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Issue 28, 2019
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Origin of the heat-induced improvement of catalytic activity and stability of MnOx electrocatalysts for water oxidation

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Abstract

Catalysis of the oxygen evolution reaction (OER) by earth-abundant materials in the near-neutral pH regime is of great interest as it is the key reaction for non-fossil fuel production. To address the pertinent stability problems and insufficiently understood structure–activity relations, we investigate the influence of moderate annealing (100–300 °C for 20 min) for two types of electrodeposited Mn oxide films with contrasting properties. Upon annealing, the originally inactive and structurally well-ordered Oxide 1 of birnessite type became as OER active as the non-heated Oxide 2, which has a highly disordered atomic structure. Oxide 2 also improved its activity upon heating, but more important is the stability improvement: the operation time increased by about two orders of magnitude (in 0.1 M KPi at pH 7). Aiming at atomistic understanding, electrochemical methods including quantitative analysis of impedance spectra, X-ray spectroscopy (XANES and EXAFS), and adapted optical spectroscopies (infrared, UV-vis and Raman) identified structure–reactivity relations. Oxide structures featuring both di-μ-oxo bridged Mn ions and (close to) linear mono-μ-oxo Mn3+–O–Mn4+ connectivity seem to be a prerequisite for OER activity. The latter motif likely stabilizes Mn3+ ions at higher potentials and promotes electron/hole hopping, a feature related to electrical conductivity and reflected in the strongly accelerated rates of Mn oxidation and O2 formation. Poor charge mobility, which may result from a low level of Mn3+ ions at high potentials, likely promotes inactivation after prolonged operation. Oxide structures related to the perovskite-like ζ-Mn2O3 were formed after the heating of Oxide 2 and could favour stabilization of Mn ions in oxidation states lower than +4. This rare phase was previously found only at high pressure (20 GPa) and temperature (1200 °C) and this is the first report where it was stable under ambient conditions.

Graphical abstract: Origin of the heat-induced improvement of catalytic activity and stability of MnOx electrocatalysts for water oxidation

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Publication details

The article was received on 15 May 2019, accepted on 17 Jun 2019 and first published on 18 Jun 2019


Article type: Paper
DOI: 10.1039/C9TA05108B
J. Mater. Chem. A, 2019,7, 17022-17036
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    Origin of the heat-induced improvement of catalytic activity and stability of MnOx electrocatalysts for water oxidation

    M. V. Abrashev, P. Chernev, P. Kubella, M. R. Mohammadi, C. Pasquini, H. Dau and I. Zaharieva, J. Mater. Chem. A, 2019, 7, 17022
    DOI: 10.1039/C9TA05108B

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