With worldwide energy demand projected to rise anywhere from 35 to 40 percent between now and 2040, the hunt is on for viable sources and solutions. Texas A&M University chemist Marcetta Darensbourg is exploring lessons provided in nature and focusing on the simplest of all molecules, hydrogen, to open related doors to inexpensive, eco-friendly, hydrogen-based energy alternatives.
Specifically, Darensbourg, an internationally respected expert in synthetic and mechanistic inorganic chemistry, is developing methods to perfect the high-stakes technology of hydrogen-powered fuel cells. Her lab is taking the novel approach of introducing Earth-abundant elements — iron, nickel and sulfur — into hydrogen-producing molecular catalysts intended to replace platinum as the kick starter in these fuel cells.
Darensbourg, a distinguished professor of chemistry since 2010, first began exploring the inorganic biocatalysts within hydrogen-controlling microorganisms for use in clean-energy initiatives nearly two decades ago. A pioneer in many areas of chemistry, she became the first-ever female recipient in 1995 of the American Chemical Society’s Distinguished Service in the Advancement of Inorganic Chemistry Award, the ACS’s top annual honor for inorganic accomplishment. (Kim Dunbar, fellow Texas A&M distinguished professor of chemistry, became the second earlier this month.)
“It will be momentous for renewable energy if we can find the most effective way of doing this, and we — by that, I mean a community of scientists dedicated to this area — are very close,” Darensbourg said. “There’s still much to be done, but I think everyone is aware enough that we can figure this out.”
Fuel cells have been heralded by the U.S. Department of Energy for their potential to provide a reliable source of heat and electricity for American homes and automobiles as well as to dramatically reduce reliance on fossil fuels. Somewhat related to batteries, they work by electrochemically combining hydrogen and oxygen to produce electricity, heat and water in a process that is both highly efficient and virtually emission-free, making them an attractive commodity in green-energy sectors and industries from automobile manufacturing to power generation. But whereas batteries have limitations — namely a fixed energy supply or, at best, a time constraint required for recharging — fuel cells can generate energy continuously, so long as they have a fuel supply.
Darensbourg believes the best such fuel supply is hydrogen. The technology, however, still faces a number of environmental and economic obstacles that she and her team are working to address.
One issue is platinum, the catalyst currently used in fuel cells to convert hydrogen and oxygen into electricity. While platinum is ideal because it can easily shuttle between oxidation states, it is expensive and resource-limited, rendering it ineffective for large-scale, global use. Further, although hydrogen is the most abundant of all elements, it is expensive to acquire from water, and generating it from non-renewable resources such as oil or natural gas places a significant burden on fossil-fuel resources while also potentially creating an undesirable byproduct: pollutants.
The Darensbourg lab’s goal of utilizing Earth-abundant transition metals, such as iron and nickel, in these reactions as an alternative to platinum could open doors to a more cost-effective, readily available source of hydrogen energy.
“We hope our work will be able two answer two things: how to make hydrogen, ultimately harvesting energy from the Sun, and how to use hydrogen,” she said.
Darensbourg’s inspiration comes from Mother Nature herself — specifically, lessons in the form of hydrogenase enzymes, which are biological catalysts found in bacteria and other microorganisms that use hydrogen as an energy vector. A chemical reaction similar to what fuel cells undergo to produce energy occurs naturally in such microorganisms that exploit hydrogenase enzymes. While fuel cells use platinum to regulate the hydrogen-oxygen reaction, which creates a huge amount of energy per reaction, hydrogenase enzymes do it more efficiently, thanks to the properties of sophisticated arrangements of iron and nickel.
Although the discovery of the role of hydrogenases in methane-producing microorganisms occurred some eight decades ago, active research into the role of metals and their catalytically active sites — the small pockets within an enzyme where molecules have the potential to undergo a chemical reaction — has taken flight only in the last 15 to 20 years following structure determinations by X-ray crystallography. Darensbourg, who is trained as an organometallic chemist, found that she could create molecular mimics of the active sites of hydrogenase enzymes and then measure their ability to produce dihydrogen and, on the other side, extract energy from the hydrogen-hydrogen bond.
The takeaway for Darensbourg is to observe how active sites operate and understand how to create better molecular models — fundamental chemistry work in which each stage and new discovery will bring the international community of scientists in this area that much closer to utilizing those compounds as viable catalytic alternatives to platinum.
“These hydrogenase enzyme active sites give us an idea of how to make hydrogen and extract electrons,” Darensbourg said. “Each step of understanding is a step forward to using synthetic analogues as catalyst materials.”
If Darensbourg can perfect this approach, the next big question will be how to store the hydrogen. Because it has a low-energy density, hydrogen must be stored and transported under high pressure, making it a highly cumbersome and volatile resource. Other chemists, including several in the Texas A&M Department of Chemistry, are working to address these problems.
The hydrogen economy has been a national priority since 2003 when then-President George W. Bush announced a $1.2 billion initiative to make it competitive for reducing dependence on foreign oil and providing a cleaner supply of energy.
While plenty of work remains, Darensbourg and her group of graduate and undergraduate students and postdoctoral fellow coworkers are optimistic about the future of a hydrogen economy.
“If we could do this effectively — take the energy of the Sun, make hydrogen, store it and use it in a fuel cell to generate electricity — that’s huge,” Darensbourg said.
A member of the Texas A&M Chemistry faculty since 1982, Darensbourg is an inaugural fellow of the American Chemical Society (2009) and a fellow of the American Academy of Arts and Sciences (2011), one the country’s oldest and most prestigious honorary learned societies. In addition, she received the Distinguished Scientist Award for 2011 from the Texas A&M chapter of Sigma Xi, The Scientific Research Society.
For more information on Darensbourg’s work, visit http://www.chem.tamu.edu/rgroup/marcetta.
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