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

Bixbyite -MnO 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 -MnO had a better performance as a bifunctional catalyst in the oxygen evolution (OER) and oxygen reduction reactions (ORR) (. 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 -MnO, and then input them into ab initio calculations. The electronic structure calculations indicate that the orthorhombic cubic phase transition causes the Mn 3 and O 2 bands to overlap and mix covalently, transforming -MnO 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.