Solar panels and wind turbines are a big part of our energy future but, to a growing number of researchers, these devices are only transition technologies.
These scientists hope to make energy directly from sunlight, water and air, the way plants do — what they call “solar fuels” — and they’re steadily coming closer to succeeding.
If they can, and they believe they will, fuel refineries and chemical plants will dispense with petroleum feedstocks. Instead, these factories will extract carbon dioxide from air, combine it with selected catalysts — or even living organisms — in the presence of sunlight and make methanol, acetate, hydrogen for fuel, electricity and other essentials.
It’s the ultimate way to recycle the world’s most pervasive greenhouse gas.
HOW PLANTS DO IT
Photosynthesis in plants is a combination of two reactions akin to breathing in animals.
The first — the “photo” part of the reaction — begins when light strikes a molecule of chlorophyll, the pigment that makes leaves green. The chlorophyll absorbs energy from the light and uses it to break water molecules into protons, electrons and oxygen, the latter being “exhaled” back out into the air.
These loose electrical charges set off a cascade of chemical reactions that result in the “synthesis” portion of the process: specialized cells use carbon dioxide from air and the chemical energy from these reactions to make glucose, the basic sugar that living things metabolize to carry out the processes of life.
In artificial photosynthesis, scientists want to replace chlorophyll and its catalogue of attendant chemicals and reactions with their own: simple catalysts that draw energy from sunshine and react with water and carbon dioxide to make commercial products. By designing the right combinations of catalysts, engineers can choose the products that result. Other researchers are using algae and bacteria instead of chemical catalysts, and a venturesome few are hoping to replicate plants’ own method -— dispensing with any intermediary bugs or chemicals.
SPLITTING WATER MAKES IT HAPPEN
A key goal of artificial photosynthesis is to break up water molecules — H2O — to harvest hydrogen for use as fuel. One way to go about it is with a photoelectrochemical cell, or PEC.
In a photovoltaic cell, sunlight falls on a silicon anode, or negative terminal. When sunlight strikes silicon, electrons in the silicon atoms are “excited” and begin to move, setting up an electric current. In a PEC, this current flows through wires to a cathode, or positive terminal, often made of a metallic catalyst that sparks the formation of hydrogen or carbon-based compounds. The electrons that moved out of the anode are recovered when the water molecules in the solution are broken apart into oxygen — which is “exhaled” back into the air — electrons, which return to the anode, and protons that form a pair of hydrogen atoms bonded together at the cathode. When hydrogen is burned for fuel, the chemical bonds holding the atoms together break and release the energy that held the atoms together.
That’s the point of it all: this side of nuclear processes, chemical bonds are the most energy-dense medium for storing power.
The problem with PECs is that they don’t last very long. Their materials are easily corroded either by the cells’ electrolyte solutions or the photochemical process, making them largely inefficient. To be commercially viable, PECs will need to surpass 10 percent efficiency, often the yardstick for market readiness. Chemists and materials engineers now are also working to reach the 10,000-hour service life that the US Department of Energy has mandated.
Other scientists are following a more promising idea – using dyes to start electrons moving. The work arose from knowledge that certain dyes improve the electrical performance of transistors.
At the University of North Carolina’s Energy Frontier Research Center, funded by the US Department of Energy’s Basic Energy Sciences Division, scientists design variations on the molecular structure of these dyes. Light travels through a clear material that is able to conduct electricity. Dyes, bound to catalytic molecules, are affixed to a thin semiconducting surface that is layered on top of a thicker conducting or semiconducting material. When light strikes the dyes, their electrons get excited and inject themselves into the thin layer of material and are ferried through the anode. Depriving the dye of an electron creates a positive charge that the dye neutralizes by stealing an electron from the catalyst. Repeating the process three more times builds up the right number of positive charges on the catalyst to split water. The oxygen bubbles off and protons pass through a membrane where they join the electrons at the cathode to form hydrogen.
The team already has solved two key problems. First, early versions of the cell had too thick a layer of the semiconducting material and the excited electrons would move backwards or get stuck on the surface instead of making their way through the anode. Using a method called “atomic layer deposition”, the group shaved the semiconducting layer from a thickness of 100,000 nanometers to three. The result: more electrons got to the conducting material and faster.
Second, the reaction works best in a neutral or alkaline solution but the dye molecules don’t stick to the surface unless the environment is acidic. Through atomic layer deposition, the researchers laid a thin layer of protective material over the dye molecules, allowing them to stay attached on the surface. This enables them to operate at higher pH, thereby boosting the rate of water-splitting by literally a million times.
Challenges remain, though. Demonstration cells still use rare earth metals, although researchers are sifting through families of organic materials, to find cheap, environmentally benign replacements. The cells’ efficiencies are stuck in the low single digits and it could be a decade before that figure crosses the crucial 10 percent threshold to commercialization.
Potential corporate partners are beginning to nibble but, as one chemist points out, “this is still a fundamental research project.”
Why be content with hydrogen alone?
Northwestern University’s Solar Fuels Institute has developed a process using water, carbon dioxide and catalysts to make methanol, a hydrocarbon that many vehicles can burn like gasoline.
In similar work, Peidong Yang and his colleagues at
Their technique makes use of two things not found in plants:
» First, sunlight falls on a “forest” of nanoscale wires made of titanium oxide and silicon. Each material absorbs a different portion of the light spectrum, generating a flow of electrons.
» Second, this microscopic forest is home to sporomusa ovata bacteria, which absorb the electrons directly from their environment and use the energy to make and excrete acetate. Nearby, select strains of E. coli bacteria can be encamped to digest the acetate and produce a range of products, including butanol (chemically similar to gasoline), a basic element for making an antimalarial drug, and PHB, a biodegradable plastic. The conversion efficiencies in tests range from 25 percent to more than 50 percent.
The system now converts sunlight to energy at 3 percent efficiency. Yang thinks that when the technique can yield 10 percent efficiency, it will be commercial — he hopes that’s within five years.
New solar cells
A key step in photosynthesis is moving electrons from one place to another, which is the definition of an electric current. This hasn’t been lost on the handful of scientists now copying plant biochemistry to create a new family of photovoltaic cells.
The leader is Swiss chemist Michael Gratzel, who launched the research in the late 1980s. At the time, people knew that when sunlight lands on silicon, the silicon absorbs the light’s energy and that generates a current. But for the process to work, the silicon had to be almost impossibly pure and the technique was inefficient besides.
Gratzel and his colleagues mimicked plants by putting dyes into solar cells, which absorb light’s energy rapidly and move it along just as chlorophyll in leaves does. In a typical version of these “dye-sensitized solar cells,” the pigments shuttle electrons to a layer of titanium dioxide that acts as the negative battery terminal, moving cells along to the positive terminal. Then, a chemical electrolyte returns the electrons to the dye to keep the process going.
Different colors of dye catch different parts of sunlight’s spectrum, but the dyes themselves are micro-thin layers, leaving the cells translucent or even almost transparent. Within a decade, we could be looking through windows that are making electricity without us being able to tell.
The Japanese company Fujikura is already mass-producing “Gratzel cells” (they look like regular solar panels) that achieve 18 percent efficiency — close to the best of today’s conventional solar cells — even with LEDs or lamplight. The manufactured cost is reportedly less than 4 cents per watt, below that of most of today’s conventional solar cells. With researchers working to substitute organic and even simple vegetable dyes for pure elements or manufactured compounds in the cells, this new family of solar-energy devices could take market share shortly after 2020.