Recent experiments at the U.S. Department of Energy’s Argonne National Laboratory have afforded researchers a greater understanding of how to manipulate photosynthesis, putting humankind one step closer to harvesting “solar fuel,” a clean energy source that could one day help replace coal and natural gas.
Lisa M. Utschig, a bio-inorganic chemist at Argonne for 20 years, said storing solar energy in chemical bonds such as those found in hydrogen can provide a robust and renewable energy source. Burning hydrogen as fuel creates no pollutants, making it much less harmful to the environment than common fossil fuel sources.
“We are taking sunlight, which is abundant, and we are using water to make a fuel,” said Utschig, who oversaw the project. “It’s pretty remarkable.” Unlike the energy derived from solar panels, which must be used quickly, hydrogen, a solar fuel, can be stored.
Sarah Soltau, a postdoctoral fellow at Argonne who conducted much of the research, said “the key finding of Argonne’s most recent research is that we were able to actually watch the processes of electrons going from a light-absorbing molecule to a catalyst that produces solar fuel. This piece of knowledge will help us develop a system to work more efficiently than the one we can create now, and, years on, may allow us to replace oil and gas.”
Argonne researchers attached a protein from spinach to both a light-absorbing molecule (called a photosensitizer) and to a hydrogen-producing catalyst. The protein helped stabilize both the catalyst and photosensitizer, allowing scientists to observe direct electron flow between the two for the first time.
Researchers used transient optical spectroscopy, a method for detecting very fast changes in the light absorption of a molecule when illuminated with a laser pulse, to observe changes in the colour of a compound as it undergoes chemical reactions. They also employed electron paramagnetic resonance, another form of spectroscopy, to study where electrons move inside a molecule.
“We don’t just see the result, the hydrogen,” Utschig said. “We are peering into this system. We are able to really see how it works and what the essential parts are. Once you know that, the next time you try and design something, you can make it better because you understand it.”
Argonne has been studying photosynthesis since the 1960s but this particular experiment has been pursued for about a year. Soltau said scientists may be several years from using these techniques to generate storable solar fuels to power cars or households, but that this could be made possible once researchers learn ways to make the process more efficient.
“We need to look at ways to make solar fuel production last longer,” she said. “Right now, the systems don’t have the stability necessary to last weeks or months.”
Nature’s Best Designs.
Peidong Yang, a professor of chemistry at U.C. Berkeley and co-director of the school’s Kavli Energy NanoSciences Institute, leads a team that has created an artificial leaf that produces methane, the primary component of natural gas, using a combination of semiconducting nanowires and bacteria. The research, detailed in the online edition of Proceedings of the National Academy of Sciences, builds on a similar hybrid system, also recently devised by Yang and his colleagues, that yielded butanol, a component in gasoline, and a variety of biochemical building blocks.
The research is a major advance toward synthetic photosynthesis, a type of solar power based on the ability of plants to transform sunlight, carbon dioxide and water into sugars. Instead of sugars, however, synthetic photosynthesis seeks to produce liquid fuels that can be stored for months or years and distributed through existing energy infrastructure.
In a round table discussion on his recent breakthroughs and the future of synthetic photosynthesis, Yang said his hybrid inorganic/biological systems give researchers new tools to study photosynthesis and learn its secrets.
“We’re good at generating electrons from light efficiently, but chemical synthesis always limited our systems in the past. One purpose of this experiment was to show we could integrate bacterial catalysts with semiconductor technology. This lets us understand and optimize a truly synthetic photosynthesis system,” he told The Kavli Foundation.
“Burning fossil fuels is putting carbon dioxide into the atmosphere much faster than natural photosynthesis can take it out. A system that pulls every carbon that we burn out of the air and converts it into fuel is truly carbon neutral,” added Thomas Moore, who also participated in the round table. Moore is a professor of chemistry and biochemistry at Arizona State University, where he previously headed the Center for Bioenergy & Photosynthesis.
Ultimately, researchers hope to create an entirely synthetic system that is more robust and efficient than its natural counterpart. To do that, they need model systems to study nature’s best designs, especially the catalysts that convert water and carbon dioxide into sugars at room temperatures.
“This is not about mimicking nature directly or literally,” said Ted Sargent, the vice-dean of research for the Faculty of Applied Science and Engineering at University of Toronto. He was the third participant in the round table.
“Instead, it is about learning nature’s guidelines, its rules on how to make a compellingly efficient and selective catalyst, and then using these insights to create better-engineered solutions.”
“Today, nature has us beat,” Sargent added. “But this is also exciting, because nature proves it’s possible.”