Relationship between Mn Oxidation State Changes and Oxygen Reduction Activity in (La,Ca)MnO3 as Probed by In Situ XAS and XES

High Resolution Image Download MS PowerPoint Slide 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 LaMnO 3 and CaMnO 3, 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 Mn 2+, Mn 3+, and Mn 4+ sites as a function of the applied potential. For LaMnO 3, XANES and XES show that Mn 3+ is formed, but Mn 4+ 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 Mn 3+ to Mn 4+ ratio increases. For CaMnO 3, XANES and XES show the formation of both Mn 3+ and Mn 2+ as the potential is made more negative, with little retention of Mn 4+ at 0.2 V. The EXAFS for CaMnO 3 also indicates the formation of oxygen vacancies, but in contrast to LaMnO 3, 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.