臨場X光吸收(XAS)應用在電化學觸媒研究

臨場X光吸收(XAS)應用在電化學觸媒研究

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In a study, in-situ X-ray absorption spectroscopy applied with electrochemical systems was used to elucidate the chemical nature of electrocatalysts under reaction conditions. XAS measurements on bifunctional manganese oxide (MnO) catalyst with high gen oxygen evolution reduction (ORR) reaction reaction (OER) was performed. 

臨場X光吸收(XAS)應用在電化學觸媒研究

Figure 1.  A schematic introduction of XAS and in situ XAS. (a) An XAS spectrum of Pt foil at the Pt L3 edge, showing that it consists of three parts, the pre-edge (red), the XANES (cyan) and the EXAFS (green). The insets show the electron transition happening in the XANES and the interference between photoelectrons and backscattered electrons in the EXAFS. Schematic illustration of (b) in situ XAS measurements under potential control, showing that XAS is a bulk per- atom averaging technique whilst electrochemical measurements are a probe of the surface (including the near-surface, few nanometers beneath the topmost layer, depending on the reaction), and (c) approaches to achieve surface-sensitive XAS for electrocatalysts. (Huang & Russell , 2021)

 

The X-ray absorption near edge (XANES) region is sensitive to the oxidation state and electronic structure of the absorber. The extended X-ray absorption fine structure (EXAFS) provides information about the local coordination of the absorber, the identity and coordination number of the neighboring atoms, and the distance between the absorber and the neighbors. In comparison to other approaches for characterizing electrocatalysts, XAS has two distinct advantages. The local structure regardless of the material's degree of crystallinity.

 

臨場X光吸收(XAS)應用在電化學觸媒研究

Figure 2.  (a) Comparison of XANES for 9 (solid lines) and 1 (dashed lines) cycle samples after exposure to ORR (red) and OER (blue) potentials. (b) CV characterization of the ORR and the OER activities, illustrating that the OER activity scales with the thickness of MnOx catalyst. (Gorlin et al., 2012)

 

Figure 2. (a) shows the oxidation state changes observed in the XANES spectrum. It was discovered that the disorder Mn  II,III,III  O  4  phase is produced by exposure to an ORR-relevant potential of 0.7 V vs RHE with minimal contributions from other phases. After the potential is raised to 1.8 V versus RHE, a highly anodic value, relevant to the OER, it was concluded that 20% of the catalytic thin film is less oxidized, possibly representing unchanged Mn 3 II,  III  , III  O  4  and 80% are oxidized to generate a mixed Mn  II,IV  O  4  oxide. 

臨場X光吸收(XAS)應用在電化學觸媒研究

References

[1] Huang, H., & Russell, AE (2021). Approaches to achieve surface sensitivity in the in situ XAS of electrocatalysts.  Current Opinion in Electrochemistry  ,  27  , 100681. https://doi.org/10.1016/j. coelec.2020.100681

[2] Gorlin, Y., Lassalle-kaiser, B., Benck, JD, Gul, S., Webb, SM, Yachandra, VK, Yano, J., & Jaramillo, TF (2012).  Ja3104632.Pdf  .

 

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