One of the problems with renewable produced hydrogen is that it uses fresh water – and with a quarter of the world’s population already facing severe water scarcity at least one month a year, the Fresh water is an increasingly limited and precious resource. Thus, technologies capable of electrolysing hydrogen from the abundant seawater that covers most of the planet is a vital area of research.
You can desalinate seawater and then divide it, but that’s not a good solution; most of your input energy is lost in the desalination process, driving up the price of the hydrogen you make. There are also many machines for direct seawater electrolysis, but most die too quickly to be useful in a commercial sense; the chloride ions in the complex churning of the ocean turn into highly corrosive chlorine gas at the anode, and it eats away at the electrodes and degrades the catalysts until the machine stops working.
Researchers at Nanjing Technological University in China believe they have found a way around 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 breaking down. They say it’s efficient, scalable and works much like a freshwater separator “without a noticeable increase in operating costs”.
The team’s electrolyser keeps seawater completely separate from the concentrated potassium hydroxide electrolyte and electrodes using inexpensive, waterproof, breathable and anti-biofouling PTFE-based membranes. These membranes prevent liquid water from passing through, but they allow water vapor to pass through. The difference in water vapor pressure between the seawater side and the electrolyte side “provides a driving force for the spontaneous gasification (evaporation) of seawater on the seawater side”.
So you get pure water which quickly evaporates out of seawater without any additional energy input, then passes through the PTFE membrane and is absorbed into the electrolyte in liquid form. According to the Nanjing team, it lets water through and blocks 100% of other ions that could damage the electrodes or the membrane.
The team tested a compact 11-cell electrolyzer box, the size of two medium-sized suitcases, in the seawater of Shenzhen Bay. It generated some 386 liters of hydrogen gas per hour throughout the 133-day test, which sounds like a lot, but if it’s at standard atmospheric pressure, 386 liters is just 31.652 grams of hydrogen. Putting this in the context of a fuel cell electric vehicle and assuming a car travels about 100 km (62 miles) on 1 kg of hydrogen, this 11-cell device generates enough hydrogen per hour to drive a car for approximately 3.2 km (2 miles). Still, this is only a small test unit.
In terms of efficiency, the electrolyser consumed about 5 kWh for each normal cubic meter (Nm3) of hydrogen produced. Since hydrogen carries approximately 3.544 kWh of energy per Nm3, this saltwater chlorinator operates at approximately 71% efficiency. It’s certainly in the ballpark of many current electrolyser technologies, although they don’t follow some emerging hyper-efficient designs, such as Hysata’s 95% efficient capillary feed design.
Importantly, the device was still operating at full capacity after four and a half months in seawater, and post-test analysis showed “no obvious 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. Researchers say there are many avenues to explore to improve performance now that the basic principle of drawing fresh water from seawater has been proven.
Moreover, it could also be developed into a lithium collection machine. Readers with better memories than mine may recall a story we published in 2020, in which a team from King Abdullah University of Science and Technology (KAUST) in Saudi Arabia developed and tested a seawater electrolysis device that also sucked lithium phosphate out of seawater using special ceramic membranes.
It’s a completely different system, but the Nanjing team did some tests to see how their evaporation process affected the lithium concentration in seawater. They found a significant 42-fold increase after a few hundred of hours, and they were able to precipitate crystals of lithium carbonate, suggesting that with further development these machines may be able to generate revenue from both hydrogen and battery metals – this could be a huge boost in terms of commercial adoption and scaling.
Very neat stuff. The research is published in the journal Nature.
Source: Nature via IEEE Spectrum