Photosynthesis is the process by which plants use the energy of the sun to convert carbon dioxide and water into carbohydrates and oxygen. Carbohydrates are built of carbon, hydrogen and oxygen and they are, of course, food. The goal of artificial photosynthesis is not only to build devices and systems to mimic biological photosynthesis, but to arrange for the end product to be manna suited to satiate the civilization’s hunger for energy rather than its need for food.
Artificial photosynthesis has the benefit of being carbon-neutral, because using the fuel that was obtained by the process returns the same amount of carbon back into the environment taken out during synthesis. Because of this, aside from the generation of energy, it has often been proposed that this process will be vital in removing excess carbon dioxide from the atmosphere.
Artificial Leafs, Bionic Leafs
Conventional solar energy employs a solar cell to create electricity, but if the energy isn’t used immediately, it must be efficiently stored or it’s lost. This has proved to be an enormous problem. In contrast, the advantage enjoyed by artificial photosynthesis is that the end-result is a stable chemical, whether carbohydrate or something else, that stores energy within its chemical bonds to be used to produce energy whenever it’s needed.
Called artificial leafs or bionic leafs, some of these devices will produce fuels that can be used directly as sources of power, to generate electricity, as an example. Others will produce energy-rich compounds that are important building blocks for industrial processes. Producing these chemical building blocks would otherwise have required the input of energy from other sources had they not been derived from this process.
Nature Lends a Hand
One of the most interesting aspects of artificial photosynthesis is that these projects—by their very nature—are collaborations between biologists and physical scientists. That’s because the star of the show is usually a bacteria.
In a means devised by Lawrence Berkeley National Laboratory chemist Peidong Yang, special wires produced via nanotechnology capture the sunlight and convert it into moving, energetic electrons. The wires protect the sensitive bacteria from potentially damaging chemicals in the air, and the system transfers the electrons to bacteria, which, in this adaptation, produces the chemical acetate. This is an important, energy-rich chemical building block for many industrial processes.
The nanowires are composed of silicon and titanium oxide, which each absorb sunlight at different wavelengths. The bacteria used were Sporomusa ovata, which has a known ability to employ mobile electrons to breakdown carbon dioxide. In this case, the electrons are captured by the nanowires, rather than by a biological organism.
In the process, the electrons from the nanowires break water into molecular oxygen and hydrogen ions. The bacteria, which are planted around the wires and insulated from the atmosphere, take up electrons and add them to the CO2 to build the aforementioned acetate. The final step converts the acetate either into fuel, as a green plant does, or into other useful compounds. The overall reaction is carbon dioxide and water, through the sun’s energy, producing acetic acid and water. Over the years, there have been numerous efforts to achieve these and similar ends through purely electrochemical methods. None have proved viable yet, and progress has only been possible through the use of a bacterial catalyst.
According to Yang, this iteration of the project only achieves solar-conversion efficiency of less than 1 percent, which is compatible to what green plants achieve in nature. He expects to shortly improve the process to 3 percent efficiency, and anticipates that an eventual 10 percent efficiency will be enough for a commercially viable product. It should also be noted that elsewhere, similar nanowires are used in systems to simply generate electricity in a manner similar to the now-familiar solar cell.
Biological Factories
Artificial photosynthesis offers other interesting possibilities. By altering the bacteria used, different chemicals aside from carbohydrate or acetate can be generated at the output. Or, the acetic acid can readily be converted to Acetyl Coenzyme A (Acetyl-CoA) through the presence of other bacterial catalysts.
Figure 1: Artificial photosynthesis being performed at the Lawrence Berkeley National Laboratory. (Source: Lawrence Berkeley National Laboratory)
Acetyl-CoA is a major factor in animal metabolism and, as the picture above describes, it may well serve as the starting point in the generation of many useful chemical compounds.
This opens another possibility for artificial photosynthesis. Even before such time as it becomes a major source of fuel and a significant absorber of CO2, artificial photosynthesis might well be useful as a source of chemicals otherwise difficult to obtain in an eco-friendly manner.
At Harvard University’s Bionic Leaf project, the hydrogen gas produced as an intermediate step is fed to a genetically engineered Ralstonia eutropha bacterium, which produces isopropanol as an end product. Isopropanol is a particularly interesting derivative, because it can be used as a fuel, in the manner of ethanol.
Hydrogen Gas at Cal Tech
The approach at Cal Tech’s Joint Center for Artificial Photosynthesis is to use two separate electrodes, yielding not a complex chemical, but hydrogen and oxygen gases. One electrode absorbs and employs the energy harvested from sunlight to break water down to its components of protons, electrons and oxygen gas. The other serves to unite the electrons and protons into hydrogen gas. The only problem is that this is the same mixture often used to send rockets into space, so—needless to say—there is an explosion hazard, which was mitigated with the introduction of a membrane that ensures these gases are isolated from one another.
There are also issues to be surmounted with the electrodes themselves, and the main one is that, when exposed to water, they develop a sort of “rust” that interferes with their function. The solution has been the deployment of a nickel-oxide coating to protect the electrodes. Most importantly, the new coating does not interfere with the function of the membrane that keeps the hydrogen and oxygen gases that have been produced. So far, testing has revealed that the system as devised will continue functioning safely and efficiently—without a breakdown—for several hundred hours.
While none of these approaches will produce a commercially available product in the very near future, the concept is now proven. It is becoming clear that we now have the option to extract vast amounts of energy from sunlight, and to store it safely and efficiently for use when it’s needed, so it’s only a matter of time before we’ve got another viable option as a power source.