Biologically Optimized Protein Films for Solar Energy Conversion.
David Cliffel, an electrochemist, and his colleagues are taking the proteins that plants use for photosynthesis and modifying them to convert light into electrical power. It’s every bit as science fiction-like as it sounds. The most innovative aspect of this Scialog-funded research, Cliffel says, is developing “the ability to interface proteins within electronic and electrochemical devices.” Many researchers today are exploring ways to grow plant material—biomass, they call it—harvest it, ferment it, and then extract the fuels from that process. But Cliffel’s thought is that all of the important work of the plant, as far as making energy, is really trapped in two major protein complexes – called photosystem I and photosystem II. “And so, everything the plant does after these two processes is really a loss,” he says. “We’re just looking for a way to short step a lot of extra work and gain a lot of efficiency by learning what nature has figured out how to do so well.” His research involves extracting the proteins from spinach. “And then once we get them purified, we characterize them to make sure that they’re still active. In the past we’ve targeted photosystem I, and we’re starting to extend our work to photosystem II.” The two systems are part of the same energy-generating membrane, Cliffel says. “The thylakoidmembrane in plant cells is a sub-cellular membrane that really does photosynthesis.” (The two systems are somewhat misnamed, if you’re looking at the flow of electrons in the photosynthetic process: photosystem I was discovered first, hence the name.) “But photosystem II is the protein that uses light to make oxygen. “It basically takes water and turns it back into oxygen,” Cliffel says. “Then the photosystem I process takes that energy and makes sugars and everything else for the plant.” He says each system effectively generates about a volt-and-a-half of energy. “And that electrical energy is then transformed chemically into either splitting water to make oxygen, or into powering enzymes that do the work in the plant, making new molecules to let the plant grow.” Cliffel and his associates have built some prototype devices using photosystem I. “They’re relatively low-power, but they really do use the native plant protein to convert light into electrical energy that we can store in an electro-chemical battery.” The work has been described as leading to something akin to an artificial leaf. “This basically just uses spinach leaves. It should, by its very definition, be biodegradable, and that may have a lot of benefits,” Cliffel says. “For example, it’s likely nontoxic, as people have been eating spinach for quite some time. And so we’re also trying to figure out what some important applications for a biodegradable solar cell might be.” While this approach is unlikely to produce as much power as conventional photovoltaics, it might be appropriate for environmental monitors scientists want to power remotely for a while without having to go back and collect them all. And while Cliffel admits that today’s widely used solar cells made of silicon don’t present much of an environmental threat, “a lot of the new materials often involve heavy metal semiconductors,” which one day could raise some issues. The field of protein electrochemistry is relatively uncharted territory at this point, so any well-thought-out experiment that Cliffel and his group performs is likely to lay some important groundwork for future bio-nanotechnology interfaces. The really tricky part of his research so far has been interfacing the proteins with mechanical devices. ”That’s one of the real challenges, to be honest,” he says. “The plant, of course, orients the proteins perfectly in its membrane,” he says. “But in our case, after you extract it, how do you really wire up something like a protein at the nanometer scale?” These protein molecules are about seven nanometers wide (the head of a pin is about one million nanometers across; a human hair is about 60,000 nanometers in diameter). About a year ago Cliffel and his group published a paper about their experience with gold leaf—“the stuff you buy at the art supply store for decorating picture frames,” he says. “It turns out that if you dissolve that in acid, you end up getting a nanostructured porous gold material, and we’ve used that with a fair degree of success, trapping the protein within it.” At three or four dollars a square foot, gold leaf is a fairly affordable substrate, Cliffel says. “The plant does its interfacing by using small proteins that come and dock into the large proteins and carry away the electrons. We’d rather get away from having to do everything that the plant does so well. We’d rather that everything be more like solid-state, in a more electronic-based device.” Coming up with a good electronic support system would enable a protein-based solar conversion device to produce higher current densities, he notes, adding, “The plant produces electrochemical energy and uses it to do chemistry, but if we really want to use that energy for power, we need a lot more electrical current. So we aim to pack these proteins much more tightly than what’s found in a conventional leaf.” To get technical about it, a plant doesn’t really produce electricity, nor does the human body for that matter. “Your car battery doesn’t have any electricity in it, either,” Cliffel says. “It just has electrochemical gradients that, when you ask for it, will turn into electricity.” In essence, then, Cliffel and his group are working at the boundary between turning chemical energy into electrical energy and vice versa. Their initial focus on photosystem I was due in part to the fact that it’s the more stable of the two protein systems. Photosystem II is more difficult to work with, Cliffel says, because it’s a lot more chemically reactive. The protein itself is less stable, more prone to denaturing. And the energetics of the oxygen reaction it creates are very tricky to control. But Cliffel says there could be big advantages to working both protein systems into an electronic device: “Photosystem II is geared to be in series with photosystem I, from an electrical point of view. So a plant ends up getting, effectively, more than two volts of energy by having two photons come in, one stimulating photosystem I to give it the first volt, and then photosystem II actually gives the second volt – even more than that, actually—and it’s tuned so that those are directly additive—they are, effectively, electrically in series.” The other important thing to keep in mind about photosystem II, as Cliffel points out, is that it’s one of the best known oxygen-producing catalysts. “And so in a world where CO2 levels are probably going to be too high going forward, artificial devices that use sunlight to more efficiently transform CO2 and water back into oxygen and fuels could be valuable.” In other words, one day spinach protein might be involved in the battle against greenhouse gasses. Popeye would’ve been proud.