There is a lot to covet in a green, leafy plant. Especially if you are scouting for sustainable solutions to the world’s worsening fossil fuel crisis. For about three billion years, plants and their photosynthetic forbearers have successfully captured sunlight and converted it to energy for their own private use.
We humans are scrambling to catch up to plants’ indigenous chemical savvy. We too desire to make renewable energy from photons of light. And we want this new energy to jive with our existing infrastructures for distributing electric power, and to lessen or mitigate greenhouse gas emissions.
This challenge entices scientists in an emerging field within chemistry, physics, and materials science known as solar fuels. In theory, solar fuels will use the portion of the sun’s electromagnetic radiation that falls to earth as light and even the near infrared, which our eyes can’t see, to drive chemical reactions which cause fuels like methane or hydrogen to form. These fuels can be burned immediately for energy or stored, taking advantage of our existing energy infrastructure.
In North Carolina, the path to solar fuels is being blazed by UNC distinguished chemist Thomas Meyer and his collaborators, who recently landed a $17.5 million grant from the U.S. Department of Energy, with funding through the American Recovery and Reinvestment Act, to fund an Energy Frontier Research Center (EFRC). Other lead investigators on the project are UNC chemists John Papanikolas (deputy director of the center), Edward Samulski and Wenbin Lin and University of Florida chemist Kirk Schanze.
The UNC initiative is one of 46 EFRCs funded nationwide, but it is the only one devoted to solar fuels and the only one located in North Carolina. It will link more than 20 faculty in UNC’s departments of chemistry, and physics and astronomy, and scientific collaborators at N.C. State, N.C. Central and Duke universities, plus the University of Florida. The Center will support a mix of about 30 postdoctoral fellows and graduate students.
The main thrusts of the UNC EFRC will be to investigate the use of light to split water molecules into hydrogen and oxygen, and to use light and water to reduce carbon dioxide to methane and/or other hydrocarbon fuels. In a second project, EFRC scientists will delve into the sister field of solar-to-electrical energy conversion by photovoltaics. There they will seek to create next-generation materials for photovoltaic cells that use inexpensive plastics and chemical materials called polymers rather than the silicon solar cells that are seen so often powering isolated street signs, for example. Success in this area could lead to “solar shingles” or even “solar paint” that could be easily applied to roofs, providing solar power to everyone.
Meyer, the Arey Distinguished Professor of Chemistry, says that the easiest of these — chemically speaking — is water splitting, which is one of the targets of artificial photosynthesis. Visualize a water molecule, with a central oxygen atom and two hydrogen atoms. To split them apart, you have to add enough energy to transfer electrons from the oxygen to the hydrogens. This can be done by applying an electric field in a process called electrolysis, “but to do this with sunlight is more subtle,” Meyer said. “You have to use a catalyst and activate it, using just the power of the sun to drive the reaction.”
He says his research team is designing “molecular assemblies” that will absorb light, become activated by transferring electrons and then catalyze water oxidation.
Meyer has the background to lead the EFRC research teams. In 1974, in a collaboration with ex-UNC faculty member David G. Whitten, his lab was the first to show that molecules could absorb light and undergo electron transfer, the two first two key steps in water splitting. In 1982, his laboratory was the first to describe a molecular catalyst for water oxidation.
He also envisions developing a process that would use light to drive a reaction between water and carbon dioxide gas (CO2), concentrated from smokestacks, to give methane (CH4) or other hydrocarbon fuels. This has special appeal because in addition to creating a new form of renewable energy, it finds a use for industrial carbon emissions that are contributing to global warming.
“What we envision is setting up these photoreactions using solar converters on site at a power utility or a factory,” Meyer said. “The waste carbon dioxide coming out of a smokestack is concentrated and piped into these converters, where water and CO2 are converted into methane or other hydrocarbons.”
Methane is the main component of natural gas, and it gives off waste carbon dioxide when burned for energy. In this scenario, methane from the solar reactor would be piped right back into the power plant, and burned for energy with the waste CO2 captured and returned to the solar converter to make more methane.
A third avenue that Meyer’s team is exploring is that of designing novel molecular assemblies to replace silicon’s role in traditional photovoltaic cells. Highly purified, semiconductor silicon is used to absorb sunlight that excites internal electrons, which are siphoned off to create electricity. Physical properties of silicon limit its efficiency to a little more than 20 percent. It is useful but expensive to produce, and other semiconductor materials are beginning to take its place.
The novel materials that Meyer and his team are brainstorming would be much cheaper and easier to make and fabricate.
Meyer’s team is entering talks with the Research Triangle Institute (RTI) about translating research results in the EFRC into marketplace products. He also is casting his eye farther into the future, toward creating an even larger partnership among researchers at UNC, Duke, RTI and N.C. State through the existing Research Triangle Energy Consortium (RTEC).
“We are now in discussion as a group about competing for a new Department of Energy program called Energy Innovation Hubs which will be five times bigger than existing EFRCs,” Meyer said. “If that happens, this would become a world center for research in solar fuels.”
[This story by DeLene Beeland appears in the fall '09 issue of Carolina Arts & Sciences mgazine.]
