In order to pave the way for an energy storage system for intermittent renewable sources based on the fuel H2, the PEM water electrolysis technology requires major improvements. One of these required improvements is a cost-effective and stable cata-lyst. The state-of-the-art OER catalyst in acidic media is iridium oxide. Considering the previous research on Ir-based catalysts for the OER outlined in Sections 1.2 and 1.3, it is clear that many indications have been found with respect to the nature of the species in iridium oxides favoring its OER activity. Nevertheless, before being able to efficiently tailor Ir-based OER catalysts, several points concerning iridium and its oxides remain controversial in the literature and require further investigation:
• For being able to identify the type of species contained in different iridium oxide configurations based on XPS measurements, how can the peculiar XPS Ir 4f line shape of iridium oxide be explained? Is the line shape due to a physical/spectro-scopic cause or are the reference materials simply composed of multiple species?
• Why are X-ray amorphous iridium oxides more active in catalyzing the OER than crystalline rutile-type IrO2and bare metallic iridium?
• What is the nature of the iridium and oxygen species present on iridium oxide surfaces during the OER and how is their presence related to the OER activity?
The objectives of this PhD thesis may be deduced from these unresolved questions:
• To elucidate the electronic structure of iridium and its oxides by combining theo-retical calculations with experimental results, thereby resolving the origin of the Ir 4f line shape of iridium oxides and identifying fingerprint features of oxygen and iridium species contained in highly and less OER-active iridium oxides.
• To investigate the reactivity of surface species contained in highly OER-active iridium oxides by using the prototypical probe molecule CO.
• To identify surface species forming on oxygen-evolving Ir surfaces and contribute to a further understanding of the OER reaction mechanism on Ir by making use ofin situphotoemission and absorption spectroscopy.
The following results part is divided into three chapters addressing these objectives.
1.5 Scientific objective and outline of this work
Chapter 2* delivers a detailed understanding of the electronic structure of iridium metal and crystalline as well as X-ray amorphous iridium oxides. To gain this un-derstanding and to resolve the controversy in literature about the peculiar Ir 4f line shape of iridium oxides, XPS and NEXAFS are combined with theoretical calculations.
First, Chapter 2 describes the thorough characterization of two reference iridium oxide powders, one crystalline and one X-ray amorphous, using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), the Brunauer–Emmett–Teller (BET) method, thermogravimetry (TG), differential scan-ning calorimetry (DSC), temperature-programmed reduction (TPR), and finally linear sweep voltammetry (LSV) to assess their OER performance. Second, Chapter 2 points out the origins of the different Ir 4f line shapes and features in the O K-edges of irid-ium metal and the two reference iridirid-ium oxide powders, which enables a speciation of present iridium and oxygen species. Finally, Chapter 2 discusses in situXRD and XPS/NEXAFS heating experiments of the amorphous iridium oxide powder that doc-ument its transformation into a crystalline configuration.
Chapter 3† uses the knowledge about the electronic structure of highly OER-active X-ray amorphous iridium oxides acquired in Chapter 2 to identify reactive oxygen species contained in these materials by employing the prototypical probe molecule CO. First, Chapter 3 demonstrates that, without the addition of external oxygen, highly OER-active X-ray amorphous iridium oxides oxidize CO to CO2 at room temperature in a stoichiometric reaction. Second, Chapter 3 asserts by a combination of quasiin situ XPS/NEXAFS measurements, ab initio calculations and a microcalorimetric analysis that the oxygen species spontaneously reacting with CO are those contained in the amorphous iridium oxide that have holes in their O 2p states, i. e. the electrophilic OI−
species. These electrophiles are susceptible to nucleophilic attack not only by CO to form CO2 but possibly also by H2O/OH during the O-O bond formation process of the OER. Finally, Chapter 3 confirms through a quasiin situXPS/NEXAFS experiment
*Reproduced from the publications
Pfeifer, V., Jones, T. E., Velasco Vélez, J. J., Massué, C., Arrigo, R., Teschner, D., Girgsdies, F., Scherzer, M., Greiner, M. T., Allan, J., Hashagen, M., Weinberg, G., Piccinin, S., Hävecker, M., Knop-Gericke, A., and Schlögl, R.2016The electronic structure of iridium and its oxides. Surf. Interface Anal., 48, 258–270, doi: 10.1002/sia.5895
with permission from John Wiley and Sons and
Pfeifer, V., Jones, T. E., Velasco Vélez, J. J., Massué, C., Greiner, M. T., Arrigo, R., Teschner, D., Girgs-dies, F., Scherzer, M., Allan, J., Hashagen, M., Weinberg, G., Piccinin, S., Hävecker, M., Knop-Gericke, A., and Schlögl, R.2016The electronic structure of iridium oxide electrodes active in water splitting.
Phys. Chem. Chem. Phys., 18, 2292–2296, doi: 10.1039/C5CP06997A with permission from the PCCP Owner Societies.
†Reproduced from the publication
Pfeifer, V., Jones, T. E., Wrabetz, S., Massué, C., Velasco Vélez, J. J., Arrigo, R., Scherzer, Piccinin, S., Hävecker, M., Knop-Gericke, A., and Schlögl, R.2016Reactive oxygen species in iridium-based OER catalysts.Chem. Sci., 7, 6791-6795, doi: 10.1039/C6SC01860B
with permission from the Royal Society of Chemistry.
that these reactive oxygen species can be replenished, which is a prerequisite for their participation in the catalytic OER cycle.
Chapter 4‡ employsin situXPS/NEXAFS cells enabling the observation of the elec-tronic structure of an iridium (oxide) surface while it evolves oxygen. To interpret the changes in the electronic structure fingerprints during the OER, Chapter 4 combines the findings from Chapters 2 and 3 and fosters that the presence of the reactive oxygen species identified in highly active X-ray amorphous iridium oxides is closely tied to the OER activity of iridium surfaces. First, Chapter 4 shows how the initially metallic iridium surface is almost completely oxidized during the OER at high overpotentials and identifies which types of iridium and oxygen species form during this process.
Second, Chapter 4 describes experiments performed at moderate OER overpotentials to determine the presence of which species is correlated with the OER activity of the material. Finally, Chapter 4 draws a parallel to photosystem II and points out that the electrophilic OI− species contained in highly active X-ray amorphous iridium oxides and accommodated during the OER on an initially metallic iridium surface are likely favoring the suggested rate- and potential-determining step of the OER, i. e. the O-O bond formation.
Chapter 5 delivers a final conclusion of the findings obtained throughout this thesis.
‡Reproduced with adaptions from the publication
Pfeifer, V., Jones, T. E., Velasco Vélez, J. J., Arrigo, R., Piccinin, S., Hävecker, M., Knop-Gericke, A., and Schlögl, R.2017In situobservation of reactive oxygen species on oxygen-evolving iridium surfaces.
Chem. Sci., 8, 2143-2149, doi: 10.1039/C6SC04622C with permission from the Royal Society of Chemistry.