Energy Unbound
Hydrogen is all around us. But how do we get at it?
If you were a science-fiction writer trying to dream up the perfect fuel, you could hardly do better than hydrogen. High in energy, it produces almost no pollution when burned. It's also the simplest and most abundant of elements, making up 90 percent of all matter. It's present in stars and living things, in fossil fuels and, most tantalizingly of all, in water.
"Water will be the coal of the future," Jules Verne wrote with confidence in 1874. But Verne didn't have to reckon with the challenges of extraction.
Today, almost all hydrogen is produced via steam reforming of natural gas at oil refineries like this Conoco plant near Denver, Colorado.Photo by Warren Gretz (DOE/NREL)
As it happens, hydrogen doesn't like to be alone. Hydrogen molecules—each composed of a pair of atoms, or H2, are always eager to bind with other molecules—with oxygen, with carbon, with almost every element.
Hydrogen may be everywhere, in other words, but it isn't floating free. In order to be used as fuel, hydrogen has to be removed from its attachments. And that removal requires energy.
Cracking hydrocarbons
Today, almost all the world's hydrogen is produced by "reforming" fossil fuels. "You can react any hydrocarbon source with steam and the products are carbon monoxide and hydrogen," explains Harold Schobert, professor of fuel science and director of the Energy Institute at Penn State. "Usually there's an easy way to convert the carbon monoxide to carbon dioxide, and then you can separate that, and you're left with fairly pure hydrogen."
Worldwide, according to the U.S. Department of Energy, some 48 percent of hydrogen is currently produced from natural gas, 30 percent from oil, and 18 percent from coal. The remaining four percent is produced from water.
Fossil-fuel reforming has definite advantages. The process is well-understood and relatively cheap. Yet producing hydogen this way still creates carbon dioxide emissions, contributes to global warming, and does nothing to reduce our dependence on foreign oil.
Even with these drawbacks, many experts suggest that fossil-fuel reforming will be an important transitional step in the long haul toward a hydrogen economy. In the U.S, there is particular interest in coal gasification as an attractive interim option, due to plentiful and relatively inexpensive coal reserves. he carbon dioxide produced by reforming could be piped underground instead of released into the atmosphere, some experts have suggested.
Harold Schobert is not convinced that coal gasification is a good idea even as a short-term strategy. "Don't get me wrong," he says. "I love coal. But when you talk about gasification, plus shifting the gas composition, plus CO2 capture, plus sequestration—all those processes require energy. And if you're going to do carbon sequestration, why stack the deck against yourself by starting with the highest carbon material?"Even natural gas reforming, Schobert suggests, may be a questionable goal. "Here's a material that's already a premium fuel, it's the cleanest burning hydrocarbon there is, it has the highest thermal energy, it's easy to transport and store. Why go to a lot of work to make it into another clean burning high-energy gas?"Maybe it makes more sense," he adds, "just to use natural gas until the technologies for extracting hydrogen from renewable sources are solid enough to take over."
Photovoltaic array used for solar-powered electrolysis to produce hydrogen for fuel-cell buses at SunLine Transit Agency in Thousand Palms, California.Photo by Leslie Eudy (DOE/NREL)
Splitting water
Chief among these renewable sources is plain old water. By passing an electric current through H2O, you can split the aqueous molecule neatly into its constituent elements. Hydrogen gas rises from the negative cathode, and oxygen gas collects at the positive anode. The process is called electrolysis, and it's been around since Jules Verne's time.
Electrolysis produces a very pure form of hydrogen and it's simple enough to be widely adapted. Some futurists envision an electrolysis box in every garage, producing hydrogen from tap water.
The catch is that electrolysis requires electricity. Lots of it. And if that electricity is being produced in the conventional fashion—from fossil fuels—then again we're just running in place in terms of producing clean energy.
Light-scattering layer
We do, of course, have a renewable source for all the electricity we could possibly want: It mounts the sky every morning. Yet whilethere has been considerable progress in solar technology over the last twenty years, researchers still haven't figured out how to harness the sun's power cost-efficiently.
It's easy to see how improving solar cells is a critical step towards an affordable hydrogen economy.What we need," says Tom Mallouk, "is good cheap materials that do what a solar cell already does pretty well."
Mallouk is Dupont professor of materials chemistry and physics, and director of Penn State's Center for Nanoscale Science. As he explains it, today's best solar cells are reasonably efficient, converting sunlight to electricity at a rate of about 25 percent. But those cells are made of single-crystal silicon, the same stuff that goes into computer chips. It's a good photovoltaic material, but it's expensive to grow.
Seeking an alternative, Mallouk has developed an inexpensive material that combines dye molecules with nanoparticles of titanium dioxide ("the stuff that's in white paint, and sunscreen—it's real cheap stuff"). Right now he's getting only about five percent efficiency with this composite, but he thinks it has potential to do significantly better. produce as high as 18 percent efficiency. He hopes to find ways to combine this cheaper material with the more efficient silicon in a composite cell that is both cheap and efficient. To do so, he's working with Greg Barber, an engineer at Penn State's Materials Research Institute who specializes in photovoltaic systems.
Another approach that Mallouk is investigating, with materials scientist Joan Redwing, involves a method for growing nanoscale wires of single-crystal silicon. Strung together in an array, he says, these tiny wires could conceivably accumulate enough voltage from sunlight to split water. "That's at a level of engineering we think we know how to do," he adds. "But first we need to clean up the growth process."
