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

Proceedings of the National Academy of Sciences - PNAS, 2018-06, Vol.115 (23), p.E5261-E5268 [Peer Reviewed Journal]

Volumes 1–89 and 106–114, copyright as a collective work only; author(s) retains copyright to individual articles ;2018 ;ISSN: 0027-8424 ;EISSN: 1091-6490 ;DOI: 10.1073/pnas.1722235115 ;PMID: 29784802

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  • Title:
    Electrochemical trapping of metastable Mn3+ ions for activation of MnO₂ oxygen evolution catalysts
  • Author: Chan, Zamyla Morgan ; Kitchaev, Daniil A. ; Weker, Johanna Nelson ; Schnedermann, Christoph ; Lim, Kipil ; Ceder, Gerbrand ; Tumas, William ; Toney, Michael F. ; Nocera, Daniel G.
  • Subjects: Physical Sciences ; PNAS Plus
  • Is Part Of: Proceedings of the National Academy of Sciences - PNAS, 2018-06, Vol.115 (23), p.E5261-E5268
  • Description: 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 oxygenbased HOMO results in the facilitation of OER on the application of anodic potentials to the δ-MnO₂ polymorph incorporating Mn3+ ions.
  • Publisher: National Academy of Sciences
  • Language: English
  • Identifier: ISSN: 0027-8424
    EISSN: 1091-6490
    DOI: 10.1073/pnas.1722235115
    PMID: 29784802
  • Source: GFMER Free Medical Journals
    PubMed Central
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