ABSTRACT

Bixbyite $\alpha$-Mn₂O₃ is an inexpensive Earth-abundant mineral that can be used to drive both oxygen evolution (OER) and oxygen reduction reactions (ORR) in alkaline conditions. It possesses a subtle orthorhombic $\to$ cubic phase change near room temperature that suppresses Jahn–Teller distortions and presents a unique opportunity to study how atomic structure affects the electronic structure and catalytic activity at a temperature range that is easily accessible in OER/ORR experiments. Previously, we observed that heat-treated $\alpha$-Mn₂O₃ had a better performance as a bifunctional catalyst in the oxygen evolution (OER) and oxygen reduction reactions (ORR) (Dalton Trans. 2016, 45, 18,494–18,501). We hypothesized that heat-treatment pinned the material into a more electrochemically active cubic phase. In this manuscript, we use high-resolution X-ray diffraction to collect the temperature-dependent structures of $\alpha$-Mn₂O₃, and then input them into ab initio calculations. The electronic structure calculations indicate that the orthorhombic $\to$ cubic phase transition causes the Mn 3d and O 2p bands to overlap and mix covalently, transforming $\alpha$-Mn₂O₃ from a semiconductor to a semimetal. This subtle change in structure also modifies Mn-O-Mn bond distances, which may improve the activity of the material in oxygen electrochemistry. OER and ORR experiments were performed using the same electrode at various temperatures. They show a jump in the exchange current density near the phase change temperature, demonstrating the higher activity of the cubic phase.

摘要

比克斯比 $\alpha$-Mn ₂ O ₃是一种廉价的地球上富足的矿物，可用于在碱性条件下驱动氧气释放（OER）和氧气还原反应（ORR）。它具有微妙的斜方晶系$\to$室温附近的立方相变抑制了Jahn-Teller畸变，为研究原子结构如何影响OER / ORR实验中容易达到的温度范围内的电子结构和催化活性提供了独特的机会。以前，我们观察到$\alpha$-Mn ₂ O ₃作为双功能催化剂在氧释放（OER）和氧还原反应（ORR）中具有更好的性能（Dalton Trans。2016，45，18,494–18,501）。我们假设热处理将材料固定在更具电化学活性的立方相中。在本手稿中，我们使用高分辨率X射线衍射来收集与温度相关的结构$\alpha$-Mn ₂ O ₃，然后将它们输入从头算。电子结构计算表明，斜方晶$\to$立方相变导致Mn 3 d和O 2 p带重叠并共价混合，从而转变为$\alpha$-Mn ₂ O ₃从半导体到半金属。这种微妙的结构变化也修饰了Mn-O-Mn键距，这可以提高材料在氧电化学中的活性。使用同一电极在不同温度下进行OER和ORR实验。它们在相变温度附近显示出交换电流密度的跃变，表明立方相的活性更高。