Ben Sherman Ties Photosynthesis to Affordable Solar Energy Cells
The chemist is working to improve photovoltaic cells that can convert sunlight.
Through photosynthesis, plants do something simple yet remarkable: They convert sunlight to energy by breaking apart water molecules. If scientists could replicate what plants do every day, the world would have a clean, endless alternative to fossil fuels.
Ben Sherman, assistant professor of chemistry in TCU’s College of Science & Engineering, develops solar cells that are inspired by plants’ energy conversion process. But for human energy needs, he said, the work isn’t so simple. “Photosynthesis is very convoluted … yet somehow this is the very basis of life.”
According to Sherman, the overall efficiency of converting sunlight into chemical energy during photosynthesis is negligible at best. Plants convert only about 1 percent of the sunlight they receive into carbon dioxide products. For a solar cell to compete with fossil fuels, it would need to achieve at least a 10 percent solar-to-fuel efficiency.
Sherman’s research involves assembling and optimizing photovoltaic cells. These devices are similar to batteries, but they produce electricity when sunlight strikes a special material that causes a chemical reaction to occur. His cells are inspired by photosystem II, a protein complex that drives photosynthesis by using the energy from sunlight and a metal-based catalyst to break apart a water molecule into hydrogen and oxygen.
“The objective has always been to mimic that process,” Sherman said. But breaking apart a water molecule, which is stable by nature, requires a potent catalyst.
One area of his research is searching for an inexpensive metal-based catalyst powerful enough to split a water molecule into its component parts. “The best catalysts we can come up with are using elements you don’t find in nature, at least [not] in photosystem II,” he said.
Sherman’s group, which includes graduate students, an undergraduate research student and a postdoctoral researcher, begins with catalysts proven to be capable of breaking apart water. But these catalysts contain iridium or ruthenium, expensive rare-earth metals. His group builds the rest of the photoelectrode using these catalysts, making sure to optimize all of the other parts. The last piece of the puzzle is changing out the catalyst for experimental ones made from cheaper, more abundant metals.
The right catalyst is just one piece of the puzzle. The kind of photovoltaic cells that Sherman’s group studies are called dye-sensitized solar cells, or Grätzel cells, after the Swedish scientist who invented them. Grätzel cells have four basic parts: a transparent glass surface that conducts electricity, a layer of porous semiconductor material coated with a layer of molecular dye that serves as an electrode, the electrolyte solution where the chemistry takes place and a counter electrode.
The dye sits on the surface of the semiconductor material. Reactions that require a catalyst — like splitting water — occur in that solution. These reactions always involve electrons moving from the semiconductor electrode to the counter electrode, creating an electrical current.
“The largest issue facing humanity is climate change. I wanted to apply myself to that issue and that problem to contribute any way I could.”
The end goal is to develop a photoelectrochemical system that is feasible on a large scale. Sherman said some existing systems are 18 percent efficient at converting sunlight to fuel, but they typically use costly semiconductors and expensive metal catalysts. “It’s tens of thousands of dollars for a square centimeter of an actual device. So even at that efficiency, it can’t compete [with fossil fuels].”
Optimizing each part of a complicated cell requires a team effort. “Collaboration is so essential,” Sherman said. “I’m doing the writing and getting the grants, but it’s the grad students that do the work, and without them involved, it would be nothing.”
In 2020, Sherman worked with two graduate students, Debora Beeri and Jackson Roye, and an undergraduate student, Maggie Purvis.
Beeri said she pursued PhD work in Sherman’s lab because she wanted to help find clean energy sources. “It’s great that the scientific world is trying to find cures for cancer and other health problems,” she said Sherman told her. “But who is going to heal the Earth that we live in?”
Beeri’s work involves optimizing the glass surface that conducts electricity, and she is investigating new types of semiconductor electrodes using less expensive materials that could eventually be used on a large scale.
Sherman said he is also interested in developing a rechargeable solar battery. “The largest issue facing humanity is climate change. I wanted to apply myself to that issue and that problem to contribute any way I could.”