With renewables becoming cheaper, there is a growing push to find ways to store them economically. Batteries can handle short-term production flows, but may not be able to handle longer-term shortages or seasonal changes in power output. Hydrogen is one of many options being considered that has the potential to serve as a longer-term bridge between periods of high renewable productivity.
But hydrogen has its own problems. Obtaining it by dividing water is quite inefficient, energy-wise, and storing it for long periods of time can be difficult. Most hydrogen-producing catalysts also work best with pure water, an element that is not necessarily easy to obtain as climate change increases the intensity of droughts.
A group of China-based researchers have now developed a device that can produce hydrogen when started with seawater. In fact, the device must be sitting in seawater to work. The key concept to making it work will be familiar to anyone who understands how most waterproof garments work.
i can breathe
The waterproof and breathable garments are based on a membrane with carefully structured pores. The membrane is made of a material that repels water. It had pores, but they are too small to let liquid water through. But they are large enough for individual water molecules to pass through. As a result, any water on the outside of the garment stays there, but any sweat inside that evaporates will continue to pass through the fabric and out into the outside world. As a result, the fabric breathes.
A similar membrane is central to the function of the new device. It prevents liquid water from passing through the membrane but allows water vapor to pass. The big difference is that there is liquid water on both sides of the membrane.
Outside there is sea water, with its standard collection of salts. Inside there is a concentrated solution of a single salt, potassium hydroxide (KOH) in this case, compatible with the process of electrolysis producing hydrogen. Immersed 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.
What happens once the hardware starts working? 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 many more concentrated than seawater). This makes it energetically favorable to move water through the membrane from seawater to dilute the KOH. And, because of the pores, it is possible, but only if the water moves in the form of vapour.
As a result, water briefly exists as a vapor inside the membrane, then quickly becomes liquid once inside the device. All the complex mixture of salts in the seawater is left outside the membrane and a constant supply of fresh water is supplied to the electrodes separating it. Importantly, all of this takes place without the use of energy normally involved in desalination, making the overall process more energy efficient than cleaning water for use in a standard electrolyser.
Real-world use
This all sounds good in principle, but does it actually work? To find out, the team assembled a device and used it on seawater in Shenzhen Bay (an inlet north of Hong Kong and Macau). And, by almost any reasonable performance metric, it performed well.
It maintained its 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 was not completely pure, so it contained low levels of ions found in seawater. But these levels did not increase over time, confirming that the system prevented water of sea to enter the electrolysis chamber. In terms of power, the system used about as much as a standard chlorinator, confirming that water purification requires no energy cost.
The KOH solution was also self-balancing, with water diffusion through the device slowing if its internal solution became too dilute. If it becomes too concentrated, the efficiency of electrolysis drops, so water removal slows down.
The authors estimate that their device would withstand pressures down to about 75 meters of seawater. The temperature at these depths, however, could be limiting, as the rate of diffusion of water through the membrane was six times 30°C than at 0°C.
Even with all of this good news, there are options to improve performance. Various salts beyond KOH are fine, and some may work better. The researchers also found that incorporating KOH into a hydrogel around the electrodes increased hydrogen production. Finally, it is possible that changing the material or structure of the electrodes used in water separation could further improve matters.
Finally, the team suggested it could be useful for things besides hydrogen production. Instead of seawater, they submerged one of the devices in a dilute lithium solution and found that 200 hours of operation increased lithium concentrations more than 40 times due to the displacement of water in the device. There are many other contexts, such as the purification of contaminated water, where this kind of concentration ability could be useful.
This does not solve all the problems associated with using hydrogen as an energy storage medium. But it certainly has the potential to allow us to cross “need for pure water” off the list of these problems.
Nature2022. DOI: 10.1038/s41586-022-05379-5 (About DOIs).
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