Temperature-dependent electronic structure of bixbyite α-Mn2O3 and the importance of a subtle structural change on oxygen electrocatalysis

Bixbyite α-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 → 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 α-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 α-Mn2O3, and then input them into ab initio calculations. The electronic structure calculations indicate that the orthorhombic → cubic phase transition causes the Mn 3d and O 2p bands to overlap and mix covalently, transforming α-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.

方铁锰矿（Bixbyite）型α-Mn₂O₃是一种廉价且地球储量丰富的矿物，可在碱性环境中催化析氧反应（oxygen evolution reaction, OER）与氧还原反应（oxygen reduction reaction, ORR）。该材料在室温附近存在细微的正交晶系→立方晶系相变，该相变可抑制姜-泰勒（Jahn–Teller）畸变，为在OER/ORR实验易实现的温度范围内研究原子结构如何影响电子结构与催化活性提供了独特契机。此前我们的研究发现，经热处理的α-Mn₂O₃作为双功能催化剂在析氧与氧还原反应中表现更优（Dalton Trans. 2016, 45, 18494–18501）。我们曾提出假设：热处理将该材料固定在了电化学活性更高的立方晶相之中。本文中，我们借助高分辨X射线衍射（high-resolution X-ray diffraction）采集了α-Mn₂O₃的温度依赖性结构数据，并将其输入至从头算（ab initio）计算当中。电子结构计算结果表明，正交晶系→立方晶系相变会使Mn 3d轨道与O 2p轨道发生重叠并产生共价混合，将α-Mn₂O₃从半导体转变为半金属。这一细微的结构变化同时还会改变Mn-O-Mn键长，有望提升该材料在氧电化学过程中的催化活性。我们在相同电极上于不同温度下开展了OER与ORR实验，结果显示在相变温度附近交换电流密度出现跃升，证实了立方晶相具有更高的催化活性。