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BIT Publishes Important Research Results in “Science Advances”

release date :2019-09-27 10:07:00  |   [ close window ]ViewCount:

     Translator: News Agency of BIT, Wu Yushan

    Editor: News Agency of BIT

  Recently, Li Xiang, a special researcher from the team of Professor Yao Yugui of the School of Physics, Beijing Institute of Technology (BIT), and Dr. Wang Hao, Dr. Li Yutao, Professor John B. Goodenough, Professor Zhou Jianshi from the University of Texas at Austin (UT-Austin) with Chang Qing and Long Youwen, researchers from the Institute of Physics of the Chinese Academy of Sciences and other collaborators have made breakthroughs in designing new oxygen evolution reactivities (OER) electrocatalysts at high pressure. They have successfully developed low-cost, high-efficiency and stable perovskite-type non-precious metal electrocatalysts CaCoO3 and SrCoO3 under extreme high pressure conditions, and found that their OER catalytic performance is much better than traditional materials. Their structure-performance relationship is further studied. It reveals a new OER reaction mechanism and opens up new avenues for exploring advanced electrochemical functional materials. The results were recently published in the “Science Advance” [Science Advances, 5, eaav6262 (2019)].

  Water electrolysis is an important way to obtain new clean energy hydrogen, mainly including two reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Due to the slow reaction kinetics, the later often requires the help of electrocatalyst to speed up the reaction. However, the commonly used OER electrocatalysts (precious metal oxides such as RuO2 and IrO2) are too expensive to be widely applied. 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 metal transition metal oxides AMO3 with perovskite structure have attracted the attention of researchers due to their excellent OER catalytic performance. 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 plurality of different ionic radii cations and transition metals of Cationic M, its flexible and variable form is beneficial to regulate the electronic configuration, coordination environment and electrochemical potential, and thus enhance the OER catalytic activity. However, many transition metal perovskites are difficult to synthesize under normal conditions due to the large mismatch in (A-O) and (M-O) bond lengths. Fortunately, as another basic control parameter independent of temperature, magnetic field and chemical composition,pressure 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 high pressure conditions.

Figure1 Crystal Structure by High Pressure Synthesis of CaCoO3 and SrCoO3

Figure2 Comparison of OER performance between CaCoO3 and SrCoO3 and other catalysts

Figure3 Electrical transport and magnetic measurement of CaCoO3 and SrCoO3

Figure4 Comparison of OER performance between CaCoO3 and SrCoO3 and other catalysts under different pH conditions

Figure5 Stability of CaCoO3 and SrCoO3

Figure6 OER catalytic mechanism of CaCoO3 and SrCoO3

  According to this idea, Li Xiang and others synthesized cobalt-based perovskite CaCoO3 and SrCoO3 under high pressure conditions using a two-stage propulsion type 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 among the known transition metal perovskites (Figure1). Then they compared CaCoO3 and SrCoO3 with the OER activity of traditional high-efficiency catalysts (as shown in Figure2-5), and found that the first two have significant advantages and remain stable in alkaline media; while CaCoO3 shows higher activity and stability. In order to further elucidate their high-efficiency catalytic mechanism, researchers measured the basic physical properties of CaCoO3 and SrCoO3, and found that they have similar initial potentials, and the Co4+ ions are in the intermediate spin state. Therefore, in the initial reaction stage, both are prone to have the reaction of detachment of H+ from OH- from the surface; however, CaCoO3, which has a smaller lattice parameter, exhibits higher activity, indicating that there are two competing reaction processes in the subsequent reaction (Figure6): one is the conventional processes, O–+OH– ="OOH–and OOH–+OH–=(O2)2–+H2O; the other is the possible process, O–+O–+OH–=(O2)2–+OH–. The former reaction is independent of the distance between separating oxygen in surface, while the latter is more likely to occur when the distance is shorter. Therefore, CaCoO3 with the shortest Co-O bond length exhibits the best performance. The above research results provide an important basis for the catalytic activity of high pressure regulated electrocatalysts.

  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).

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