Nov 9 2018
Visualize a day when emissions spilling from heavy industry and power plants are trapped and conveyed into catalytic reactors that chemically convert greenhouse gases, such as carbon dioxide (CO2), into chemicals or industrial fuels that release only oxygen.
It’s a future that Haotian Wang feels may be closer than many recognize.
A fellow at the Rowland Institute at Harvard, Wang and colleagues have built an improved system to use renewable electricity to reduce carbon dioxide into carbon monoxide (CO)—a crucial commodity used in several industrial processes. The system is detailed in a November 8th paper published in Joule, a newly introduced sister journal of Cell Press.
“The most promising idea may be to connect these devices with coal-fired power plants or other industry that produces a lot of CO2,” Wang said. “About 20 percent of those gases are CO2, so if you can pump them into this cell … and combine it with clean electricity, then we can potentially produce useful chemicals out of these wastes in a sustainable way, and even close part of that CO2 cycle.”
The new system, Wang said, signifies a major step forward from the one he and colleagues first talked about in a 2017 paper in Chem.
The old system was hardly the size of a cellphone and depended on two electrolyte-filled chambers, each of which contained an electrode. The new system is cheaper and depends on high concentrations of CO2 gas and water vapor to function more efficiently. Just one 10-by-10-centimeter cell can generate as much as four liters of CO per hour, said Wang.
The new system solves the two critical challenges—scalability and cost—that were observed as limiting the preliminary approach, he said.
“In that earlier work, we had discovered the single nickel atom catalysts which are very selective for reducing CO2 to CO … but one of the challenges we faced was that the materials were expensive to synthesize,” Wang said. “The support we were using to anchor single nickel atoms was based on graphene, which made it very difficult to scale up if you wanted to produce it at gram or even kilogram scale for practical use in the future.”
To address that issue, he said, his team looked to a commercial product, which is thousands of times less expensive than graphene, as an alternative support—carbon black.
Employing a process similar to electrostatic attraction, Wang and colleagues are able to absorb single nickel atoms (positively charged) into defects (negatively charged) in carbon black nanoparticles, with the ensuing material being both economical and extremely selective for CO2 reduction.
“Right now, the best we can produce is grams, but previously we could only produce milligrams per batch,” Wang said. “But this is only limited by the synthesis equipment we have; if you had a larger tank, you could make kilograms or even tons of this catalyst.”
The other challenge facing Wang and colleagues was linked to the fact that the original system only functioned in a liquid solution.
The original system functioned by using an electrode in one chamber to divide water molecules into oxygen and protons. As the oxygen bubbled away, protons conducted through the liquid solution would travel into the second chamber, where—with the assistance of the nickel catalyst—they would bind with CO2 and split the molecule, leaving water and CO. That water could then be sent back into the first chamber, where it would again be divided, and the process would restart.
“The problem was that the CO2 we can reduce in that system are only those dissolved in water; most of the molecules surrounding the catalyst were water,” he said. “There was only a trace amount of CO2, so it was pretty inefficient.”
Although it may be tempting to just increase the voltage applied on the catalyst to boost the reaction rate, that can have the unplanned consequence of splitting water, not reducing CO2, Wang said.
“If you deplete the CO2 that’s close to the electrode, other molecules have to diffuse to the electrode, and that takes time,” Wang said. “But if you’re increasing the voltage, it’s more likely that the surrounding water will take that opportunity to react and split into hydrogen and oxygen.”
The solution proved to be comparatively simple—to avoid splitting water, the team removed the catalyst out of solution.
“We replace that liquid water with water vapor, and feed in high-concentration CO2 gas,” he said. “So if the old system was more than 99 percent water and less than 1 percent CO2, now we can completely reverse that, and pump 97 percent CO2 gas and only 3 percent water vapor into this system. Before those, liquid water also functioned as ion conductors in the system, and now we use ion exchange membranes instead to help ions move around without liquid water.”
“The impact is that we can deliver an order of magnitude higher current density,” he continued. “Previously, we were operating at about 10 milliamps-per-centimeter squared, but today we can easily ramp up to 100 milliamps.”
Wang said there are still some system challenges to overcome—mainly related to stability.
“If you want to use this to make an economic or environmental impact, it needs to have a continuous operation of thousands of hours,” he said. “Right now, we can do this for tens of hours, so there’s still a big gap, but I believe those problems can be addressed with more detailed analysis of both the CO2 reduction catalyst and the water oxidation catalyst.”
Eventually, Wang said, the day may arrive when industry will be able to trap the CO2 that is currently discharged into the atmosphere and convert it into useful products.
“Carbon monoxide is not a particularly high-value chemical product,” Wang said. “To explore more possibilities, my group has also developed several copper-based catalysts that can further reduce CO2 into products that are much more valuable.”
Wang credits the freedom he enjoyed at the Rowland Institute to helping lead to breakthroughs like the innovative system.
“Rowland has provided me, as an early career researcher, a great platform for independent research, which initiates a large portion of the research directions my group will continue to push forward,” said Wang, who recently accepted a position at Rice University. “I will definitely miss my days here.”
This research received funding from the Rowland Fellows Program, the Center for Nanoscale Systems, NSERC, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada, the University of Saskatchewan, the National Science Foundation, the China Scholarship Council, and Rice University.