A third avenue that Mallouk and his students have long been pursuing bypasses electricity altogether, creating hydrogen and oxygen directly from water using only sunlight and a photocatalyst.
Call it artificial photosynthesis: You find a compound such as ruthenium trisbipyridine—"Ru-bipy" to chemists—that is energized by sunlight. You suspend it in an aqueous solution, along with a semiconducting material that is doped, or treated, with a catalyst. When sunlight hits, the electrons given off by the Ru-bipy are taken up by the semiconductor and—spurred by the catalyst—they join with hydrogen ions already present in the water to form hydrogen gas. Meanwhile, a second catalyst pulls electrons from adjacent water molecules, generating oxygen molecules and hydrogen ions. The hydrogen ions float off to further the cycle.
For this chain to play out smoothly, everything depends on having the right materials: an initial compound that gives off enough energy, a semiconductor that will hold up under water, and a catalyst that will drive the necessary reaction at a fast enough rate. And, says Mallouk, the catalysts need to be sequestered in such a way as to prevent them from contacting both products of the reaction (hydrogen and oxygen molecules). "A good catalyst for one reaction, such as water splitting, is also a good catalyst for the reverse reaction," he explains. "It is important to prevent that reverse reaction from happening, because it would convert all the stored energy into heat."
So far, Mallouk says, no one has pulled it off. After years of experiment, "We think we have components now that are viable," he adds, "but we haven't actually assembled them."
A set of bio-reactors used for photobiological hydrogen production by the green alga, Chlamydomonas Reinhardtii.Photo by Warren Gretz (DOE/NREL)
The power of green
Hydrogen is also produced by natural photosynthesis. Biologists know that certain species of green algae and photosynthetic bacteria can make hydrogen using sunlight. Engineers are currently working on adapting this process by growing algae in bioreactors. As of yet, though, the conversion efficiency of sunlight to hydrogen is only about one percent. At that rate, the land and water requirements for growing algae would be prohibitive.
A green option that avoids the space problem is fermentation, which works by introducing hydrogen-producing bacteria-like Clostridia into water spiked with organic matter for it to feed on.
A team headed by Bruce Logan, Kappe professor of environmental engineering and director of the Penn State Hydrogen Energy Center, has successfully demonstrated fermentation using wastewater samples from several Pennsylvania food-processing plants. For their catalyst, the Logan team used ordinary garden soil that had been heat-treated to kill all the bacteria it contained except hydrogen-producing spores. When the spores were introduced to the wastewater, they began to grow, feeding on the organic material in the water and producing a biogas of 60 percent hydrogen in the headspace of the test flasks. In a second stage, Logan's students added another type of bacteria to the same water, which generated methane while consuming the leftovers.
"Using this continuous fermentation process, we can strip nearly all of the energy out of the wastewater," said Steven Van Ginkel, the doctoral student who conducted the tests. In addition, the fermentation process acts as wastewater treatment, substantially removing the need for costly aeration, Logan says.
The method holds promise, he notes, but is not yet efficient. Penn State colleagues John Regan in civil engineering and Mark Guiltinan in horticulture have embarked with Logan on a project to genetically engineer a Clostridium acetobutylicum strain that will produce more hydrogen.
Meanwhile, there are still other ways to tap the energy contained in biomass. Wood chips and agricultural waste can be converted via the same gasification techniques used to extract hydrogen from natural gas and coal. These techniques can be adapted to convert biofuels, like alcohol made from corn. Researchers— including Chunshan Song, professor of energy and geo-environmental engineering at Penn State—are working on developing better catalysts for these reactions.
Think global, act local?
"Not all hydrogen will be produced in one way," Logan told the audience gathered at University Park last fall for Hydrogen Day. "It will depend on where you live—how much sun, wind, and biomass are around." Nuclear power will also likely play a role in generating the necessary electricity. "We have to look at all our existing resources," Logan said, "and at what new technologies may emerge along the way."
A 270-kilowatt wind turbine with advanced airfoils.Photo by Warren Gretz (DOE/NREL)
Mallouk chaired a Department of Energy panel last spring that was charged with outlining the technological obstacles to a hydrogen economy. Though he agrees in principal with Logan's assessment, he cautions, "To make hydrogen efficiently on a large scale, to avoid runaway global warming, we will ultimately need solar. Nuclear, biomass, wind, hydroelectric—they don't have enough capacity to make a dent in global energy needs. With solar, there's an unlimited supply."
The real problem, Mallouk says, is cost. And that's a problem that only grows tougher beyond our borders— borders which, when it comes to energy solutions, are increasingly permeable.
"Even if we in the U.S. can learn to sequester carbon, or subsidize solar to keep the environment clean," he says, "there are plenty of developing countries that cannot afford to do this. To make a successful transition, we will need to develop cost-efficient solutions for the rest of the world."
Harold Schobert, Ph.D., is professor of fuel science and director of the Energy Institute, hxs3@psu.edu. Thomas Mallouk, Ph.D., is Dupont professor of materials chemistry and physics, and director of Penn State's Center for Nanoscale Science, tom@chem.psu.edu. Bruce E. Logan, Ph.D. is Kappe professor of environmental engineering and director of the Hydrogen Energy Center, bel3@psu.edu.
