New breakthrough in the study of hydrogen production by electrolysis of water
In December 2018, the National Development & Reform Commission and the National Energy Administration jointly issued the “Clean Energy Consumption Action Plan (2018-2020)” (hereinafter referred to as the “Plan”). The “Plan” pointed out that exploring the conversion of surplus renewable energy into electricity Heat energy, cold energy, hydrogen energy, realize the efficient use of renewable energy in multiple ways.
Hydrogen generation by electrolysis of water is a good way to convert surplus electricity into hydrogen energy. Recently, Chen Liang’s team, a researcher from the Institute of New Energy, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, provided a highly efficient acidic oxygen evolution electrocatalyst and proposed a corresponding mechanism explanation, which to a certain extent promoted the generation of hydrogen from acidic electrolyzed water. Related research is published in “Nature-Communication”.
Hydrogen generation by electrolysis of water to achieve surplus power conversion
Both hydrogen energy and electric energy are important secondary energy sources and are also the main green and clean energy sources in the future. Hydrogen has no pollution and zero emissions, and will play an extremely important role in human life and production in the future.
Hydrogen energy has the characteristics of long-distance transportation, large-scale storage and hydrogen-electricity interchange. The current main generation methods include hydrogen generation from fossil fuels, hydrogen generation from water electrolysis, and industrial by-product hydrogen.
Chen Liang, a researcher at the Ningbo Institute of Materials Technology and Engineering of the Chinese Academy of Sciences and the corresponding author of the paper, explained to the China Science Daily in an interview: “At present, the industry mainly uses fossil fuel reforming to produce hydrogen. Fossil fuels can be natural gas, petroleum and coal. The energy contained in hydrogen produced by this method is lower than the energy contained in the original fossil fuel because of the heat loss. In addition, the hydrogen produced by this method does not reduce carbon dioxide emissions, because the carbon dioxide emitted by the process of reforming hydrogen is the same as the carbon dioxide emitted by direct burning of fossil fuels.”
At present, the methods for hydrogen generation by reforming alternative fossil fuels under research in the laboratory stage include biological hydrogen generation, hydrogen generation by electrolysis of water, photoelectrochemical hydrogen generation and photoelectrocatalytic hydrogen generation. Among them, the technology of hydrogen generation by electrolysis has already reached a certain scale application in industry.
The main purpose of hydrogen generation by electrolysis of water is to convert surplus electricity into hydrogen energy. China has the world’s largest wind power and solar power generation, but solar and wind power have intermittent problems, limited by day and night changes and climate factors.
“The storage of electricity has always been a problem. The electricity that is not used up must be input to the national grid or converted into energy, otherwise it can only be wasted. Because the electricity generated by solar and wind energy is unstable and if directly input into the grid it will cause a series of problems. Therefore, we need to vigorously develop surplus power conversion technology.” Chen Liang explained.
New high-efficiency acidic oxygen evolution electrocatalyst.
According to different electrolytes, hydrogen generation by electrolysis of water can be divided into alkaline electrocatalytic hydrogen generation and acidic electrocatalytic hydrogen generation. Chen Liang explained that the electrolysis of water involves two half reactions-the hydrogen evolution reaction on the cathode and the oxygen evolution reaction on the anode. According to different electrolytes, it is divided into alkaline electrolyzed water and acidic electrolyzed water. For alkaline electrolyzed water, the difficulty is hydrogen evolution on the cathode; for acidic electrolyzed water, the difficulty is oxygen evolution on the anode.
According to reports, the industry has been relatively thorough in the research on alkaline electrolyzed water, and it has certain applications in industry. However, compared with alkaline electrolyzed water, acidic electrolyzed water is more popular. The reason is that the reaction rate of acidic electrolyzed water is 2 to 3 orders of magnitude faster, with fewer by-products, and proton exchange membrane (PEM) can be used, which in turn makes the stack very portable.”
The bottleneck restricting the development of acidic electrolyzed water is the oxygen evolution reaction on the anode, and there is still a lack of efficient acidic oxygen evolution electrocatalysts. This time, Chen Liang’s team provided a highly efficient new type of acidic oxygen evolution electrocatalyst—CrO2-RuO2 solid solution material, and proposed a corresponding mechanism explanation.
Dr. Lin Yichao in the team successfully prepared a new type of CrO2-RuO2 solid solution material based on Cr-based metal-organic framework materials by adsorption of RuCl3 precursors and annealing. The structure of the CrO2-RuO2 solid solution was determined by PXRD crystal repair and Vegard’s law verification, and the Cr and Ru atoms were uniformly distributed in the same nano-single crystal through atomic-resolving spherical aberration electron microscope.
