The greenhouse gas carbon dioxide is of huge environmental concern but for one scientist from Harvard, it’s the perfect raw material.
Over 2 million pounds of carbon dioxide is pumped into the atmosphere on any given day, from factories, emissions from cars and trucks and the burning of gas and coal to generate electricity. Hoatian Wang and his research team have developed a system that could turn this carbon dioxide into carbon monoxide – a key commodity used in a number of industrial processes – using renewable electricity.
The process is akin to photosynthesis and electrochemically converts carbon dioxide to carbon monoxide using a device a little smaller than a smartphone.
Basically, what this is, is a form of artificial photosynthesis. In a plant, sunlight, CO2 and water become sugar and oxygen. In our system, the input is sunlight, CO2 and water, and we produce CO and oxygen.
Hoatian Wang, Fellow, The Rowland Institute at Harvard
The reaction takes place in an unassuming-looking device which includes two electrolyte-filled chambers separated by an ion exchange membrane. On one side, an electrode powered by renewable energy oxidizes water molecules into oxygen gas and releases free protons. These protons move to the other chamber where they bind to carbon dioxide molecules with the help of a carefully engineered metal single atom catalyst. Water and carbon monoxide are created as a result.
"The challenge is that most catalysts that are known tend to produce hydrogen gas," Wang explains. "So it's difficult, when you split water, to prevent those protons from combining together to form hydrogen gas. What we needed was a catalyst that can prevent hydrogen evolution and instead can efficiently inject those protons into CO2, therefore achieving a high selectivity for CO2 reduction."
The two best known such catalysts are gold and silver but these precious metals are expensive and would not be cost effective on a large scale.
So we began by looking at low-cost materials like nickel, iron and cobalt, which are all Earth-abundant. But the problem is that they are all very good hydrogen catalysts, so they want to produce hydrogen gas. In addition, they can all very easily be poisoned by carbon monoxide. Even if you manage to use them to reduce CO2, the resulting CO bonds very strongly to the surface, preventing any further reactions from taking place.
Kun Jiang, Postdoctoral Fellow in Wang’s group and First Author of the team’s paper
Wang and his team turned to colleagues Professors Yi Cui and Jens Nørskov from Stanford University to help solve the problem. They needed to ‘tune’ the electronic properties of the metal, so Dr Samira Siahrostami, a staff scientist in Nørskov’s group rationalized the nature of active sites by atomic scale modeling. She discovered that dispersing nickel metals into isolated single atoms – which are trapped in graphene vacancies – produced a material that was eager to react with carbon dioxide and willing to release the resulting carbon monoxide.
This carbon monoxide can then be used in a host of industrial processes, says Wang, "Carbon monoxide is a very important industry product. It can be used in plastics production, to make hydrocarbon products or can be burned as a fuel itself. It's widely used in industry."
The scientists hope that their system could one day be scaled up enough to scrub carbon dioxide from the atmosphere in an effort to combat global climate change.
The basic idea was if we can capture existing CO2 and use renewable electricity, from solar or wind power, to reduce it into useful chemicals, then we can possibly form a carbon loop.