Julius Scholz

Oxygen Evolution at Manganite Perovskite Ruddlesden-Popper Type Particles: Trends of Activity on Structure, Valence and Covalence

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.

Ex-Situ Analysis and In-Situ Environmental TEM Studies of Manganite Perovskites for Catalytic Water Splitting

Catalytic water splitting, i.e. the electrochemical decomposition of water into molecular hydrogen and oxygen allows for conversion and storage of solar or electrical energy in chemical energy and, thus, represents an important contribution to the development of renewable energy sources. The oxygen evolution half reaction 2H2O+4h4H + +O2 is the bottleneck of water-splitting since it requires the coupled transfer of 4 electron holes from the catalyst surface to the water molecule. This half reaction was comprehensively studied on perovskite catalysts [1], because they provide high chemical flexibility and the opportunity to selectively tune and adjust the electronic structure to the water oxidation potential by Aand B-site doping. Furthermore, electronic and electron-lattice correlations influence the electronic and crystal structure and may result in the formation of polaronic charge carriers, well known in particular for manganites. However, the impact of such correlation effects on catalytic activity is not yet understood.