Verena Pfeifer

Photoelectron Spectroscopy at the Graphene–Liquid Interface Reveals the Electronic Structure of an Electrodeposited Cobalt/Graphene Electrocatalyst

Electrochemically grown cobalt on graphene exhibits exceptional performance as a catalyst for the oxygen evolution reaction (OER) and provides the possibility of controlling the morphology and the chemical properties during deposition. However, the detailed atomic structure of this hybrid material is not well understood. To elucidate the Co/graphene electronic structure, we have developed a flow cell closed by a graphene membrane that provides electronic and chemical information on the active surfaces under atmospheric pressure and in the presence of liquids by means of X-ray photoelectron spectroscopy (XPS). We found that cobalt is anchored on graphene via carbonyl-like species, namely Co(CO)x , promoting the reduction of Co(3+) to Co(2+), which is believed to be the active site of the catalyst.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4 Nanorods

The electronic and structural properties of vanadium-containing phases govern the formation of isolated active sites at the surface of these catalysts for selective alkane oxidation. This concept is not restricted to vanadium oxide. The deliberate use of hydrothermal techniques can turn the typical combustion catalyst manganese oxide into a selective catalyst for oxidative propane dehydrogenation. Nanostructured, crystalline MnWO4 serves as the support that stabilizes a defect-rich MnOx surface phase. Oxygen defects can be reversibly replenished and depleted at the reaction temperature. Terminating MnOx zigzag chains on the (010) crystal planes are suspected to bear structurally site-isolated oxygen defects that account for the unexpectedly good performance of the catalyst in propane activation.

High-Performance Supported Iridium Oxohydroxide Water Oxidation Electrocatalysts

The synthesis of a highly active and yet stable electrocatalyst for the anodic oxygen evolution reaction (OER) remains a major challenge for acidic water splitting on an industrial scale. To address this challenge, we obtained an outstanding high-performance OER catalyst by loading Ir on conductive antimony-doped tin oxide (ATO)-nanoparticles by a microwave (MW)-assisted hydrothermal route. The obtained Ir phase was identified by using XRD as amorphous (XRD-amorphous), highly hydrated IrIII/IV oxohydroxide. To identify chemical and structural features responsible for the high activity and exceptional stability under acidic OER conditions with loadings as low as 20 μgIr  cm-2 , we used stepwise thermal treatment to gradually alter the XRD-amorphous Ir phase by dehydroxylation and crystallization of IrO2 . This resulted in dramatic depletion of OER performance, indicating that the outstanding electrocatalytic properties of the MW-produced IrIII/IV oxohydroxide are prominently linked to the nature of the produced Ir phase. This finding is in contrast with the often reported stable but poor OER performance of crystalline IrO2 -based compounds produced through more classical calcination routes. Our investigation demonstrates the immense potential of Ir oxohydroxide-based OER electrocatalysts for stable high-current water electrolysis under acidic conditions.

Trends in reactivity of electrodeposited 3d transition metals on gold revealed by operando soft x-ray absorption spectroscopy during water splitting

We activated gold electrodes for their use as electrocatalyst for water splitting by electrodepositing Cu, Ni and Co. A combination of operando x-ray absorption spectroscopy and potentiometric control under aqueous conditions revealed the trends in reactivity yielded by these electrodes, which are directly associated with the cross- and overpotentials as well as the occupancy of the 3d orbitals. It was found that under anodic polarization the materials electrodeposited on gold suffer from a lack of stability, while under cathodic polarization they exhibit stable behavior. The observed activity is strongly related to the lack of stability shown by these composites under anodic polarization revealing a dynamic process ruled by corrosion. By operando x-ray absorption, we established that the overall enhancement of the activity for the oxygen evolution reaction is directly attributable to the cross-potential and corrosion process of the electrodeposited materials. It is associated with the high potential deposition, which is the origin of the incipient oxidation-corrosion resistance of the lattice. We conclude that the observed trends in the total current are directly associated with the loss of oxygen in the metal-oxide lattice and the subsequent dissolution of metallic ions in the electrolyte under anodic polarization.

