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Zusammenfassung:
The oxygen evolution reaction (OER) is of vital importance for electrochemical energy conversion technologies. The use of metal-organic framework (MOF) catalysts after postprocessing is an important method to optimize catalytic activity; however, little attention has been paid to the transformation of the electronic structure and coordinated ions under operando conditions. Here, we focus on a Prussian blue analogue (PBA) with the formula Na2Co2+[Fe2+(CN)(6)]center dot nH(2)O after postprocessing at a temperature of 400 degrees C. The catalysts exhibit a significant improvement in the OER activity with an overpotential of 254 mV against 320 mV for the initial PBA at 10 mA cm(-2) in 1 M KOH. Our ex situ soft X-ray absorption spectroscopy (XAS) results demonstrate that some of the high-spin Co2+ and low-spin Fe2+ ions of PBA exhibit exchange with N and C ligands. Our fast operando hard XAS study revealed a dynamic evolution process of the transformation of ligands from N(C) in the initial Na2Co[Fe(CN)(6)]center dot nH(2)O to oxygen and the formation of pure low-spin Co3+ oxide and high-spin Fe3+ oxide after OER. The amorphous (Co,Fe)OOH chemical state was identified as the catalytically active state based on our operando X-ray spectroscopy results. The results of this study provide insights into a dynamic evolution process of MOFs after postprocessing and the formation of real active sites during electrocatalysis.
