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 better performance as a bifunctional catalyst in the oxygen evolution (OER) and oxygen reduction reactions (ORR) (Dalton Trans. 2016, 45, 18494-18501). 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）。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，并提供了一个独特的机会来研究在OER / ORR实验容易达到的温度范围内原子结构如何影响电子结构和催化活性。以前，我们观察到$$\alpha$$-Mn ₂ O ₃在氧气析出（OER）和氧气还原反应（ORR）中作为双功能催化剂具有更好的性能（Dalton Trans .2016，45，18494-18501）。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。电子结构计算表明正交晶$$\to$$立方相变导致Mn 3 d和O 2 p带重叠并共价混合，$$\alpha$$-Mn ₂ O ₃从半导体到半金属。这种微妙的结构变化也改变了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。