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

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 ?-MnO₂ polymorph incorporating Mn³⁺ ions.

SUBMITTER: Morgan Chan Z

PROVIDER: S-EPMC6003334 | biostudies-literature | 2018 Jun

REPOSITORIES: biostudies-literature

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

Morgan Chan Zamyla Z Kitchaev Daniil A DA Nelson Weker Johanna J Schnedermann Christoph C Lim Kipil K Ceder Gerbrand G Tumas William W Toney Michael F MF Nocera Daniel G DG

Proceedings of the National Academy of Sciences of the United States of America 20180521 23

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+ p ...[more]

PMID: 29784802

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Project description:Transition metal oxides are characterized by an acute structure and composition dependent electrocatalytic activity toward the oxygen evolution (OER) and oxygen reduction (ORR) reactions. For instance, Mn containing oxides are among the most active ORR catalysts, while Ni based compounds tend to show high activity toward the OER in alkaline solutions. In this study, we show that incorporation of Ni into α-MnO2, by adding Ni precursor into the Mn-containing hydrothermal solution, can generate distinctive sites with different electronic configurations and contrasting electrocatalytic activity. The structure and composition of the Ni modified hollandite α-MnO2 phase were investigated by X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), transmission electron microscopy coupled to energy-dispersive X-ray spectroscopy (TEM-EDX), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and X-ray photoelectron spectroscopy (XPS). Our analysis suggests that Mn replacement by Ni into the α-MnO2 lattice (site A) occurs up to approximately 5% of the total Mn content, while further increasing Ni content promotes the nucleation of separate Ni phases (site B). XAS and XRD show that the introduction of sites A and B have a negligible effect on the overall Mn oxidation state and bonding characteristics, while very subtle changes in the XPS spectra appear to suggest changes in the electronic configuration upon Ni incorporation into the α-MnO2 lattice. On the other hand, changes in the electronic structure promoted by site A have a significant impact in the pseudocapacitive responses obtained by cyclic voltammetry in KOH solution at pH 13, revealing the appearance of Mn 3d orbitals at the energy (potential) range relevant to the ORR. The evolution of Mn 3d upon Ni replacement significantly increases the catalytic activity of α-MnO2 toward the ORR. Interestingly, the formation of segregated Ni phases (site B) leads to a decrease in the ORR activity while increasing the OER rate.

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

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Electrochemical trapping of metastable Mn<sup>3+</sup> ions for activation of MnO<sub>2</sub> oxygen evolution catalysts.

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