Winter 2011

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Something New Under the Sun

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For a dream team of northwestern researchers, the sun provides powerful answers to the world’s looming energy issues.

by Sean Hargadon

When Tobin Marks thinks about the future of renewable energy, just one word comes to mind — plastics.

Well, plastic solar cells, actually.

He imagines reams and reams of photovoltaic solar cells — “thousands of miles of them” — printed on large, flexible sheets, rolling off of industrial-size presses around the world. These photovoltaic cells, made by printing semiconducting polymers onto plastic, can turn radiant light into electricity.

“You could easily put them on the roof of your home, or maybe you could manufacture roofing shingles with the solar cell already built into them,” Marks, a Northwestern chemistry and materials science and engineering professor, says excitedly. “Imagine in the Third World, where a plastic solar cell could be made in a village printing plant with the right kinds of inks, and it could be used to cover people’s roofs.”

That’s one of the science fiction–like solutions being imagined at the Argonne-Northwestern Solar Energy Research Center (ANSER), a Department of Energy–funded collaboration that brings together an interdisciplinary dream team of chemists, materials scientists, engineers and nanotechnologists to explore solutions to the world’s energy crisis, “the grand challenge for our time,” says Michael Pellin (WCAS74), deputy director of ANSER and a chemist at Argonne National Laboratory.

Population growth and development around the globe have resulted in an insatiable demand for energy. Experts say that by the time the population tops 9 billion in 2050, we’re going to need twice as much energy as we’re using today and about triple that much by 2100. The question is, where will all that energy come from?

The mature energy technologies — nuclear and hydroelectric power and, of course, fossil fuels — all have limitations.

The disaster at the Fukushima Daiichi reactor following the earthquake in Japan last spring highlighted the safety issues with nuclear power.

Hydropower “is maxed out, and it wreaks havoc on the ecology of any place where you install it,” says Northwestern chemistry professor Joseph Hupp. Prime example: “Salmon can’t leap an 800-foot dam.”

The supply of oil and coal, which provide nearly 80 percent of the world’s energy but are increasingly

difficult and costly to find and extract, will last for, perhaps, a few hundred more years. And, of course, these fossil fuels are implicated in climate change — an issue “the scientific community is pretty convinced we need to deal with now or it may be, ultimately, too late,” says Northwestern chemistry professor and ANSER director Michael Wasielewski.

By most accounts we would begin to address climate change concerns if we could get at least 25 percent of our energy from renewable sources in the next 25 years, Wasielewski says. (Renewable energy, including hydropower, accounted for about 10 percent of energy produced in 2009). “I think that’s a realistic goal,” he adds. “That assumes that we’ll have the political will to recognize that climate change is a reality.”

Among renewables, wind power has seen incredible growth in the past decade and has even greater potential if developers can overcome technological limitations, practical barriers and environmental concerns, including inefficient storage and harm to birds and other wildlife.

Solar energy — presently a fringe contributor to the energy portfolio, even among renewables — is relatively efficient, but silicon solar cells, the most common photovoltaic cells, are fragile, difficult to manufacture and expensive to install.

The technology works, but “the fundamental problem for those materials is that you have to take sand —silicon oxide — and make it into silicon crystals. That’s expensive in terms of energy, so expensive that it’s hard to imagine you can ever get that investment in energy back,” says Pellin, director of the materials science division at Argonne National Laboratory.

Yet solar energy is “the ultimate solution,” says Wasielewski, “with more than enough capacity to meet our needs beyond our wildest imaginations,” if we can only find a way to harness its abundant potential.

If we could capture all the sunlight that hits the surface of the Earth for one hour, we would have enough energy for the entire global population for a whole year. “Those numbers are flabbergasting, they’re absolutely incredible,” says Torsten Fiebig, director of operations at ANSER. “You can’t make such a comparison with any other source.”

Drawing inspiration from photosynthesis and mimicking natural systems, the ANSER researchers are dreaming up solutions for solar fuels and inexpensive plastic solar cells made from Earth-abundant materials.

“The stakes are very high,” says Fiebig, a research associate professor of chemistry at Northwestern. “You have to think about solving some of these problems now and not procrastinate, because it takes so long to translate the fundamental science into a product. We’re looking at a timeframe of 15 to 20 years before we can implement these solutions.”

Mimicking Nature

For a billion years plants have been splitting water to produce oxygen and protons to power their metabolism. That’s the inspiration for solar fuels.

“In terms of the laws of the universe, it’s completely plausible to use sunlight to make water into oxygen and hydrogen,” says Hupp, the Charles E. and Emma H. Morrison Professor of Chemistry and a member of the ANSER team. “But it’s a wickedly difficult problem. I don’t think anyone would work on it except that plants have already proven that it can be done.”

