Metal-Organic and Oxide Nanostructures for Electrocatalyzing the Oxygen Half-Cell Reaction

Energy Storage in the chemical bonds of water, a process accessible through electrochemical conversion, is a simple approach to compensate supply fluctuation common to sustainable energy production such as wind and solar. However, the water cycle is limited by the oxygen half-cell reaction hindered by large overpotentials and suitable electrocatalysts are necessary to enhance electrochemical conversion. For the tailored design of electrocatalysts, fundamental insight into catalytic processes as well as the interaction between individual components of the catalytic material is crucial. This thesis focuses on the investigation of transition metal electrocatalysts involving organic or purely inorganic components. The catalysts are structurally and chemically characterized before and after electrocatalysis by a complementary scanning tunneling microscopy and x-ray spectroscopy approach. The sample preparation in ultra-high vacuum is bridged to ambient electrochemical conditions by a home-built transfer system.

The first part concentrates on Co, Cu and Fe stabilized within a porphyrin environment forming metal-organic coordination networks which are bifunctionally active towards oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Co sublimation onto a Fe/Cu metalloporphyrin network on a Au(111) substrate drives an irreversible on-surface transmetalation process with an exchange rate of ~50% at room temperature. Supporting DFT calculations suggest a redox transmetalation, in which Fe is reduced and expelled from the macrocycle while oxidized Co accommodates within the porphyrin cage. The disposability of a second coordination site in the form of a peripheric pyridyl group is not inhibiting the transmetalation.

The bimetallic Co/Fe tetrapyridylporphyrin network catalyzes the ORR successfully. Both the local structural integrity of the self-assembled network as well as its chemical stability is confirmed for up to 10 electrocatalytic cycles. In contrast, the opposite reaction direction of OER is more aggressive leading to an entirely modified surface structure already after the first cycle. The applied potential induces the formation of Co/Fe oxides which are determined as actual catalytically active phase.

Subsequently, in situ prepared oxide structures are explicitly investigated in the transfer system, starting from Co cluster. Exposure to 1 bar Ar atmosphere partially oxidizes Co, presumably due to adventitious oxygen or water. Aqueous solutions like water or 0.1 M NaOH promote the oxidation process of Co which goes along side with sintering of the cluster. Oxidized Co cluster successfully catalyze the OER reducing the overpotential of Au(111) by approximately 100 mV.

Last the integrity of Fe3O4(001) in the transfer system was investigated, which could be utilized for stabilizing catalytically active single transition metal atoms. The reconstruction persists both Ar and water exposure, but disappears upon 0.1 M NaOH contact which is explained by the formation of a hydroxo complex. Preliminary experiments on Ag adatoms/Fe3O4(001) leave the ORR unaffected but x-ray spectroscopy confirms the stability of the adatoms after ORR.

In conclusion, this thesis demonstrates the importance of catalyst characterization on the molecular scale. The influence of the potential and the electrolyte on catalyst structure should not be underestimated and might change the as-synthesized material to the actual electroactive composition.