Caltech-based startup company Captura, which captures carbon dioxide (CO2) from ocean water to combat climate change, has been awarded $1 million from the XPRIZE Carbon Removal competition.
Captura was co-founded by Harry Atwater, the Howard Hughes Professor of Applied Physics and Materials Science; and Chengxiang "CX" Xiang, research professor of applied physics and materials science; and is based on technology they recently patented and are licensing from Caltech. It has the potential to scale up to harvesting gigatons of carbon dioxide—that is, billions of tons—from the ocean every year.
For reference, the Intergovernmental Panel on Climate Change (IPCC) estimates that to limit climate change to around 1.5 degrees Celsius, a goal set by the Paris Accords, humanity needs to not only stop adding greenhouse gases to the atmosphere but also to remove CO2 from the ocean and atmosphere at a rate of 10 gigatons per year by the year 2050.
"Currently, we're at orders of magnitude less than that," Xiang says. "It's going to require multiple solutions to pull this off, not just Captura's. But the concept we have is truly scalable to significant levels. We don't use any rare earth materials, we don't make any byproducts, and we rely only on the rapidly growing capacity of renewable energy. There's nothing really limiting us."
The project began a year ago with a Department of Energy (DOE) Advanced Research Projects Agency–Energy (ARPA-E) funded effort. The ARPA-E project also includes a co-PI, Shane Ardo, associate professor of chemistry from the University of California-Irvine, who is also a scientific advisor to Captura. At the time, Xiang, Atwater and Ardo were primarily interested in CO2 conversion—trying to turn CO2 into useful products, such as fuels.
"The challenge we face with carbon dioxide conversion is also one of scale," says Atwater, who is also Otis Booth Leadership Chair of the Division of Engineering and Applied Science and director of the Liquid Sunlight Alliance (LiSA). "Right now, we are pursuing the science of CO2 conversion in the laboratory using pure carbon dioxide from tanks, and we want to ultimately scale up that conversion to the gigaton level. However, conversion generates at best zero emissions from a CO2 generation source. A full climate solution for the planet will also require negative emissions by capture of dilute CO2, both to compensate for previous historical emissions and future emissions from hard-to-decarbonize sectors." Atwater says.
Atwater and Xiang could have pulled CO2 from a smokestack, but such a strategy would be carbon neutral, not carbon negative, because pulling already-emitted CO2 out of the environment would simply stop more from going in. Instead, they chose to target either the air or the ocean. And the ocean, it turns out, is easier: CO2 is about 120 times more concentrated in the ocean than in the air by volume. Also, culling carbon from the ocean has the additional benefit of reducing ocean acidification, which protects marine life like coral reefs. (Carbon dioxide from the air bleeds into the ocean, which is actually the world's largest carbon sink.)
"As far as we can tell, Captura is one the very few companies that is doing carbon capture from ocean water," Xiang says. Further, their strategy manages to avoid producing any unwanted chemical byproducts; other techniques can produce hydrochloric acid or chlorine as waste, which makes these efforts difficult to scale up, as such byproducts must be disposed of.
So, how does it work? Atwater and Xiang's team developed what they call an "electrochemical pH swing process." The acidity or alkalinity or a substance, known as its pH, is recorded on a scale from 0 to 14, with 0 being the most acidic and 14 the most alkaline (or "basic"). Chemically, acidity is the relative concentration of protons (positively charged hydrogen ions) while alkalinity is the relative concentration of hydroxyls (negatively charged molecules of hydrogen and oxygen).
Before the Industrial Revolution, the pH of ocean water was 8.2. Today, it is about 8.1, thanks to all of the CO2 that the ocean has absorbed from the air. While that may not seem like a major difference, it represents an approximate 30 percent increase in acidity since the pre-industrial time—enough to start irreversibly dissolving coral reefs and impacting other ocean life.
Atwater, Xiang, and their colleagues developed a material called a bipolar membrane. When ocean water is passed through the membrane, the water molecules are broken apart into protons and hydroxyls. In a given sample of water to be processed, a small amount (less than 1 percent) is passed through the bipolar membrane, stripping out the protons. These protons are then added back into the bulk of the water sample (the other 99+ percent) and acidify it very slightly.
The vast majority of the carbon in ocean water exists in the form of bicarbonate, a negatively charged salt molecule made from one carbon atom, one hydrogen atom, and three oxygen atoms. Even a slight acidification causes the bicarbonate to convert into dissolved CO2, which can be extracted easily from the water.
After extracting CO2, the Captura team recombines the decarbonated water with hydroxyls and discharges the oceanwater with slightly higher pH. This decarbonated oceanwater is then capable of absorbing still more carbon dioxide from air. Meanwhile, the extracted carbon dioxide can be sequestered in geological formations, such as depleted oil and gas reservoirs.
With the help of Caltech's Office of Technology Transfer and Corporate Partnerships (OTTCP), Atwater and Xiang launched Captura one year ago. Steve Oldham, former CEO of direct-air carbon capture company Carbon Engineering, joined as the Captura's CEO.
For now, the work takes place in a Pasadena garage workspace about 10 minutes from Caltech, where Xiang and Atwater remain busy when not on campus. Xiang says the XPrize is a wonderful first step, but adds much more work remains to be done.
"The next steps are to raise additional money and hire more great people. And we need to start by sequestering 1,000 tons of carbon dioxide in a year," he says. To launch that effort, Captura is planning to first build a small pilot device that can sequester 1 ton of carbon dioxide per year at Caltech's Kerckhoff Marine Laboratory in Newport Beach. Next, the company plans to build a scaled-up facility that is capable of removing 100 tons of carbon dioxide per year at a desalination plant, and then on to removing kilotons and beyond.
Image: Artist's illustration of what a large-scale carbon capture plant could one day look like. (Artist: Darius Siwek. Courtesy of Chengxiang Xiang/Captura)