American Scientist

For Jillian Dempsey, the most eye-opening statement to come out of COP26, the November 2021 United Nations Climate Change Conference held in Glasgow, was that a number of countries made promises to help limit global temperature rise to below 2 degrees Celsius based on technologies that don’t yet exist. “That’s where I think we, as scientists, come in,” says Dempsey, who is the Deputy Director of the Center for Hybrid Approaches in Solar Energy to Liquid Fuels (CHASE) at the University of North Carolina at Chapel Hill. Dempsey says she feels some responsibility for filling that technology gap to make climate change reduction possible.

But even considering any number of renewable energy sources—such as wind, hydroelectric, geothermal, or biomass—all those combined don’t come close to meeting the planet’s energy demands, Dempsey says. The only possibly sufficient source is solar—if it can be properly harnessed and stored. Although photovoltaic technology has improved greatly in efficiency while decreasing in cost, the rate of conversion from solar to stored energy is still low. “A strategy that’s incredibly attractive to me is storing solar energy in the form of a chemical fuel, which is exactly what nature is doing in the process of photosynthesis in green leaves,” Dempsey says. She and her colleagues are exploring a simpler form of artificial photosynthesis, using solar energy to split water into hydrogen and oxygen, then pull in other common molecules such as carbon from carbon dioxide to make more complex fuels.

Dempsey spoke about her research on January 25 at Science by the Slice, a monthly lunch series cohosted by Science Communicators of North Carolina (SCONC) and Sigma Xi. (See the recorded talk below, and scroll down for live tweets of the event from SCONC intern Kirsten Giesbrecht.) “If we take a glass of water and stick it out in the sunshine, it doesn't spontaneously split into oxygen and hydrogen, and thank goodness it doesn't,” Dempsey said. “There’s a high energy barrier to drive this splitting.” Dempsey and her colleagues are developing chemical catalysts to facilitate these reactions.

The approach that Dempsey and her colleagues take to producing fuels from solar energy is to integrate a catalyst with a semiconductor that can capture solar photons. Right now the catalyst they are working with is based on rhenium, but they are hoping to eventually develop catalysts based on more common elements such as iron. The solar photons would be the sole energy source to turn small molecules found in the air into a more complex fuel that can be stored easily. “Those small molecules are going to grab the carbon dioxide out of solution, stick it to the catalyst, pump in the electrons, and bring in the hydrogen ion that allows us to reduce that carbon dioxide all the way down to something like methanol, or perhaps reduce nitrogen all the way down to ammonia, which could be another fuel,” said Dempsey.

The first step that Dempsey’s group has been working on is to take the catalysts that they had already developed and figure out how to attach them to a semiconductor without damaging their function, which was not a simple process. Dempsey’s team had to add a carbon group to the end of their catalyst, then etch a silicon surface to make sure it was reactive, and super-cool it to prevent any new reactions. They add their catalyst on top and use a sonicator—a sound device that creates what’s called acoustic cavitation, a variation in pressure that can activate bonds—to induce the end of the catalyst to bond to the silicon surface. They follow up with a different reaction to ensure that no further surface bonding takes place. The team was able to verify that the molecular catalyst remained intact during this complex process.

Preliminary tests of this hybrid photoelectrode with a solar simulator, in which Dempsey’s team measured all the gases produced, showed a direct link between charges passing through the silicon and the reduction of carbon dioxide to carbon monoxide with about a 69 percent efficiency. “Of course, this product is not yet the liquid fuel that we’re targeting, but it’s the first step that we would integrate into one of these cascade technologies,” Dempsey says.

In the long term, perhaps 10 to 15 years, Dempsey envisions that a more advanced form of this technology could be used to convert carbon dioxide in the atmosphere into more complex carbon-based fuels. The more bonds a molecule contains, the more energy it can potentially store in each of its bonds. “I think methanol is a really incredible target to think about because it’s a liquid fuel. We could go even further and think about stitching together carbon atoms from two different carbon dioxide molecules to make something like ethanol or butanol, or even longer chain hydrocarbons, that again increase in energy density.”

Tweets highlighting the talk follow below.

This blog was produced in collaboration with Science Communicators of North Carolina.