Sunlight is by far the most abundant source of energy, and finding efficient ways to capture and store solar energy on a global scale is a challenge of paramount importance.One of the few viable ways to store energy on the required TW-scale is in the form of a chemical fuel. A promising route towards such ‘solar fuels’ is the production of hydrogen by light-driven electrolysis of water.

In order to achieve the required high efficiencies of such photo-electrochemical devices, semiconducting light absorbers have to be combined with highly active catalysts for the hydrogen- and oxygen-evolution reactions (HER, OER).This is especially important for the oxygen evolution reaction, since its slow kineticslimit the overall efficiency of water electrolysis. Developing low cost, earth-abundant and highly efficient oxygen evolving catalysts is thus an important challenge.

Nature uses transition metal-protein complexes in photosystems I and II of the thylakoid membrane to reduce protons and evolve oxygen from water. The oxygen evolving complex of PS-II is composed of a Mn₃CaO₃MnO cluster, which shows turnover frequencies > 100 s^‑1 at modest overpotentials. Our aim is to mimic the high catalytic activity of these clusters with an inorganic approach. To this end, different manganese oxides and calcium manganates have been synthesized, and their catalytic behavior with respect to the oxygen evolution reaction (OER) are investigated.

Thin layers of manganese oxides were prepared onto F:SnO₂-glass substrates by electrodeposition and by RF magnetron sputtering under various process conditions. Furthermore, cobalt oxides, which are known to have a high activity for the OER, were electro-deposited as a reference material.

Electrochemical characterization was done by cyclic voltammetry and electrochemical mass spectroscopy. We employ in-situ X-ray absorption spectroscopy (EXAFS and XANES) and in-system synchrotron radiation photoelectron spectroscopy (SR-XPS) as tools to investigate the electrochemical behavior of these catalysts at the electrode/electrolyte interface as a function of applied potential. We find that a-Mn₂O₃ and Co₃O₄ show the highest activities for the OER, with current densities of ~1 mA/cm² at an overpotential of 320 mV and at pH 14. The samples show remarkable stability at this pH. We will discuss the structural features of the manganese oxides and calcium manganates with respect to their electrochemical behavior, and compare these with the structure of the Mn₃CaO₄MnO cluster in PS-II. These findings offer useful clues for the design of efficient artifical electrocatalysts for the OER and ORR reactions.