Ebrahimizadeh Abrishami Majid, Risch Marcel, Scholz Julius, Roddatis Vladimir, Osterthun Norbert, Jooss Christian
Institute of Materials Physics, University of Goettingen, Friedrich-Hund-Platz 1, Goettingen 37077, Germany.
Materials (Basel). 2016 Nov 14;9(11):921. doi: 10.3390/ma9110921.
An improved understanding of the correlation between the electronic properties of Mn-O bonds, activity and stability of electro-catalysts for the oxygen evolution reaction (OER) is of great importance for an improved catalyst design. Here, an in-depth study of the relation between lattice structure, electronic properties and catalyst performance of the perovskite CaPrMnO₃ and the first-order RP-system CaPrMnO₄ at doping levels of x = 0, 0.25 and 0.5 is presented. Lattice structure is determined by X-ray powder diffraction and Rietveld refinement. X-ray absorption spectroscopy of Mn-L and O-K edges gives access to Mn valence and covalency of the Mn-O bond. Oxygen evolution activity and stability is measured by rotating ring disc electrode studies. We demonstrate that the highest activity and stability coincidences for systems with a Mn-valence state of +3.7, though also requiring that the covalency of the Mn-O bond has a relative minimum. This observation points to an oxygen evolution mechanism with high redox activity of Mn. Covalency should be large enough for facile electron transfer from adsorbed oxygen species to the MnO₆ network; however, it should not be hampered by oxidation of the lattice oxygen, which might cause a crossover to material degradation. Since valence and covalency changes are not entirely independent, the introduction of the energy position of the pre-edge peak in the O-K spectra as a new descriptor for oxygen evolution is suggested, leading to a volcano-like representation of the OER activity.
深入理解锰氧键的电子性质、析氧反应（OER）电催化剂的活性和稳定性之间的相关性对于改进催化剂设计至关重要。在此，本文对钙钛矿CaPrMnO₃和一级RP体系CaPrMnO₄在x = 0、0.25和0.5掺杂水平下的晶格结构、电子性质与催化剂性能之间的关系进行了深入研究。通过X射线粉末衍射和Rietveld精修确定晶格结构。Mn-L和O-K边的X射线吸收光谱可获取Mn的化合价和Mn-O键的共价性。通过旋转环盘电极研究测量析氧活性和稳定性。我们证明，对于Mn价态为+3.7的体系，活性和稳定性最高，不过还要求Mn-O键的共价性具有相对最小值。这一观察结果指向一种具有高Mn氧化还原活性的析氧机制。共价性应足够大，以便吸附的氧物种能方便地向MnO₆网络转移电子；然而，它不应受到晶格氧氧化的阻碍，否则可能导致材料降解。由于化合价和共价性变化并非完全独立，因此建议引入O-K光谱中预边缘峰的能量位置作为析氧的新描述符，从而得到OER活性的类似火山的表示。
