Electricity from a beet
by E. Kirsten Peters | © Washington State University
Chemists around the world are looking to the plant kingdom for ideas about harvesting the energy of sunlight. Plants, after all, have been making a living exploiting sunbeams for almost four billion years. And part of what plants accomplish each day creates a tiny flow of electrons—a form of electricity.
The familiar solar-electric panels on the roofs of RVs depend on
pure silicon crystals, which are produced in an energy-intensive
manufacturing process. The crystals are semiconductors "doped" with
special impurities to make them work—impurities that are often
toxic metals requiring special mining to unearth. These
first-generation panels certainly work, but the electrical power we
can create from them is costly to us and to the environment.
Just a few years ago, hopes were high for second-generation panels made of thin flexible films. These devices now make up roughly 10 percent of the market for solar panels. They are often used for powering satellites because they weigh little. But the cost of second-generation panels has not dropped as engineers and investors alike had hoped, so the price of electricity from them is still far from economic.
Jeanne McHale, a Washington State University chemistry professor, and her team of students hope to better learn how molecules from plants can promote electron flow. If she is successful, a potential third generation of solar panels could be manufactured in wholly new ways and at vastly lower cost.
McHale's professional interest lies in fundamental science. "In physical chemistry, we don't usually produce 'deliverables,'" she says with a laugh. Her research includes manipulating materials on the nano-scale and studying them via advanced spectroscopy. But the work always begins quite simply—by going to the grocery store and buying beets.
The red color of beets comes from a molecule called betanin, a natural anti-oxidant. Anti-oxidants, as the name implies, moderate cellular processes of oxidation, or the loss of electrons. Betanin is also a natural dye that interacts with light. Dyes are strongly colored substances, the color stemming from which part of sunlight's spectrum they reflect while absorbing all the other wavelengths. Betanin isn't directly a part of beet photosynthesis; in fact, its function is to protect cells from too much light, like a natural sunscreen. But betanin does react to the photons of sunlight, the keystone to any hopes for solar power.
The fundamental trick for "dye-sensitive" third-generation solar panels is to get the dye molecules to give up an electron when they are hit by sunlight and then not instantly take it back, as they are wont to do. If they give without taking back, the dyes produce a tiny but steady stream of electrons.
It might be surprising that a chemical reaction in a beet can create a tiny electrical current. But, of course, it's a chemical reaction in the battery of a car that creates the current that flows to the car's starter motor. Chemical reactions are simply the rearrangement of chemical bonds, and bonds are formed by electrons. What's different about the dye-sensitive case is that the electron flow is powered by incoming photons of sunlight.
McHale and her team take the betanin from beets and put it onto a layer of a mineral called anatase. Nano-particles of anatase create a good surface to which the betanin can cling, and the anatase also acts as a semiconductor. The anatase sits on a conducting layer below.
When the photons from sunlight hit this layered sandwich, a little bit of net electron flow results. Even a small amount of electron flow is fine, because with manufacturing materials as cheap as beets and minerals, society can afford a great number of solar panels. Indeed, if this route to the third generation works out, making electricity via betanin could be cheaper than burning coal.
"And if dye-sensitized panels were 10 percent efficient, as we think reasonable, you could generate all the electricity the U.S. uses each year with about 2 percent of our land devoted to such panels," McHale says.
But there's a long row to hoe in the beet field before third-generation panels are covering roofs near you.
Complex organic molecules don't typically last for a long time if they are taken out of their natural realm, and the betanin in beets is no exception. So, for the moment, the beet-powered solar cells are up and running for only a few hours before they peter out.
"One of the challenges is to understand what's going on. We want the chemistry to be regenerative," McHale says.
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