On Jan. 31, the first snow of the year fell briefly on campus. Earlier that same day, Dr. Jillian Dempsey dropped by to talk about her recent solar energy research.
Dempsey is an associate professor of chemistry from the University of North Carolina at Chapel Hill, and her research group aims to find more efficient ways to store solar energy so that it may still be a viable energy option even on the days that are plagued by clouds—or snow.
North Carolina is the second largest solar state in the country with over 150 solar facilities and more than 4,300 megawatts of installed solar capacity. That’s enough to power over 486,000 individual homes, and Dempsey wants to see these numbers grow even more.
The photovoltaic cells that make up solar panels work by capturing protons from sunlight and converting them to electricity. Dempsey said one of the biggest obstacles facing widespread solar energy usage is that electricity can only be generated when the sun is shining.
“…If you’re anything like me, it’s when the sun goes down at night that you want to turn on your lights and watch trashy television,” said Dempsey. “So what we need to do is think about how we can store the sun’s energy for the times that the sun is not shining.”
According to Dempsey, there are a number of potential ways to do this ranging from large battery storage centers to a combination of water pumps and electrical turbines. The idea that Dempsey finds most intriguing, however, is storing solar energy in the form of a chemical fuel.
“This is exactly what plants are doing in the process of photosynthesis, where they take the energy from solar photons, carbon dioxide, and water to generate oxygen and sugars,” said Dempsey. “These reduced carbon products, the sugars, are nature’s stored chemical fuel.”
She envisions a very similar process, in which energy from the sun is used to split water into oxygen and hydrogen. Hydrogen, in this case, would act as the sugar of photosynthesis; it would store the energy taken from sunlight.
“Let’s focus on hydrogen, which is a great form of fuels,” said Dempsey. “We know that if we take a glass of water and stick it out into the sun, you don’t just split it into hydrogen gas and oxygen gas. There’s a kinetic activation to splitting water. We don’t just need the energy input of the sun, we need catalysts as well.”
She said there are many metal catalysts that are able to generate hydrogen fuel from protons and electrons, although they aren’t able to do it on an efficient scale. Dempsey’s research group is interested in developing better catalysts for this activity, but must first figure out exactly how the catalysts work.
“One thing we do know is that most of them go through a metal hydride intermediate,” said Dempsey. “So the transfer of the first proton and electron forms this metal hydride intermediate, and then that metal hydride takes another proton and electron [to finish making the hydrogen fuel].”
To form the metal hydride, both a proton and an electron must be added to the metal catalyst. Maybe the proton is added first and the electron is added afterwards, or maybe the electron is added first and the proton is second. In both of these cases, the reaction is said to proceed via a ‘stepwise’ pathway because it took two steps to complete.
Alternatively, the proton and the electron can be attached in a single step. Dempsey called that a ‘concerted’ pathway, and said it is energetically more efficient.
Every reaction can be thought of as hiking on a mountain, where the reactants and products are represented by two valleys on either side of the mountain path and the path itself represents the amount of energy needed to continue the reaction. At the very top of the mountain pass, where the most energy is needed, is where a reactant is turned into a product.
If a proton and electron are transferred separately, the reactant must climb up and over two different mountains to get to the final product valley because there are two steps in the reaction.
“But maybe if you were going on that mountain pass trail with some really experienced Sherpas,” said Dempsey, “they might know this shortcut that doesn’t involve going quite all the way to the top and dropping back down [both] mountain[s]. So maybe you won’t be as tired when you get to the other side.”
The idea of using concerted pathways to conserve energy is important when driving these chemical fuel reactions with solar photons. Ideally, all of the energy from the photon is stored in the fuel, and as little as possible is used up in the reaction process itself. The efficiency of this single chemical step could have a large impact on the efficiency of the entire chemical system and solar energy as a whole.
“I think a long-term vision for our country is to be able to do these and couple these reactions with carbon dioxide so that we can form something like methanol, which is a liquid fuel [that is transported more easily than gaseous hydrogen fuel],” said Dempsey. “Our work studying simple systems like hydrogen is super fundamental so that long-term we are able to do similar chemistry when coupling it with carbon dioxide reduction.”