Fill UpThe general idea is to use energy from sunlight to split water. A separate catalyst would then convert the protons into hydrogen gas. The hydrogen could then be combined with carbon monoxide, which could be produced by a solar-driven catalyst from carbon dioxide captured from fossil fuel–based power-generating facilities, to create a liquid hydrocarbon fuel, “sort of solar gasoline, as it were,” says Wasielewski, the Clare Hamilton Hall Professor of Chemistry. “That way you can use today’s infrastructure and nothing changes, but you have a renewable source, which essentially recycles the carbon that you’re putting into the atmosphere by burning fossil fuels.”

There’s plenty of carbon dioxide to work with. Carbon dioxide emissions from the consumption of coal alone topped 13.4 billion metric tons in 2009. The Intergovernmental Panel on Climate Change estimates that carbon capture and sequestration technologies could reduce carbon dioxide emissions from a conventional power plant by as much as 90 percent.

The conceptual idea for solar fuels is to study how nature does certain photochemical conversions and to use those concepts as a template for an artificial process, using nanotechnology as a tool. Wasielewski, a physical organic chemist, has studied photosynthesis for more than 35 years, unraveling the mysteries of this process in “exquisite molecular detail.” It’s satisfying, he says, to mimic the process in meaningful ways with nonbiological systems that use the same concepts but are much more robust.

Wasielewski and his colleagues hope to design an integrated system that could couple the catalysts that split the water and do the proton reduction chemistry and combine them with molecules that actually capture the light and generate the charges to drive the catalysts. “It’s an important problem because frequently the two communities develop good systems on their own, but when you try to couple them, to use the light-driven process to generate the fuels, they don’t work well together. This kind of system integration approach is really what’s needed to develop a meaningful working solar fuels system.”

Hupp is also taking cues from nature in his work to overcome bottlenecks on dye-sensitized solar cells (DSSCs), a possible alternative to silicon-based solar cells due to their inexpensive processing costs, ease of manufacturing and respectable energy efficiency. These photovoltaic cells work when light-harvesting dye -molecules absorb energy from sunlight and pass it onto nano-structured titanium dioxide, where the electrons conduct around a circuit.

Venessa Williams

Vennesa Williams, a fourth-year graduate student advised by Northwestern chemistry professor Joe Hupp and ANSER deputy director Michael Pellin, examines dye-sensitized solar cells.

Hupp hopes to improve light harvesting, electron transfer and the overall longevity and durability of DSSCs. The best dye-based photovoltaics can garner 12 percent efficiency but last only 18 months.

“Our approach is to hijack the first stages of photosynthesis, but in principle we should be able to do better,” says Hupp. “That sounds kind of audacious, but our only goal is to hijack the first couple of steps, which are electron transfer, to make sunlight into electricity. We don’t have to optimize the system for everything else, and we don’t have to keep an organism alive. So, it’s not crazy to think that we could outperform Mother Nature and her billions of years of evolution if we’re only picking one part of the system to optimize.”

Hupp says the DSSCs could break through in niche markets in a year or two with some minor advances. Some companies, such as Massachusetts-based Konarka, are already seeking to adapt the technology to produce novelty items such as jackets with flexible solar cells to power personal electronics. Hupp says broader commercial electricity applications for DSSCs are up to a decade away.

Tiny Pores Have A Big Job

One big problem for solar fuels is storage. To be used as a fuel, hydrogen must be compressed and stored under extremely high pressure, a process that uses energy and has explosive implications. “You can’t keep a weather balloon full of hydrogen in your garage and then go out and light up a cigar,” Hupp says. “You’d blow up the neighborhood. That’s not state of the art for storage, but we’re not far from that.”

Hupp’s colleague Omar Farha, a research assistant professor of chemistry, is developing a solar fuel storage solution that uses sticky, sponge-like metal-organic frameworks (MOFs) — materials with an incredible amount of surface area — to soak up hydrogen.

MOFs, which also have potential applications in catalysis, sensing and carbon dioxide sequestration, can hold gases in nanoscopic pores. So a fuel tank can hold more hydrogen, perhaps 10 times as much of the gas, at a much lower, much safer pressure. “We’re making materials that have porosity, large cavities where those gases can go and hide,” says Farha. “We’re asking, ‘What can we do inside these materials, how can we decorate these materials, to make them even stickier?’”

Farha, a materials and inorganic chemist, in collaboration with Randall Snurr, professor of chemical and biological engineering, designed and created NU-100, the MOF that holds the National Renewable Energy Laboratory–verified record for the highest hydrogen uptake. If you take one gram of that material — with its innumerable nanoscale pores and cavities — and unfold it, Farha says, the surface area could cover an entire soccer field.

VIDEO: Created for the 2011 National Science Foundation's International Science & Engineering Visualization Challenge, this video shows how self-assembled porous materials can lead to fuel tanks that are able to highly concentrate gaseous fuels and thus hold a lot of energy in a small space. Cutting edge algorithms and materials are depicted, drawing from research and development in the previous two years at Northwestern University in the Snurr and Hupp laboratories.

Solar Cells Go Organic

The current solar photovoltaic systems — installed small scale on rooftops or on the utility scale in large sun-drenched expanses — are increasingly efficient but also expensive to manufacture and install.