Reactive Electrophilic OI− Species Evidenced in High-Performance Iridium Oxohydroxide Water Oxidation Electrocatalysts

Abstract Although quasi‐amorphous iridium oxohydroxides have been identified repeatedly as superior electrocatalysts for the oxygen evolution reaction (OER), an exact description of the performance‐relevant species has remained a challenge. In this context, we report the characterization of hydrothermally prepared iridium(III/IV) oxohydroxides that exhibit exceptional OER performances. Holes in the O 2p states of the iridium(III/IV) oxohydroxides result in reactive OI− species, which are identified by characteristic near‐edge X‐ray absorption fine structure (NEXAFS) features. A prototypical titration reaction with CO as a probe molecule shows that these OI− species are highly susceptible to nucleophilic attack at room temperature. Similarly to the preactivated oxygen involved in the biological OER in photosystem II, the electrophilic OI− species evidenced in the iridium(III/IV) oxohydroxides are suggested to be precursors to species involved in the O−O bond formation during the electrocatalytic OER. The CO titration also highlights a link between the OER performance and the surface/subsurface mobility of the OI− species. Thus, the superior electrocatalytic properties of the iridium (III/IV) oxohydroxides are explained by their ability to accommodate preactivated electrophilic OI− species that can migrate within the lattice.

Identification of reactive oxygen species in iridium-based OER catalysts by in situ photoemission and absorption spectroscopy

A change in our energy supply from fossil fuels to intermittent renewables requires energy storage capacity. Part of this capacity may be delivered from chemical energy conversion relying on H2 as basic fuel. Renewable H2 can be generated using proton exchange membrane (PEM) electrolyzers able to adapt to the varying voltage inputs of intermittent sources. Such PEM electrolyzers operate in acidic environments requiring corrosion-resistant catalysts for the H2 and O2 evolution reactions (HER & OER). Especially the OER is challenging and typical catalysts use rare and precious iridium oxides. Minimizing costs by reducing the iridium usage requires knowledge-based catalyst design: Favorable iridium oxide surface configurations need to be identified. X-ray photoemission and Near-edge X-ray absorption fine structure spectroscopy (XPS & NEXAFS) are powerful techniques to characterize surfaces. Therefore, having observed the increased catalytic OER activity of X-ray amorphous IrOx when compared to rutile-type IrO2, these techniques were combined with theory to identify respectively present iridium and oxygen species based on their signatures in the electronic structure. While rutile-type IrO2 was confirmed to consist only of Ir and OII−, the more active amorphous IrOx was observed to contain Ir and OI− in addition. The electron deficiency of the OI− species led to the suspicion that they may be good electrophiles and enhance the OER activity. Therefore, their character and reactivity were tested with the prototypical probe molecule CO. By monitoring both the gas phase composition and the spectroscopic fingerprint of OI− in the NEXAFS of the O Kedge, the spontaneous reaction between OI− and CO to form CO2 at room temperature was observed, confirming the electrophilic character and exceptional reactivity of OI−. To test the involvement of OI− species in OER catalysis, an electrochemical in situ cell was employed to monitor the electronic structure of an oxygen-evolving iridium surface by XPS and NEXAFS. These experiments confirmed the formation of a mixedvalent Ir matrix hosting both OII− and electrophilic OI− species during the OER. Measurements near the onset of iridium’s OER activity yielded a linear correlation between OI− concentration and OER activity. Further, major parts of the OI− contribution could be reversibly switched on and off when turning on and off the applied potential. These observations further indicated the intimate relationship between the presence of electrophilic OI− species and the OER activity of iridium-based catalysts. This connection may be understood by analogy with photosystem II: Electrophilic oxygen species can facilitate the nucleophilic attack of water during the O-O bond formation. This thesis demonstrates that the integration of electrophilic OI− species is a crucial design criterion for OER catalysts and explains why iridium is a good choice: It has the propensity to form electrophilic OI− species enhancing the O-O bond formation.