title: Nature of Activated Manganese Oxide for Oxygen Evolution.
abstract: Electrodeposited manganese oxide films (MnOx) are promising stable oxygen evolution catalysts. They are able to catalyze the oxygen evolution reaction in acidic solutions but with only modest activity when prepared by constant anodic potential deposition. We now show that the performance of these catalysts is improved when they are "activated" by potential cycling protocols, as measured by Tafel analysis (where lower slope is better): upon activation the Tafel slope decreases from ∼120 to ∼70 mV/decade in neutral conditions and from ∼650 to ∼90 mV/decade in acidic solutions. Electrochemical, spectroscopic, and structural methods were employed to study the activation process and support a mechanism where the original birnessite-like MnOx (δ-MnO2) undergoes a phase change, induced by comproportionation with cathodically generated Mn(OH)2, to a hausmannite-like intermediate (α-Mn3O4). Subsequent anodic conditioning from voltage cycling or water oxidation produces a disordered birnessite-like phase, which is highly active for oxygen evolution. At pH 2.5, the current density of activated MnOx (at an overpotential of 600 mV) is 2 orders of magnitude higher than that of the original MnOx and begins to approach that of Ru and Ir oxides in acid.
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University , Cambridge, Massachusetts 02138, United States.
authors.forename: Michael
authors.initials: M
authors.lastname: Huynh
authors.affiliation: Department of Applied Physics and Applied Mathematics, Columbia University , New York, New York 10027, United States.
authors.forename: Chenyang
authors.initials: C
authors.lastname: Shi
authors.affiliation: Department of Applied Physics and Applied Mathematics, Columbia University , New York, New York 10027, United States.|Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory , Upton, New York 11973, United States.
authors.forename: Simon J L
authors.initials: SJ
authors.lastname: Billinge
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University , Cambridge, Massachusetts 02138, United States.
authors.forename: Daniel G
authors.initials: DG
authors.lastname: Nocera