Electrolyzed water, which splits water into hydrogen and oxygen, is a simple and long-standing idea. Most of the existing electrolyzed water technology is based on pure water, and more than 95% of the earth's water resources-seawater, has received little attention.
Recently, Beijing University of Chemical Technology and Stanford University have published a research paper titled "Solar-driven, Continuous and Stable Seawater Decomposition to Produce Hydrogen" in the Proceedings of the National Academy of Sciences in the United States. A new method for producing hydrogen by electrolyzing seawater, and has a stability of thousands of hours under the current density of industrial electrolysis. "This new type of electrocatalyst not only solves the engineering problem of electrolysis of seawater to produce hydrogen, but also prompts the possibility of preparing hydrogen, oxygen, and salt from seawater in the future." The first author of the paper, Beijing University of Chemical Technology Kwong Yun, associate professor of the State Key Laboratory and Beijing Advanced Innovation Center for Soft Matter Science and Engineering, said.
Sleepy Sea
Hydrogen is the substance with the highest energy density known on earth. Combustion does not emit carbon dioxide, which can alleviate the problem of global warming. It is one of the solutions for future clean energy. Hydrogen production from electrolyzed water is a potential solution for hydrogen production on a large scale: a power source is connected to two electrodes placed in water. When the power is turned on, hydrogen gas emerges from the cathode (negative electrode), and breathable oxygen gas emerges from the anode (positive electrode). In theory, this method provides power for cities and cars. But at present, electrolytic hydrogen needs to prepare seawater into high-purity water, and the production cost is very high. "So much hydrogen is needed globally, so it is impossible to use pure water," said Dai Hongjie, a professor at the Stanford University Department of Chemistry, a member of the National Academy of Sciences, and one of the authors of the paper. "California has little water to meet current needs." Undoubtedly, the use of abundant sea water resources to produce hydrogen can solve this problem. However, during the electrolysis process, chloride ions in seawater easily interact with the metal current collector to dissolve the metal, resulting in corrosion and limiting the life of the system. According to Michael J. Kenney, a graduate student and one of the authors of the chemistry department at Stanford University, ordinary anodes can only work in seawater for about 12 hours. "Then the entire electrode shattered." "This is mainly because sodium chloride in seawater can cause severe chlorine evolution side reactions and electrode corrosion in the anode. The scientific research community has always hoped to explore the prevention and control of chlorine gas by regulating the electrolysis system and electrode structure. A new method of anode generation. "Kuang Yun said.
Protect the electrode
The researchers found that adjusting the pH of seawater to alkaline can inhibit the oxidation of chloride ions, making oxygen more likely to be produced at the anode. According to Kwong Yun, under alkaline conditions, nickel iron hydroxide is the most stable and stable catalyst for oxygen evolution. "Choosing nickel iron hydroxide as an electrocatalyst may improve the selectivity and stability of the electrode at the same time." In response to the corrosive problems caused by chloride ions, the researchers grew nickel sulfide on a foamed nickel conductor and used nickel iron The hydroxide catalyst grows on top of nickel sulfide, forming a multilayer structure. Kwong Yun stated that nickel foam acts as a conductor-transmitting electrical energy and triggering electrolysis. In the electrolysis process, the intermediate layer of nickel sulfide will evolve into a negatively charged layer, protecting the anode. Just as the negative poles of the two magnets push each other, the negatively charged layer repels chloride ions and prevents them from reaching the metal conductor part inside the electrode. Michael J. Kenney said: "With this protective layer, it can run for more than 1000 hours." In addition, in previous studies that attempted to decompose seawater into hydrogen fuel, corrosion was extremely likely to occur at higher currents because Therefore, the current in the actual electrolysis process is very small, but this often affects the electrolysis efficiency. This time, the researchers used multi-layer electrodes to increase the electrolysis current by more than 10 times in the past to achieve hydrogen gas from seawater at a faster rate. "I think we have set a record of sea water flow." Dai Hongjie said, "Now that we have found a new method of electrolysis of seawater, this may open the door to increase the availability of hydrogen fuel driven by solar or wind energy." In order to verify the practical application of this method The researchers also designed a demonstration device for a solar-powered electrolysis system to electrolyze hydrogen and oxygen from seawater collected in the San Francisco Bay. Ali Javey, deputy editor of the American Chemical Society's "Nano" magazine, a professor at the University of California at Berkeley, and the director of the electronic materials project at Lawrence Berkeley National Laboratory, said that solar-powered seawater decomposition has always been the goal of scientific research, but little progress has been made over the years The study demonstrated its feasibility for the first time.
Breathe in the sea like a fish
What is the practical difficulty and cost of this method? The researchers further verified that the method can be based on the existing electrolyzer system, using industrial current to develop work, and the electrolysis speed is fast. Feng Xinliang, director of the Department of Molecular Functional Materials at Dresden University of Technology, commented that this research provides an economical and low-cost electrocatalyst solution for the development of stable seawater electrolysis, which is of great significance for the production of hydrogen fuel. "At present, there are still many engineering details to be studied in this study, and there is still a period of time before practical application." Kuang Yun introduced that on the one hand, the use of solar energy, wind energy and other renewable energy to electrolyze seawater is facing the problem of energy input fluctuations, and is different from The long-term continuous electrolysis in the laboratory runs, and the actual electrolysis system will face frequent switches. The actual conditions of these projects have put forward new requirements for the stability of the electrode, and it is urgent for scientific researchers to tackle the problem; on the other hand, the laboratory has now carried out methods Proof of concept, scale-up and industrialization require scale-up experiments, and a truly practical seawater electrolysis system that is not laboratory-simulated is needed. It is worth mentioning that this new method in the future is expected to be used for new purposes other than power generation. Kwong Yun said that because this process produces breathable oxygen, divers or submarines can take the equipment into the ocean and use it to generate oxygen on the bottom of the sea to achieve ventilation without the need to float to the surface for ventilation. (Author: Ye Bu)
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