Sebastian Watzele

To optimize the performance of catalytic materials, it is paramount to elucidate the dependence of the chemical reactivity on the atomic arrangement of the catalyst surface. Therefore, identifying the nature of the active sites that provide optimal binding of reaction intermediates is the first step toward a rational catalyst design. In this work, we focus on the oxygen reduction reaction (ORR), an essential constituent of several energy provision and storage devices. Among the state-of-the-art ORR catalysts are platinum (Pt) and its alloys. The latter benefit from the so-called ligand and strain effects, which influence the electronic properties of the surface. Here, we “visualize” the active sites on Pt₃Ni(111) in an acidic medium with a lateral resolution in the nanometre regime via an in situ technique based on electrochemical scanning tunnelling microscopy. In contrast to pure Pt, where the active sites are located at concave sites close to steps, Pt₃Ni(111) terraces contain the most active centres, while steps show activity to a comparable or lesser extent. We confirm the experimental findings by a model based on alloy- and strain-sensitive generalized coordination numbers. With this model, we are also able to assess both the composition and the geometric configuration of optimal catalytic active sites on various Pt alloy catalysts. In general, the interplay of ligand effects and lattice compression resulting from the alloying of Pt with 3d transition metals (Ti, Co, Ni, Cu) gradually increases the generalized coordination number of surface Pt atoms, thereby making (111) terraces highly active. This combination of theoretical and experimental tools provides clear strategies to design more efficient Pt alloy electrocatalysts for oxygen reduction.

Efficient electrocatalysis is most likely an answer to recent energy related challenges. Countless studies have been trying to find the links between the electrode/electrolyte interface structure, its composition, and the resulting activity in order to improve the performance of numerous devices, such as electrolyzers, fuel cells, and certain types of batteries. However, this scientific field currently meets serious complications associated with the prediction and explanation of an unexpected influence of seemingly inert electrolyte components on the observed activity. Herein, we investigate various electrocatalytic systems using a unique laser‐induced current transient technique to answer a long‐lasting fundamental question: How can “inert” electrolytes change the activity so drastically? Different metal electrodes in contact with various aqueous solutions and two energy important reactions were used as model systems. We experimentally determine the potential of maximum entropy of the electrodes and find the connections between its position and the electrocatalytic performance.

The oxygen evolution reaction (OER) is a key process for renewa-ble energy storage. However, developing non-noble metal OER electrocatalysts with high activity, long durability and scalability remains a major challenge. Herein, high OER activity and stability in alkaline solution were discovered for mixed nickel/cobalt hy-droxide electrocatalysts, which were derived in one-step procedure from oriented surface-mounted metal-organic framework (SURMOF) thin films that had been directly grown layer-by-layer on macro- and microelectrode substrates. The obtained mass activi-ty of ~2.5 mA∙μg-1 at the defined overpotential of 300 mV is one order of magnitude higher than that of the benchmarked IrO2 elec-trocatalyst and at least 3.5 times higher than the mass activity of any state-of-the-art NiFe-, FeCoW- or NiCo-based electrocatalysts reported in the literature. The excellent morphology of the SURMOF-derived ultrathin electrocatalyst coating led to a high exposure of the most active Ni- and Co-based sites.

The oxygen reduction reaction (ORR) is of great interest for future sustainable energy conversion and storage, especially concerning fuel cell applications. The preparation of active, affordable and scalable electrocatalysts and their application in fuel cell engines of hydrogen cars is a prominent step towards the reduction of air pollution, especially in urban areas. Alloying nanostructured Pt with lanthanides is a promising approach to enhance its catalytic ORR activity, whereby the development of a simple synthetic route turned out to be non-trivial endeavor. Herein, for the first time we present a successful single-step, scalable top-down synthetic route for Pt-lanthanide alloy nanoparticles, as witnessed by the example of Pr-alloyed Pt nanoparticles. The catalyst was characterized by high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and photoelectron spectroscopy and its electrocatalytic oxygen reduction activity was investigated using a rotating disk electrode technique. PtxPr/C showed ~3.5-times higher (1.96 mA/cm2, 0.9 V vs reversible hydrogen electrode (RHE)) specific activity and ~1.6-times higher (696 mA/mgPt, 0.9 V vs RHE) mass activity compared to commercial Pt/C catalysts. Based on previous findings and characterization of the PtxPr/C catalyst, the activity improvement over commercial Pt/C originates from a lattice strain introduced by the alloying process.