Hg is one of the most important clean energy sources in today's society. As an ideal hydrogen production method, electrolytic water technology is limited by the high theoretical voltage of the anode OER, complex reaction process and slow kinetics, resulting in its high electrolytic water voltage. Small molecule oxidation reactions (including electrosynthesis reactions and sacrificial agent oxidation reactions) have low theoretical voltages and often produce high value-added products during the reaction, which is ideal for replacing OER.
Recently, Nankai University Professor Jiao Lifang Professor Jiao Lifang published a review article titled "Progress in Hydrogen Production Coupled with Electrochemical Oxidation of Small Molecules" on Angewandte Chemie International Edition (Figure 1). This review first analyzes how to select suitable small molecules and design high-performance catalyst from a theoretical perspective. Then, some common small molecule oxidation reactions in recent years were further summarized from the two aspects of electrosynthesis reaction and sacrificial agent oxidation reaction. Finally, the small molecule oxidation reaction is prospected to promote its practical application in the field of electrolyzed hydrogen production.
Figure 1. Small molecule oxidation assisted electrolysis of hydrogen production by water
1. Research background of small molecule oxidation
Energy shortage and environmental pollution are regarded as two major problems affecting the sustainable development of society, and are also challenges facing each country at present. Using the electrical energy generated by renewable energy to drive electrolytic water reactions to prepare green hydrogen is of great significance to solving my country's energy shortage and environmental pollution problems and achieving the "dual carbon" goal. The theoretical voltage of the anode OER is too high, the reaction process is complex and the kinetics are the main bottlenecks that limit the high efficiency of electrocatalytic water decomposition. Although researchers have spent a lot of effort on developing high-performance, low-cost, and high-stability OER catalysts, high voltages are still required to drive the reaction. At the same time, explosive hydrogen and oxygen mixture is easily generated during the reaction process, and reactive oxygen species can easily be generated, which will lead to membrane decomposition and reduce the life of the electrolytic cell.
Therefore, in recent years, researchers have been committed to coupling the cathode HER reaction with small molecules (alcohols, aldehydes, hydrazine hydrate, urea , etc.) oxidation reactions with low theoretical voltage and thermodynamics, to reduce the tank voltage of hydrogen produced by electrolyzing water, as shown in Figure 2. Moreover, small molecule oxidation will generate high value-added products (electric synthesis reaction) or alleviate water resource pollution (sacrificial agent oxidation reaction), so it has received widespread attention.
Figure 2. Advantages of hydrogen production by small molecule oxidation assisted electrolysis by water
2. Summary of small molecule oxidation reaction
2.1 Electrochemical synthesis reaction
Figure 3. Prospects of hydrogen production by electrosynthesis reaction assisted electrolysis by water
Electrochemical synthesis reaction is an attractive and efficient hydrogen production strategy. Because electrochemical synthesis reactions usually have low theoretical voltages, they can use the electricity generated by renewable energy to convert the rich biomass on the earth into valuable chemicals and fuels under mild conditions, and have important applications in the fields of organic synthesis of , pharmaceuticals, polymer production, etc. (Figure 3). Compared with the traditional chemical oxidation of and thermally catalytic oxidation, electrosynthesis reactions have the advantages of environmentally friendly and mild reaction conditions. They have received widespread attention in recent years and have achieved a series of research results. This review mainly summarizes the electrosynthesis reactions carried out by alcohols, aldehydes, carbohydrates, primary amines, and sulfides as anode substrates widely reported in recent years.
2.2 sacrificial agent oxidation reaction
In addition to the electrochemical synthesis reaction, sacrificial agent oxidation assists in hydrogen production is another type of small molecule oxidation reaction.Although it is necessary to consume reactant molecules (urea, hydrazine hydrate, etc.) during the reaction process to produce low value-added products, it has shown great potential in efficient hydrogen production and fuel cell (direct urea fuel cell and direct hydrazine hydrate fuel cell). Furthermore, since urea and hydrazine hydrate are considered common pollutants in wastewater, their oxidation reactions have the potential to achieve the treatment of water pollutants. In this review, we provide a detailed summary of the urea oxidation reaction and hydrazine hydrate oxidation reaction reported in recent years.
