Converting carbon dioxide emissions into clean fuel
Haotian Wang, a fellow at Harvard University’s Rowland Institute, and his team have recently detailed improvements they made to a system they first outlined in 2017. This system was already capable of reducing carbon dioxide into carbon monoxide, a reactant commonly used in a variety of industrial processes, but the the team has made it drastically more practical and effective.
Wang, who is slated to join Rice University as an assistant professor of chemical and biomolecular engineering, has developed a series of small reactors which help catalyze various greenhouse gases into carbon monoxide, by using single atoms of nickel. In the research paper, Wang describes his plan to connect his system to power plants or other industrial sites which emit a great deal of pollutants. Since carbon dioxide makes up about 20 percent of these pollutants, Wang hopes it could one day be pumped into his system and converted into useful chemicals when combined with clean electricity.
The old system was roughly as large as a cell phone and consisted of two electrodes in two electrolyte-filled chambers. The new system is far smaller – a mere 10-by-10 centimeter cell. It is also far more efficient; as much as four liters of carbon monoxide per hour can be emitted from this type of cell. Rather than electrodes and electrolyte-filled chambers, the new system instead relies on enormous concentrations of water vapor and carbon dioxide. Cost and scalability were the main issues with the old system, but Wang says the new system addresses these issues.
“The support we were using to anchor single nickel atoms was based on graphene, which made it difficult to scale up if you wanted to produce it at gram or even kilogram scale for practical use in the future,” says Wang. Graphene is prohibitively expensive, which is why Wang’s team made the decision to switch to carbon black instead. Carbon black is thousands of times cheaper than graphene but anchors nickel atoms almost as effectively. Positively charged nickel atoms can be absorbed into negatively charged defects in carbon black nanoparticles, resulting in an inexpensive material that is still highly selective for reduction of carbon dioxide.
According to Wang, the system currently in his laboratory can only produce several grams of carbon monoxide per batch. He claims that this is due to the synthesis equipment owned by Wang and his team; if someone utilized a larger tank instead, he speculates that “kilograms or even tons of this catalyst” could be produced.
A major issue Wang’s team encountered was that the old system only functioned in a liquid solution. The original system works by splitting water molecules into oxygen and protons in one chamber, then allowing the oxygen to leave the chamber and the protons to move into the second chamber. In the second chamber, the protons bind with carbon dioxide with the assistance of the nickel catalyst and break the carbon dioxide into carbon monoxide and water. This water is then driven back into the first chamber. However, since the only carbon dioxide capable of being reduced is that which is dissolved in water, only trace amounts of carbon dioxide would actually be reduced. According to Wang, “most of the molecules surrounding the catalyst were water.”
Wang’s team was unable to increase the voltage applied to nickel because that would likely cause water molecules to split instead. Thus, they eventually decided to take out the liquid water out of the system entirely and replaced it with water vapor. Since water vapor does not act as an ion conductor like liquid water does, Wang’s team can enormously increase the voltage and, in turn, the efficiency of the overall system. Ion exchange membranes take the place of the liquid water in moving ions in the chamber.
Although Wang’s team has already patched up many of the challenges faced by their original system, Wang believes that their main challenge concerns the stability of the system. The system can currently be run for tens of hours, but it has to be developed to a point where it can continuously operate for thousands of hours to have a tangible environmental or economic impact. In spite of this massive gap, Wang is confident that this goal will eventually become a reality through careful analysis of “both the carbon dioxide reaction catalyst and the water oxidation catalyst.”
In addition to their nickel-based reactor, Wang and his team have developed copper-based systems dedicated to further reducing carbon dioxide. Carbon monoxide’s value is quite low compared to other products which can be acquired through further reduction, making these copper-based catalysts more ideal to produce even more valuable products.