Molecular Approach to Converting Solar Energy into Chemical Fuel.
Alan Heyduk sees a bit more detail in a drop of oil than your average nonscientist: “Millions of years ago, plants captured sunlight, turned it into sugars, which were turned into starches, which were used to make plants. The plants died, got buried under tons of rock, compressed so that the water came out, and that left behind the hydrocarbons, basically, the oil.” Heyduk describes himself as the kind of scientist who loses interest in a problem once he knows the answer. “I want to move on to whatever problem I don’t know the answer to. I love figuring things out.” He especially likes tinkering with molecules. In this Scialog project, he and his colleagues are focused on approximating the plant kingdom’s big talent, converting the kinetic energy of photons – the basic particles of sunlight—into a form of potential energy – what we call fuel. “In theory we know how to do it,” he says. “But in practice we don’t really know how it works. And so my project with Scialog is aimed at understanding that fundamental transformation. It really comes down to any form of kinetic energy – how do you turn it into chemical, stored, energy?” Heyduk is betting chemical fuels are the best way to store energy. “It’s all about energy density,” he says. “If you think about the different ways to store energy, the simplest physical way to do it is to pump water up a hill, and then you let it run back down to turn a turbine. In terms of the amount of energy you can store, chemicals have an energy density orders of magnitude higher for the same amount of material as water pumped up a hill.” He extends the metaphor to describe his work: “You can store solar energy in chemical bonds by driving any reaction uphill.” By that he means photons can add some of their energy to the electrons that hold various atoms together in molecules. Heyduk’s molecule of choice in this project is a substance called catechol. Small amounts of catechol occur naturally in fruits and vegetables as feathery white crystals that are very rapidly soluble in water. “Catechol is benzene with two hydroxyl groups on it,” Heyduk says. Benzene is a colorless, flammable liquid that has become an important industrial solvent. It is a natural constituent of crude oil. “Hydroxyl” refers to a compound containing an oxygen atom bound “covalently” – that is, characterized by the sharing of pairs of electrons between atoms—with a hydrogen atom. “So we’re just trying to pull the hydrogen off that molecule,” Heyduk says. “And the action of pulling it off and separating it stores energy.” But Heyduk is also looking to design a process that also retains the oxygen atom on the catechol molecule. Why? “An important thing we often lose sight of is that you have to have a fuel (in this case, hydrogen) and an oxidant to store energy,” he says. “In elementary school we learn about combustion – in order for combustion to occur you have to have a fuel and an oxidant. Hydrogen by itself is stable forever. It’s not until it mixes with oxygen and there’s a spark that you have a problem.” The specific reaction he’s looking to drive is the conversion of catechol into hydrogen and an oxidant called a “quinone.” A quinone is an aromatic compound with two oxygen atoms present in the molecule. Your body uses catechols and quinones as electron shuttles, to store energy for short periods of time. Oxygen alone is incredibly reactive and causes all kinds of problems, Heyduk notes. “And so if we can keep the oxygen in these quinone molecules, it really simplifies the problem – because you don’t have to worry about separating the hydrogen and the oxygen gas.” Assuming he can do this, the ultimate goal would be to recombine the quinone and the hydrogen in a fuel cell “and release that energy again to provide electricity.” Although he’s quick to add that creating the fuel cell for this process is “a whole ‘nother big problem,” Heyduk says one “beautiful” aspect of this possible energy-producing process “is that it would be a closed-loop system—there are no byproducts, per se. You just recycle the fuel. So basically you take your catechol, run it through our process, which would make hydrogen and quinone – your fuel and oxidant—run the hydrogen and quinone into your fuel cell, get energy out to power the process, which would regenerate the catechol.” OK, so Heyduk admits that’s not really a closed-loop system, since it would rely on a constant stream of sunlight to break off the hydrogen atoms. “Actually, it’s probably the ultimate open-loop system, since the sun is beating us with vast amounts of energy. This system would just act as a storage mechanism for that energy – it’s the exact same thing, in principle, as pumping that water up and down the hill.” What makes this project unique, he adds, is that “we’re trying to build a single molecule that carries out all of the functions of photosynthesis – captures light, achieves charge separation, makes fuel and makes oxidant, all in a single molecule that you measure in nanometers – that’s 10 to the minus nine meters.” If he manages to pull that off, Heyduk admits, he’ll probably feel like he’s hit a grand-slam homerun “on a 3-2 count with two outs in the bottom of the ninth inning of the seventh game of the World Series.” Obviously an iffy proposition at best – what’s known in science-policy circles these days as high-risk, potentially high-reward research. Much has been written lately about federal science-funding agencies’ general aversion to supporting high-risk projects. But Heyduk says whether an experiment is risky depends on how you frame the problem. “To go back to the baseball analogy, if you swing in a way that that leads to either a homerun or a strikeout, you’re going to strike out a lot more than you’re going to hit a homer run—that’s not a great way to do research. But you can put together a research program, you can tackle a problem, in a way that you learn things from failing—you can learn why it failed, rather than just that it failed. And if you learn that ‘why,’ then it gives you a data point, or information, to go back and modify your hypothesis, modify your approach for the next time. So you increase your chance the next time of hitting that home run.” When it comes to replacing oil as a major global power source, let’s just hope Heyduk does it in mid-game, rather than the bottom of the ninth.