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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.
Bibliographical noteFunding Information:
V.C., A.E.R., and D.J.F. kindly thank the UK Catalysis Hub for resources and support provided via the membership of the UK Catalysis Hub Consortium (EPSRC grants EP/K014706/1 and EP/K014714/1). The authors wish to acknowledge the Diamond Light Source for provision of beamtime (SP16479). EXAFS experiments were performed on the SAMBA beamline at SOLEIL Synchrotron, France (proposal number 20160321). We are grateful to Dr Andrea Zitolo for assistance and to the SOLEIL staff for smoothly running the facility. V.C. is in debt to the STFC Batteries Network (ST/N002385/1) for the support through the STFC Batteries Early Career Award. H.H. acknowledges the China Scholarship Council (201608440295) and the University of Southampton and A.S.L. acknowledges City Technology and the EPSRC CASE award scheme and D.I. Johnson Matthey and the EPSRC CASE award scheme for support of their Ph.D. studentships.
- perovskite oxides
- oxygen reduction reaction
- X-ray absorption/emission
- Manganese Redox
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