Department of Chemical Engineering, Stanford University, Stanford, CA 94305.
Physical Biosciences Division, Lawrence Berkeley National Laboratory Berkeley, CA 94720.
J Am Chem Soc. 2013 Jun 12;135(23):8525-34. doi: 10.1021/ja3104632. Epub 2013 Jun 3.
In situ X-ray absorption spectroscopy (XAS) is a powerful technique that can be applied to electrochemical systems, with the ability to elucidate the chemical nature of electrocatalysts under reaction conditions. In this study, we perform in situ XAS measurements on a bifunctional manganese oxide (MnOx) catalyst with high electrochemical activity for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), we find that exposure to an ORR-relevant potential of 0.7 V vs RHE produces a disordered Mn3(II,III,III)O4 phase with negligible contributions from other phases. After the potential is increased to a highly anodic value of 1.8 V vs RHE, relevant to the OER, we observe an oxidation of approximately 80% of the catalytic thin film to form a mixed Mn(III,IV) oxide, while the remaining 20% of the film consists of a less oxidized phase, likely corresponding to unchanged Mn3(II,III,III)O4. XAS and electrochemical characterization of two thin film catalysts with different MnOx thicknesses reveals no significant influence of thickness on the measured oxidation states, at either ORR or OER potentials, but demonstrates that the OER activity scales with film thickness. This result suggests that the films have porous structure, which does not restrict electrocatalysis to the top geometric layer of the film. As the portion of the catalyst film that is most likely to be oxidized at the high potentials necessary for the OER is that which is closest to the electrolyte interface, we hypothesize that the Mn(III,IV) oxide, rather than Mn3(II,III,III)O4, is the phase pertinent to the observed OER activity.
原位 X 射线吸收光谱（XAS）是一种强大的技术，可应用于电化学系统，能够阐明反应条件下电催化剂的化学性质。在这项研究中，我们对具有高电化学活性的双功能锰氧化物（MnOx）催化剂进行了原位 XAS 测量，该催化剂可用于氧还原反应（ORR）和氧析出反应（OER）。通过 X 射线吸收近边结构（XANES）和扩展 X 射线吸收精细结构（EXAFS），我们发现，在与 ORR 相关的 0.7 V vs RHE 电势下暴露会产生无序的 Mn3(II,III,III)O4 相，几乎没有其他相的贡献。当电势增加到与 OER 相关的 1.8 V vs RHE 的高阳极值时，我们观察到大约 80%的催化薄膜被氧化形成混合 Mn(III,IV)氧化物，而薄膜的其余 20%由氧化程度较低的相组成，可能对应于未改变的 Mn3(II,III,III)O4。两种具有不同 MnOx 厚度的薄膜催化剂的 XAS 和电化学表征表明，在 ORR 或 OER 电势下，厚度对测量的氧化态没有明显影响，但证明 OER 活性与薄膜厚度成正比。这一结果表明，这些薄膜具有多孔结构，不会限制电催化作用仅限于薄膜的最上层几何层。由于在 OER 所需的高电势下最有可能被氧化的催化剂薄膜部分是最接近电解质界面的部分，因此我们假设 Mn(III,IV)氧化物而不是 Mn3(II,III,III)O4 是与观察到的 OER 活性相关的相。