Chen Liang pointed out that the preparation process is very simple, and the most important thing is to choose a suitable Cr-based metal organic framework material that can adsorb a large amount of RuCl3. “Tens of thousands of metal-organic framework materials have been reported so far, and there are also thousands of Cr-based materials. If blindly screening, the workload is very large. Thanks to the accumulation of research on metal-organic framework materials in the past ten years, rapid entry has been achieved.
The results show that the CrO2-RuO2 solid solution material is used as the oxygen evolution electrocatalyst on the acidic electrocatalytic hydrogen generation anode, reducing the overpotential of the reaction, that is, reducing the energy consumption of the reaction. The material has an overpotential of only 178mV at a current density of 10mA·cm-2, and after 10,000 cycles, the overpotential has only increased by 11mV, which is far better than commercial RuO2. Through the synchrotron radiation near-side absorption test, it is found that the valence state of Ru atoms in the crystal structure is slightly higher than +4 due to the strong electron withdrawing effect of 4-valent Cr, and the bond length of Ru-O becomes shorter.
Through density functional simulation calculations , Dr. Ziqi Tian of this team found that it is precisely the electron withdrawing effect of +4-valent Cr in the crystal lattice that the catalytic activity of Ru becomes higher and the reaction energy barrier is lowered. In addition, it is worth noting that the content of precious metal Ru in the solid solution material is only 40%, which can significantly reduce the cost of the catalyst.
According to reports, RuO2 and IrO2 and their derivatives are currently recognized catalysts with acidic oxygen evolution electrocatalytic activity. The acidic oxygen evolution activity of IrO2-based materials is very stable, but the price of Ir is very expensive. The current market price of Ir metal is about 390 yuan/g. In contrast, Ru metal is the cheapest platinum group element, the price is about 60 yuan / gram. Although RuO2-based materials have high electrocatalytic activity for acidic oxygen evolution, they are very unstable. The new CrO2-RuO2 solid solution material reported this time has the highest acidic oxygen evolution electrocatalytic activity, and can be stable for 10 hours at a current density of 10mA·cm-2, far superior to commercial RuO2.
Chen Liang gave the following figures: “The molecular formula of the new material is Cr0.6Ru0.4O2, and the price of Cr is 0.4 yuan/gram, which is almost negligible compared to the price of Ru. Therefore, simply estimate the cost from the element composition, it can be reduced by about 60%. Of course, the premise is that large-scale preparation methods for new materials must also be developed.”
Hydrogen energy is the most promising secondary energy
The global hydrogen industry is developing rapidly, with the market size growing from US$187.082 billion in 2011 to US$251.493 billion in 2017, a growth rate of 34.4%. Among them, the United States is the largest importer of industrial hydrogen, while the Netherlands is the largest exporter of industrial hydrogen.
According to data, in 2017, more than 96% of the world’s main artificial hydrogen generation raw materials were derived from the thermochemical reforming of traditional fossil resources, and only about 4% were derived from electrolyzed water. Coal and natural gas are also the main raw materials for artificial hydrogen generation in our country, accounting for 62% and 19% respectively. According to the “Blue Book of Infrastructure Development of China’s Hydrogen Energy Industry (2018)”, China’s hydrogen generation in 2016 was approximately 21 million tons, of which coal produced hydrogen accounted for 62%, which was the main source of hydrogen; natural gas produced hydrogen followed by 19 %.
Zou Caineng, academician of the Chinese Academy of Sciences and vice president of the China Petroleum Exploration and Development Research Institute, analyzed that although coal gasification hydrogen generates a large amount of carbon dioxide, it is still a large-scale, low-cost and best way to artificial hydrogen generation due to its abundant raw materials and low prices.
Blast furnace flue gas, chemical exhaust gas, etc. can achieve low-cost recovery of hydrogen through pressure swing adsorption (PSA) technology. Solar hydrogen generation technology (photocatalysis, photopyrolysis) is the ideal hydrogen generation technology in the future, but it is subject to problems such as conversion efficiency and cost are expected to be difficult to achieve large-scale before 2030.
Among all the artificial hydrogen generation methods, hydrogen generation by electrolysis of water will run through the entire process of hydrogen energy development, and it is one of the main sources of industrial hydrogen for the construction of a future “hydrogen energy society”. With the continuous development of water electrolysis hydrogen generation technology and the gradual reduction of costs, electrolysis water hydrogen generation will gradually meet the requirements of commercialization and realize distributed hydrogen generation.
Chen Liang said that their team will continue to optimize the preparation method of CrO2-RuO2 solid solution materials in the future. At present, the metal-organic framework material is still in the laboratory stage and has not been commercialized. The team will try to use other commercially available and cheap Cr-based raw materials. In addition, they will also try to use the same strategy to prepare other rutile solid solution materials, such as MnO2-RuO2 solid solution, in order to obtain higher performance acidic oxygen evolution electrocatalysts.