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The problem with renewable hydrogen is that it uses fresh water, and with a quarter of the world’s population already facing severe water shortages for at least one month each year, fresh water is becoming an even more limited and expensive resource. So technologies that can electrolyze hydrogen from the abundant seawater that covers most of the planet are an important area of ​​research.

You can desalinate seawater and then split it, but that’s not a great solution; Most of your input energy is lost in the purification process, and this raises the price of the hydrogen you produce. There are also many direct seawater electrolysis machines, but most die too quickly to be commercially useful. The chloride ions in the ocean’s complex concoction turn into highly corrosive chlorine gas at the anode, which eats away at the electrodes and corrodes the catalysts until the machine stops working.

Researchers at Nanjing University of Technology in China believe they have found a way to solve this problem. in a study published in Nature Last month, the Nanjing team demonstrated a direct seawater electrolysis machine that ran for more than 3,200 hours (133 days) without failure. They say it’s efficient, scalable and performs like a fresh water separator “with no noticeable increase in operating cost.”

The team’s electrolyzer keeps seawater completely separate from the concentrated potassium hydroxide electrolyte and electrodes using inexpensive, waterproof, breathable, antibiotic, PTFE-based membranes. These membranes stop liquid water from penetrating, but they allow water vapor to pass through. The difference in water vapor pressure between the seawater and the electrolyte side “provides a driving force for the spontaneous gasification (evaporation) of seawater.

When water escapes from the electrolyte into hydrogen and oxygen gases, it creates a vapor pressure difference between the electrolyte and the seawater, causing the seawater to spontaneously evaporate and pass through the waterproof membrane.

Nanjing Technical University

So what you get is pure water that evaporates quickly from the seawater without any additional energy input, then passes through the PTFE membrane and is absorbed into the electrolyte as a liquid. According to the Nanjing team, it releases water and blocks 100% of other ions that could damage the electrodes or membrane.

The team tested the compact 11-cell electrolyzer box, about the size of several medium-sized suitcases, in seawater in Shenzhen Bay. It produced about 386 liters of hydrogen gas per hour during the 133-day test, which sounds like a lot, but at standard atmospheric pressure, 386 liters is only 31,652 grams of hydrogen. Putting it in the context of a fuel cell EV and assuming the car drives about 100 km (62 miles) on 1 kg of hydrogen, this 11 cell device produces enough hydrogen per hour to drive the car about 3.2 km (2 miles). Still, it’s just a small test unit.

In terms of efficiency, the electrolyser consumed about 5 kWh per normal cubic meter (Nm3:) produced hydrogen. Because hydrogen carries about 3,544 kWh of energy per Nm3:, this marine electrolyzer operates at about 71% efficiency. This is certainly within the realm of many electrolyzer technologies, although it falls short of some emerging hyper-efficient designs such as Hysata’s 95%-efficient capillary feed design.

Left.  The 11-cell test rig has been in operation for more than four months.  Right: Structure of each cell
Left. The 11-cell test rig has been in operation for more than four months. Right: Structure of each cell

Nanjing Technical University

Importantly, the device was still operating at full capacity after four-and-a-half months in seawater, and post-test analysis showed a “clear increase in impurity ions” in the electrolyte, “suggesting 100% ion blocking efficiency of the PTFE membrane.” , and there was no visible corrosion on the catalyst layers. The researchers say there are now many avenues they can explore to improve performance once the basic principle of extracting fresh water from seawater has been proven.

Furthermore, it can also be developed into a lithium harvesting machine. Readers with a better memory than mine may recall a story we published back in 2020 where a team at King Abdullah University of Science Ant Technology (KAUST) in Saudi Arabia developed and tested a seawater electrolyser that also absorbs lithium from seawater. phosphate. using special ceramic membranes.

It’s a completely different system, but the Nanjing team did a little experiment to see how their evaporation process affected the lithium concentration in seawater. They found a significant 42-fold increase after a few hundred hours, and they were able to deposit some lithium carbonate crystals, suggesting that with further development, these cars could generate revenue from both hydrogen and battery metals, which could. be a huge boost in terms of commercial uptake and scale.

Very neat stuff. The research is published in the journal Nature.

Source: Nature via IEEE Spectrum


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