New device can make hydrogen when dunked in salt water
New device can make hydrogen when dunked in salt water

          
          
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.
Real-world use
All of this sounds great in principle, but does it actually work? To find out, the team assembled a device and put it to use on Shenzhen Bay seawater (an inlet north of Hong Kong and Macau). And, by nearly every reasonable performance measure, it worked well.
It maintained performance even after 3,200 hours of use, and electron microscopy of the membrane after use indicated that the pores remained unblocked at this point. The KOH used for the system wasn't completely pure, so it contained low levels of the ions found in seawater. But those levels didn't increase over time, confirming that the system kept the seawater out of the electrolysis chamber. Power-wise, the system used about as much energy as a standard electrolyzer, confirming that the water purification wasn't exacting any energetic cost.

          
                  