While it is amazing what photoautotrophs have been able to accomplish with photosynthesis, simple sugars are not quite energy dense enough to fuel the demands of human society. Scientists and engineers are taking the blueprint of photosynthesis and scaling it up to human industrial needs. In terms of energy systems, it is clear that artificial photosynthesis research must focus on three areas: light harvesting complexes, proton-coupled electron transfer, and redox catalysts. In the simplest biomimetic approach, this involves using a photosensitizer, a water oxidation catalyst and a hydrogen evolving catalyst.
When a photosensitizer is hit with a photon it oxidizes, transfering photoexcited electrons to the hydrogen catalyst. This drives a reduction reaction whereby the water splitting catalyst strips electrons from water to fill the deficit in the photosensitzer just as in nature. Meanwhile the hydrogen catalyst receives the energized electrons from the photosensitizer and the free floating protons from the water splitting step and generates hydrogen gas (H2). The key to artificial photosynthesis lies in finding the right catalyst to drive each step of the reaction, and deciding whether hydrogen fuel is the end goal, or fixing carbon dioxide and generating useful hydrocarbons is preferred.
Water Splitting Catalysts. In order to replicate photosystem II, researchers are interested in catalysts that facilitate the electrolysis of water when excited by a photon. Some of the best candidates for solar activated water splitting include manganese, dye-sensitized titanium dioxide (TiO2), and cobalt oxide (CoO).
Manganese Catalysts. Manganese is the natural world's catalyst of choice for light activated splitting of water, so it stands to reason that creating an industrial catalyst based off of manganese is the most direct approach to mimicking the magic of photosynthesis. In practice this involves embedding manganese complexes within a nafion matrix to produce nanoparticles of manganese oxide. In this form the manganese catalyst can more readily be integrated into a photochemical cell.
Dye-Sensitized Titanium Dioxide. Titanium dioxide (TiO2) is the photocatalyst of choice for dye-sensitized solar cells, which we will go into further detail later in this article. The anatase form of this crystalline mineral owes its strong oxidative potential to the positive holes within its atomic matrix. Doping the oxide with carbon can greatly improve titanium dioxide's efficiency in cleaving the hydrogen oxygen bonds in water. Adding disorder to the lattice structure of the surface layer of TiO2 nanocrystals can also broaden the spectrum of light absorption to include infrared.
Cobalt Oxide (CoO) Nanoparticles. When it comes to hydrogen generation from the photocatalysis of water, conventional catalysts usually require additional reagents and external biases to produce stoichiometric amounts of hydrogen and oxygen gas. In 2013, researchers at the University of Houston discovered that Cobalt Oxide (CoO) nanoparticles were capable of decomposing water under visible light without the need of co-catalysts, reagents, or other additives. Current state of the art water splitting devices sport terribly low solar-to-hydrogen conversion efficiencies, on the order of 0.1% but these CoO nanoparticles achieved 5% conversion efficiency in the lab. A relatively new discovery, it will likely be some time before we see the first photochemical cells using CoO in the field, but the initial test results have the artificial photosynthesis community excited.
CO2 Reduction Catalysts. Since photoautotrophs evolved in a reducing environment virtually all of photosynthesis in nature relies on the inefficient enzyme RuBisCO as part of the Calvin Cycle. It was unnecessary for natural selection to evolve a better catalyst because of the abundance of CO2 in the environment. RuBisCO is too slow for industrial processes, only sequestering a few molecules of CO2 per minute into ribulose-1,5-bisphosphate per minute. Researchers are instead turning to metal catalysts like transition metal polyphosphine complexes which are still in development, but aim to be able to reduce CO2 from air at atmospheric pressure at a faster rate
Photoelectrochemical Cells. Photoelectrochemical cells or PECs are mankind's best attempt at artificial photosynthesis. They use solar cells to power the electrolysis of water to produce hydrogen and oxygen gas. In its simplest implementation, incoming photons excite four surface electrons of a silicon electrode, which in turn flow through wires to a stainless steel electrode to where they split four water molecules into two hydrogen molecules (H2) and 4 hydroxide ions (OH-). The four OH- groups pass through the liquid electrolyte back to the surface of the silicon electrode and refill the four electron holes created when the previous four photoelectrons left the cell. This results in the production of two water molecules and two oxygen molecules. The direct conversion of sunlight into a fuel makes PECs a near mirror image of the photosynthetic process.
Dye Sensitized Solar Cells. Dye sensitized solar cells are low cost solar cells that combine a photosensitized anode with an electrolyte to create an artificial photosynthetic system. In the most common implementation, a porous layer of TiO2 nanoparticles is impregnated with a molecular dye that absorbs a targeted spectrum of light. The TiO2 layer is immersed in an electrolyte solution and forms the anode of the system. A platinum cathode is placed on the other side of the electrochemical cell completing a circuit. The oxidation reduction reaction is driven by the photoexcitation of electrons within the dye, and the semiconducting layer serves as the scaffold which supports a large number of dye molecules in a 3-D matrix increasing the amount of active material per surface area of the cell. By itself, the dye sensitized solar cell mimics the photoexcited locomotion of electrons within the thylakoid of a chloroplast. To completely mimic photosynthesis, the current produced by the cell can be used to power the electrolysis of water into hydrogen and oxygen gas. However power from a dye sensitized solar cell can just as easily be stored in a battery or integrated directly into the electrical grid, as with conventional solar cells.
Helioculture. While most researchers are content with making hydrogen, some scientists are taking biomimetics of photosynthesis a step further by making carbon fuels from the sun. The company Joule Unlimited has genetically engineered a bacterium that can take CO2 out of the atmosphere and secrete useful hydrocarbons. Not to be confused with biodiesel which must be refined in a batch process to create fuel, helioculture involves using photosynthetic microbes to directly produce ethanol, olefins, and other hydrocarbons without the need for further refining. The key is to manipulate the genes related to enzymatic mechanisms within microorgansims that enable the direct synthesis of these fuels.
The Future of Artificial Photosynthesis. One approach to artificial photosynthesis is to replace the expensive semi-conductors in photovoltaic cells with the isolated protein complexes of photosystems I and II to create cheaper and renewable solar cells. In August of 2014, researchers from the Ruhr-University Bochum (RUB) in Germany revealed that they had successfully transplanted photosystem 1 (PS1) protein complex from a cyanobacterium into a solar cell. The discovery marks the first time a semi artificial leaf was able to transfer photoelectrons faster than "natural" photosynthesis. Mankind has never been closer to unlocking the power of photosynthesis for human use.
The future of artificial photosynthesis is looking bright.
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