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Relationship between Mn oxidation state changes and oxygen reduction activity in (La,Ca)MnO3 as probed by in situ XAS and XES

DOI: 10.1021/acscatal.1c00997 DOI Help

Authors: Veronica Celorrio (Diamond Light Source) , Andrew S. Leach (University of Southampton) , Haoliang Huang (University of Southampton) , Shusaku Hayama (Diamond Light Source) , Adam Freeman (Diamond Light Source) , David W. Inwood (University of Southampton) , David J. Fermin (University of Bristol) , Andrea E. Russell (University of Southampton)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Acs Catalysis

State: Published (Approved)
Published: May 2021
Diamond Proposal Number(s): 16479

Open Access Open Access

Abstract: In situ X-ray absorption and emission spectroscopies (XAS and XES) are used to provide details regarding the role of the accessibility and extent of redox activity of the Mn ions in determining the oxygen reduction activity of LaMnO3 and CaMnO3, with X-ray absorption near-edge structure (XANES) providing the average oxidation state, extended X-ray absorption fine structure (EXAFS) providing the local coordination environment, and XES providing the population ratios of the Mn2+, Mn3+, and Mn4+ sites as a function of the applied potential. For LaMnO3, XANES and XES show that Mn3+ is formed, but Mn4+ ions are retained, which leads to the 4e– reduction between 0.85 and 0.6 V. At more negative potentials, down to 0.2 V, EXAFS confirms an increase in oxygen vacancies as evidenced by changes in the Mn–O coordination distance and number, while XES shows that the Mn3+ to Mn4+ ratio increases. For CaMnO3, XANES and XES show the formation of both Mn3+ and Mn2+ as the potential is made more negative, with little retention of Mn4+ at 0.2 V. The EXAFS for CaMnO3 also indicates the formation of oxygen vacancies, but in contrast to LaMnO3, this is accompanied by loss of the perovskite structure leading to structural collapse. The results presented have implications in terms of understanding of both the pseudocapacitive response of Mn oxide electrocatalysts and the processes behind degradation of the activity of the materials.

Journal Keywords: perovskite oxides; oxygen reduction reaction; X-ray absorption/emission; LaMnO3; CaMnO3; manganese redox

Subject Areas: Materials, Chemistry, Energy


Instruments: I20-Scanning-X-ray spectroscopy (XAS/XES)

Other Facilities: SAMBA at SOLEIL

Added On: 24/05/2021 10:44

Documents:
acscatal.1c00997.pdf

Discipline Tags:

Energy Storage Energy Physical Chemistry Catalysis Energy Materials Chemistry Materials Science

Technical Tags:

Spectroscopy X-ray Absorption Spectroscopy (XAS) X-ray Emission Spectroscopy (XES) High Energy Resolution Fluorescence Detected XAS (HERFD-XAS)