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Temperature-Dependent Electronic Structure of Bixbyite α-Mn2O3 and the Importance of a Subtle Structural Change on Oxygen Electrocatalysis

preprint
submitted on 14.02.2019 and posted on 14.02.2019 by Junais Mokkath, Maryam Jahan, Masahiko Tanaka, Satoshi Tominaka, Joel Henzie

α-Mn2O3 is an inexpensive Earth-abundant mineral that is used as an electrode material in various kinds of electrochemical devices. The complex bixbyite structure of α-Mn2O3, and its subtle orthorhombic → cubic phase transformation near room temperature has made it challenging to accurately determine its electronic proper- ties. We used high-resolution X-ray diffraction to study the temperature-dependent structures of phase-pure α-Mn2O3 prisms. Our measurements show a clear change in the crystal phase from orthorhombic → cubic between 293K and 300K. We input the Rietveld refined high-resolution crystal structures collected at various temperatures (273, 293, 300, 330K) directly into density functional theory (DFT) calculations to model their electronic properties. These calculations indicate that the orthorhombic phase α-Mn2O3 is a narrow bandgap semiconductor as expected. However, temper- atures higher than 300K transform the α-Mn2O3 into the cubic phase, causing the molecular orbitals of the Mn 3d and O 2p bands to overlap and mix covalently, mak- ing the material behave as a semimetal. This subtle change in crystal structure will affect the bulk conductivity of the material as well as Mn-O-Mn bond distances that influence the quality of its catalytic active sites for oxygen electrochemistry. Elec- trochemical oxygen evolution (OER) and oxygen reduction reaction (ORR) experi- ments performed at various temperatures (∼ 288K to 323K) using the same prepared electrode show a marked enhancement in both OER and ORR performance that is attributed to the higher activity of the cubic phase.

Funding

JASRI 2015B4503

JSPS Bilateral Research Grant

Study on unknown structure analysis method of nanomaterial: Construction of analysis algorithm using two-body distribution function

Japan Society for the Promotion of Science

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History

Email Address of Submitting Author

joelhenzie@protonmail.com

Institution

National Institute for Materials Science (NIMS)

Country

Japan

ORCID For Submitting Author

0000-0002-9190-2645

Declaration of Conflict of Interest

n/a

Version Notes

14 February 2019 (12:00 JST)

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