Hillebrand, Philipp: Structure-function relationships of cobalt and manganese oxides as catalysts for (photo-) electrochemical water splitting = Struktur-Funktions-Beziehungen von Kobalt- und Manganoxid als Katalysatoren für die (photo-) elektrochemische Wasserspaltung. , 2015
Technische Universität Berlin
Open Access Version
Photoelectrochemical water splitting represents a promising route for producing sustainable and potentially cheap hydrogen as an alternative energy carrier to replace fossil fuels. However, several scientific and technological challenges are still to be solved. One of the key issues is the availability of high efficient catalysts for the oxygen evolution reaction (OER) at the anode side of the system, to minimize energetic losses in the process of charge transfer. In this work cobalt and manganese oxides were investigated as cheap and abundant catalysts for the OER in alkaline media. The main focus was put on electrochemically deposited cobalt oxide (CoOx) on FTO substrates, which is easy to fabricate and does not need the application of high temperatures to form the catalyst. On the contrary, annealing of the as-deposited amorphous thin films has led to the formation of crystalline spinel phase Co3O4 correlated to a decrease in activity. In the manganese oxide system on the other hand, crystalline α-Mn2O3, which formed after annealing the galvanostactically deposited film at 500°C in air, showed the best catalytic performance. This material is characterized by (partially) strongly distorted [MnO6] coordination octahedrons with a wide gamut of different interatomic bonding distances. It is assumed that the availability of various kinds of energetically slightly different binding sites at the surface of amorphous CoOx and crystalline α-Mn2O3 is responsible for the high catalytic activity. Overpotentials of 370 and 360 mV at 10 mA/cm2 have been achieved for CoOx and α-Mn2O3, respectively, which are comparable to the best catalysts based on abundant transition metal compounds1. In the synthesis of CoOx electrodes, a systematic increase of the charge during film formation, which is a measure for the amount of the deposited catalyst, had a pronounced effect on the catalytic performance. This behavior was attributed to microporosity of the films, which is supported by TEM imaging. Differential electrochemical mass spectroscopy (DEMS) confirmed that the faradaic current at anodic potentials above 1.5 V (RHE) is related to oxygen evolution. Measurements in buffered aqueous solutions at pH7 revealed acidification inside the pores, if the buffer concentration is too low. In general, the activity at neutral pH is significantly lower compared to alkaline media. In-line synchrotron X-ray photoelectron spectroscopy (SXPS) was applied to study the surface state of CoOx and α-Mn2O3 after the application of different anodic potentials. CoOx showed a partial oxidation of Co2+ to Co3+ at moderate potentials, but no Co4+ was detected in the OER potential range. This was surprising, since electrochemical experiments and literature data strongly suggest the formation of Co4+. In-situ UV/Vis measurements showed a pronounced electrochromic effect and gave further evidence for the appearance of Co4+ at potentials above 1.5 V. Furthermore, UV/Vis studies provided indications for the instability of Co4+, if the potential is removed, which is the reason for not appearing in the X-ray spectra. Instead, α-Mn2O3 showed a clear share of Mn4+ after application of a potential in the OER regime. This difference in stability of the tetravalent oxidation state is interpreted as a sign for a possible different oxygen evolution reaction mechanism at the electrode/electrolyte interface.