Abstract
An improved understanding of the correlation between the electronic properties of Mn-O bonds, activity and stability of electro-catalysts for the oxygen evolution reaction (OER) is of great importance for an improved catalyst design. Here, an in-depth study of the relation between lattice structure, electronic properties and catalyst performance of the perovskite Ca1−xPrxMnO3 and the first-order RP-system Ca2−xPrxMnO4 at doping levels of x = 0, 0.25 and 0.5 is presented. Lattice structure is determined by X-ray powder diffraction and Rietveld refinement. X-ray absorption spectroscopy of Mn-L and O-K edges gives access to Mn valence and covalency of the Mn-O bond. Oxygen evolution activity and stability is measured by rotating ring disc electrode studies. We demonstrate that the highest activity and stability coincidences for systems with a Mn-valence state of +3.7, though also requiring that the covalency of the Mn-O bond has a relative minimum. This observation points to an oxygen evolution mechanism with high redox activity of Mn. Covalency should be large enough for facile electron transfer from adsorbed oxygen species to the MnO6 network; however, it should not be hampered by oxidation of the lattice oxygen, which might cause a crossover to material degradation. Since valence and covalency changes are not entirely independent, the introduction of the energy position of the eg↑ pre-edge peak in the O-K spectra as a new descriptor for oxygen evolution is suggested, leading to a volcano-like representation of the OER activity.
Highlights
The oxygen evolution reaction (OER) is the bottleneck in electro-chemical water splitting.Transforming two H2 O molecules into molecular O2 via a four-step electron transfer reaction is demanding and requires large overpotentials
For RP25, we found the best fit for the tetragonal space group, I/4 mmm, with lattice parameters close to that reported by Daoudi et al [44] and in contrast to the orthorhombic charge ordered structure, which was found at room temperature in reference [45]
It is quite natural that the space group of the RP-systems is changed by because of different of the
Summary
The oxygen evolution reaction (OER) is the bottleneck in electro-chemical water splitting.Transforming two H2 O molecules into molecular O2 via a four-step electron transfer reaction is demanding and requires large overpotentials. Metal oxides are a natural choice for catalysts [9,10,11,12,13,14,15,16,17] because even noble metals such as Pt form surface oxides under the highly oxidizing conditions of the OER [18]. Many binary metal oxides are insulators [19], either due to the ionic nature of the metal-oxygen bond or electronic correlations. Such materials are not suitable for the catalysis of electron transfer reactions because of poor electrical conductivity. Based on the assumption that metals represent the Materials 2016, 9, 921; doi:10.3390/ma9110921 www.mdpi.com/journal/materials
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
