Correlative ex-situ analysis and operando ellipsometric investigation during oxygen evolution reaction of mesoporous iridium oxide model catalysts
In the face of rising energy demand and the impending climate change, the development of sustainable, fossil-free fuel and chemical production is of global importance. One way to achieve these goals is the development of electrochemical conversion processes using catalysts. Key to the development of improved catalysts is a better understanding of the relations between their performance, stability, and physicochemical properties. However, the complex morphology of such catalysts constitutes a challenge even for modern analytical techniques. For example, standard rotating disc electrode experiments are performed to study the impact of the catalysts structure on electrochemical processes, but do not provide any information about changing material properties during the experiment. Ellipsometry is a very versatile spectroscopic method, often used to study film thickness and refractive index, but also providing access to electrical and electronic properties. Spectroscopic ellipsometry (SE) further offers the possibility to study materials under different environments. However, since material properties cannot be taken directly from the measured spectra, suitable models have to be developed and validated. In this thesis, a model for ellipsometric fit studies of a calcination series of mesoporous iridium oxide films (300 °C – 600 °C) was developed and validated in terms of film thickness and porosity as well as electrical and electronic properties using various ex-situ characterization methods such as scanning electron microscopy (SEM), X-ray reflectometry (XRR), conductivity measurements, and UV-VIS-NIR absorption spectroscopy. SE data also offer the possibility to derive valence electron energy loss spectra (VEELS), which provide further information about the electronic structure of the materials. Comparison of the data from the VEEL spectra with activity measurements of the oxygen evolution reaction (OER) suggests that the intrinsic activity (surface charge normalized activity) of IrOx scales with interband transition energies between the Ir 5d t2g sub-level and the O 2p orbital. For SE studies under operando conditions, an environmental cell setup was developed that allows simultaneous spectroscopic characterization while measuring the catalytic activity of a reaction. Analyses on a platinum surface successfully demonstrate the oxidation and reduction of a platinum oxide layer as a function of the potential. Electrochemical SE studies (ECSE) on mesoporous IrOx films show a dependence of the electrical and electronic properties on the applied potential. Furthermore, the degree of filling of the pores with the generated gas (oxygen) can be detected. The influence of the calcination temperature can also be related to the potential dependent changes of the electrical and electronic properties as well as the pore filling degree. This work further discusses a possible band structure of the IrOx materials as a function of calcination temperature as well as potential and calcination temperature dependent changes during OER catalysis. Furthermore, a time-resolved and potential-dependent quantification analysis of the gas accumulation in the pores of the catalyst systems is discussed.
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