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Abstract:
To minimize the consequences from the progressing global warming, the way energy is roduced, stored, and released must change. Thanks to recent progress, the fraction of electricity from renewable energy sources like wind- and solar power increases steadily. However, one main disadvantage of both methods is the dependability on external energy sources like sun and wind which prove to be unreliable at times. Currently it remains impossible to provide energy from renewable sources on demand. Moreover, frequently, excessive renewable electricity from solar and wind power cannot be used and simply expires. Here, the production of green hydrogen through water electrolysis from excessive, renewable electricity is a promising way of storing power chemically. However, the presently implemented precious metal catalysts made of Ir will not stay economically viable with respect to the required necessary number of electrolyzers. Instead, the more abundant Co oxides are an appealing alternative with lower costs of acquisition which can be implemented in alkaline systems.
This thesis focuses on the anodic side reaction of water electrolysis, the oxygen evolution reaction (OER). Chapter 1 elaborates in more detail on the motivation for a hydrogen economy. Chapter 2 provides a scientific perspective of the current state of science for alkaline OER catalysts and leads to open questions to optimize catalyst design. The utilized methodologies are explained in Chapter 3 and experimental results discussed in Chapters 4-7.
Chapter 4 is a fundamental study of the active state during OER on the basis of size-dependent trends in Co oxide nanoparticles (NPs). Here, operando X-ray absorption spectroscopy investigates the catalyst during reaction and the chemical and electronic structure is identified through comparison with density functional theory (DFT) calculations. Chapters 5 and 6 discuss the role of the CoFe composition in OER catalysts and correlate it with the OER activity and structural changes. This is done on a local scale in Chapter 5 where the effect of inhomogeneities in the nanoscale on the catalysis is investigated. Chapter 6 describes the influence of the Co:Fe ratio on OER activity and correlated structural trends we also illustrated. Finally, Chapter 7 covers the effect that Fe impurities have on Co3O4 catalysts. It has been well documented that the (un-)intentional addition of Fe to Co oxides improves the OER activity. However, the role of Fe and the interaction with Co is hardly understood. The final Chapter 8 provides an overview of the results and an outlook what can be done based on the herein presented work.
