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BIT Publishes Important Research Results in the Sub-journal of Science

release date :2019-09-27 08:03:00  |   [ close window ]ViewCount:

     Translator: News Agency of BIT, Pang Yu

    Editor: News Agency of BIT

       

Recently, Li Xiang, a special researcher in the team of Professor Yao Yugui, School of Physics, BIT, and Dr. Wang Hao, Dr. Li Yutao, Professor John B. Goodenough and Professor Zhou Jianshi of the University of Texas at Austin in the United States, jointly with Jin Changqing, Professor Long Youwen, Institute of Physics, Chinese Academy of Sciences, and other collaborators have made breakthroughs in designing new oxygen evolution (OER) electrocatalysts at high pressure. They successfully developed low-cost, high-efficiency and stable perovskite-type non-noble metal electrocatalysts CaCoO3 and SrCoO3 under extreme high pressure, and found that their OER catalytic performance was far superior to that of traditional materials. Through in-depth study of their structure-performance relationship, a new OER reaction mechanism was revealed to explore advanced electricity, which will open up new avenues for exploring advanced electrochemical functional materials.

Electrolyzed water is an important way to obtain new clean energy hydrogen, including two half-reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Because of the slow reaction kinetics, the latter usually needs the help of electrocatalysts to accelerate the reaction rate. However, OER electrocatalysts (such as RuO2 and IrO2 are both noble metal oxides) are expensive, which severely limits their large-scale applications. Therefore, exploring a new type of OER electrocatalyst with low cost, high activity and stability has become a key issue in the research of electrolyzed water hydrogen production technology. In recent years, some non-noble transition metal oxides like AMO3 with perovskite structure have attracted the attention of researchers because of their excellent catalytic performance for OER. In these electrocatalysts, the transition metal cation M forms a co-angled MO6 octahedron with the oxygen ion O, and the A site cation is located in the middle of the void of the MO6 octahedron. The structure can accommodate a variety of A cation and transition metal cation M with different ion radius. Its flexible form is conducive to the regulation of electronic configuration, coordination environment and electrochemical potential, thereby enhancing the catalytic activity of OER. However, many transition metal perovskites are difficult to synthesize under conventional conditions, which is due to the large mismatch between (A-O) and (M-O) bond lengths. Fortunately, pressure as another basic control parameter independent of temperature, magnetic field and chemical composition can effectively adjust the atomic spacing and electron orbital overlap, thus greatly expanding the stability of the perovskite structure. This provides a unique idea for designing new perovskite-type non-precious metal electrocatalysts under the condition of high pressure.

Fig. 1. Crystal structure of CaCoO3 and SrCoO3 synthesized under high pressure

Fig. 2. Comparison of OER performance of CaCoO3 and SrCoO3 and other catalysts

Fig. 3. Electrical transport and magnetic measurement of CaCoO3 and SrCoO3

Fig. 4. Comparison of OER performance of CaCoO3 and SrCoO3 and other catalysts under different pH

Fig. 5. Stability of CaCoO3 and SrCoO3

Fig. 6. OER catalytic mechanism of CaCoO3 and SrCoO3

In accordance with this idea, Li Xiang et al. synthesized cobalt-based perovskite CaCoO3 and SrCoO3 under high pressure using a two-stage propulsion ultra-high pressure experimental device. The results of structural refinement indicate that CaCoO3 has a smaller lattice constant, and its Co-O bond length is the shortest known transition metal perovskite of the same kind (Fig. 1). They then compared the OER activity of CaCoO3 and SrCoO3 with that of traditional high-efficiency catalysts (as shown in Figure 2-5), and found that the former two have significant advantages and remained stable in alkaline medium, while CaCoO3 shows higher activity and stability. In order to further elucidate its high-efficiency catalytic mechanism, they measured the basic physical properties of CaCoO3 and SrCoO3, and found that they have similar initial potentials, and Co4+ ions are in the intermediate spin state. Therefore, in the initial reaction stage, both of them were prone to H+ detachment from surface OH-. However, CaCoO3, which has a small lattice parameter, exhibits higher activity, indicating that there are two competing reactions in the subsequent reaction (Fig. 6): one is the traditional process O+OH="OOH–and OOH+OH=(O2)2–+H2O; the other process is possible O+O+OH=(O2)2–+OH. The former is independent of the distance of surface oxygen separation, while the latter is more likely to occur when the surface oxygen separation distance is shorter, so CaCoO3 with the shortest Co-O bond length exhibits optimal catalytic performance. The above results provide an important basis for regulating the catalytic activity of electrocatalysts under high pressure.

The work was supported by BIT.

[1] Xiang Li, Hao Wang, Zhiming Cui, Yutao Li, Sen Xin, Jianshi Zhou, Youwen Long, Changqing Jin, John B. Goodenough; “Exceptional oxygen evolution reactivities on CaCoO3 and SrCoO3”, Science Advances, 5, eaav6262 (2019).

 

Link to the article: https://advances.sciencemag.org/content/5/8/eaav6262

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