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

doi:10.1073/pnas.1722235115

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

Proc Natl Acad Sci U S A. 2018 Jun 5;115(23):E5261-E5268. doi: 10.1073/pnas.1722235115. Epub 2018 May 21.

Authors

Zamyla Morgan Chan¹, Daniil A Kitchaev², Johanna Nelson Weker³, Christoph Schnedermann¹, Kipil Lim^{3

4}, Gerbrand Ceder^{5

6

7}, William Tumas⁸, Michael F Toney⁹, Daniel G Nocera¹⁰

Affiliations

¹ Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138.
² Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.
³ Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA 94025.
⁴ Department of Materials Science, Stanford University, Menlo Park, CA 94025.
⁵ Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; gceder@berkeley.edu Bill.Tumas@nrel.gov mftoney@slac.stanford.edu dnocera@fas.harvard.edu.
⁶ Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
⁷ Department of Materials Science and Engineering, University of California, Berkeley, CA 94720.
⁸ National Renewable Energy Laboratory, Golden, CO 80401 gceder@berkeley.edu Bill.Tumas@nrel.gov mftoney@slac.stanford.edu dnocera@fas.harvard.edu.
⁹ Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA 94025; gceder@berkeley.edu Bill.Tumas@nrel.gov mftoney@slac.stanford.edu dnocera@fas.harvard.edu.
¹⁰ Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138; gceder@berkeley.edu Bill.Tumas@nrel.gov mftoney@slac.stanford.edu dnocera@fas.harvard.edu.

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⁺(T_d) 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.

Keywords: catalysis; manganese oxide; polymorph; renewable energy storage; water splitting.

Publication types

Research Support, U.S. Gov't, Non-P.H.S.