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Authors: Wang, Li
Title: Development and investigation of oxygen evolution reaction catalysts for proton exchange membrane electrolyzers
Other Titles: Entwicklung und Untersuchung von Katalysatoren für die Sauerstoffentwicklungsreaktion bei Polymer-Elektrolyt-Membran-Elektrolyseuren
Issue Date: 2018 Dissertation xx, 173
Abstract: Hydrogen as an energy carrier is expected to play a vital role in the future renewable dominated energy system. One of the promising technologies to produce high purity H2 is the proton exchange membrane (PEM) electrolyzer. However, its large-scale commercialization is hindered by the high investment cost, even though the technology is becoming mature. One main cost contributor is the anode catalyst, which requires Ir-based materials with a high loading to overcome the high overpotential of oxygen evolution reaction (OER). Aiming to address the challenge to reduce the amount of Ir required in the anodes, several OER catalysts were developed in this work by adopting different strategies. On the one hand, Ir nanoparticles were supported on electro-conductive ceramic materials, including Magnéli phase Ti4O7 and SnO2:Sb aerogel, to increase the Ir utilization. Both Ir/Ti4O7 and Ir/SnO2:Sb aerogel catalysts show significantly improvement in Ir mass activity. In particular the latter one achieved the same performance compared to the unsupported counterpart while only containing ca. 30 wt.% Ir on the electrode. On the other hand, to increase the specific OER activity for each active site, one unsupported Ir-rich catalyst was derived from IrRuOx by electrochemical Ru leaching. After stabilization of Ru leaching, it demonstrates a 13-fold OER activity greater than state-of-the-art, rutile phase IrRuO2. The developed catalysts were physically characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). Subsequently, the materials were electrochemically evaluated by rotating disc electrode (RDE) technique, consisting of cyclic voltammetry (CV), linear scanning voltammetry (LSV) and chronopotentiometry. Besides, CO-stripping and Cu underpotential deposition (Cu-UPD) were employed to investigate their electrochemical surface area (ECSA). Advanced operando techniques were used to investigate the stabilization and catalysis mechanisms of the given catalysts. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) was applied on rutile phase RuO2 and IrRuO2. The results unveil that the abundant unstable Ru(OH)x formation on the electrode surface leads to the fast degradation of RuO2 while Ir prevents its formation in the case of IrRuO2 thereby stabilizes the electrode. Additionally, the amorphous IrOx and rutile phase IrO2 were comparatively studied by both NAP-XPS and soft X-ray adsorption near edge structure (XANES). A correlation between the concentration of anion OI- species and the OER activity on both Ir@IrOx and IrO2 electrodes was observed, suggesting the OER catalysis mechanism on Ir oxides is likely to involve anion rather than cation red-ox chemistry regardless of the oxide structure (amorphous vs. rutile). At the end, the above developed Ir-rich catalyst that shows promising performance from RDE measurements was tested in a PEM electrolyzer. It demonstrated an apparent higher OER activity compared to IrRuO2 and no cell potential increase was observed for ca. 400 h of electrolysis operation. To correlate the fundamental understanding and those acquirements from engineering perspective, a simulation model based on Butler-Volmer equation was established to extract the key kinetic parameters for the anode catalyst from the electrolyzer cell test. Ir-rich catalyst and IrRuO2 were implemented with the model, their exchange current density (i_oa) and charge transfer coefficient (α_an) were obtained, then the corresponding Tafel slopes were calculated, with which the OER catalysis mechanisms are elucidated accordingly.
Appears in Collections:04 Fakultät Energie-, Verfahrens- und Biotechnik

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