Plus, “the silicon solar cell technology is old, which is not bad, but it’s as good as it’s ever going to get,” Hupp says. “There’s essentially no room left to improve.”

Other photovoltaic systems use newer materials that have lower costs and simpler production processes, but those systems rely on rare, and in some cases toxic, materials.

Flexible solar cells, created from inexpensive, nontoxic, Earth-abundant materials (elements that are plentiful, because limited supplies might prevent a technology from coming into widespread use) and printed with inks on long sheets of plastic, are a growing focus at the Argonne-Northwestern Solar Energy Research Center.

Silicon solar cells generally get 14 to 18 percent efficiency — a measure of the amount of the sunlight striking the cell that is turned into electricity. Other lower-price inorganics garner 8 to 10 percent efficiency.

“Plastic ones will be in that ballpark for efficiency, and the projections are that they’ll be a lot cheaper and nontoxic,” says Tobin Marks, the Vladimir N. Ipatieff Professor of Catalytic Chemistry and professor of materials science and engineering (see "Molecule Master," summer 2004).

Solar HarvestThe cost of plastic organic solar cells — a so-called third generation solar technology — could be as low as one-tenth the cost of a silicon solar cell to produce the same amount of energy, says Marks, whose research group, in collaboration with researchers from the University of Chicago, recently set new records for efficiency for organic photovoltaics (called “organic” because the polymers are carbon based, unlike traditional solar cells that rely on silicon, an inorganic semiconductor).

“All of a sudden, people are realizing there’s going to be an industry, there’s going to be a technology,” says Marks, who became interested in organic solar cells while working on organic light emitting diodes (OLEDs)— an increasingly popular material for displays for cellphones, laptops, tablets and other electronics.

OLEDs and solar cells “are structurally quite similar, and we thought, ‘If our molecular engineering works for OLEDs, let’s try them on solar cells. … Why hasn’t anyone else thought of this?’ And it worked.”

In research related to OLEDs, Marks studies printable electronics. These new organic materials can be dissolved in a solvent to produce an “ink” that allows electronic devices, such as transistors or solar cells, to be printed using ink-jet printing or other roll-to-roll printing processes. The photo-reactive, semiconducting polymers are coated onto a plastic substrate, similar to the way newspapers are printed on large rolls of paper.

In 2005–06 Marks launched the startup Polyera with one of his former postdoctoral students, Antonio Facchetti, an adjunct professor of chemistry. With 35 employees in nearby Skokie, Ill., Polyera creates customized ink packages for electronics manufacturers for products such as foldable e-readers — think collapsible Kindles — or radio-frequency identification tags, an I-PASS–like super-barcode that can function much like an electronic toll collection transponder for grocery store operators, for example, providing detailed information on a product’s price, date and location of manufacture, time on the shelf and other inventory information and allow cashiers to scan an entire cart at once, rather than item by item.  

The solar technology company Konarka converted an old Polaroid film plant to make solar cells using organic inks. One of Konarka’s early products is a briefcase with a flexible solar cell to recharge a cellphone or a laptop. Konarka also put plastic solar cells on the rooftops of San Francisco transit stops to light the stop and power a display with route information.

“It’s moving very fast,” Marks says. “I’m very optimistic that in 20, 30, 40 years, people are going to look back at how we generate electricity now and wonder, ‘Why didn’t they wake up and see there were better ways to do it, cleaner ways, more environmentally acceptable ways?’”

Off Into the Sunset

Solar energy first came to the fore during the oil crisis in the early 1970s, but the solar movement has suffered from sporadic effort. “It’s been fits and starts for the last 40-some years,” says Wasielewski. “We’re always in crisis mode, always responding instead of making steady progress.”

Sustained research and development and engineering are going to be required along with continued federal support and investment from the private sector, including the energy industry, to make solar energy a success, Wasielewski says. With belt-tightening in Washington, D.C., and the toxic political environment on Capitol Hill, federal funds for solar energy might be drying up, though it’s not the time to be cutting back on solar energy research, the ANSER director adds.

“Hopefully the legs won’t be cut out from under these efforts,” says Wasielewski. “It’s kind of scary. We’re at the crest of the wave of federal funding that came about three years ago. Hopefully it’s not all downhill from here.”

Wasielewski thinks progress will require an infusion of private capital. In that effort, Northwestern is establishing the Solar Fuels Institute (SOFI), a privately funded global consortium of research organizations, created to translate basic science research into functional solutions by engaging industry and private enterprise to push new technologies into the mainstream. SOFI will likely be in operation in the next six months.

“I am optimistic,” Wasielewski says. “The developments, especially over the last five years, have been spectacular, and it looks like the forces have been marshaled to actually get the job done.”

His Argonne counterpart, Pellin, agrees. “ANSER has made discoveries about how several different photovoltaics work and how catalysts can work on solar fuels. Those discoveries give you confidence that, going forward, discoveries will continue to happen,” he says. “With science work, you turn over a stone, you learn something, and that leads you to the next step. That’s the process we’re in now.”