Record droughts in the U.S. have sparked new interest in using this potentially renewable resource to power the nation's transportation. But creating what is, in effect, a bionic leaf poses formidable challenges.
After all, when it comes to photosynthesis, plants have had eons to perfect their technique. They’ve evolved to gather energy from the sun and used it to reshuffle the molecules in water and carbon dioxide to create fuel in the form of a sugar molecule, or carbohydrate. The holy grail of artificial photosynthesis is to turn the same three ingredients that plants use -- sunlight, water, and carbon dioxide -- directly into a cost-effective transportation fuel.
But though natural photosynthesis serves as the original inspiration, researchers working on an artificial version can't simply mimic plants to create a prototype.
"It's the difference between birds and airplanes," says Tanja Cuk, PhD, an assistant professor of chemistry at the University of California at Berkeley, who is conducting basic research into artificial photosynthesis. "They both fly. But airplanes don't work simply by mimicking the movements of birds."
For all its challenges, the benefits of artificial photosynthesis, if realized, would be enormous.
As a renewable energy, artificial photosynthesis could offer several key advantages over other technologies. Unlike biofuels, artificial photosynthesis doesn't require arable land, so it wouldn’t compete with food crops -- a crucial consideration as the world's population grows and the pressure on water resources intensifies.
Even an artificial photosynthesis system with relatively modest solar conversion efficiency could provide energy for all the nation's transportation needs using an area of non-arable land roughly equal to that currently used by the country's interstate highway system, according to Heinz Frei, PhD, a senior scientist at Lawrence Berkeley National Labs. Although the process requires water, it gives off equal amounts of water, so it is entirely renewable.
“While water is an essential reactant, it is not needed at high concentrations,” says Frei, who has been involved in solar photochemistry and artificial photosynthesis research for several decades. “It could be in the form of water vapor no higher in concentration than in typical air. Also, water is regenerated when the fuel is consumed, so there is no net consumption of water.”
Solar fuel vs electric cars
Photovoltaic cells can already convert sunlight into electricity, but using electricity to run vehicles requires a new infrastructure of vehicles and charging stations. Also, batteries as a source of stored electricity are not suitable for powering airplanes, ships, or heavy trucks. Artificial photosynthesis, in contrast, can produce a storable and stable fuel that could theoretically be used for transportation using the existing infrastructure of airplanes, cars, trucks, and filling stations.
The problem is producing it in a way that is scalable, with components that can be manufactured using affordable and widely-available materials. In addition, the devices have to be capable of generating fuel on the scale needed for transportation.
Experts in the field acknowledge that we are still years, even decades away from filling our gas tanks with solar fuel. The LBNL’s Frei also directs the Joint Center for Artificial Photosynthesis’s (JCAP) science-based scale-up efforts. The goal of JCAP is to have a scalable working prototype within five to 10 years, but developing systems to produce the most desirable solar fuel for pipe distribution will require more time, he says. According to some skeptics, cost-effective solar fuels may not be ready for several decades.
To be economically viable, artificial photosynthesis must be far more efficient at using sunlight to create fuel than plants. (This may not be that hard a step, since some artificial photosynthesis projects can already produce fuel from sunlight up to ten times more efficiently than plants.) But the technology, composed of non-biological materials, must also be durable enough to work for years under the glaring sun with minimal maintenance. And it must be cost-effective to manufacture on a large scale.
Nanotechnology front and center
Scientists have been pursuing the dream of artificial photosynthesis since the 1960s. The first proof of concept came in the late 1990s, when researchers at the National Renewable Energy Laboratory demonstrated the world's first integrated device that converted sunlight into fuel.
That breakthrough demonstrated that artificial photosynthesis was possible--but not yet practical. The first prototype used rare earth materials and methods for manufacturing on the computer chip scale, far from the scale needed to produce transportation fuel on a national or even global scale. What's more, its components disintegrated within hours.
