can point to this sort of thing and say "If you just wait a few more years, you'll have a magic technology that will fix all your climate problems forever without needing to spend any money on it, so you shouldn't spend any money on the technology we already have that might fix it".

      The only thing that stops a bad guy with a compiler is a good guy with a compiler.
    • (Score: 4, Interesting) by Immerman on Monday December 05, @04:22PM

      by Immerman (3985) on Monday December 05, @04:22PM (#1281279)

      To be clear, it removes all ocean salts, by causing water to travel across a membrane into an even saltier solution that is beneficial to the electrolysis process

      They're not talking about desalinating drinking water - and I suspect removing KOH is no more energetically favorable than removing the ocean salts, or else the initial ocean-salt-desalination would likely not have been energetically favorable in the first place. Thermodynamics tends to be a real stickler about not allowing energy-positive shortcuts. The closest you get is usually analogs to catalysts, which reduce the intermediate energy barrier between A and B, and thus the energy lost as waste heat, without actually changing the energy difference between the end points.

      This is strictly a method to remove the salts that interfere with electrolysis - which is a much bigger problem that just corrosion, since the salts themselves can be electrically decomposed into often very nasty substances that can contaminate the rest of the process. Salts are only held together with ionic bonds after all, far weaker than the covalent bonds holding together water molecules, in fact the ions are already separated when they dissolve in water.

      Assuming they are correct, they manage to remove all those problematic salts without *any* energy cost or membrane fouling. It's hard to understate how huge of a step forward that is for electrolysis options.

      If you want pure drinking water, then capture the exhaust when you use the hydrogen. Electrolysis is a horribly inefficient way to desalinate drinking water, but if your primary goal is energy storage in hydrogen, then generating pure water from seawater is a heck of a free bonus.

    • (Score: 2) by Rich on Monday December 05, @07:34PM

      by Rich (945) on Monday December 05, @07:34PM (#1281309) Journal

      I also only have very cursory knowledge of the subject. I guess the goal was to use salt water as basis for electrolysis, because that's much easier available than clean water, particularly in hot areas of the world where most of the sun shines to provide PV power. But if you naively electrolyze salt water, apart from hydrogen, you end up with sodium hydroxide solution - and chlorine gas. Especially the latter isn't really desirable, but you can dump it into Ethen from the refinery, get PVC, and even sell that. This kind of keeps a balance now, as a byproduct of NaOH production, but with manufacturing moving to halogen free materials, and for a full hydrogen economy, it's probably not really sustainable.

      So it looks they have looked for and found some practical way for salt water electrolysis into hydrogen and oxygen, and found a possible side process that could increase lithium concentration as a "free" byproduct, which looks like a path to explore given the current state of battery lithium mining and refining.

  • (Score: 2) by looorg on Monday December 05, @09:45PM

    by looorg (578) on Monday December 05, @09:45PM (#1281327)

    Does it scale to ocean level? Like build a really big one and stick it in the Atlantic or I guess Israel and Jordan could push one into the Dead sea. That ought to be salty enough.