Question: What happens when you put together salty water, silicate minerals and some electrical current?
Answer: If a new technique pans out, a potential solution to some of our most vexing energy problems.
A new study, published this week in the journal Proceedings of the National Academy of Sciences, outlines a way to produce hydrogen while also capturing carbon dioxide and producing a base that could be used to offset or neutralize ocean acidification. Hydrogen is an ideal fuel source since its only byproduct is water.
It goes like this: First, you apply electricity to salty water. This well-studied technique, called electrolysis, breaks water into oxygen and hydrogen gas, said Greg Rau, study co-author and researcher at Lawrence Livermore National Laboratory in Livermore, California.
While this is happening, acid is produced at the negative end of the electrode, called the anode. Usually, after the current stops being applied, the acid recombines with the base (hydroxide) produced at the electrode's positive end (cathode), turning back into water.
But Rau's team added another step and encased the anode in a silicate rock, one of the most abundant types of rock on Earth. The acid reacted with the basic rock, creating salts and more water.
By using up the acid produced during electrolysis, the water then became quite basic, filling up with hydroxide ions, Rau said. Ordinary air was then bubbled through the solution. The carbon dioxide present in the air reacted with the hydroxide to create bicarbonate, another base present in the bodies of many marine animals like corals and oyster shells.
In the course of the experiment, the carbon content of the water increased by 45 times, harnessed from carbon dioxide. This process could possibly be used to offset ocean acidification in certain important areas like oyster farms or coral reefs, he said.
The acidity of the world's oceans has increased by about 30 percent since the beginning of the Industrial Revolution. This threatens the health of organisms whose bodies contain carbonate and bicarbonate, which is broken down by carbonic acid, the acid created when carbon dioxide dissolves in water.
On slow, geologic timescales, weakly acidic rainwater weathers rocks, which consumes carbon dioxide. Rau said his new technique is meant to mimic this natural process, but to speed it up.
Rau calls the hydrogen produced via this process "supergreen," because carbon is consumed. Most methods for producing hydrogen are carbon neutral, at best, and most involve releasing some of the greenhouse gas, he said.
There are, of course, many caveats. So far this process has only been demonstrated at the scale of a laboratory, and many promising technologies fail to make the leap from lab to factory. Scaling up brings many difficulties, not the least of which is cost; Rau is currently looking for funding for further researcher to iron out kinks in the process.
Then there's perhaps the most important practical question: Where does the electricity come from? Obviously if the process is to be carbon negative, it must rely on a renewable source that doesn't emit carbon dioxide, like wind, solar or nuclear, Rau said.
Hydrogen gas is also not the easiest gas to store or transport. And if ordinary sea water is used, electrolysis typically produces chlorine gas, which is poisonous. Rau said that future work should be able to overcome this obstacle, either by using a different salt besides sodium chloride (which gives rise to chlorine) or tweaking the electrode in a way that doesn't produce the noxious gas, he said.
Still, preliminary calculations suggest the process could be cheaper and more effective than current carbon sequestration techniques, especially when used during off-peak hours. Many existing sequestration processes involve condensing carbon dioxide to a gas or super-cooled liquid, which is costly and potentially dangerous. There's also the risk that carbon dioxide, once stored underground, could leak out, Rau said.
The beauty of this technique is that the carbon would be stored in a stable solid—carbonate or bicarbonate. This chemical could then be added to the ocean, where it could prevent further carbon dioxide from leaving the ocean and entering the atmosphere.
"We think this suggests a process that is significantly cheaper and safer than other chemical air capture methods that have been proposed," Rau said.