"This is a very ineffective proof-of-concept system. But in the end, catalytic systems with a solar conversion efficiency of 10-15 percent are achievable," says the lead researcher.
Trees and algae do this. Even some bacteria and fungi, but scientists have had a great challenge in developing methods to convert sunlight into usable fuel. Now, scientists have presented a facility that allows proof of the conceptual ability to break down water to obtain recyclable hydrogen.
"This is a very ineffective proof-of-concept system. But in the end, catalytic systems with a solar conversion efficiency of 10-15 percent are achievable," says Thomas Malouk, the professor of chemistry and materials physics at DuPont. If the idea comes to fruition, water splitting will make it possible to obtain a clean and safe source of hydrogen fuel from water and sunlight." Although solar cells can currently generate electricity from visible light with an efficiency higher than ten percent, hydrogen solar cells - such as those developed by Craig Grimes, professor of electrical engineering at Penn University - are limited due to the poor spectral response of the semiconductors that make them up.
Basically, molecular photoreceptors can use a wider part of the visible light spectrum in a process that mimics the process of photosynthesis in nature. Photosynthesis uses chlorophyll and other color compounds to absorb visible light. So far, experiments with natural and synthetic color compounds have resulted in obtaining by-products that originate from the reaction of the various reactants with the hydrogen or oxygen released in the process, and therefore a continuous and complete process is not yet available. These processes are also still more expensive than electric discharge of water. One of the main reasons for this stems from the fact that as soon as hydrogen and oxygen are separated separately, they react together very easily to get water back. The catalysts used in the study of the separate oxygen and hydrogen reactions are also effective catalysts in the connection reaction between them.
Malouk and Justin Youngblood, a postdoctoral fellow in chemistry, in collaboration with researchers from the University of Arizona, developed a catalytic system which, in combination with a dye compound, can mimic the processes of electron transfer and water oxidation that occur in plants during photosynthesis. They reported the results of their experiments at the annual meeting of the "American Association for the Advancement of Science" in Boston on February 17. The key to the success of their process lies in a tiny coupling (complex) of compounds with a center where there is an iridium oxide catalyst surrounded by orange-red color compounds. The diameter of these aggregates is about two nanometers when the catalyst components and the color compounds are essentially the same size. The researchers chose orange-red color compounds because they absorb sunlight in the blue range, which has the highest energy. These compounds have also been properly studied in previous experiments to develop artificial photosynthesis processes. The researchers were able to build an array in which the color compounds are arranged around the catalytic center with an interval that allows the required chemical reaction to take place. When visible light hits the dye compound, its energy excites electrons in the compound and these, with the help of the catalyst, can cause the water to dissolve to obtain free oxygen.
"Each catalytic region of the iridium atom is able to carry out this oxidation reaction at a rate of 50 times per second," says the researcher. "This rate is faster, by about three orders of magnitude, than the best synthetic catalysts, and is comparable to the rate of catalysis of photonic system II in the photosynthesis processes of green plants." The type II photonic system is the protein coupling in plants that is able to oxidize water and start the entire photosynthesis process. The researchers connected their new array to a titanium-oxide anode and a platinum cathode. In the next step, they immersed the electrodes in a salt solution but spaced them apart to prevent reconnection of the resulting hydrogen and oxygen. All that is needed now for the system to work is a light beam that will reach the titanium oxide anode with the catalytic array. This type of cell is similar to those that produce electricity (electrolytic cell), but the addition of the catalytic array allows the reaction to break down the water into its gaseous components - oxygen and hydrogen. Discharging the water requires an energy of 1.23 volts, but the technical configuration of the experiment does not allow receiving this amount of energy and required the researchers to add about 0.3 volts from an external source. Their current system has an efficiency of only about 0.3 percent.
"Nature is only 1-3 percent efficient in its photosynthesis process," says the lead researcher. "This is why we cannot expect that only from the grass in the garden of our house will it be possible to operate our house and our car. Researchers have a variety of ideas regarding improving the efficiency of the process. They plan to conduct experiments to improve the efficiency of the dye compound, improve the catalytic system and adjust the relative spatial arrangement of the system components. Instead of spherical catalytic arrays, it is possible that their other spatial arrangements will allow greater absorption of the beam that hits them. Improvements in the overall geometry of the system can also streamline the process. "At each stage of the process there are several choices," says the researcher. "The main question is how the electrons can be "forced" to go through the required path and not waste their energy without helping the chemical process." The physical distance between the compounds is extremely important in terms of controlling the rate of electron transfer and their efficient direction. By reducing the distances in different paths and increasing others, non-essential, a greater number of electrons will be transferred in the appropriate path so that more of their energy will be used to discharge the water and obtain clean hydrogen.
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Nitpok: *all* mosses are capable of photosynthesis (they are multicellular plants after all).