Scientists have found a clever way to generate hydrogen straight from salty seawater. This could be another step towards a clean energy future, if renewables power the process.
The new device makes a few chemical modifications to existing technologies, making it possible to extract hydrogen from untreated, unpurified seawater – which could alleviate concerns about using precious water supplies.
"We have split natural seawater into oxygen and hydrogen… to produce green hydrogen by electrolysis, using a non-precious and cheap catalyst in a commercial electrolyzer," explains chemical engineer Shizhang Qiao of the University of Adelaide in Australia.
Traditionally, hydrogen fuel has been made using natural gas, but it can also be made through electrolysis.
Electrolysis is a water-splitting reaction that uses electricity to bump hydrogen atoms out of elbow-shaped water molecules, and an electrolyzer is a device in which that happens.
Right now, this process can be achieved using electricity from fossil fuels or from renewable energy sources, but both systems require fresh water. Finding a way to achieve electrolysis with seawater could make the future of green hydrogen fuel production far more sustainable.
Researchers have been trying to develop an alternative to commercial electrolyzers, which only work with purified fresh water, out of concern for water shortages.
Accessible freshwater makes up just 1 percent of Earth's total water, but there is a virtually limitless supply of seawater that could be used.
While concerns about water scarcity are valid, recent estimates suggest that the amount of water needed to sustain future hydrogen use is far, far less than the trillions of liters of water used to extract and burn fossil fuels today.
Scientists are still mindful of the environmental impacts, though. For decades they have been trying to develop devices to make hydrogen from seawater but kept running into several hurdles.
When popped into an electrolyzer, unwanted chlorine ions in seawater erode the catalyst materials meant to drive the hydrogen-yielding, water-splitting reaction. Massive insoluble precipitates also form, blocking reaction sites and hindering large-scale production.
The new system developed by Qiao and colleagues avoids both these problems.
As described in their new paper, the researchers layered a hard Lewis acid substance over a series of common cobalt oxide catalysts to split water molecules. In a series of tests, the modified catalysts resisted chlorine attack and prevented any precipitates from forming.
"This is a general strategy that can be applied to different catalysts without the need for specifically engineered catalysts and electrolyzer design," write the researchers in their published paper.
While it sounds promising, the decades-long effort to develop seawater electrolyzers should serve as a reminder of the challenges ahead in commercializing this or any other technology.
"Direct seawater electrolysis without the purification process and chemical additives is highly attractive and has been investigated for about 40 years, but the key challenges of this technology remain in both catalyst engineering and device design," the researchers note.
For instance, scientists from China and Australia recently developed a prototype device designed to float on the ocean surface and split hydrogen from seawater using solar energy. Another prototype in the works takes a totally different approach, harvesting water from humid air before extracting hydrogen.
Of course, prototypes are a far cry from industrial-scale methods, so it's good to have a healthy mix of potential systems in the pipeline to see which ones deliver.
Qiao and colleagues are working on scaling up their system by using a larger electrolyzer. But many factors can make or break a potential technology.
Commercializing any process boils down to the cost of materials, energy inputs, and efficiency at scale – where small gains can make a big difference in how much hydrogen is produced.
Cobalt, the material used in the metal oxide catalysts, is also not without its problems. Like any precious metal used in batteries or solar panels, it must be mined sustainably and recycled where possible.
Having tested the durability of their set-up, Qiao and colleagues think their modified catalysts can go the distance. Their system can deliver similar outputs to a commercial electrolyzer under the same low temperatures and operational conditions.
But with other researchers making strides to steadily improve the efficiencies of conventional electrolyzers, it really is anyone's game.
The research has been published in Nature Energy.