Water
electrolyzers are promising devices that use electricity to split water into
hydrogen and oxygen. If powered by solar cells or other renewable energy
technologies, they could contribute to the sustainable production of hydrogen
used to make ammonia, which in turn is used to produce fertilizers and other
valuable compounds.
Most water electrolyzers require
purified water, even though seawater is far more abundant on Earth. This is
because the salts in seawater can damage the devices and interfere with
electrolysis, the process by which water is split into hydrogen and oxygen.
Researchers at the Chinese Academy of
Sciences recently developed a system that could produce hydrogen from seawater
while also desalinating it to produce fresh water. A 20-kilowatt pilot system,
introduced in a paper published in Nature
Energy, operated stably for 100 days, reliably producing both hydrogen and
fresh water.
"Hydrogen production from seawater
is attractive given its abundance, but direct seawater electrolysis is
challenging due to the poor stability and low energy efficiency," wrote
Shang Jiang, Peixin Zhu and their colleagues in their paper. "We report
the co-production of hydrogen and fresh water from seawater by coupling alkaline water electrolysis with
low-temperature desalination, wherein low-grade waste heat from the electrolyzer
drives seawater distillation to generate fresh water for both electrolysis and
external use."
Turning seawater into hydrogen and fresh water
The system developed by Jiang, Zhu
and their colleagues uses heat released during hydrogen production to remove
salt from seawater. This process yields fresh water that can then be fed to an
electrolyzer to produce hydrogen or set aside for other uses.
The team initially built a
20-kilowatt industrial pilot system and monitored its performance for 100 days.
They then scaled the system up to 250 kilowatts and measured the hydrogen and
fresh water it produced.
"A 20-kW industrial pilot
system achieved co-productivities of 3.8 Nm3 h−1 H2 and 1.2 kg h−1 fresh water with stable operation for 100 days," wrote Jiang, Zhu and
their colleagues.
"Scaling up to 250 kW achieved
48 Nm3 h−1 H2 and 31.6 kg h−1 fresh water, with a 14.4%
improvement in system electrical efficiency relative to alkaline electrolysis
of fresh water alone. Techno-economic analysis of the coupled process indicates
higher profitability than the traditional tandem process of desalination
followed by electrolysis."
Scaling up production and running additional tests
This study introduced a promising
approach for producing hydrogen from seawater without feeding it directly to an
electrolyzer. The researchers' analyses also suggest that their two-process
approach could be more profitable than desalinating seawater before feeding it to a separate electrolyzer.
That advantage remains a
projection, as the team has not yet demonstrated the system's commercial
potential. In the future, their methods could be refined to improve stability
or increase the amount of hydrogen and fresh water produced.
The new system could be tested in other settings to further validate its performance and identify possible shortcomings. Eventually, it may contribute to commercially viable ways to produce hydrogen and fresh water from abundant seawater.
Source: Scaled-up seawater system produces both hydrogen and fresh water with higher efficiency
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