Electrochemical trapping of metastable Mn3+ ions for activation of MnO2 oxygen evolution catalysts

Zamyla Morgan Chan, Daniil A. Kitchaev, Johanna Nelson Weker, Christoph Schnedermann, Kipil Lim, Gerbrand Ceder, William Tumas, Michael F. Toney, Daniel G. Nocera

Research output: Contribution to journalArticle

54 Citations (Scopus)

Abstract

Electrodeposited manganese oxide films are promising catalysts for promoting the oxygen evolution reaction (OER), especially in acidic solutions. The activity of these catalysts is known to be enhanced by the introduction of Mn3+. We present in situ electrochemical and X-ray absorption spectroscopic studies, which reveal that Mn3+ may be introduced into MnO2 by an electrochemically induced comproportionation reaction with Mn2+ and that Mn3+ persists in OER active films. Extended X-ray absorption fine structure (EXAFS) spectra of the Mn3+-activated films indicate a decrease in the Mn–O coordination number, and Raman microspectroscopy reveals the presence of distorted Mn–O environments. Computational studies show that Mn3+ is kinetically trapped in tetrahedral sites and in a fully oxidized structure, consistent with the reduction of coordination number observed in EXAFS. Although in a reduced state, computation shows that Mn3+ states are stabilized relative to those of oxygen and that the highest occupied molecular orbital (HOMO) is thus dominated by oxygen states. Furthermore, the Mn3+(Td) induces local strain on the oxide sublattice as observed in Raman spectra and results in a reduced gap between the HOMO and the lowest unoccupied molecular orbital (LUMO). The confluence of a reduced HOMO–LUMO gap and oxygen-based HOMO results in the facilitation of OER on the application of anodic potentials to the δ-MnO2 polymorph incorporating Mn3+ ions.

Original languageEnglish
Pages (from-to)E5261-E5268
JournalProceedings of the National Academy of Sciences of the United States of America
Volume115
Issue number23
DOIs
Publication statusPublished - Jun 5 2018

Keywords

  • Catalysis
  • Manganese oxide
  • Polymorph
  • Renewable energy storage
  • Water splitting

ASJC Scopus subject areas

  • General

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