One problem with renewable hydrogen is its use of fresh water — and with a quarter of the world’s population facing acute water shortages for at least one month each year, fresh water is a more limited and precious resource than ever. Therefore, technologies capable of electrolyzing hydrogen from the abundant seawater that blankets most of the planet are an important area of ​​inquiry.

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

Researchers at Nanjing Tech University 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 operated for more than 3,200 hours (133 days) without failure. It’s efficient, scalable and works like a fresh water dispenser, they say, “without significantly increasing operating costs”.

The team’s electrolyzer uses seawater as a fully concentrated potassium hydroxide electrolyte and electrodes with inexpensive, waterproof, breathable, anti-biofouling, PTFE-based membranes. These membranes stop liquid water from entering, but allow water to evaporate. The water vapor pressure difference between the seawater side and the electrolyte side “provides the driving force for spontaneous seawater gas (vapor) through the seawater.”

When water from the electrolyte splits into hydrogen and oxygen gases, the vapor pressure difference between the electrolyte and the seawater creates a vapor pressure difference, which causes the seawater to suddenly evaporate and pass through the waterproof layer.

Nanjing Tech University

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

The team tested an 11-cell electrolyzer box compacted into two medium-sized suitcases in the seawater of Shenzhen Bay. During the 133-day test, it produced 386 liters of hydrogen gas per hour, which sounds like a lot, but at normal atmospheric pressure, 386 liters represents only 31.652 grams of hydrogen. Putting that in the context of a fuel cell EV and assuming that a car can drive 100 km (62 miles) on 1 kilo of hydrogen, this 11-cell device generated enough hydrogen per hour to drive a car about 3.2 km (2 miles). Still, it’s a bit of a test piece.

In terms of efficiency, an electrolyzer consumes about 5 kW for every standard cubic meter (Nm)3) production of hydrogen. Since hydrogen carries about 3.544 kWh of energy in Nm3This seawater electrolyzer operates at 71% efficiency. That’s certainly in the ballpark of a lot of current electrolyzer technology, although it doesn’t quite keep up with some emerging high-efficiency designs like Hysata’s 95% efficient capillary-feed design.

Left: The 11-cell test rig took more than four months.  Right: the structure of each cell
Left: The 11-cell test rig took more than four months. Right: the structure of each cell

Nanjing Tech University

Importantly, the device was operating at full capacity after four and a half months in seawater, and a post-test analysis showed “no apparent increase in purity” in the electrolyte “suggesting 100% ion-blocking efficiency” due to the PTFE coating, and No corrosion was observed on the catalyst layers. The researchers said there are many avenues open to investigate performance improvements since the basic principle of extracting fresh water from seawater has been proven.

Moreover, it can also be developed into a lithium harvesting machine. Readers with better memories than I do. You may remember a story we published back in 2020, where a team from Saudi Arabia’s King Abdullah Science and Technology Group (KAUST) invented and tested seawater electrolysis to leach lithium phosphate from seawater. Using special ceramic coatings.

It’s a different system altogether, but the Nanjing team did a little experiment to see how their evaporation process affected the concentration of lithium in seawater. After a few hundred hours, they achieved a significant increase of 42 times and were able to generate some lithium carbonate crystals, which suggests that with further development these machines could generate income from hydrogen and battery metals – which is possible. Be a big boost in terms of business activity and scale.

Very beautiful things. The study was published in the journal Nature.

Source: Nature by IEEE Spectrum

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