It’s a dream that seems to be getting closer to reality: can we reverse the destructive trends of the modern world and convert carbon dioxide back into usable fuels? Artificial photosynthesis has long been a goal of scientists, dating back to Professor Giacomo Ciamician.
In 1912, he had this dream of the future:
“Glass buildings will rise everywhere; inside of these will take place the photochemical processes that hitherto have been the guarded secret of the plants, but that will have been mastered by human industry which will know how to make them bear even more abundant fruit than nature, for nature is not in a hurry and mankind is… life and civilization will continue as long as the sun shines!”
Traditionally, efforts towards converting carbon dioxide into fuels have focused on creating hydrocarbon chains like ethanol, or sometimes into flammable gases like methane which can be burned as fuels. These processes often involve photocatalysis powered by solar panels (in artificial photosynthesis), or genetically-engineered bacteria that do the chemical heavy lifting. Now, a new procedure discovered by a collaboration including scientists from the U.S. Department of Energy's (DOE) Brookhaven National Laboratory suggests an intriguing alternative; convert carbon dioxide into carbon monoxide.
The research, published in Energy and Environmental Science, demonstrates that single atoms of nickel can be used as an electrocatalyst that efficiently converts carbon dioxide into carbon monoxide. Carbon monoxide is highly energetic, and useful energy can be harnessed in a number of different ways.
There are many ways to use CO. You can react it with water to produce energy-rich hydrogen gas, or with hydrogen to produce useful chemicals, such as hydrocarbons or alcohols. If there were a sustainable, cost-efficient route to transform CO2 to CO, it would benefit society greatly.
Eli Stavitski, Author
That sustainable and cost-efficient route has proved elusive to previous scientists. Typically, scientists have used noble metal nanoparticles as electrocatalysts. But this is difficult in the case of converting carbon dioxide to carbon monoxide because a competing chemical reaction – ‘water splitting’, a process that splits hydrogen into protons and electrons or vice versa – takes precedent over the conversion of CO2 to CO.
Some metals can avoid this ‘hydrogen evolution reaction’, but, unfortunately, the best that have been identified are gold and platinum, which puts paid to the “cost-efficient” use of these materials as electrocatalysts.This is why the discovery that individual atoms of Nickel can serve as electrocatalysts for this reaction is a big step forward.
Haotian Wang is a Rowland Fellow at Harvard University, and served as a corresponding author for the paper; he describes how nickel’s usefulness here is somewhat counterintuitive: "Nickel metal, in bulk, has rarely been selected as a promising candidate for converting CO2 to CO. One reason is that it performs HER very well, and brings down the CO2 reduction selectivity dramatically. Another reason is because its surface can be easily poisoned by CO molecules if any are produced."
However, individual atoms can produce different results to the metal in bulk, due to the different surfaces; the surface of the bulk metal is at a uniform, single energy potential, while the potential energy surface of individual atoms varies considerably across the atom:
"Single atoms prefer to produce CO, rather than performing the competing HER, because the surface of a bulk metal is very different from individual atoms," Stavitski said.
A surrounding sheet of graphene was used, which interacted with the nickel atoms and enabled the scientists to tune the catalyst and suppress the unwanted hydrogen evolution reaction. To analyze the reactions more closely, the scientists used scanning emission electron microscopy to resolve the system at the atomic level. This system’s resolution is powerful enough to allow individual nickel atoms to be resolved on the graphene layer. It was provided by Brookhaven’s Centre for Functional Nanomaterials (CFN).
Single atoms are usually unstable and tend to aggregate on the support. However, we found the individual nickel atoms were distributed uniformly, which accounted for the excellent performance of the conversion reaction.
Dong Su, Co-Author & CFN scientist
Based on the results from this microscopy and a beamline analysis that involved bombarding the material with ultra-high-energy X-rays to probe its structure, the scientists were able to conclude that single nickel atoms were catalyzing the reaction from CO2 to CO with a near-maximal 97% efficiency. The next step is to see how scalable the production of the catalyst is and how much carbon monoxide can be produced by the nanomaterial, as Professor Wang describes:
"To apply this technology to real applications in the future, we are currently aimed at producing this single atom catalyst in a cheap and large-scale way, while improving its performance and maintaining its efficiency."
Given that many of the more optimistic projections for climate change suggest we need to actively suck carbon dioxide out of the atmosphere – and given that fossil fuels aren’t being replenished any time soon – such innovation comes not a second too soon.