In photosynthesis nature uses transition metal complexes as catalysts to evolve oxygen and hydrogen from water. In this process the catalytic centers are separated from the light capturing and absorbing co-factors in photosystem I and II integrated in the thylakoid membrane. To develop bio-inspired catalysts and to mimic e.g. the Mn₃CaO₃MnO complex in PSII, different manganese oxides as well as alkaline metal and earth alkali metal manganates were investigated with respect to the oxygen evolution reaction (OER) in the process of water oxidation. In this contribution special attention will be turned to the structure - morphology– function relationship of these materials.

Manganese oxide electrodes have been prepared by reactive magnetron sputtering from a Mn target in an Ar/O₂ atmosphere as well as by anodic electrodeposition and a subsequent annealing step. Besides amorphous MnO_x obtained at low temperatures, crystallized oxides, such as γ-MnO₂ and α-Mn₂O₃, were tested as electrocatalysts. Thin (60-70 nm) and dense layers were deposited by reactive magnetron sputtering using conductive glass (FTO) slides as substrates. Cross section transmission electron micrograph (Figure 1a) clearly revealed that the sputtered α-Mn₂O₃ layers consist of nanocrystals of
10 - 20 nm edge length. These layers show current densities of 10 mA/cm^-2 at an overpotential of 370 mV (pH 13.8). Similar overvoltages were also obtained with electrodeposited α-Mn₂O₃ layers (10mA/cm^-2 at 340 mV overvoltage). In contrast, the electrochemically deposited α-Mn₂O₃ layers are several 100 nm thick and appear highly porous (Figure 1b). Obviously, the specific activity related to the active surface area of the sputtered material is about one order of magnitude higher than the one of the electrodeposited samples. Nevertheless, higher current densities can be achieved with the electrodeposited material because high electrochemically active surface areas can be provided.

This behavior will be discussed as a function of defect chemistry and will be compared with the structure – function relationship of other OER electrocatalysts.

References

[1] A. Ramírez, P. Hillebrand, D. Stellmach, M. May, P. Bogdanoff, S. Fiechter; J. Phys Chem. C, 118 (2014) 14073-14081 and SI.

[2] M.M. Najafpour, T. Ehrenberg, M. Wiechen, P. Kurz; Angew. Chem. Int. Ed., 49 (2010) 2233-2237.

Figure 1: α-Mn₂O₃ deposited a) by reactive magnetron sputtering and b) by electrochemical deposition.