Since then, advances on several fronts have moved the dream of artificial photosynthesis closer to reality, says LBNL’s Frei.
"First, we've seen tremendous progress in understanding natural photosynthesis. Not to mimic Nature, but to take advantage of its design principles. At the same time, the explosive growth of nanotechnologies starting in the mid-1990s has provided essential new tools. The natural process of photosynthesis is controlled on a nanometer scale," explains Frei. "For the first time, nanotechnology allows us to engineer, control and manipulate the process of artificial photosynthesis at this critical length scale."
Recognizing that the time is ripe for progress in the field, the U.S. Department of Energy established the Joint Center for Artificial Photosynthesis in 2010. The Center, an energy innovation hub, brings together scientists from California Institute of Technology and Lawrence Berkeley National Laboratory. Its mission: to develop working prototypes that use widely-available materials that can be scaled up to generate large amounts of fuel from sunlight efficiently and cost-effectively. In October 2012, LBNL broke ground for a new bricks-and-mortar home for Joint Center’s work, the Solar Energy Research Center building, which is scheduled to be completed in late 2014.
In one of eight scientific and engineering projects now underway, JCAP researchers are currently testing a prototype device, about the size of a laptop computer, which represents a crucial leap over the first proof of concept. "If you look under the hood, the control is entirely on the molecular and nanoscale," says Frei.
High hopes, enormous challenges
But while the device efficiently converts sunlight and water into the components necessary for making fuel, it’s still far from economically viable.
The components of an artificial photosynthesis system are fairly basic. The process requires a photovoltaic material that absorbs light energy from the sun. This energy must then be directed to two separate catalysts--one that splits water into protons and oxygen and another that converts carbon-dioxide and protons into hydrocarbons. Because the two catalytic processes compete for electrical charges, the system requires a membrane to separate the two chemical reactions. Initial targets are basic fuels such as methane or methanol, which can be used to replace fossil fuels.
Achieving all that is currently possible -- using layered photovoltaics and expensive, rare materials such as iridium or platinum as catalysts. "Now the challenge is to create little photovoltaics connected to little catalysts on a nanoscale, and to use materials that are abundant and scalable," says Cuk.
In the laboratory where Cuk works at the University of California, behind drapes of heavy black curtains used to protect scientists' eyes, lasers shoot high-energy beams of light through intricate mazes of instruments. These detect precisely how chemical bonds in water molecules change on the surfaces of different catalysts. Insights from this kind of basic research, Cuk believes, will be crucial to solving these last remaining challenges.
"I like to use the analogy of the transistor," she explains. "We were able to go from the original vacuum tubes to millions of tiny transistors in a single integrated circuit because we had a good basic understanding of the principles at work. We may not need as new a principle as the p-n junction was to the vacuum tube. But the insights we can get from basic research into how highly active catalysts work will help lead us to better nanostructure solutions for photosynthesis."
At the Caltech site of the hub, for example, scientists have come up with a process to develop millions of different variations of possible catalysts almost simultaneously – each sample of which is as tiny as a pixel on a screen. Rather than a few discoveries of new catalysts a year, researchers can now have new candidates every few milliseconds.
Cuk, who received her PhD in physics, shifted her focus to artificial photosynthesis because of its enormous promise as a renewable energy source. "I wanted to be involved in research that could make a real difference in the world," she says. As a Miller Research fellow at Berkeley, she worked closely with Frei, who she regards as a mentor.
Today, young scientists like Cuk inspire Frei to hope that, after decades of slow but steady progress, the development of artificial photosynthesis is poised to shift into high gear. "That's what makes this field so exciting," he says. "The Joint Center for Artificial Photosynthesis comes at a moment when we can start putting things together to see how poorly they work. That's the way you improve and get to a viable technology -- by seeing what you need to solve. Fortunately, we have young scientists who are looking for those solutions, exploring fresh ideas that will hopefully lead to new designs that haven't even thought about."