Mimicking Nature Backwards and Forwards
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Photo courtesy of Pacific Northwest National Laboratory
R. Morris Bullock directs the Center for Molecular Electrocatalysis, an Energy Frontier Research Center that specializes in devising new synthetic catalysts for energy applications based on biological models.
Remember the days when wind-up toy cars were found in cereal boxes and kids' fast food meals? Pull the car backwards and the motion of the turning wheels was stored as potential energy in a coiled spring. Let the car go, and the spring would unwind, providing kinetic energy to propel the wheels forward. Store the energy, then use the energy—over and over again.
Wouldn't it be wonderful if today's automobiles could run on a similar reversible system—storing energy in one cycle and using it in another? In fact, such a reversible storage system would also be ideal for smoothing out renewable energy sources such as wind and solar power—storing energy when the wind blows and sun shines and releasing energy when the wind dies down and the sun sets.
The simplest energy storage system that one could envision would involve the simplest molecule possible—a combination of only two of the smallest available atom, hydrogen, into the simple molecule, H2, or "dihydrogen."
Much as the toy car's spring converts kinetic energy into potential energy and stores it until the car is released, the chemical "spring" (or bond) that links the two hydrogen atoms of dihydrogen stores chemical energy until it is ready to be released as electrical energy in the form of two electrons (2 e-) and two protons (2 H+). If it were made reversible, the back-and-forth conversion between H2 and 2 e- + 2 H+ could be used to store and release energy over and over again.
However, while this hydrogen/proton system might seem simple on paper, the real-life application of such a system is much more complex. Typically the storage of energy in H-H bonds has required one catalyst to promote the formation of dihydrogen and a different catalyst to split the H2 to form electrons and protons. The need for two different catalysts means a significant increase in complexity, and cost, of the system.
"The mechanism is remarkably complicated," said R. Morris Bullock, Laboratory Fellow at the Pacific Northwest National Laboratory. "There is a lot of detail in this process: taking the hydrogen apart, moving protons and electrons, and putting it back together."
Recently, Bullock and his colleagues have developed the first synthetic catalyst capable of forming dihydrogen and splitting it apart reversibly. The team performed the work as part of the Center for Molecular Electrocatalysis (CME), which Bullock directs. CME is one of 46 DOE Energy Frontier Research Centers established by the DOE Office of Science in 2009 at universities, national laboratories, and other institutions around the country to accelerate basic research on energy.
In order to develop an effective catalyst, the CME team began by studying how nature does the job. In the world of bacteria and other microbes, where hydrogen is used as an important source of energy, hydrogen-converting enzymes, or "hydrogenases," successfully perform the operation reversibly, and do so at ambient conditions and pressures, using abundant metals such as iron and nickel in their active sites. This "biomimetic" approach to the design of new catalysts—basing designs on biological models—is a hallmark of CME's research.
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Image courtesy of Pacific Northwest National Laboratory
The new catalyst enables hydrogen to reversibly store and release electrical energy, much as an old-fashioned friction-based toy car stores and releases kinetic energy.
Bullock and the CME team began their studies with a focus on the metal core of the catalyst. Iron- and nickel-based complexes were chosen for their low cost and abundance as well as their similarity to biological systems. The researchers attached molecular strands called "ligands" to the metal active site. These ligands function as arms, transporting molecules, protons, and electrons to and from the active site. From a combination of theoretical and experimental studies, the team systematically explored how changing the size, structure, and electronic behavior of the ligands affected the energy of hydrogen addition to the catalyst. The ability to vary these properties, by changing the groups on the ligand led to the design of catalysts that are biased toward either hydrogen production or proton formation. They characterized each version of the catalyst using nuclear magnetic resonance (NMR) spectroscopy, elemental analysis, and electrochemical measurements.
Having characterized several catalysts as candidates, they tested the ability of each to drive the reaction both forward and backward. The tests involved measuring the electric current produced by adding hydrogen to the catalyst. Using complex mathematical formulas, they determined the speed and efficacy of the reactions. One catalyst, specifically, [Ni(PPh2NR2)2](BF4)2 (where Ph = phenyl group and R = CH2CH2OCH3) was found to display reversible electrocatalytic hydrogen oxidation and production activity with high efficiency. Previously, only one synthetic dihydrogen production catalyst has been shown to also produce protons, although this reaction was slow and the reaction was not truly catalytic (i.e., the reaction happened only once per metal complex instead of many times as is the case for catalytic processes). The CME discovery is the first example of electrocatalysis which comprises reversible proton and dihydrogen production and is shown by a non-enzymatic complex under homogeneous conditions.
The catalyst proved very efficient, wasting little energy. Energy waste is measured by determining the so-called "overpotential," a ratio of the actual energy used under real world conditions versus the energy that would be required under perfect conditions.
"This [catalyst] has a lower overpotential than we usually find," said Bullock.
"Sadly," said Bullock, "the catalyst is slow."
So the next challenge for the researchers will be to increase the speed of the catalyst by tweaking the molecular structure of the ligands to transport protons to and from the metal center more quickly.
"We'll figure out what the slow step is and then figure out how to speed it up. Then, we'll take on the next slowest step, and so on, until we get the speed we need," said Bullock.
What Bullock and his team have produced so far is essentially a proof of concept. But it's an important first step. A practical reversible catalyst for the reaction could lead to a significant advance in our capabilities for energy storage, with potentially major consequences for renewable energy in transportation and a range of other sectors.
—Carol A. Bessel, DOE Office of Science, [email protected]
(Some material has been adapted with permission from the website of the Pacific Northwest National Laboratory at http://www.pnnl.gov/science/highlights/highlight.asp?id=1117.)
Research Funding
DOE Office of Science, Office of Basic Energy Sciences
Publication
S. E. Smith, J. Y. Yang, DL DuBois, and R. M. Bullock. 2012. "Reversible Electrocatalytic Production and Oxidation of Hydrogen at Low Overpotentials by a Functional Hydrogenase Mimic," Angewandte Chemie International Edition 51, 3152 (2012).