3. Conclusion and Prospect
This article reviews several small molecule oxidation reactions that can replace OER to produce hydrogen by electrolyzing water, and focuses on discussing the catalyst regulation strategies, various possible small molecules and corresponding catalytic reaction mechanisms. Pairing small molecule oxidation reaction with HER can not only reduce the electrolytic voltage, but also inhibit the production of explosive hydrogen and oxygen mixtures and reactive oxygen species, obtain high-value-added products or alleviate wastewater pollution problems. Although significant progress has been made in small molecule oxidation-assisted high-efficiency hydrogen production technology, industrial applications still have a long way to go. In this context, our outlook for future research is as follows (Figure 4): Design of
catalyst: Currently, the design strategies for small molecule oxidation reaction catalysts mostly come from the traditional electrolytic water catalyst method, and there is still a lack of targeted design of small molecule oxidation reaction catalysts. Therefore, it is necessary to have a deep understanding of the relevant catalytic reaction mechanisms, realize the targeted design of each small molecule oxidation reaction, and develop the potential of the catalyst. Machine learning can be an effective tool for rapid screening of small molecule oxidation catalysts. In addition, small molecules usually contain a variety of functional groups . A deep understanding of the relationship between different functional groups and catalysts can also help design efficient catalysts.
mechanism analysis: At present, the mechanism of these small molecules oxidation reactions is still controversial. Therefore, it is necessary to develop advanced in-situ characterization techniques to analyze key intermediates and actual catalytic components in real time. In addition, theoretical calculations are a powerful tool for analyzing catalytic reaction paths. In the future, advanced in-situ characterization technology will be combined with more complex theoretical simulations to more reliably analyze the structural evolution and real reaction mechanisms in the reaction process.
High current density: In commercial applications, high selectivity needs to be achieved at high current density and high efficiency, but this is a difficult task. Assembly of flow electrolytic cells can promote the transport of reactants, quickly separate the product from the catalyst, maximize contact between the reactants and the active site, and provide a large current density at low voltages, thus preventing competitive OER at high operating voltages. In addition, the design is easier to scale to larger modules with great commercial application prospects.
Product isolation: The isolation and purification of products are also very important for industrial applications. Therefore, there is a need to develop methods that can effectively isolate the target products. In addition, exploring small molecular substrates that can be oxidized to high value-added products of hydrophobicity is also a feasible strategy.
Figure 4. Outlook analysis of small molecule oxidation reaction
Author introduction:
Jiao Lifang, Professor of the School of Chemistry, Nankai University, and Ph.D. supervisor. National Outstanding Youth Science Foundation winner, chief scientist of key R&D program projects. He won the first prize in natural sciences in Tianjin (first person completing the game), and the 18th Chinese Young Female Scientist Award . He serves as an editorial committee member of eScience, Chinese Chemical Letters journal, and vice chairman of the electrospinning committee of the China Association of China. The main research directions focus on efficient energy storage and electrocatalytic conversion: design and synthesize key electrode materials for high-performance lithium/sodium/potassium ion batteries, revealing the energy storage mechanism of new materials; design and develop cheap electrocatalytic water decomposition catalysts with high catalytic activity, good stability and strong selectivity.More than 4,280 SCI papers have been published in journals such as Angew. Chem. Int. Ed., Chem. Soc. Rev., Adv. Mater., Adv. Energy Mater., Nano Lett., etc., with a total of more than 16,300 citations and H factor 68.
References:
Wang T. et al. Progress in Hydrogen Production Coupled with Electrochemical Oxidation of Small Molecules. Angew. Chem. Int. Ed., 2022.
https://onlinelibrary.wiley.com/doi/10.1002/anie.202213328
